1. What are the characteristics of the primary crystalline structure of the weld?
Answer: The crystallization of the welding pool also follows the basic law of general liquid metal crystallization, which involves the formation of a crystal nucleus and subsequent growth of the nucleus.
As the welding pool’s liquid metal solidifies, the semi-molten grains present in the base metal of the fusion zone typically transform into nuclei.
The crystal nucleus attracts atoms from the surrounding liquid and grows as a result. As the crystal grows in the opposite direction to heat conduction, it grows on both sides. However, when adjacent crystals are formed, it is blocked, resulting in a columnar shape crystal.
Furthermore, under specific conditions, self-generating nuclei are formed in the liquid metal during solidification in the molten pool. If heat dissipation occurs in all directions, crystals grow uniformly in all directions, forming grain-like crystals, known as equiaxed crystals.
Columnar crystals are usually observed in the weld joint. Under certain conditions, equiaxed crystals may also emerge at the center of the weld joint.
2. What are the characteristics of the secondary crystallization structure of the weld joint?
After the primary crystallization, the metal in a weld joint continues to cool below the phase transformation temperature, leading to changes in its metallographic structure.
For instance, in the case of low carbon steel welding, austenite grains are the primary crystallization grains.
As the metal cools down below the phase transformation temperature, austenite breaks down into ferrite and pearlite.
Thus, the structure after secondary crystallization mostly comprises ferrite with a small proportion of pearlite.
However, the welding process involves rapid cooling, resulting in a higher pearlite content than that in the equilibrium structure.
The cooling speed affects the amount of ferrite and pearlite, with higher cooling rates resulting in higher pearlite content, higher hardness, and strength, and lower plasticity and toughness.
The actual structure at room temperature is obtained after secondary crystallization.
Different welding process conditions result in different weld microstructures for various types of steel.
3. Take low carbon steel as an example to explain what structure is obtained after secondary crystallization of weld joint metal?
Take low plastic steel as an example, the primary crystallization structure is austenite, and the solid-state phase transformation process of the weld joint metal is called secondary crystallization.
The microstructure of secondary crystallization consists of ferrite and pearlite. In the equilibrium structure of low-carbon steel, the carbon content of the weld joint metal is very low, and its structure is coarse columnar ferrite plus a small amount of pearlite.
Due to the high cooling rate of the weld, ferrite cannot precipitate completely according to the iron-carbon phase diagram. As a result, the content of pearlite is generally higher, and the microstructure contains more of it.
A high cooling rate also refines the grain, and improves the hardness and strength of the metal. The decrease of ferrite and the increase of pearlite will increase the hardness and decrease the plasticity.
Therefore, the final microstructure of the weld joint is determined by the metal composition and cooling conditions. Due to the characteristics of the welding process, the microstructure of the weld joint metal is fine, resulting in better microstructure and properties than in the casting state.
4. What are the characteristics of dissimilar metal welding?
The characteristics of dissimilar metal welding are mainly attributed to the distinct differences in the alloy composition of the deposited metal and the weld joint. In addition, the behavior of the welding pool is inconsistent with the shape of the weld joint, the thickness of the base metal, the electrode coating or flux, and the type of shielding gas.
Consequently, the amount of base metal melted during the welding process is also different, and the mutual dilution effect of the concentration of the chemical composition in the melting area of the deposited metal and the base metal will vary accordingly.
It can be observed that the degree of non-uniformity of the chemical composition in each area of the dissimilar metal welded joint depends not only on the original composition of the weldment and filler material but also on the specific welding processes employed.
2）After a welding thermal cycle, different metallographic structures appear in each area of the welded joint, which are related to the chemical composition of the base metal and filler material, welding method, level, process, and heat treatment.
3）The non-uniformity of performance, due to the varying chemical composition and metal structure of the joint, results in different mechanical properties of the joint. There are significant differences in strength, hardness, plasticity, and toughness in each area of the joint. The impact value even differs by several times in the heat-affected zone on both sides of the weld joint. The creep limit and rupture strength at high temperatures also differ greatly due to different compositions and microstructures.
