Handheld Laser Welding Machine

Handheld Laser Welding Machine

Automatic Laser Welding Machine

Automatic Laser Welding Machine

Robotic Laser Welding Machine

Robotic Laser Welding Machine

80 FAQs That Welding Engineers Must Know

Table of Contents

1. What are the hazards of pores formed during welding?

The existence of pores in the weld will weaken the effective working section of the weld, thus reducing the mechanical properties of the weld and reducing the plasticity, especially the bending and impact toughness of the weld metal.

When the pores are serious, the metal structure will be damaged during operation, especially in the environment with alternating stress (water hammer or mechanical vibration, temperature change, etc.).

2. What are the reasons and prevention methods for the non-compliance of weld surface size with the requirements?

Causes: incorrect groove angle of weldment, uneven assembly gap, improper welding speed or incorrect electrode transportation method, improper selection or change of electrode and angle.

Prevention method: select appropriate groove angle and assembly clearance;

Correctly select the welding process parameters, especially the welding current value, and adopt appropriate strip transportation methods and angles to ensure uniform weld formation.

3. What are the causes and prevention methods of undercut?

Causes: it is mainly due to improper selection of welding process parameters, too large welding current, too long arc, improper strip transportation speed and electrode speed, etc.

Prevention method: select the correct welding current and welding speed, the arc shall not be pulled too long, and master the correct strip transportation method and angle.

4. What are the functions of drug skin?

The coating layer pressed on the surface of the welding core is called coating.

Its function is

(1) Improve the stability of welding arc;

(2) Protect the molten metal from the influence of external air;

(3) Transition alloy elements make the weld obtain the required properties

(4) Improve welding process performance and welding productivity.

5. What is the purpose of quenching?

The heat treatment process of heating steel parts to a temperature above Ac3 or Ac1, maintaining it for a certain time, and then cooling at an appropriate speed to obtain martensite or bainite is called quenching.

The purpose is to improve the hardness, strength and wear resistance of steel.

6. What is the purpose of steel annealing?

① Reduce the hardness and improve the plasticity of steel, so as to facilitate cutting and cold deformation processing;

② Refine the grain size, homogenize the structural composition of the steel, improve the properties of the steel or prepare for future heat treatment;

③ Eliminate residual internal stress in steel to prevent deformation and cracking.

7. What are the general principles for groove selection?

① It can ensure the workpiece penetration (the penetration depth of manual arc welding is generally 2mm-4mm) and is convenient for welding operation.

② The groove shape shall be easy to process.

③ Improve welding productivity and save welding rods as much as possible.

④ Minimize the deformation of workpiece after welding.

8. What are the characteristics of CO2 gas shielded welding? What is the cause of splash?

It is characterized by:

(1) Oxidation of CO2 gas;

(2) Pores due to the cooling effect of air flow, the molten pool solidifies rapidly, which is easy to produce pores in the weld. However, it is conducive to thin plate welding and small deformation after welding.

(3) Cold crack resistance due to the low hydrogen content of the welded joint, CO2 gas shielded welding has high cold crack resistance.

(4) Spatter is the main disadvantage of carbon dioxide gas shielded welding.

The causes of splash are as follows: 

1) Splash caused by CO gas;

2) Splash caused by speckle pressure;

3) Splash caused by short circuit.

9. How to check the air tightness of welded containers with ammonia inspection method?

Fill the pressure element with compressed air mixed with 1% ammonia, and stick the paper or bandage soaked in 5% mercuric nitrate aqueous solution on the outside of the weld (white paper soaked in phenolphthalein reagent can also be pasted).

In case of leakage, black spots (red spots when phenolphthalein paper is used) will appear on the corresponding position of the paper strip or bandage.

This method is more accurate and efficient. It is suitable for checking the compactness of the weld when the ambient temperature is low.

10. What are the types of welding methods?

According to the adopted energy and process characteristics, welding is divided into three categories: fusion welding, pressure welding and brazing, and each category is divided into various welding methods.

Fusion welding is divided into arc welding, gas welding, thermit welding, electroslag welding, electron beam welding and laser welding.

Pressure welding is divided into resistance spot seam welding, resistance butt welding, ultrasonic welding, explosive welding, diffusion welding, friction welding and high-frequency welding. Brazing includes flame brazing, induction brazing, furnace brazing, salt solution brazing and electron beam brazing.

Arc welding is divided into electrode arc welding, stud welding, gas shielded welding Submerged arc welding, plasma arc welding

Gas shielded welding is divided into argon arc welding, titanium dioxide arc welding and atomic hydrogen welding.

Metal thermal cutting, spraying and carbon arc gouging are metal processing methods similar to welding methods, which usually belong to the technical scope of welding specialty.

11. What are the effects of groove angle, root clearance and blunt edge?

Groove angle refers to the included angle between two groove surfaces.

Root gap refers to the gap reserved between the root of the joint before welding.

Its function is to ensure root penetration during backing welding.

Blunt edge refers to the straight edge of the end face along the root of the joint groove of the weldment when the weldment is beveled, which is used to prevent the root from burning through.

12. What should be paid attention to in the use and maintenance of AC arc welding machine?

(1) It shall be used according to the rated welding current and load duration of the welding machine, and shall not be overloaded.

(2) The welder shall not be short circuited for a long time.

(3) The regulating current shall operate at no load.

(4) Frequently check the wire contact, fuse, grounding, regulating mechanism, etc. and try to make them intact.

(5) Keep the welding machine clean, dry and ventilated to prevent the invasion of dust and rain.

(6) Place stably and cut off the power after work.

(7) The welding machine shall be overhauled regularly.

13. Safe operation technology of manual arc welding?

(1) Note that the no-load voltage does not exceed the rated value, i.e. AC 60V, DC 90V

(2) Protective glasses shall be provided for removing welding slag

(3) Construction areas with a large number of people shall be provided with shutters to prevent household arc radiation

(4) Welding tongs with welding rods shall not be placed at will

(5) Welding rod heads shall not be thrown casually, but shall be stacked in a centralized manner. Pay special attention to fire prevention

(6) When welding non-ferrous metals, galvanized pipes or alloys, masks shall be worn to prevent zinc oxide from being sucked into the body

(7) Clean the site, tools, etc. and cut off the power supply at the end of the work

(8) Welding machine shell grounding

(9) When the weldment needs vegetation ash for thermal insulation and slow cooling, pay attention to the surrounding objects to prevent fire

(10) Disconnect the power supply when moving the welding machine or changing the wiring

(11) Welding shall be carried out inside the vessel and supervised outside

(12) When welding and repairing containers or pipes containing flammable and explosive media, they shall be replaced and qualified, and all covers shall be opened

14. What are the functions of flux in the welding process?

In welding, flux is the main factor to ensure welding quality.

It has the following functions:

(1) After melting, the flux floats on the surface of the molten metal to protect the molten pool and prevent the erosion of harmful gases in the air.

(2) The flux can deoxidize and alloying, and cooperate with the welding wire to obtain the required chemical composition and mechanical properties of the weld metal.

(3) Make the weld well formed.

(4) Slow down the cooling rate of molten metal and reduce defects such as pores and slag inclusion.

