1. What are the characteristics of the primary crystalline structure in welds?
Answer: The crystallization of the weld pool follows the fundamental principles of liquid metal crystallization: nucleation and grain growth. During solidification, partially melted grains from the base metal in the fusion zone typically serve as nuclei.
These nuclei then grow by adsorbing surrounding liquid atoms. Since crystals grow in the direction opposite to heat conduction while simultaneously expanding laterally, they form columnar crystals—known as columnar grains—due to obstruction from adjacent growing crystals.
Additionally, under certain conditions, spontaneous nucleation may occur in the liquid metal during solidification within the weld pool. If heat dissipation occurs uniformly in all directions, crystals grow isotropically in all directions, forming granular crystals known as equiaxed crystals. While columnar crystals are commonly observed in welds, equiaxed crystals may also appear at the weld center under specific conditions.
2. What are the characteristics of the secondary recrystallization microstructure in welds?
Answer: After primary crystallization, as the weld metal continues cooling below the phase transformation temperature, further microstructural changes occur. For example, in low-carbon steel welding, the primary crystallized grains are all austenite. When cooled below the phase transformation temperature, austenite decomposes into ferrite and pearlite. Thus, the post-secondary crystallization structure is predominantly ferrite with a small amount of pearlite.
However, due to the rapid cooling rate in welds, the resulting pearlite content is generally higher than in equilibrium structures. Faster cooling yields greater pearlite content and less ferrite, leading to increased hardness and strength but reduced plasticity and toughness. The microstructure obtained after secondary crystallization represents the actual structure at room temperature. The microstructure of welds varies depending on the steel grade and welding process conditions employed.
3. Using low-carbon steel as an example, what microstructure is obtained after secondary crystallization of the weld metal?
Answer: Taking low-carbon steel as an example, the primary crystallization results in an austenitic microstructure. The solid-state phase transformation process of the weld metal is termed secondary crystallization. The microstructure after secondary crystallization consists of ferrite and pearlite.
In the equilibrium microstructure of low-carbon steel, the weld metal contains very low carbon content, resulting in a coarse columnar ferrite structure with a small amount of pearlite. Due to the high cooling rate during welding, ferrite cannot fully precipitate according to the iron-carbon phase diagram. Consequently, the pearlite content is generally higher than in equilibrium structures. The high cooling rate also refines the grain size, increasing the hardness and strength of the metal. The reduction in ferrite and increase in pearlite also contribute to higher hardness but lower plasticity.
Therefore, the final microstructure of the weld metal is determined by its composition and cooling conditions. Due to the characteristics of the welding process, the weld metal exhibits a finer microstructure, resulting in superior mechanical properties compared to the as-cast state.
4. What are the characteristics of dissimilar metal welding?
Answer: 1) The characteristics of dissimilar metal welding primarily involve significant differences in the alloy composition of the deposited metal and the weld. The behavior of the weld pool varies depending on the weld shape, base metal thickness, welding rod coating or flux, and shielding gas type. Consequently, the amount of base metal melted also differs, and the dilution effect on the chemical composition concentration between the deposited metal and the melted base metal region will change. Thus, the degree of heterogeneity in dissimilar metal welding depends not only on the original compositions of the workpiece and filler material but also varies with different welding processes.
2) Microstructural Heterogeneity: After undergoing the welding heat cycle, distinct metallographic microstructures emerge in different regions of the welded joint. These are influenced by the chemical compositions of the base metal and filler material, the welding method, the number of weld passes, the welding process, and heat treatment.
3) Inhomogeneity of Properties: Differences in chemical composition and microstructure across the joint result in varying mechanical properties. Strength, hardness, ductility, and toughness exhibit significant variations throughout the joint regions. Impact values in the heat-affected zones (HAZ) on either side of the weld can differ by several orders of magnitude. Creep limits and creep strength at elevated temperatures also show substantial disparities due to compositional and microstructural differences.
4) Non-uniform stress field distribution: Residual stresses in dissimilar metal joints exhibit uneven distribution, primarily determined by the differing plasticity of each joint region. Additionally, variations in thermal conductivity between materials cause changes in the temperature field during the welding thermal cycle. Factors such as differing linear expansion coefficients across regions also contribute to the non-uniform stress field distribution.
