Wondershare EdrawMind
MindMap Gallery Mechanical Engineering-Welding Mind Map of Alloy Structures
- 68
Mechanical Engineering-Welding Mind Map of Alloy Structures
This is a mind map about mechanical engineering-alloy structure welding, including the classification and properties of alloy structural steel, the welding of hot-rolled, normalized steel and controlled-rolled steel, etc.
Edited at 2023-12-01 15:14:13
- bacteria
This is a mind map about bacteria, and its main contents include: overview, morphology, types, structure, reproduction, distribution, application, and expansion. The summary is comprehensive and meticulous, suitable as review materials.
- Plant asexual reproduction
This is a mind map about plant asexual reproduction, and its main contents include: concept, spore reproduction, vegetative reproduction, tissue culture, and buds. The summary is comprehensive and meticulous, suitable as review materials.
- Reproductive development of animals
This is a mind map about the reproductive development of animals, and its main contents include: insects, frogs, birds, sexual reproduction, and asexual reproduction. The summary is comprehensive and meticulous, suitable as review materials.
Mechanical Engineering-Welding Mind Map of Alloy Structures
- bacteria
This is a mind map about bacteria, and its main contents include: overview, morphology, types, structure, reproduction, distribution, application, and expansion. The summary is comprehensive and meticulous, suitable as review materials.
- Plant asexual reproduction
This is a mind map about plant asexual reproduction, and its main contents include: concept, spore reproduction, vegetative reproduction, tissue culture, and buds. The summary is comprehensive and meticulous, suitable as review materials.
- Reproductive development of animals
This is a mind map about the reproductive development of animals, and its main contents include: insects, frogs, birds, sexual reproduction, and asexual reproduction. The summary is comprehensive and meticulous, suitable as review materials.
- Recommended to you
- Outline
Welding of alloy structures
Classification and properties of alloy structural steel
Classification of alloy structural steel
Strength steel
Press tempering state
non-quenched and tempered steel
Hot rolled steel (yield strength 295~390MPa)
normalized steel
controlled rolling steel
Quenched and tempered steel (QT)
According to the yield strength grade and heat treatment status of steel
Hot-rolled and controlled-rolled, normalized and controlled-rolled steel
It is widely used in some stressed structures working at room temperature, such as pressure vessels, power equipment, construction machinery, bridges, building structures and pipelines, etc.
Low carbon quenched and tempered steel
The carbon content is low (generally the mass fraction of carbon is less than 0.22%), which has high strength, good plasticity and toughness. It can be welded directly in the quenched and tempered state, and does not need to be quenched and tempered after welding.
Used in large engineering machinery, pressure vessels and shipbuilding, etc.
Medium carbon quenched and tempered steel
It has a high carbon content (the mass fraction of carbon is 0.25%~0.5%) and is a heat-treated strengthened steel. The hardenability is much higher than that of low carbon quenched and tempered steel. It has high hardness and strength, but its toughness is relatively low, which brings great difficulties to welding.
Used for products or components with high strength requirements, such as rocket engine casings, aircraft landing gear, etc.
Low and medium alloy special steel
Mainly used for mechanical parts and engineering structures working under certain conditions
Pearlitic heat-resistant steel
Low and medium alloy steel based on Cr and Mo, as the working temperature increases To improve the strength, V, W, Nb, B and other alloying elements can also be added to have better high-temperature strength and high-temperature oxidation properties.
Mainly used for high temperature equipment with working temperature of 500-600℃, such as thermal power equipment and chemical equipment, etc.
low temperature steel
Most of the low-temperature steels are Ni-containing or Mi-free low-alloy steels, which are generally used in normalized or quenched and tempered states.
Mainly used in various low-temperature devices (-40~-196℃) and some engineering structures in severe cold areas, such as liquefied petroleum Gas, natural gas storage containers, etc. Compared with ordinary low alloy steel, low temperature steel must ensure that it has sufficient High low temperature toughness, no special requirements for strength
Low alloy corrosion resistant steel
In addition to general mechanical properties, it must also have the special requirement of corrosion resistance.