(5) Prevent splashing, reduce loss and improve bonding coefficient.

15. What are the advantages and disadvantages of manual arc welding?

Advantage:

(1) Flexible process and strong adaptability;

(2) Good quality;

(3) It is easy to control deformation and improve stress through process adjustment;

(4) The equipment is simple and easy to operate.

Disadvantages:

(1) The requirements for welders are high, and the operation technology and experience of welders directly affect the quality of products. (2) Poor working conditions; (3) Low productivity.

16. What is the meaning and influencing factors of welding thermal cycle?

In the welding process, the heat source moves along the weldment.

Under the action of the welding heat source, the temperature of a point on the weldment changes with time, which is called the welding thermal cycle of the point.

Influencing factors:

(1) Welding process parameters and linear energy;

(2) Preheating and interpass temperature; Plate thickness, joint form and thermal conductivity of materials.

17. What are the causes and prevention methods of hot cracks?

Causes:

It is the result of the tensile stress when the molten pool cools and crystallizes, and the liquid thin layer formed by low melting point eutectic during solidification.

Prevention methods:

① Control the content of harmful impurities in the weld, that is, the content of carbon, sulfur and phosphorus, and reduce the formation of eutectic at the bottom melting point in the molten pool.

② Preheating: to reduce the cooling rate and improve the stress condition.

③ Alkaline electrode is used because the slag of alkaline electrode has strong desulfurization and dephosphorization ability.

④ Control the weld shape and try to avoid deep and narrow welds.

⑤ The hand arc plate is used to lead the arc pit to the outside of the weldment. Even if the arc pit crack occurs, it will not affect the weldment itself.

18. What is the relationship between weld forming factor and weld quality?

During fusion welding, the ratio between the weld width (b) and the calculated weld thickness (H) on the cross section of a single weld, i.e.  ф= B / h, is called the weld forming factor.

The smaller the weld forming coefficient is, the narrower and deeper the weld is, so pores, slag inclusions and cracks are easy to occur in the weld.

Therefore, the weld forming factor should be kept at a certain value.

19. What are the causes and prevention methods of pores during welding?

The causes are:

(1) Rust and moisture;

(2) Welding method;

(3) Type of welding rod;

(4) Current type and polarity;

(5) Welding process parameters;

Prevention methods:

(1) 10mm on both sides of manual arc welding weld, buried

Within 20mm on both sides of automatic arc welding, carefully remove rust and other dirt on the surface of weldment.

(2) The welding rod and flux shall be dried strictly according to the regulations before welding, and stored in the insulation barrel, so that they can be used and taken at any time.

(3) Adopt appropriate welding process parameters. When using alkaline electrode for welding, short arc welding must be carried out.

20. What are the alloying methods of weld metal?

The alloying of weld metal is to transfer the required alloy elements to the weld metal (surfacing metal) through the welding material to make the weld metal composition meet the required requirements.

Alloying methods mainly include:

1) Application of alloy welding wire;

2) Apply flux cored wire or flux cored electrode;

3) Use alloy coating or ceramic flux;

4) Application of alloy powder;

5) Apply displacement reaction.

21. What are the causes of cold cracks?

The causes of cold cracks mainly include the following three aspects:

(1) The greater the hardening tendency of hardened steel is, the easier it is to produce cold cracks for materials with carbon content exceeding that of 16MnR steel;

(2) Role of hydrogen: during welding, the weld metal absorbs more hydrogen. Due to the rapid cooling rate of the weld, some hydrogen remains in the weld metal;

(3) Welding stress: hydrogen, hardened structure and stress are the main causes of cold cracks.

Cold cracks are easy to occur in welding low-alloy high-strength steel, medium carbon steel, alloy steel and other types of steel, but less in welding low-carbon steel and austenitic stainless steel.

22. What are the advantages of using pulsed arc in mixed gas shielded welding?

Pulse arc is adopted in mixed gas shielded arc welding, which is a leap in the development of gas shielded arc technology.

It expands the application scope of gas electric welding and has the following advantages:

(1) The utility model has the advantages of short-circuit transition and jet transition, which can weld both thin plates and thick plates, and is suitable for all position welding.

(2) It can effectively control the heat input to the base metal to improve the performance of welded joints.

(3) Wide current regulation range and strong adaptability.

23. What is the difference between acid electrode and alkaline electrode?

Acid electrode has good process performance, beautiful shape, insensitive to rust, grease, moisture, etc., and low moisture absorption.

It can be used with AC and DC power supply.

Its disadvantages are incomplete desulfurization and deaeration, no dephosphorization, poor crack resistance and low mechanical properties.

Alkaline electrode has good crack resistance, thorough deaeration, easy slag removal, beautiful weld formation and high mechanical properties.

Its disadvantages are strong moisture absorption and poor pore resistance.

Generally, it can only use DC power supply, but if an appropriate amount of arc stabilizer is added to the coating, it can use AC and DC.

24. How to improve the connection strength of lap joints?

The measures to improve the connection strength of lap joints are:

(1) On the premise that the structure allows, lap joints with both side fillet welds and front fillet welds shall be used as far as possible to reduce stress concentration and improve stress distribution;

(2) Plug weld and slotted weld shall be added at the lap weld;

(3) Straight seam single-sided lap joint can adopt sawtooth seam lap shape.

25. What is magnetic bias blowing? How to overcome?

During DC arc welding, the arc partial blow caused by the action of electromagnetic force in the welding circuit is called magnetic partial blow.

 Measures to prevent magnetic bias blowing generally include:

① Welding with short arc and small current;

② Properly change the position of grounding wire on weldment and use double grounding wire;

③ Adjust the electrode angle to tilt to the side of partial blowing;

④ When welding small workpieces, the welding cable can be wound around the weldment for two or three turns to generate a magnetic field in the opposite direction of the magnetic bias blowing magnetic field.

26. What is the main function of preheating?

Preheating is an effective measure to reduce the cooling rate after welding.

It can not only prolong the cooling time within the range of austenite transformation temperature and reduce the hardening tendency, but also prolong the cooling time when the maximum heating temperature of welding reaches 100 ℃, which is conducive to the escape of hydrogen.

In addition, preheating can also reduce welding stress and prevent cold cracks.

27. What are the causes of slag inclusion? How to prevent?

The causes of slag inclusion are:

① Dirt exists at the edge of the joint;

② The groove is too small, the electrode diameter is too thick, and the welding current is too small;

③ During welding, the welding angle and strip transportation method are inappropriate, and the slag and molten iron can not be distinguished clearly: the molten metal and slag are mixed together;

④ The cooling speed of weld is too fast, and the slag has no time to float up;

⑤ Improper chemical composition of base metal and welding materials. For example, when the molten pool contains more oxygen, nitrogen and other components, the chance of forming inclusions increases.

Measures to prevent slag inclusion include:

① Remove the oxide scale at the bottom slag shell and groove edge;

② Carry the bar correctly, swing the welding rod regularly and stir the molten pool to separate the molten slag from the molten iron.

③ Improve the condition of slag floating: slow down the welding speed and increase the welding current to prevent the weld metal from cooling too fast.