4）The distribution of residual stress in dissimilar metal joints is uneven, mainly determined by the different plasticity of each region of the joint. Additionally, the differences in material thermal conductivity cause changes in the welding thermal cycle temperature field. The uneven distribution of the stress field is due to the different coefficient of linear expansion in each region.
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:
1）When there are no cracks or defects in the welded joint, the welding material with good plasticity should be selected, even if the strength and plasticity of the weld joint metal are not a significant consideration.
2）The properties of the weld joint metal of dissimilar steel welding materials should meet at least one of the two base metals to meet technical requirements.
3）The welding materials should have excellent process performance and result in a visually appealing weld joint formation. The welding materials should also be cost-effective and readily available for purchase.
6. How is the weldability of pearlite steel and austenitic steel?
Pearlitic steel and austenitic steel are two distinct types of steel with varying structures and compositions. When these two steels are welded together, the resulting weld joint metal is formed by fusing two different base materials and filler materials. This presents challenges in terms of the weldability of these two steels.
1) Dilution of the weld joint.
The low gold content in pearlitic steel results in a dilution effect on the overall alloy of the weld joint metal. This dilution effect leads to a reduction in the content of austenite-forming elements in the weld joint. Consequently, the weld joint may exhibit a martensite structure, which can compromise the quality of the weld and even cause cracking.
2) Form transition layer.
During the welding thermal cycle, the degree of mixing between molten base metal and filler metal varies at the edge of the molten pool.
The liquid metal at the edge of the molten pool has a lower temperature, poor fluidity, and a shorter residence time in the liquid state.
Due to the chemical composition differences between pearlitic steel and austenitic steel, the molten base metal and filler metal cannot fuse well at the edge of the molten pool on the pearlitic side. As a result, the weld joint on one side of pearlitic steel will have a larger proportion of pearlitic base metal, and the proportion of base metal will increase as it gets closer to the fusion line.
This creates a transition layer with a different internal composition of weld joint metal.
3) A fusion zone diffusion layer is formed.
In the weld joint composed of two types of steel, the high carbon content of pearlitic steel and the presence of high and low alloy elements result in a concentration difference of carbon and carbide forming elements on one side and two sides of the pearlitic steel in the fusion zone, in contrast to austenitic steel.
Over time, when the joint is exposed to temperatures higher than 350-400℃, carbon diffusion occurs in the fusion zone, causing carbon to diffuse from the pearlitic steel side to the austenitic weld.
This results in the formation of a decarburization softening layer on the base metal of the pearlitic steel near the fusion zone, and a carburizing layer corresponding to decarburization on one side of the austenitic weld joint.
4）The physical properties of pearlitic steel and austenitic steel are vastly different, and the composition of the weld joint is also dissimilar. As a result, heat treatment cannot eliminate the welding stress in this type of joint, and can only cause stress redistribution. This differs significantly from welding the same metal.
5）Delayed cracking can occur during the crystallization process of the welding pool of dissimilar steel. This is because the pool consists of both austenitic and ferritic structures which are in close proximity. As a result, gas can diffuse, and hydrogen can accumulate, leading to delayed cracks.
What factors should be considered when selecting the repair welding method of cast iron?
Answer: when selecting the welding method of gray cast iron, the following factors must be considered:
The condition of the welded casting depends on various factors, including its chemical composition, microstructure, and mechanical properties. Additionally, the size, thickness, and structural complexity of the casting are also important considerations.
2) Defects of castings.
Before welding, it is important to have an understanding of the type of defects (such as cracks, lack of material, wear, air pockets, sand holes, insufficient pouring, etc.), their size, the stiffness of the position, and the cause of the defects.
3）After welding, it is important to consider the quality requirements of the welded joints, including their mechanical properties, machinability, and color. It is crucial to understand the necessary standards for the appearance and tightness of the weld joints.
