Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3)

Table of Contents show

Welding Training Series:

Common welding defects

1. Shape defect of the weld (Fig. 6-1)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 1
6-1

2. Unqualified weld size (Fig. 6-2)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 2
6-2

3. Undercut (Fig. 6-3)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 3
6-3

A groove created along the toe or root of a weld.

1) Excessive welding current;

2) The welding arc is too long;

3) The electrode angle is incorrect.

4. Incomplete penetration (Fig. 6-4)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 4
6-4

Incomplete penetration of joint root during welding.

1) Incorrect groove size;

2) Improper selection of welding process parameters;

3) The electrode deviates from the groove center or the angle is incorrect.

5. No fusion (Fig. 6-5)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 5
6-5

Incomplete fusion and bonding between weld metal and base metal or weld bead metal.

1) The welding current is too small or the welding speed is too high;

2) Unqualified cleaning before welding;

3) The electrode deviates from the weld center.

6. Crater (Fig. 6-6)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 6
6-6

A depression formed at the end of a weld or at a joint.

7. Burnthrough (Fig. 6-7)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 7
6-7

During welding, molten metal flows out from the back of the groove to form perforation.

8. Overlap (Fig. 6-8)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 8

A metal nodule formed when molten metal flows to the unmelted base metal outside the weld.

9. Slag inclusion and inclusion

Slag or non-metallic impurities left in the weld after welding.

10. Air hole (Fig. 6-10)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 9

A hole formed by gas remaining in the weld after welding.

Gas source forming pore:

1) Outside air;

2) Moisture;

3) Oil contamination and impurities.

11. Welding cracks (Fig. 6-11)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 10

(1) According to welding position

(2) According to the crack direction

① The longitudinal crack is parallel to the weld

② Transverse crack perpendicular to weld

(3) According to the conditions of crack generation

① Hot crack Crack near the solidus temperature of weld and heat affected zone

② A crack cooled below the martensitic transformation temperature

③ Reheat crack

④ Ladder shaped cracks along the rolling direction of plate due to lamellar tearing

12. Splash

In CO2 welding, most of the molten metal of welding wire can be transferred to the molten pool, and some of the molten metal of welding wire flies out of the molten pool.

The metal flying out of the molten pool is called splash. Especially when large parameters are welded by CO2 gas shielded welding with thick welding wire, the spatter is more serious, and the spatter rate can reach more than 20%.

At this time, normal welding is impossible.

Splash is harmful. It not only reduces welding productivity and affects welding quality, but also worsens working conditions.

Splash hazard

The metal spatter loss in CO2 gas shielded welding process accounts for about 10% of the welding wire molten metal, and the serious loss can reach 30-40%.

Under the best condition, the spatter loss can be controlled within 2-4%.

1. The increase of spatter loss will reduce the deposition coefficient of welding wire, thus increasing the consumption of welding wire and electric energy, and reducing the welding productivity and welding cost.

2. The splashed metal sticks to the end face of the contact nozzle and the inner wall of the nozzle, which will make the wire feeding not smooth and affect the stability of the arc, reduce the protective effect of the shielding gas, and deteriorate the weld forming quality.

3. The splashed metal sticks to the contact nozzle, nozzle, weld and weldment surface, which needs to be cleaned after welding, which increases the auxiliary working hours for welding.

4. The metal splashed out during welding is easy to burn the welder’s work clothes, even burn the skin, and deteriorate the working conditions.

Due to the above problems caused by metal spatter, how to prevent and reduce metal spatter has always been a problem that must be paid attention to when using CO2 gas shielded welding.

Measures to reduce splash

(1) Correct selection of process parameters

1. Welding current and voltage In CO2 arc, there is a certain rule between the spatter rate and welding current for each diameter of welding wire.

In the low current area (short circuit transition area), the spatter rate is small, and after entering the high current area (fine particle transition area), the spatter rate is also small, while in the middle area, the spatter rate is the largest.

The spatter rate is small when the current is less than 150A or more than 300A, and the spatter rate between the two is large.

When selecting welding current, avoid the current area with high spatter rate as far as possible.

After the current is determined, the appropriate voltage shall be matched to ensure the minimum splash rate

2. Angle of welding gun When the welding gun is vertical, the spatter amount is minimum, the inclination angle is maximum, and the spatter is more.

It is better not to tilt the welding gun forward or backward more than 20 degrees.

3. The extension length of the welding wire also affects the spatter.

The length of welding wire shall be as short as possible.

(2) Select appropriate welding wire material and shielding gas composition.

For example:

1. Steel welding wire with low carbon content shall be selected as far as possible to reduce CO gas generated during welding.

The practice shows that when the carbon content in the welding wire is reduced to 0.04%, the spatter can be greatly reduced;

2. Use tubular welding wire for welding.

As the flux core of tubular welding wire contains deoxidizer and arc stabilizer, the gas slag joint protection makes the welding process very stable and the spatter can be significantly reduced. The metal spatter rate of flux cored wire is about 1/3 of that of solid wire

(3) CO2 mixture is used as shielding gas during long arc welding.

Although the spatter rate can be reduced through reasonable selection of specification parameters and the use of submerged arc method, the spatter amount is still large.

Adding a certain amount of Ar gas into CO2 gas is the most effective method to reduce metal spatter caused by excessive welding of particles.

The above physical and chemical properties of pure carbon dioxide gas are changed after adding Ar into the CO2 gas.

With the increase of Ar gas ratio, the spatter decreases gradually.

The CO2+Ar mixed gas can not only overcome the spatter, but also improve the weld formation, affecting the weld penetration, weld height and reinforcement.

When the content is 60%, the size of the transfer droplet can be obviously reduced, and even the spray transfer can be obtained, which improves the droplet transfer characteristics and reduces the metal splash.

Diagram of welding defects

1. Weld scale

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 11

Repair method

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 12

Weld surface after descaling

2. Air hole

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 13

Repair method: Grind and remove the weld and re weld.

3. Crater needle shaped air hole

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 14

4. Air hole (sand hole)

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 15

5. Shrinkage cavity

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 16

6. End crack/weld crack

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 17

7. Appearance of bad welds

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 18

8. Overlap and flash

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 19
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 20

9. Undercut

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 21
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 22
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 23

10. Uneven weld

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 24
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 25

11. Poor appearance

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 26
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 27
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 28
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 29

Weld symbol and mark

1. Basic symbols

The weld symbol consists of a basic symbol and a leader line, and if necessary, additional symbols, supplementary symbols, and weld size symbols.

