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

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In this comprehensive guide, we cover everything from welding methods and materials to common welding defects and how to prevent them. With detailed diagrams and expert tips, you’ll be able to identify and fix any welding issues that come your way.

Plus, we’ll teach you how to read and interpret welding symbols, so you can take your welding skills to the next level. Don’t miss out on this essential resource for any aspiring welder!

Welding Training Series:

• Welding Training 101: Welding Method (1)
• Welding Training 101: Welding Materials (2)
• Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3)

Common welding defects

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

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

3. Undercut (Fig. 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)

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)

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)

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

7. Burnthrough (Fig. 6-7)

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

8. Overlap (Fig. 6-8)

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)

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)

(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 melted metal from the welding wire is transferred to the weld pool, but some of it escapes and forms splatter. When using thick welding wire for CO₂ gas shielded welding with large parameters, the splatter can become particularly severe, with a rate as high as 20%.

This results in an inability to perform normal welding. The splatter is harmful, as it decreases welding efficiency, impacts the quality of the weld, and creates poor working conditions.

Splash hazard

The metal spatter loss in CO₂ gas shielded welding can account for anywhere from 10% to 30-40% of the melted metal from the welding wire. The ideal loss is controlled to 2-4%.

This loss has several negative impacts:

• It increases the consumption of welding wire and electricity, reducing efficiency and driving up costs.
• The spattered metal can clog the contact nozzle and nozzle, causing feeding issues and undermining the stability of the arc, as well as reducing the protective effect of the shielding gas and the quality of the weld.
• The spattered metal must be cleaned after welding, which adds to the overall working hours.
• The spatter can even burn the welder’s clothing and skin, making the work environment hazardous.

Preventing and reducing metal spatter is a crucial consideration in CO₂ gas shielded welding.

Measures to reduce splash

（1） Correct selection of process parameters

1. Welding Current and Voltage in CO2 Arc Welding:

There is a correlation between the spatter rate and welding current for each diameter of welding wire in CO2 arc welding. In the low current area (short circuit transition area), the spatter rate is low. When the current enters the high current area (fine particle transition area), the spatter rate decreases again. However, the spatter rate is highest in the middle area.

The spatter rate is low when the welding current is less than 150A or more than 300A, and it is high between these two values. To minimize the spatter rate, it is best to avoid selecting welding currents in this high spatter rate area.

Once the welding current has been determined, the appropriate voltage should be chosen to ensure the lowest possible spatter rate.

2. Angle of the Welding Gun:

The spatter amount is at its minimum when the welding gun is held vertically. As the inclination angle of the gun increases, the spatter rate also increases. It is recommended not to tilt the welding gun forward or backward by more than 20 degrees.

3. Extension Length of the Welding Wire:

The spatter rate is also affected by the extension length of the welding wire. It is best to keep the length of the welding wire as short as possible to minimize spatter.

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

For example:

1. To minimize the generation of CO gas during welding, it is advisable to select steel welding wire with a low carbon content as much as possible.

The experience shows that when the carbon content in the welding wire is reduced to 0.04%, spatter can be significantly reduced.

2. Opt for tubular welding wire when welding.

The flux core in tubular welding wire includes deoxidizers and arc stabilizers, providing gas slag joint protection, making the welding process more stable, and reducing spatter significantly. The metal spatter rate of flux cored wire is approximately one-third 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 the proper selection of specification parameters and the use of the submerged arc method, the amount of spatter produced is still significant.

Incorporating a certain amount of Argon (Ar) gas into Carbon Dioxide (CO₂) gas is the most effective method to reduce metal spatter that is caused by excessive welding of particles.

The physical and chemical properties of pure CO₂ gas are altered when Argon is added to the mix.

As the ratio of Argon gas increases, the amount of spatter decreases gradually.

The CO₂+Ar mixed gas not only reduces spatter but also improves weld formation, influencing weld penetration, height, and reinforcement.

When the Argon content reaches 60%, the size of the transfer droplets can be noticeably reduced and even spray transfer can be achieved, thus improving droplet transfer characteristics and reducing metal splash.

Diagram of welding defects

1. Weld scale

Repair method

Weld surface after descaling

2. Air hole

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

3. Crater needle shaped air hole

4. Air hole (sand hole)

5. Shrinkage cavity

6. End crack/weld crack

7. Appearance of bad welds

8. Overlap and flash

9. Undercut

10. Uneven weld

11. Poor appearance

Weld symbol and mark

1. Basic symbols

The weld symbol is made up of a basic symbol and a leader line, and if necessary, additional symbols, supplementary symbols, and symbols indicating the size of the weld.

The basic symbol represents the cross-sectional shape of the weld and is similar to the symbol for the cross-sectional shape of the weld found in Table 4-2.

