1. What are the characteristics of the primary crystalline structure of the weld seam?
The solidification of a welding molten pool also follows the fundamental principles of general liquid metal solidification, which involve the formation of crystal nuclei and their growth.
As the liquid metal in the welding pool cools and solidifies, the partially melted grains on the base metal in the fusion zone typically serve as the sites for crystal nucleation.
These crystal nuclei subsequently attract and absorb the atoms of the surrounding liquid, leading to crystal growth.
Since crystals grow in the opposite direction of heat conduction and in two opposing directions, but are obstructed by neighboring growing crystals, the resulting crystals take on a columnar shape and are referred to as columnar crystals.
Under specific circumstances, the liquid metal in the molten pool can also undergo spontaneous nucleation during solidification.
If heat dissipation occurs in all directions, crystals will grow uniformly in all directions, forming equiaxed crystals.
While columnar crystals are typically observed in the weld seam, equiaxed crystals may also appear in the center of the weld under certain conditions.
2. What are the characteristics of the secondary crystallization structure of the weld?
After primary crystallization, the weld metal continues to cool below the phase transformation temperature, causing a change in its metallographic structure.
For instance, 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 mostly composed of ferrite with a small amount of pearlite.
However, due to the rapid cooling rate of the weld, the obtained pearlite content is generally higher than that found in the equilibrium structure. The higher the cooling speed, the more significant the pearlite content.
Reduced ferrite content results in higher 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 types of steel can produce varying weld microstructures.
3. Take low-carbon steel as an example to explain what structure is obtained after the secondary crystallization of weld metal?
Let’s take low plastic steel as an example, which has an austenitic primary crystalline structure.
The process of solid phase transformation in the weld metal is known as the secondary crystallization of 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 ferrite structure with a small amount of pearlite.
However, due to the high cooling rate during welding, ferrite cannot precipitate completely according to the iron-carbon phase diagram, resulting in a higher content of pearlite 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 increased hardness and strength, but decreased ferrite and increased pearlite content can lead to reduced plasticity.
Therefore, the final microstructure of the weld is determined by the composition of the metal and the cooling conditions during welding.
Due to the nature of the welding process, the weld metal structure is fine, resulting in better microstructure and properties than the casting state.
4. What are the characteristics of dissimilar metal welding?
1）The characteristics of dissimilar metal welding are primarily defined by the significant differences in the alloy composition of the deposited metal and the weld. The behavior of the welding pool varies depending on the shape of the weld, the thickness of the base metal, the coating or flux of the electrode, 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 dissimilar metal welding joints with respect to 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 welding thermal 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 composition and metal 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 vastly different. In the heat-affected zone on both sides of the weld, the impact value can differ by several times. The creep limit and endurance strength under high temperature can also vary greatly due to differences in composition and structure.
4）The non-uniformity of the stress field distribution and the residual stress distribution in the dissimilar metal joint is mainly determined by the different plasticity of each area of the joint.
In addition, the difference in material thermal conductivity can cause a change in the welding thermal cycle temperature field. The different coefficient of linear expansion in each region and other factors are reasons for the uneven distribution of the stress field.
5. What are the selection principles of welding materials for dissimilar steel welding?
The selection principles for dissimilar 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 dissimilar steel welding materials should meet the technical requirements of at least one of the two base metals.
- The welding materials should have good process performance and produce visually appealing welds.The welding materials should be affordable and readily available.
Related reading: Weldability of Metal Materials
6. How about the weldability of pearlitic steel and austenitic steel?
Pearlitic steel and austenitic steel are two distinct types of steel with different structures and components. When welding these two types of steel together, the weld metal is created by fusing two different types of base metals and filler materials, which can lead to challenges with weldability.
1) Dilution of welds.
Pearlitic steel contains low-alloy elements, which can dilute the overall weld metal alloy.
The dilution effect of pearlitic steel reduces the content of austenite-forming elements in the weld.