The basic symbol is the symbol representing the cross section shape of the weld, which is similar to the cross section shape symbol of the weld, Table 4-2.

2. Auxiliary symbols and supplementary symbols

Auxiliary symbols are symbols that represent the shape characteristics of the weld surface.

They can be omitted when it is not necessary to specify the surface shape of the weld.

The supplementary symbols are used to supplement the symbols used for some characteristic surfaces of the weld, and their representation methods are shown in Table 4-3.

3. Size symbol of weld

When the weld size needs to be indicated in design or production, it is indicated by the weld size symbol, as shown in Table 4-4.

Table 4-2 Basic Symbols of Weld Forms

Serial NoWeld nameWeld typeBasic symbols
1I-shaped weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 30 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 31 
2V-shaped weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 32 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 33 
3Blunt V weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 34 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 35 
4Unilateral V-shaped weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 36 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 37 
5Single V-shaped weld with blunt edgeWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 38 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 39 
6U-shaped weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 40 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 41 
7Unilateral U-shaped weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 42 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 43 
8Flare weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 44 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 45 
9Fillet weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 46 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 47 
10Plug weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 48Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 49 
11Spot weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 50Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 51 
12Seam weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 52 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 53 
13Back beadWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 54 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 55 

Table 4-3 Auxiliary Symbols and Supplementary Symbols of Welds

Serial NoNameTypeAuxiliary symbolExplain
1Plane symbolWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 56Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 57 Indicates that the weld surface is flush
2Depression symbolWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 58Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 59 Denotes weld surface depression
3Raised symbolWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 60Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 61 Indicating weld surface bulge
Serial NoNameTypeSupplementary symbolExplain
1Symbol with backing plateWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 62Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 63 Indicates that there is a backing plate at the bottom of the weld
2Three side weld symbolWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 64Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 65 It is required that the opening direction of the three side weld symbol is basically consistent with the actual direction of the three side weld
3Peripheral weld symbolWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 66 Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 67 Indicates welding around the workpiece
4Site SymbolsWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 68Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 69 Indicates welding at site or construction site

Table 4-4 Size Symbols of Welds

 SymbolNameSketch Map
δSheet thicknessWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 70 
αGroove angleWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 71 
bButt clearanceWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 72 
pHeight of blunt edgeWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 73 
cWeld widthWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 74 
KFillet sizeWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 75 
dNugget diameterWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 76 
SEffective thickness of weldWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 77 
NNumber of identical welds symbolWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 78 
KFillet sizeWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 79 
RRoot radiusWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 80 
lWeld lengthWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 81 
nNumber of weld segments
HGroove depthWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 82 
hWeld reinforcementWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 83 
βGroove face angleWelding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 84 

4. Leader

(1) The leader consists of an arrow line with an arrow and two reference lines (one is thin solid line and the other is dotted line).

(2) The dotted line can be drawn on the upper or lower side of the thin and solid line.

The datum line is generally parallel to the long side of the title block, or it can be perpendicular to the long side of the title block if necessary.

The arrow line is drawn with a thin solid line, and the arrow points to the relevant weld seam. If necessary, the arrow line can be bent once.

When the welding method needs to be described, the tail symbol can be added at the end of the reference line.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 85
Fig. 4-1 Leader of weld symbol
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 86

5. Dimensioning method of common welds

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 87

(1) The dimensions on the cross section of the weld are marked on the left side of the basic symbol.

(2) The dimension along the length of the weld, marked on the right side of the basic symbol

(3) Groove angle α,  Groove face angle β, the root gap b is marked on the upper or lower side of the basic symbol.

(4) The same weld quantity and welding method code are marked at the tail.

(5) When there are many dimension data to be marked and it is difficult to distinguish, corresponding dimension symbols can be added in front of the data.

Table 12-1 Weld Symbols and Marking Methods

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 88

Welding joint and groove type

The common welded joints are butt joint, T-joint, corner joint and lap joint, as shown in the figure.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 89

Selection of welded joints: mainly based on the welding structure, weldment thickness, weld strength requirements and construction conditions.

Specified drawing method of weld

The seam formed after the workpiece is welded is called weld seam.

If it is necessary to draw the weld simply in the drawing, it can be represented by view, section view or section view, or it can be represented by axonometric diagram.

The specified drawing method of weld is shown in the figure.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 90

Welding stress and deformation

Welding deformation and stress always occur during structural welding.

In the welding process, the deformation and internal stress generated in the weldment that change with time are called transient deformation and welding transient stress respectively.

The deformation and stress remaining in the weldment when the temperature is cooled to room temperature after welding are called welding residual deformation and welding residual stress respectively.

3.1 Causes of welding stress and deformation

Uneven heating and cooling of the weld zone is the root cause of welding stress and deformation.

During welding, the weldment is heated locally, and the deformation is generated according to the characteristics of metal expansion and contraction.

However, the steel plate is a whole, and this extension cannot be realized freely.

The end of the steel plate can only be extended evenly Δι。

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 91
Stress and deformation during butt welding of flat plate

(a) During welding;
(b) After welding.

During cooling, since the metal near the weld has undergone irrecoverable compressive plastic deformation during welding, it is also constrained by the metal on both sides.

In order to maintain the overall consistency, Δι‘ is shrunk evenly, and a certain amount of elastic tension is generated in the weld area, and a certain amount of elastic compression is generated in the metal on both sides.

Therefore, there is tensile stress in the weld zone and the metal near it, and there is compressive stress in the metal on both sides.

The stress in the member is in equilibrium. It can be seen that after butt welding of flat plate, Δι‘ is shorter than that before welding.

At the same time, tensile stress is generated in the weld zone, and the metal pressure stress on both sides far away from the weld.

That is, welding stress and deformation are retained at room temperature – welding residual stress and deformation.

3.2 Distribution, influence and elimination of welding residual stress

Welding stress can be divided into thermal stress, restraint stress, phase change stress and welding residual stress. The welding residual stress is often very large.

In the welding structure with large thickness, the welding residual stress can generally reach the yield limit of the material.

1. Classification of welding stress

(1) Longitudinal stress: stress along the weld length;

(2) Transverse stress: the stress perpendicular to the weld length and parallel to the component surface;

(3) Stress in thickness direction: the stress perpendicular to the weld length direction and the component surface.