28. How to reduce the stress concentration of welded joints?

The measures to reduce the stress concentration of welded joints are:

(1) Butt joints shall be used as far as possible, the reinforcement value of butt joints shall not be too large, and the weld toe shall transition smoothly as far as possible;

(2) For T-joint (cross joint), groove shall be made or deep penetration welding shall be adopted to ensure full penetration;

(3) Reduce or eliminate welding defects, such as cracks, incomplete penetration, undercut, etc;

(4) When steel plates of different thicknesses are butt jointed, the thick plates shall be thinned;

(5) The welds shall not be too dense to ensure the minimum distance;

(6) Welds shall be avoided at the turning of the structure as far as possible.

29. What are the factors affecting the mechanical properties of welded joints?

The mechanical properties of weld metal depend on the chemical composition, fusion ratio, welding layer and welding line energy of filler material.

The mechanical properties of heat affected zone depend on the welding line energy.

The mechanical properties of the whole welded joint are also related to whether heat treatment is carried out after welding.

30. What process measures should be taken when welding 15CrMo pearlite heat resistant steel to prevent cold cracks?

In order to prevent cold cracks during welding of 15CrMo steel, the following process measures shall be taken:

(1) The weldment shall be preheated to 150 ~ 300 ℃ for welding before welding, but the preheating temperature can be reduced or not preheated during argon arc welding backing and CO2 gas shielded welding

(2) Immediately cover the weld and heat affected zone with asbestos cloth after welding to cool it slowly

(3) High temperature tempering at 680 ~ 700 ℃ shall be carried out immediately after welding.

31. Why is the welding process of surfacing transition layer often used in the welding of dissimilar steel (metal)?

When dissimilar steel (metal) is welded, the welding process of surfacing transition layer is used to obtain high-quality joint quality and performance.

For example, when austenitic stainless steel and blow stabilized pearlite steel are welded, the purpose of surfacing the transition layer on one side of Pearlite Heat-resistant steel is to reduce the size of diffusion layer and reduce the tendency of cracks;

When steel is welded with copper and its alloys, the purpose of surfacing the transition layer on steel or copper and copper alloys is to prevent penetration cracks and improve the performance of the joint.

32. How to improve the quality of weld metal through the metallurgical reaction between slag and molten pool metal?

A series of metallurgical reactions, such as deoxidation, dephosphorization, desulfurization and alloying, will occur during temperature regulation of slag and molten pool metal.

Correct control of these metallurgical reactions can greatly improve and improve the weld quality. If a certain amount of deoxidizer is added to the electrode coating, it can deoxidize well in the molten pool;

If the necessary alkaline oxide is added to the electrode coating or flux, the sulfur and phosphorus in the molten pool can be removed to a certain extent;

These added alloy components can also be transferred to the weld by using the alloy elements in the flux or flux or directly using the alloy welding core or welding wire.

Therefore, these metallurgical reactions are widely used in the process of fusion welding.

33. When welding stainless steel, which shielding gas is more suitable?

When TIG welding stainless steel, pure argon can be used for protection.

When stainless steel is welded by MIG welding, if pure argon is used for protection, the weld appearance is not ideal due to cathode drift.

At this time, argon rich mixed gas can be used for protection, that is, 1 ~ 2% oxygen or 1 ~ 2% oxygen + 5% carbon dioxide can be added to argon.

34. What kind of groove can be selected when full penetration welding is required for butt joint of small-diameter pipeline?

When small-diameter pipes are butt jointed, because the internal welding cannot be carried out, the single-sided groove can only be selected according to the welding process of single-sided welding and double-sided forming.

If the wall thickness is not too thick, V-groove can be selected;

When the wall thickness is thick and the processing conditions permit, U-groove can be selected.

If necessary, the bottom locking joint or the joint with lining ring at the bottom can also be selected.

35. What is the basis for selecting the welding current during welding?

Generally, the selection of welding current is based on:

(1) Electrode diameter;

(2) Type of electrode coating;

(3) Welding space position;

(4) Weldment size and joint type;

(5) Type and polarity of welding power supply;

(6) Welding site and ambient temperature.

36. Why should welding electrodes be dried before use?

Due to the influence of factors such as electrode coating composition and performance, air humidity, storage mode and storage time, the process performance of the electrode is often deteriorated due to moisture absorption, resulting in unstable arc, increased spatter, and prone to defects such as pores and cracks.

Therefore, the electrode shall be dried before use.

37. What are the advantages of argon arc welding of aluminum and aluminum alloys?

The main advantages of argon arc welding of aluminum and aluminum alloys are that argon is an inert gas, with good protection effect, stable arc and beautiful weld formation.

When the power supply adopts AC, the cathode crushing effect can be used to effectively remove the alumina film on the surface of the molten pool.

There is no slag during welding, and the corrosion of the joint by the residue after welding will not occur.

Argon flow has a scouring effect on the welding area, which accelerates the cooling rate of the welded joint, improves the microstructure and properties of the joint, and reduces the residual deformation of the weldment after welding.

38. Which welding method can be selected for horizontal butt welding of pipes?

Generally speaking, because the pipe wall is not thick, when all position welding is realized, the weld formation will be deteriorated due to the flowing of molten iron.

At this time, the ideal welding method is pulsed argon arc welding, that is, tungsten pulsed argon arc welding or melting electrode pulsed argon arc welding, especially the former is more mature at present.

Its characteristic is that the welding process is easy to realize automation, which reduces the labor intensity and the requirements for welders’ operation skills;

It is beneficial to realize single-sided welding and double-sided forming; Good weld quality and beautiful appearance.

39. What problems should be paid attention to when using rigid fixation method to reduce welding residual deformation?

Using rigid fixation method to reduce welding residual deformation, attention should be paid to:

(1) The rigid fixation method can not completely eliminate the welding residual deformation, but can only reduce part of the residual deformation, because when the external restraint is removed, there will still be some residual deformation on the weldment;

(2) Rigid fixation method will produce large welding stress in welded joints, so it should be used with caution for some crack prone materials.

40. What is a weld “joint”? What are the key points of joint operation?

During electrode arc welding, due to the limited length of electrode, a weld is often welded by multiple electrodes, or due to the welding process requirements, a weld is connected by several short welds.

The connection between electrode and electrode, short weld and short weld is called the “joint” of weld.

The following operation essentials shall be mastered when connecting:

(1) The slag covered at the crater shall be removed before “joint”

(2) The shorter the welding rod replacement or arc self extinguishing time, the better;

(3) During the joint, the arc shall be struck at 15-20mm in front of the arc pit, and the arc shall be pulled back to the arc pit for a short time for preheating, and then the arc shall be shortened to melt the original arc pit.

After it is filled to the required height and width, the welding shall continue according to the original strip transportation method.

41. What is the effect of welding current on welding quality?

The welding current has a direct impact on the welding quality. Increase the welding current to increase the weld penetration and ensure the penetration.

However, if the current is too large, it is easy to cause weld undercut, burn through and splash, overheat the weld metal structure and increase the grain size.

The welding rod is easy to turn red, the coating is easy to fall off and the protection performance is reduced.

If the current is too small, it is easy to cause slag inclusion, incomplete welding and other defects.