4) Site equipment conditions and economy.
The primary objective of casting welding repair, while ensuring quality requirements after welding, is to utilize the simplest methods, most commonly used welding equipment and process equipment, and achieve the lowest cost while maximizing its economic benefits.
7. What are the measures to prevent cracks during repair welding of cast iron?
(1) Preheating before welding and slow cooling after welding.
Preheating the weldment as a whole or locally before welding, and slow cooling after welding, can reduce not only the tendency for white mouth in the weld joint but also welding stress, and prevent cracking of the weldment.
(2) Arc cold welding is adopted to reduce welding stress.
To prevent cracks in welded materials, it is important to select materials with good plasticity, such as nickel, copper, nickel-copper, and high vanadium steel. These materials allow the weld metal to relax stress through plastic deformation.
Using a fine diameter electrode, low current, and intermittent welding techniques can also help to prevent cracks. Dispersed welding (skip welding) can reduce the temperature difference between the weld joint and the base metal, further reducing welding stress.
In addition to these techniques, hammering the weld can help eliminate stress and prevent cracks.
(3) Other measures:
To reduce the brittleness temperature range of the weld joint metal, the chemical composition should be adjusted. This can be achieved by adding rare earth elements to enhance the desulfurization and dephosphorization metallurgical reactions of the weld. Additionally, powerful grain refining elements can be added to refine the weld grain.
In certain scenarios, the heating method is utilized to lessen the stress at the welding repair location. This approach can effectively prevent the occurrence of cracks.
8. What is stress concentration? What are the factors causing stress concentration?
Due to the shape and characteristics of the weld joint, discontinuities in the collective shape can appear, which, when loaded, cause uneven distribution of the working stress of the welded joint. This results in local peak stress σ Max, which is much higher than the specific average stress σ M, leading to stress concentration.
There are several reasons for stress concentration in welded joints. The main causes include:
(1) Process defects, such as air holes, slag inclusions, cracks, and incomplete penetration in the weld joint. Stress concentration caused by welding cracks and incomplete penetration is particularly serious.
(2) Unreasonable weld joint shape, such as excessive reinforcement of butt welds and excessive fillet weld toes.
Unreasonable street design, such as sudden changes in street interface or butt joints with cover plates, can also cause stress concentration.
Unreasonable weld arrangement, such as T-joints with only store welds, may also produce stress concentration.
9. What is plastic failure and what is its harm?
Plastic failure can be categorized into two types: plastic instability (yield or significant plastic deformation) and plastic fracture (edge fracture or ductile fracture).
The process starts with the welded structure undergoing elastic deformation under load, followed by yield, plastic deformation (plastic instability), micro crack or micro void formation, macro crack development, instability propagation, and finally, fracture.
Compared to brittle fracture, plastic failure causes relatively small damage, which includes:
(1) Irreversible plastic deformation after yielding, leading to the scrapping of welded structures with high dimensional requirements.
(2) For pressure vessels made of high toughness and low strength materials, failure is not determined by the fracture toughness of the materials but rather by plastic instability failure caused by insufficient strength.
The ultimate outcome of plastic failure is the failure of welded structures or catastrophic accidents that affect enterprise production, result in unnecessary casualties, and seriously impede national economic development.
10. What is brittle fracture and what is its harm?
Generally, brittle fracture refers to fractures that occur by dissociation (including quasi-dissociation) and grain boundary (intergranular) splitting along a certain crystal surface.
Cleavage fracture is a type of intracrystalline fracture that occurs due to separation along a particular crystallographic plane.
Under specific conditions, such as low temperatures, high strains, and high stress concentrations, cleavage fracture can occur when the stress reaches a certain value.
There are several models regarding the generation of cleavage fracture, most of which are related to dislocation theory.