As a consequence, the weld may develop a martensitic structure, which can negatively impact the quality of the joint and even lead to cracks.
2) Formation of transition layers.
During the welding thermal cycle, the degree of mixing between the melted base metal and filler metal varies at the edge of the molten pool.
At this location, the liquid metal is characterized by low temperature, poor fluidity, and a short residence time in the liquid state.
Due to the significant differences in chemical composition between pearlitic steel and austenitic steel, the molten base metal and filler metal cannot be adequately fused at the edge of the molten pool on the pearlitic side.
Consequently, welds on the pearlitic steel side contain a significant proportion of pearlitic base metal, with the proportion increasing closer to the fusion line.
This creates 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, the pearlitic steel has a higher carbon content, but lower alloy elements than the austenitic steel.
Conversely, in the fusion zone, the concentration difference of carbon and carbide-forming elements is formed on both sides of the pearlitic steel side in the case of austenitic steel.
When the joint works at a temperature higher than 350-400 ℃ for a long time, the fusion zone will exhibit obvious carbon diffusion, i.e., diffusion from the pearlitic steel side to the austenitic weld through the fusion zone.
As a result, a decarburized and softened layer is formed on the pearlitic steel base metal 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 composition of the weld 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 process of crystallization, the molten pool created by welding dissimilar steel contains both austenite and ferrite structures that are closely related to each other.
Since gas can diffuse easily in this process, diffusible hydrogen may accumulate and lead to delayed cracking.
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 it after welding, can reduce the tendency of weld porosity and minimize welding stress, thereby preventing the weldment from cracking.
(2) Arc cold welding is adopted to reduce welding stress.
To prevent cracks, 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.
Reducing welding stress can be achieved by using fine diameter electrodes, small current, intermittent welding, and scattered welding techniques to decrease the temperature difference between the weld and base metal. Additionally, hammering the weld can help eliminate stress and prevent cracks.
(3) Other measures: adjust the chemical composition of weld metal to narrow its brittle temperature range;
To enhance the metallurgical reaction of desulfurization and dephosphorization in the weld, rare earth elements should be added. Additionally, adding Zengna’s refining grain elements will refine the weld 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 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 the weld seam itself, there is a discontinuity in the collective shape, 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 peak stress 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 inlets, slag inclusions, cracks, and incomplete penetration in 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 weld toes of fillet welds, and unreasonable street design, such as sudden changes in street interfaces or the use of butt joint street with cover plates.
Unreasonable weld joint arrangements can also cause stress concentration. For example, T-joints with only shop welds can lead to stress concentration.
9. What is plastic failure and what are its hazards?
Plastic failure can result in plastic instability (yielding or significant plastic 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, macro cracks, unstable growth, and ultimately fracture.
In comparison 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, low-strength materials, failure is not controlled by the fracture toughness of the materials, but is caused by plastic instability due to insufficient strength.
Plastic damage can render the welded structure invalid, leading to catastrophic accidents that impact enterprise production, result in unnecessary casualties, and seriously hinder national economic development.
10. What is brittle fracture and what is its harm?
Brittle fracture generally refers to dissociation fracture (including quasi-dissociation fracture) and grain boundary (intergranular) fracture that split 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, metal materials can experience cleavage fracture when the stress reaches a certain value.
There are several models that explain 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 severely hindered, the materials cannot conform to external stress through deformation and instead undergo separation, leading to cleavage cracks.
Inclusions, brittle precipitates, and other defects in metals also play an important role in the generation of cleavage cracks.
Brittle fracture typically occurs when the stress is not higher than the design allowable stress of the structure and there is no significant plastic deformation. It can quickly propagate throughout the structure, causing sudden damage that is difficult to detect and prevent in advance, often resulting in personal injury and property loss.
11. What role does welding crack play in structural brittle fracture?
Out of all defects, cracks are the most hazardous. When subjected to external loads, a small amount of plastic deformation occurs near the crack front, and a certain amount of opening displacement happens at the tip, causing the crack to develop gradually.