42. How to distinguish between welding and brazing? What are the characteristics of each?

The characteristic of welding is the combination of atoms between weldments, while fiber welding connects weldments with solder, an intermediate medium with lower melting point than weldments.

Welding is characterized by high mechanical properties of the joint and high productivity when connecting thick and large parts.

The disadvantage is that the stress and deformation are large, and the microstructure of the heat affected zone changes;

Brazing is characterized by low heating temperature, flat and smooth joint, beautiful appearance, small stress and deformation.

The disadvantage is low joint strength and high requirements for assembly clearance during assembly.

43. What should be paid attention to at the beginning of electrode arc welding?

At the beginning of the weld, the temperature of the weldment is low, and the temperature cannot rise rapidly after arc striking.

Therefore, the penetration depth of this part of the weld is shallow, which reduces the weld strength.

Therefore, when using acid electrode, the arc shall be slightly lengthened after arc striking, the weld end shall be preheated, and then the arc length shall be shortened for normal welding; When using alkaline electrode, after striking the arc in front of the starting point, return the electrode to the starting point for normal welding, so as to improve the shortcomings of insufficient penetration at the starting point and low joint strength.

44. What is the effect of oxygen in the welding area on the weld?

Effect of oxygen: with the increase of oxygen content in weld, its strength, hardness and plasticity decrease significantly.

At the same time, it will also cause hot embrittlement, cold embrittlement and age hardening of weld metal.

Oxygen also affects the physical and chemical properties of weld metal, such as reducing the conductivity, magnetic conductivity and corrosion resistance of weld metal.

The oxygen dissolved in the molten pool is also easy to form carbon monoxide pores, burn the beneficial alloy elements in the welding material, and deteriorate the weld performance.

In the droplet, when there is too much oxygen and carbon, it is easy to cause spatter and affect the stability of the welding process.

45. What are the main difficulties in sheet welding? How to overcome these difficulties?

The main difficulties in thin plate welding are burn through, post weld deformation and poor weld formation, followed by porosity.

(1) When welding thin plates, carefully clean the dirt and water stains in the joint area, otherwise it is easy to produce pores.

The plate edge shall be processed accurately, and the misalignment between the two plates shall not be greater than 0.5mm, otherwise it is easy to burn through.

The welding process parameters shall be accurately controlled. If there is a large fluctuation, there will also be defects such as burn through, poor forming or incomplete penetration

(2) For thin plate welding, press horse, iron or fixed welding around can be used for rigid fixation to reduce welding deformation.

Segmented welding method (such as skip welding method) shall be adopted for long welds;

(3) The spacing of tack welding shall not be too large, and dense spot tack welding shall be used (each section is about 10 ~ 15mm long).

DC reverse connection method shall be adopted, short arc fast linear weld bead. When production conditions permit, the weldment can be inclined 15 ° ~ 20 ° for downhill welding to improve the welding speed, so as to prevent burn through and reduce deformation. Intermittent arc quenching or vertical downward vertical welding can also be used.

At the same time, the reasonable welding sequence shall be strictly observed.

46. What are the measures to prevent welding residual deformation? What are the welding process parameters?

(1) Select reasonable assembly and welding sequence; Select reasonable welding method and sequence; Inverse deformation method;

Rigid fixation method; Heat dissipation method.

(2) Welding current, welding voltage, welding speed and linear energy.

47. What are the requirements for groove cleaning before welding?

(1) The groove shall be processed according to the specified shape and size.

(2) Before welding, the rust, oil stain, water, paint and other sundries on and around the groove surface shall be carefully removed. Residues left by carbon arc gouging must be removed.

(3) Welding shall be carried out in time after cleaning.

If the groove is damp or rusted due to non welding for other reasons, it shall be cleaned again before welding.

When welding is carried out in a very humid climate, or there is dew and frost on and around the groove surface, it shall be dried before welding.

48. Why do stresses and deformations occur during welding?

Because during the welding process, the weldment is subject to local and uneven heating and cooling.

Therefore, the degree of metal thermal expansion and cold contraction at each part of the welded joint is different.

As the weldment itself is a whole, all parts are interrelated and restrict each other, and cannot be extended or shortened freely, stress and deformation will occur in the welding process.

49. Why is alkaline electrode beneficial to prevent welding cold crack and hot crack?

The alkaline electrode has good De-S and P properties, so it is beneficial to resist hot cracks.

The coating contains a large amount of alkaline slag forming substances and a certain amount of deoxidizer and alloying agent.

It combines with hydrogen to synthesize hydrogen cyanide (HF) at high temperature, reducing the hydrogen content in the weld, so it is beneficial to resist cold cracks.

50. What are the characteristics of ultrasonic flaw detection compared with radiographic flaw detection?

Compared with radiographic inspection, ultrasonic inspection has the following characteristics:

(1) Insensitive to thin parts and near surface defects; Applicable to thick parts;

(2) Short flaw detection cycle; Simple equipment; Low cost; No harm to human body;

(3) The nature of welding defects cannot be directly judged.

51. What are the causes of incomplete penetration?

The blunt edge of weld groove is too large, the groove angle is too small, the weld root is not cleaned, the gap is too small, the angle of electrode or welding wire is incorrect, the current is too small, the speed is too fast, the arc length is too large, and there is magnetic bias blowing during welding;

Or the current is too large, and the welding rod has melted rapidly when the metal of the weldment has not been fully heated;

The rust, oxide scale and oil stain between layers or at the edge of base metal are not removed, the welding position is poor, and the welding accessibility is poor.

52. Why is electrode forward welding suitable for sheet welding?

When tilting forward, the weld forming coefficient increases, the penetration depth is shallow, and the weld is wide, which is suitable for welding thin plates.

When tilting forward, the effect of arc force on the back row of molten pool metal is weakened, and the liquid metal at the bottom of molten pool is thickened, which hinders the heating effect of arc on base metal, so the weld thickness is reduced.

At the same time, the preheating effect of arc on the unmelted base metal in front of the molten pool is strengthened, so the weld width increases and the reinforcement decreases. Suitable for welding thin plates.

53. What are the measures to prevent air holes?

Preventive measures include:

(1) within 10mm on both sides of manual arc welding weld and 20mm on both sides of automatic submerged arc welding, carefully remove rust and other dirt on the surface of weldment.

(2) the welding rod and flux shall be dried strictly according to the regulations before welding, and stored in the insulation barrel, so that they can be used and taken at any time.

(3) adopt appropriate welding process parameters. When alkaline electrode is used for grounding, short arc welding must be used.

54. What are the advantages and disadvantages of argon arc welding and carbon dioxide gas shielded welding?

Argon arc welding has small weld penetration and small working deformation.

The weld density is high, and it is not easy to form slag inclusion, air hole, undercut and other defects.

It meets the high requirements of nondestructive testing, and the weld has good strength, toughness and plasticity.

The tensile, bending and Impact Indexes of mechanical properties are better than other welding methods.

It can better meet the requirements of single-sided welding and double-sided forming and thin-wall welding.

The disadvantages are low work efficiency and high processing cost. Moreover, the cost varies greatly with the market.