2. Distribution of welding residual stress

(1) Longitudinal stress of weld σ x

The stress along the longitudinal direction of the weld is called longitudinal stress σ x;

The stress perpendicular to the longitudinal direction of the weld is called transverse stress σ y .

In the compression plastic deformation zone of the weld and its vicinity σ X is the tensile stress, which can generally reach the yield strength of the material.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 92
Distribution of weld sections

(2) Transverse stress of weld

The figure shows the distribution of transverse stress σy in a plate weld with a certain length.

σy is tensile stress in the weld and the compressive plastic deformation zone near the weld, and the two ends are compressive stress.

The farther away from the weld center, the faster σy decays.

In addition to the longitudinal and transverse stresses, there are also stresses along the thickness direction in thick plate welded structures.

The stress distribution in the three directions is extremely uneven in the thickness direction.

Three axial tensile stress appears in the weld center of thick plate electroslag welding, which increases with the increase of plate thickness, but it is compressive stress on the surface.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 93

3. Effect of welding residual stress

(1) Influence on structural strength and stability of compression parts

When the component bears tensile load, the welding residual internal stress will be superimposed with the load stress, thus affecting the strength of the component.

(2) Influence on brittle fracture of components

The increase of the nominal stress of the component, together with the decrease of the toughness of the materials in the welding joint area and the generation of welding defects, will promote the brittle fracture of the component under low stress under low external load.

(3) Effect on fatigue strength

The residual tensile stress in the weld zone can increase the average tensile stress value of the structure and reduce the fatigue life.

(4) Influence on machining accuracy and dimensional stability of weldments

(5) Effect on crack propagation

Welding residual stress must be considered when evaluating the crack state of welding zone.

When calculating the driving force of crack growth – stress intensity factor KI, the residual stress σr uses the equivalent value of tensile stress σ3 to consider the contribution of residual stress to crack growth, namely:

 σ3 = αrσr

Among them, σr is related to the type of crack (through crack, buried crack, surface crack) and crack direction (cracks parallel to the fusion line, cracks perpendicular to the fusion line, and fillet weld cracks).

4. Measures and methods to reduce and eliminate welding residual stress

Reduce welding residual stress from design and welding process

(1) The core of reducing welding stress in design is to correctly arrange the weld, so as to avoid stress superposition and reduce the peak stress.

① Minimize the number of welds, and reduce the size and length of welds.

② The weld should avoid excessive concentration (figure), have enough distance, and avoid crossing as much as possible to avoid three-dimensional complex stress.

③ Welds shall not be arranged in areas with high stress and abrupt changes in cross section to avoid stress concentration.

④ The table type of less rigid joint is adopted.

The flanging replaces the insertion tube.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 94
Two Splicing Methods of Spherical Vessels
(a) Staggered welds; (b) Weld intersection

(2) Methods to reduce welding stress in process

① Adopt reasonable welding sequence and direction. Allow most welds to be welded with less rigidity.

② Reduce the temperature difference between the welding area and the whole structure, so as to reduce the welding internal stress. Overall preheating, using less linear energy.

③ Hammer weld. Reduce welding stress and deformation.

④ Reduce hydrogen content and eliminate hydrogen.

(3) The method to eliminate residual stress is mainly to eliminate residual stress after welding. For pressure components of boilers and pressure vessels with thickness exceeding a certain size, post welding heat treatment shall be carried out to eliminate internal stress.

Generally, the workpiece will be deformed after welding. If the deformation exceeds the allowable value, the use will be affected.

The main reason for deformation is uneven local heating and cooling of the weldment.

Because during welding, the weldment is only heated to high temperature in local areas, but the metal in the heating area cannot expand freely because it is prevented by the metal with lower temperature around;

When cooling, it can not shrink freely due to the containment of surrounding metal.

As a result, this part of the heated metal has tensile stress, while other parts of the metal have compressive stress in balance with it.

When these stresses exceed the yield limit of the metal, welding deformation will occur;

Cracks occur when the strength limit of the metal is exceeded.

3.3 Forms, influencing factors and control methods of welding deformation

1. Welding deformation forms

The forms of welding deformation may be varied.

The most common forms are five basic forms or combinations of these forms.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 95
Basic forms of welding deformation

Fig. (a) shows the longitudinal and transverse shrinkage deformation of the flat plate after butt welding;

Fig. (b) shows the angular deformation of the flat plate after docking;

Fig. (c) shows the bending deformation caused by the deviation of the weld arrangement of the cylinder from the centroidal axis of the weldment;

Fig. (d) shows the wavy deformation of thin-walled weldment after welding.

In addition, the beam column structure is prone to distortion during welding.

The shrinkage deformation and bending deformation belong to the overall deformation, while the other forms are local deformation.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 96

2. Influencing factors of welding deformation

(1) Influence of weld position on welding deformation

The welds are arranged symmetrically in the structure, and only longitudinal and transverse shortening occurs.

However, when the welds are arranged asymmetrically in the structure, bending deformation will occur;

When the center of gravity of the weld section deviates from the center of gravity of the joint section, angular deformation will occur.

(2) Influence of structural rigidity

Under the same force, the structure with large rigidity has small deformation, while the structure with small rigidity has large deformation.

Welding deformation is always carried out along the direction with the minimum constraint of structure or weldment rigidity.

(3) Influence of assembly and welding sequence

The size of the rigid constraint when welding a strip weld depends on the assembly welding procedure.

For welding structures with symmetrical sections and welds, the method of first assembling into a whole can be adopted.

For complex welding structures, because there are many welds, the deformation caused by each weld affects each other and is difficult to control, so the sequence of partial assembly, welding, reassembly and re welding must be adopted to control the overall welding deformation.

(4) Other influencing factors

Deformation is also closely related to groove type, assembly clearance, welding specification and welding method.

3. Methods for controlling welding deformation

In order to control and reduce welding deformation, necessary reasonable design scheme and process measures shall be adopted.

(1) The number, length and size of welds shall be reduced as much as possible under the condition that the reasonable design ensures the bearing capacity.

The position of welds shall be reasonably arranged so that all welds in the structure are symmetrical to or close to the neutral axis of the section as far as possible to reduce the deformation of the weldment.

(2) Necessary process measures

① Reserve shrinkage allowance.

Add a certain shrinkage allowance when preparing the workpiece.

Generally, the longitudinal shrinkage of the weld is calculated according to the length of the weld.

The value is related to groove, joint type and plate thickness.

② The reverse deformation method uses experience or calculation method.

It is necessary to first judge the size and direction of possible deformation of the workpiece after welding.

