References:
Welding metallurgy-material welding machinery industry press
Meng Qingsen, Basic Chemical Industry Press, Weldability of Metals.
Metallurgical and Mechanical Industry Press Cui Zhongqi Qin Yaochun
Metal Technology Harbin Institute of Technology Press Xing Renxue
Li rongxue, metal material welding process machinery industry press
Metal material welding process chemical industry press Lei Yucheng
Jing Hongyang, structural steel welding metallurgical industry press.
Development and application of low alloy steel
With the development of science and technology, the design of welded structure is developing in the direction of high parameter, light weight and large scale, which puts forward higher and higher requirements for the performance of steel. Low alloy steel has been widely used in welded structures because of its excellent properties and remarkable economic benefits.
The development of low alloy steel has roughly gone through three stages. Before the 1920s, steel structures were mainly manufactured by riveting, and the design parameters were mainly tensile strength. The strengthening of steel mainly depends on carbon and single alloying elements, such as manganese, silicon and chromium. , the total mass fraction reached 2%~3%, or even higher. In the 1920s and 1960s, welding technology was gradually adopted in the manufacture of steel structures, and the requirements of yield strength, toughness and weldability of materials should be considered in the design parameters. In order to prevent welding cracks, the chemical composition of steel is low carbon multi-element alloying. Generally, the mass fraction of carbon is below 0.2%, and it contains 2~4 kinds of alloying elements which are beneficial to welding. It is paved by heat treatment, strengthening and other technological measures. After 1970s, low-alloy high-strength steel developed rapidly, and the carbon content in steel dropped below 0. 1%, and some steels developed to ultra-low carbon content. Ti, V, Nb and other alloying trace elements have gradually attracted attention and are developing in the direction of multi-element composite alloying.
Great progress of modern low alloy steel. Since 1970s, the development of low-alloy high-strength steel in the world has entered a new era. Based on controlled rolling technology and microalloying metallurgy, a new concept of modern low-alloy high-strength steel, namely microalloyed steel, has been formed. In 1980s, with the help of metallurgical technical achievements, the variety development involving a wide range of industrial fields and special materials reached its peak. In the four-in-one relationship of chemical composition, process, microstructure and properties of steel, the dominant position of steel microstructure and microstructure is highlighted for the first time, which also shows that the basic research of low alloy steel has matured and alloy design has been carried out with an unprecedented new concept.
Application of low alloy steel and low alloy steel in buildings and bridges. Construction machinery and other industries can not be widely used. When alloy steel is used in important welded structures such as bridges, offshore buildings and hoisting machinery, the requirements of impact toughness should be put forward according to the lowest temperature of the structure. For low-alloy structural steel working in atmospheric environment, the impact absorption work (0℃, V-notch impact sample) should meet the requirements of at least 27 J.
For the moving structures of vehicles, ships and construction machinery, reducing self-weight can save energy and improve carrying capacity and industrial efficiency. Therefore, the use of low carbon quenched and tempered steel with good weldability can promote the development of engineering structures in the direction of mass production, lightweight and high efficiency. Due to the reduction of wall thickness, weight and welding workload, it creates conditions for on-site construction and hoisting. This kind of steel has good strength, toughness and comprehensive properties, which can greatly improve the durability of equipment and prolong its service life. WCF-80 steel is a high strength welded structural steel developed after WCF-62 in China. This kind of steel has high cold cracking resistance and low temperature toughness, and is mainly used in large hydropower stations, petrochemicals and open-pit coal mines.
Low-carbon quenched and tempered steel with tensile strength of 700MPa has good notch impact toughness, and can be used for welding structures in low temperature service such as large excavators and electric wheel dump trucks in open-pit coal mines. Low-carbon quenched and tempered steel with tensile strength of 800MPa is mainly used to manufacture construction machinery and mining machinery, such as bulldozers, construction cranes, heavy trucks, roller drilling rigs and so on. Low-carbon quenched and tempered steel with tensile strength above 10000MPa is mainly used for high-strength and wear-resistant parts of construction machinery, nuclear power plants and navigation and aerospace equipment.
