1. What are the characteristics of the primary crystalline structure of solder?
To respond:
Solidification of a weld molten pool also follows the fundamental principles of general liquid metal solidification, which involves the formation of crystalline nuclei and their growth.
As the liquid metal in the weld pool cools and solidifies, partially molten grains in the base metal in the fusion zone typically serve as sites for crystal nucleation.
These crystalline nuclei subsequently attract and absorb atoms from the surrounding liquid, leading to crystal growth.
Because the crystals grow in the opposite direction of heat conduction and in two opposite directions but are obstructed by neighboring growing crystals, the resulting crystals take on a columnar shape and are called columnar crystals.
Under specific circumstances, the liquid metal in the molten pool may also undergo spontaneous nucleation during solidification.
If heat dissipation occurs in all directions, the crystals will grow uniformly in all directions, forming equiaxed crystals.
Although columnar crystals are typically observed in the weld seam, equiaxed crystals can also appear in the center of the weld under certain conditions.
2. What are the characteristics of the secondary crystallization structure of solder?
To respond:
After primary crystallization, the weld metal continues to cool below the phase transformation temperature, causing a change in its metallographic structure.
For example, when welding low carbon steel, the primary crystalline grains are austenitic. As the temperature drops below the transformation point, austenite decomposes into ferrite and pearlite. Consequently, the structure after secondary crystallization is mainly composed of ferrite with a small amount of pearlite.
However, due to the rapid cooling rate of the weld, the pearlite content obtained is generally higher than that found in the equilibrium structure. The higher the cooling speed, the more significant the perlite content will be.
The reduced ferrite content results in increased hardness and strength, but a decrease in plasticity and toughness. The actual structure at room temperature is obtained after secondary crystallization.
Different welding conditions and steel types can produce varying weld microstructures.
3. Let us take low carbon steel as an example to explain which structure is obtained after secondary crystallization of the weld metal?
To respond:
Take low plastic steel as an example, which has an austenitic primary crystalline structure.
The process of transforming the solid phase into the weld metal is known as secondary crystallization of the weld metal, which results in a microstructure of ferrite and pearlite.
In the equilibrium structure of low carbon steel, the carbon content in the weld metal is very low, resulting in a coarse columnar structure of ferrite with a small amount of pearlite.
However, due to the high cooling rate during welding, ferrite cannot completely precipitate according to the iron-carbon phase diagram, resulting in a higher pearlite content than in the flat structure.
The cooling rate during welding also determines the grain size, hardness and strength of the metal. Finer grains are obtained with higher cooling rates, resulting in greater hardness and strength, but decreasing ferrite and increasing pearlite content can lead to reduced plasticity.
Therefore, the final microstructure of the weld is determined by the metal composition and cooling conditions during welding.
Due to the nature of the welding process, the structure of the weld metal is fine, resulting in better microstructures and properties than in the as-cast state.
4. What are the characteristics of welding dissimilar metals?
To respond:
1）The characteristics of dissimilar metal welding are mainly defined by the significant differences in the alloy composition of the deposited metal and the weld. The behavior of the welding puddle varies depending on the shape of the weld, the thickness of the base metal, the electrode coating or flux, and the type of shielding gas used.
As a result, the melting amount of the base metal is different and the mutual dilution of the concentration of chemical components in the melting area of the deposited metal and the base metal is also affected. Therefore, the degree of non-uniformity of welded joints of dissimilar metals with respect to the regional chemical composition depends not only on the original composition of the weldments and filler materials, but also on the welding process used.
2）After the thermal welding cycle, different metallographic structures will appear in each area of the welded joint due to the inhomogeneity of the structure. This 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, resulting from the different chemical compositions and metallic structure of the joint, leads to significant differences in the mechanical properties of the joint.
The strength, hardness, plasticity and toughness of each area along the joint can be very different. In the heat-affected zone on both sides of the weld, the impact value may differ several times. Creep limit and high temperature resistance can also vary greatly due to differences in composition and structure.
4）The non-uniformity of stress field distribution and residual stress distribution in different metal joint is mainly determined by the different plasticity of each area of the joint.
Furthermore, the difference in the thermal conductivity of the material can cause a change in the temperature field of the welding thermal cycle. The different coefficients of linear expansion in each region and other factors are reasons for the uneven distribution of the stress field.
5. What are the principles of selecting welding materials for welding different steels?
To respond:
The selection principles for different steel welding materials mainly include the following four points:
- When the strength and plasticity of the weld metal are not critical, select welding materials with good plasticity, ensuring that the welded joint does not develop cracks or other defects.