CO2 gas shielded welder has high efficiency and low cost, and can meet many on-site operations under general inspection requirements.

It is especially suitable for using manpower to catch up with the progress.

However, the disadvantages are also obvious.

The use is generally restricted in occasions where the state has strict weld quality control.

55. What welding wires and electrodes are used for welding 55.12cr1mov and 20 steel?

First of all, we consider what kind of dissimilar steel is welded. If it is carbon structural steel, it should be low strength matching (that is, select the welding rod of the steel with low welding strength).

If one of the two heterogeneous steels is alloy steel, in order to make up for the burning loss of alloy elements in the welding process, the electrode for welding alloy steel shall be selected!

56. What is weldability? What is the weldability of carbon steel?

Weldability refers to the ability of materials to be welded into components according to the specified design requirements under limited construction conditions and meet the predetermined service requirements.

Weldability is affected by four factors: material, welding method, component type and service requirements. Carbon steel is based on iron, iron carbon alloy.

Carbon is an alloy element, and the mass fraction of carbon is not more than 1%.

In addition, the mass fraction of manganese is not more than 1.2%, and the mass fraction of silicon is not more than 0.5%.

The latter two are not used as alloy elements.

Other elements, such as Ni, Cr and Cu, are controlled within the limit of residual amount, and are not used as alloy elements.

Impurity elements such as S, P, O and N are strictly limited according to different steel varieties and grades.

Therefore, the weldability of carbon steel mainly depends on the carbon content.

With the increase of carbon content, the weldability gradually deteriorates, and the weldability of low carbon steel is the best, as shown in Table 1.

Table 1 Relationship between weldability and carbon content of carbon steel

name Mass fraction of carbon (%) Typical hardness Typical use Weldability
mild steel ≤0.15 60HRB Plates,, and excellent
0.15~0.25 90HRB Structural profiles, plates and bars good
Medium carbon steel 0.25~0.60 25HRC Machine parts and tools Medium (heat required, post heat, low hydrogen welding method)
High carbon steel ≥0.60 4OHRC Spring, mold, rail Inferior (preheating and post heating are required, and hydrogen welding method is required for the core)

57. Welding process of pearlite heat resistant steel?

Steel with sufficient strength and oxidation resistance at high temperature is called heat-resistant steel.

Low alloy heat-resistant steel with Cr and Mo as the main alloy elements.

The matrix structure is pearlite (or pearlite + ferrite), which is called Pearlite Heat-resistant steel. The commonly used steel grades are 15CrMo, 12CrMoV, 12Cr2MoWVTiB, 14mnmov, 18mnmonb and 13mnnimonb.

Because Pearlite Heat-resistant steel contains a certain amount of Cr, Mo and other alloy elements, hard and brittle martensite structure will be produced in the heat affected zone.

Cold cracks are easy to form when welding at low temperature or welding rigid structures. Therefore, the following process measures shall be taken during welding:

(1) Preheating

Preheating is an important process measure for welding Pearlite Heat-resistant steel.

In order to ensure the welding quality, the weldment shall be preheated and maintained at 80 ~ 150 ℃ during tack welding or formal welding.

When backing with argon arc welding and CO2 gas shielded welding, the preheating temperature can be reduced or not preheated.

(2) Slow cooling after welding

Immediately after welding, cover the weld and heat affected zone with asbestos cloth to cool it slowly.

(3) Post weld heat treatment

High temperature tempering shall be carried out immediately after welding to prevent delay cracks, eliminate stress and improve microstructure.

The post weld heat treatment temperature should be avoided in the temperature range of 350 ~ 500 ℃, because the Pearlite Heat-resistant steel has strong fire brittleness in this temperature range.

The post weld heat treatment temperature of several common pearlitic heat-resistant steels is shown in Table 2.

Table 2 post weld heat treatment temperature of pearlite heat resistant steel

Steel gradeThickness to be heat treated (m)High temperature tempering temperature after welding (℃)
15CxMo12Cx1MoV20CxMo12Cx212Cx3MoVSiTiB>10> 6 Any Thickness any thickness680 ~ 700720 ~ 760720 ~ 760760 ~ welding and cutting union 780740 ~ 780

58. How to weld low carbon steel at low temperature?

When welding low-carbon steel structure in severe winter, due to the fast cooling speed of the welded joint, the crack tendency increases, especially the first weld of thick and large structure is easy to crack.

Therefore, the following process measures must be taken:

1) Preheat before welding, and strictly keep the interpass temperature not lower than the preheating temperature during welding.

2) Use low hydrogen or ultra-low hydrogen welding materials.

3) During tack welding, increase the welding current, slow down the welding speed, appropriately increase the cross-sectional area and length of tack weld, and preheat if necessary.

4) The whole weld shall be continuously welded as far as possible to avoid interruption.

5) Arc striking shall not be carried out on the base metal outside the groove surface, and the arc pit shall be filled during arc extinguishing.

6) Bending, straightening and assembly of weldments at low temperature shall not be carried out as far as possible.

See Table 3 for preheating temperature during low temperature welding of various metal structures. See Table 4 for preheating temperature during low temperature welding of pipes and pressure vessels.

Table 3 preheating temperature of low temperature welding of low carbon steel metal structure

Thickness of weldment (mm)Preheating temperature at various temperatures
<3031~5051~70Do not preheat when it is not lower than – 30 ℃; Preheating below – 30 ℃; Do not preheat when 100 ~ 150 ℃ is not lower than 10 ℃; Preheating when the temperature is lower than 10 ℃; Do not preheat when 100 ~ 150 ℃ is not lower than 0 ℃; Preheat 100 ~ 150 ℃ when it is lower than 0 ℃

Table 4 preheating temperature of low temperature welding of low carbon steel pipeline and pressure vessel

Thickness of weldment (mm)Preheating temperature at various temperatures
<16173031^4041~50Do not preheat when it is not lower than – 30 ℃; Preheat at 100 ~ 150 ℃ below – 30 ℃ and not below – 20 ℃; Preheating below – 20 ℃; Do not preheat when 100 ~ 150 ℃ is not lower than 10 ℃; When the temperature is lower than – 10 ℃, the pre heating temperature is not lower than 100c150 ℃, and when there is no – 0 ℃, the pre heating is not required; Preheat 100 ~ 150 ℃ when it is lower than 0 ℃

59. How to correctly select welding materials when welding low carbon steel?

(1) Selection of welding rod for manual arc welding: the average tensile strength of common low carbon steel Q235 is 417.5mpa.

According to the principle of equal strength, the matching welding rod shall be E43 series.

See Table 5 for the selection of electrodes for manual arc welding of low carbon steel with several different steel grades.

Table 5 selection of electrodes for low carbon steel manual arc welding

Steel grade Model of welding rod selected for general structure Dynamic load, complex, thick plate structure, boiler Welding conditions
    Model of welding rod for pressure vessel and low temperature welding  
Q235 E4313,E4303,E4301,E4320,E4311 E4316,E4315(E5016,E5015) Generally not preheated
Q255 Generally not preheated
Q275 E4316,E4315 E5016,E5015 Preheating of thick plate structure above 150 ℃
08、10、15、20 E4303,E4301,E4320,E4311 E4316,E4315(E5016,E5015) Generally not preheated
25 E4316,E4315 E5016,E5015 Preheating of thick plate structure above 150 ℃
20g22g E4303,E4301 E4316,E4315(E5016,E5015) The preheating of thick plate structure is 100 ~ 150 ℃, and the welding and cutting alliance is generally not pre executed
20R E4303,E4301 E4316,E4315(E5016,E5015)

Note: the model of welding rod in brackets in the table indicates that it can be used instead.