2. Auxiliary symbols and supplementary symbols

Auxiliary symbols are symbols that indicate the shape characteristics of the weld’s surface. These symbols can be omitted if specifying the surface shape of the weld is not necessary.

Supplementary symbols are used to complement the symbols that represent certain characteristics of the weld surface. The methods of representing these symbols are shown in Table 4-3.

3. Size symbol of weld

If the size of the weld needs to be specified during design or production, it is indicated by the weld size symbol, as illustrated in Table 4-4.

Table 4-2 Basic Symbols of Weld Forms

Serial No	Weld name	Weld type	Basic symbols
1	I-shaped weld
2	V-shaped weld
3	Blunt V weld
4	Unilateral V-shaped weld
5	Single V-shaped weld with blunt edge
6	U-shaped weld
7	Unilateral U-shaped weld
8	Flare weld
9	Fillet weld
10	Plug weld
11	Spot weld
12	Seam weld
13	Back bead

Table 4-3 Auxiliary Symbols and Supplementary Symbols of Welds

Serial No	Name	Type	Auxiliary symbol	Explain
1	Plane symbol			Indicates that the weld surface is flush
2	Depression symbol			Denotes weld surface depression
3	Raised symbol			Indicating weld surface bulge

Serial No	Name	Type	Supplementary symbol	Explain
1	Symbol with backing plate			Indicates that there is a backing plate at the bottom of the weld
2	Three side weld symbol			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
3	Peripheral weld symbol			Indicates welding around the workpiece
4	Site Symbols			Indicates welding at site or construction site

Table 4-4 Size Symbols of Welds

Symbol	Name	Sketch Map
δ	Sheet thickness
α	Groove angle
b	Butt clearance
p	Height of blunt edge
c	Weld width
K	Fillet size
d	Nugget diameter
S	Effective thickness of weld
N	Number of identical welds symbol
K	Fillet size
R	Root radius
l	Weld length
n	Number of weld segments
H	Groove depth
h	Weld reinforcement
β	Groove face angle

4. Leader

(1) The leader line is made up of an arrow line with an arrowhead, and two reference lines (one is a thin solid line, and the other is a dotted line).

(2) The dotted line can be located either above or below the thin solid line.

The datum line is usually parallel to the long side of the title block, but it can also be perpendicular to the long side of the title block if required.

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

If it is necessary to describe the welding method, a tail symbol can be added at the end of the reference line.

5. Dimensioning method of common welds

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

(2) The dimensions along the length of the weld are marked on the right side of the basic symbol.

(3) The groove angle (α), groove face angle (β), and root gap (b) are marked either above or below the basic symbol.

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

(5) If there is a large amount of dimension data to be marked and it becomes difficult to distinguish, corresponding dimension symbols can be added in front of the data to help clarify the information.

Table 12-1 Weld Symbols and Marking Methods

Welding joint and groove type

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

The selection of welded joints is primarily based on the structure of the welding, the thickness of the weldment, the strength requirements of the weld, and the conditions under which the construction is taking place.

Specified drawing method of weld

The line formed after welding the workpieces together is referred to as the weld seam.

If a simple representation of the weld is needed in a drawing, it can be depicted using a view, section view, or axonometric diagram.

The specific method of representing the weld in a drawing is shown in the figure.

Welding stress and deformation

Structural welding always results in welding deformation and stress.

During the welding process, the deformation and internal stress generated in the weldment that change over time are referred to as transient deformation and transient welding stress, respectively.

The deformation and stress that remain in the weldment after the temperature has cooled to room temperature after welding are known as residual welding deformation and residual welding stress, respectively.

3.1 Causes of welding stress and deformation

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

During the welding process, the weldment is heated locally, causing deformation to occur due to the metal’s characteristic of expanding and contracting.

However, the steel plate is a solid piece, and this expansion cannot occur freely.

The end of the steel plate can only expand evenly by an amount of Δι.

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

During cooling, the metal near the weld has undergone permanent compressive plastic deformation during welding and is also restricted by the metal on both sides.

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

As a result, there is tensile stress in the weld zone and the surrounding metal, and there is compressive stress in the metal on both sides.

The stress in the member is in a state of balance. It can be observed that after butt welding a flat plate, the length of Δι‘ is shorter than it was before welding.

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

In other words, the welding stress and deformation are maintained at room temperature and are known as residual welding stress and deformation.

3.2 Distribution, influence and elimination of welding residual stress

Welding stress can be divided into four categories: thermal stress, restraint stress, phase change stress, and residual welding stress. The residual welding stress is often very high.

In structures with thick welding, the residual welding stress can usually reach the yield strength of the material.

1. Classification of welding stress

(1) Longitudinal stress: Stress along the length of the weld.

(2) Transverse stress: Stress perpendicular to the length of the weld and parallel to the surface of the component.