5. What are the principles for selecting welding materials when welding dissimilar steels?
Answer: The principles for selecting welding materials for dissimilar steel joints primarily include the following four points:
1) When ensuring no cracks or defects in the welded joint, prioritize welding materials with superior ductility if both weld metal strength and ductility cannot be achieved simultaneously.
2) The weld metal properties of dissimilar steel welding materials need only meet the requirements of one of the two base metals to be considered technically acceptable.
3) Welding materials should exhibit good processability and produce aesthetically pleasing weld bead formation.
4) Welding materials should be economical and readily available.
6. How is the weldability of pearlitic steel compared to austenitic steel?
Answer: Pearlitic steel and austenitic steel are two distinct steel grades differing in both microstructure and composition. 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. This presents the following weldability issues:
1) Weld dilution. Due to the lower gold content in pearlite steel, it dilutes the alloying elements throughout the weld metal. This dilution reduces the austenite-forming element content in the weld, potentially leading to martensite formation within the weld. This degrades joint quality and may even cause cracking.
2) Formation of a transition layer. Under the thermal cycling of welding, the degree of mixing between the molten base metal and filler metal varies at the edges of the molten pool. At the pool edges, the liquid metal experiences lower temperatures, reduced fluidity, and shorter residence times. Due to the significant chemical composition differences between pearlitic and austenitic steels, the molten base metal and filler metal fail to fuse adequately at the pearlitic side of the molten pool edge. resulting in a higher proportion of pearlitic base metal within the weld on the pearlitic steel side. This proportion increases closer to the fusion line. This creates a transition layer with differing internal composition within the weld metal.
3) Formation of a diffusion layer in the fusion zone. In weld metal composed of these two types of steel, the pearlitic steel has a higher carbon content than the austenitic steel. This creates a concentration gradient of carbon and carbide-forming elements on both sides of the pearlitic steel within the fusion zone. When the joint operates continuously at temperatures above 350°C to 400°C, significant carbon diffusion occurs within the fusion zone—specifically, carbon diffusing from the pearlitic steel side through the fusion zone into the austenitic weld metal. Consequently, a decarburized softened layer forms on the pearlitic base metal adjacent to the fusion zone, while a corresponding carburized layer develops on the austenitic weld metal side.
4) Due to the significant differences in physical properties between pearlitic and austenitic steels, as well as substantial compositional variations within the weld, heat treatment cannot eliminate welding stresses in such joints. Instead, it only redistributes stresses—a fundamental difference from welding dissimilar metals.
5) Delayed cracking. During solidification of the weld pool in these dissimilar steel welds, both austenitic and ferritic structures coexist in close proximity. This allows gas diffusion, enabling hydrogen accumulation and subsequent delayed cracking.
7. What measures prevent cracking during cast iron repair welding?
Answer: 1) Preheating before welding and controlled cooling after welding. Preheating the entire or partial workpiece before welding and implementing controlled cooling afterward not only reduces the tendency for white cast iron formation in the weld but also minimizes welding stresses and prevents cracking.
2) Employ arc cold welding to reduce welding stresses. Select welding materials with good plasticity, such as nickel, copper, nickel-copper alloys, and high-vanadium steel as filler metals. This allows the weld metal to relax stresses through plastic deformation, preventing cracks. Using fine-diameter electrodes, low current, intermittent welding (intermittent welding), and spot welding (skip welding) to minimize temperature differences between the weld zone and base metal, thereby reducing welding stresses. Hammering the weld can eliminate stresses and prevent cracking.
3) Additional measures include adjusting the chemical composition of weld metal to narrow its brittle temperature range, incorporating rare earth elements to enhance desulfurization and dephosphorization metallurgical reactions and adding grain-refining elements to refine weld grain structure. In certain cases, employing the heated zone method to reduce stresses at the repair site can also effectively prevent crack formation.
8. What is stress concentration? What factors contribute to stress concentration?
Answer: Due to the shape of the weld and the characteristics of the weld zone, discontinuities in the shape occur. When loaded, this causes uneven distribution of working stresses in the welded joint, resulting in local peak stresses significantly higher than the average stress. This phenomenon is known as stress concentration. In welded joints, stress concentration arises from multiple causes, the most significant being:
(1) Process defects within the weld, such as porosity, slag inclusions, cracks, and lack of fusion. Among these, cracks and lack of fusion cause the most severe stress concentration.