If the external load increases to a critical level, the crack will expand at a high velocity. At this point, if the crack is situated in a high-value tensile stress zone, it often results in the brittle fracture of the entire structure.
However, if the extended crack enters an area with low tensile stress, there will be 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) where it receives greater resistance.
If the crack cannot continue to expand, the damage caused by the crack will reduce 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
The material’s micro-elastic deformation ability is particularly poor, especially at the tip of the notch.
Brittle failure under low stress usually occurs at lower temperatures, and materials’ toughness sharply decreases as the temperature decreases.
Furthermore, with the advancement of low-alloy, high-strength steel, the strength index is increasing, while plasticity and toughness are decreasing.
In many cases, brittle fracture originates from the welding zone, making the lack of toughness in the weld and heat-affected zone the primary cause of brittle failure under low stress.
13. What main factors should be considered when designing welded structures?
The main considerations for welded joints are as follows:
- The welded joint should possess sufficient stress and rigidity to ensure a long service life.
- The working medium and conditions of welded joints should 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.
- The weldment should require little or no machining.
- Welding work should be minimized.
- The deformation and stress of welded structures should be reduced to a minimum.
- The welded structure should be easy to construct and create good working conditions for construction.
- New technology and mechanized and automatic welding should be adopted to improve labor productivity.
- The weld should be easily inspected 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 requirements for gas cutting are as follows:
(1) The ignition point of the metal should be lower than its melting point.
(2) The melting point of the metal oxide should be lower than that of the metal itself.
(3) The burning of metal in oxygen should produce a substantial amount of heat.
(4) The thermal conductivity of the metal should be low.
Red copper cannot be cut using an oxygen-acetylene flame because the amount of heat generated by copper oxide (CuO) is very small, and the thermal conductivity of copper is very high. As a result, the heat cannot be concentrated near the incision, making it impossible to cut with gas.
15. What is the main function of gas welding powder?
The primary purpose of welding powder is to generate slag by reacting with metal oxides or non-metallic impurities present in the molten pool, thereby facilitating the process of slagging.
Simultaneously, the generated slag covers the surface of the molten pool and acts as a barrier, insulating the molten pool from the surrounding air. This insulation prevents the metal present in the molten pool from continuously oxidizing at high temperatures.
16. What are the process measures to prevent weld porosity in manual arc welding?
(1) Welding rods and fluxes must be stored in a dry environment and dried before use if necessary.
(2) The welding wire and weldment’s surfaces must be kept clean, free of water, oil, rust, or any other contaminants.
(3) The welding specification must be accurately selected, taking into account factors such as appropriate welding current and welding speed.
(4) The correct welding method must be employed, including the use of alkaline electrodes for manual arc welding and short arc welding, reducing the electrode swing range, slowing down the electrode’s moving speed, and controlling short arc starting and stopping.
(5) The assembly clearance of the control weldment must 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 prevent chill during cast iron welding?
(1) The use of graphitized electrodes is highly recommended. These electrodes are made of 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) Prior to welding, preheating is necessary 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. By doing so, it extends the time when the fusion zone is in the red hot state, making the graphitization sufficient and reducing thermal stress.
(3) Consider using a brazing process to achieve optimal results.
18. Try to describe the role of flux in the welding process?
Flux plays a crucial role in ensuring the welding quality. It serves the following functions:
- Upon 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 aids in deoxidation and alloying, and in conjunction with the welding wire, ensures the weld metal attains the necessary chemical composition and mechanical properties.
- Ensures the weld has a well-formed appearance.
- Slows down the cooling rate of the molten metal, thereby reducing defects like porosity and slag inclusion.
- Prevents splashing, reduces loss, and enhances binding coefficient.
19. What should be paid attention to in the use and maintenance of AC arc welding machine?
(1) The welding machine should be operated according to its rated welding current and load duration, and must not be overloaded.
(2) Prolonged short-circuiting of the welding machine must be avoided.
(3) The regulating current should be operated without any load.
