1 What is welding? What are the common types of welding methods?
A process known to join a workpiece by heating or pressurizing, or a combination of both, and with or without a filler material, is referred to as welding.
The workpiece can be made of various metal materials of the same kind or different types, such as metal, non-metal materials (plastic, graphite, ceramic, glass, etc.), or a metal and a non-metal material. Metal welding has a wide range of applications in modern industry, so in a narrow sense, welding generally refers to the welding of metallic materials.
According to the state of the metal materials in the welding process, the welding methods are currently classified into the following three categories:
(1) Fusion welding In the welding process, the weldment joint is heated to a molten state, and the method of completing the welding without pressure is called fusion welding. Commonly used welding methods are arc welding, gas welding, electroslag welding, and the like.
(2) Pressure welding During the welding process, pressure (heating or no heating) must be applied to the weldment to complete the welding method called pressure welding. Commonly used pressure welding methods include resistance welding (butt welding, spot welding, seam welding), friction welding, rotating arc welding, ultrasonic welding, and the like.
(3) In the brazing welding process, a metal material having a lower melting point than the base material is used as the brazing material, and the weldment and the brazing material are heated to a temperature higher than the melting point of the brazing material and lower than the melting point of the base material, and the base material is wetted by the liquid brazing material. The method of filling the joint gap and inter-diffusion with the base material to connect the weldment is called brazing. Common brazing methods include flame brazing, induction brazing, furnace brazing, salt bath brazing, and vacuum brazing.
2 What gases are in the weld zone? What is the source?
During the welding process, the welding zone is filled with a large amount of gas. When welding with acid electrode, the main gas components are CO, H2, H2O; when welding with alkaline electrode, the main gas components are CO and CO2; when submerged arc welding, the main gas components are CO and H2.
The gas in the welding zone is mainly derived from the following aspects: First, in order to protect the welding area from air intrusion, artificially add a protective gas in the welding area, such as gas generating agent in the coating (starch, wood flour, marble). Etc.) Gas generated by thermal decomposition, protective gas used for gas shielded welding (CO2 gas, Ar gas), etc.; secondly, when it is welded with a wet electrode or flux, the gas evolved, the air that is not protected from intrusion, the welding wire, and Impurities (oil, rust, paint, etc.) on the surface of the base metal are heated by heat, and gases generated by high temperature evaporation of metal and slag.
3 Describe the effect and influence of nitrogen, hydrogen and oxygen on weld metal
(1) Nitrogen Nitrogen mainly comes from the air around the weld area. In hand arc welding, the surfacing metal contains approximately 0.025% nitrogen. Nitrogen is an element that increases the strength of weld metal, reduces ductility and toughness, and is one of the main causes of pores in welds.
(2) Hydrogen Hydrogen is mainly derived from the electrode coating, the moisture in the flux, the organic matter in the coating, the dirt on the weldment and the surface of the wire (rust, oil), and moisture in the air. All kinds of welding methods increase the hydrogen in the weld, but the degree of hydrogen increase is different: the weld seam welded with the cellulose coating electrode in hand arc welding is 70 times higher than the base material; only the low hydrogen type electrode is used. When welding, the hydrogen content of the weld is relatively low; while with CO2 gas shielded welding, the hydrogen content is the lowest.
Hydrogen severely degrades the plasticity of the weld metal, causing porosity and time-delay in the welded joint, and also forms white spots on the section of the tensile specimen.
(3) Oxygen Oxygen is mainly derived from oxides, moisture and oxides on the surface of solder materials in air, coating and flux. As the oxygen content in the weld increases, its strength, hardness and plasticity decrease significantly, which can also cause hot brittle, cold brittle and age hardening of the metal, and is also the main cause of the formation of pores (CO pores) in the weld. One.
In short, nitrogen, hydrogen, and oxygen entering the weld metal are all harmful elements.
4 Why is the welding area protected? How to protect?
The purpose of protecting the welded area is to prevent air from intruding into the droplets and the molten pool, and to reduce the nitrogen and oxygen content in the weld metal. There are three ways to protect:
(1) Gas protection For example, gas shielded welding uses a shielding gas (CO2, H2, Ar) to isolate the welded area from the air.