- The weld metal properties of different steel welding materials must meet the technical requirements of at least one of the two base metals.
- Welding materials must have good process performance and produce visually attractive welds. Welding materials must be affordable and readily available.
Related reading: Weldability of metallic materials
6. What about the weldability of pearlitic steel and austenitic steel?
To respond:
Pearlitic steel and austenitic steel are two distinct types of steel with different structures and components. When welding these two types of steel, the weld metal is created by fusing two different types of base metals and filler materials, which can lead to weldability challenges.
1) Dilution of welds.
Pearlitic steel contains low-alloy elements, which can dilute the overall alloy of the weld metal.
The dilution effect of pearlitic steel reduces the content of austenite-forming elements in the weld.
As a consequence, the weld can develop a martensitic structure, which can negatively impact the quality of the joint and even cause cracking.
2) Formation of transition layers.
During the welding thermal cycle, the degree of mixing between the molten base metal and the filler metal varies at the edge of the molten pool.
In this location, the liquid metal is characterized by low temperature, low fluidity and short residence time in the liquid state.
Due to significant differences in chemical composition between pearlitic steel and austenitic steel, the molten base metal and filler metal cannot be adequately melted at the edge of the molten pool on the pearlitic side.
Consequently, the welds on the pearlitic steel side contain a significant proportion of pearlitic base metal, with the proportion increasing near the fusion line.
This creates a transition layer with different internal weld metal components.
3) A diffusion layer is formed in the melting zone.
In the weld metal composed of these two types of steels, pearlitic steel has a higher carbon content, but a lower content of alloy elements than austenitic steel.
On the other hand, in the melting zone, the concentration difference of carbon and carbide forming elements is formed on both sides of pearlitic steel in case of austenitic steel.
When the joint works at a temperature higher than 350-400 ℃ for a long period, the fusion zone will exhibit obvious carbon diffusion, that is, diffusion from the pearlitic steel side to the austenitic weld through the fusion zone.
As a result, a softened decarburized layer is formed on the base metal of the pearlitic steel near the fusion zone, and a decarburized layer corresponding to decarburization is formed on one side of the austenitic weld.
4) Because the physical properties of pearlitic steel and austenitic steel are very different, the weld composition is also very different.
This type of joint cannot be heat treated to eliminate welding stress. Heat treatment can only cause stress redistribution, which is very different from welding the same metal.
5) Delayed cracking.
During the crystallization process, the molten pool created by welding different steels contains austenite and ferrite structures that are closely related to each other.
Because gas can easily diffuse in this process, diffusible hydrogen can accumulate and lead to delayed cracking.
7. What are the measures to prevent cracks during cast iron repair welding?
To respond:
(1) Preheating before welding and slow cooling after welding
Preheating the weldment as a whole or partially before welding, and slowly cooling it after welding, can reduce the porosity tendency of the weldment and minimize welding stress, thus preventing the weldment from cracking.
(2) Cold arc welding is adopted to reduce welding stress.
To prevent cracking, welding materials with good plasticity such as nickel, copper, nickel-copper and high vanadium steel are selected as filler metals. This allows the weld metal to relieve stress through plastic deformation.
Welding stress reduction can be achieved by using thin diameter electrodes, small current, intermittent welding, and scatter welding techniques to decrease the temperature difference between the weld and the base metal. Additionally, hammering the weld can help eliminate stress and prevent cracking.
(3) Other measures: adjust the chemical composition of the weld metal to narrow its brittle temperature range;
To enhance the metallurgical reaction of desulfurization and dephosphorization in the weld, rare earth elements must be added. Furthermore, adding Zengna refined grain elements will refine the solder grains.
In some cases, the heating stress zone method is used to reduce stress in the area where the weld is being repaired. This method is also effective in preventing cracks from occurring.
Related reading: How to weld cast iron?
8. What is stress concentration? What are the factors that cause stress concentration?
To respond:
Due to the unknown characteristics of the weld bead shape and the weld bead itself, a discontinuity in the collective shape occurs, which leads to stress concentration when loaded. This stress concentration causes an uneven distribution of the working stress of the welded joint, resulting in a local stress peak σmax that is much higher than the average stress σm.
There are many reasons for stress concentration in welding, with process defects being a significant contributor. Air ingress, slag inclusions, cracks and incomplete penetration into the weld can cause stress concentration, with welding cracks and incomplete penetration being the most severe.
