1. What are the characteristics of the primary crystalline structure of the weld seam?
The crystallization of welding molten pool also follows the basic law of general liquid metal crystallization: formation of crystal nucleus and growth of crystal nucleus.
When the liquid metal in the welding pool solidifies, the semi melted grains on the base metal in the fusion zone usually become crystal nuclei.
Then the crystal nucleus adsorbs the atoms of the surrounding liquid to grow up.
Because the crystal grows in the opposite direction to the direction of heat conduction, and it also grows in the direction of both sides, but it is blocked by the adjacent growing crystals, so the crystals that form a columnar shape are called columnar crystals.
In addition, under certain conditions, the liquid metal in the molten pool will also produce spontaneous nucleation during solidification.
If the heat dissipation is carried out in all directions, the crystal will grow into crystalline crystal uniformly in all directions, which is called equiaxed crystal.
The columnar crystal is usually seen in the weld seam, under certain conditions, the equiaxed crystal will also appear in the weld center.
2. What are the characteristics of the secondary crystallization structure of the weld?
For the structure of weld metal, after the primary crystallization, the metal continues to cool below the phase transformation temperature, and then the metallographic structure changes.
For example, during the welding of low carbon steel, the primary crystalline grains are austenitic grains. When the cooling temperature is lower than the phase transformation temperature, the austenite is decomposed into ferrite and pearlite.
Therefore, the structure after secondary crystallization is mostly ferrite plus a small amount of pearlite.
However, due to the fast cooling speed of the weld, the pearlite content obtained is generally higher than that in the equilibrium structure.
The faster the cooling speed is, the higher the pearlite content is.
The less the ferrite content is, the higher the hardness and strength are, while the plasticity and toughness are reduced.
After secondary crystallization, the actual structure at room temperature was obtained.
Different steels have different weld microstructures under different welding conditions.
3. Take low-carbon steel as an example to explain what structure is obtained after the secondary crystallization of weld metal?
Take low plastic steel as an example. The primary crystalline structure is austenite.
The solid phase transformation process of weld metal is called secondary crystallization of weld metal.
The microstructure of secondary crystallization is ferrite and pearlite.
In the equilibrium structure of low-carbon steel, the carbon content of weld metal is very low, and its structure is coarse columnar ferrite with a small amount of pearlite.
Due to the high cooling rate of the weld, the ferrite can not be completely precipitated according to the iron carbon phase diagram, and the pearlite content is generally higher than that in the flat structure.
The higher the cooling rate, the finer the grains, and the higher the hardness and strength of the metal.
The decrease of ferrite and the increase of pearlite will also increase the hardness and decrease the plasticity.
Therefore, the final structure of the weld is determined by the composition of the metal and cooling conditions.
Due to the characteristics of the welding process, the weld metal structure is fine, so the weld metal has better microstructure and properties than the casting state.
4. What are the characteristics of dissimilar metal welding?
1) The characteristics of dissimilar metal welding mainly lie in the obvious difference between the alloy composition of the deposited metal and the weld.
The behavior of the welding pool varies with the weld shape, base metal thickness, electrode coating or flux, and the type of shielding gas.
Therefore, the melting amount of the base metal is also different, and the mutual dilution of the concentration of chemical components in the melting area of the deposited metal and the base metal will also change.
It can be seen that the degree of non-uniformity of dissimilar metal welding joints with regional chemical composition depends on not only the original composition of weldments and filler materials, but also the welding process.
2) After the welding thermal cycle, different metallographic structures will appear in each area of the welded joint due to the inhomogeneity of the structure, which is related to the chemical composition, welding method, welding layer, welding process and heat treatment of the base metal and filler material.
3) The non-uniformity of performance, due to the different chemical composition and metal structure of the joint, results in the different mechanical properties of the joint.
The strength, hardness, plasticity, toughness, etc. of each area along the joint are very different.
In the heat affected zone on both sides of the weld, the impact value is even several times different.
The creep limit and endurance strength under high temperature will also vary greatly due to the different composition and structure.
4) The non-uniformity of the stress field distribution and the non-uniformity of the residual stress distribution in the dissimilar metal joint are mainly determined by the different plasticity of each area of the joint.
In addition, the difference of material thermal conductivity will cause the change of welding thermal cycle temperature field.
The different coefficient of linear expansion in each region and other factors are the reasons for the uneven distribution of the stress field.