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

2. Distribution of welding residual stress

(1) Longitudinal stress of weld σ x

The stress along the longitudinal direction of the weld is referred to as longitudinal stress (σ x).

The stress perpendicular to the longitudinal direction of the weld is referred to as transverse stress (σ y).

In the zone of compressive plastic deformation near the weld, the longitudinal stress (σ x) is tensile stress, which can typically reach the yield strength of the material.

(2) Transverse stress of weld

The figure illustrates the distribution of transverse stress (σy) in a plate weld of a certain length.

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

The further away from the center of the weld, the quicker σy decreases.

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 highly uneven in the thickness direction.

Thick plate electroslag welding results in three axial tensile stresses at the center of the weld, which increase with the increase in plate thickness, but the surface experiences compressive stress.

3. Effect of welding residual stress

(1) Impact on the Strength and Stability of Compressive Parts

When the component is under tensile load, the residual welding stress will be added to the load stress, affecting the component’s strength.

(2) Influence on Brittle Fracture of Components

The increase in the nominal stress of the component, combined with the decrease in material toughness in the welding joint area and the presence of welding defects, will increase the likelihood of brittle fracture under low external loads.

(3) Effect on Fatigue Strength

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

(4) Impact on Machining Accuracy and Dimensional Stability of Weldments

(5) Effect on Crack Propagation

When evaluating the crack state of the welding zone, the residual welding stress must be taken into consideration.

When calculating the stress intensity factor (KI) that drives crack growth, the residual stress (σ_r) is taken into account by using the equivalent tensile stress (σ₃), which represents the contribution of residual stress to crack growth:

σ₃ = α_rσ_r

Where σ_r is related to the type of crack (through crack, buried crack, surface crack) and the direction of the crack (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

Reducing Welding Residual Stress through Design and Welding Process

(1) The key to reducing welding stress in design is to properly arrange the welds to avoid stress overlap and reduce peak stress.

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

② The welds should be spaced out sufficiently and avoid crossing as much as possible to prevent complex three-dimensional stress.

③ Welds should not be located in areas with high stress and abrupt changes in cross-section to avoid stress concentration.

④ The more flexible table type joint should be used, and flanging should replace the insertion tube.

(2) Techniques for Reducing Welding Stress in the Process

① Adopt a reasonable welding sequence and direction, and perform most welds with less rigidity.

② Minimize the temperature difference between the welding area and the entire structure to reduce internal welding stress. Use overall preheating and low linear energy.

③ Utilize hammer welding to reduce welding stress and deformation.

④ Decrease the hydrogen content and eliminate hydrogen.

(3) The method for eliminating residual stress primarily involves eliminating residual stress after welding. For boilers and pressure vessels with a pressure component thickness exceeding a certain size, post-welding heat treatment is required to eliminate internal stress.

Generally, welding causes deformation of the workpiece. If the deformation exceeds the acceptable limit, it will affect functionality.

The main cause of deformation is uneven heating and cooling of the weldment during welding.

During welding, the weldment is only heated in local areas, but the metal in the heated area cannot expand freely due to the metal with lower temperature around it.

When cooling, it cannot shrink freely due to containment by surrounding metal.

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

When these stresses exceed the yield limit of the metal, welding deformation occurs.

Cracks appear 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 can be varied. The most common forms are five basic forms, or combinations of these forms.

Figure (a) illustrates the longitudinal and transverse shrinkage deformation in a flat plate after butt welding;

Figure (b) illustrates the angular deformation in a flat plate after docking;

Figure (c) illustrates the bending deformation caused by the deviation of the welding arrangement in a cylinder from the centroidal axis of the weldment;

Figure (d) illustrates the wavy deformation in a thin-walled weldment after welding.

Additionally, beam-column structures are susceptible to distortion during welding.

Shrinkage deformation and bending deformation are forms of overall deformation, while the other forms are considered local deformation.

2. Influencing factors of welding deformation

(1) The Effect of Weld Position on Welding Deformation

When the welds are symmetrically arranged in the structure, only longitudinal and transverse shortening occurs. However, if the welds are arranged asymmetrically in the structure, bending deformation will occur. Angular deformation will occur when the center of gravity of the weld section deviates from the center of gravity of the joint section.

(2) Influence of Structural Rigidity

Under the same force, structures with large rigidity have less deformation, while structures with low rigidity have more deformation. Welding deformation is always carried out in the direction with the least constraint of structural or weldment rigidity.

(3) Effect of Assembly and Welding Sequence

The rigidity constraint when welding a strip weld depends on the assembly and welding procedure. For structures with symmetrical sections and welds, a method of first assembling into a whole can be used. For complex welding structures, due to the multiple welds, the deformation caused by each weld affects the other welds, making it difficult to control. Thus, a procedure of partial assembly, welding, reassembly, and re-welding must be adopted to control the overall welding deformation.

