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28 Key Questions on Advanced Welder Welding Knowledge
1. What are the characteristics of the primary crystallization structure of the weld?
Answer: The crystallization of the weld pool also follows the fundamental laws of liquid metal crystallization: nucleation and crystal growth. When the liquid metal in the weld pool solidifies, the partially melted grains in the fusion zone of the base metal usually become the nucleation sites. The nuclei then adsorb surrounding liquid atoms to grow. Because crystals grow in the direction opposite to the heat conduction direction and also grow laterally—but are hindered by adjacent growing crystals—they form columnar crystals. Furthermore, under certain conditions, the liquid metal in the weld pool can also form spontaneous nuclei during solidification. If heat dissipation occurs uniformly in all directions, the crystals grow uniformly in all directions, forming granular crystals known as equiaxed crystals.
Columnar crystals are commonly observed in welds; under specific conditions, equiaxed crystals can also appear in the center of the weld.
2. What are the characteristics of the secondary crystallization structure of the weld?
Answer: After the primary crystallization, the weld metal continues to cool below the phase transition temperature, resulting in further changes in its metallographic structure. For example, in low-carbon steel welding, the primary crystals are all austenite grains. When cooled below the phase transition temperature, the austenite decomposes into ferrite and pearlite. Thus, the structure after secondary crystallization is mostly ferrite with a small amount of pearlite. However, due to the relatively fast cooling rate of the weld, the pearlite content is generally higher than that in the equilibrium structure. The faster the cooling rate, the higher the pearlite content and the lower the ferrite content, resulting in increased hardness and strength, but reduced plasticity and toughness. After secondary crystallization, the actual structure at room temperature is obtained. The weld structure varies depending on the steel type and welding process conditions.
3. Using low-carbon steel as an example, what structure is obtained after the secondary crystallization of the weld metal?
Answer: Taking low-carbon steel as an example, the primary crystallization structure is austenite. The solid-state phase transformation process of the weld metal is called the secondary crystallization of the weld metal. The microstructure after secondary crystallization is ferrite and pearlite.
In the equilibrium structure of low-carbon steel, the carbon content of the weld metal is very low, and its structure consists of coarse columnar ferrite and a small amount of pearlite. Due to the high cooling rate of the weld, ferrite cannot fully precipitate according to the iron-carbon phase diagram, resulting in a generally higher pearlite content than in the equilibrium structure. A high cooling rate also refines the grains, increasing the metal's hardness and strength. The reduction in ferrite and increase in pearlite further enhance hardness while reducing plasticity.
Therefore, the final structure of the weld is determined by the metal's composition and cooling conditions. Due to the characteristics of the welding process, the weld metal structure is finer, resulting in better performance compared to the as-cast structure.
4. What are the characteristics of dissimilar metal welding?
Answer: 1) The primary characteristic of dissimilar metal welding lies in the significant difference in alloy composition between the deposited metal and the weld. The behavior of the welding pool varies with weld shape, base metal thickness, type of electrode coating or flux, and shielding gas. Consequently, the amount of melted base metal differs, leading to variations in the mutual dilution of chemical compositions between the deposited metal and the melted zone of the base metal. Thus, the degree of chemical inhomogeneity across different zones of a dissimilar metal welded joint depends not only on the original compositions of the workpiece and filler material but also on the welding process employed.
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Microstructural inhomogeneity: After undergoing the welding thermal cycle, different zones of the welded joint exhibit varying metallographic structures. These structures are influenced by the chemical compositions of the base metal and filler material, welding method, welding sequence, welding process, and heat treatment.
Performance inhomogeneity: Variations in chemical composition and metallic structure lead to divergent mechanical properties throughout the joint. Significant differences in strength, hardness, plasticity, and toughness are observed across its various zones. The impact values in the heat-affected zones on both sides of the weld can vary by a factor of several times, while the creep limit and stress rupture strength at elevated temperatures also differ substantially due to variations in composition and microstructure.
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Stress field inhomogeneity: The residual stress distribution in a dissimilar metal joint is uneven, primarily due to the varying plasticity of different zones in the joint. Additionally, differences in material thermal conductivity cause changes in the temperature field of the welding thermal cycle. Variations in the coefficient of linear expansion across different zones are also factors contributing to uneven stress distribution.
5. What are the principles for selecting welding materials when welding dissimilar steels?
Answer: The principles for selecting welding materials for dissimilar steels mainly include the following four points:
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On the premise that no cracks or other defects occur in the welded joint, if both the strength and plasticity of the weld metal cannot be satisfied simultaneously, welding materials with better plasticity should be selected.