This type of steel is mainly used for various mechanical equipment and equipment working in rotten media such as atmosphere, seawater, petrochemical industry, etc. Welded construction. Due to different media, the types and compositions of corrosion-resistant steel are also different. The most widely used corrosion-resistant steel is steel that is resistant to atmospheric and seawater corrosion.
summary
Basic properties of alloy structural steel
chemical composition
The chemical composition of low carbon steel is: Wc=0.10%~0.25%, Wsi≤0.3%, WMn=0.5%~0.8% Elements added to low alloy steel: Mn.Si.Cr, Ni, Mo, V, Nb, B. Cu
The total mass fraction of alloy elements in low and medium alloy steel used for welded structures generally does not exceed 10%
The comprehensive influence of various elements on the lower critical point temperature A₁ (℃) of alloy structural steel can be expressed by the following formula, A₁=720 28WSi 5WCr 6WCo 3WTi-5WMn-10WNi-3WV
when it dissolves In alloy structural steel, nitrogen is widely used as an alloying element. Nitrogen plays a similar role to carbon in steel; When in iron, the Y zone will be expanded. Nitrogen can form stable nitrides with other alloying elements in steel. These nitrides Dispersed particle distribution, thereby refining the grains and improving the yield point and brittle fracture resistance of steel. The effect of nitrogen depends on its content The amount also depends on the type and amount of other alloying elements present in the steel
In addition, some alloying elements are added, such as Mn, Cr, Ni, Mo, V, Nb, B, Cu, etc., mainly to improve Hardenability of steel and tempering stability of martensite. These elements can delay the transformation of pearlite and bainite, resulting in martensitic The critical cooling rate for bulk transformation is reduced
Mechanical properties
The higher the strength of alloy structural steel, the smaller the difference between yield strength and tensile strength. The ratio of yield strength to tensile strength is called the yield ratio.
Notch toughness is an indicator of a material’s resistance to brittle failure.
Absorbed energy can reflect the transition phenomenon of sharp changes in toughness in a certain temperature range.
Microstructure
According to the different structural characteristics of the heat-affected zone, the welding heat-affected zone of low-alloy steel with non-hardenability tendency is divided into fusion zone, coarse-grained zone, fine-grained zone, incomplete recrystallization zone and tempering zone.
The microstructure in the heat affected zone of low alloy steel is mainly low carbon martensite, bainite, M-A component and pearlite-like structure, resulting in different hardness, strength properties, plasticity and toughness
Welding of hot rolled, normalized steel and controlled rolled steel
Composition and properties of hot rolled, normalized and controlled rolled steel
hot rolled steel
Ordinary low-alloy steel with a yield strength of 295 to 390MPa belongs to hot-rolled steel. This type of steel ensures the strength of the steel through solid solution strengthening of alloying elements such as Mn and Si on the basis of Wc ≤ 0.2%. , which belongs to C-Mn or Mn-Si series steel. V and Nb can also be added to achieve grain refinement and precipitation strengthening.
Hot-rolled steel is usually aluminum-killed fine-grained ferrite and pearlite steel, which is generally used in the hot-rolled state.
normalized steel
Normalizing steel is based on solid solution strengthening, adding some carbon and nitrogen compound forming elements (such as V, Nb, Ti and Mo, etc.) to strengthen and refine the grains through precipitation, further improving the strength of the steel and ensuring toughness.
The steel used in the normalized state is mainly steel containing V, N b, Ti, such as Q390, Q345, etc. The main feature is that the yield strength ratio is high.
Mo-containing steel used in normalizing and tempering conditions, such as 14 MnMoV, 18MnMoNb, etc.
Z-direction steel resistant to lamellar tearing, yield strength Rm≥343MPa
Microalloy controlled rolling steel
Steels that add trace alloying elements with a mass fraction of about 0.1% that have a significant or special impact on the structural properties of the steel are called microalloyed steels.
It uses technologies such as microalloying (adding trace amounts of Nb, V, Ti) and controlled rolling to achieve a combination of grain refinement and precipitation strengthening.