(2) Unreasonable weld geometry, such as excessive root fill height in butt welds or excessive toe height in fillet welds.
Unreasonable joint design, such as abrupt changes at the joint interface or the use of butt joints with cover plates. Improper weld layout can also cause stress concentration.
9. What is plastic failure, and what are its hazards?
A: Plastic failure encompasses plastic instability (yielding or significant plastic deformation) and plastic fracture (ductile fracture). The process involves a welded structure undergoing elastic deformation → yielding → plastic deformation (plastic instability) → formation of microcracks or microvoids → development of macroscopic cracks → unstable propagation → fracture. Compared to brittle fracture, plastic failure poses relatively minor risks, specifically:
(1) Irreversible plastic deformation after yielding renders welded structures with stringent dimensional requirements unusable.
(2) For pressure vessels made of high-toughness, low-strength materials, failure is not controlled by fracture toughness but rather by insufficient strength leading to plastic instability failure.
The ultimate consequence of plastic failure is the failure of welded structures or catastrophic accidents, disrupting industrial production and causing unnecessary casualties.
10. What is brittle fracture, and what hazards does it pose?
Answer: Brittle fracture typically refers to cleavage fractures (including quasi-cleavage fractures) along specific crystallographic planes and grain boundary (intragranular) fractures. Cleavage fracture occurs when separation takes place along a specific crystallographic plane within a grain, representing an intragranular fracture. Under specific conditions—such as low temperatures, high strain rates, or high stress concentrations—metallic materials may undergo cleavage fracture when stress reaches a certain threshold.
Numerous models exist to explain cleavage fracture mechanisms, most linked to dislocation theory. It is widely accepted that when plastic deformation becomes severely impeded, the material adapts to applied stress through separation rather than deformation, leading to cleavage cracking. Inclusions, brittle precipitates, and other defects within metals also significantly influence the initiation of cleavage cracks.
Brittle fracture typically occurs when stresses do not exceed the structure’s allowable design stress and without significant plastic deformation. It propagates instantaneously throughout the entire structure, exhibiting sudden failure characteristics that are difficult to detect or prevent in advance. Consequently, it often results in significant casualties and substantial property damage.
11. What role do welding cracks play in brittle fracture of structures?
Answer: Among all defects, cracks pose the greatest danger. Under external loading, minor plastic deformation occurs near the crack tip, accompanied by a certain amount of opening displacement at the tip, allowing the crack to propagate slowly. When the external load increases to a critical value, the crack propagates at high speed. If located in a region of high tensile stress, this often leads to brittle fracture of the entire structure. However, if the propagating crack enters an area with lower tensile stress, it may lack sufficient energy to sustain further growth. Alternatively, if the crack enters a material with better toughness (or the same material at a higher temperature where toughness increases), it encounters significant resistance and cannot continue propagating. In such cases, the hazard posed by the crack is correspondingly reduced.
12. What causes brittle fracture in welded structures?
Answer: The causes of fracture can generally be summarized into three aspects:
(1) Insufficient toughness of the material. Particularly at the notch tip, the material exhibits poor micro-plastic deformation capability. Low-stress brittle failure typically occurs at lower temperatures, where material toughness declines sharply. Furthermore, with the development of low-alloy high-strength steels, strength properties have increased while plasticity and toughness have decreased. Brittle fracture often originates in the weld zone, making insufficient toughness in welds and heat-affected zones the primary cause of low-stress brittle failure.
(2) Presence of defects such as microcracks. Fracture always initiates at defects, with cracks being the most hazardous. Welding is the primary cause of crack formation. Although welding technology has largely controlled cracks, their complete avoidance remains challenging.
(3) Specific stress levels. Improper design and poor manufacturing processes are the primary sources of residual welding stresses. Therefore, for welded structures, in addition to operational stresses, residual welding stresses, stress concentration levels, and additional stresses caused by factors like poor assembly must also be considered.
13. What are the main factors to consider when designing welded structures?
Answer: The primary factors to consider are as follows:
1) The welded joint must ensure sufficient strength and stiffness to guarantee an adequate service life.
2) Consider the working medium and conditions of the welded joint, such as temperature, corrosion, vibration, and fatigue.
3) For large structural components, minimize the amount of preheating and post-weld heat treatment required.
4) Welded components should require no or minimal machining.
5) Welding workload should be minimized.
6) Deformation and residual stresses in the welded structure should be minimized.
