1. What are the characteristics of the primary crystal structure of the weld?
Answer: The crystallization of the welding pool also follows the basic rules of general liquid metal crystallization: the formation of crystal nuclei and the growth of crystal nuclei. When the liquid metal in the welding pool solidifies, the semi-molten grains on the parent material in the fusion zone usually become crystal nuclei.
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Then the crystal nucleus absorbs the atoms of the surrounding liquid and grows. Since the crystal grows in the direction opposite to the heat conduction direction, it also grows in both directions. However, due to being blocked by the adjacent growing crystals, the crystal forms Crystals with columnar morphology are called columnar crystals.
In addition, under certain conditions, the liquid metal in the molten pool will also produce spontaneous crystal nuclei when solidifying. If the heat dissipation is carried out in all directions, the crystals will grow uniformly into grain-like crystals in all directions. This kind of crystal is called It is an equiaxed crystal. Columnar crystals are commonly seen in welds, and under certain conditions, equiaxed crystals may also appear in the center of the weld.
2. What are the characteristics of the secondary crystallization structure of the weld?
Answer: The structure of the weld metal. After primary crystallization, the metal continues to cool below the phase transformation temperature, and the metallographic structure changes again. For example, when welding low carbon steel, the grains of the primary crystallization are all austenite grains. When cooled below the phase transformation temperature, austenite decomposes into ferrite and pearlite, so the structure after secondary crystallization is mostly ferrite and a small amount of pearlite.
However, due to the faster cooling rate of the weld, the resulting pearlite content is generally greater than the content in the equilibrium structure. The faster the cooling rate, the higher the pearlite content, and the less ferrite, the hardness and strength are also improved. , while the plasticity and toughness are reduced. After secondary crystallization, the actual structure at room temperature is obtained. The weld structures obtained by different steel materials under different welding process conditions are different.
3. Taking low carbon steel as an example to explain what structure is obtained after secondary crystallization of weld metal?
Answer: Taking low plastic steel as an example, the primary crystallization structure is austenite, and the solid-state phase transformation process of the weld metal is called secondary crystallization of the weld metal. The microstructure of 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 is coarse columnar ferrite plus a small amount of pearlite. Due to the high cooling rate of the weld, the ferrite cannot be completely precipitated according to the iron-carbon phase diagram. As a result, the content of pearlite is generally larger than that in the smooth structure. A high cooling rate will also refine the grains and increase the hardness and strength of the metal. Due to the reduction of ferrite and the increase of pearlite, the hardness will also increase, while the plasticity will decrease.
Therefore, the final structure of the weld is determined by the composition of the metal and the cooling conditions. Due to the characteristics of the welding process, the weld metal structure is finer, so the weld metal has better structural properties than the cast state.
4. What are the characteristics of dissimilar metal welding?
Answer: 1) The characteristics of dissimilar metal welding mainly lie in the obvious difference in the alloy composition of the deposited metal and the weld. With the shape of the weld, the thickness of the base metal, the electrode coating or flux, and the type of protective gas, the welding melt will change. Pool behavior is also inconsistent,
Therefore, the amount of melting of the base metal is also different, and the mutual dilution effect of the concentration of the chemical components of the deposited metal and the melting area of the base metal will also change. It can be seen that the dissimilar metal welded joints vary with the uneven chemical composition of the area. The degree not only depends on the original composition of the weldment and filler material, but also varies with different welding processes.
2) Inhomogeneity of the structure. After experiencing the welding thermal cycle, different metallographic structures will appear in each area of the welded joint, which is related to the chemical composition of the base metal and filler materials, welding method, welding level, welding process and heat treatment.
3) Non-uniformity of performance. Due to the different chemical composition and metal structure of the joint, the mechanical properties of the joint are different. The strength, hardness, plasticity, toughness, etc. of each area along the joint are very different. In the weld The impact values of the heat-affected zones on both sides are even several times different, and the creep limit and lasting strength at high temperatures will also vary greatly depending on the composition and structure.
4) Non-uniformity of stress field distribution. The residual stress distribution in dissimilar metal joints is non-uniform. This is mainly determined by the different plasticity of each area of the joint. In addition, the difference in thermal conductivity of materials will cause changes in the temperature field of the welding thermal cycle. Factors such as differences in linear expansion coefficients in various regions are the reasons for the uneven distribution of the stress field.
5. What are the principles for selecting welding materials when welding dissimilar steels?
Answer: The selection principles for dissimilar steel welding materials mainly include the following four points:
1) On the premise that the welded joint does not produce cracks and other defects, if the strength and plasticity of the weld metal cannot be taken into account, welding materials with better plasticity should be selected.
2) If the weld metal properties of dissimilar steel welding materials only meet one of the two base materials, it is considered to meet the technical requirements.
3) The welding materials should have good process performance and the welding seam should be beautiful in shape. Welding materials are economical and easy to purchase.