(2) Slag protection The molten metal surface is covered with a layer of molten slag to separate it from the air, such as electroslag welding and submerged arc welding.
(3) Gas-slag joint protection The protective gas and slag are simultaneously protected by molten metal, such as hand arc welding.
5 How to reduce the oxygen content in the weld metal?
Protecting the weld zone and preventing the air from coming into contact with the molten metal is an important measure to control the oxygen content in the weld metal, but it cannot solve the problem fundamentally, because oxygen can also enter the weld through many other channels, and these channels should be completely blocked. In fact, it is impossible, so currently only measures can be taken to deoxidize the oxygen that has entered the molten metal.
6 What are the common deoxidation methods for weld metal?
The use of slag or core (wire) metal to interact with molten metal for deoxidation is a common deoxidation method for weld metal.
(1) Diffusion deoxidation When the temperature drops, the FeO originally melted in the molten pool will continuously diffuse to the slag, thereby reducing the oxygen content in the weld. This deoxidation method is called diffusion deoxidation.
If there is a strong acidic oxide SiO2, TiO2, etc. in the slag, they will form a complex with FeO, and the reaction formula is
(SiO2+FeO)= FeO·SiO2
(TiO2+FeO)= FeO·TiO2
As a result of the reaction, the free FeO in the slag is reduced, which causes the [FeO] in the molten pool metal to continuously diffuse into the slag, and the content in the weld metal is thus reduced.
The acidic slag (such as the slag formed by melting the electrode J422 and the flux HJK431) contains a large amount of SiO2 and TiO, so the deoxidation method is mainly diffusion deoxidation. However, under the welding conditions, due to the fast cooling rate of the molten pool, the interaction time between the slag and the liquid metal is short, and the diffusion deoxidation is insufficient, so the weld seam welded by the acid electrode (agent) has an oxygen content. Higher, the weld metal has lower plasticity and toughness.
(2) Deoxidation with a deoxidizer adds an element to the core, the coating or the wire to cause it to be oxidized during the welding process, thereby ensuring that the metal to be welded and its alloy elements are not oxidized or the metal that has been oxidized is reduced. This element for deoxidation is called a deoxidizer. Commonly used deoxidizers are carbon, manganese, silicon, titanium and aluminum.
The deoxidizer of the alkaline electrode is added to the coating in the form of an iron alloy, such as ferromanganese, ferrosilicon, and the like.
Submerged arc welding often uses alloy welding wire, such as H08MnA, H10MnSi and so on.
The effect of deoxidizing with deoxidizer is much better than that of diffusion deoxidation. Therefore, the weld welded by alkaline electrode has lower oxygen content than that of acid electrode, and the plasticity and toughness are correspondingly improved. Therefore, the basic electrode is commonly used. To weld alloy steel and important welded structures.
7 How to reduce the hydrogen content in the weld metal?
Common measures to reduce the hydrogen content in weld metal are:
1) drying the flux of the electrode;
2) Remove impurities on the weldment and the surface of the wire;
3) Add appropriate amount of fluorspar (CaF2) and silica sand (SiO2) to the coating and flux, both of which have good dehydrogenation effect;
4) Immediately after welding, the weldment is heated and post-heat treated;
5) Low hydrogen type electrode, ultra low hydrogen type electrode and alkaline flux.
8 Explain the hazard of sulfur in weld metal. How to desulfurize?
Sulfur is one of the most harmful elements in the weld. Sulfur promotes hot cracking of the weld metal, reduces impact toughness and corrosion resistance, and promotes segregation. Sulfur can also cause lamellar tears when thick plates are welded. Sulfur exists in the form of FeS in the liquid metal, and Mn, MnO and CaO in the slag have a certain desulfurization effect; the reaction formula is as follows
[Mn]+[FeS] =[MnS]+[Fe]
[MnO]+[FeS]=[MnS]+[FeO]
[CaO]+[FeS] =[CaS]+[FeO]
The generated MnS and CaS enter the slag. Since MnO and CaO are both alkaline oxides and are contained in the alkaline slag, the desulfurization ability of the alkaline slag is higher than that of the acidic slag.