7) Construction should be facilitated, creating favorable working conditions.
8) New technologies and mechanized/automated welding should be adopted whenever possible to enhance labor productivity.
9) Welds should be easily accessible for inspection to ensure joint quality.
14. Describe the fundamental conditions for oxy-fuel cutting. Can copper be cut using an oxy-acetylene flame? Why?
Answer: The fundamental conditions for oxy-fuel cutting are:
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) The metal must release significant heat when burning in oxygen.
4) The metal’s thermal conductivity should be low.
Copper cannot be cut using an oxygen-acetylene flame because copper forms oxide (CuO) with minimal heat generation. Additionally, its excellent thermal conductivity prevents heat concentration near the cut, making gas cutting impractical.
15. What is the primary function of flux in gas welding?
Answer: The primary function of flux is slag formation. It reacts with metal oxides or non-metallic impurities in the molten pool to create slag. Simultaneously, the formed slag covers the molten pool surface, isolating it from the air. This prevents further oxidation of the molten metal at high temperatures.
16. What are the process measures to prevent porosity in welds during manual metal arc welding?
Answer:
1) Keep electrodes and flux dry, bake them according to specifications before use.
2) Maintain cleanliness of welding wire and workpiece surfaces, remove water, oil, rust, and other contaminants.
3) Select appropriate welding parameters, such as avoiding excessive welding current and maintaining suitable welding speed.
4) Employ correct welding techniques: use alkaline electrodes for manual arc welding, apply short-arc welding, minimize electrode oscillation, reduce travel speed, and control arc striking and extinguishing.
5) Control workpiece assembly gaps to prevent excessive clearance.
6) Do not use electrodes with cracked, peeling, deteriorated, off-center coatings, or rusted cores.
17. What are the primary measures to prevent white structure formation during cast iron welding?
Answer: 1) Use graphitizing electrodes, i.e., cast iron electrodes with high graphitizing elements (such as carbon or silicon) added to the coating or wire, or employ nickel-based and copper-based cast iron electrodes.
2) Preheat before welding, maintain temperature during welding, and cool slowly after welding to reduce the cooling rate in the weld zone. This prolongs the time the fusion zone remains red-hot, allowing sufficient graphitization and reducing thermal stress.
3) Employ brazing techniques.
18. What is the role of flux in the welding process?
Answer: In welding, flux is a primary factor ensuring weld quality. It serves the following functions:
1) After melting, the flux floats on the molten metal surface, protecting the weld pool and preventing corrosion from harmful gases in the air.
2) Flux deoxidizes and promotes alloying, working in conjunction with the welding wire to achieve the desired chemical composition and mechanical properties in the weld metal.
3) It facilitates proper weld bead formation.
4) It slows the cooling rate of molten metal, reducing defects such as porosity and slag inclusions.
5) It prevents spatter, minimizes material loss, and improves the fusion efficiency.
19. What precautions should be observed in the use and maintenance of AC arc welding machines?
Answer: 1) Operate within the machine’s rated welding current and duty cycle, avoid overloading.
2) Prohibit prolonged short-circuiting of the welding machine.
3) Adjust the current while the machine is operating under no-load conditions.
4) Regularly inspect and test the integrity of wire connections, fuses, grounding, and adjustment mechanisms.
5) Keep the welder clean, dry, and well-ventilated to prevent dust and rainwater ingress.
6) Place the welder on a stable surface and disconnect the power supply after use.
7) Schedule regular maintenance for the welder.
20. What are the hazards of brittle fracture?
Answer: Due to its sudden nature, brittle fracture cannot be detected or prevented in time. Once it occurs, the consequences are severe, causing not only significant economic losses but also endangering human life. Therefore, brittle fracture in welded structures is a critical issue that warrants serious attention.
21. What are the characteristics and applications of plasma spraying?
Answer: Plasma spraying features extremely high flame temperatures capable of melting nearly all refractory materials, enabling its application to a wide range of substrates. The high velocity of the plasma jet effectively accelerates particles, resulting in high coating bond strength. It is widely used and considered the optimal method for spraying various ceramic materials.
