Heat-resistant steel refers to the steel that works at high temperature and has excellent thermal strength and thermal stability. Thermal strength refers to the ability to resist creep and fracture at high temperature, and thermal stability refers to the ability to resist oxidation and corrosion of gaseous media at high temperature. People usually refer to the heat-resistant steel with thermal strength as heat-resistant steel and heat-resistant steel with thermal stability as heat-stable steel. Heat-resistant steels are mainly used in power and energy engineering, such as in the manufacture of oil refining equipment, boilers, nuclear vessels, steam turbines, synthetic chemical vessels, aerospace equipment and other high-temperature processing equipment. It should be noted that many stainless steels (309, 310H) also have heat resistance and are sometimes referred to as “heat resistant stainless steel”.
The welded joints of heat resistant steel shall have substantially the same high temperature oxidation resistance as the base metal. The alloy composition and content of weld metal should be basically consistent with the base metal, such as Cr, Mo, W and other major elements, while impurities such as P and S should be controlled at a low level as far as possible to reduce the tendency of hot crack. In order to improve the weldability, the C content of the welding material can be slightly lower than that of the base metal to ensure the high temperature performance. The strength of the weld metal shall be similar to that of the base metal to be welded. Heat-resistant steel welded joints shall not only have short-term strength at room temperature and high temperature basically equal to that of the base metal, but also, more importantly, have high temperature creep properties similar to that of the base metal. The performance requirements of new heat-resistant steel joints for ultra-supercritical boilers are shown in the following table.
|Grades||T.S σb MPa||Y.Sσs MPa||Elongation δ％||AkvJ||Allowable stress at operating temperature,MPa||Hardness, HB|
Although most of heat resistant steel welding structure is working under high temperature, but the final inspection for pressure vessels and piping requirements, usually at room temperature to 1.5 times the working pressure experiment hydraulic or pneumatic pressure test, the operation of pressure equipment or maintenance have to undergo the cold start process, so the heat resistant steel welding joint is also should have certain resistance to brittle fracture. For martensite and austenite heat resistant steels, the content of δ Ferrite in the deposited metal should be strictly controlled to ensure the creep property of the welded joints during the long time running at high temperature.
P92/T92, P122/T122 martensitic steel welding
Both P92 and P122 are martensitic steels, which have cold cracking tendency and hot cracking tendency during welding. In order to prevent cold cracks in welding, it is necessary to preheat before welding. The preheat temperature is not less than 150℃ for TIG welding and not less than 200℃ for electrode arc welding and submerged arc welding. In order to prevent hot crack and coarse grain, the welding line energy should be strictly controlled during the welding process, the interlayer temperature should be less than 300℃, and the tungsten electrode argon arc welding with small welding heat input is preferred. Multilayer and multi-pass welding should be paid attention to when welding electrode arc welding. The welding pass thickness should not be greater than the electrode diameter. The welding pass width should not be more than 3 times the electrode diameter and it is recommended that the electrode diameter should not be more than 4mm.For the workpiece with large wall thickness, submerged arc welding can be used for welding, but fine wire submerged arc welding should be used, and the diameter of the welding wire should be less than 3mm. When welding T122 and T92 small diameter tubes, the back side should be filled with argon during the whole welding process. For large-diameter thick-walled pipes, argon gas protection is required on the back of the first three layers of welds at the root. After weld welding, use asbestos insulation and slow cooling and stay between 100 ~ 150℃ for at least 1 ~ 2 hours, until the metallography is completely transformed into martensite, then can carry out post-weld heat treatment. For the wall thickness of the workpiece is greater than 40mm, after welding with asbestos insulation slow cooling, 100 ~ 150℃ at least stay 1 ~ 2 hours, if not immediately heat treatment, should be heated to 200 ~ 300℃ insulation 2 hours and then slow cooling to room temperature.
SUPER 304H, SA-213 TP310HCBN Austenitic steel welding
Austenitic steel has good weldability and no cold cracking tendency, so it does not need preheating. However, austenitic steel has hot cracking tendency during welding, so attention should be paid to the control of welding heat input and interlayer temperature. In the welding process, the welding method of welding line energy is smaller, such as manual TIG, automatic cold wire TIG welding or hot wire TIG welding. Generally, the interlayer temperature should be controlled not more than 150℃. For automatic cold wire TIG welding or hot wire TIG welding, the continuous welding process requires interlayer water cooling of the welded weld. In order to prevent intergranular corrosion, the chloride ion content in the cooling water should be controlled. In order to prevent the oxidation of alloying elements in the high temperature zone, the back surface should be filled with argon during the whole welding process. In order to ensure good fusion on both sides of groove, groove Angle of austenitic steel should be larger than that of general ferrite steel. For dissimilar steel welding with ferrite materials, ernicR-3 or EnICRFE-2 welding wire or electrode is recommended. When dissimilar steel is welded (with ferrite steel) and used at high temperatures, the expansion coefficient of both materials must be taken into account.