(2) The matching selection of welding wire and flux for submerged arc welding is low carbon steel.

See Table 6 for the matching selection of welding wire and flux for submerged arc welding.

Table 6 matching selection of low carbon steel submerged arc welding wire and flux

Steel grade Welding wire Flux
Q234 HO8A HJ430HJ431
Q255 HO8A
Q275 HOBMnA
15、20 H08A,HO8MnA HJ430HJ431HJ330
25 HO8MnA,H10Mn2
20g,22g HO8HnA,HO8MnSi,h10Mn2
20R H08MnA

(3) Selection of CO2 welding wire: the grades of solid core welding wire are H08Mn2Si and H08Mn2SiA, and the strength of deposited metal after welding is high. The grades of flux cored wire are YJ502-1, YJ506-2, YJ506-3 and YJ506-4.

(4) For the matching of welding wire and flux for electroslag welding, the temperature of the molten pool of electroslag welding is lower than that of submerged arc welding, so the reduction effect of silicon and manganese in the flux is weak, and the welding wire with high manganese and silicon content shall be selected.

H10Mn2, h10mnsi welding wire and flux HJ360 or h10MnSi welding wire and flux HJ431 are often selected.

60. What are the limitations of using carbon equivalent value to evaluate the weldability of steel?

The carbon equivalent value can only be within a certain range. The weldability of steel can be evaluated generally and relatively because:

1) If the carbon equivalent values of the two steels are equal, but the carbon content is different, the steel with higher carbon content is easy to produce hardened structure during welding, and its crack tendency is obviously greater than that of the steel with lower carbon content, with poor weldability.

Therefore, when the carbon equivalent value of steel is equal, it can not be regarded as exactly the same weldability.

2) The calculated value of carbon equivalent only expresses the influence of chemical composition on weldability, without considering that different cooling speeds can obtain different structures.

When the cooling speed is fast, it is easy to produce hardened structures, and the weldability will become worse.

3) In addition to the chemical composition and cooling rate, the factors affecting the weld metal structure and weldability, as well as the maximum heating temperature and residence time at high temperature in the welding cycle, are not expressed in the calculation formula of carbon equivalent value.

Therefore, the calculation formula of carbon equivalent value can only evaluate the weldability of steel in a certain range of steel grades, and can not be used as an accurate evaluation index.

61. Welding process of 18MnMoNb steel?

The yield point of 18mnmonb steel is equal to 490MPa (belonging to 490mpa grade steel).

Due to the high content of carbon and alloy steel elements, the quenching hardening tendency and cold cracking tendency are greater than that of 16Mn steel.

Key points of welding process:

1) In addition to electroslag welding, preheating measures shall be taken for weldments before welding, and the preheating temperature shall be controlled at 150 ~ 180 ℃.

For joints with high stiffness, the preheating temperature shall be increased to 180 ~ 230 ℃. After welding or interruption of welding, post heat treatment at 250 ~ 350 ℃ shall be carried out immediately.

2) Selection of welding materials.

3) In order to ensure the performance and quality of the joint, the welding line energy shall be properly controlled.

For example, during manual arc welding, the welding line energy shall be controlled below 24kj / cm;

During submerged arc welding, the welding line energy shall be controlled below 35kJ / cm. However, the welding line energy should not be too small, otherwise the welded joint is prone to harden structure and reduce toughness.

At the same time, the interlayer temperature shall be controlled between the preheating temperature and 300 ℃.

4) Heat treatment shall be carried out after welding. The heat treatment method of electroslag welded joint is normalizing at 900 ~ 980 ℃ and tempering at 630 ~ 670 ℃.

Manual arc welding and submerged arc welding joints shall be subject to high-temperature tempering treatment to eliminate welding residual stress.

The tempering temperature is about 30 ℃ lower than that of general steel.

62. Selection of annealing temperature and time for welded steel parts?

Stress relief annealing is heating at 450 ~ 650 ℃ below the abnormal point for a period of time and then slowly cooling to room temperature, which can eliminate the residual stress generated in the process of cutting, stamping, casting and welding.

For carbon steel, the reference heating temperature is 625 ± 25 ℃; For alloy steel, the reference heating temperature is 700 ± 25 ℃.

The holding time will also vary. For carbon steel, the holding time is 1 hour for every 25mm thickness;

For alloy steel, the holding time is 2 hours per 25mm thickness, and the cooling rate is cooled at a cooling rate of less than 275 ℃ hours every 25mm.

Generally speaking, the heat treatment process mainly depends on empirical values.

It can’t be solved by copying. The composition of materials is different and very different.

63. What are the gas sources in the welding area?

During welding, various gases are filled around the solution pool, which mainly come from the following aspects:

(1) Gas generated by gas generating agent in electrode coating or flux;

(2) Ambient air;

(3) Residual gas of welding core, welding wire and base metal during smelting;

(4) Gas formed by decomposition of crystal water remaining in electrode coating or flux at high temperature;

(5) Rust, moisture, paint, etc. not removed from the surface of base metal, and gas decomposed under the action of electric arc.

64. Performance and application of HJ431 flux?

HJ431 is a melting type high manganese and high silicon flux.

The chemical composition of the flux is shown in table 24.

The color of the flux is red brown or light yellow, in the form of glassy particles with a particle size of 0.45 ~ 2.5mm.

The power supply can be used for both AC and DC, and the reverse connection is used for DC power supply.

The flux has good process performance, stable arc, beautiful fish scale ripple in the weld, but the rust resistance is general. The main chemical reactions with molten metal during welding are as follows:

MnO + Fe = FeO + mnsio2 + 2Fe = 2FeO + SiCaFe + H2O = CaO + 2HF ↑

CaF2 + 2H = Ca + 2HF ↑

The reduced Mn and Si infiltrate into the weld metal, which can improve the mechanical properties of the weld metal.

The escape of HF can reduce the hydrogen content of the weld metal and improve the anti porosity ability.

Table 7 chemical composition (mass fraction) of HJ431 (%)

Si0MnOCaFMgoCa0AlOFe0SP
40~4434~383~75~8≤6≤4≤1.8≤0 .06≤0.08

HJ431 is used together with H08A and H08MnA welding wires to weld important components of low carbon steel and low alloy steel.

65. Preparation method of sintered flux brand?

According to the regulations in the product sample of welding materials, the sintered flux is represented by the letter SJ followed by three digits:

1) The first digit indicates the slag system of flux slag, as shown in Table 8.