Strong.
9 Explain the hazards of phosphorus in welding metals. How to dephosphorize?
Phosphorus is also one of the most harmful elements in the weld. Phosphorus increases the cold brittleness of the steel, greatly reduces the impact toughness of the weld metal, and increases the brittle transition temperature. Phosphorus also causes hot cracking in the weld metal when the austenitic steel or weld has a high carbon content.
Phosphorus is present in the liquid metal in the form of Fe2P, P2O5. The dephosphorization reaction can be carried out in two steps: the first step is to oxidize phosphorus to P2O5; the second step is to form a stable complex with the basic oxide CaO in the slag into the slag. Its reaction formula is
2[Fe2P]+5(FeO=P2O5+11[Fe]
P2O5+3(CaO)=(CaO)3·P2O5
P2O5+4(CaO)=(CaO)4·P2O5
Since the alkaline slag contains more CaO, the dephosphorization effect is better than the acidic slag.
However, in fact, whether it is alkaline slag or acidic slag, the final desulfurization and dephosphorization effects are still not satisfactory. Therefore, the current control of sulfur and phosphorus content in the weld can only be restricted.
Method for sulfur and phosphorus content in raw materials (base metal, welding rod, welding wire).
10 What is the alloying of weld metal? What are the common alloying methods?
Alloying is the transition of the required alloying elements into the weld metal (or surfacing metal) through the welding material.
The purpose of alloying: 1) compensate for the loss of alloying elements due to oxidation, evaporation, etc. during the welding process; 2) improve the microstructure and properties of the weld metal; 3) obtain surfacing metal with special properties. Commonly used alloying methods include: application of alloy welding wire; application of flux cored wire or flux cored electrode; application of alloy coating or bonding flux; application of alloy powder; application between slag and metal
Displacement reaction.
11 What is the transition factor of alloying elements?
During the welding process, some of the alloying elements are lost due to oxidation, evaporation, etc., and it is impossible to completely transition into the weld. The transition coefficient of the alloying element refers to the percentage of the alloying element in the welding material that transitions to the surfacing metal and its original content, ie
CF
η=───
CT
Where η - the transition coefficient (%) of an alloying element;
CF—the content of an alloying element in the surfacing metal;
CT—The original total content of an alloying element in the electrode (wire, flux).
When the hand arc welding adopts different welding rod models, the transition coefficient η of the alloy elements and the transition coefficient of the alloy elements of the basic electrode (low sodium hydroxide type) are higher than those of the acidic electrode (titanium calcium type).
12 What is the primary crystallization of the weld pool? What are the characteristics?
After the heat source leaves, the process of transforming the metal of the weld pool from a liquid state to a solid state is called primary crystallization of the weld pool. The primary crystallization of the weld pool has the following characteristics:
(1) The volume of the molten pool is small and the cooling rate is large. When arc welding, the volume of the molten pool is about 30 cm 3 at the maximum, and the mass of the liquid metal is not more than 200 g (single wire automatic submerged arc welding). Because the volume of the molten pool is small and the surrounding area is surrounded by cold metal, the cooling rate of the molten pool is very high, up to 100 ° C / s, which is hundreds to 10,000 times larger than the cooling rate of the ingot, which makes the carbon A high-volume steel with a large amount of alloying elements exhibits hardened hard structure (martensite) and crystal cracks in the welded joint.
(2) The liquid metal in the molten pool is in a superheated state For low-carbon and low-alloy steels, the average temperature of the molten pool during arc welding is (1770±100) °C, which exceeds the melting point of the material and is in an overheated state. Therefore, the burning loss of alloying elements is more serious.
(3) When the molten pool is crystallized and welded under motion, the molten pool moves at the same speed with the heat source, and the melting and crystallization of the metal in the molten pool is simultaneously performed, that is, the first half of the molten pool is in the melting process, and the latter half is at the same time. During the crystallization process, the molten metal in the molten pool crystallizes under motion.
