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: formation of crystal nucleus and growth of crystal nucleus.
When the liquid metal in the welding pool solidifies, the semi-molten grains on the base metal in the fusion zone usually become nuclei.
Then the crystal nucleus adsorbs the atoms of the surrounding liquid to grow.
Because the crystal grows in the direction opposite to the direction of heat conduction, it also grows in the direction of both sides.
However, because it is blocked by the adjacent growing crystals, the crystal forming a columnar shape is called columnar crystal.
In addition, under certain conditions, the liquid metal in the molten pool will also produce self-generating nuclei during solidification.
If the heat dissipation is carried out in all directions, the crystals will grow evenly into grain-like crystals in all directions, which is called equiaxed crystals.
Columnar crystals are usually seen in the weld joint. Under certain conditions, equiaxed crystals will also appear in the center of the weld joint.
2. What are the characteristics of the secondary crystallization structure of the weld joint?
Answer: for the structure of weld joint metal, after the primary crystallization, the metal continues to cool below the phase transformation temperature, and the metallographic structure changes.
For example, when low carbon steel is welded, the primary crystallization grains are austenite grains.
When it is cooled below the phase transformation temperature, austenite decomposes into ferrite and pearlite.
Therefore, most of the structure after secondary crystallization is ferrite plus a small amount of pearlite.
However, due to the faster cooling speed of the weld joint, the pearlite content obtained is generally larger than that in the equilibrium structure.
The faster the cooling speed, the higher the pearlite content, the less the amount of ferrite, the higher the hardness and strength, and the lower the plasticity and toughness.
After secondary crystallization, the actual structure at room temperature was obtained.
The weld microstructure of different steels is different under different welding process conditions.
3. Take low carbon steel as an example to explain what structure is obtained after secondary crystallization of weld joint metal?
Answer: take low plastic steel as an example, the primary crystallization structure is austenite, and the solid-state phase transformation process of weld joint metal is called secondary crystallization of weld joint metal.
The microstructure of secondary crystallization is ferrite and pearlite.
In the equilibrium structure of low carbon steel, the carbon content of 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 can not precipitate completely according to the iron carbon phase diagram.
As a result, the content of pearlite is generally gentle, and the content in the microstructure is large.
High cooling rate will also refine the grain, and improve the hardness and strength of the metal. The decrease of ferrite and the increase of pearlite will also 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 welding process, the microstructure of weld joint metal is fine, so the microstructure and properties of weld joint metal are better than those in the casting state.
4. What are the characteristics of dissimilar metal welding?
1) The characteristics of dissimilar metal welding mainly lie in the obvious difference between the alloy composition of the deposited metal and the weld joint.
The behavior of the welding pool is also 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.
Therefore, the melting amount of the base metal 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 also change.
It can be seen 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 different welding processes.
2) After welding thermal cycle, different metallographic structures will appear in each area of the welded joint, which is related to the chemical composition of base metal and filler material, welding method, welding level, welding process and heat treatment.
3) The non-uniformity of performance, due to the different chemical composition and metal structure of the joint, leads to the different mechanical properties of the joint.
There are great differences in strength, hardness, plasticity and toughness along each area of the joint.
In the heat affected zone on both sides of the weld joint, the impact value is even several times different.
The creep limit and rupture strength at high temperature will also vary greatly due to different composition and microstructure.
4) The distribution of residual stress in dissimilar metal joints is uneven, which is mainly determined by the different plasticity of each region of the joint.
In addition, the difference of material thermal conductivity will cause the change of welding thermal cycle temperature field.
The different coefficient of linear expansion in each region is the reason for the uneven distribution of stress field.
5. What are the selection principles of welding materials for dissimilar steel welding?
Answer: the selection principles of dissimilar steel welding materials mainly include the following four points:
1) On the premise of no crack and other defects in the welded joint, if the strength and plasticity of the weld joint metal can not be considered, the welding material with good plasticity shall be selected.
2) The weld joint metal properties of dissimilar steel welding materials meet only one of the two base metals, that is, they are considered to meet the technical requirements.
3) Welding materials shall have good process performance and beautiful weld joint formation. Welding materials are economical and easy to purchase.