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If the properties of the weld metal from the selected welding materials meet the technical requirements of only one of the two base metals, it is considered acceptable.
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The welding materials should have good process performance and produce aesthetically shaped welds. The welding materials should be economical and readily available.
6. What is the weldability between pearlitic steel and austenitic steel?
Answer: Pearlitic steel and austenitic steel are two types of steel with different structures and compositions. Therefore, when these two types of steel are welded together, the weld metal is a fusion of two different types of base metals and filler materials, which presents the following issues regarding their weldability:
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Weld dilution: Since pearlitic steel has lower alloy element content, it dilutes the overall alloy composition of the weld metal. This dilution reduces the content of austenite-forming elements in the weld, potentially resulting in martensite formation in the weld, which deteriorates the quality of the welded joint and may even cause cracks.
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Formation of a transition layer: Under the influence of the welding thermal cycle, the degree of mixing between the melted base metal and filler metal varies at the edges of the molten pool. At the edges of the molten pool, the temperature of the liquid metal is lower, fluidity is poorer, and the residence time in the liquid state is shorter. Due to the significant difference in chemical composition between pearlitic steel and austenitic steel, the melted base metal and filler metal do not mix well at the edge of the molten pool on the pearlitic steel side. As a result, the proportion of pearlitic base metal is higher in the weld on the pearlitic steel side, and this proportion increases closer to the fusion line. This forms a transition layer within the weld metal with varying compositions.
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Formation of a diffusion layer in the fusion zone: In the weld metal composed of these two types of steel, pearlitic steel has higher carbon content but lower alloy element content, while austenitic steel is the opposite. This creates a concentration gradient of carbon and carbide-forming elements on either side of the fusion zone on the pearlitic steel side. When the joint operates for extended periods at temperatures above 350-400°C, significant carbon diffusion occurs in the fusion zone, i.e., carbon diffuses from the pearlitic steel side through the fusion zone into the austenitic weld. This results in the formation of a decarburized softening layer in the base metal of the pearlitic steel near the fusion zone and a corresponding carburized layer on the austenitic weld side.
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Due to the significant differences in physical properties between pearlitic steel and austenitic steel and the substantial compositional differences in the weld, such joints cannot be stress-relieved through heat treatment methods. Heat treatment can only lead to a redistribution of stress, which is significantly different from welding similar metals.
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Hydrogen-induced delayed cracks: During the crystallization process of the welding pool for these dissimilar steels, both austenite and ferrite structures are present and close to each other, allowing gases to diffuse. This enables the accumulation of diffusible hydrogen, leading to the formation of hydrogen-induced delayed cracks.
7.What are the measures to prevent cracks during cast iron repair welding?
Answer: (1) Preheating before welding and slow cooling after welding: Preheating the entire weldment or locally before welding and implementing slow cooling after welding can not only reduce the tendency for white iron formation in the weld but also minimize welding stress and prevent cracking of the weldment.
(2) Using arc cold welding to reduce welding stress: Selecting welding materials with good plasticity, such as nickel, copper, nickel-copper, or high-vanadium steel, as filler metals allows the weld metal to relieve stress through plastic deformation, thereby preventing cracks. Using small-diameter electrodes, low current, intermittent welding, and skip welding methods can reduce the temperature difference between the weld and the base metal, thus minimizing welding stress. Stress can also be relieved and cracks prevented by peening the weld.
(3) Other measures: Adjusting the chemical composition of the weld metal to narrow its brittle temperature range; adding rare earth elements to enhance desulfurization and dephosphorization metallurgical reactions in the weld; adding elements that refine the grain structure to achieve finer grains in the weld. In certain cases, using the heated stress zone method to reduce the stress applied to the welded area can also effectively prevent crack formation.
8. What is stress concentration? What factors cause stress concentration?
Answer: Due to the characteristics of weld shape and arrangement, geometric discontinuities occur. When under load, this leads to an uneven distribution of working stress in the welded joint, causing local peak stress σmax to be much higher than the average stress σm. This phenomenon is known as stress concentration. In welded joints, there are many reasons for stress concentration, the most significant of which are: (1) Process defects in the weld, such as porosity, slag inclusions, cracks, and lack of penetration, among which welding cracks and lack of penetration cause the most severe stress concentration. (2) Unreasonable weld geometry, such as excessive reinforcement in butt welds or overly high weld toes in fillet welds. Unreasonable joint design, such as abrupt changes in joint cross-section or the use of cover plates in butt joints, can also cause stress concentration. Improper weld placement can lead to stress concentration as well, for example, in T-joints with only fillet welds.