Controlled rolling steel has the advantages of high strength, high toughness and good weldability.
The main problem in the welding of controlled rolling pipeline steel is that the grain size in the overheated zone is coarse, which reduces the impact resistance. The improvement measures are to add precipitation strengthening elements (forming TiO₂, TiN) to the steel to prevent grain growth, and to optimize the welding process and specifications.
Welding properties of hot rolled, normalized and controlled rolled steels
Cold cracks and influencing factors
carbon equivalent
Hardening tendency (for any steel with a large hardening tendency, the continuous cooling transition curve will shift to the right)
The carbon equivalent of hot rolled steel with yield strength 295-390MPa is generally less than 0.4%, good weldability, except when the steel plate is very thick and the ambient temperature is very low. In addition, preheating and strict control of welding heat input are generally not required.
Normalized steel with a yield strength of 420~490MPa, such as Q420, has a tendency to harden. As the plate thickness increases, certain preheating measures need to be taken.
The Ceq of 18MnMoNb is above 0.5%, and the cold cracking sensitivity is large, which is To avoid the occurrence of cold cracks, stricter process measures need to be taken, such as strict Control heat input, preheating, post-weld heat treatment, etc.
Reducing the cooling rate is beneficial to reducing the hardenability of the heat-affected zone and the maximum hardness of the heat-affected zone, and can reduce the tendency of cold cracks
There is a direct relationship between the maximum hardness of the heat-affected zone and the probability of cracks under the weld bead.
Thermal cracking and stress crack relief
Hot cracks in welds are mainly related to the high content or severe segregation of C, S, P and other elements in hot-rolled and normalized steel.
Reheat cracks generally occur in the coarse-grained area of the heat-affected zone
Structure and toughness of non-quenched and tempered steel welds
Toughness is a property that characterizes a metal's ease of generating and propagating brittle cracks.
The toughness of the weld depends on the proportion of acicular ferrite (AF) and proeutectoid (PF) ferrite structures
The yield ratio of weld metals dominated by acicular ferrite structure is generally greater than 0.8. The yield-to-strength ratio of weld metals dominated by proeutectoid ferrite structure is usually below 0.8. When there is upper bainite in the weld metal, the yield ratio is less than 0.7
Heat affected zone embrittlement
Embrittlement of the coarse-grained area: The overheated zone of the heat-affected zone heated to above 1200°C may cause embrittlement of the coarse-grained area and the toughness will be significantly reduced.
The use of small welding heat input is an effective measure to avoid embrittlement in such hot areas.
Thermal strain embrittlement: occurs in the welding fusion zone and the subcritical heat-affected zone where the maximum heating temperature is lower than AC1
Lamellar tearing (a special form of crack that mainly occurs in thick plate structures that require penetration of corner joints or T-joints)
The occurrence of lamellar tearing is not limited by the type and strength level of steel. Considering the z-direction binding force, lamellar tearing is closely related to the plate. It is related to the thickness. Generally, lamellar tearing will not occur when the plate thickness is below 16mm.
From the nature of steel, it mainly depends on the quality of refining. The flake sulfide and layered silicate in the steel or a large number of flakes are concentrated in Oxide inclusions in the same plane reduce Z-direction plasticity, leading to lamellar tearing, among which lamellar sulfides have the most serious impact.
Sulfur content and Z-direction area shrinkage are the main indicators to evaluate the lamellar tear sensitivity of steel.
Reasonably select steel materials with low lamellar tear sensitivity and improve joint forms to reduce the stress and strain in the Z direction of the steel plate. Under the premise of meeting product usage requirements, welding materials with lower strength levels should be selected and auxiliary measures such as preheating and hydrogen reduction should be adopted. Helps prevent lamellar tears from occurring
Welding processes for hot rolled, normalized and controlled rolled steel
Grooving, assembly and tack welding
Grooving processing can be done by mechanical processing, which has high processing accuracy, or flame cutting or carbon arc gouging can be used.
The assembly gap of welded parts should not be too large, and strong assembly should be avoided as much as possible to reduce welding stress.