When assembling before welding, place the weldment in the opposite direction of the deformation or make artificial deformation in advance.

Proper control can make the workpiece get the correct shape and prevent residual deformation.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 97
Anti deformation control of butt welding of 8~12mm thick steel plate

③ Select reasonable welding methods and specifications

Adopt energy concentrated heat source and fast welding method

④ Reasonable assembly and welding sequence

The large structure shall be properly divided into several parts, assembled and welded separately, and then assembled and welded into a whole.

⑤ Rigid fixation

The structure shall be fixed and clamped before welding, and the welding deformation shall be reduced by external constraints.

However, the rigid clamping prevents the free shrinkage of the weldment, which will produce large internal stress in the component.

The weldment material and structure shall be carefully selected.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 98

⑥ Use reasonable welding sequence

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 99

4. Correction of welding deformation

Even if the deformation control method is adopted, it is difficult to avoid deformation after welding.

When the weldment exceeds the welding deformation allowed by the product technical requirements, it is required to rectify after welding to make it meet the product quality requirements.

The essence of correction is to make the welding components produce new deformation to offset the deformation during welding.

The process of correcting welding deformation often increases the internal stress of components.

Therefore, it is better to eliminate the welding residual stress before correcting the deformation, so as to avoid local fracture of the component during the correction of deformation.

Common mechanical correction and flame heating correction in production

(1) Mechanical correction method:

Mechanical correction method is to use mechanical pressure or hammer cold deformation method to produce plastic deformation to correct welding deformation.

(2) Flame correction method:

The flame heating correction method uses the cooling shrinkage of the flame after local heating to offset the elongation and deformation of this part.

The heating position must be correct, and the heating temperature for flame heating correction is generally 600~800 ℃.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 100
Correction of T-beam welding deformation by flame heating
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 101
Correction of upper arch deformation
Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 102
Correction of angular deformation

(3) Special attention shall be paid to the steel type during orthopedic:

It is not suitable to hammer corrosion resistant equipment to prevent stress corrosion;

The stainless steel with intergranular corrosion tendency and the steel with higher hardening tendency should not be flame corrected;

For high strength steel with high cold cracking tendency, mechanical method should be used less, because it is easy to produce cold work hardening.

Welding process elements and specifications

Welding technology is the key factor to control the welding quality of joints.

In the factory, the elements of the welding process are specified in the detailed rules of the welding process.

The preparation of the detailed welding procedure card is based on the corresponding welding procedure qualification test results.

Welding process elements specified in the detailed welding procedure card:

① Preparation before welding;

② Brand and specification of welding materials;

③ Welding procedure specification parameters;

④ Operation technology;

⑤ Post welding inspection, etc.

2. Welding electrical parameters

(1) When using continuous AC and DC welding, the electrical parameters in the welding specification are mainly welding voltage and welding current.

(2) When pulse current welding is adopted, the electrical parameters also include the alternating frequency, on-off ratio, basic current and peak current value of the current.

(3) The selection principle of welding specification parameters is firstly to ensure the penetration and crack free of the joint and to obtain a well formed weld bead.

At the same time, the performance of the joint shall meet the requirements specified in the technical conditions.

Therefore, the influence of welding heat input on the performance of the joint shall be considered when selecting electrical parameters.

See Table 4-8 for diameter selection of manual arc welding electrode and corresponding welding current range

Table 4-8 Selection of electrode diameter and welding current for manual arc welding

Thickness of steel parts (mm)1.5234~56~89~1212~1516~20>20
Electrode diameter (mm)1.6233~444~555~66~10
Welding current (A)25~4040~6565~100100~160160~210160~250200~270260~300320~400

Table 4-9 Selection of double side submerged arc automatic welding specifications for beveled workpieces

Automatic submerged arc welding

Groove form

Diameter of welding wire (mm)

Weld sequence

welding current

(A)

Arc voltage

(V)

Welding speed

(m/h)

14

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 103

5

positive

830~850

36~38

5

negative

600~620

36~38

16

5

positive

830~850

36~38

5

negative

600~620

36~38

18

5

positive

830~850

36~38

5

negative

600~620

36~38

22

6

positive

1050~1150

38~40

5

negative

600-620

36~38

24

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 104

6

positive

1100

38~40

5

negative

800

36~38

30

6

positive

100~1100

36~40

5

negative

900~1000

36~38

Welding cracks and control

Welding crack refers to the metal material separation phenomenon (local fracture) within the scope of welding joint due to welding reasons (metallurgy, materials or internal and external forces) during or after welding.

Crack is one of the most dangerous welding defects, which is characterized by sharp end and much smaller separation width (opening displacement) than crack length.

Prevention of welding cracks is an important content in the design and manufacture of welding structures.

1. Classification of welding cracks

There are many kinds of welding cracks, and their classification methods are also very different with the deepening of people’s understanding of the essence of cracks.

The following table is a general classification method based on the time and location of cracks.

Table 4-11 Current crack classification method

Crack occurrence period

Occurrence site

Name

Welding process

Near the solid line

weld line

Solidification crack

Hot crack

Heat affected zone

Liquefaction crack

Below the solid phase line

weld line

Polygonal crack

Near recrystallization temperature T

Heat affected zone

High temperature plastic crack

Near room temperature

Heat affected zone

Cold crack

Heat affected zone and base metal rolling layer

Lamellar tearing

During re high temperature tempering heating after welding

Heat affected zone

Reheat crack

During use of corrosive medium

Welds, heat affected zone

Stress corrosion cracking

2. General conditions for welding crack formation

High strength steel bridges, shipbuilding steel structures, cold cracks, accounting for 90%.

In petrochemical plants or power equipment, hot cracks are the majority.

Pearlitic heat-resistant steel is easy to appear reheat cracks.

There are two reasons for cracks.

(1) The stress and strain caused by restraint is one of the main causes of cracking.

The cracking process must require a certain amount of stress, and the local uneven heating process in the welding process will inevitably cause the joint to be subject to tensile stress and strain due to the restraint of the whole structure during the cooling process of welding.

(2) In a certain temperature range, due to the existence of brittleness factors, specific parts of the joint will crack under tensile stress.

3. Welding cracks

1. Hot cracks

(1) Characteristics of welding hot cracks Hot cracks have the following morphological characteristics, which are different from other cracks:

① Most of the cracks open on the weld surface and have oxidation color.

② Cracks often occur at the junction of dendrites and along the longitudinal direction at the center of the weld cross section.