(4) Other Influential Factors

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

3. Methods for controlling welding deformation

To control and minimize welding deformation, it is essential to adopt appropriate design schemes and process measures.

(1) Reduce the number, length, and size of welds as much as possible while ensuring reasonable design for bearing capacity.

Arrange the position of the welds in a reasonable manner, such that all welds in the structure are symmetrical to, or as close as possible to, the neutral axis of the section. This will help to reduce the deformation of the weldment.

(2) Necessary Process Measures:

① Reserve Shrinkage Allowance:

When preparing the workpiece, add a suitable shrinkage allowance.

Typically, the longitudinal shrinkage of the weld is calculated based on the length of the weld and depends on factors such as groove, joint type, and plate thickness.

② Reverse Deformation Method:

Employ experience or calculation methods to determine the reverse deformation method.

Before welding, it’s crucial to assess the size and direction of potential deformation of the workpiece. To prevent residual deformation, place the weldment in the opposite direction of the deformation or apply artificial deformation beforehand during assembly. Proper control will help ensure that the workpiece attains the correct shape.

③ Select Appropriate Welding Methods and Specifications:

Utilize energy-concentrated heat sources and fast welding methods to reduce deformation.

④ Optimal Assembly and Welding Sequence:

Divide the large structure into smaller parts, assemble and weld each part separately, and then join the parts together into a complete whole.

⑤ Sturdy Fixation:

Fix and clamp the structure before welding to reduce deformation through external constraints. However, rigid clamping can prevent the free shrinkage of the weldment, leading to high internal stress within the component.

Therefore, it is crucial to carefully select the weldment material and structure.

⑥ Use reasonable welding sequence

4. Correction of welding deformation

Despite adopting deformation control methods, it is still challenging to avoid deformation after welding. When the deformation of the weldment exceeds the limits specified in the product technical requirements, it is necessary to perform post-weld correction to meet product quality standards.

The objective of correction is to induce new deformation in the welding components to counteract the deformation that occurred during welding. However, the correction process often increases internal stress in the components.

To avoid local fractures during correction, it’s advisable to relieve welding residual stress before correcting deformation. This will help to ensure the integrity and stability of the component.

Common Methods of Mechanical and Flame Correction in Production:

(1) Mechanical Correction Method:

The mechanical correction method involves using mechanical pressure or cold hammering to produce plastic deformation and correct welding deformation.

(2) Flame Correction Method:

The flame correction method uses the contraction caused by local heating with a flame to counteract elongation and deformation in the affected area. It is crucial to correctly identify the heating position, and the heating temperature for flame correction is typically between 600-800°C.

(3) Pay Special Attention to Steel Type during Correction:

When performing correction, it’s important to be mindful of the type of steel being used:

• Avoid using hammering for corrosion-resistant equipment to prevent stress corrosion.
• Steels with a tendency towards intergranular corrosion and high hardening should not be corrected using flame correction.
• For high strength steels with a high tendency towards cold cracking, it’s best to minimize the use of mechanical methods as they can easily cause cold work hardening.

Welding process elements and specifications

Welding technology is a critical factor in ensuring the quality of welded joints. In a manufacturing setting, the elements of the welding process are outlined in the detailed welding procedure guidelines.

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

The elements specified in the detailed welding procedure card include:

① Preparation prior to welding;

② Brand and specifications of welding materials;

③ Welding procedure specification parameters;

④ Welding technique;

⑤ Post-weld inspection, and so on.

Welding Electrical Parameters:

(1) When using continuous AC or DC welding, the main electrical parameters in the welding specifications are the welding voltage and current.

(2) For pulse current welding, additional electrical parameters include the alternating frequency, on-off ratio, basic current, and peak current value.

(3) The principle for selecting welding specification parameters is to ensure proper penetration and a weld bead free from cracks, while also meeting the performance requirements specified in the technical conditions.

When selecting electrical parameters, it’s important to consider the impact of welding heat input on joint performance.

Refer to Table 4-8 for the selection of manual arc welding electrode diameters and the corresponding welding current range.

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

Thickness of steel parts (mm)	1.5	2	3	4~5	6~8	9~12	12~15	16~20	>20
Electrode diameter (mm)	1.6	2	3	3~4	4	4~5	5	5~6	6~10
Welding current (A)	25~40	40~65	65~100	100~160	160~210	160~250	200~270	260~300	320~400

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		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		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 cracks refer to the separation of metal material (local fracture) within the weld joint due to welding-related causes, such as metallurgy, materials, or internal and external forces, during or after welding.

Cracks are one of the most dangerous welding defects, characterized by sharp ends and a much smaller separation width (opening displacement) than crack length.

Preventing welding cracks is a crucial aspect in the design and production of welding structures.