5. What are the selection principles of welding materials for dissimilar steel welding?
The selection principle of dissimilar steel welding materials mainly includes the following four points:
1) If the strength and plasticity of the weld metal cannot be taken into account, the welding materials with good plasticity shall be selected on the premise that the welded joint does not produce cracks and other defects.
2) The weld metal properties of dissimilar steel welding materials only meet one of the two base metals, which is considered to meet the technical requirements.
3) Welding materials shall have good process performance and beautiful weld formation.
Welding materials are economical and easy to purchase.
Related reading: Weldability of Metal Materials
6. How about the weldability of pearlitic steel and austenitic steel?
Pearlitic steel and austenitic steel are two kinds of steel with different structures and components.
Therefore, the two types of steel are welded together, and the weld metal is formed by the fusion of two different types of base metals and filler materials, which raises the following problems for the weldability of these two types of steel:
1) Dilution of welds.
As pearlitic steel contains low gold elements, it has a dilution effect on the alloy of the whole weld metal.
Because of this dilution effect of pearlitic steel, the content of austenite-forming elements in the weld decreases.
As a result, martensite structure may appear in the weld, which worsens the quality of the weld joint, even cracks.
2) Formation of transition layers.
Under the action of welding thermal cycle, the mixing degree of the melted base metal and filler metal is different at the edge of the molten pool.
At the edge of the molten pool, the liquid metal has low temperature, poor fluidity and short residence time in the liquid.
Due to the great difference in chemical composition between pearlitic steel and austenitic steel, the molten base metal and filler metal at the edge of the molten pool on the pearlitic side can not be well fused.
As a result, the pearlitic base metal accounts for a large proportion in the welds on the pearlitic steel side, and the closer to the fusion line, the larger the proportion of base metal.
This forms a transition layer with different internal components of the weld metal.
3) A diffusion layer in the fusion zone is formed.
In the weld metal composed of these two types of steels, pearlitic steel has higher carbon content, but higher alloy elements, but less alloy elements.
On the contrary, for austenitic steel, the concentration difference of carbon and carbide forming elements is formed on both sides of the pearlite steel side in the fusion zone.
When the joint works at a temperature higher than 350-400 ℃ for a long time, the fusion zone will show obvious carbon diffusion, that is, diffusion from the pearlite steel side to the austenitic weld through the fusion zone.
Results a decarburized and softened layer was formed on the pearlitic steel base metal near the fusion zone, and a decarburized layer corresponding to decarburization was formed on one side of the austenitic weld.
4) Because the physical properties of pearlitic steel and austenitic steel are very different, the composition of the weld is also very different.
Therefore, this type of joint cannot be heat treated to eliminate the welding stress, which can only cause the redistribution of stress, which is very different from the welding of the same metal.
5) Delayed cracking.
In the process of crystallization, the welding molten pool of this kind of dissimilar steel has both austenite and ferrite structures, which are close to each other.
The gas can be diffused, so the diffusible hydrogen can be gathered to produce delayed cracks.
7. What are the measures to prevent cracks during the repair welding of cast iron?
(1) Preheating before welding and slow cooling after welding
Preheating the weldment as a whole or partially before welding and slow cooling after welding can not only reduce the tendency of the white mouth of the weld, but also reduce the welding stress and prevent the weldment from cracking.
(2) Arc cold welding is adopted to reduce welding stress.
Welding materials with good plasticity are selected, such as nickel, copper, nickel copper, high vanadium steel, etc. as filler metals, so that the weld metal can relax stress through plastic deformation to prevent cracks.
The welding stress can be reduced by using fine diameter electrodes, small current, intermittent welding (intermittent welding) and scattered welding (skip welding) to reduce the temperature difference between the weld and the base metal, stress can be eliminated and cracks can be prevented by hammering the weld.
(3) Other measures: adjust the chemical composition of weld metal to narrow its brittle temperature range;
Add rare earth elements to enhance the desulfurization and dephosphorization metallurgical reaction of the weld, and add the refining grain elements of Zengna to refine the weld grains.
In some cases, the heating stress zone method is used to reduce the stress at the weld repair area, which can also effectively prevent the occurrence of cracks.
Related reading: How to Weld Cast Iron?
8. What is stress concentration? What are the factors that cause stress concentration?
Due to the unknown characteristics of the weld seam shape and weld seam, there is a discontinuity in the collective shape.