13 What is segregation? What kinds of segregation occurs in the weld?
The phenomenon in which the constituent elements of the alloy are unevenly distributed during crystallization is called segregation. During the primary crystallization of the weld pool, due to the fast cooling rate, the chemical composition of the solidified weld metal is too late to spread, resulting in uneven distribution and segregation.
There are three types of segregation in the weld:
(1) In the primary crystallization of the microsegregation bath, the crystallographic center metal of the first crystal is the purest, the latter crystal part contains slightly higher alloying elements and impurities, and the final crystal part, that is, the outer and leading edges of the crystal, and other alloying elements The highest impurity. The phenomenon of uneven distribution of chemical composition inside a columnar grain and between grains is called microsegregation.
(2) Regional segregation When the molten pool is crystallized at one time, due to the continuous growth and movement of the columnar crystals, the impurities will be "caught" to the center of the molten pool, so that the impurity content in the center of the molten pool is more than other parts. This phenomenon is called regional segregation. The cross-sectional shape of the weld has a great influence on the distribution of regional segregation. Narrow and deep welds, the boundary of each columnar crystal is at the center of the weld, so the center of the weld is concentrated with more impurities, and the weld is highly susceptible to thermal cracking at its center. Wide and shallow welds, impurities accumulate in the upper part of the weld, see Figure 1b, this weld has a high resistance to thermal cracking.
(3) Layered segregation In the process of primary crystallization, the latent heat of crystallization is continuously released. When the latent heat of crystallization reaches a certain value, the crystallization of the molten pool temporarily pauses. Later, as the molten pool dissipates heat, the crystallization begins again, forming periodic crystals, accompanied by periodic fluctuations in the concentration of impurities in the liquid metal at the crystallization front, and periodic segregation is called layered segregation. Layered segregation concentrates some harmful elements, so defects often appear in layered segregation. Porosity caused by layered segregation.
14 How to improve the primary crystal structure of the weld? What is metamorphic treatment?
By adding some alloying elements such as V, Mo, Ti, Nb, A1, B, N, etc. to the molten pool by welding materials (solders, fluxes), the crystal grains can be refined to obtain a fine-grained structure, thereby ensuring strength and Plasticity, which can improve crack resistance, is called metamorphism. The modification treatment is very effective for improving the primary crystal structure of the weld. For example, the E5015MoV electrode is based on the original E5015 electrode, and a small amount of ferromolybdenum and vanadium iron is added to the coating, which has higher crack resistance.
15 What is the secondary crystallization of the weld metal?
After the completion of one crystallization, the molten pool is transformed into a solid weld. When the high temperature weld metal cools to room temperature, it undergoes a series of structural phase transformation processes called secondary crystallization of the weld metal.
At the end of the secondary crystallization of the low carbon steel weld metal, the structure is ferrite plus pearlite. It can be seen from the iron-carbon alloy state diagram that ferrite accounts for about 82%, pearlite accounts for about 18%, and weld metal has a hardness of about 83 HBS. However, the iron-carbon alloy state diagram is obtained under the extremely slow cooling condition of the material. In fact, the cooling rate of the weld metal during secondary crystallization is relatively fast, so the pearlite content in the structure increases, and the cooling rate is higher, the pearlite The more the content, the hardness and strength of the weld increase. For example, when the cooling rate of the weld metal is 110 ° Cs, the hardness can reach 96 HBS, which is why when the weld metal is low carbon steel, cooling Although the quenched structure does not appear, the hardness will increase.
16 Why can multi-layer multi-pass welding improve the plasticity of weld metal?
Multi-layer multi-pass welding can improve the quality of weld metal, especially plasticity, because the back layer (channel) weld has a heat treatment effect on the front layer (channel) weld, which is equivalent to the front layer (channel) weld A normalizing treatment was performed, thus improving the secondary structure. For the last weld, an additional weld bead can be applied to the weld. In some factories, when the bending specimen test of the welded joint fails, the measures to change the original welding process parameters are adopted, and the single-layer weld is changed into a multi-layer weld, and the rapid welding is performed with a small current to improve the bending test. The test pass rate (plasticity index) has a certain effect.
