1. What are the characteristics of the primary crystal structure of solder?
Answer: Solder pool crystallization also follows the basic rules of general crystallization of liquid metals: the formation of crystal nuclei and the growth of crystal nuclei. When the liquid metal in the weld pool solidifies, the semi-molten grains of the parent material in the fusion zone usually become crystalline nuclei.
Then the crystal nucleus absorbs atoms from the surrounding liquid and grows. Since the crystal grows in the direction opposite to the direction of heat conduction, it also grows in both directions. However, due to blockage by adjacent growing crystals, the crystal forms. Crystals with columnar morphology are called columnar crystals.
Additionally, under certain conditions, the liquid metal in the molten pool will also produce spontaneous crystal nuclei upon solidification. If heat dissipation is carried out in all directions, the crystals will grow uniformly into granular crystals in all directions. of the crystal is called It is an equiaxed crystal. Column-shaped crystals are commonly seen in welds, and under certain conditions equiaxed crystals may also appear in the center of the weld.
2. What are the characteristics of the secondary crystal structure of the solder?
Answer: The structure of molten metal. After primary crystallization, the metal continues to cool below the phase transformation temperature and the metallographic structure changes again. For example, when welding low carbon steel, the primary crystallization grains are all austenite grains. When cooled below the phase transformation temperature, austenite decomposes into ferrite and pearlite, so the structure after secondary crystallization consists mainly of ferrite and a small amount of pearlite.
However, due to the faster cooling rate of the weld, the resulting pearlite content is generally higher than the content in the equilibrium structure. The faster the cooling rate, the higher the pearlite content, and the less ferrite, the higher the hardness and strength. enhanced, while plasticity and toughness are reduced. After secondary crystallization, the real structure at room temperature is obtained. The weld structures obtained by different steel materials under different welding process conditions are different.
3. Let’s take the example of low carbon steel to explain what structure is obtained after the secondary crystallization of the molten metal?
Answer: Taking low plastic steel as an example, the primary crystallization structure is austenite, and the solid-state phase transformation process of molten metal is called secondary crystallization of molten metal. The secondary crystallization microstructure is ferrite and pearlite.
In the equilibrium structure of low carbon steel, the carbon content of the weld metal is very low, and its structure consists of coarse columnar ferrite plus a small amount of pearlite. Due to the high cooling rate of the solder, the ferrite cannot be completely precipitated according to the iron-carbon phase diagram. As a result, the perlite content is generally higher than that of the smooth structure. A high cooling rate will also refine the grains and increase the hardness and strength of the metal. Due to the reduction of ferrite and increase of pearlite, the hardness will also increase, while the plasticity will decrease.
Therefore, the final weld structure is determined by the metal composition and cooling conditions. Due to the characteristics of the welding process, the structure of the welded metal is finer, so the welded metal has better structural properties than the cast state.
4. What are the characteristics of welding different metals?
Answer: 1) The characteristics of dissimilar metal welding are mainly obvious differences in the alloy composition of the weld metal and weld with the weld shape, base metal thickness, coating or flux of the electrode and the type of welding. protective gas. The solder puddle also behaves inconsistently. Therefore, the melting degree of the base material is also different, and the mutual dilution effect of the concentration of chemical components in the molten zone of the deposited metal and the base material will also change. It can be seen that the degree of irregularity when welding dissimilar metals depends not only on the original composition of the weldment and filler material, but also on changes due to different welding processes.
2) Inhomogeneity of the structure. After undergoing the welding thermal cycle, different metallographic structures will appear in each area of the welded joint, related to the chemical composition of the base metal and filler materials, welding method, welding level, welding process and heat treatment.
3) Non-uniformity of performance. Due to the different chemical composition and metal structure of the gasket, the mechanical properties of the gasket are different. weld The impact values of the heat-affected zones on the two sides are even several times different, and the creep limit and durable resistance at high temperatures also vary greatly depending on the composition and structure.
4) Non-uniformity of the stress field distribution. The distribution of residual stresses in different metal joints is not uniform. This is mainly determined by the different plasticity of each area of the joint. In addition, the difference in thermal conductivity of materials will cause changes in the temperature field of the welding thermal cycle. Factors such as differences in linear expansion coefficients in various regions cause the uneven distribution of the stress field.
5. What are the principles of selection of welding materials when welding different steels?
Answer: The selection principles of different steel welding materials mainly include the following four points:
1) Under the premise that the welded joint does not produce cracks and other defects, if the strength and plasticity of the welded metal cannot be taken into account, welding materials with better plasticity should be selected.