2. Brief introduction of low carbon steel
Low alloy steel is made by adding a certain amount of alloying elements on the basis of carbon steel, and the mass fraction of alloying elements is generally less than 5%, which is used to improve the strength of steel, ensure its plasticity and toughness, or make steel have some special properties, such as low temperature resistance, high temperature resistance or corrosion resistance. Low-alloy steels commonly used to make welded structures can be divided into four types: high-strength steel, low-temperature steel, corrosion-resistant steel and pearlite heat-resistant steel. Among them, high-strength steel is the most widely used. According to the yield strength of steel and the heat treatment state when it is used, it can be divided into the following three types:
A welding of low-alloy high-strength structural steel with yield strength of 295 ~ 490 MPa is used for hot rolling, controlled cooling and controlled rolling and normalizing (or normalizing+tempering).
B. Low-carbon and low-alloy quenched and tempered steel with yield strength of 490 ~ 980 MPa is welded and used in quenched and tempered state.
C.w(C) is 0.25 ~ 0.50%, and the yield strength is 880 ~ 1 176 MPa.
The classification of steel in the standard is based on its mechanical properties. The grade of steel consists of three parts: the Chinese phonetic letter Q representing the yield point, the yield point value and the quality grade symbol. According to the yield strength of steel, low-alloy high-strength steel is divided into five strength grades, which are 295MPa, 345MPa, 390MPa, 420MPa and 460MPa respectively. According to the requirements of impact absorption work, each strength grade is divided into five quality grades: A, B, C, D and E, which respectively represent different impact toughness requirements.
W(c) in low-alloy high-strength steel is generally controlled below 0.20%. In order to ensure the strength and toughness of steel, the steel has excellent comprehensive mechanical properties by adding appropriate alloying elements Mn and Mo and microalloying elements V, Nb, Ti and al. Adopt appropriate rolling process or heat treatment process. Low-alloy high-strength steel is widely used to manufacture pressure vessels, vehicles, bridges, buildings, machinery, offshore structures and ships because of its good weldability, excellent formability and low manufacturing cost, and has become one of the most important structural materials in large-scale welded structures.
The strengthening mechanism of low alloy high strength steel is different from that of carbon steel. Carbon steel is mainly strengthened by the formation of pearlite, bainite and martensite by carbon content in steel. The strengthening of low-alloy high-strength steel is mainly realized by grain refinement, precipitation hardening and substructure change.
Low alloy steel with yield strength of 295 ~ 390 MPa is mostly hot rolled steel, and its high strength is obtained by solid solution strengthening of alloying element manganese. For example, when Q345 steel is used as steel or thick plate structure for low-temperature pressure vessels, it can also be used after normalizing to improve low-temperature toughness. The microalloyed low alloy steels such as Q345 and Q390 are based on Q345 steel, and a small amount of Nb(0.0 15%~0.06%) or V(0.02%~0.20%) which can refine grains and strengthen precipitation is added. The properties of these steels are unstable in hot rolling state. Normalization treatment makes their grains refined and carbides uniformly dispersed and precipitated, thus obtaining higher plasticity and toughness. Therefore, it is more reasonable to adopt the normalized Q345 and Q390 steels.
Low alloy steel with yield strength greater than 390MPa generally needs to be normalized or normalized and tempered, such as Q420. Carbon and nitrogen compounds formed after normalizing are precipitated from solid solution in the form of fine particles, which improves the strength of steel and ensures certain plasticity and toughness. With the further improvement of steel strength, it is necessary to add a certain amount of Mo to steel. Mo can not only refine the structure and improve the strength, but also improve the medium temperature properties of steel.
Low-alloy high-strength steel can also be divided into boiler steel, pipeline steel, container steel, shipbuilding steel and bridge steel according to their uses. In addition, there is Z-direction steel with good delamination resistance in normalized steel, which is mainly used in large thick plate structures such as offshore oil production platforms, nuclear reactors and submarines.