When loaded, it causes the uneven distribution of the working stress of the welded joint, making the local peak stress σmax much higher than the average stress σm.
This is called stress concentration.
In the welding street, there are many reasons for stress concentration, among which the main reason is:
(1) The process defects, air inlets, slag inclusions, cracks and incomplete penetration in the weld, among which the stress concentration caused by welding cracks and incomplete penetration is the most serious.
(2) Unreasonable weld appearance, such as excessive reinforcement of butt weld and high weld toe of fillet weld
Unreasonable street design, such as sudden change of street interface, use of butt joint street with cover plate, etc.
Unreasonable weld joint arrangement will also cause stress concentration, for example, T-joints with only shop welds.
9. What is plastic failure and what are its hazards?
Plastic failure includes plastic instability (yielding or significant plastic deformation) and plastic fracture (edge fracture or ductile fracture).
The process is that the welded structure first undergoes elastic deformation under load → yielding → plastic deformation (plastic instability) → microcracks or voids → macro cracks → unstable growth → fracture.
Compared with brittle fracture, the plastic fracture is less susceptible to cold, specifically as follows:
(1) Unrecoverable plastic deformation occurs after yielding, which makes welded structures with high size requirements scrapped.
(2) For pressure vessels made of high-toughness and low-strength materials, the failure is not controlled by the fracture toughness of the materials, but caused by plastic instability due to insufficient strength.
The final result of plastic damage is that the welded structure will be invalid or catastrophic accidents will occur, which will affect the production of enterprises, cause unnecessary casualties, and seriously affect the development of the national economy.
10. What is brittle fracture and what is its harm?
Generally, brittle fracture refers to dissociation fracture (including quasi dissociation fracture) and grain boundary (intergranular) fracture splitting along a certain crystal plane.
Cleavage fracture is a kind of intragranular fracture, which is formed by separating along a certain crystallographic plane in the crystal.
Under certain conditions, such as low temperature, high strain rate and high stress concentration, metal materials will undergo cleavage fracture when the stress reaches a certain value.
There are many models about the generation of cleavage fracture, most of which are related to dislocation theory.
It is generally believed that when the plastic deformation process of materials is seriously hindered, the materials can not conform to the external stress by deformation but by separation, thus cleavage cracks occur.
Inclusions, brittle precipitates and other defects in metals also have an important influence on the generation of cleavage cracks.
Brittle fracture generally occurs when the stress is not higher than the design allowable stress of the structure and there is no significant plastic deformation, and it extends to the whole structure instantaneously.
With the nature of sudden damage, it is not easy to detect and prevent in advance, so it often causes personal injury and property loss.
11. What role does welding crack play in structural brittle fracture?
Among all the defects, cracks are the most dangerous. Under the action of external load, a small amount of plastic deformation will occur near the crack front, and a certain amount of opening displacement will occur at the tip, making the crack develop slowly;
When the external load increases to a certain critical value, the crack will expand at a high speed.
At this time, if the crack is located in a high value tensile stress zone, it will often cause brittle fracture of the whole structure.
If the extended crack enters the area with low tensile stress, the reputation is that there is enough energy to maintain the further expansion of the crack, or the crack enters a material with better toughness, (or the same material with higher temperature and increased toughness) receives greater resistance.
If the crack cannot continue to expand, the damage of the crack will be reduced accordingly.
12. What are the causes of brittle fractures of welded structures?
The causes of fracture can be summarized into three aspects:
(1) The humanity of materials is insufficient
Especially at the tip of the notch, the material has poor micro elastic deformation ability.
Low-stress brittle failure usually occurs at a lower temperature, and the toughness of materials decreases sharply with the decrease in temperature.
In addition, with the development of low alloy high strength steel, the strength index is increasing, while the plasticity and toughness are decreasing.
In most cases, the brittle fracture starts from the welding zone, so the lack of toughness of the weld and heat-affected zone is often the main reason for brittle failure at low stress.
