What are the welding processes for low alloy steel?

Low alloy steel is made by adding a certain amount of alloying elements to carbon steel. The mass fraction of the alloying elements generally does not exceed 5% to improve the strength of the steel and protect it from certain plasticity and toughness, or Give steel some special properties, such as low temperature resistance, high temperature resistance or corrosion resistance. Low alloy steel commonly used to make welded structures can be divided into four types: high-strength steel, low-temperature steel, corrosion-resistant steel and pearlitic heat-resistant steel.
1. During the welding process of low alloy steel, the following types of electrodes are mainly used:

1) Ultra-low hydrogen electrode (J×××H) When welding with ultra-low hydrogen electrode, due to the extremely low diffusible hydrogen content in the weld metal, the crack resistance can be significantly improved and the preheating temperature before welding can be reduced. Therefore, in order to improve the crack resistance and simplify the welding process when welding important low alloy steel products, the application of ultra-low hydrogen electrodes in welding production at home and abroad has received widespread attention.

2) High-toughness welding rod (J×××GR) Some important low-alloy steel products require good low-temperature toughness, but the low-temperature toughness of the deposited metal of general low-hydrogen welding rods cannot meet this requirement. In order to meet the needs of low-alloy steel welded structures such as offshore oil platforms and pressure vessels with sub-poor working conditions, high-toughness welding rods have been developed and produced at home and abroad, such as J507GR (also known as J507NiTiB) and other welding rods. The deposited metal  quote of the welding rod is -40℃ V-shaped. The notched Charpy impact absorbed energy is not less than 47J. In order to improve the safety and resistance to brittle fracture of the welded structure, in addition to requiring the weld metal to have good low-temperature impact toughness, the weld is also required to have good fracture toughness. For example, offshore oil production platform structures require a critical COD value or NDT temperature of -10°C for the weld metal. When using general J507 low-hydrogen welding rods to weld Q345R and Q420R steel pressure vessels, the critical COD value of the weld metal is often lower than the base metal. However, using high-toughness welding rods can ensure the -40°C impact performance and COD value of the weld metal. Reach the level of parent material.

3) High-toughness ultra-low hydrogen welding rod (J×××RH). This type of welding rod has both good crack resistance and low-temperature toughness of the weld, such as J506RH, J556RH, J607RH and J707RH. These high-toughness welding rods and high-toughness ultra-low hydrogen welding rods can be used for the welding of offshore oil production platforms, pressure vessels, ships, engineering mining machinery, and important low-alloy steel structures serving in severe cold areas.

4) Moisture absorption resistant welding rod (J×××LMA) Moisture absorption resistance refers to the ability of the coating to resist moisture absorption in humid air. For low-hydrogen moisture-absorption-resistant welding rods, it is required that the moisture content of the coating should still meet the specified requirements after the welding rod is dried and taken out of the oven for use in a high-temperature environment. For example, after J506LMA moisture-resistant welding rod is dried at 350°C for 2 hours and used in an environment where the relative temperature is greater than 80%, the moisture content of the coating still meets the requirements within 8 hours. , there are a variety of moisture-absorbing and low-hydrogen welding rods abroad that have replaced the general low-hydrogen welding rods.
2. Principles for selecting welding materials for low alloy steel 1) Select welding materials according to the product’s performance requirements for the weld. When welding high-strength steel, welding materials that are equivalent in strength to the base metal should generally be selected. The toughness and plasticity of the weld metal must be comprehensively considered. and intensity. As long as the weld strength or the actual strength of the welded joint is not lower than the product requirements. If the strength of the weld metal is too high, it will lead to a decrease in the toughness, plasticity and crack resistance of the weld, thereby reducing the safety of the production and use of the welded structure. This requires high toughness of the welded joint, and the base material has poor crack resistance. The welding of alloy steel structures is particularly important. The laser welding of large  materials selected for marine 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.

2) When selecting welding materials, the influence of process conditions should also be considered. ① The influence of groove and joint form. When the same welding material is used to weld the same steel type, if the groove form is different, the performance of the weld will be different.

② Influence of post-weld processing technology Regarding the conditions for hot coiling or heat treatment after welding, the impact on the performance of the weld metal after it has been subjected to high-temperature heat treatment must be considered. It should be ensured that the weld still has the required strength, plasticity and toughness after heat treatment. Generally, the strength of the weld after normalizing treatment is lower than that in the welded state. For welds that need to undergo normalizing treatment after welding, alloy separation should be used. Higher welding. The weldment must be subjected to stress relief heat treatment after welding. Generally, the strength of the weld metal will be reduced. At this time, welding materials with higher alloy composition should also be selected. For weldments that undergo cold rolling or cold stamping after welding, the weld is required to have high plasticity.