Other factors that contribute to stress concentration include unreasonable weld appearance, such as excessive reinforcement of butt welds and high ends of fillet welds, and unreasonable street design, such as sudden changes in street interfaces or the use of gasket joints. top with cover plates.
Unreasonable solder joint arrangements can also cause stress concentration. For example, T-joints with only welds can lead to stress concentration.
9. What is plastic failure and what are its dangers?
To respond:
Plastic failure can result in plastic instability (significant plastic yield or deformation) or plastic fracture (edge fracture or ductile fracture).
The process begins with elastic deformation of the welded structure under load, followed by yielding, plastic deformation (plastic instability), microcracks or voids, macrocracks, unstable growth, and finally fracture.
Compared to brittle fracture, plastic fracture is less likely to occur in cold environments for the following reasons:
(1) Unrecoverable plastic deformation occurs after yielding, rendering welded structures with high size requirements unusable.
(2) For pressure vessels made of high toughness and low strength materials, failure is not controlled by the fracture toughness of materials, but is caused by plastic instability due to insufficient strength.
Plastic damage can invalidate the welded structure, leading to catastrophic accidents that affect business production, result in unnecessary victims and seriously harm national economic development.
10. What is a brittle fracture and what are its harms?
To respond:
Brittle fracture generally refers to dissociation fracture (including quasi-dissociation fracture) and grain boundary (intergranular) fracture that splits along a certain crystal plane.
Cleavage fracture, on the other hand, is a type of intragranular fracture that occurs when materials separate along a specific crystallographic plane within the crystal.
Under certain conditions, such as low temperature, high strain rate and high stress concentration, metallic materials may undergo cleavage fracture when the stress reaches a certain value.
There are several models that explain the generation of cleavage fractures, many of which are related to the theory of dislocations.
It is generally believed that when the plastic deformation process of materials is severely impaired, the materials cannot conform to external stresses through deformation and instead undergo separation, leading to cleavage cracks.
Inclusions, brittle precipitates and other defects in metals also play an important role in generating cleavage cracks.
Brittle fracture typically occurs when the stress is not greater than the allowable design stress of the structure and there is no significant plastic deformation. It can quickly spread throughout a structure, causing sudden damage that is difficult to detect and prevent in advance, often resulting in personal injury and property loss.
11. What is the role of welding cracks in structural brittle fracture?
To respond:
Of all the defects, cracks are the most dangerous. When subjected to external loads, a small plastic deformation occurs near the front of the crack, and a certain amount of opening displacement occurs at the tip, causing the crack to develop gradually.
If the external load increases to a critical level, the crack will expand at high speed. At this point, if the crack is located in a zone of high tensile stress, it often results in brittle fracture of the entire structure.
However, if the extended crack enters an area with low tensile stress, there will be sufficient energy to maintain further expansion of the crack, or the crack enters a material with better toughness (or the same material with higher temperature and greater toughness). ) where it receives greater resistance.
If the crack cannot continue to expand, the damage caused by the crack will be reduced accordingly.
12. What are the causes of brittle fractures of welded structures?
To respond:
The causes of fracture can be summarized in three aspects:
(1) The humanity of materials is insufficient
The microelastic deformation capacity of the material is particularly weak, especially at the notch tip.
Brittle failure under low stress generally occurs at lower temperatures, and the toughness of materials decreases drastically as the temperature decreases.
Furthermore, with the advancement of low-alloy and high-strength steels, the strength index is increasing, while plasticity and toughness are decreasing.
In many cases, brittle fracture originates in the weld zone, making lack of toughness in the weld and heat-affected zone the main cause of brittle failure under low stress.
13. What are the main factors that must be considered when designing welded structures?
To respond :
The main considerations for welded joints are as follows:
- The welded joint must have sufficient tension and rigidity to ensure a long service life.
- The working environment and conditions of the welded joints must be considered, such as temperature, corrosion, vibration and fatigue.
- The workload of pre-welding preheating and post-welding heat treatment should be minimized for large structural members.
- Welding should require little or no machining.
- Welding work should be minimized.
- Deformation and stress of welded structures must be kept to a minimum.
- The welded structure must be easy to construct and create good working conditions for construction.
- New technologies and mechanized and automatic welding must be adopted to improve labor productivity.
- The weld must be easily inspected to ensure the quality of the joint.