3. The following mainly introduces the weldability of low alloy high strength steel.
Low-alloy high-strength steel contains a certain amount of alloying elements and microalloying elements, and its weldability is different from that of carbon steel, mainly because the change of microstructure and properties in welding heat affected zone is sensitive to welding heat input, and the hardening tendency of heat affected zone is increased, which is sensitive to hydrogen-induced cracking. Low-alloy high-strength steel containing carbon and nitrogen compound forming elements is also at risk of reheat cracking. Only on the basis of mastering the weldability characteristics and laws of different low-alloy high-strength steels can the correct welding process be formulated to ensure the welding quality of low-alloy high-strength steels.
1) Microstructure and properties of welding heat affected zone
According to the different peak temperature of the welding heat affected zone, the welding heat affected zone can be divided into fusion zone (1350 ~ 1450℃), coarse-grained zone (1000 ~ 1300℃) and fine-grained zone (800 ~1) The microstructure and properties of heat affected zone in different parts depend on the chemical composition of steel and the heating and cooling rates during welding. For some low-alloy high-strength steels, if the welding cooling rate is not properly controlled, hardened or brittle structures will be produced locally in the welding heat affected zone, which will lead to the decrease of crack resistance or toughness.
When welding low-alloy high-strength steel, the coarse grain zone in the heat-affected zone heated above 1 100℃ and the incomplete phase transformation zone heated to 700 ~ 800℃ are the two weak areas of the welded joint. When welding hot rolled steel, if the welding heat input is too large, the toughness of coarse grain area will be reduced due to serious grain growth or widmanstatten structure. If the welding heat input is too small, the toughness will decrease due to the increase of martensite proportion in coarse-grained structure. When welding normalized steel, the welding heat input has a greater influence on the microstructure and properties of coarse grain zone. In the incomplete phase transformation zone of the welding heat affected zone, only a part of carbon-rich components in this zone undergo austenite transformation during welding heating, and in the subsequent welding cooling process, this part of carbon-rich austenite will be transformed into high-carbon twin martensite, and the final transformation temperature (Mf) of this part of high-carbon martensite is lower than room temperature, and a considerable part of austenite remains around the martensite island, forming the so-called M-A component. The formation of M-A component is the main reason for the embrittlement of this region. The measures to prevent embrittlement in incomplete phase transformation zone are to control the welding cooling rate and avoid the generation of brittle martensite.
Softening of welding heat affected zone is the main problem encountered in welding controlled rolling and controlled cooling steel. When submerged arc welding, electroslag welding and flash butt welding are used, the softening of welding heat affected zone of controlled rolling and controlled cooling steel becomes very prominent. The softening of welding heat affected zone makes the strength of welded joint obviously lower than that of base metal, which damages the fatigue performance of welded joint. In addition, welding heat input also affects the microstructure and toughness of heat affected zone of controlled rolling and controlled cooling steel. When welding with small wire energy, the lower bainite structure is obtained in the heat affected zone due to the faster cooling rate of welding, which has good toughness. However, with the increase of welding heat input, the welding cooling rate decreases, and the upper bainite or side plate ferrite structure is obtained in the heat affected zone, which significantly reduces the toughness.
2) thermal strain embrittlement
In C-Mn low alloy steel with high free nitrogen content, thermal strain embrittlement often occurs in the fusion zone of welded joints and the subcritical heat affected zone with the highest heating temperature lower than Ac 1. It is generally believed that this embrittlement is caused by nitrogen and carbon atoms gathering around dislocations and pinning them. Thermal strain embrittlement easily occurs in the subcritical heat affected zone, and the maximum heating temperature ranges from 200℃ to 400℃. If there is notch effect, thermal strain embrittlement is more serious, and notch defects often exist in the fusion zone. When continuous welding thermal strain is applied around the defect, thermal strain embrittlement tends to be greater due to strain concentration and unfavorable structure, so thermal strain embrittlement also easily occurs in the fusion zone. In the paper "Study on Thermal Strain Brittleness in Welding Zone of Domestic Low Alloy Structural Steel Q345 and Q420", the thermal strain embrittlement of Q345 and Q420 steel is analyzed, and it is found that Q345 steel has a great tendency of thermal strain embrittlement. It is considered that V and N in Q420 steel form nitrides, thus reducing the tendency of thermal strain embrittlement, while Q345 steel does not contain nitride forming elements. After annealing at 600℃× 1h, the toughness of Q345 steel embrittled by thermal strain is remarkably restored.