It is commonly believed that when the plastic deformation process of a material is significantly hindered, the material cannot comply with the applied stress through deformation, but instead separates, resulting in cleavage cracks.
Inclusions, brittle precipitates, and other defects in metals also have a significant influence on the generation of cleavage cracks.
Brittle fracture usually occurs when the stress is not higher than the design allowable stress of the structure, and there is no significant plastic deformation, and it propagates through the entire structure instantaneously.
It is characterized by sudden damage and is difficult to detect and prevent in advance, often resulting in personal casualties and significant property loss.
11. What role does welding crack play in structural brittle fracture?
Answer: among all defects, cracks are the most dangerous.
Under external load, a small amount of plastic deformation occurs near the crack front, causing a certain amount of opening displacement at the tip and slowing down the crack’s development.
When the external load increases to a critical value, the crack expands at high speed. If the crack is located in a high tensile stress area, it often results in the brittle fracture of the entire structure.
If the crack extends into a low tensile stress area, there may be enough energy to maintain further crack expansion. Alternatively, if the crack enters a material with good toughness or the same material with higher temperature and increased toughness, it will experience greater resistance and cannot continue to expand.
At this point, the crack’s harm is reduced accordingly.
12. What are the reasons for brittle fracture of welded structure?
Answer: the causes of fracture can be basically summarized into three aspects:
(1) Insufficient human nature of materials
The material’s micro deformation ability is particularly poor at the tip of the notch.
Brittle failure under low stress typically occurs at lower temperatures, and the material’s toughness sharply decreases as the temperature drops.
Furthermore, with the advancements in low alloy high strength steel, the strength index has risen while the plasticity and toughness have decreased.
In most scenarios, brittle fracture initiates from the welding zone. Therefore, the inadequate toughness of the weld and the heat-affected zone is frequently the primary cause of low stress brittle failure.
(2) There are microcracks and other defects
Fractures typically initiate from defects, with cracks being the most perilous of these defects. Welding represents the primary source of cracks. Despite advances in welding technology that have made it possible to largely mitigate cracks, it remains challenging to eliminate them entirely.
(3) Certain stress level
The primary causes of welding residual stress are incorrect design and poor manufacturing processes. Thus, when dealing with welded structures, it is essential to consider not only the working stress but also the welding residual stress, stress concentration, and any additional stress caused by inadequate assembly.
13. What main factors should be considered when designing welded structures?
Answer: the main considerations are as follows:
1）The welded joint must have sufficient strength and rigidity to ensure a long service life.
2）Consider the working medium and conditions of the welded joints, including factors such as temperature, corrosion, vibration, and fatigue.
3）For large structural parts, the workload of pre-welding preheating and post-welding heat treatment should be minimized as much as possible.
4）Weldments may no longer require machining, or only a minimal amount of machining may be needed.
5）The amount of welding work should be minimized.
6）Minimize deformation and stress in the welded structure.
7）Ensure ease of construction and create good working conditions for construction.
8）Utilize new technology and mechanized and automatic welding to improve labor productivity.
9）Ensure that the weld is easily inspected to guarantee the quality of the joint.
14. Please describe the basic conditions of gas cutting. Can oxygen acetylene flame gas cutting be used for red copper? Why?
Answer: the basic conditions of gas cutting are:
(1) The ignition point of a metal should be lower than its melting point.
(2) The melting point of a metal oxide should be lower than that of the metal itself.
(3) A significant amount of heat can be released when metals burn in oxygen.
(4) The thermal conductivity of a metal should be low.
The oxygen-acetylene flame gas cutting cannot be used for red copper because copper oxide (CuO) generates little heat, and its thermal conductivity is very high, which means that the heat cannot be concentrated near the notch and, therefore, cannot be gas cut.
15. What is the main function of gas welding powder?
The primary purpose of welding powder is to create slag, which occurs when the powder interacts with metal oxides or non-metallic impurities in the molten pool.