Table 8 first digit series of sintered flux brand

Flux brand Range of main components of slag (number of quality classification types) (%)
SJ1 ×× SJ2 ×× SJ3 ×× ST4 ×× sJ5 ×× SJ6 ×× Calcium fluoride type; High aluminum type; Silicon calcium type; Silicon manganese type; Aluminum titanium type; Other types CaF2≥15;
CaO+MgO+MnO+CaF2>50;
SiO2≤20;
Al3O2≥20;
Al3O2+CaO+MgO>45CaO+MgO+SiO2>60MnO+SiO2>50A12O3+TiO2>45

2) The second and third digits represent different brands of flux in the same slag system type, which are arranged in the order of 01, 02 and „.

66. Preparation method of melting flux brand?

According to the regulations in the product sample of welding materials, the melted flux is represented by the letter HJ followed by three digits:

1) The first digit indicates the content of MnO in the flux, and its series arrangement is shown in Table 9.

Table 9 first digit series of melting flux brand

Flux brandFlux TypeMass fraction of MnO (%)
H order 1 ×× H2 ×× H Ding 3 ×× H Ding 4 ××No manganese; Low manganese; Medium manganese; High manganese<22~51

2) The second digit indicates the content of SiO2 and CaF2 in the flux, and its series arrangement is shown in table 10.

Table 10 second digit series of melting flux brand

Flux brandFlux TypeMass fraction of si0caf (%)
× one × HJ × two × H × three ×× 4XHJ × five × H Ding × six × HJ × seven × HJ × eight × H × nine ×Low silicon and low fluorine; Medium silicon and low fluorine; High silicon and low fluorine; Fluorine in low silicon; Medium silicon and fluorine; Fluorine in high silicon; Low silicon and high fluorine; Medium silicon and high fluorine; otherSiO2<10;CaF<10SiO2 10~30;CaF<10SiO2>30;CaF<10SiO<10;CaF210~30SiO210~30;CaF210~30SiO2>30;CaF10~30SiO<10;CaF<30SiO10~30;CaF>30 

3) The third digit indicates different grades of the same type of flux, which are arranged in the order of 0, 1, 2 and „. 4) When two particle sizes are produced for the same brand of flux, in the case of fine particles (the particle size of flux is

0.45 ~ 2.4mm) flux brand followed by“ ×” Word.

67. Performance and application of sj501 flux?

SJ501 is a sintered acid flux.

The chemical composition of the flux is shown in Table 11. The power supply can be used for both AC and DC.

The reverse connection is adopted during DC welding, and the maximum welding current can reach 1200A.

The color is silver white. It has strong pore resistance during high-speed welding and is not sensitive to a small amount of rust film and high-temperature oxide film.

It can be used to weld low-carbon steel and some low-alloy steel structures with H08A and H08MnA welding wires.

It can be used for multi wire rapid welding, especially for double-sided single pass welding.

Table 11 chemical composition (mass fraction) of SJ501 (%)

Si0+Ti0AlO+MnOCaF2SP
30598.80.0390.041

68. What are the common forms of pad joints? What are its advantages and disadvantages?

A backing plate with the same composition as the base metal is placed on the back of the groove so that a fully welded weld can be obtained during welding without burning through the root.

This joint is called backing plate joint. Common joint forms of backing plate include: I-shaped belt backing plate groove, V-shaped belt backing plate groove, Y-shaped belt backing plate groove, single-sided V-shaped belt backing plate groove, etc. see Fig. 6.

a) I-shaped groove with backing plate

b) V-shaped groove with backing plate

c) Y-shaped groove with backing plate

d) Single sided groove with backing plate

The operation skills of the base plate joint are simpler and easier to master than that of single-sided welding and double-sided forming.

It is often used when the back cannot be welded (such as the circumferential seam of small-diameter cylinder and the circumferential seam of jacketed container).

The disadvantage is that when the ellipticity of the base plate and the cylinder is inconsistent, there will be a gap when they are assembled together.

During welding, the molten slag cannot float up when flowing into this gap, so it is easy to form slag inclusion.

JB4708-92 welding procedure qualification of steel pressure vessels stipulates that the bending angle of single-sided welding with backing can be based on the bending angle standard of double-sided welding.

69. Compare the advantages and disadvantages of Y-shaped groove, U-shaped groove with blunt edge and double Y-shaped groove?

When the thickness of the weldment is the same, the geometry of the three grooves is shown in Fig. 5.

a) Y-groove  b) U-groove with blunt edge   c) Double Y-groove

(1) Y-groove

1) Groove surface processing is simple.

2) It can be welded on one side without turning over.

3) When the space area of welding groove is large, there are many filling materials and the thickness of weldment is large, the productivity is low.

4) Large welding deformation.

(2) U-shaped groove with blunt edge

1) It can be welded on one side without turning over.

2) When the space area of the welding groove is large, the filling material is small, and the thickness of the weldment is large, the productivity is higher than that of the Y-shaped groove.

3) Large welding deformation.

4) It is difficult to process the root radius of the groove surface, which limits the popularization and application of this groove.

(3) double Y-shaped groove

1) Double sided welding, so the weldment needs to turn over during welding, but the welding deformation is small.

2) Although the processing of groove surface is slightly more complex than that of Y-shaped groove, it is simpler than that of U-shaped groove with blunt edge.

3) The groove area is between Y-shaped groove and U-shaped groove with blunt edge, so the productivity is higher than Y-shaped groove and the filling material is less than Y-shaped groove.

70. Try to describe the representation of supplementary symbols in weld symbols?

Supplementary symbols are used to supplement some characteristics of welds, as shown in table.

Name Sketch Map Symbol interpretative statement
Symbol with backing plate Indicates that there is a backing plate at the bottom of the weld
Three side weld symbol Indicates that there are welds on three sides
Surrounding weld symbol Indicates the weld around the weldment
Field symbol   Indicates welding on site or on site
Tail coincidence   The welding process and method can be marked with reference to gb185-85

71. What are weld symbols? How many parts does the weld symbol consist of?

The code indicating the welding method, weld form and weld size on the drawing is called weld symbol.

According to GB324-88 weld symbol representation, weld symbol is generally composed of basic symbol and leader.

Auxiliary symbols, supplementary symbols and weld size symbols can also be added if necessary.

72. Describe the type of weld?

The joint formed in the weldment after welding is called weld.

According to the combination form, welds can be divided into butt welds, fillet welds, plug welds and end welds.

(1) Butt weld

The welds that make up the butt joint are called butt welds. Butt weld can be formed by butt joint or T-joint (cross joint), which refers to the weld with zero weld leg after full penetration welding after beveling.

(2) Fillet weld

The joint surface of the two weldments constitutes a weld welded by straight or close to straight. At the same time, the weld composed of butt weld and fillet weld is called combined weld.

The weld with full penetration welding and certain weld leg after beveling of T-joint (cross joint) is combined weld.

The weld in the groove is butt weld, and the weld connecting two weldments outside the groove is fillet weld.

(3) Plug weld

It refers to the weld that fills the round hole formed by overlapping two weldments, one of which is opened with a round hole, and then welded in the round hole

(4) End weld

Welds forming termination joints.

73. Describe the influence of welding process parameters on weld shape?

During welding, the general name of various physical quantities (such as welding current, arc voltage, welding speed, linear energy, etc.) selected to ensure welding quality is welding process parameters.