3) cold crack sensitivity
Welding hydrogen-induced crack (usually called welding cold crack or delayed crack) is the most easily produced and harmful process defect in welding of low-alloy high-strength steel, and is often the main reason for the failure of welding structure. Hydrogen-induced cracks in low-alloy high-strength steel mainly appear in the heat affected zone of welding, and sometimes in weld metal. According to the type of steel, hydrogen content and stress level in the welding zone, hydrogen-induced cracks may appear immediately below 200℃ after welding or within a period of time after welding.
A large number of studies show that low-alloy high-strength steel is sensitive to hydrogen-induced cracks when hardened M or M+B+F structure is produced in the heat affected zone of welding. However, when B or B+F structure is produced, it is insensitive to hydrogen-induced cracks. The highest hardness of heat affected zone can be used to roughly evaluate the sensitivity of hydrogen-induced cracking in welding. For general low-alloy high-strength steel, in order to prevent hydrogen-induced cracking, the hardness of welding heat affected zone should be controlled below 350HV. The hardening tendency of heat affected zone can be evaluated by carbon equivalent formula.
The hardening tendency of hot rolled steel with lower strength grade is slightly greater than that of low carbon steel because of less alloying elements. For example, when welding Q345 steel and 15MnV steel, hardened martensite structure may appear during rapid cooling, and the tendency of cold cracking will increase. However, due to the low carbon equivalent of hot rolled steel, there is usually little tendency to cold crack. However, when the ambient temperature is very low or the thickness of steel plate is large, measures should be taken to prevent cold cracks.
The carbon content and carbon equivalent of controlled rolling and controlled cooling steel are very low, and its cold crack sensitivity is low. Except for the ultra-thick welding structure, the welding of 490MPa grade controlled rolling and controlled cooling steel generally does not need preheating.
The content of alloying elements in normalized steel is high, and the hardening tendency of welding heat affected zone increases. For normalized steel with low strength grade and carbon equivalent, the tendency of cold cracking is not great. However, with the increase of strength grade and plate thickness, its hardening ability and cold cracking tendency increase. It is necessary to take measures such as controlling welding heat input, reducing hydrogen content, preheating and timely post-heating to prevent cold cracking.
4) Thermal crack sensitivity
Compared with carbon steel, low-alloy high-strength steel has lower w(C) and w(S), higher w(Mn) and less hot cracking tendency. However, sometimes there will be hot cracks in the weld, for example, in the welding production of thick-walled pressure vessels, hot cracks are easy to appear in the root bead of multi-layer and multi-pass submerged arc welding or the high dilution bead near the edge of the groove; In electroslag welding, if the base metal contains high carbon content and Nb, splayed hot cracks may appear in the electroslag welding seam. In addition, the root weld of low-carbon controlled rolling and controlled cooling pipeline steel often appears welding hot cracks, which is related to the high dilution rate of base metal and fast welding speed of root weld. Using welding material with high Mn: Si content, reducing welding heat input, reducing the fusion ratio of base metal in weld and improving the weld forming coefficient (that is, the width-height ratio of weld) is beneficial to prevent weld metal from hot cracking.
5) reheat crack sensitivity
Reheat cracks in welded joints of low alloy steel, also known as stress relief cracks, appear during post-weld stress relief heat treatment. Reheat crack belongs to intergranular fracture, which generally appears in the coarse grain zone of heat affected zone and sometimes in weld metal. Its formation is related to the grain boundary embrittlement caused by the segregation of impurity elements P, Sn, Sb and As at the grain boundary of primary austenite, and also to the composite strengthening of V, Nb and other elements.