It should be noted that multi-layer multi-pass welding is effective in improving the quality of hand arc welding. In submerged arc welding, since the thickness of each layer of weld bead can reach 6~10mm, the thermal effect of the next layer of weld is only 3~mm, so the heat treatment effect is poor.
17 What is the welding thermal cycle? What are the main parameters of the welding thermal cycle?
The process of changing the temperature of a point on a weldment with time under the action of a welding heat source is called the welding thermal cycle at that point.
When the heat source approaches the point, the temperature of the point increases until the maximum value is reached; as the heat source leaves, the temperature gradually decreases, and the whole process can be represented by a curve.
This curve is called the welding thermal cycle curve, see Figure 3. Obviously, the thermal cycles experienced are different at different points on the sides of the weld from the weld. The closer to the weld, the higher the maximum temperature reached by heating, and the farther the point is heated. The lower the temperature.
The main parameters of the welding thermal cycle are the heating rate, the maximum temperature reached by heating, the time spent above the tissue transition temperature, and the cooling rate.
18 What is the welding line energy? How to calculate?
In fusion welding, the energy input from the welding energy source to the weld per unit length is called the welding line energy and is expressed as
IU
q= ───
Ï…
Where I - welding current (A);
U——Arc voltage (V);
Υ——welding speed (cm/s);
q——Line energy (J/cm).
For example, the plate thickness is 12 mm, and the double-sided I-shaped groove submerged arc welding is performed, the welding wire is Ñ„4 mm, I=650 A, U=38 V, and Ï…=0.9 cm/s. , the welding line energy q is
IU 650×38
q = ── = ────── = 27444 J/cm
Ï… 0.9
The line energy combines the effects of welding process parameters, arc voltage and welding speed on the welding thermal cycle. When the line energy is increased, the width of the heat-affected zone is increased, the area heated to a high temperature is widened, the residence time at a high temperature is increased, and the cooling rate is slowed down.
19 How to choose line energy when welding?
In the production, according to different material composition, under the premise of ensuring good weld formation, the welding process parameters are properly adjusted and welded with appropriate line energy to ensure good performance of the welded joint. For example, when the weldment is assembled and positioned, the weld length is short, the cross-sectional area is small, the cooling speed is fast, and the weld is prone to cracking, especially for some steels with a higher tendency to harden. The line energy is welded to prevent cracking of the weld. However, for low-alloy steels and low-temperature steels with higher strength grades, the line energy must be strictly controlled because the increase in line energy leads to a decrease in the ductility and toughness of the welded joint. Especially when welding austenitic stainless steel, in order to improve the corrosion resistance of the welded joint, it is necessary to use the process parameters of small current and rapid welding to keep the line energy at the lowest value.
20 What is preheating? What is the role of preheating?
The process of heating the entire weldment or the weld zone locally before welding is called preheating. For steels with a high level of weld strength, a tendency to harden, a material with particularly good thermal conductivity, a weld with a large thickness, and when the ambient temperature around the weld is too low, it is often necessary to preheat the weld before welding. The main purpose of preheating is to reduce the cooling rate of the welded joint. The preheating temperature is shown in Table 3. As can be seen from the table, preheating can reduce the cooling rate, but does not substantially affect the time spent at high temperatures, which is ideal. Therefore, when welding a steel material having a tendency to harden, the main process measure for lowering the cooling rate and reducing the tendency of hardening is to perform preheating instead of increasing the line energy.
21 What is the interlayer temperature? How to correctly choose the interlayer temperature?
When multi-layer welding is applied to the weldment, the minimum temperature of the weld before the weld is called the inter-layer temperature. For materials requiring preheating welding, when multi-layer welding is required, the interlayer temperature should be equal to or slightly higher than the preheating temperature. If the interlayer temperature is lower than the preheating temperature, the preheating should be performed again. When welding the low-profile stainless steel, in order to maintain the high corrosion resistance of the welded joint, a relatively fast cooling speed is required. Therefore, it is necessary to control the lower interlayer temperature at this time, that is, the front weld is cooled to a lower temperature. Then, the welding of the subsequent welds is carried out.