Selection of welding materials
1. There should be no welding defects such as cracks. 2. Can meet the performance requirements.
Select the corresponding grade of welding materials that match the mechanical properties of the base metal
Also consider the effects of fusion ratio and cooling rate
Consider the effect of post-weld heat treatment on the mechanical properties of the weld
Determination of welding parameters
Welding heat input (depending on whether cold cracking and heat-affected zone embrittlement occurs in the joint area)
Welding rod arc welding is suitable for welding seams of various irregular shapes and various welding positions.
Automatic welding, hot rolling and normalizing steel. Commonly used automatic welding methods are submerged arc welding, electroslag welding, carbon dioxide gas shielded welding, etc.
Argon arc welding, used for bottom welding of some important low alloy steel multi-layer welds, pipeline bottom welding or pipe-plate welding to ensure the welding quality at the root of the weld
Preheating and post-weld heat treatment (the purpose is mainly to prevent cracks, but also to improve the structure and performance to a certain extent)
Preheating, preheating temperature is related to factors such as the hardenability, plate thickness, restraint and hydrogen content of the steel.
Post weld heat treatment
Do not exceed the original tempering temperature of the base material to avoid affecting the performance of the base material itself.
For materials with temper brittleness, avoid the temperature range where temper brittleness occurs.
Mechanical properties of welded joints
Welding of Pearlitic Heat Resistant Steel
Pearlite heat-resistant steel is mainly composed of Cr-Mo and Gr-Mo based multi-component alloy steel, with alloying elements Cr, Mo, V, and sometimes a small amount of W, Ti, Nb, B, etc. added. The total mass fraction of alloying elements is less than 10 %
Composition and properties of pearlitic heat-resistant steel
The mass fraction of Cr in pearlitic heat-resistant steel is generally 0.5%~9%, and the mass fraction of M0 is generally 0.5% or 1%. As the Cr and Mo content increases, the oxidation resistance, high temperature strength and sulfide corrosion resistance of steel also increase.
Solid solution strengthening of the matrix: adding alloying elements to strengthen the ferrite matrix. Commonly used elements such as Cr, Mo, W, and Nb can To improve thermal strength. Among them, the solid solution strengthening effect of Mo and W is the most significant; the strengthening effect of Cr is already very significant when WCr=1%. Obviously, the strengthening effect of continuing to increase the Cr content is not significant, but the lasting strength can be improved.
Second phase precipitation strengthening In heat-resistant steel with ferrite as the matrix, the strengthening phase is mainly alloy carbide (V4C3 Or VC, NbC, TiC, etc.). The precipitation strengthening effect can be maintained up to 0.7TM (TM is the melting point), and the solid solution strengthening effect is 0.6TM, the above is significantly weakened. However, the type, shape and dispersion of carbides have a great influence on the thermal strength. Among them, body-centered cubic crystals Carbides of the system such as V4C3, NbC, TiC, etc. are the most effective; Mo2C has a certain precipitation strengthening effect when the temperature is lower than 520°C; Cr7C3 and Cr23C6. It is extremely unstable at around 540℃ and easy to aggregate.
Grain boundary strengthening adds trace elements (RE, B, Ti B, etc.) that can be adsorbed on the grain boundaries and delay the alloying elements along the grain. diffusion of grain boundaries, thus strengthening the grain boundaries
Weldability Analysis of Pearlitic Heat-Resistant Steel
Heat affected zone hardening and cold cracking
Cold cracks may occur during welding of pearlitic heat-resistant steel with high hardenability. The tendency of cracks generally increases with the increase of Cr and Mo content in the steel.
The factors that affect the occurrence of cold cracks in the welding of heat-resistant steel include the hardenability of the steel (structural factors), the diffusible hydrogen content of the weld, and the degree of restraint of the joint (stress state).
reheat crack
Reheat cracks in pearlitic heat-resistant steel appear in the coarse-grained area of the welding heat-affected zone, which is related to the welding process and welding residual stress.