6) delamination tearing tendency
When welding large thick plate welded structures (such as marine engineering, nuclear reactors and ships, etc.). ), if they are subjected to large tensile stress in the thickness direction of steel, stepped delamination tearing may occur along the rolling direction of steel. This kind of crack often appears in corner joints or T-joints that need penetration. Selecting layered tear-resistant steel; Improve the joint form to slow down the stress and strain of steel plate in z direction; On the premise of meeting the requirements of product use, choose low-strength welding materials or use low-strength welding materials for pre-stacking; Measures such as preheating and hydrogen reduction are beneficial to prevent delamination and tearing.
4. Specific welding process, mainly introducing the welding process of Q345 steel.
I. Introduction of materials
Analysis of chemical composition and mechanical properties of (1) materials
Table 1Q345( 16Mn) Chemical Composition of Materials
Chemical composition of steel grade
comment
CSiMnSPCrMoVNi
Q345≤0.2≤0.55 1.00 ~ 1.60≤0.045≤0.045 _ _ 0.02 ~
0. 15 _
Table 2 Material Mechanical Properties of Q345 (16mn) [2]
Mechanical properties of steel grades
comment
δb/MPaδs/MPaδ(%)AKV/J
Q345A 470 ~ 630 345 2 1 _ GB/t 1s 9 1—94
(2) Welding characteristics of 2)Q345 steel
Calculation of carbon equivalent;
CEQ = C+Mn/6+Ni/ 15+Cu/ 15+Cr/5+Mo/5+V/5
Ceq=0.49%, exceeding 0.45%. It can be seen that the weldability of Q345 steel is not very good, and strict technological measures need to be formulated during welding.
(3) Problems easily occurred in welding of 3)Q345 steel.
1 During welding cooling. In Q345 steel, the quenched structure-martensite is easy to form in the heat affected zone, which increases the hardness and reduces the plasticity in the near-seam zone. As a result, cracks appeared after welding.
2. The welding cracks of 2.Q345 steel are mainly cold cracks.
Second, the welding construction process
Groove processing → spot welding → preheating → root cleaning (carbon arc gouging) → external welding → internal welding → self-inspection/special inspection → post-weld heat treatment → nondestructive testing (weld quality reaches Grade I).
Third, the selection of welding process parameters
By analyzing the weldability of Q345 steel, the following measures are formulated:
1. Selection of welding materials:
Welding materials should be selected according to the requirements of products for welding performance. The selection of welding materials for low-alloy high-strength steel should first ensure that the strength, plasticity and toughness of the weld metal meet the technical requirements of the product, and at the same time consider the crack resistance and welding production efficiency. Because low-alloy high-strength hydrogen-induced cracking is sensitive, low-hydrogen covered electrode and submerged arc welding flux with moderate alkalinity should be preferred when selecting welding materials. Covered electrode, flux before use should be according to the provisions of the manufacturer or process procedures for drying. The high strength of weld metal will reduce the toughness, plasticity and crack resistance of weld, thus reducing the safety of production and use of welded structures, which is particularly important for the welding of low alloy steel structures with high toughness and poor crack resistance of base metal. In order to ensure that the welded joint has the same impact toughness as the base metal, the welding materials of normalized steel and controlled rolling and controlled cooling steel are preferred to use high toughness welding materials, and the correct welding process is adopted to ensure that the weld metal and heat affected zone have excellent impact toughness. The welding materials selected for offshore engineering, ultra-high strength steel shells and pressure vessels should also ensure that the weld metal has corresponding special properties such as low temperature, high temperature and corrosion resistance. Because Q345 steel is prone to cold cracking, low-hydrogen welding materials should be selected. Considering the principle that the strength of welded joints should be equivalent to that of base metal, E50 15 (J507) covered electrode should be selected.
2. Groove form:
When welding the same steel with the same welding material, the weld performance will be different if the groove forms are different. If HJ43 1 flux is used for submerged arc welding of Q345 steel without groove, the mechanical properties of the weld can be satisfied by using H08A welding wire with low alloy composition in combination with HJ43 1, because the base metal is dissolved in the weld metal. However, if the combination of H08-HJ43 1 is still used when welding the groove butt joint of Q345 steel thick plate, the weld strength will be lower due to the small fusion of base metal. At this time, HJ43 1 should be combined with other high alloy welding wires. The cooling rate of fillet joint is higher than that of butt joint, so the combination of H08A welding wire with low alloy composition and HJ43 1 flux should be used in fillet welding of Q345 steel to obtain weld metal with better comprehensive mechanical properties. If H08MnA or H 10Mn2 welding wire with high alloy composition is used, the plasticity of fillet weld is low.