22 What is the welding affected zone? What are its characteristics?
In the welding (or cutting) process, the area where the base metal of the weld adjacent to the weld is affected by heat (but not melted) and the mechanical properties of the metallographic structure change is called the weld heat affected zone.
In fusion welding, the welded joint consists of two parts that are interconnected and whose structure and properties are different, namely the weld zone and the heat affected zone. Practice has shown that the quality of the welded joint is determined not only by the weld zone, but also by the heat affected zone to a considerable extent. Sometimes the problem of the heat affected zone is more complicated than the weld zone, especially when alloy steel is welded. . Therefore, it is of great significance to study and master the changes in the microstructure and properties of the heat affected zone during the welding process.
23 Describe the characteristics of the weld heat affected zone of solid unstructured transition materials.
Solid metal unstructured transition of pure metals (such as A1, Cu, Ni, MoT W, etc.) and single-phase solid solution alloys (such as Zn mass fraction <39% alpha brass, Ni-Cu alloy and ultra-low carbon chromium nickel austenite Body stainless steel and ultra-low carbon high-chromium pure ferritic stainless steel, etc.) do not undergo structural transformation during heating and cooling, so the welding heat affected zone is very simple, only the superheat zone and the recrystallization zone (the base metal is cold before welding) Rolled state) two sections.
(1) Superheated zone Since there is no structural transformation during the cooling process of such materials, the crystal grains grown during the heating process will not undergo recrystallization refinement caused by the structural transformation during the cooling process, so the grains in the superheated zone It grows very large and cannot be refined by heat treatment (such as normalizing of steel). The plasticity and toughness of the material in the overheated zone are very poor. For this reason, small line energy should be used for welding, and repeated welding should be prevented as much as possible to prevent the grain from growing longer.
(2) Recrystallization zone If the base metal is cold-rolled before welding, there is a recrystallized zone with finer grains between the superheated zone and the base metal after welding. However, in the recrystallization zone, since the base material structure in the cold-rolled state is recrystallized, the cold work hardening effect in the original cold rolling process completely disappears, so the strength is lowered but the plasticity is improved.
If the base metal is in annealed state after hot rolling or after cold rolling, there is no recrystallization zone in the heat affected zone after welding.
24 Describe the characteristics of the weld heat affected zone of pure metal or single phase alloy with solid isomeric transformation.
Metals such as Fe, Mn, Ti, and Co are pure metals with solid isomeric transitions in the solid state, and single-phase alloys with isomeric transformations formed by these metals. The heat affected zone can be divided into superheated zones and heavy Crystallization zone, incomplete recrystallization zone (single phase alloy) and several sections of recrystallization zone.
In particular, in addition to the superheat zone and recrystallization zone mentioned in the 23 questions, there is a recrystallization zone caused by the homologous transformation, which is located between the superheat zone and the recrystallization zone. The grain refinement caused by the transformation of the recrystallized structure, that is, the fine grain structure obtained after the normalizing treatment of the steel material,
This section has a high impact toughness.
If the base metal is a single-phase alloy, such as α-Ti and pure Ti, there is only one isomeric transformation of α β in the solid state, which are both β phases at high temperatures, and at low temperatures.
The alpha phase, except that the homologous transformation of the pure metal is carried out at a fixed temperature, and the allo-isomeric transformation of the single-phase alloy is carried out in a certain temperature range, so the heat is The recrystallization zone of the affected zone can be further divided into two parts, a recrystallization zone II and an incomplete recrystallization zone II'.
In addition, some pure metals with isomeric transformations, such as Ti and Co, single-phase alloys such as α-Ti, will produce martensite transformation under rapid cooling conditions, such as pure Ti and α-Ti alloys, rapid cooling. At the time of welding heat affected zone, β→α′ transition can be found, and α′ is called titanium martensite.