1. Classification of welding cracks

There are various types of welding cracks, and their classification methods have evolved as our understanding of the nature of cracks has deepened.

The following table provides a general classification based on the timing 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

Cracks in high-strength steel bridges and shipbuilding steel structures are primarily cold cracks, accounting for 90% of all cracks. In petrochemical plants and power equipment, hot cracks are more prevalent. Pearlitic heat-resistant steel is prone to reheat cracks.

There are two main reasons for cracking:

(1) The stress and strain resulting from restraint is a major cause of cracking. A certain level of stress is required for cracking to occur, and the uneven heating process during welding can lead to tensile stress and strain in the joint due to the restraint of the entire structure during the welding cooling process.

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

3. Welding cracks

1. Hot cracks

(1) Characteristics of Hot Welding Cracks:

Hot cracks have the following morphological characteristics, which distinguish them from other cracks:

① Most cracks open on the weld surface and have an oxidized color.

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

③ Cracks are typically intergranular and exhibit high-temperature intergranular fracture properties.

④ They mostly occur during and after solidification.

(2) Formation Mechanism:

In the solidification process of welding, when there is low melting point eutectic present, the fast welding cooling speed can cause the grain boundary to be pulled apart and form cracks when the grain has solidified and the grain boundary is still in a liquid state with almost zero deformation resistance, and the welding tensile strain is high.

(3) Influencing Factors:

① Effect of Weld Chemical Composition:

Many eutectic crystals in welding are the result of welding metallurgical reactions.

Elements that can produce eutectic are elements that promote hot cracking.

Elements that can refine grains, produce high melting point compounds, or distribute low melting point eutectic in spherical or blocky forms are effective in inhibiting hot cracking.

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

Seriously affect the formation of hot cracks	A small amount has little effect, while a large amount promotes hot cracking	Reduce the hot cracking tendency of weld	Undetermined
Carbon, sulfur, phosphorus, copper, hydrogen, nickel, niobium	Silicon (>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:

Hot cracks are prone to form in deep and narrow welds due to the macro segregation that concentrates in the middle of the weld. Therefore, when performing automatic submerged arc welding on thick plates, it’s crucial to adjust the welding current and arc voltage proportion to ensure 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, making it less likely to cause deep and narrow welds.

③ Influence of Welding Process and Weldment Structure:

The weldment structure and welding process directly impact the restraint of the welded joint, which is reflected in the welding tensile strain. Its effect on hot cracks is considered a mechanical factor.

(4) Measures to Prevent Hot Welding Cracks:

① The basic measures to prevent hot cracks are to tightly control the chemical composition of the weld, limit the content of carbon, sulfur, and phosphorus impurities, and add sufficient desulfurizers to the welding materials.

② Implement process measures such as preheating before welding, heat tracing, and welding with high wire energy (ensuring the weld shape factor is not too small).

③ Reduce the rigidity of the weldment as much as possible to minimize the internal stress of welding.

2. Cold cracks

(1) Characteristics of Cold Cracks:

Cold cracks are the most commonly produced welding defects when welding low-alloy high-strength steel, medium-alloy steel, medium-carbon steel, and other easily quenched steels.

① They occur after solidification of the weld metal, usually below the martensite transformation temperature or at room temperature.

② They mainly occur in the heat-affected zone and rarely in the weld zone.

③ They are often delayed.

(2) Cause: The root cause of cold cracks is the combined effect of low plasticity structure (hardening structure) in the heat-affected zone of the weldment, hydrogen in the welded joint, and welding stress.

(3) Influencing Factors:

① Hardening Effect:

When easily quenched steel is welded, the overheated zone forms a coarse martensite structure, reducing the plasticity of the metal in the heat-affected zone and increasing its brittleness. This makes it prone to cracking under high welding tensile stress.

② Role of Hydrogen:

Cold cracks induced by hydrogen exhibit the characteristics of delayed fracture, from latency to initiation, propagation, and cracking. The length of the delay time is related to the hydrogen concentration and the stress level of the welded joint.

③ Effect of Welding Stress:

Cold cracks are more likely to occur when welding stress is tensile stress and happens simultaneously with hydrogen precipitation and material hardening.

Welding of thick plates is more susceptible to cold cracks at the root. This is due to the rigidity of the thick plate and the fast cooling, which leads to the formation of a quenching structure and results in high welding stress.

3. Reheat cracks

(1) Characteristics of Reheat Cracks

① Reheat cracks occur in the temperature range of 540-930°C after post-welding stress relief heat treatment.

② The cracks propagate along the grain boundaries in the coarse grain zone of the heat affected zone.

③ Intergranular cracks with a branching shape will stop when they reach the fine grain area of the weld or base metal.

(2) Mechanism of Reheat Crack Formation

After the post-welding stress relief heat treatment and reheating, alloy carbides are dispersed and precipitated on dislocation lines after heat preservation at 550-700°C, which strengthens the intragranular structure.