9. What is plastic failure, and what are its hazards?
Answer: Plastic failure includes plastic instability (yielding or significant plastic deformation) and plastic fracture (brittle fracture or ductile fracture). The process involves the welded structure undergoing elastic deformation under load → yielding → plastic deformation (plastic instability) → formation of micro-cracks or micro-voids → development of macroscopic cracks → unstable propagation → fracture. Compared to brittle fracture, the hazards of plastic failure are relatively smaller, specifically including the following: (1) Yielding results in irreversible plastic deformation, rendering welded structures with high dimensional accuracy requirements unusable. (2) For pressure vessels made of high-toughness, low-strength materials, failure is not controlled by the material's fracture toughness but by insufficient strength leading to plastic instability failure.
The ultimate outcome of plastic failure is the failure of the welded structure or the occurrence of catastrophic incidents, which impacts enterprise production, causes unnecessary casualties, and severely hinders the development of the national economy.
10. What is brittle fracture, and what are its hazards?
Answer: Brittle fracture typically refers to cleavage fracture (including quasi-cleavage fracture) along specific crystallographic planes and intergranular fracture. Cleavage fracture is a type of intragranular fracture that occurs along certain crystallographic planes within the grain. Under specific conditions, such as low temperature, high strain rates, and high stress concentration, metallic materials may undergo cleavage fracture when stress reaches a certain value. Numerous models have been proposed to explain the occurrence of cleavage fracture, most of which are related to dislocation theory. It is generally believed that when the plastic deformation process is severely hindered, the material cannot accommodate external stress through deformation but instead separates, leading to the formation of cleavage cracks. Inclusions, brittle precipitates, and other defects in metals also significantly influence the initiation of cleavage cracks.
Brittle fractures generally occur when the stress does not exceed the design allowable stress of the structure and there is no significant plastic deformation. They propagate instantaneously throughout the entire structure, exhibiting sudden failure characteristics that are difficult to detect and prevent in advance. As a result, brittle fractures often cause personal injuries, fatalities, and significant property losses.
11. What role do welding cracks play in the brittle fracture of structures?
Answer: Among all defects, cracks are the most dangerous. Under external loads, a small amount of plastic deformation occurs near the crack tip, accompanied by a certain amount of crack tip opening displacement, causing the crack to propagate slowly. When the external load increases to a critical value, the crack begins to propagate at high speed. If the crack is located in an area of high tensile stress, it often leads to the brittle fracture of the entire structure. If the propagating crack enters an area of lower tensile stress, where there is insufficient energy to sustain further crack growth, or if the crack enters a material with better toughness (or the same material at a higher temperature where toughness is increased) and encounters greater resistance, preventing continued propagation, then the hazard posed by the crack is correspondingly reduced.
12. Why are welded structures prone to brittle fracture?
Answer: The causes of fracture can be essentially summarized into three aspects:
(1) Insufficient Toughness of the Material: Particularly, the material's microscopic capacity for plastic deformation at the crack tip is poor. Low-stress brittle fractures generally occur at relatively low temperatures, and as the temperature decreases, the material's toughness drops sharply. Furthermore, with the development of high-strength low-alloy steels, strength indicators have continuously risen, while plasticity and toughness have somewhat decreased. Brittle fractures in most cases initiate from the welded area. Therefore, insufficient toughness in the weld and heat-affected zone is often the main cause of low-stress brittle failure.
(2) Presence of Defects such as Micro-cracks: Fractures always start from defects, among which cracks are the most dangerous. Welding is a primary source of cracks. Although welding technology has advanced to the point where cracks can be largely controlled, completely avoiding cracks remains relatively difficult.
(3) A Certain Level of Stress: Incorrect design and poor manufacturing practices are the main causes of welding residual stress. Therefore, for welded structures, in addition to service stress, it is necessary to consider welding residual stresses, the degree of stress concentration, and additional stresses arising from factors such as poor assembly.
13. What main factors should be considered when designing a welded structure?
Answer: The main factors to consider are as follows:
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The welded joint must ensure sufficient strength and stiffness, guaranteeing a adequately long service life.
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Consider the working medium and conditions of the welded joint, such as temperature, corrosion, vibration, fatigue, etc.
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For large structural components, strive to minimize the workload for preheating before welding and post-weld heat treatment.
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The welded component should require little or no subsequent machining.
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The welding workload should be minimized.
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Deformation and stress in the welded structure should be minimized.
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The design should facilitate construction and create good working conditions.
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Adopt new technologies and mechanized/automated welding as much as possible to improve labor productivity.