2) If the weld metal properties of different steel welding materials only meet one of the two base materials, it is considered to meet the technical requirements.
3) The welding materials should have good process performance, and the weld seam should be in beautiful shape.
4) Welding materials are economical and easy to purchase.
6. What is the weldability of pearlitic steel and austenitic steel?
Answer: Pearlitic steel and austenitic steel are two types of steel with different structures and compositions. Therefore, when these two types of steel are welded together, the weld metal is formed by the fusion of two different types of base metals and filler materials. raises the following questions for the weldability of these two types of steel:
1) Solder dilution. Since pearlitic steel contains fewer gold elements, it has a diluting effect on the alloy of the entire molten metal. Due to this dilution effect of pearlitic steel, the content of austenite-forming elements in the weld is reduced. weld, A martensitic structure may appear, thus deteriorating the quality of the welded joint, or even causing cracks.
2) Formation of an excessive layer. Under the action of welding thermal cycle, the mixing degree of molten base metal and filler metal is different at the edge of the molten pool, the temperature of the liquid metal is lower, the fluidity is. bad, and the residence time in the liquid state is shorter since pearlitic steel and austenitic steel. The chemical composition is very different. At the edge of the weld pool on the pearlitic side, the molten base metal and filler metal cannot fuse well. Therefore, in the weld on the pearlitic steel side, the pearlitic base metal matters. for a greater proportion, and the proportion is greater. Closer to the melting line, the proportion of base material is greater. This forms a transition layer with different internal compositions of the molten metal.
3) Form a diffusion layer in the fusion zone. In the weld metal composed of these two types of steel, since pearlitic steel has a higher carbon content, and the opposite is true for austenitic steel, a difference in concentration of carbon-forming elements Carbon and carbide form on both sides of the pearlitic steel. of the fusion zone. When the joint operates at a temperature above 350°C to 400°C for a long time, obvious carbon diffusion occurs in the melting zone, that is, the pearlitic steel side through the melting zone up to the austenite. weld. As a result, a decarburized softening layer is formed on the pearlitic steel base metal near the fusion zone, and a carburized layer corresponding to the decarburization is produced on the austenitic weld side.
4) Since the physical properties of pearlitic steel and austenitic steel are very different, and the weld composition is also very different, this type of joint cannot eliminate the welding stress by heat treatment , and can only cause the redistribution of constraints. is very different from welding the same metal.
5) Delayed cracking. During the crystallization process of the welding molten pool of this different type of steel, there are both austenitic structure and ferrite structure, both are close to each other, and the gas can diffuse , so diffused hydrogen can accumulate and cause delayed cracking. .
7. What measures should be taken to avoid cracking when repair welding cast iron?
Answer: (1) Preheating before welding and slow cooling after welding. Preheating all or part of the solder before welding and slowly cooling it after welding can not only reduce the tendency of the solder to turn white, but also reduce the welding stress and prevent the solder from cracking.
(2) Use cold arc welding to reduce welding stress and choose welding materials with good plasticity, such as nickel, copper, nickel-copper and high vanadium steel as filler metal, so that the welded metal can release stress through plastic deformation and avoid cracking. , using small diameter welding rods, low current welding, intermittent (intermittent welding), dispersed welding (jump welding) methods can reduce the temperature difference between the weld and the base metal and reduce the welding stress, which can be removed by hammering the weld. stress and prevent cracking.
(3) Other measures include adjusting the chemical composition of the molten metal to reduce its brittle temperature range; adding rare earth elements to enhance the metallurgical reactions of desulfurization and dephosphorization of the solder and adding powerful grain refining elements to crystallize the solder; Grain refinement. In some cases, the heating stress zone method is used to weaken the stress on the welding repair area, which can also effectively prevent the occurrence of cracks.
8. What is stress concentration? What are the factors that cause stress concentration?
Answer: Due to the shape of the weld and the unknown characteristics of the weld, a discontinuity in the collective shape appears when loaded, it causes uneven distribution of the working stress in the welded joint, making the stress maximum local well above the average stress. . , it is the concentration of stress. There are many reasons for stress concentration during welding, the most important of which are:
(1) Process defects produced in the weld, such as air inlet holes, slag inclusions, cracks and incomplete penetration, etc. Among them, the stress concentration caused by welding cracks and incomplete penetration is the most serious.
(2) Unreasonable weld shape, such as the reinforcement of the butt weld is too large, the butt of the fillet weld is too high, etc.