25 Describe the characteristics of the weld heat affected zone of hard-to-harden steel.
Hard-to-harden steel, such as low-carbon steel and low-alloy high-strength steel with less alloying elements (16Mn, 15MnTi, 15MnV steel), in addition to homogenous isomerization in the solid state alloy, there are compositional changes and second phase precipitation , ie, eutectoid transformation and precipitation of Fe3C, the welding heat affected zone can be divided into four sections: superheat zone, recrystallization zone, incomplete recrystallization zone and recrystallization zone.
(1) Superheated zone (also known as coarse grain zone) This zone is adjacent to the weld. The temperature range is from the temperature at which the grain grows sharply to the temperature range of the solidus line, and is 1100 to 1490 °C for the low carbon steel. The ferrite and pearlite in the base metal of this zone all become austenite, and the austenite grains grow very coarse. After cooling, the impact toughness of the metal is greatly reduced, generally 25% to 30% lower than the base metal. Is a weak link in the heat affected zone.
(2) Recrystallization zone (also known as normalized zone or fine grain zone) Refers to the zone below the superheat zone, where the heating temperature is above A3, and 900 to 1100 °C for mild steel. After air cooling, uniform and fine ferrite and pearlite are obtained, which is equivalent to the normalized structure in the heat treatment. Because of the fine and uniform grain size, the recrystallization zone has both high strength and good plasticity and toughness, which is the best comprehensive mechanical properties in the heat affected zone. However, since the performance of the entire welded joint depends on the weakest area in the joint, the performance of the area is good, but it does not work.
(3) Incomplete recrystallization zone (also known as incomplete normalization zone or partial phase change zone) refers to the zone where the heating temperature is between Ac1 and Ac3, and is 750 to 900 °C for low carbon steel. In the base metal of the area
All pearlite and part of the ferrite are transformed into fine austenite grains, but some ferrite is still retained. When cooled, the austenite transforms into fine ferrite and pearlite, while the ferrite that is not dissolved in austenite does not change, and the crystal grains are coarse, so the grain size of the cooled tissue is extremely uneven. Therefore, the mechanical properties are not uniform and the strength is reduced.
(4) Recrystallization zone means a zone where the heating temperature is between 450 ° C and Ac1 and 450 to 750 ° C for the low carbon steel. For the base material which has been subjected to press working, that is, plastic deformation, the crystal grains are broken, and in this temperature region, they become complete crystal grains again, which is called recrystallization. There is no homogeneous transformation in the region, and there is no change in the structure. Therefore, the mechanical properties of the metal change little, and only the plasticity is slightly improved. For the base metal that has not been plastically deformed before welding, this area does not appear.
26 What is Wei's organization? How does it affect the performance of welded joints?
In the overheated zone in the heat-affected zone of hard-to-harden steel, because the austenite grains grow very coarse, this coarse austenite forms a special over-structure at a faster cooling rate, and its microstructure is characterized by A plurality of parallel ferrite needles are formed in a coarse austenite grain, and the remaining austenite between the ferrite needles is finally converted into pearlite. This superheated structure is called Wei's structure. The Wei's structure is not only coarse in grain, but also due to the fragile surface formed by a large number of ferrite needles, the toughness of the metal is drastically reduced, which is one of the main reasons why the hardened steel welded joint becomes brittle.
The formation of the Wei's organization depends on the degree of superheat in the superheated zone, that is, the time the metal stays at high temperatures. In the case of hand arc welding, the heat-affected zone stays at a high temperature for a short period of time, and the grain grows and
Not serious; while electroslag welding, the heat-affected zone stays at high temperature for a long time, and the grain grows seriously. Therefore, electroslag welding is prone to coarse Weiss structure than hand arc welding. For the same welding method, the higher the line energy used during welding, the longer the residence time at high temperature, the more severe the overheating, the coarser the austenite grains grow, the easier it is to obtain the Wei's structure, and the performance of the welded joint is The worse, this is a major problem that causes the performance of the heat affected zone to deteriorate when the mild steel is welded.