14. Describe the basic conditions of gas cutting. Can red copper be cut with oxygen and acetylene flame? Why?
To respond:
The basic requirements for gas cutting are as follows:
(1) The ignition point of the metal must be lower than its melting point.
(2) The melting point of the metal oxide must be lower than that of the metal itself.
(3) Burning metal in oxygen must produce a substantial amount of heat.
(4) The thermal conductivity of the metal must be low.
Red copper cannot be cut with oxygen-acetylene flame because the amount of heat generated by copper oxide (CuO) is too small and the thermal conductivity of copper is too high. As a result, heat cannot be concentrated near the incision, making gas cutting impossible.
15. What is the main function of gas welding powder?
To respond:
The main purpose of welding powder is to generate slag through reaction with metallic oxides or non-metallic impurities present in the molten pool, thus facilitating the slagging process.
Simultaneously, the generated slag covers the surface of the molten pool and acts as a barrier, isolating the molten pool from the surrounding air. This insulation prevents the metal present in the weld pool from continually oxidizing at high temperatures.
16. What are the process measures to prevent weld porosity in manual arc welding?
To respond:
(1) Welding rods and fluxes should be stored in a dry environment and dried before use if necessary.
(2) The welding wire and welded surfaces must be kept clean, free from water, oil, rust or any other contaminants.
(3) The welding specification must be selected accurately, taking into account factors such as appropriate welding current and welding speed.
(4) The correct welding method must be employed, including using alkaline electrodes for manual arc welding and short arc welding, reducing the electrode oscillation range, slowing down the electrode movement speed, and controlling the starting and short arc stop.
(5) The assembly gap of control welding should not be too large.
(6) Welding rods with cracked coatings, peeling, deterioration, eccentricity and corroded cores should not be used.
17. What are the main measures to avoid cold when welding cast iron?
To respond:
(1) The use of graphitized electrodes is highly recommended. These electrodes are manufactured from cast iron with a high concentration of graphitized elements (such as carbon, silicon, etc.) added to the coating or welding wire. Alternatively, nickel or copper based cast iron electrodes can also be used.
(2) Before welding, preheating is required to prepare the materials. During welding, it is important to maintain heat preservation, and after welding, slow cooling is recommended to reduce the cooling rate of the weld zone. Doing so prolongs the time that the fusion zone is in the incandescent state, making graphitization sufficient and reducing thermal stress.
(3) Consider using a brazing process for optimal results.
18. Try to describe the role of flux in the soldering process?
To respond:
Flux plays a crucial role in ensuring welding quality. It serves the following functions:
- 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.
- The welding flux assists in deoxidation and alloying and, in conjunction with the welding wire, ensures that the weld metal reaches the required chemical composition and mechanical properties.
- Ensures the weld has a well-formed appearance.
- Slows down the cooling rate of molten metal, thus reducing defects such as porosity and slag inclusion.
- Prevents splashes, reduces losses and increases the connection coefficient.
19. What should be paid attention to in the use and maintenance of AC arc welding machine?
To respond:
(1) The welding machine must be operated in accordance with its rated welding current and charging duration and must not be overloaded.
(2) Prolonged short circuit of the welding machine should be avoided.
(3) The regulating current must be operated without any load.
(4) Regularly inspect the wire contact, fuse, grounding and regulation mechanism and make sure they are in good condition.
(5) Keep the welding machine clean, dry and well ventilated to prevent dust and rain from entering.
(6) Place the machine in a stable position and turn off the power supply after use.
(7) It is necessary to carry out regular maintenance and inspection of the welding machine.
20. What are the dangers of brittle fracture?
To respond:
Brittle fracture is a sudden phenomenon that cannot be detected and prevented in time. When it occurs, the consequences can be serious, leading to significant economic losses and endangering human safety.
As a result, the issue of brittle fracture in welded structures must receive greater attention.
21. Features and application of plasma spraying?
To respond:
Plasma spraying is characterized by a high plasma flame temperature that can melt almost all refractory materials, making it suitable for a wide range of spraying applications. It also features high plasma flame flow speed, excellent powder particle acceleration effect and superior coating bonding strength.
Due to its versatility, plasma spraying is ideal for various ceramic materials and has a wide range of applications, making it the best method for spraying ceramic materials.
22. Welding procedure card preparation procedure?
To respond:
To prepare the welding procedure card, the corresponding welding procedure qualification should be identified, and a joint sketch should be drawn up based on the product assembly drawing, parts processing drawing and its technical requirements.
The welding procedure card must include the welding procedure qualification number, welding procedure card number, drawing number, joint name, joint number and welder certificate items.