6. What is the weldability of 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 formed by the fusion of two different types of base metals and filler materials. This raises the following questions for the weldability of these two types of steel:
1) Dilution of the weld. Since pearlitic steel contains lower gold elements, it has a diluting effect on the alloy of the entire weld metal. Due to this dilution effect of pearlitic steel, the content of austenite-forming elements in the weld is reduced. As a result, in the weld, A martensite structure may appear, thereby deteriorating the quality of the welded joint and even causing cracks.
2) Formation of excessive layer. Under the action of welding heat cycle, the degree of mixing of the molten base metal and filler metal is different at the edge of the molten pool. At the edge of the molten pool, the temperature of the liquid metal is lower, the fluidity is poor, and the residence time in the liquid state is shorter. Due to the huge difference in chemical composition between pearlitic steel and austenitic steel, the molten base metal and filler metal cannot be well fused at the edge of the molten pool on the pearlitic side. As a result, in the weld on the pearlitic steel side, the pearlitic base metal The proportion is larger, and the closer to the fusion line, the greater the proportion of the base material. This forms a transition layer with different internal compositions of the weld metal.
3) Form a diffusion layer in the fusion zone. In the weld metal composed of these two types of steels, since pearlitic steel has higher carbon content but higher alloying elements but less alloying elements, while austenitic steel has the opposite effect, so on both sides of the pearlitic steel side of the fusion zone A concentration difference between carbon and carbide-forming elements is formed. When the joint is operated at a temperature higher than 350-400 degrees for a long time, there will be obvious diffusion of carbon in the fusion zone, that is, from the pearlite steel side through the fusion zone to the austenite welding zone. seams spread. As a result, a decarburized softening layer is formed on the pearlitic steel base metal close to the fusion zone, and a carburized layer corresponding to decarburization is produced on the austenitic weld side.
4) Since the physical properties of pearlitic steel and austenitic steel are very different, and the composition of the weld is also very different, this type of joint cannot eliminate the welding stress by heat treatment, and can only cause the redistribution of stress. It is very different from welding of the same metal.
5) Delayed cracking. During the crystallization process of the welding molten pool of this kind of dissimilar steel, there are both austenite structure and ferrite structure. The two are close to each other, and the gas can diffuse, so that the diffused hydrogen can accumulate and cause delayed cracks.
25. What factors should be considered when choosing a cast iron repair welding method?
Answer: When choosing a gray cast iron welding method, the following factors must be considered:
1) The condition of the casting to be welded, such as the chemical composition, structure and mechanical properties of the casting, the size, thickness and structural complexity of the casting.
2) Defects of the cast parts. Before welding, you should understand the type of defect (cracks, lack of flesh, wear, pores, blisters, insufficient pouring, etc.), the size of the defect, the stiffness of the location, the cause of the defect, etc.
3) Post-weld quality requirements such as mechanical properties and processing properties of the post-weld joint. Understand the requirements such as weld color and sealing performance.
4) On-site equipment conditions and economy. Under the condition of ensuring the post-weld quality requirements, the most basic purpose of welding repair of castings is to use the simplest method, the most common welding equipment and process equipment, and the lowest cost to achieve greater economic benefits.
7. What are the measures to prevent cracks during repair welding of cast iron?
Answer: (1) Preheat before welding and slow cooling after welding. Preheating the weldment in whole or in part before welding and slow cooling after welding can not only reduce the tendency of the weld to become white, but also reduce the welding stress and prevent the cracking of the weldment. .
(2) Use arc cold welding to reduce welding stress, and choose welding materials with good plasticity, such as nickel, copper, nickel-copper, high vanadium steel, etc. as filler metal, so that the weld metal can relax stress through plastic deformation and prevent cracks. , using small diameter welding rods, small current, intermittent welding (intermittent welding), dispersed welding (jump welding) methods can reduce the temperature difference between the weld and the base metal and reduce the welding stress, which can be eliminated by hammering the weld. stress and prevent cracks.
(3) Other measures include adjusting the chemical composition of the weld metal to reduce its brittleness temperature range; adding rare earth elements to enhance the desulfurization and dephosphorization metallurgical reactions of the weld; and adding powerful grain-refining elements to make the weld crystallized. Grain refinement.
In some cases, heating is used to reduce the stress on the welding repair area, which can also effectively prevent the occurrence of cracks.
8. What is stress concentration? What are the factors that cause stress concentration?
Answer: Due to the shape of the weld and the characteristics of the weld, discontinuity in the collective shape appears. When loaded, it causes uneven distribution of working stress in the welded joint, making the local peak stress σmax higher than the average stress σm. More, this is stress concentration. There are many reasons for stress concentration in welded joints, the most important of which are:
(1) Process defects produced in the weld, such as air inlets, slag inclusions, cracks and incomplete penetration, etc. Among them, the stress concentration caused by welding cracks and incomplete penetration is the most serious.
(2) Unreasonable weld shape, such as the reinforcement of butt weld is too large, the weld toe of fillet weld is too high, etc.
Unreasonable street design. For example, the street interface has sudden changes, and the use of covered panels to connect to the street. Unreasonable weld layout can also cause stress concentration, such as T-shaped joints with only storefront welds.