22. What is the procedure for compiling a welding procedure specification?
Answer: The procedure for preparing welding procedure cards should be based on the product assembly drawings, component machining drawings, and their technical requirements. Identify the corresponding welding procedure qualification, draw a simplified joint diagram, and provide the welding procedure card number, drawing number, joint name, joint number, welding procedure qualification number, and welder certification items. Develop the welding sequence based on the welding procedure qualification, actual production conditions, technical requirements, and production experience. Establish specific welding parameters according to the welding procedure qualification. Determine the inspection authority, inspection methods, and inspection ratio for the product based on the requirements of the product drawings and product standards.
23. Why is a certain amount of silicon and manganese added to the welding wire in CO₂ gas shielded arc welding?
Answer: Carbon dioxide is an oxidizing gas that causes oxidation of alloy elements in the weld during welding, significantly reducing the mechanical properties of the weld. This oxidation leads to porosity and spatter. Adding silicon and manganese to the welding wire serves as a deoxidizer, effectively addressing oxidation and spatter issues.
24. What is the explosion limit of a combustible mixture, and what factors influence it?
A: The concentration range within which combustible gases, vapors, or dust in a mixture can explode is termed the explosion limits.
The lower concentration limit is called the lower explosion limit (LEL), and the upper concentration limit is called the upper explosion limit (UEL). Explosion limits are influenced by factors such as temperature, pressure, oxygen content, and container diameter. As temperature increases, the explosion limits decrease and when pressure increases, the explosion limits also decrease, as the oxygen concentration in the mixture rises, the lower explosion limit decreases. For combustible dust, its explosion limits are influenced by factors such as dispersion, humidity, and temperature.
25. What measures should be taken to prevent electric shock when performing welding work inside boiler drums, condensers, oil tanks, oil reservoirs, and other metal containers?
Answer: 1) During welding, welders should avoid contact with iron components, stand on rubber insulating mats or wear rubber insulating shoes, and wear dry work clothes.
2) A supervisor visible and audible to the welder should be stationed outside the container, equipped with a switch to cut power upon the welder’s signal.
3) Portable lamps used inside containers must not exceed 12 volts. The transformer casing must be reliably grounded, autotransformers are prohibited.
4) Neither portable lamp transformers nor welding transformers may be brought into boilers or metal containers.
26. How do you distinguish fusion welding from brazing? What are the characteristics of each?
Answer: Fusion welding involves the atomic bonding of the workpieces, while brazing uses a filler material—a medium with a lower melting point than the workpieces—to join them. Advantages of fusion welding include higher mechanical properties of the welded joint and greater productivity when joining thick or large components. Disadvantages are significant residual stresses, deformation, and microstructural changes in the heat-affected zone. Advantages of brazing include lower heating temperatures, resulting in flat, smooth, aesthetically pleasing joints with minimal stress and deformation. Disadvantages are lower joint strength and stringent requirements for assembly tolerances.
27. Both carbon dioxide and argon are protective gases. What are their properties and uses?
Answer: Carbon dioxide is an oxidizing gas. When used as a shielding gas in the welding zone, it causes intense oxidation of molten droplets and the molten pool metal, leading to burn-off of alloying elements. It also has poor processability, often resulting in porosity and significant spatter. Therefore, it is currently only suitable for welding low-carbon steel and low-alloy steel, and is not applicable for welding high-alloy steel or non-ferrous metals. Particularly for stainless steel, it causes carbon enrichment in the weld, reducing resistance to intergranular corrosion, making its use even more limited. Argon is an inert gas that does not react chemically with molten metal. Consequently, the chemical composition of the weld remains largely unchanged, resulting in high-quality welds. It is suitable for welding various alloy steels, stainless steels, and non-ferrous metals. As the price of argon continues to decrease, it is also increasingly used for welding low-carbon steel.
28. What are the weldability and welding characteristics of 16Mn steel?
Answer: 16Mn steel is produced by adding approximately 1% manganese to Q235A steel, with a carbon equivalent of 0.345% to 0.491%. Therefore, it exhibits good weldability. However, its hardening tendency is slightly higher than that of Q235A steel. When welding thick sections or rigid structures with small parameters and narrow welds, cracking may occur, particularly under low-temperature conditions. In such cases, appropriate preheating before welding is recommended.
For manual arc welding, use E50 grade electrodes. For submerged arc automatic welding without a bevel, use H08MnA welding wire with flux 431. For beveled joints, use H10Mn2 welding wire with flux 431. For CO₂ gas shielded welding, use H08Mn2SiA or H10MnSi welding wire.