The influence of process parameters on weld shape is as follows:

(1) Welding current

When other conditions remain unchanged, with the increase of welding current, the weld thickness and reinforcement increase, while the weld width remains almost unchanged (or slightly increased).

(2) Arc voltage

When other conditions remain unchanged, the arc voltage increases, the weld width increases significantly, and the weld thickness and reinforcement decrease slightly.

(3) Welding speed

When other conditions remain unchanged, the welding speed increases and the weld width, weld thickness and reinforcement decrease.

Welding current, arc voltage and welding speed are the three welding process parameters during welding.

When selecting, the proper cooperation among the three should be considered in order to obtain a weld with good shape and meet the requirements.

74. What are the technical requirements for butt welding of weldments?

The requirements for butt joint of weldments are as follows:

1) When steel plates with different thicknesses are butted, if the thickness difference of steel plates on both sides is too large, the section at the connection will change greatly after connection, which will cause serious stress concentration.

Therefore, for important welded structures, such as pressure vessels, thick plates should be thinned.

According to relevant technical standards: when the thickness of the thin plate is ≤ 10mm and the thickness difference between the two plates exceeds 3mm, or when the thickness of the thin plate is > 10mm and the thickness difference between the two plates is greater than 30% of the thickness of the thin plate or more than 5m, the edge of the thick plate shall be thinned, and the thinned length shall be greater than or equal to 3 times of the thickness difference.

2) When the straight-line weldment is butted with the curved weldment, the weld is just at the junction, resulting in large welding stress and becoming the weak surface of the whole structure. Therefore, the curved weldment at the butt joint shall have a straight section, so that the weld is in the flat butt position.

75. Try to describe the representation method and application of the leader in the weld symbol?

The leader is generally composed of a leader with an arrow (hereinafter referred to as the arrow line) and two datum lines (one is a solid line and the other is a dotted line), as shown in Fig. 17.

When using the leader, it shall match with the basic symbol:

1) If the weld is on the arrow side of the joint, mark the basic symbol on the solid line side of the datum line, as shown in Figure 18a.

2) If the weld is on the non arrow side of the joint, mark the basic symbol on the dotted line of the reference line, as shown in Figure 18b.

3) When marking symmetrical welds and double-sided welds, dotted lines may not be added, as shown in Fig. 18c and Fig. 18d.

a) The weld is on the arrow side of the joint

b) The weld is on the non arrow side of the joint

c) Symmetrical weld

d) Double side weld work

76. Try to describe the principle and quality standard of magnetic particle flaw detection.

Magnetic particle testing is a non-destructive testing method that uses the phenomenon of magnetic particle attracted by the leakage magnetic field generated by the surface defects of ferromagnetic materials in a strong magnetic field.

Principle of magnetic particle flaw detection: firstly, magnetize the inspected weld locally, and there will be magnetic lines of force passing through the weld.

For welds with the same section size and uniform internal materials, the distribution of magnetic lines of force is uniform.

When there are cracks, pores, slag inclusions and other defects on the surface or inside of the weld, the magnetic line of force will bypass the defects with large magnetic resistance and produce bending, as shown in Fig. 5A.

At this time, the magnetic particle is sprinkled on the weld surface, and the magnetic force line will pass through the magnetic particle on the surface defect to form “magnetic flux leakage”, and the magnetic particle is adsorbed on the defect.

The size and position of the defect can be judged according to the shape, number and thickness of the adsorbed magnetic particle.

Because the internal defects are far away from the weld surface, magnetic flux leakage will not be formed on the magnetic line of force, and the magnetic particles cannot be absorbed and there is no accumulation, so the defects cannot be exposed. Common magnetic particles are ferric oxide (Fe3O4) and ferric oxide (Fe2O3).

Defects can be divided into three types according to shape:

(1) The display length of linear defect magnetic trace is more than 3 times of its width.

(2) Circular defect magnetic trace is a defect magnetic trace other than linear defect magnetic trace.

(3) Scattered defect magnetic trace magnetic trace with several defects in a certain area at the same time.

Quality standard: according to the provisions of ZBJ04006-87 standard, the grade of defective magnetic trace is divided into 7 levels.

77. Describe the representation of basic symbols in weld symbols?

The basic symbol is a symbol representing the cross-sectional shape of the weld. Several common basic symbolic representations are shown in table.

Name Sketch Map Symbol
I-shaped weld
Y-shaped weld
Single side V-shaped weld with blunt edge
U-shaped weld with blunt edge
Back bead
Fillet weld
Plug weld

78. Principle of radiographic inspection and image characteristics of welding defects.

X-ray and Y-ray can be used for radiographic inspection respectively.

When the ray passes through the metal material, part of the energy is absorbed to attenuate the ray.

If the thickness of the penetrating metal material is different (cracks, pores, incomplete penetration and other defects, where holes occur, making the material thinner) or the volume and mass are different (slag inclusion), the attenuation is also different.

The attenuation is large when passing through thick or large objects, so the intensity on the substrate is weak, the sensitivity of the negative is small, and the blackness obtained after development is shallow;

On the contrary, the blackness is deep.

According to the images with different blackness on the negative, the defects can be clearly displayed.

79. Try to describe the principle and quality standard of ultrasonic flaw detection.

The non-destructive testing method using ultrasonic to detect the internal defects of materials is called ultrasonic flaw detection.

The principle of ultrasonic flaw detection is to find defects by using the different acoustic impedance (the product of material volume mass and sound velocity) between defects in welds and normal tissues and the reflection phenomenon of sound waves on heterogeneous interfaces with different acoustic impedance.

During flaw detection, the piezoelectric transducer in the probe emits pulse ultrasonic.

It is transmitted to the weldment through the acoustic coupling medium (water, oil, glycerol or paste, etc.), and the reflected wave is generated after encountering the defect.

Then another similar probe or the same probe is used to receive the reflected sound wave, which is converted into an electrical signal by the transducer, amplified and displayed on the fluorescent screen or printed on the paper tape.

The defect position can be obtained according to the probe position and the propagation time of sound wave (echo position on the fluorescent screen);

The amplitude of the reflected wave can approximately evaluate the size of the defect.

Quality standard: the more directions of ultrasonic detection welds, the greater the probability that the beam is perpendicular to the defect plane, the higher the detection rate of defects, and the more accurate the evaluation results.

According to GB11345-89 manual ultrasonic flaw detection methods and classification of flaw detection results for steel welds, ultrasonic damage is divided into three inspection levels A, B and C according to the number of detection directions of welds.

The perfection of inspection increases step by step, and level B is suitable for pressure vessels.

80. Why should the welding area be protected? How to protect?

The purpose of protecting the welding area is to prevent air from invading the droplet and pool and reduce the nitrogen and oxygen content in the weld metal.

There are three ways of protection:

⑴ Gas protection.

For example, during gas shielded welding, shielding gas (CO2, H2, Ar) is used to isolate the welding area from the air.

(2) Slag protection cover a layer of slag on the metal surface of the molten pool to separate it from the air, such as electroslag welding and submerged arc welding.

(3) Gas slag combined protection uses shielding gas and slag to protect molten metal at the same time, such as manual arc welding.

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