Simultaneously, the slag that forms helps to shield the surface of the molten pool and isolates it from the air. This action is critical as it prevents the metal in the molten pool from continuous oxidation at high temperatures.
16. What are the process measures to prevent weld porosity in manual arc welding?
(1) Electrodes and fluxes must be kept dry and dried as required before use.
(2) The surface of the welding wire and weldment should be kept clean, free from water, oil stains, and rust.
(3) Correct welding specifications should be selected, such as avoiding using excessive welding current and maintaining an appropriate welding speed.
(4) Correct welding methods should be adopted, such as using alkaline electrodes for manual arc welding, short arc welding, reducing the swing range of the electrode, slowing down the speed of electrode transportation, and controlling the start and end of short arc.
(5) The assembly clearance of the weldments should be controlled to prevent it from being too large.
(6) Do not use electrodes with cracked coatings, peeling, deterioration, eccentricity, or corrosion of the welding core.
17. What are the main measures to prevent white cast iron during cast iron welding?
(1) Graphitized electrodes are utilized, which are cast iron electrodes with a high proportion of graphitized elements, such as carbon, silicon, etc., added to coatings or welding wires. Alternatively, nickel-based and copper-based cast iron electrodes can also be used.
(2) Prior to welding, preheating is required to maintain the temperature, and during welding, heat preservation is necessary to slow down the cooling process of the weld area. This helps in prolonging the duration for which the fusion area remains in the red hot state, ensuring full graphitization, and reducing thermal stress.
(3) The brazing process is employed.
18. Describe the role of flux in the welding process?
In welding, flux is a crucial factor in ensuring welding quality as it serves the following functions:
(1) Upon melting, the flux floats on the surface of the molten metal to protect the molten pool from harmful gases in the air and prevent erosion.
(2) The flux can deoxidize and alloy, working with the welding wire to achieve the required chemical composition and mechanical properties of the weld metal.
(3) It helps to shape the weld joint properly.
(4) By slowing down the cooling rate of the molten metal, it reduces defects such as pores and slag inclusion.
(5) It prevents splashing, reduces losses, and improves the bonding coefficient.
19. What should be paid attention to in the use and maintenance of AC arc welding machine?
(1) The welding machine must be operated according to its rated welding current and load duration, and should not be overloaded.
(2) The welder should not be left in a short-circuited state for an extended period.
(3) The current regulator should only be operated when there is no load.
(4) Regularly inspect and ensure the wire contact, fuse, grounding, and regulating mechanism are intact.
(5) Keep the welding machine clean, dry, and well-ventilated to prevent dust and rain from entering.
(6) Place the welding machine on a stable surface and switch off the power after use.
(7) Regularly carry out maintenance and inspection of the welding machine.
20. What are the hazards of brittle fracture?
Due to the sudden occurrence of brittle fractures, there is no time to identify and prevent them. Once they occur, the consequences are severe, leading not only to significant economic losses but also endangering human safety.
Therefore, it is crucial to pay great attention to the issue of brittle fracture in welded structures.
21. Characteristics and application of plasma spraying?
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 objects. The process also features high plasma flame flow speed and a good powder particle acceleration effect, resulting in high coating bonding strength.
Due to its excellent properties, plasma spraying has a wide range of applications and is considered the best way to spray various types of ceramic materials.
22. Preparation procedure of welding process card?
The preparation procedure for the welding process card should identify the relevant welding process qualification and create the joint diagram based on the product assembly drawing, parts processing drawing, and technical requirements.
Include the welding process card number, drawing number, joint name, joint number, welding process qualification number, and welder certificate items.
Establish the welding sequence based on the welding procedure qualification, actual production conditions, technical elements, and production experience.
Determine the specific welding process parameters in accordance with the welding process qualification. Specify the product inspection authority, inspection method, and inspection proportion based on the requirements of product drawings and standards.
23. Why should a certain amount of silicon and manganese be added to the welding wire of carbon dioxide gas shielded welding?