The welding sequence should be prepared based on the qualification of the welding procedure, actual production conditions, technical personnel and production experience.
Specific welding process parameters should also be included based on the welding process qualification.
The inspection authority, inspection method and inspection proportion of products should be determined according to the requirements of product drawings and standards.
23. Why should a certain amount of silicon and manganese be added to the CO2 gas shielded welding wire?
To respond :
Carbon dioxide is an oxidizing gas that can burn the alloy elements of a welding seam during the welding process, significantly reducing the mechanical properties of the weld. This oxidation can result in the formation of pores and splashes.
To solve these problems, silicon and manganese can be added to the welding wire to play a deoxidizing role, preventing oxidation and welding spatter.
24. What is the explosion limit of the combustible mixture and what factors affect it?
To respond:
The concentration range of combustible gas, vapor or dust present in a combustible mixture that can cause an explosion is called the explosion limit.
The lower concentration limit is known as the lower explosive limit, and the upper limit is known as the upper explosive limit.
Various factors such as temperature, pressure, oxygen content, container diameter and others can influence the explosion limit. An increase in temperature results in a decrease in the explosion limit, and the same occurs when there is an increase in pressure.
Furthermore, an increase in the oxygen concentration in the gas mixture causes a decrease in the lower explosive limit.
For combustible dust, factors such as dispersion, humidity and temperature can also affect its explosive limit.
25. What measures should be taken to avoid electric shocks when welding on a boiler drum, condenser, oil tank, oil tank and other metal containers?
To respond:
(1) Welders must avoid contact with iron parts during electric welding. They should stand on rubber isolation pads or wear rubber isolation shoes and dry work clothes.
(2) There must be a supervisor outside the vessel who can observe and listen to the welder's work. A switch must be installed to cut off power based on the signal from the welder.
(3) The voltage of portable lamps used in containers should not exceed 12V. The portable lamp transformer housing must be reliably grounded, and the use of an autotransformer is prohibited.
(4) Portable lamp transformers and welding transformers should not be placed in boilers and metal containers.
26. How to distinguish fusion welding from brazing? What are the characteristics of each one?
To respond:
Fusion welding involves bonding atoms between weldments, while brazing connects weldments using filler metal, an intermediate medium with a lower melting point than welding.
Fusion welding offers several advantages, such as high mechanical properties of welded joints and high productivity when joining thick and large parts. However, it also has some disadvantages, such as large stresses and deformations and microstructural changes in the heat-affected zone.
Brazing, on the other hand, has advantages such as low heating temperature, flat and smooth joints, and beautiful appearance. It also results in small stresses and deformations. However, its disadvantages include low joint strength and high assembly clearance requirements during assembly.
27. Both carbon dioxide and argon belong to protective gases. What are its properties and uses?
To respond:
Carbon dioxide is an oxidizing gas. When used as a shielding gas in welding, it can cause severe oxidation of molten droplets and puddle metal, resulting in burning loss of alloying elements. Furthermore, it has low processability and can cause pores and large splashes.
Therefore, it is currently only suitable for welding low-carbon steels and low-alloy steels, and is not recommended for high-alloy steels and non-ferrous metals. Especially when welding stainless steel, it can cause carburization of the weld and reduce resistance to intergranular corrosion, making it less widely used.
Argon, on the other hand, is an inert gas that does not chemically react with the molten metal, resulting in minimal changes in the chemical composition of the weld seam. Welded seams produced with argon have good quality and can be used on various alloy steels, stainless steels and non-ferrous metals.
As the price of argon gradually decreases, it is becoming a popular option for welding a large number of low-carbon steels.
28. Try to describe the weldability and welding characteristics of 16Mn steel?
To respond:
16Mn steel contains approximately 1% Mn in addition to Q235A steel, resulting in a carbon equivalent of 0.345%~0.491%. As a result, the steel has good welding performance. However, the hardening tendency of 16Mn steel is slightly higher than that of Q235A steel, therefore, when welding thick and rigid structures, lower parameters must be used to avoid cracking, especially at low temperatures. In such cases, adequate preheating can be applied before welding.
For manual arc welding, the use of E50 welding rods is recommended. In cases where the groove cannot be opened for automatic submerged arc welding, H08MnA welding wire with flux 431 can be used. When beveling, H10Mn2 welding wire with flux 431 must be used. During welding with gas protection CO2, H08Mn2SiA or H10MnSi welding wire must be used.