3. Selection of welding method:
Low-alloy high-strength steel can be welded by all common fusion welding and pressure welding methods, such as covered electrode arc welding, MIG welding, submerged arc welding, TIG welding, gas-electric vertical welding and electroslag welding. The specific welding method depends on the structure, plate thickness, stack performance requirements and production conditions of the welded products. Among them, covered electrode arc welding, submerged arc welding, solid wire and flux-cored wire gas shielded welding are commonly used welding methods. For the welding of low-alloy high-strength steel with strong sensitivity to hydrogen-induced cracking, no matter which welding process is adopted, low-hydrogen process measures should be taken. Circular and long straight-line welds of low-alloy high-strength steel structures with a thickness greater than 100mm are often submerged arc welded with single wire or double wire load gap. When electroslag welding, gas-electric vertical welding and multi-wire submerged arc welding are used, the toughness of weld metal and heat affected zone should meet the requirements before use. Q345 steel can be welded by arc welding, CO gas shielded welding and electroslag welding, but this design adopts manual arc welding.
4. Control of welding heat input:
The change of welding heat input will change the welding cooling rate, thus affecting the microstructure of weld metal and heat affected zone, and finally affecting the mechanical properties and crack resistance of welded joints. The weld metal of low-alloy high-strength steel with yield strength less than 500MPa has excellent strength and toughness if it can obtain fine and uniform acicular ferrite structure. The formation of acicular ferrite structure needs to control the welding cooling rate. Therefore, in order to ensure the toughness of weld metal, excessive welding heat input should not be used. Transverse swing and arc welding are not needed in welding operation as far as possible, and multi-layer narrow pass welding is recommended.
Heat input also has a significant effect on the crack resistance and toughness of welding heat affected zone. Brittleness or softening of microstructure in low alloy high strength heat affected zone is related to welding cooling rate. Because the strength and thickness of low-alloy high-strength steel vary widely, the alloy system and alloy content are very different, and the state of steel is different when welding, it is difficult to make a unified regulation on welding heat input. When welding various low-alloy high-strength steels, the appropriate welding heat input should be selected according to its own weldability characteristics, combined with the specific structural form and plate thickness. Compared with normalized or normalized+tempered steel and controlled rolling and controlled cooling steel, hot rolled steel can adapt to greater welding heat input. When welding hot rolled steel (09Mn2, 09MnNb, etc.) with low carbon content. ) and 16Mn steel with low carbon content have no strict restrictions on welding heat input. Because these steels are not easy to embrittle and cold crack in the heat affected zone of welding However, when welding 16Mn steel with high carbon content, in order to reduce the hardening tendency and prevent the occurrence of cold cracks, the welding heat input should be large.
Normalized steel with high content of carbon and alloying elements and yield strength of 490MPa, such as 18MnMoNb, etc. When selecting heat input, we should consider both the hardening tendency of steel and the overheating tendency of coarse grain zone in heat affected zone. In general, in order to ensure the toughness of the heat affected zone, low heat input should be selected, low hydrogen welding method should be adopted, and appropriate preheating or timely post-welding hydrogen elimination treatment should be used to prevent welding cold cracks. The carbon content and carbon equivalent of Q345 steel are low, so it is insensitive to hydrogen-induced cracking. In order to prevent the softening of the heat affected zone and improve the toughness of the heat affected zone, it is advisable to use small line energy welding, and the welding cooling time is controlled within 10s.
5. Mechanical properties of welded joints
The mechanical properties of weld metal and heat affected zone are the basic properties that affect the reliability of street use, and strength and toughness are the key assessment factors, especially for alloy structural steel streets. The following table shows the mechanical properties of several typical hot rolled and normalized steel welded joints.