6. How is the weldability of pearlite steel and austenitic steel?
Answer: pearlitic steel and austenitic steel are two kinds of steel with different structures and compositions.
Therefore, these two kinds of steels are welded together, and the weld joint metal is fused by two different types of base materials and filler materials, which puts forward the following problems for the weldability of these two kinds of steels:
1) Dilution of the weld joint.
Due to the low gold content of pearlitic steel, it has a dilution effect on the alloy of the whole weld joint metal.
Due to this dilution effect of pearlitic steel, the content of austenite forming elements in the weld joint is reduced.
As a result, martensite structure may appear in the weld joint, which worsens the quality of the welded joint and even cracks.
2) Form transition layer.
Under the action of the welding thermal cycle, the mixing degree of molten base metal and filler metal is different at the edge of the molten pool.
At the edge of the molten pool, the liquid metal has low temperature, poor fluidity and short residence time in the liquid.
Due to the difference in chemical composition 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.
Results in the weld joint on one side of pearlitic steel, the proportion of pearlitic base metal is larger, and the closer it is to the fusion line, the larger the proportion of base metal is.
This forms a transition layer with a different internal composition of weld joint metal.
3) A fusion zone diffusion layer is formed.
In the weld joint metal composed of these two types of steels, due to the high carbon content of pearlitic steel, but high alloy elements and less alloy elements, on the contrary of austenitic steel, the concentration difference of carbon and carbide forming elements is formed on one side and two sides of pearlitic steel in the fusion zone.
When the joint works for a long time at a temperature higher than 350-400 ℃, there will be obvious carbon diffusion in the fusion zone, that is, it diffuses from the pearlite steel side to the austenitic weld through the fusion zone.
Results a decarburization softening layer was formed on the base metal of pearlite steel near the fusion zone, and a carburizing layer corresponding to decarburization was formed on one side of the austenitic weld joint.
4) Because the physical properties of pearlitic steel and austenitic steel are very different, and the composition in the weld joint is also very different, this kind of joint can not eliminate the welding stress by heat treatment, but can only cause the redistribution of stress, which is very different from the welding of the same metal.
5) Delayed crack. During the crystallization process of the welding pool of this kind of dissimilar steel, there are both austenitic structure and ferrite structure.
They are close to each other. The gas can diffuse so that the diffused hydrogen can accumulate and produce 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:
1) The condition of the welded casting, such as the chemical composition, microstructure and mechanical properties of the casting, the size, thickness and structural complexity of the casting.
2) Defects of castings.
Before welding, the type of defect (crack, lack of meat, wear, air hole, sand hole, insufficient pouring, etc.), the size of the defect, the stiffness of the position, the cause of the defect, etc. shall be understood.
3) Quality requirements after welding, such as mechanical properties and machinability of welded joints. Understand the requirements of weld joint color and tightness.
4) Site equipment conditions and economy.
Under the condition of ensuring the quality requirements after welding, the most basic purpose of casting welding repair is to use the simplest method, the most common welding equipment and process equipment, and the lowest cost to make it play a greater economic benefit.
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 not only reduce the white mouth tendency of the weld joint, but also reduce the welding stress and prevent the weldment from cracking.
(2) Arc cold welding is adopted to reduce welding stress.
Welding materials with good plasticity are selected, such as nickel, copper, nickel copper and high vanadium steel, so that the weld metal can relax stress through plastic deformation and prevent cracks.
Fine diameter electrode, low current and intermittent welding (intermittent welding) are used.
Dispersed welding (skip welding) can reduce the temperature difference between the weld joint and the base metal and reduce the welding stress.
By hammering the weld, the stress can be eliminated and cracks can be prevented.
(3) Other measures:
Adjust the chemical composition of weld joint metal to reduce its brittleness temperature range;
Rare earth elements are added to enhance the desulfurization and dephosphorization metallurgical reaction of the weld, and powerful grain refining elements are added to refine the weld grain.
In some cases, the heating method is used to reduce the stress at the welding repair place, which can also effectively prevent the occurrence of cracks.