13. What main factors should be considered when designing welded structures?
The main considerations are as follows:
1) The welded joint shall have enough stress and rigidity to ensure a long service life;
2) Consider the working medium and working conditions of welded joints, such as temperature, corrosion, vibration, fatigue, etc;
3) The workload of pre-welding preheating and post-welding heat treatment shall be minimized for large structural members;
4) The weldment can no longer need or only need a small amount of machining;
5) Welding work can be minimized;
6) The deformation and stress of welded structure shall be reduced to the minimum;
7) Easy to construct and create good working conditions for construction;
8) Try to adopt new technology and mechanized and automatic welding to improve labor productivity;
9) The weld is convenient for inspection to ensure joint quality.
14. Please describe the basic conditions of gas cutting. Can red copper be cut with oxygen acetylene flame? Why?
The basic conditions for gas cutting are:
(1) The ignition point of metal shall be lower than the melting point of metal
(2) The melting point of metal oxide shall be lower than that of the metal itself;
(3) Metal burning in oxygen should be able to give off a lot of heat;
(4) The thermal conductivity of metal should be small.
Red copper cannot be cut with oxygen acetylene flame, because the heat generated by copper oxide (CuO) is very little, and its thermal conductivity is very good (the heat cannot be concentrated near the incision), so it cannot be cut with gas.
15. What is the main function of gas welding powder?
The main function of welding powder is slagging, which generates slag by reacting with metal oxides or non-metallic impurities in the molten pool.
At the same time, because the generated slag covers the surface of the molten pool and insulates the molten pool from the air, it prevents the metal in the molten pool from being continuously oxidized at high temperatures.
16. What are the process measures to prevent weld porosity in manual arc welding?
(1) Welding rods and fluxes shall be kept dry and dried as required before use;
(2) The surface of welding wire and weldment shall be kept clean and free of water, oil, rust, etc;
(3) The welding specification shall be correctly selected, for example, the welding current shall not be too large, and the welding speed shall be appropriate;
(4) Adopt correct welding method, use alkaline electrode for manual arc welding, short arc welding, reduce the swing range of electrode, slow down the electrode moving speed, and control short arc starting and stopping;
(5) The assembly clearance of control weldment shall not be too large;
(6) Welding rods with cracked coating, peeling, deterioration, eccentricity and corroded core shall not be used.
17. What are the main measures to prevent chill during cast iron welding?
(1) The right to use graphitized electrodes is strong, that is, cast iron electrodes with a large number of graphitized elements (such as carbon, silicon, etc.) added to the coating or welding wire, or nickel base and copper base cast iron electrodes;
(2) Preheating before welding, heat preservation during welding, and slow cooling after welding to reduce the cooling rate of the weld zone, extend the time when the fusion zone is in the red hot state, make the graphitization sufficient, and reduce the thermal stress;
(3) Use brazing process.
18. Try to describe the role of flux in the welding process?
Flux is the main factor to ensure the welding quality in welding. It has the following functions;
(1) After melting, the flux floats on the surface of molten metal to protect the molten pool and prevent the erosion of harmful gases in the air.
(2) The welding flux has the functions of deoxidation and alloying, and cooperates with the welding wire to make the weld metal obtain the required chemical composition and mechanical properties.
(3) Make the weld well formed.
(4) Slow down the cooling rate of molten metal and reduce defects such as porosity and slag inclusion.
(5) Prevent splashing, reduce loss and improve binding coefficient.
19. 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 welding machine shall not be short circuited for a long time.
(3) The regulating current shall operate without load.
(4) Frequently check the wire contact, fuse, grounding, regulating mechanism, etc. and make sure they are in good condition.
(5) Keep the welding machine clean, dry and ventilated to prevent dust and rain intrusion.
(6) Place it stably and cut off the power supply after work.
(7) The welding machine shall be overhauled regularly.
20. What are the hazards of brittle fracture?
Because brittle fracture occurs suddenly and cannot be detected and prevented in time, once it happens, the consequences will be very serious, not only causing significant economic losses, but also endangering human life safety.
Therefore, brittle fracture of welded structures should be paid more attention to.
21. Characteristics and application of plasma spraying?
Plasma spraying is characterized by high plasma flame temperature, which can melt almost all refractory materials, so it has a wide range of spraying objects, high plasma flame flow speed, good powder particle acceleration effect, and high coating bonding strength.
It has a wide range of applications and is the best method for spraying various ceramic materials.