Preventive measures for reheat cracking
Use welding materials with higher high-temperature plasticity than the base metal, and limit the alloy composition of the base metal and welding materials, especially Strictly limit the content of V, Ti, Nb and other alloying elements to the minimum level
Temper brittleness in heat affected zone
The brittleness of chromium-molybdenum heat-resistant steel and its welded joints occurs during long-term operation in the temperature range of 300 to 500°C, which is called temper brittleness.
2.25Cr-1Mo steel anti-temper brittleness characteristics
Whether it is brittle or not can be compared by comparing the changes in the ductile-brittle transition temperature in the impact test before and after tempering.
Low-alloy steel containing impurity elements such as P, Sb, Sn, As, etc. is easily damaged when heated for a long time in the temperature range of 375~575℃. Embrittlement occurs. The impact fracture of the embrittled specimen starts from the original austenite grain boundary. The embrittled steel is heated to a certain temperature to on, resilience can be restored
In addition to the above impurity elements, Mn, Si, Cr, and Ni also intensify embrittlement, while Mo and W can delay the embrittlement process.
For steels with the same chemical composition, the embrittlement degree decreases with different structures in the following order: martensite, bainite Pearlite. If the austenite grains are coarse, the degree of embrittlement will also be greater.
Welding process characteristics of pearlitic heat-resistant steel
Common welding methods and welding materials
Welding methods: electrode arc welding, submerged arc welding, melting and gas shielded welding, electroslag welding, tungsten arc welding, etc. can be used for welding pearlitic heat-resistant steel
Selection of welding materials: The alloy composition and strength performance of the weld metal at the operating temperature should be consistent with the corresponding indicators of the base metal, or meet the minimum performance indicators proposed by the product technical conditions.
Controlling the moisture content of welding materials is one of the main measures to prevent welding cracks, and the welding rods and fluxes used in pearlitic heat-resistant steel are easy to absorb moisture.
Preheating and post-weld treatment
Post-heat dehydrogenation treatment is one of the important measures to prevent cold cracking
Welding of medium carbon quenched and tempered steel
Composition and properties of medium carbon modulated steel
The yield strength of medium carbon modulated steel is over 880~1176 MPa
The main characteristics of medium carbon quenched and tempered steel are high specific strength and high hardness (for example, it can be used as rocket shell and armor steel, etc.). The hardenability of medium carbon quenched and tempered steel is much higher than that of low carbon quenched and tempered steel, and it can reach a very high value after heat treatment. strength and hardness, but relatively low toughness, which brings great difficulties to welding
Alloy systems for medium carbon tempered steels
40Cr
35CrMoA and 35CrMoVA
30CrMnSiA, 30CrMnSiNi2A and 40CrMnSiMoVA
40CrNiMoA and 34CrNi3MoA
Weldability Analysis of Medium Carbon Modulated Steel
Thermal cracks in welds
Medium carbon quenched and tempered steel has a high carbon content and alloying element content. When the weld solidifies and crystallizes, the solid-liquid temperature range is large, and the tendency of crystallization segregation is serious. Crystallization cracks are prone to occur during welding, and it has a greater sensitivity to hot cracks.
Welding materials with low carbon content and low S and P impurities should be used as much as possible
Hardenability and cold cracking
The hardening tendency of medium carbon quenched and tempered steel is very obvious, and the hard and brittle martensite structure is prone to appear in the welding heat-affected zone, which increases the tendency of cold cracking in the welded joint area.
The higher the carbon content of the base metal, the greater the hardenability and the greater the tendency of welding cold cracks.
Embrittlement and Softening of the Heat Affected Zone
Embrittlement in the heat-affected zone. Medium-carbon quenched and tempered steel has a considerable hardening tendency due to its higher carbon content and more alloying elements. It has a low martensite transformation temperature and no "self-tempering" process, but should be used during welding. The heat-affected zone is prone to produce a large amount of brittle and hard martensite structure, resulting in embrittlement of the heat-affected zone.
The heat-affected zone softens. When a steel material that is in a quenched and tempered state before welding is heated above the tempering temperature of the quenched and tempered steel during welding, a softened zone will appear in the welding heat-affected zone with lower strength and hardness than the base metal.