8. What is stress concentration? What are the factors causing stress concentration?
Answer: due to the shape and characteristics of the weld joint, the discontinuity of the collective shape appears.
When loaded, it causes the uneven distribution of the working stress of the welded joint, resulting in the local peak stress σ Max specific average stress σ M is much higher, which is stress concentration.
There are many reasons for stress concentration in welded joints, among which the main reasons are:
(1) Process defects, air holes, slag inclusions, cracks and incomplete penetration in the weld joint, among which the stress concentration caused by welding cracks and incomplete penetration is the most serious.
(2) Unreasonable weld joint shape, such as excessive reinforcement of butt weld and excessive toe of fillet weld
Unreasonable street design, such as sudden change of street interface, butt joint with cover plate, etc.
Unreasonable weld arrangement will also produce stress concentration, such as T-joints with only store welds.
9. What is plastic failure and what is its harm?
Answer: plastic failure includes plastic instability (yield or significant plastic deformation) and plastic fracture (edge fracture or ductile fracture).
The process is that the welded structure first produces elastic deformation under load → yield → plastic deformation (plastic instability) → micro crack or micro void → macro crack → instability propagation → fracture.
Compared with brittle fracture, the damage of plastic failure is relatively small, including the following:
(1) Unrecoverable plastic deformation occurs after yielding, which makes the welded structure with high dimensional requirements scrapped.
(2) For pressure vessels made of high toughness and low strength materials, the failure is not controlled by the fracture toughness of the materials, but the plastic instability failure caused by insufficient strength.
The ultimate result of plastic failure is the failure of welded structure or catastrophic accident, which affects the production of enterprises, causes unnecessary casualties, and seriously affects the development of the national economy.
10. What is brittle fracture and what is its harm?
Answer: generally, brittle fracture refers to dissociation fracture (including quasi dissociation fracture) and grain boundary (intergranular) fracture splitting along a certain crystal surface.
Cleavage fracture is a kind of intracrystalline fracture formed by separation along a certain crystallographic plane.
Under certain conditions, such as low temperature, high strain and high stress concentration, cleavage fracture will occur when the stress reaches a certain value.
There are many models about the generation of cleavage fracture, most of which are related to dislocation theory.
It is generally believed that when the plastic deformation process of the material is seriously hindered, the material can not comply with the applied stress by deformation, but by separation, resulting in cleavage cracks.
Inclusions, brittle precipitates and other defects in metals also have an important influence on the generation 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 extends to the whole structure instantaneously.
It has the nature of sudden damage and is not easy to find and prevent in advance. Therefore, it often causes personal casualties and huge loss of property.
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 will occur near the crack front, and there is a certain amount of opening displacement at the tip, which makes the crack 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 the high tensile stress area, it will often cause the brittle fracture of the whole structure.
If the extended crack enters the area with low tensile stress, it is reputed that there is enough energy to maintain the further expansion of the crack, or the crack enters the material with good toughness, (or the same material but with higher temperature and increased toughness) receives greater resistance and cannot continue to expand.
At this time, the harm of crack will be 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
Especially at the tip of the notch, the micro deformation ability of the material is poor.
Low stress brittle failure usually occurs at lower temperature, and the toughness of the material decreases sharply with the decrease of temperature.
In addition, with the development of low alloy high strength steel, the strength index is rising, but the plasticity and toughness are reduced.
In most cases, brittle fracture starts from the welding zone, so the insufficient toughness of weld and heat affected zone is often the main reason for low stress brittle failure.
(2) There are microcracks and other defects
The fracture always starts from the defect, and the crack is the most dangerous defect. Welding is the main cause of cracks.
Although with the development of welding technology, cracks can be basically controlled, it is difficult to completely avoid cracks.
(3) Certain stress level
Incorrect design and poor manufacturing process are the main causes of welding residual stress. Therefore, for welded structures, in addition to working stress, welding residual stress, stress concentration and additional stress caused by poor assembly must also be considered.