27 Describe the characteristics of the weld heat affected zone of easily hardened steel.
Easy-hardened steel includes carbon steel (35, 40, 45, 50 steel), low carbon quenched and tempered high strength steel (ωC (1) ≤ 0.25%), medium carbon quenched and tempered high strength steel (ωC is 0.25% to 0.45%), heat resistant steel and In low-temperature steel, the heat-affected zone can also obtain martensite under welding air-cooling conditions and is in a quenching state. If the base metal is in an annealed state before welding, the microstructure of the heat affected zone after welding can be divided into two sections: a completely quenched zone and an incompletely quenched zone. If the base metal is quenched before welding, a tempering zone is formed. .
(1) Complete quenching zone Refers to the section where the heating temperature exceeds Ac3. After the welding, the austenite is completely transformed into martensite, including the superheating zone and the recrystallization zone in the heat affected zone of the low carbon steel welding. Since the quenched structure exists in this zone, the strength and hardness are increased, the ductility and toughness are lowered, and cold cracks are likely to occur.
(2) Incomplete quenching zone refers to the section where the heating temperature is between Ac1 and Ac3. After welding, the austenite transforms into martensite, the original ferrite remains unchanged, only grows to varying degrees, and finally martensite is formed. - the organization of ferrite. The structure and properties of this section are very uneven, with reduced plasticity and toughness.
(3) Tempering zone If the base metal is in a quenching state before welding, a tempering process of a different degree occurs in a section where the temperature is lower than Ac1, which is called a tempering zone. The hardness of the tempering zone decreases and the plasticity increases.
28 Describe the carbon diffusion and its effects in the heat affected zone of dissimilar steel welding.
When the dissimilar steel is welded, the base material composition and the weld bead composition are greatly different, and the carbon will diffuse from the base metal to the weld bead, and a “decarburization layer†of 1 to 2 grain widths is formed in the vicinity of the base material fusion line.
A "carbonation layer" appears correspondingly on the weld side. The factors that promote the diffusion of carbon from the base metal to the weld are:
When the weld is in a liquid state, since the deep solution of carbon in the liquid metal is greater than that of the solid metal, the carbon is caused to migrate and migrate from the base metal near the weld line to the weld.
(1) Heating temperature and time have a great influence on the diffusion of carbon. In the dissimilar steel joints of Q235-A and Cr25Ni13, the decarburization layer was only found when heated to 350 °C. When it was added above 550 °C, the decarburization layer was remarkable. After 600 °C, it was more serious, especially At 800 ° C. The effect of heating temperature and time on the width (B) of the decarburized layer during welding of Q235-A and Cr25Ni13 dissimilar steel is shown in Fig. 8. Therefore, it is generally difficult to form a carbon diffusion layer during single pass welding, which is usually obvious when the joint is subjected to post-weld heat treatment or high temperature long-term work.
The carbon diffusion layer is a weak link in the welded joint of dissimilar steel. It has little effect on the instantaneous mechanical properties of the joint at normal temperature and high temperature, but it will reduce the high temperature endurance of the joint, which is generally reduced by about 10% to 20%.
29 What defects will occur during the crystallization of the weld pool?
The resulting defects are:
(1) Crystal crack (solidification crack) During the crystallization of the weld pool, the tensile stress generated by the metal shrinkage causes the low-melting eutectic liquid film on the grain boundary to be pulled apart to cause crystal cracking. Crystal cracks mainly occur in weld metal containing more impurities, single-phase austenitic steel, nickel-based alloy, aluminum alloy, molybdenum alloy, and the like.
(2) Porosity A large amount of hydrogen, oxygen, and nitrogen are melted in the weld pool at high temperatures. During the rapid cooling process, these gases do not come out, and remain in the weld metal (internal or surface) to form holes.
(3) Cinder slag The phenomenon that the welding slag remains in the weld metal.
(4) Segregation Due to the rapid crystallization rate of the weld pool, the distribution of chemical elements in the weld is uneven, resulting in segregation.
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