Aluminium (for utensils)
Welding process-overheated zone of weld metal performance
/MPa
/MPa
(%)
talent
(%)/j . cm
-20℃ -40℃
-20℃ -40℃
Q345 submerged arc welding (δ= 16mm, V-butt welding) H08MnA+HJ250 Welding status 50435130.265.31661kloc-0/75.
Submerged arc welding (δ= 12mm, I-shaped butt joint) H08MnA+HJ43 1 as-welded 576 400 30.7 67 84 33Q73
CO gas shielded welding H08Mn2SiA welding status 540 390 24 6 1 78
6. Welding current:
In order to avoid coarse weld structure and reduce impact toughness, small-scale welding must be adopted. The specific measures are as follows: select small-diameter covered electrode, narrow bead, thin welding layer and multi-layer and multi-pass welding process (see Figure 1 for welding sequence). The weld bead width is not more than 3 times the width of covered electrode, and the thickness of welding layer is not more than 5 mm The first to third layers adopt Ф 3.2 covered electrode, and the welding current is100-130a; Covered electrode Ф 4.0 is used for the fourth to sixth floors, and the welding current is 120- 180A.
7. Preheating temperature: Preheating and interlayer temperature:
1) preheating temperature
Preheating can control the welding cooling speed, reduce or avoid the generation of hardened martensite in heat affected zone and reduce the hardness of heat affected zone. At the same time, preheating can also reduce the welding stress and help hydrogen escape from the welded joint. Therefore, preheating is an effective measure to prevent hydrogen-induced cracks in welding of low-alloy high-strength steel. However, preheating usually worsens working conditions and complicates the production process. Unreasonable and excessive preheating and interlayer temperature will also damage the performance of welded joints. Therefore, whether it is necessary to preheat before welding and the reasonable preheating temperature need to be carefully considered or determined by experiments.
The determination of preheating temperature depends on the composition of steel (carbon equivalent), the thickness of plate, the structural shape and constraint degree of weldment, the ambient temperature and the content of welding materials used. With the increase of carbon equivalent, plate thickness, structural constraints, hydrogen content in welding materials and the decrease of ambient temperature, the preheating temperature before welding should be increased accordingly. For multi-pass and multi-layer welding of thick plates, in order to promote the escape of hydrogen in the welding zone and prevent the generation of hydrogen-induced cracks in the welding process, the temperature between passes should be controlled not to be lower than the preheating temperature, and the necessary intermediate hydrogen elimination heat treatment should be carried out. So the icon below is the preheating state of Q345.
Preheating temperature of plate thickness (mm) under different air temperatures.
≤ 10, not less than-26 oC, no preheating.
10 ~ 16 shall not be lower than-10oC without preheating, and lower than-10oC when preheating15oc.
16 ~ 14 is not less than -5oC, and the preheating below -5oC is 100oC ~ 150oC.
25 ~ 40 shall not be lower than 0oC, without preheating, 100oC ~ 150oC shall be preheated to below 0oC.
≥ 40, preheating 100 oc ~ 150 oc.
2) Interlayer temperature
Excessive interlayer temperature will cause coarse grains in the heat affected zone, which will reduce the weld strength and low temperature impact toughness. If it is lower than the preheating temperature, cracks may occur during welding. Therefore, it is stipulated that the temperature between lanes shall not be lower than the preheating temperature, and the maximum temperature shall not be higher than a certain limit. For Q345, the interlayer temperature Ti is ≤ 400℃.
8. Post-weld heat treatment parameters:
In electroslag welding, except for the serious overheating in the joint area, other welding conditions should be judged according to the use requirements. Hot rolled steel and normalized steel in low-carbon alloy high-strength steel do not need post-weld heat treatment, but high-temperature tempering is needed to relieve stress for welding mechanism, welding structure used at low temperature and plate thickness structure. Principles for determining tempering temperature after welding;
1) Do not exceed the original tempering temperature of the wood, so as not to affect the performance of the parent material itself.