③ The cracks are generally intergranular, with high-temperature intergranular fracture property.

④ Most occur during and after solidification.

(2) Formation mechanism:

When there is low melting point eutectic in the solidification process of the weld, because the welding cooling speed is fast, when the grain has been solidified and the grain boundary is in the liquid state, and the deformation resistance is almost zero, if the welding tensile strain is large, the grain boundary may be pulled apart, forming cracks.

(3) Influencing factors

① Effect of weld chemical composition

Many eutectic crystals in welding are the products of welding metallurgical reaction.

All elements that can produce eutectic are elements that promote hot cracking;

All elements that can refine grains or produce high melting point compounds or make low melting point eutectic into spherical or blocky distribution are effective in inhibiting hot cracking.

Table 4-12 Effect of alloy elements on hot crack tendency

Seriously affect the formation of hot cracksA small amount has little effect, while a large amount promotes hot crackingReduce the hot cracking tendency of weldUndetermined
Carbon, sulfur, phosphorus, copper, hydrogen, nickel, niobiumSilicon (>0.4%) Manganese (>0.8%) Chromium (>0.8%)Titanium, zirconium, aluminum, rare elements, manganese (within 0.8%)Nitrogen, oxygen, arsenic

② Influence of weld section shape

As the macro segregation of deep and narrow welds is mainly concentrated in the middle of the weld, it is easy to form hot cracks.

Therefore, during automatic submerged arc welding of thick plates, special attention should be paid to adjusting the proportion of welding current and arc voltage, so that the weld shape coefficient is greater than 1.3~1.5.

In manual arc welding, the weld section is small and the current is low, so it is not easy to cause deep and narrow weld.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 105

③ Influence of welding process and weldment structure

The structure and welding process of weldment directly affect the restraint of welded joint, which is reflected in the welding tensile strain, and its influence on hot cracks belongs to mechanical factors.

(4) Measures to prevent welding hot cracks

① The basic measures to prevent hot cracks are to strictly control the chemical composition of the weld, limit the content of carbon, sulfur and phosphorus impurities, and also add enough desulfurizer in the welding materials.

② Take process measures, such as preheating before welding, heat tracing, welding with large wire energy (it shall be ensured that the weld shape factor is not small).

③ The rigidity of weldment shall be reduced as far as possible to reduce the internal stress of welding.

2. Cold cracks

(1) Characteristics of cold cracks

Cold crack is the most easily produced welding defect when welding low alloy high strength steel, medium alloy steel, medium carbon steel and other easily quenched steels.

① It occurs after solidification of weld metal, generally below martensite transformation temperature or at room temperature.

② It mainly produces heat affected zone, and the possibility of producing weld zone is very small.

③ It is often delayed.

(2) Cause: The essence of cold crack is the comprehensive effect of low plasticity structure (hardening structure) in heat affected zone of weldment, hydrogen in welded joint and welding stress.

(3) Influencing factors

① Hardening action

When the easily quenched steel is welded, the overheated zone will produce coarse martensite structure, which will reduce the plasticity of the metal in the heat affected zone and increase the brittleness.

When it is subjected to large welding tensile stress, it is easy to crack.

② Role of hydrogen

The cold cracks induced by hydrogen have the characteristics of delayed fracture from latency, initiation, propagation to cracking.

The length of the delay time is related to the hydrogen concentration and the stress level of the welded joint.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 106
TW – Austenitic transformation isothermal surface of the weld;
TH – Austenitic transformation isothermal surface in heat affected zone
Corresponding induced diffusion of hydrogen

③ Effect of welding stress

When the welding stress is tensile stress and occurs at the same time with hydrogen precipitation and material hardening, cold cracks are very easy to occur.

The thick plate welding is more likely to produce cold cracks at the root.

First, the thick plate is rigid, and second, the thick plate is cooled fast, which promotes the generation of quenching structure, resulting in greater welding stress.

3. Reheat cracks

(1) Characteristics of reheat cracks

① The reheat cracks are generated in the range of 540~930 ℃ after post welding stress relief heat treatment;

② The crack propagates along the grain boundary of coarse grain zone in heat affected zone;

③ Intergranular crack with branching shape will terminate when the crack extends to the fine grain area of weld or base metal.

(2) Mechanism of reheat crack formation

After the post welding stress relief heat treatment and reheating, the alloy carbides are dispersed and precipitated on the dislocation line after heat preservation at 550~700 ℃, which strengthens the intragranular.

At the same time, the strength of the grain boundary in the coarse grain area is low and the plasticity is poor.

During the reheating process, the residual stress is released, and the strength of the grain boundary is lower than that in the grain, which leads to the grain boundary cracking.

(3) Influencing factors

There are many factors affecting reheat cracks:

Such as the chemical composition, restraint state, welding specification, welding rod strength, stress relief specification and service temperature of the base metal.

① Chemical composition mainly affects grain boundary plasticity in heat affected zone;

② Restraint state and welding specification affect welding residual stress;

③ The specification of stress relief heat treatment or service temperature mainly affects the plastic strain and the degree of alloy carbide precipitation caused by reheating.

Therefore, the plastic deformation ability of coarse grain zone in heat affected zone, welding residual stress and plastic strain caused by reheat are three basic factors affecting reheat cracks.

(4) Measures to prevent reheat cracks

One is to improve the plasticity of coarse grain zone in welding heat affected zone;

The second is to reduce welding residual stress.

① The basic measure is to select the base metal with small reheat crack sensitivity

② Take all measures conducive to reducing residual stress.

③ Avoid the combination of welding residual stress and other stresses (structural stress, thermal stress during reheating, etc.)

④ The use of low matching welding materials is conducive to absorbing deformation.

⑤ Under the premise of ensuring the effect of stress relief, lower reheat temperature and shorter holding time shall be adopted as far as possible.

If reheating can be replaced by afterheat slightly lower than the preheating temperature, the afterheat is better.

4. Lamellar tear

(1) Characteristics of lamellar tearing

① In the process of rapid cooling of the weld, under the welding tensile stress in the direction of plate thickness, cracks parallel to the rolling surface of the base metal are generated in the steel plate, which often occur in T-shaped and K-shaped thick plate joints;

② Layered tearing is a kind of crack that occurs at room temperature, most of which occur after cooling to below 150 ℃ or room temperature after welding.