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

During the reheating process, residual stress is released and the strength of the grain boundary is weaker than that of the grain, resulting in grain boundary cracking.

(3) Influencing Factors

There are several factors that affect reheat cracks:

These include chemical composition, restraint state, welding specifications, welding rod strength, stress relief specifications, and service temperature of the base metal.

① Chemical composition mainly affects the plasticity of grain boundaries in the heat affected zone.

② Restraint state and welding specifications affect welding residual stress.

③ Stress relief heat treatment specifications and service temperature mainly affect the plastic strain and degree of alloy carbide precipitation caused by reheating.

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

(4) Measures to Prevent Reheat Cracks

1. Improve the plasticity of the coarse grain zone in the welding heat affected zone.
2. Reduce welding residual stress.

① The primary measure is to select a base metal with low sensitivity to reheat cracks.

② Take all necessary steps to reduce residual stress.

③ Avoid combining welding residual stress with other stresses, such as structural stress and thermal stress during reheating.

④ The use of low-matching welding materials helps absorb deformation.

⑤ Under the condition of ensuring stress relief, use the lowest possible reheat temperature and shortest holding time.

If possible, replace reheating with afterheat slightly lower than the preheating temperature for better results.

4. Lamellar tear

(1) Characteristics of Lamellar Tearing

① During rapid cooling of the weld, cracks parallel to the rolling surface of the base metal occur in the steel plate due to welding tensile stress in the direction of plate thickness. These cracks are known as lamellar tearing and often occur in T-shaped and K-shaped thick plate joints.

② Lamellar tearing is a type of crack that occurs at room temperature, typically after cooling to below 150°C or room temperature after welding. However, when structural restraint is very high and the steel is highly sensitive to lamellar tearing, it may also occur at temperatures between 300-250°C.

(a) Typical Position of Lamellar Tearing in “T” Joint

(b) Lamellar Tearing in Downcomer Joint of Boiler Drum

(2) Main Factors Causing Lamellar Tearing

① Influence of Inclusions

Inclusions are the main cause of steel anisotropy and the origin of lamellar tearing.

② Effect of Base Metal Properties

The plasticity and toughness of the metal matrix itself have a significant impact on lamellar tearing. Poor plasticity and toughness result in poor resistance to lamellar tearing.

③ Influence of Restraint Stress

All welding cracks occur under the action of tensile stress and lamellar tearing is no exception. Lamellar tearing is only caused when corner joints and T-joints are prone to forming large two-way restraint stress.

(3) Precautions for Lamellar Tearing

Lamellar tearing is difficult to repair, so preventing this defect is the main objective.

① When the welded joint is prone to causing lamellar tearing, evaluate the lamellar tearing sensitivity of the steel plate used and choose a steel plate with low sensitivity.

② Adopt a reasonable groove type to align the fusion line of the weld as closely as possible with the steel plate.

③ For steel grades that are sensitive to lamellar tearing, if possible, use welding materials with a lower strength grade, better plasticity, and toughness to reduce stress in the thickness direction of the steel plate.

④ For steel grades with high sensitivity to lamellar tearing, pre-deposit several layers of low strength weld metal on the steel plate surface at the welding groove.

Workmanship of welding structure

The arrangement of the weld seams in a welding structure has a significant impact on the quality and efficiency of the welded joints.

General principles of weld joint arrangement:

1. Weld Arrangement Should Facilitate Welding Operations

The weld arrangement must provide clear space for welders to work freely and for welding equipment to operate normally.

When performing submerged arc welding, consider the ease of storing the welding flux.

For spot welding and seam welding, the ease of inserting electrodes should be taken into consideration.

2. Weld Position Should Avoid Maximum Stress and Stress Concentration

For components with large and complex stress, welds should not be placed at positions with maximum stress and stress concentration.

For instance, the splicing weld for a large-span welded steel beam and plate should not be located in the center of the beam, but instead, an additional weld should be added.

3. Decentralized Weld Arrangement Reduces Welding Stress and Deformation

Dense or cross welds can cause overheating, enlarge the heat affected zone, and weaken the structure.

Typically, the distance between two welds should be more than three times the plate thickness and not less than 100mm.

4. Welds Should Avoid Machined Surfaces Whenever Possible

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

On surfaces with high machining requirements, it is best to avoid setting welds.

5. Welding End Transition Design Should Be Smooth and Avoid Melting

To prevent melting during welding, there should be no sharp angles at the end of the weldment. The transition between two welding joints should be smooth to avoid stress concentration.