Carbon dioxide is an oxidizing gas that can cause the alloy elements of the welding seam to burn during the welding process, significantly reducing the mechanical properties of the weld. This can lead to the formation of pores and splashes.
To address these issues, welding wire can be formulated with silicon and manganese to help deoxidize and prevent oxidation and splashing during the welding process.
24. What is the explosion limit of the combustible mixture and what factors affect it?
The concentration range of combustible gas, vapor, or dust present in a combustible mixture is known as the explosion limit. The lower limit of the concentration range is referred to as the lower explosion limit, while the upper limit is called the upper explosion limit.
The explosion limit is influenced by several factors such as temperature, pressure, oxygen content, and vessel diameter. An increase in temperature causes a decrease in the explosion limit. Similarly, an increase in pressure leads to a decrease in the explosion limit. Moreover, an increase in the concentration of oxygen in the mixed gas results in a decrease in the lower explosion limit.
For combustible dust, the explosion limit is influenced by factors such as dispersion, humidity, and temperature, among others.
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) When performing electric welding, welders should avoid contact with iron parts, stand on rubber insulation pads or wear rubber insulation shoes, and wear dry work clothes.
(2) A supervisor, who can see and hear the welder’s work, should be stationed outside the container. A switch should be installed to cut off the power supply according to the welder’s signal.
(3) The voltage of the portable lamp used in the container should not exceed 12V. The shell of the portable lamp transformer should be reliably grounded, and the use of autotransformers is prohibited.
(4) Transformers for portable lamps and welding transformers should not be taken into boilers and metal containers.
26. How to distinguish between fusion welding and brazing? What are the characteristics of the two?
Fusion welding involves the bonding of atoms between weldments, whereas brazing utilizes solder, an intermediate medium with a lower melting point than weldments, to connect them.
The advantages of fusion welding include high mechanical properties of welded joints and high productivity when connecting thick and large parts. However, its disadvantages include large stress and deformation and microstructure changes in the heat-affected zone.
On the other hand, brazing offers several benefits such as low heating temperature, flat and smooth joint, and beautiful appearance. It also causes small stress and deformation. However, it has some drawbacks as well, including low joint strength and high requirements for assembly clearance during assembly.
27. Both carbon dioxide and argon are protective gases. What are their properties and uses?
Carbon dioxide is an oxidizing gas. When it is used as the shielding gas in welding, it can strongly oxidize the droplet and molten pool metal, resulting in a loss of alloy elements, poor manufacturability, and the formation of pores and large splashes.
Therefore, currently, it is only suitable for welding low carbon steel and low alloy steel, and not recommended for use on high alloy steel and non-ferrous metals, particularly stainless steel.
Related reading: Ferrous vs Non-ferrous Metals
It is used less frequently because it can cause carburization of the weld joint and reduce resistance to intergranular corrosion.
Argon, being an inert gas, does not react with molten metal, thereby preserving the chemical composition of the weld joint.
The resulting weld quality is good and can be utilized for welding a variety of materials, including alloy steel, stainless steel, and non-ferrous metals.
As the cost of argon is decreasing, it is also becoming more commonly used for welding low carbon steel.
28. Describe the weldability and welding characteristics of 16Mn steel?
The 16Mn steel is produced by adding approximately 1% Mn to Q235A steel, resulting in a carbon equivalent of 0.345% to 0.491%. This enhances its welding performance. However, the hardening tendency of 16Mn steel is slightly higher than that of Q235A steel.
When welding on thick and rigid structures with small parameters and passes, there may be a risk of cracks, especially at low temperatures. Preheating before welding is recommended to prevent this issue.
For manual arc welding, it is recommended to use an E50 welding rod. For automatic submerged arc welding without a groove, H08MnA welding wire with flux 431 can be used. When beveling, H10Mn2 welding wire and flux 431 are recommended. Finally, H08Mn2SiA or H10Mnsi welding wire should be used during CO2 gas shielded welding.
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