3) For thick plates, welded structures with high restraint and high cold cracking tendency, ultra-low hydrogen welding materials should be used to improve crack resistance and reduce preheating temperature. For thick plates and highly constrained weldments, cracks are most likely to occur in the first layer of primer welds. In this case, low-hydrogen or ultra-low-hydrogen welding materials with slightly lower strength, good plasticity and toughness can be used.

4) For important welding products such as offshore oil production platforms, pressure vessels and ships, in order to ensure the safety of product use, the welding seam should have high toughness welding materials, such as high alkalinity flux, high toughness welding wire, welding rod, high purity The shielding gas is Ar+CO2 mixed gas shielded welding, etc.

5) In order to increase the production rate, high-efficiency iron powder welding rods, gravity casting rods, flux-cored welding wires with high deposition rates and high-speed flux can be used. Vertical downward welding rods can be used for vertical fillet welding, high-speed flux can be used for large-diameter pipe joints, and high-speed flux can be used for small-diameter pipes. The joints can be made with bottom welding rods.

6) In order to improve sanitary conditions, low-dust and low-toxic welding rods should be used when welding in poorly ventilated products (such as ship warehouses, pressure vessels, etc.).
3. Strengthening mechanism of low alloy steel The strengthening mechanism of low alloy high strength is different from that of carbon steel. Carbon steel is strengthened mainly through the formation of pearlite, bainite and martensite through the carbon content in the steel; while low alloy high strength The strengthening of steel is mainly achieved through grain refinement, precipitation hardening and changes in substructure.
4. Selection of welding consumables for low alloy steel 1) Select welding materials according to the product’s performance requirements for the weld. When welding high-strength steel, welding materials that are equivalent in strength to the base metal should generally be selected. The toughness, plasticity and strength of the weld metal must be comprehensively considered. . As long as the weld strength or the actual strength of the welded joint is not lower than the product requirements. If the strength of the weld metal is too high, it will lead to a decrease in the toughness, plasticity and crack resistance of the weld, thereby reducing the safety of the production and use of the welded structure. This requires high toughness of the welded joint, and the base material has poor crack resistance. The welding of alloy steel structures is particularly important. The welding materials selected for marine 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.

2) When selecting welding materials, the influence of process conditions should also be considered. ① The influence of groove and joint form. When the same welding material is used to weld the same steel type, if the groove form is different, the performance of the weld will be different.

② Influence of post-weld processing technology Regarding the conditions for hot coiling or heat treatment after welding, the impact on the performance of the weld metal after it has been subjected to high-temperature heat treatment must be considered. It should be ensured that the weld still has the required strength, plasticity and toughness after heat treatment. Generally, the strength of the weld after normalizing treatment is lower than that in the welded state. For welds that need to undergo normalizing treatment after welding, alloy separation should be used. Higher welding. The weldment must undergo stress relief heat treatment after welding. Generally, the strength of the weld metal will be reduced. At this time, welding materials with higher alloy composition should also be selected. For weldments that undergo cold rolling or cold stamping after welding, the weld is required to have high plasticity.

3) For thick plates, welded structures with high restraint and high cold cracking tendency, ultra-low hydrogen welding materials should be used to improve crack resistance and reduce preheating temperature. For thick plates and highly constrained weldments, cracks are most likely to occur in the first layer of primer welds. In this case, low-hydrogen or ultra-low-hydrogen welding materials with slightly lower strength, good plasticity and toughness can be used.

4) For important welding products such as offshore oil production platforms, pressure vessels and ships, in order to ensure the safety of product use, the welding seam should have high toughness welding materials, such as high alkalinity flux, high toughness welding wire, welding rod, high purity The shielding gas is Ar+CO2 mixed gas shielded welding, etc.

5) In order to increase the production rate, high-efficiency iron powder welding rods, gravity welding rods, flux-cored welding wires with high deposition rates and high-speed flux can be used. Vertical downward welding rods can be used for vertical fillet welding, high-speed flux can be used for large-diameter pipe joints, and high-speed flux can be used for small-diameter pipes. The joints can be made with bottom welding rods.

6) In order to improve sanitary conditions, low-dust and low-toxic welding rods should be used when welding in poorly ventilated products (such as ship warehouses, pressure vessels, etc.).
5. Classification of low alloy high strength steel

Low alloy high-strength steel can also be divided according to its use: boiler steel, pipeline steel, container steel, steel making steel and bridge steel, etc.