13. What main factors should be considered when designing welded structures?
Answer: the main considerations are as follows:
1) The welded joint shall have enough emphasis and rigidity to ensure long enough service life;
2) Consider the working medium and working conditions of welded joints, such as temperature, corrosion, vibration, fatigue, etc;
3) The workload of pre welding preheating and post welding heat treatment shall be reduced as much as possible for large structural parts;
4) Weldments may no longer require or require only a small amount of machining;
5) The amount of welding work can be minimized;
6) The deformation and stress of welded structure shall be minimized;
7) Easy to construct and create good working conditions for construction;
8) Try to adopt new technology and mechanized and automatic welding to improve labor productivity;
9) The weld is convenient for inspection to ensure 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 metal shall be lower than the melting point of metal,
(2) The melting point of metal oxide shall be lower than that of the metal itself,
(3) A large amount of heat can be released when metals are burned in oxygen,
(4) The thermal conductivity of metal shall be small.
Oxygen acetylene flame gas cutting can not be used for red copper, because copper oxide (CuO) generates little heat, and its thermal conductivity is very good (the heat cannot be concentrated near the notch), so it cannot be gas cut.
15. What is the main function of gas welding powder?
The main function of welding powder is slagging. It reacts with metal oxides or non-metallic impurities in the molten pool to form slag.
At the same time, the generated slag covers the surface of the molten pool and isolates the molten pool from the air, which prevents the continuous oxidation of the molten pool metal at high temperature.
16. What are the process measures to prevent weld porosity in manual arc welding?
(1) Electrodes and fluxes shall be kept dry and dried as required before use;
(2) The surface of welding wire and weldment shall be kept clean without water, oil stain and rust.
(3) Correctly select welding specifications, such as welding current should not be too large, welding speed should be appropriate, etc;
(4) Adopt correct welding methods, use alkaline electrode for manual arc welding, short arc welding, reduce the swing range of electrode, slow down the speed of electrode transportation, and control the start and end of short arc;
(5) Control that the assembly clearance of weldments shall not be too large;
(6) Electrodes with cracked coating, peeling, deterioration, eccentricity and corrosion of welding core shall not be used.
17. What are the main measures to prevent white cast iron during cast iron welding?
(1) Graphitized electrodes are used, that is, cast iron electrodes with a large amount of graphitized elements (such as carbon, silicon, etc.) added to coatings or welding wires, or nickel based and copper based cast iron electrodes;
(2) Preheating before welding, heat preservation during welding and slow cooling after welding to reduce the cooling rate of the weld area, prolong the time when the fusion area is in the red hot state, fully graphitize and reduce thermal stress;
(3) Brazing process is adopted.
18. Describe the role of flux in the welding process?
In welding, flux is the main factor to ensure welding quality, which has the following functions;
(1) After melting, the flux floats on the surface of the molten metal to protect the molten pool and prevent the erosion of harmful gases in the air.
(2) The flux can deoxidize and alloying, and cooperate with the welding wire to obtain the required chemical composition and mechanical properties of the weld metal.
(3) Make the weld joint well-formed.
(4) Slow down the cooling rate of molten metal and reduce defects such as pores and slag inclusion.
(5) Prevent splashing, reduce loss and improve bonding coefficient.
19. What should be paid attention to in the use and maintenance of AC arc welding machine?
(1) It shall be used according to the rated welding current and load duration of the welding machine, and shall not be overloaded.
(2) The welder shall not be short circuited for a long time.
(3) The regulating current shall operate at no load.
(4) Frequently check the wire contact, fuse, grounding, regulating mechanism, etc. and try to make them intact.
(5) Keep the welding machine clean, dry and ventilated to prevent the invasion of dust and rain.
(6) Place stably and cut off the power after work.
(7) The welding machine shall be overhauled regularly.
20. What are the hazards of brittle fracture?
Answer: because brittle fracture occurs suddenly, there is no time to find and prevent it.
Once it occurs, the consequences are very serious, which will not only cause significant economic losses, but also endanger human life safety.
Therefore, the brittle fracture of welded structures is a problem that should be paid great attention to.
21. Characteristics and application of plasma spraying?
Answer: plasma spraying is characterized by high plasma flame temperature, which can melt almost all refractory materials, so it has a wide range of spraying objects, high plasma flame flow speed and good powder particle acceleration effect, so the coating bonding strength is high.