Unreasonable joint design, such as sudden changes in joint interface, use of butt joints with cover plates, etc. Unreasonable arrangement of welds can also lead to stress concentration.
9. What is plastic damage and what is its danger?
Answer: Plastic damage includes plastic instability (elasticity or significant plastic deformation) and plastic fracture (edge fracture or ductile fracture). The process is that the welded structure first undergoes elastic deformation → elasticity → plastic deformation (plastic loss) under load. Stable) → microcracks or microvoids → macrocracks → unstable expansion → fracture. Compared to brittle fracture, plastic damage is relatively low and the most specific are as follows:
(1) Unrecoverable plastic deformation occurs after yielding, leading to the scrapping of welded structures with high size requirements.
(2) The failure of pressure vessels made of high toughness and low strength materials is not controlled by the toughness of the material, but is caused by plastic instability failure due to insufficient strength.
The end result of plastic damage is the failure of the welded structure or a catastrophic accident that affects the company’s production and causes unnecessary losses.
10. What is a brittle fracture and what damage does it cause?
Answer: Generally, brittle fracture refers to divisional dissociation fracture (including quasi-dissociation fracture) and grain boundary (intergranular) fracture along a certain crystal plane. Cleavage fracture is a fracture formed by separation along a certain crystallographic plane within the crystal. Under certain conditions, such as low temperature, high strain rate and high stress concentration, cleavage and fracture will occur in metallic materials when the stress reaches a certain value.
There are many models for the generation of cleavage fractures, most of which are related to dislocation theory. It is generally believed that when the plastic deformation process of a material is greatly hindered, the material cannot adapt to external stresses by deformation but by separation, resulting in cleavage cracks. Inclusions, brittle precipitates and other defects in metals also have a significant impact on the occurrence of cleavage cracks.
Brittle failure generally occurs when the stress is not greater than the design allowable stress of the structure and there is no significant plastic deformation, and spreads instantly throughout the structure. It has the nature of sudden destruction and is difficult to detect and prevent in advance. , this often causes personal injury and enormous property damage.
11. What role do welding cracks play in brittle structural failure?
Answer: Among all defects, cracks are the most dangerous. Under the action of external load, a small plastic deformation will occur near the crack front, and at the same time there will be a certain opening displacement at the tip, causing the crack to develop slowly; when the external load increases to a certain critical value. When , the crack expands at high speed. At this time, if the crack is located in a large value tensile stress zone, it often causes the totality. Brittle fracture of the structure, if the expanding crack enters a region with lower tensile stress, there is not enough energy to keep the crack expanding further, or the crack enters a material with better toughness (or the same material but with higher temperature and temperature). increased toughness). When it reaches greater strength, it cannot continue to expand, the risk of cracking is reduced accordingly.
12. What is the reason why welded structures are prone to brittle fracture?
Answer: The reasons for the fracture can be summarized in three aspects:
(1) The material is not human enough. Especially at the end of the notch, the microscopic deformation capacity of the material is poor. Low-stress brittle fracture generally occurs at lower temperatures, and as the temperature decreases, the toughness of the material decreases sharply. In addition, with the development of high-strength low-alloy steels, the strength index continues to increase, while plasticity and toughness have decreased. In most cases, brittle fracture starts from the weld zone, so insufficient toughness of the weld and heat affected zone is often the main cause of low stress brittle fracture.
(2) There are defects such as microcracks. Fracture always begins with a defect and cracks are the most dangerous defects. Welding is the main cause of cracks. Although cracks can in principle be controlled through the development of welding technology, it remains difficult to completely avoid cracks.
(3) A certain level of stress. Incorrect design and poor manufacturing processes are the main causes of welding residual stress. Therefore, for welded structures, in addition to working stresses, welding residual stresses and stress concentration, as well as additional stresses caused by poor assembly, must also be considered.
13. What are the main factors to consider when designing welded structures?
Answer: The main factors to consider are:
1) The welded joint must guarantee sufficient stress and rigidity to guarantee a sufficiently long lifespan;
2) Consider the working environment and working conditions of the welded joint, such as temperature, corrosion, vibration, fatigue, etc. ;
3) For large structural parts, the workload of preheating before welding and heat treatment after welding should be reduced as much as possible;
4) The welded parts require no more or little mechanical treatment;
5) Welding workload can be reduced to a minimum;
6) Minimize the deformation and stresses of the welded structure;
7) Easy to build and creates good working conditions for construction;
8) Try to adopt new technologies, mechanization and automated welding to improve labor productivity;
9) Welds are easy to inspect to ensure joint quality.