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 shorter side of the angle should be used as the reference edge and its length can be extended to a designated reference point. In this case, the datum point should be marked on the drawing. Refer to Table 2 for tolerances. Figures 1 to 5 provide 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 listed in Table 3 apply to all dimensions of weldments, welded assemblies, or welded components, as well as the dimensions marked on the drawings. Coaxiality and symmetry tolerances are not specified. If these tolerances are necessary for production, they should 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 Sheet Metal Thickness and Quality The thickness and quality of the sheet metal must comply with the national standard, and the performance test report and manufacturer’s certificate for the sheet metal used must be provided.

1.1.2 Material Appearance The material must be flat, free of rust, cracks, and deformations.

1.1.3 Dimensions The dimensions must conform to the drawings or technical requirements. If not provided by our company, they must comply with the current national standards.

1.2 Plastic powder

1.2.1 Plastic Powder Consistency The entire batch of plastic powder must have good consistency, with a factory certificate and inspection report that includes the powder number, color number, and various inspection parameters.

1.2.2 Trial Requirements The plastic powder must meet product requirements after trial, including color, luster, leveling, adhesion, etc.

1.3 General hardware and fasteners

1.3.1 Appearance The surface must be free of embroideries and burrs, and the appearance of the entire batch of incoming materials must be consistent.

1.3.2 Size The size must meet the requirements of the drawings and national standards.

1.3.3 Performance The performance must meet product requirements after trial assembly and service performance.

2. Process quality inspection standards

2.1 Blanking inspection standard

All sharp corners, edges, and rough surfaces that may cause harm must be deburred.

The burrs produced from stamping should not have any noticeable protrusions, indentations, roughness, scratches, rust, or other imperfections on the exposed and visible surfaces of door panels and panels.

Burrs: After blanking, the burr height must not exceed 5% of the plate thickness (t).

Scratches and knife marks: The product is considered qualified if it does not have any visible scratches when touched by hand and the scratches should not be greater than 0.1.

The specifications for surface tolerance are presented in Table I.

Attached Table 1. Flatness Tolerance Requirements

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

2.2 Bending inspection standard

2.2.1 Burr: The height of the extruded burr after bending must not exceed 10% of the plate thickness (t). Unless otherwise specified, the bending radius must be R1.

2.2.2 Indentation: The product may have visible creases, but they should not be noticeable when touched. The product can be compared to a reference sample.

2.2.3 Bending Deformation Standards: The standards for bending deformation must be in accordance with Tables II, III, and IV.

2.2.4 Bending Direction and Size: The direction and size of the bending must be consistent with the drawings.

Attached Table 2: Diagonal Tolerance Requirements

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

2.3 Angle:

The angle must be checked and measured according to the specifications in the drawings. The tolerance for the angle is presented 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 size	Dimensional 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 must be strong and consistent, without defects such as inadequate welding, cracks, incomplete penetration, welding penetration, notches, or undercuts.

The length and height of the welds must not exceed 10% of the required length and height.

2.5.2 Welding Point Requirements: The length of each welding point must be between 8mm and 12mm, with a distance of 200 ± 20mm between two welding points. The V welding point must be symmetrical and have uniform upper and lower positions.

If the processing drawings have special requirements for welding points, these requirements take precedence.

2.5.3 The distance between spot welds must be less than 50mm, the spot weld diameter must be less than φ5, and the spot welds must be evenly spaced. The indentation depth on the spot welds must not exceed 15% of the actual plate thickness, and no noticeable welding scars should remain after welding.

2.5.4 After welding, no other non-welding parts can be damaged by welding slag or arc, and all surface welding slag and spatter must be removed.

2.5.5 After welding, the outer surface of the parts must be free of slag inclusions, air holes, overlaps, bulges, depressions, or any other defects. The defects on the inner surface must not be noticeable and must not affect the assembly.

The post-welding stress of important parts such as door panels and panels must also be relieved to prevent workpiece deformation.

2.5.6 The external surface of the welding parts must be smoothed by grinding. For powder-sprayed parts and electroplated parts, the roughness after grinding must be Ra3.2-6.3, and for painted parts, it must 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 must be degreased, derusted, phosphatized, and cleaned.

3.1.2 The workpiece surface must not have any watermarks or residual cleaning solution.

3.1.3 The workpiece surface must not have any oil stains, dust, fibers, or other undesirable phenomena that may affect the quality or adhesion of the sprayed surface.

3.1.4 The color must match the sample plate (no obvious color difference should be observed under natural light or a 60w fluorescent lamp for normal vision), and there must be no color difference for the same batch of products (note: color difference includes color and glossiness).

3.1.5 The coating surface must be smooth, flat, and even, and must not have the following defects:

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

Sagging: there are liquid protrusions on the surface that are bead-shaped at the top;

Particles: the surface has a sand-like appearance and feels blocked to the touch;

Orange peel: the surface appears uneven and irregular like the skin of an orange;

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

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

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

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

Inclusion: there are foreign objects in the coating;

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

3.1.6 Surface Grade Classification Standards:

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

Grade B Surface: the surface that is seldom seen but can be seen under certain conditions, such as the inner accessories, reinforcing ribs, and inner side of the gate that can be seen after opening.