It has a wide range of applications and is the best way to spray all kinds of ceramic materials.
22. Preparation procedure of welding process card?
Answer: the preparation procedure of welding process card shall find out the corresponding welding process qualification and draw the joint diagram according to the product assembly drawing, parts processing drawing and its technical requirements;
Give the welding process card number, drawing number, joint name, joint number, welding process qualification number and welder certificate items;
Prepare welding sequence according to welding procedure qualification, actual production conditions, technical elements and production experience;
Prepare specific welding process parameters according to welding process qualification; Determine the product inspection authority, inspection method and inspection proportion according to the requirements of product drawings and product standards.
23. Why should a certain amount of silicon and manganese be added to the welding wire of carbon dioxide gas shielded welding?
A: carbon dioxide is an oxidizing gas, which will burn the alloy elements of the welding seam during the welding process, thus greatly reducing the mechanical properties of the weld.
Among them, oxidation will lead to pores and splash.
Adding silicon and manganese into the welding wire can deoxidize and solve the problems of welding oxidation and splash.
24. What is the explosion limit of the combustible mixture and what factors affect it?
Answer: the concentration range of combustible gas, vapor or dust contained in combustible mixture is called explosion limit.
The lower limit of the contained concentration is called the lower explosion limit, and the upper limit of the contained concentration is called the upper explosion limit.
The explosion limit is affected by temperature, pressure, oxygen content and vessel diameter. When the temperature increases, the explosion limit decreases;
When the pressure increases, the explosion limit will also decrease;
When the concentration of oxygen in the mixed gas increases, the lower explosion limit decreases.
For combustible dust, its explosion limit is affected by dispersion, humidity, temperature and other factors.
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) During 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 shall be set outside the container, and a switch shall be set to cut off the power supply according to the welder’s signal.
(3) The voltage of the portable lamp used in the container shall not exceed 12V. The shell of the portable lamp transformer shall be reliably grounded, and the autotransformer is not allowed.
(4) Transformers for portable lamps and welding transformers shall not be carried into boilers and metal containers.
26. How to distinguish between fusion welding and brazing? What are the characteristics of the two?
Answer: fusion welding is characterized by the combination of atoms between weldments, while brazing is to connect weldments with solder, an intermediate medium with a lower melting point than weldments.
The advantages of fusion welding are the high mechanical properties of welded joints and high productivity when connecting thick and large parts.
The disadvantages are large stress and deformation and microstructure changes in heat-affected zone;
The advantages of brazing are low heating temperature, flat and smooth joint, beautiful appearance, small stress and deformation, and the disadvantages are 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?
Answer: carbon dioxide is an oxidizing gas.
When it is used as the shielding gas in the welding area, it will fiercely oxidize the droplet and molten pool metal, cause the burning loss of alloy elements, and have poor manufacturability, resulting in pores and large splash.
Therefore, at present, it can only be used to weld low carbon steel and low alloy steel, not high alloy steel and non-ferrous metals, especially stainless steel.
It is less used because it will cause weld joint carburization and reduce the resistance to intergranular corrosion.
Argon is an inert gas. Because it does not react with molten metal, the chemical composition of the weld joint is basically unchanged.
The weld quality after welding is good and can be used to weld all kinds of alloy steel, stainless steel and non-ferrous metals.
Because the price of argon is gradually decreasing, it is also used to weld low carbon steel.
28. Describe the weldability and welding characteristics of 16Mn steel?
Answer: 16Mn steel adds about 1% Mn on the basis of Q235A steel, and the carbon equivalent is 0.345% ~ 0.491%.
Therefore, the welding performance is good.
However, the hardening tendency is slightly larger than that of Q235A steel.
When welding with small parameters and small pass on large thickness and large rigidity structure, cracks may occur, especially under low temperature conditions.
At this time, proper preheating can be adopted before welding.
E50 welding rod shall be used for manual arc welding;
H08MnA welding wire can be used with flux 431 when automatic submerged arc welding does not have groove;
H10Mn2 welding wire and flux 431 shall be used for beveling;
During CO2 gas shielded welding, welding wire H08Mn2SiA or H10Mnsi shall be used.