However, when the structural restraint is very high and the layered tearing sensitivity of steel is high, it may also occur at 300~250 ℃.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 107

(a) Typical position of laminar tearing in “T” joint;
(b) Layer tearing in downcomer joint of boiler drum

(2) Main factors causing lamellar tearing

① Influence of inclusions.

Inclusion is the main cause of steel anisotropy, and also the origin of lamellar tearing.

② Effect of base metal properties.

The plasticity and toughness of metal matrix itself have an important influence on lamellar tearing.

Poor plasticity and toughness mean poor resistance to lamellar tearing.

③ The influence of restraint stress.

Any welding crack occurs under the action of tensile stress, and lamellar tearing is no exception.

Only when corner joint and T-joint are easy to form large two-way restraint stress, layered tear will be caused.

(3) Precautions for lamellar tearing

It is difficult to repair the lamellar tear, and the prevention of such defects is the main task.

① When the restraint degree of the welded joint may cause lamellar tearing, the lamellar tearing sensitivity of the steel plate used shall be evaluated, and the steel plate with low lamellar tearing sensitivity shall be selected.

② Reasonable groove type shall be adopted to make the fusion line of the weld at the same angle with the steel plate as far as possible.

③ For steel grades sensitive to lamellar tearing, if the design allows, welding materials with lower strength grade, better plasticity and toughness can be used to reduce the stress in the thickness direction of the steel plate.

④ The layered tearing sensitivity of steel grades is high, and several layers of low strength weld metal can be pre deposited on the steel plate surface at the welding groove.

Workmanship of welding structure

Whether the weld seam arrangement of the welding structure is reasonable has a great influence on the quality and productivity of welded joints.

General principles of weld joint arrangement:

1. The weld arrangement shall be convenient for welding operation

The weld arrangement must ensure that there are conditions for free operation of welders and normal operation of welding devices around the weld.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 108

When submerged arc welding, the convenience of storing welding flux shall be considered.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 109

For spot welding and seam welding, convenient electrode insertion shall be considered.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 110

2. The weld position shall avoid the maximum stress and stress concentration

For welding components with large and complex stress, welds shall not be arranged at the position of maximum stress and stress concentration.

For example, the splicing weld of large-span welded steel beam and plate should not be placed in the middle of the beam, but rather an additional weld.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 111

3. The arrangement of welds shall be decentralized to reduce welding stress and deformation

Due to the dense or cross weld, the metal will be overheated, the heat affected zone will be enlarged, and the structure will be deteriorated.

Generally, the distance between two welds shall be more than 3 times of the plate thickness and not less than 100mm.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 112

4. Welds shall avoid machined surfaces as much as possible

If machining is required before welding, the weld position shall be designed as far away from the machined surface as possible.

On surfaces with high machining requirements, try not to set welds.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 113

5. The design of welding end transition shall be smooth and avoid melting during welding

There shall be no acute angle at the end of the weldment to avoid melting during welding, and the two welding joints shall have smooth transition to avoid stress concentration.

Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3) 114

GB/T 19804-2005/ISO 13920:1996

(1) General dimensional tolerance and geometric tolerance range of welded structures

Table 1 Linear Dimension Tolerance      Unit: mm    

Range of nominal size l

Tolerance class

A

B

C

D

2~30

Tolerance t

± 1

>30~120

± 1

± 2

± 3

± 4

>120~400

± 1

± 2

± 4

± 7

>400~1000

± 2

± 3

± 9

± 6

>1000~2000

±3

±4

±8

±12

>2000~4000

±4

±6

±11

±16

>4000-~8000

±5

±8

±14

±21

>8000~12000

±6

±10

±18

±27

>12000~16000

±7

±12

±21

±32

>16000~20000

±8

±14

±24

±36

>20000

±9

±16

±27

±40

(2) Angular dimension tolerance

The short side of the angle shall be used as the reference edge, and its length can be extended to a specific reference point.

In this case, the datum point shall be marked on the drawing.

See Table 2 for tolerances.

Figures 1 to 5 show specific examples.

Table 2 Tolerance of Angular Dimensions

Tolerance class

Nominal size (workpiece length or short side length) range/mm

0~400

>400~1000

>1000

Tolerance in angle △ a/(°)

A

± 20

Scholars 15

±10

B

± 45

±30

± 20

C

± 1 °

± 45

± 30

D

±130

Shi 115

Soil 1

Tolerance in length t/(mm/m)

A

Soil 6

Soil 4.5

±3

B

Scholars 13

±9

Scholars 6

C

Scholar 18

Scholars 13

±9

D

Scholars 26

Soil 22

Soil 18

(3) Straightness, flatness and parallelism

The straightness, flatness and parallelism tolerances specified in Table 3 are applicable to all dimensions of weldments, welded assemblies or welded components, as well as the dimensions marked on the drawings.

Other geometric tolerances, such as coaxiality and symmetry tolerances, are not specified.

If such tolerance is required in actual production, it shall be marked on the drawing according to GB/T1182.

Table 3 Tolerances for Straightness, Flatness and Parallelism       Unit: mm

 

Public grade

E

F

G

H

Range of nominal dimension l (corresponding to the longer side of the surface)

>30~120

Tolerance t

± 0.5

±1

± 1.5

± 2.5

>120~400

±1

± 1.5

±3

±5

>400~1000

±1.5

±3

± 5.5

±9

>1000~-2000

±2

± 4.5

±9

±14

>2000~4000

±3

±6

±11

±18

>4000~8000

±4

±8

±16

±26

>8000~-12000

±5

±10

± 20

±32

>12000~16000

±6

±12

±22

±36

>16000~20000

±7

±14

± 25

±40

>20000

±8

±16

± 25

±40

Inspection of sheet metal riveting welding section

1. Raw material inspection standard

1.1 Metal materials

1.1.1 The thickness and quality of sheet metal shall conform to the national standard, and the performance test report and manufacturer’s certificate shall be presented for the sheet metal used.

1.1.2 Material appearance: flat without rust, crack and deformation.

1.1.3 Dimensions: according to the drawings or technical requirements, and those not provided by our company shall be subject to the current national standards.

1.2 Plastic powder

1.2.1 The whole batch of plastic powder is in good consistency, with factory certificate and inspection report, including powder number, color number and various inspection parameters.

1.2.2 Meet the product requirements after trial (including color, luster, leveling, adhesion, etc.).

1.3 General hardware and fasteners

1.3.1 Appearance: the surface shall be free of embroideries and burrs, and the appearance of the whole batch of incoming materials shall be consistent.