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Welds should be easily inspectable to ensure joint quality.
14. Please describe the basic conditions for gas cutting. Can pure copper be cut using an oxy-acetylene flame? Why or why not?
Answer: The basic conditions for gas cutting are:
(1) The metal's ignition point should be lower than its melting point.
(2) The melting point of the metal oxide should be lower than the melting point of the metal itself.
(3) The metal must release a significant amount of heat when burning in oxygen.
(4) The metal's thermal conductivity should be low.
Pure copper cannot be cut using an oxy-acetylene flame because the oxide formed (CuO) has very low heat of formation, and simultaneously, copper's thermal conductivity is very high (preventing heat from concentrating near the cut), thus making gas cutting impossible.
15. What is the main function of flux in gas welding?
Answer: The main function of welding flux is slag formation. It reacts with metal oxides or non-metallic impurities in the molten pool to form slag. Simultaneously, the slag formed covers the surface of the molten pool, isolating it from the air. This prevents the molten pool metal from continuing to oxidize at high temperatures.
16. What are the process measures to prevent weld porosity in manual arc welding?
Answer: (1) Keep electrodes and fluxes dry, baking them according to regulations before use.
(2) Maintain the cleanliness of the welding wire and workpiece surface, ensuring they are free from water, oil, rust, etc.
(3) Correctly select welding parameters, such as avoiding excessively high welding currents, using appropriate welding speeds, etc.
(4) Employ correct welding techniques: use basic electrodes for manual arc welding, maintain a short arc, reduce electrode oscillation amplitude, slow down the travel speed, control arc initiation and termination with a short arc, etc.
(5) Control the assembly gap of the workpieces to prevent it from being too large.
(6) Do not use electrodes with cracked, peeling, deteriorated, eccentric coating, or rusted core.
17. What are the main measures to prevent the formation of chilled/white iron during cast iron welding?
Answer: (1) Use strongly graphitizing electrodes, i.e., cast iron electrodes with coatings or filler wires containing high amounts of graphitizing elements (such as carbon, silicon), or use nickel-based or copper-based cast iron electrodes.
(2) Apply preheating before welding, maintain interpass temperature during welding, and use slow cooling after welding to reduce the cooling rate in the weld zone, prolong the time the fusion zone remains at elevated temperatures, allow full graphitization, and reduce thermal stresses.
(3) Employ brazing techniques.
18. What is the role of flux in the welding process?
In welding, flux is a key factor ensuring weld quality. Its functions include the following:
(1) The melted flux floats on the surface of the molten metal, protecting the molten pool from infiltration by harmful gases in the air.
(2) Flux has deoxidizing and alloying functions. Working in conjunction with the welding wire, it enables the weld metal to obtain the required chemical composition and mechanical properties.
(3) It promotes good weld bead appearance.
(4) It slows down the cooling rate of the molten metal, reducing defects such as porosity and slag inclusions.
(5) It prevents spatter, reduces material loss, and improves deposition efficiency.
19. What precautions should be noted for the use and maintenance of AC arc welding machines?
(1) Use the machine according to its rated welding current and duty cycle; do not overload it.
(2) Do not allow the welder to be in a prolonged short-circuit state.
(3) Current adjustment should be performed while the machine is under no load.
(4) Regularly check cables for proper connection, fuses, grounding, and adjustment mechanisms to ensure they are intact and functional.
(5) Keep the welder clean, dry, and well-ventilated; prevent the ingress of dust and rainwater.
(6) Place it stably. Cut off the power supply after work is completed.
(7) Perform regular maintenance and inspection of the welder.
20. What are the hazards of brittle fracture?
Answer: Because brittle fracture occurs suddenly, allowing no time for detection or prevention, its consequences are extremely severe once it happens. It not only causes significant economic losses but can also endanger human lives. Therefore, the brittle fracture of welded structures is an issue that demands serious attention.
21. What are the characteristics and applications of plasma spraying?
Answer: The characteristics of plasma spraying are its high plasma flame temperature, capable of melting almost all refractory materials, thus making the range of sprayable objects very broad. The high velocity of the plasma jet provides good particle acceleration, resulting in high coating bond strength. Its applications are extensive, and it is the best method for spraying various ceramic materials.
22. What is the procedure for developing a welding procedure specification (WPS)?
Answer: The procedure for developing a WPS should be based on the product assembly drawings, component drawings, and their technical requirements. Identify the corresponding welding procedure qualification. Draw a joint sketch; provide the WPS number, drawing number, joint name, joint identification, welding procedure qualification number, and welder certification items. Based on the welding procedure qualification, actual production conditions, technical requirements, and production experience, establish the welding sequence. Based on the welding procedure qualification, specify detailed welding parameters. According to the product drawing requirements and product standards, determine the inspection items, inspection methods, and inspection proportion for the product.