9. What is plastic damage and what harm does it have?
Answer: Plastic damage includes plastic instability (yield or significant plastic deformation) and plastic fracture (edge fracture or ductile fracture). The process is that the welded structure first undergoes elastic deformation → yield → plastic deformation (plastic instability) under the action of load. ) → produce micro cracks or micro voids → form macro cracks → undergo unstable expansion → fracture.
Compared with brittle fracture, plastic damage is less harmful, specifically the following types:
(1) Irrecoverable plastic deformation occurs after yielding, causing welded structures with high size requirements to be scrapped.
(2) The failure of pressure vessels made of high-toughness, low-strength materials is not controlled by the fracture toughness of the material, but is caused by plastic instability failure due to insufficient strength.
The final result of plastic damage is that the welded structure fails or a catastrophic accident occurs, which affects the production of the enterprise, causes unnecessary casualties, and seriously affects the development of the national economy.
10. What is brittle fracture and what harm does it have?
Answer: Usually brittle fracture refers to splitting dissociation fracture (including quasi-dissociation fracture) along a certain crystal plane and grain boundary (intergranular) fracture.
Cleavage fracture is a fracture formed by separation along a certain crystallographic plane within the crystal. It is an intragranular fracture. Under certain conditions, such as low temperature, high strain rate and high stress concentration, cleavage and fracture will occur in metal materials when the stress reaches a certain value.
There are many models for the generation of cleavage fractures, most of which are related to dislocation theory. It is generally believed that when the plastic deformation process of a material is severely hindered, the material cannot adapt to the external stress by deformation but by separation, resulting in cleavage cracks.
Inclusions, brittle precipitates and other defects in metals also have an important impact on the occurrence of cleavage cracks.
Brittle fracture generally occurs when the stress is not higher than the design allowable stress of the structure and there is no significant plastic deformation, and instantly extends to the entire structure. It has the nature of sudden destruction and is difficult to detect and prevent in advance, so it often causes personal casualties. and huge damage to property.
11. What role do welding cracks play in structural brittle fracture?
Answer: Among all defects, cracks are the most dangerous. Under the action of external load, a small amount of plastic deformation will occur near the crack front, and at the same time there will be a certain amount of opening displacement at the tip, causing the crack to develop slowly;
When the external load increases to a certain critical value, the crack will expand at a high speed. At this time, if the crack is located in a high tensile stress area, it will often cause brittle fracture of the entire structure. If the expanding crack enters an area with low tensile stress, The reputation has enough energy to sustain the further expansion of the crack, or the crack enters a material with better toughness (or the same material but with higher temperature and increased toughness) and receives greater resistance and cannot continue to expand. At this time, the hazard of the crack becomes decrease accordingly.
12. What is the reason why welded structures are prone to brittle fracture?
Answer: The reasons for fracture can basically be summarized into three aspects:
(1) Insufficient humanity of materials
Especially at the tip of the notch, the microscopic deformation ability of the material is poor. Low-stress brittle failure generally occurs at lower temperatures, and as the temperature decreases, the toughness of the material decreases sharply. In addition, with the development of low-alloy high-strength steel, the strength index continues to increase, while the plasticity and toughness have decreased. In most cases, brittle fracture starts from the welding zone, so insufficient toughness of the weld and heat-affected zone is often the main cause of low-stress brittle fracture.
(2) There are defects such as micro cracks
Fracture always starts from a defect, and cracks are the most dangerous defects. Welding is the main cause of cracks. Although cracks can basically be controlled with the development of welding technology, it is still difficult to completely avoid cracks.
(3) Certain stress level
Incorrect design and poor manufacturing processes are the main causes of welding residual stress. Therefore, for welded structures, in addition to working stress, welding residual stress and stress concentration, as well as additional stress caused by poor assembly, must also be considered.
13. What are the main factors that should be considered when designing welded structures?
Answer: The main factors to consider are as follows:
1) The welded joint should ensure sufficient stress and stiffness to ensure a long enough service life;
2) Consider the working medium and working conditions of the welded joint, such as temperature, corrosion, vibration, fatigue, etc.;
3) For large structural parts, the workload of preheating before welding and post-welding heat treatment should be reduced as much as possible;
4) The welded parts no longer require or require only a small amount of mechanical processing;
5) The welding workload can be reduced to the minimum;
6) Minimize the deformation and stress of the welded structure;
7) Easy to construct and create good working conditions for construction;
8) Use new technologies and mechanized and automated welding as much as possible to improve labor productivity; 9) Welds are easy to inspect to ensure joint quality.
14. Please describe the basic conditions for gas cutting. Can oxygen-acetylene flame gas cutting be used for copper? Why?
Answer: The basic conditions for gas cutting are:
(1) The ignition point of metal should be lower than the melting point of metal.
(2) The melting point of the metal oxide should be lower than the melting point of the metal itself.
(3) When metal burns in oxygen, it must be able to release a large amount of heat.
(4) The thermal conductivity of metal should be small.
Oxygen-acetylene flame gas cutting cannot be used on red copper, because the copper oxide (CuO) generates very little heat, and its thermal conductivity is very good (the heat cannot be concentrated near the incision), so gas cutting is not possible.
Post time: Nov-06-2023