14. Please describe the basic conditions for gas cutting. Can oxygen-acetylene flame gas cutting be used for copper? For what?
Answer: The basic conditions for gas cutting are:
(1) The flash point of the metal should be lower than the melting point of the metal;
(2) The melting point of the metal oxide should be lower than the melting point of the metal itself;
(3) The metal must be capable of releasing a large amount of heat when burning in oxygen;
(4) The thermal conductivity of the metal should be low.
Oxygen-acetylene flame gas cutting cannot be used on red copper, because copper oxide (CuO) generates very little heat and its thermal conductivity is very good (heat cannot be concentrated near of the incision), gas cutting is therefore not possible.
15. What is the main function of gas welding powder?
Answer: The main function of welding powder is to create slag. It reacts with metal oxides or non-metallic impurities in the molten pool to generate molten slag. At the same time, the generated molten slag covers the surface of the molten pool and isolates the molten pool from air, thereby preventing the metal in the molten pool from being oxidized at high temperature.
16. What are the process measures to avoid weld porosity during manual arc welding?
Answer: (1) The welding rod and flux should be kept dry and dried according to regulations before use;
(2) The surfaces of welding wires and welded constructions must be kept clean and free from water, oil, rust, etc. ;
(3) Correctly select the welding specifications, such as the welding current should not be too large, the welding speed should be appropriate, etc.
(4) Use the correct welding method. Use alkaline electrodes for manual arc welding, short arc welding, reduce the oscillation amplitude of the electrode, slow down the rod transport speed, control the starting and closing of the short bow, etc.
(5) Check that the weld assembly gap is not too large;
(6) Do not use electrodes with cracked, peeled, deteriorated, eccentric coatings or corroded weld cores.
17. What are the main measures to avoid white spots when welding cast iron?
Answer: (1) Use solid graphitized welding rods, that is, use cast iron welding rods with a large amount of graphitizing elements (such as carbon, silicon, etc.) added to the paint or wire welding, or use nickel-based welding rods and copper-based cast iron welding rods;
(2) Preheat before welding, maintain heat during welding, and cool slowly after welding to reduce the cooling rate of the welding zone, extend the time the fusion zone is in the red-hot state, fully graphitize and reduce thermal stress;
(3) Use a brazing process.
18. Describe the role of flux in the welding process?
Answer: In welding, flux is the main factor ensuring weld quality. It has the following functions:
(1) After the flux is melted, it floats on the surface of the molten metal to protect the molten pool and prevent erosion caused by harmful gases in the air.
(2) The flux has the functions of deoxidation and alloying and cooperates with the welding wire to obtain the required chemical composition and mechanical properties of the welded metal.
(3) Make the weld well formed.
(4) Slow down the cooling rate of molten metal and reduce defects such as pores and slag inclusions.
(5) Prevent splashing, reduce losses and improve welding coefficient.
19. What points should be paid attention to when using and maintaining AC arc welding machines?
Answer: (1) It should be used according to the rated welding current and charging time of the welding machine, and not overload.
(2) The welding machine should not be short-circuited for a long time.
(3) The regulating current must operate without load.
(4) Always check wire contacts, fuses, grounding, adjustment mechanisms, etc. and make sure they are in good condition.
(5) Keep the welding machine clean, dry and ventilated to prevent dust and rain from entering.
(6) Place it stably and turn off the power supply after finishing the work.
(7) The welding machine should be inspected regularly.
20. What are the risks of brittle fracture?
Answer: Since brittle fracture occurs suddenly and cannot be discovered and avoided in time, once it occurs, the consequences will be very serious, not only causing major economic losses, but also endangering life human. Therefore, brittle fracture of welded structures is a problem that must be taken seriously.
21. What are the characteristics and applications of plasma projection?
Answer: The characteristics of plasma spraying are that the plasma flame temperature is high and can melt almost all refractory materials, so it can be sprayed on a wide range of objects, the plasma flame speed is high and the particle acceleration effect is good, therefore. the bond strength of the coating is high. It has a wide range of uses and is the best way to spray various ceramic materials.