22. Preparation procedure of welding procedure card?
The preparation procedure of the welding procedure card shall find out the corresponding welding procedure qualification and draw the joint sketch according to the product assembly drawing, parts processing drawing and its technical requirements;
Give the welding procedure card number, drawing number, joint name, joint number, welding procedure qualification number and welder certificate items;
Prepare welding sequence according to welding procedure qualification, actual production conditions, technical personnel and production experience;
Prepare specific welding process parameters according to welding process qualification;
Determine the inspection authority, inspection method and inspection proportion of products according to the requirements of product drawings and product standards.
23. Why should a certain amount of silicon and manganese be added to the welding wire of CO2 gas shielded welding?
Carbon dioxide is an oxidizing gas, which will burn the alloy elements of the welding seam during welding, thus greatly reducing the mechanical properties of the welding seam.
Among them, oxidation will lead to pores and splashes.
Adding silicon and manganese to the welding wire plays a deoxidization role, which can solve the problems of welding oxidation and splashes.
24. What is the explosion limit of combustible mixture and what factors affect it?
The concentration range of combustible gas, vapor or dust contained in combustible mixture is called explosion limit.
The lower limit of the contained concentration is called the lower explosion limit, and the upper limit of the contained concentration is called the upper explosion limit.
The explosion limit is affected by temperature, pressure oxygen content, container diameter and other factors. When the temperature rises, the explosion limit decreases;
When the pressure increases, the explosion limit will also decrease;
When the concentration of oxygen in the mixed gas increases, the lower explosion limit decreases.
For combustible dust, its explosion limit is affected by dispersion, humidity, temperature and other factors.
25. What measures should be taken to prevent electric shock when welding in boiler drum, condenser, oil tank, oil tank and other metal containers?
(1) Welders shall avoid contact with iron parts during electric welding, stand on rubber insulation pads or wear rubber insulation shoes, and wear dry work clothes.
(2) Outside the vessel, there shall be a supervisor who can see and hear the welder’s work, and a switch shall be set to cut off the power supply according to the welder’s signal.
(3) The voltage of portable lamps used in containers shall not exceed 12V.
The shell of the portable lamp transformer shall be reliably grounded, and auto transformer is not allowed.
(4) Transformers for portable lamps and welding transformers shall not be carried into boilers and metal containers.
26. How to distinguish fusion welding from brazing? What are the characteristics of each?
Fusion welding is characterized by the combination of atoms between weldments, while brazing is to connect weldments with filler metal, an intermediate medium with a lower melting point than the weldment.
The advantages of fusion welding are high mechanical properties of welded joints, high productivity when connecting thick and large pieces, while the disadvantages are large stress and deformation, and the microstructure changes in the heat affected zone;
The advantages of brazing are low heating temperature, flat and smooth joints, beautiful appearance, small stress and deformation, while the disadvantages are low joint strength and high requirements for assembly clearance during assembly.
27. Both carbon dioxide and argon belong to protective gases. What are their properties and uses?
Carbon dioxide is an oxidizing gas. When it is used as the shielding gas in the welding area, it will cause the molten droplets and molten pool metal to be severely oxidized, resulting in the burning loss of alloy elements.
Moreover, it has poor processability, which will produce pores and large splashes.
Therefore, at present, it can only be used for welding low carbon steel and low alloy steel, not for welding high alloy steel and non-ferrous metals, especially for stainless steel, because it will cause weld carburetion and reduce resistance to intergranular corrosion, so it is less used.
Argon is an inert gas. Since it does not react with molten metal in any chemical way, the chemical composition of the weld seam has little change.
The quality of the weld seam after welding is good, and it can be used to weld various alloy steels, stainless steels and non-ferrous metals.
As the price of argon is gradually decreasing, it is also used to weld a large number of low-carbon steels.
28. Try to describe the weldability and welding characteristics of 16Mn steel?
16Mn steel adds about 1% Mn on the basis of Q235A steel, with carbon equivalent of 0.345%~0.491%.
Therefore, the welding performance is good.
However, the hardening tendency is slightly larger than that of Q235A steel, and small parameters are used on thick and rigid structures.
Cracks may occur when small weld passes are welded, especially at low temperatures.
At this time, proper preheating can be used before welding.
E50 welding rod shall be used for manual arc welding;
H08MnA welding wire with flux 431 can be used when submerged arc automatic welding cannot open the groove;
H10Mn2 welding wire with flux 431 shall be used for beveling;
During CO2 gas-shielded welding, welding wire H08Mn2SiA or H10MnSi shall be used.