Grade C Surface: the surface that is generally not seen or only seen during assembly, such as the contact surface between the carriage and the guide rail in the cabinet.

3.1.7 Inspection Conditions:

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

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

3.1.8 Inspection Standards:

The grade surface of the product must be distinguished according to the light source standard.

The coating film of all grade surfaces must not have any base material exposure, peeling, or other defects, and all surfaces must not have any scratches, bubbles, pinholes, powder accumulation, or other undesirable phenomena.

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

Acceptance must be conducted according to the sample, without any obvious color difference (no more than 3 degrees), and the grain must match the sample.

The inspection must be conducted at the eye distance level, scanning 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

Project	Outdoor powder	Indoor powder	Painting	Test method
Product surface thickness	60~120	50~100	40~70	Coating thickness gauge
Inside thickness of product	60~100	50~80	30~60	Coating thickness gauge

3.3 Coating gloss and color detection

3.3.1 Fabrication of spraying color plate

A. During baking, two color plates should be created for each furnace to conduct a performance test. The metal plate used should be of the same material as the product, with a size of 80 × 120, and it should be added to the product under normal conditions. The powder number, curing conditions, date, and time should be clearly marked and signed by the Quality Engineer (QE).

After confirmation, the number, name, and registration should be recorded and managed. One plate should be kept for testing purposes and the other for archiving.

B. The validity period of the color plate used in the powder spray manufacturing process is two years, and it should be stored at room temperature (70 ± 15%) in an environment that is free from light. The storage environment should also maintain a consistent temperature and humidity level.

3.3.2 Gloss and color detection method

Gloss: The gloss should be evaluated using a glossmeter with an incidence angle of 60° and an error tolerance of ± 5%. If the results meet these criteria, the product is considered qualified.

Color: The color of the product must match the design drawing or be no significantly different from the standard color plate.

3.4 Coating adhesion test

3.4.1 Baige test method

After the spraying process, a furnace color plate should be taken and 11 layers of coating should be carved on the surface in both a vertical and horizontal manner, with an interval of 1mm. The carving should be done with appropriate strength, such that the scratch does not reach the substrate.

Next, the coating surface should be divided into 100 squares, and then secured with a strong transparent adhesive at a 45-degree angle. The adhesive should then be suddenly removed. At this point, the contents within each square should be checked to see if they have fallen off.

Each grid represents 1 percent, and the acceptance standard is Level 5, meaning that the number of squares with falling contents should not exceed 5.

3.4.2 Assessment method

Grade 0: There should be no shedding at any intersections.

Grade 1: Less than 5% of the contents at intersections should have fallen off.

Grade 2: Between 5% and 15% of the contents at intersections should have fallen off.

Grade 3: Between 15% and 25% of the contents at intersections should have fallen off.

Grade 4: Between 25% and 35% of the contents at intersections should have fallen off.

Grade 5: More than 35% of the contents at intersections should have fallen off.

3.4.3 Judgment method

When the coating thickness is less than 40μm, the side length of each square should not exceed 1mm and must meet the requirements of Grade 2.

When the coating thickness is between 40μm and 90μm, the side length of each square should be between 1mm and 2mm and must meet the requirements of Grade 3.

When the coating thickness is between 90μm and 120μm, the side length of each square should be 2mm and must meet the requirements of Grade 4.

When the coating thickness exceeds 120μm, the adhesion may be reduced. Generally, it is preferred that the coating thickness does not exceed 120μm.

As a tentative standard, if there is a complete square with falling contents, the product will be considered unqualified.

3.5 Bending plate test method

After the spraying process, a furnace color plate should be taken and bent 180 degrees, so that the internal bend angle is equal to the thickness (r=t). Alternatively, the color plate can be bent 90 degrees once, and the coating should not fall off.

3.6 Alcohol solvent resistance test of coating

The coating surface should be repeatedly wiped with a white cotton cloth dipped in alcohol for 10 times (without excessive pressure). After wiping, there should be no visible coating that has fallen off on the cotton cloth. Once the alcohol has fully evaporated, there should be no noticeable difference in color or shine between the wiped area and the non-wiped area.

3.7 Impact resistance test

Using the test equipment, a 500g heavy hammer should be dropped freely from a height of 500mm. The evaluation criteria are as follows: after impacting a quarter of the punch on the front, there should be no cracks or film falling on the surface coating.

3.8 Hardness test

A sharpened 2H pencil should be used to form a 45-degree angle with the film surface and pushed forward along a ruler for 15-30mm. The film surface should then be checked after wiping off the resulting mark with a rubber. The product will be considered qualified if no substrate is exposed.