2) For tempered brittle materials, avoid the temperature range where tempered brittleness occurs. Such as low alloy steel containing V or V+Mo, the cooling rate should be increased during tempering to avoid staying in the temperature range of about 600℃ for too long, so as to avoid the precipitation of secondary carbides of V and embrittlement;
If heat treatment cannot be carried out in time after welding, it should be kept at 200~350℃ for 2~6h immediately, so that hydrogen in the welding zone can diffuse and escape. In order to eliminate welding stress, the surface of weld metal should be tapped immediately after welding, but this is not applicable to steel parts with poor plasticity. For welded structural parts with high strength grade or important strength grade, mechanical method should be adopted to correct the weld shape, so that it can smoothly transition to the parent metal with less stress concentration. Q345 post-weld heat treatment process parameters are shown in the following table:
Brightness level
Typical steel preheating temperature δs/MPa/℃
Arc welding electroslag welding
345 Q345 100 ~ 150
δ≥ 16 mm is generally not performed.
Or tempering at 600-650℃ and normalizing at 900-930℃
Tempering at 600~650℃
In order to reduce the welding residual stress, reduce the hydrogen content in the weld and improve the metal structure and properties of the weld, the weld should be heat treated after welding. The heat treatment temperature is 600-640℃, the constant temperature time is 2 hours (when the plate thickness is 40mm), and the heating and cooling rate is125℃/h.
9. Welding process:
1) preheating before welding
Before welding the flange, preheat the flange and keep it at a constant temperature for 30 minutes before welding. The preheating, interlayer temperature and heat treatment of welding are automatically controlled by the heat treatment temperature control cabinet. Using far infrared crawler heating plate, microcomputer automatically sets and records curves, and thermocouple measures temperature. During preheating, the measuring point of thermocouple is 15mm-20mm away from the edge of groove.
2) Welding
(1) To prevent welding deformation, each column joint is symmetrically welded by two people, and the welding direction is from the middle to both sides. When welding the inner mouth (the inner mouth is a groove near the web), the first to third layers must be operated in small specifications, because its welding is the main reason that affects the welding deformation. After welding one or three layers, clean the back. After the completion of carbon arc gouging, the weld must be mechanically polished, and the surface of the weld must be cleaned to carburize and expose the metallic luster, so as to prevent the surface from cracking due to serious carbonization. The welding of the outer port shall be completed at one time, and then the remaining inner ports shall be welded.
(2) when welding the second layer, the welding direction should be opposite to the first layer, and so on. The welded joints of each layer shall be staggered by 15-20mm.
(3) the welding current, welding speed and welding layers of two welders should be consistent.
(4) When welding, it should start from the arc striking plate and end on the arc closing plate. Cutting and polishing after welding.
summary
Through the understanding of low carbon steel and the study of welding process of Q345 steel, we have a general understanding of its welding process, so after the above description, we summarized the welding process of Q345, as shown in the following table:
Joint form weld thickness /mm welding sequence (grade) welding wire diameter /mm welding current /a welding voltage /mm welding speed/
Welding wire and flux
No fracture (double-sided welding) 8 positive electrode
Resistance 4.0 550 ~ 580
600 ~ 650 34 ~ 36 34.5 h08a+HJ 43 1
10 ~ 12 positive
Resistance 4.0 620 ~ 680
680 ~ 700 36 ~ 38 32h 08a+HJ 43 1
V-groove (double-sided welding) α = 60 ~ 70 14 ~ 16 positive
Prevention 4.0 600 ~ 640
29.5 h08a+HJ 43 1 620 ~ 680 34 ~ 36
18 ~ 20 plus
Resistance 4.0 680 ~ 700
700 ~ 720 36 ~ 38 27.5h 08 mna+HJ 43 1
22 ~ 25 positive
Anti-4.0 700 ~ 720
720 ~ 740 36 ~ 38 2 1.5h 08 mna+HJ 43 1
T-joint without groove (double-sided welding)16 ~18 (2) 4.0 600 ~ 650
680~720 32~34
36~38 34~38
h08a+HJ 43 1 24～29h 08
20~25 (2) 4.0 600~700
720~760 32~34
36~36 30~36
2 1 ~ 26h 08 a+HJ 43 1