1.3.2 Size: according to the requirements of drawings and national standards.

1.3.3 Performance: trial assembly and service performance meet product requirements.

2. Process quality inspection standards

2.1 Blanking inspection standard

Sharp corners, edges and rough surfaces that may cause damage shall be deburred.

The burr produced by stamping shall be free of obvious bulge, depression, roughness, scratch, rust and other defects on the exposed and visible surfaces of door panels and panels.

Burr: after blanking, the burr height L ≤ 5% t (t is the plate thickness).

Scratches and knife marks: it is qualified if it is not scratched by touching with hands, and should be ≤ 0.1.

The requirements for plane tolerance are shown in Table I.

Attached Table 1. Flatness Tolerance Requirements

Surface dimension (mm)Deformation size (mm)
Below 3Less than ±0.2
More than 3 but less than 30Less than ±0.3
More than 30 but less than 400Less than ±0.5
More than 400 but less than 1000Less than ±1.0
More than 1000 but less than 2000Below ±1.5
More than 2000 but less than 4000Less than ± 2.0

2.2 Bending inspection standard

2.2.1 Burr: the height of extruded burr after bending L ≤ 10% t (t is the plate thickness). Unless otherwise noted, the bending fillet is R1.

2.2.2 Indentation: creases can be seen, but can not be felt by touching (can be compared with the limit sample).

2.2.3 The bending deformation standard shall be in accordance with Table II, Table III and Table IV

2.2.4 The bending direction and size shall be consistent with the drawings.

Attached Table 2: Diagonal Tolerance Requirements

Diagonal dimension (mm)Dimension difference of diagonal (mm)
Below 300Below ±0.3
More than 300 but less than 600Less than ±0.6
More than 600 but less than 900Less than ±0.9
More than 900 but less than 1200Less than ±1.2
More than 1200 but less than 1500Less than ± 1.5
More than 1500 but less than 1800Less than ± 1.8
More than 1800 but less than 2100Below±2.1
More than 2100 but less than 2400Below±2.4
Over 2400 to 2700Below ±2.7

2.3 Angle:

The angle shall be inspected according to the drawing requirements, and the angle tolerance is shown in Table III.

Limit deviation value of angular dimension

Limit deviation value of angular dimension

Tolerance class

Basic size segmentation

0-10

>10-50

>50-120

>120-400

>400

Precision f

± 1 °

±30’

+20’

±10’

±5’

Medium m

Coarse c

+1°30

+1°

+30

+15′

+10’

Coarsest v

+3°

±2°

+1°

+30’

+20’

2.4 Inspection standards for sheet metal workpieces

The dimensions shall be inspected according to the drawing requirements, and the dimensional tolerance is shown in Table IV.

Attached Table 4: Dimensional Tolerance Requirements

Standard sizeDimensional tolerance (mm)
Below 3±0.2
More than 3 but less than 30±0.3
More than 30 but less than 400± 0.5
More than 400 but less than 1000±1.0
More than 1000 but less than 2000± 1.5
More than 2000 but less than 4000± 2.0

2.5 Welding

2.5.1 Welds shall be firm and uniform, and shall be free of defects such as insufficient welding, cracks, incomplete penetration, welding penetration, notch, undercut, etc.

The weld length and height shall not exceed 10% of the length and height requirements.

2.5.2 Requirements for welding point: the length of welding point is 8~12mm, the distance between two welding points is 200 ± 20mm, the position of V welding point should be symmetrical, and the upper and lower positions should be unified.

If there are special requirements for welding points on the processing drawings, the drawings shall prevail.

2.5.3 The spot welding distance is less than 50mm, the spot welding diameter is less than φ5, the spot welding is evenly arranged, the indentation depth on the spot welding is not more than 15% of the actual thickness of the plate, and no obvious welding scar can be left after welding.

2.5.4 After welding, other non welding parts are not allowed to be damaged by welding slag and arc, and the surface welding slag and spatter must be removed.

2.5.5 After welding, the outer surface of the parts shall be free of slag inclusion, air hole, overlap, bulge, depression and other defects, and the defects on the inner surface shall not be obvious and will not affect the assembly.

The post welding stress of important parts such as door panel and panel shall also be removed to prevent workpiece deformation.

2.5.6 The external surface of the welding parts shall be ground smooth. For powder sprayed parts and electroplated parts, the grinding roughness after welding shall be Ra3.2~6.3, and that of the painted parts shall be Ra6.3~12.5.

3. Inspection standards for sprayed parts

3.1 Appearance inspection (inspection method: visual inspection and hand feeling)

3.1.1 Before spraying, the workpiece surface shall be degreased, derusted, phosphatized, cleaned, etc.

3.1.2 There is no watermark or residual cleaning solution on the workpiece surface.

3.1.3 There shall be no oil stain, dust, fiber and other undesirable phenomena that may affect the quality or adhesion of the sprayed surface.

3.1.4 The color shall conform to the sample plate (no obvious color difference is observed under natural light or fluorescent lamp 60w for normal vision), and no color difference is found for the same batch of products (note: color difference includes color and glossiness);

3.1.5 The coating surface shall be smooth, flat and even, and the surface shall be free of the following defects:

Non drying and back sticking: the surface is dry, but actually not completely dry, with (or easy to produce) grain marks on the surface and fabric fluff;

Sagging: there are liquid flowing protrusions on the surface, and the top is bead shaped;

Particles: the surface is in the shape of sand particles, and it feels blocked when touching;

Orange peel: the appearance is uneven and irregular like orange peel;

Bottom leakage: the surface is transparent and the color of the substrate is exposed;

Pits: small holes (pits) on the surface due to shrinkage, also called pinholes;

With pattern: the surface color varies in depth, showing patterns;

Wrinkle: locally piled and raised, showing wrinkles (except for wrinkle powder);

Inclusion: there are sundries in the coating;

Mechanical damage: scratches, scratches, abrasion and bruises caused by external forces.

3.1.6 Classification standard of grade plane:

Level A surface: the external surface often seen after assembly, such as the cabinet panel, cabinet door, the sides around the cabinet, the top surface visible to ordinary people and the low surface visible without bending over.

Grade B surface: the surface that is seldom seen but can be seen under certain conditions.

For example, the inner accessories, reinforcing ribs and the inner side of the gate can be seen after opening.

Grade C surface: the surface that cannot be seen generally or can only be seen during assembly. The contact surface between the carriage and the guide rail in the cabinet.