23. Why are certain amounts of silicon and manganese added to the wire in CO2 gas shielded welding?
Answer: Carbon dioxide is an oxidizing gas. During the welding process, it can cause the burning loss of alloying elements in the weld metal, significantly reducing the mechanical properties of the weld. Specifically, this oxidizing action can lead to porosity and spatter. Adding elements like silicon and manganese to the wire serves as a deoxidizer, which can mitigate the issues of oxidation and spatter.
24. What is the explosion limit of a combustible mixture, and what factors influence it?
Answer: The concentration range of combustible gases, vapors, or dust within a combustible mixture that can cause an explosion is called the explosion limit.
The lower concentration limit is called the Lower Explosion Limit (LEL), and the upper concentration limit is called the Upper Explosion Limit (UEL). The explosion limit is influenced by factors such as temperature, pressure, oxygen content, and container diameter. When temperature increases, the explosion limit range typically widens (note: common interpretation is lower LEL and higher UEL, but context might imply increased hazard). When pressure increases, the explosion limit range also generally widens. As the oxygen concentration in the mixture gas increases, the lower explosion limit decreases. For combustible dusts, the explosion limit is influenced by factors such as particle size distribution (dispersion), humidity, and temperature.
25. What measures should be taken to prevent electric shock when performing welding work inside metal containers such as boiler drums, condensers, fuel tanks, oil troughs, etc.?
Answer: (1) When arc welding, the welder should avoid contact with the metal structure. Stand on a rubber insulating mat or wear rubber insulating shoes, and wear dry work clothes.
(2) A supervisor, who can see and hear the welder working, should be stationed outside the container and be equipped with a switch to cut off the power supply upon the welder's signal.
(3) The voltage of lights used inside the container must not exceed 12 volts. The casing of the lighting transformer must be reliably grounded, and autotransformers must not be used.
(4) Neither the lighting transformer nor the welding transformer should be brought inside the boiler or metal container.
26. How to distinguish between fusion welding and brazing? What are the characteristics of each?
Answer: The characteristic of fusion welding is that it achieves atomic bonding between the workpieces, while brazing joins the workpieces using a filler metal (brazing material) that has a melting point lower than that of the workpieces themselves.
The advantage of fusion welding is the high mechanical strength of the welded joint and high productivity when joining thick or large components. Its disadvantages include the generation of significant stress and distortion, and microstructural changes in the heat-affected zone.
The advantages of brazing are the low heating temperature required, resulting in smooth, neat, and aesthetically pleasing joints with minimal stress and distortion. Its disadvantages are the relatively low strength of the brazed joint and the high requirement for precise fit-up and clearance during assembly.
27. Both carbon dioxide and argon are shielding gases. Describe the properties and applications of each.
Answer: Carbon dioxide is an oxidizing gas. When used as a shielding gas in the welding zone, it causes vigorous oxidation of the molten droplet and weld pool metal, leading to the burning loss of alloying elements. It also offers poorer arc characteristics, which can cause porosity and significant spatter. Therefore, it is currently only suitable for welding mild steel and low-alloy steel, and is not applicable for welding high-alloy steels and non-ferrous metals. Particularly for stainless steel, its use is even less common because it can cause carbon pick-up in the weld, reducing resistance to intergranular corrosion.
Argon is an inert gas. As it does not undergo any chemical reaction with the molten metal, the chemical composition of the weld remains largely unchanged, resulting in high-quality welds. It can be used to weld various alloy steels, stainless steels, and non-ferrous metals. As the cost of argon is gradually decreasing, it is also widely used for welding mild steel.
28. Discuss the weldability and welding characteristics of 16Mn steel.
Answer: 16Mn steel is based on Q235A steel with the addition of about 1% Mn. Its carbon equivalent is 0.345%–0.491%. Therefore, its weldability is relatively good. However, its hardening tendency is slightly greater than that of Q235A steel. When welding with low heat input and small weld beads on structures with large thickness and high rigidity, there is a risk of cracking, especially under low-temperature welding conditions. In such cases, appropriate preheating before welding can be employed.
For manual metal arc welding, E50-grade electrodes should be used. For submerged arc welding without groove preparation, H08MnA wire combined with Flux 431 can be used. When welding with grooved preparations, H10Mn2 wire combined with Flux 431 should be used. For CO2 gas shielded welding, H08Mn2SiA or H10MnSi wire should be used.
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