Welding process characteristics of medium carbon modulated steel
Welding in annealed or normalized condition
Medium carbon quenched and tempered steel is best welded in the annealed or normalized state. After welding, the overall modulation process is used to obtain a welded joint with satisfactory performance.
When selecting welding materials, in addition to the processing requirements to ensure that no hot and cold cracks occur, there are also some special requirements, that is, the modulation and processing specifications of the weld metal should be consistent with that of the base metal to ensure that the joint performance after modulation is also the same as the base metal.
In the case of post-weld quenching and tempering, the determination of welding parameters is mainly to ensure that no cracking occurs before the quenching and tempering treatment, and the joint performance This can be guaranteed by post-weld heat treatment. Therefore, very high preheating temperature (200~350℃) and interlayer temperature can be used. in addition, In many cases, it is often too late to carry out quenching and tempering treatment immediately after welding. In order to ensure that the welded joint is cooled to room temperature and treated at the quenching and tempering place. In order to avoid delayed cracks before welding, an intermediate heat treatment must be carried out promptly after welding. This heat treatment is generally maintained for a period of time at or above the preheating temperature after welding. The purpose is to achieve two aspects: Prevent delayed cracks: first, it plays the role of diffusion and removal of hydrogen; second, it transforms the structure into a structure with low sensitivity to cold cracking.
When using local preheating, the preheating temperature range should be no less than 100 mm from both sides of the weld. If it cannot be tempered in time after welding, it should be tempered at 680°C.
Welding in quenched and tempered state
Embrittlement and hardening caused by high carbon martensite can be solved by post-weld tempering treatment
In order to prevent welding cold cracks, austenitic electrodes with good plasticity and toughness can also be used
For welding that must be in a modulated state, the smallest possible welding heat input should be used
Welding methods and welding materials
Welding method Commonly used welding methods for medium carbon quenched and tempered steel include arc welding, gas shielded welding, submerged arc welding, etc. Using methods such as pulsed argon arc welding, plasma arc welding and electron beam welding with concentrated heat, It is beneficial to reduce the width of the welding heat-affected zone, obtain fine-grained structure, and improve the mechanical properties of the welded joint. Some thin plate welding methods mostly use gas shielded welding, tungsten arc welding and micro-beam plasma arc welding.
Welding materials Medium carbon quenched and tempered steel welding materials should use low carbon alloy system to reduce the S and P impurity content of the weld metal to ensure the toughness, plasticity and strength of the weld metal and improve the crack resistance of the weld metal. For components that require heat treatment after welding, the chemical composition of the weld metal should be similar to that of the base metal. Welding materials should be selected according to the stress conditions, performance requirements and post-weld heat treatment conditions of the weld.
Preheating and post-weld heat treatment Preheating and post-weld heat treatment are important process measures for medium-carbon quenched and tempered steel. Whether to preheat and the level of preheating temperature depend on the structure of the weldment and production conditions. In addition to being less restrictive, Except for thin-walled shells or weldments with simple structures that do not require preheating, in general, preheating or timely postheating measures must be taken when welding medium carbon quenched and tempered steel. The preheating temperature is generally 200~350℃.
Welding of low carbon quenched and tempered steel
Types, composition and properties of low carbon tempered steel
Generally speaking, the effect of alloying elements on the plasticity and toughness of steel is opposite to its strengthening effect.
After the steel is quenched, whether it is tempered at high temperature or tempered at low temperature, it is called "quenched and tempered". The steel that has been heat treated by "quenching and tempering" is called "quenched and tempered steel".
Low carbon steel (mass fraction of carbon is not greater than 0.22%)
High-strength structural steel (Rm=600~800MPa) is mainly used in engineering welded structures. Welds and welding areas mostly bear tensile loads.
High-strength wear-resistant steel (Rm≥1000MPa) is mainly used for high-strength wear-resistant engineering structures and parts that are required to withstand impact and wear resistance.