22. What is the procedure to prepare the welding process sheet?
Answer: The welding process map preparation program should determine the corresponding welding process assessment based on the product assembly drawings, part processing drawings and their technical requirements, and draw a diagram simplified joint with welding process card number, drawing number and joint; name, joint number, welding procedure qualification number and certification elements of the welder; prepare the welding sequence based on the qualification of the welding procedure and actual production conditions, technical experts and production experience; prepare the specific parameters of the welding process according to the qualification of the welding procedure; product drawing requirements and product The standard determines the inspection agency, inspection method and product inspection rate.
23. Why do we need to add a certain amount of silicon and manganese to the carbon dioxide gas shielded welding wire?
Answer: Carbon dioxide is an oxidizing gas. During the welding process, the welding metal elements will be burned, thereby significantly reducing the mechanical properties of the weld. Among them, oxidation will cause pores and spatter. . It has a deoxidizing effect and can solve oxidation and welding spatter problems.
24. What is the explosive limit of flammable mixtures and what factors influence it?
Answer: The concentration range in which flammable gases, vapors or dusts contained in a flammable mixture can occur is called the explosive limit.
The lower limit of the concentration is called the lower explosion limit and the upper limit of the concentration is called the upper explosion limit. The explosion limit is affected by factors such as temperature, pressure, oxygen content and container diameter. As the temperature increases, the explosion limit decreases as the pressure increases, the explosion limit also decreases as the concentration of oxygen in the mixed gas. increases, the lower explosive limit decreases. For combustible dusts, their explosive limit depends on factors such as dispersion, humidity and temperature.
25. What measures should be taken to avoid electric shock when welding in boiler drums, condensers, fuel oil tanks, fuel oil tanks and other metal containers?
Answer: (1) When welding, welders should avoid contact with iron parts, stand on rubber insulating mats or wear rubber insulating shoes, and wear dry work clothes.
(2) There should be a guard outside the container who can see and hear the welder’s work, as well as a switch to cut off the power supply based on the welder’s signal.
(3) The voltage of street lights used in containers must not exceed 12 volts. The shell of the portable light transformer must be reliably grounded, and the use of autotransformers is not permitted.
(4) Transformers for portable lamps and welding transformers must not be transported in boilers and metal containers.
26. How to distinguish between welding and brazing? What are the characteristics of each?
Answer: The characteristic of fusion welding is the bonding of atoms between the parts to be welded, while brazing uses an intermediate medium with a lower melting point than the parts to be welded – a brazing material to connect the parts to be welded . The advantage of fusion welding is that the mechanical properties of the welded joint are high and the productivity when connecting thick and bulky parts is high. The disadvantage is that stresses and strains are large and structural changes occur in the heat-affected zone. area ; the advantage of brazing is that the heating temperature is low, the joint is flat, smooth, beautiful in appearance, with low stress and deformation. The disadvantage is that the joint strength is low and the assembly space needs to be high. assembly.
27. Carbon dioxide and argon are both protective gases. Please describe their properties and uses.
Answer: Carbon dioxide is an oxidizing gas. When used as a shielding gas in the welding zone, it violently oxidizes the droplets and metal in the molten pool, causing combustion loss of the alloying elements. will be produced. Therefore, it can currently only be used for welding low carbon steel and low alloy steel, and is not suitable for welding high alloy steel and non-ferrous metals, especially stainless steel, as it will cause carbonization of the weld and reduce the anti. -resistance to interstitial corrosion, it is used less. Argon is an inert gas. Because it does not react chemically with the molten metal, the chemical composition of the weld is basically unchanged. The quality of the weld after welding is good. It can be used to weld various alloy steels, stainless steels and. non-ferrous metals. Because the price of argon is gradually decreasing, so it is also used in large quantities for welding mild steel.
28. Describe the weldability and welding characteristics of 16Mn steel?
Answer: 16Mn steel is based on Q235A steel with about 1% Mn added, and the carbon equivalent is 0.345% to 0.491%. Welding performance is therefore better. However, the hardening tendency is slightly higher than that of Q235A steel. When welding with small parameters and small weld passes on a large thickness and a large rigid structure, cracks may appear, especially when welding at low temperatures. In this case, appropriate measures can be taken. can be taken before preheating the floor.
When manual arc welding, E50 quality electrodes are used; When automatic submerged arc welding does not require chamfering, H08MnA welding wire and 431 flux can be used when chamfering, H10Mn2 welding wire and 431 flux should be used when CO2 gas shielded welding ; H08Mn2SiA welding wire is used or H10MnSi.
Daguang focuses on providing solutions such as precision CNC machining services (3-axis, 4-axis, 5-axis machining), CNC milling, 3D printing and rapid prototyping services.