3.1.7 Inspection conditions

A Light source requirements: Arctic daylight or indoor high efficiency fluorescent lamp with two light sources (illuminance of 1000 lumens).

Visual inspection distance: 300mm for Class A surface, 500mm for Class B surface and 1000mm for Class C surface.

3.1.8 Inspection standards

According to the requirements of the light source standard, the grade surface of the product shall be distinguished.

The coating film of all grade surfaces shall be free of base material exposure, peeling and other defects, and all surfaces shall be free of scratches, bubbles, pinholes, powder accumulation and other undesirable phenomena.

Color and pattern: the manufacturer shall make samples as required, which shall be confirmed by both parties.

The acceptance shall be conducted according to the sample, without obvious color difference (no more than 3 degrees), and the grain shall conform to the sample.

At the standard of eye distance level, scan at a speed of 3m/min.

3.1.9 Appearance defect standard

See Attached Table 5 for the determination criteria.

Attached Table 5: Judgment Criteria for Surface Defects

Serial No

Defect type

Specification value (mm)

Area limit (mm2)

Inspection tools

Below 100

100-300

Above 300

A

B

C

A

B

C

AB

 

C

1

Abrasion, scratch, scratch

10 in length and less than 0.1 in width

0

2

2

0

3

1

4

 

4

Vernier tape

Length: 10, width: less than 0.15

0

1

1

0

2

21

3

 

3

15 in length and less than 0.1 in width

0

0

0

0

1

1

1

2

2

More than 0.15 wide

0

0

 

0

0

0

0

1

1

2

Foreign particle

Below 1

1

2

3

2

3

4

3

4

5

vernier

Below 1.5

0

1

2

1

2

3

2

3

4

Below 2

0

0

1

0

1

2

0

2

3

3

Shrinkage cavity

Below φ0.3

1

1

2

2

2

3

3

3

4

vernier

Below φ0.5

0

0

1

1

1

2

2

2

3

Above φ0.5

0

0

0

0

0

1

0

0

2

4

Black dot White dot Other color dots

Below 0.3

1

2

2

2

3

3

3

4

4

vernier

5.

Bending indentation

3 in length and less than 0.2 in width.

2

3

3

3

4

4

4

5.

5.

vernier

Length: 5, width: less than 0.2

1

2

2

2

3

3

3

4

4

More than 5 long

0

1

1

1

2

2

2

3

3

More than 0.2 wide.

0

0

1

1

0

2

0

2

3

6.

color and lustre

In addition to the upper and lower limits of the specified color palette, no mixed colors and shedding are allowed

Visual inspection

7.

gloss

There shall be no unevenness as specified in the design.

Visual inspection

8.

Oil stains and stains

No

Visual inspection

Remarks: The values in the bold black boxes are the judgment criteria. For example, “2” means that under the specified conditions, no more than 2 points are allowed.:

3.2 Coating thickness inspection standard

unit: µm

ProjectOutdoor powderIndoor powderPaintingTest method
Product surface thickness60~12050~10040~70Coating thickness gauge
Inside thickness of product60~10050~8030~60Coating thickness gauge

3.3 Coating gloss and color detection

3.3.1 Fabrication of spraying color plate

A. During baking, 2 color plates shall be made for each furnace for performance test.

Take the metal plate of the same material as the product, size 80 × 120, which is added with the product under normal conditions

The powder number, curing condition, date and time shall be marked and signed by QE

Number, name, register and manage after confirmation. One for testing and one for archiving.

B. The validity period of the powder spraying color plate in the manufacturing process is two years, and it is stored at room temperature with a temperature and humidity of 70 ± 15%. The storage environment is free of any light.

3.3.2 Gloss and color detection method

Gloss: judged by glossmeter, with an incidence angle of 60 ° and an error of ± 5%, it is qualified.

Color: the color must conform to the design drawing or have no obvious difference from the standard color plate.

3.4 Coating adhesion test

3.4.1 Baige test method

After spraying, take a furnace color plate, and carve 11 layers on the coating surface in a vertical and horizontal parallel manner at an interval of 1mm.

With proper strength (the scratch shall be subject to the exposure of the substrate), cut the coating surface into 100 squares, and then cover it with strong transparent adhesive to tighten it at an angle of 45 degrees.

Then suddenly tear it off. At this time, check whether the objects in the square fall off.

One grid is one percent, and the acceptance standard is level 5, that is, the number of falling off is not more than 5 squares.

3.4.2 Assessment method

Grade 0: there should be no falling off (including crossing);

Grade 1: less than 5% shedding at intersections;

Grade 2: shedding of more than 5% but less than 15% at intersections;

Grade 3: shedding of more than 15% but less than 25% at intersections;

Grade 4: Shedding greater than 25% but less than 35% at intersections;

Grade 5: more than 35% shedding at intersections (including intersections);

3.4.3 Judgment method

When the coating thickness is less than 40μm, the side length of the square shall not be greater than 1mm, meeting the requirements of Grade 2;

When the coating thickness is>40μm and<90μm, the side length of the square is>1mm and<2mm, meeting the requirements of Grade 3;

When the coating thickness is>90μm and<120μm, the side length of the square is 2mm, meeting the requirements of Grade 4;

When the coating thickness is more than 120μm, the adhesion will be reduced.

Generally, the coating thickness is required to be no more than 120μmm.

At this time, the judgment standard is tentatively determined as: if there is a complete grid falling off, it is unqualified.

3.5 Bending plate test method

After spraying, take a furnace color plate, bend it 180 degrees, and make the internal bending angle equal to the thickness (r=t) or bend it 90 degrees for one time, and the coating will not fall off.

3.6 Alcohol solvent resistance test of coating

Wipe the coating surface repeatedly with white cotton cloth dipped in alcohol for 10 times (without pressing hard), and there should be no visible coating falling off on the cotton cloth; After the alcohol is completely volatilized, there shall be no difference in color and luster between the wiped part and the non wiped part.

3.7 Impact resistance test

Using the test equipment, use a 500g heavy hammer to drop freely from a height of 500mm.

The judgment standard is: after a quarter of the punch is impacted on the front, the surface coating is free of cracking and film falling.

3.8 Hardness test

Use a sharpened 2H pencil to form a 45 degree angle with the film surface, and push forward along the ruler for 15-30mm.

Check the film surface after wiping the slip mark with a rubber.

The judgment standard is: it is qualified when no substrate is exposed.

Leave a Comment

Your email address will not be published. Required fields are marked *