High-strength and high-toughness steel (Rm≥700MPa, this kind of steel requires high strength and high toughness at the same time, and is mainly used for high-strength and high-toughness welded structures)
Weldability Analysis of Low Carbon Modulated Steel
The mass fraction of low carbon quenched and tempered carbon steel does not exceed 0.18%, and the welding performance is much better than that of medium carbon quenched and tempered steel.
Weld strength and toughness matching (weld strength matching coefficient S=(Rm)w/(Rm)b) is one of the parameters that characterizes the mechanical heterogeneity of the joint.
When (Rm)w/(Rm)b>1, it is called a “super strong match”
When (Rm)w/(Rm)b=1, it is called “equally strong matching”
When (Rm)w/(Rm)b<1, it is called "low-strength matching"
cold crack
The alloying principle of low carbon modulated steel is based on low carbon, by adding a variety of alloying elements to improve hardenability to ensure high strength and good toughness low carbon "self-tempering" martensite and part of lower bainite. mixed tissue
Hot cracking and reheat cracking
Influence of chemical composition: C content is low, Mn content is high, and the content of S, The control of P is also stricter, so the tendency of hot cracking is smaller. However, steel types with high Ni and low Mn have a certain sensitivity to hot cracking. It is related to C, Mn/S and N, and is mainly produced in the overheated area of the heat affected zone. Called liquefaction cracks.
The greater the welding heat input, the coarser the grains in the heat-affected zone, and the greater the grain boundary melting. Seriously, the longer the liquid intergranular layer between crystal grains exists, the more liquid The greater the tendency of cracking. To prevent the occurrence of liquefaction cracks: small heat input and control should be adopted in the process. Control the shape of the molten pool and reduce the concavity of the fusion zone
V has the greatest impact on reheat cracking, followed by Mo. When V and Mo are the same It will be more sensitive when added. The impact of Cr is related to content (1%)
Heat affected zone performance changes
Microstructure characteristics of heat affected zone of quenched and tempered steel
Heat affected zone embrittlement
Under the action of welding heat cycle, when t8/5 continues to increase, the overheated zone of the heat affected zone of low carbon modulated steel is prone to embrittlement, that is, the impact toughness is significantly reduced.
The cause of embrittlement in the heat affected zone is not only the coarsening of austenite grains, but also the formation of upper bainite and M-A components.
Heat affected zone softening
The peak temperature of the heat-affected zone of low-carbon modulated steel is higher than the tempering temperature of the base metal to Ac1, and softening (strength and hardness are reduced) will occur.
Welding process characteristics of low carbon modulated steel
1. It is required that the cooling rate during martensite transformation should not be too fast, so that the martensite has a self-tempering effect to prevent the occurrence of cold cracks. 2. The cooling rate between 800 and 500℃ is required to be greater than the critical speed for producing brittle mixed structure.
Welding methods and selection of welding materials
Prevent cracks
While ensuring that high strength requirements are met, the toughness of the weld metal and heat-affected zone is improved.
Generally, automated or semi-automatic mechanized welding methods such as gas metal arc welding or active gas arc welding are used.
Selection of welding parameters
Determination of welding heat input
The determination of welding heat input E is based on the requirements for crack resistance and toughness of the heat affected zone.
For low-alloy steel with low carbon content, increasing the cooling rate (reducing heat input) has formed low-carbon martensite, which is beneficial to ensuring toughness.
Preheating temperature and post-weld heat treatment
The purpose of preheating is to reduce the cooling rate during martensite transformation and improve the crack resistance through the "self-tempering" effect of martensite.
Low carbon quenched and tempered steel welded structures are generally used in the welded state, and post-weld heat treatment is not performed under normal circumstances. Unless the strength and toughness of the joint area after welding are too low, Post-weld heat treatment is only carried out when the welded structure is under heavy stress or is subjected to stress corrosion and requires high-precision processing after welding to ensure the structural dimensions.
Mechanical properties of low carbon quenched and tempered steel welded joints
The embrittlement elements harmful to the weld metal of low carbon modified steel are S, P, N, O, and H, which must be restricted.