Welding of low alloy heat resistant steel

• Low alloy heat-resistant steel for special equipment, in order to improve its weldability, the carbon mass fraction are controlled below 0.2%, or even below 0.15%, and such steels are usually delivered in annealed or normalised and tempered condition. As the alloy content of 2.5% of the following low alloy heat-resistant steel with pearlite and ferrite organisation, it is also often referred to as pearlite heat-resistant steel, such as 15CrMoR, 12CrlMoV. alloy content of 3% ~ 5% of the low alloy heat-resistant steel supply state for the bainite and ferrite organisation, it is also known as bainite heat-resistant steel, such as 12Cr2MolR, 12Cr2MoWVTiB. Pressure equipment using low alloy heat-resistant steel is mainly to add chromium and molybdenum elements or supplemented by the addition of a small amount of vanadium, titanium and other elements to improve the creep strength of the steel and organisational stability, so it is also often called Cr-Mo heat-resistant steel or Cr-Mo-V system heat-resistant steel. This type of steel in the high temperature resistance at the same time has good hydrogen corrosion resistance, for this reason, for Cr-Mo or Cr-Mo-V system of low alloy heat-resistant steel is often called hydrogen-resistant steel.
• In addition, heat-resistant steel and medium alloy heat-resistant steel (such as 1Cr5Mo) and high alloy heat-resistant steel (such as 08Cr13Al), due to the special equipment is not common, this article mainly describes the low alloy heat-resistant steel. For low alloy heat-resistant steel, in order to ensure that it is in the high temperature, high pressure and a variety of complex media under long-term safe operation, it is necessary to consider it has the basic performance requirements, mainly; adequate room temperature and high temperature short-term strength; high temperature strength and creep strength; corrosion resistance, hydrogen resistance and oxidation resistance; resistance to brittle fracture; processability, including cold, hot forming properties, welding performance, and so on.

• the welding characteristics of low alloy heat-resistant steel

According to its alloy (mainly carbon) content, there are different degrees of hardening tendency. Most of the heat-resistant steel contains chromium, molybdenum, vanadium, niobium, titanium and other strong carbide-forming elements, so that the superheated area of the joint has a different degree of susceptibility to reheat cracking. Some heat-resistant steel welded joints, when the total amount of harmful residual elements over the permissible limit will appear tempering brittleness or long-time embrittlement.

• low alloy heat-resistant steel welding consumables selection

Weld metal and base material strength principle is still the basic principle of low alloy heat-resistant steel welding consumables selection, at this time, not only to consider the weld metal and base material strength at room temperature, but also to make its high temperature strength is not lower than the lower limit of the base material standard requirements. In order to make its weld metal has the same performance with the base material, the requirements of the weld metal of chromium, molybdenum content shall not be lower than the lower limit of the base material standard value. In order to ensure that the weld metal has the same small temper brittleness, the content of oxygen, silicon, phosphorus, antimony, tin, arsenic and other trace elements in the weld metal should be strictly limited. In order to improve the crack resistance of the weld, the carbon content in the welding consumables should be controlled to be lower than the carbon content of the base material. However, it should be noted that; the carbon content is too low, after a long time of post-weld heat treatment will promote the formation of ferrite, which leads to a decline in toughness. Therefore, for low alloy heat-resistant steel weld metal carbon content is best controlled in the range of 0.08% to 0.12%, so that the weld metal has a high impact toughness and high-temperature creep strength comparable to the base material.

(A) low alloy heat-resistant steel welding rod selection

Low alloy heat-resistant steel welding rod that is chromium-molybdenum steel welding rod, the molten metal has four strength levels and different chromium and molybdenum content.

(B) low-alloy heat-resistant steel submerged arc welding flux and wire selection

Selection of low alloy heat-resistant steel welding material is the basic principle: the alloy composition and mechanical properties of the weld metal and base material should be basically the same, or to achieve the minimum performance indicators proposed by the product specifications; if the weldments need to be welded by annealing, normalising and other heat treatment or thermal processing of forming, the alloy composition should be used or a higher level of strength of the welding material. In order to improve the weld metal resistance to thermal cracking performance, the carbon content of the welding material should be controlled at a slightly lower than the carbon content of the base material. For 1.25Cr-Mo and 2.25Cr-Mo heat-resistant steel, the optimum carbon content of the weld metal should be close to 0.10%, at which time the weld metal has a high impact toughness and creep strength comparable to the base material. In order to reduce the temper brittleness of the weld metal, low phosphorus content of sintered flux should be used, while the content of phosphorus, sulfur, tin, antimony and arsenic and other harmful impurities in the welding wire should be strictly limited to meet the stringent requirements for thick-walled containers against temper brittleness.

(C) low alloy heat-resistant steel gas shielded welding welding material selection

Cr-Mo heat-resistant steel wire selection: first of all, to ensure that the mechanical properties of the weld with the base material as far as possible, the weld metal alloy composition is not lower than the lower limit of the standard value of the base material. So that the weld in the working temperature, with good temper brittleness, oxidation resistance, resistance to gas medium corrosion resistance and a certain high temperature strength. Secondly, the welding performance of the material should be considered to avoid the selection of higher impurity content or higher strength of the welding material.Cr-Mo heat-resistant steel weld carbon content is generally controlled between 0.07% to 0.12%, the carbon content is too low will reduce the high temperature strength of the weld, carbon content is too high and easy to weld crystalline cracks; selection of ultra-low-carbon Cr-Mo heat-resistant steel weld material (such as ER55-B2L) Mainly to improve the weld crack resistance.

Cr-Mo heat-resistant steel welding method: at present the more commonly used or electrode arc welding and submerged arc welding. But with the progress of technology, CO2 or Ar + CO2 fusion electrode gas shielded welding method is gradually expanding the application. Some important high-temperature, high-pressure heat-resistant steel pipe welding, commonly used tungsten argon arc welding bottom welding, solid wire melting electrode gas-shielded welding filler or electrode arc welding filler (large-diameter thick-walled pipe will also be used to fill the submerged arc welding). Solid wire fusion electrode gas shielded welding, the shielding gas used is Ar + CO2 (2% to 5%). The choice of shielding gas is mainly to consider the transition form of the molten droplet and the stable combustion of the arc, as well as to ensure penetration and good weld channel formation.