(4) Regularly inspect the wire contact, fuse, grounding, regulating mechanism, and ensure they are in good condition.
(5) Keep the welding machine clean, dry and well-ventilated to prevent the intrusion of dust and rain.
(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 hazards of brittle fracture?
Brittle fracture is a sudden phenomenon that cannot be detected and prevented in time. Once it occurs, the consequences can be severe, leading to significant economic losses and endangering human safety.
As a result, the issue of brittle fracture in welded structures should be given greater attention.
21. Characteristics and application of plasma spraying?
Plasma spraying is characterized by a high plasma flame temperature that can melt nearly all refractory materials, making it suitable for a wide range of spraying applications. It also boasts 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 ceramic material spraying.
22. Preparation procedure of welding procedure card?
To prepare the welding procedure card, the corresponding welding procedure qualification must be identified, and a joint sketch should be drawn based on the product assembly drawing, parts processing drawing, and its technical requirements.
The welding procedure card should 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 welding procedure qualification, 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 in accordance with 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 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 address these issues, silicon and manganese can be added to the welding wire to play a deoxidizing role, preventing welding oxidation and splashes.
24. What is the explosion limit of combustible mixture and what factors affect it?
The range of concentration of combustible gas, vapor, or dust present in a combustible mixture that can cause an explosion is referred to as the explosion limit.
The lower limit of concentration is known as the lower explosion limit, and the upper limit is known as the upper explosion limit.
Several 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 concentration of oxygen in the mixed gas causes a decrease in the lower explosion limit.
For combustible dust, factors like dispersion, humidity, and temperature can also affect its explosion limit.
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 must avoid contact with iron parts during electric welding. They should stand on rubber insulation pads or wear rubber insulation shoes and dry work clothes.
(2) There should be a supervisor outside the vessel who can observe and listen to the welder’s work. A switch should be installed to cut off the power supply based on the welder’s signal.
(3) The voltage of portable lamps used in containers should not exceed 12V. The shell of the portable lamp transformer should be grounded reliably, and the use of an auto transformer is prohibited.
(4) Transformers for portable lamps and welding transformers should not be brought into boilers and metal containers.
26. How to distinguish fusion welding from brazing? What are the characteristics of each?
Fusion welding involves the bonding of atoms between weldments, whereas brazing connects weldments using filler metal, an intermediate medium with a lower melting point than the weldment.
Fusion welding offers several advantages, such as high mechanical properties of welded joints and high productivity when connecting thick and large pieces. However, it also has some drawbacks, such as large stress and deformation and microstructure changes in the heat-affected zone.
Brazing, on the other hand, has advantages such as low heating temperature, flat and smooth joints, and a beautiful appearance. It also results in small stress and deformation. However, its disadvantages include 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 used as a shielding gas in welding, it can cause severe oxidation of molten droplets and pool metal, resulting in burning loss of alloy elements. Additionally, it has poor processability and can lead to pores and large splashes.
Therefore, it is currently only suitable for welding low carbon steel and low alloy steel, and not recommended for high alloy steel and non-ferrous metals. Especially when welding stainless steel, it can cause weld carburetion and reduce resistance to intergranular corrosion, making it less commonly used.
Argon, on the other hand, is an inert gas that does not react with molten metal in any chemical way, resulting in minimal changes to the chemical composition of the weld seam. Welded seams produced with argon have good quality and can be used for various alloy steels, stainless steels, and non-ferrous metals.
As the price of argon is gradually decreasing, 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?
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, so when welding thick and rigid structures, smaller parameters should be used to avoid cracks, especially at low temperatures. In such cases, proper preheating can be applied before welding.
For manual arc welding, it is recommended to use E50 welding rods. In cases where the groove cannot be opened for submerged arc automatic welding, H08MnA welding wire with flux 431 can be used. When beveling, H10Mn2 welding wire with flux 431 should be used. During CO2 gas-shielded welding, welding wire H08Mn2SiA or H10MnSi should be used.