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.

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.

The connection form of heat exchange tube and tube plate mainly includes expansion, welding, expansion welding, etc. The strength expansion joint refers to the expansion of the sealing performance and tensile strength of the connection between the heat exchange tube and the tubesheet. It relies on the plastic deformation of the tube end to withstand the pulling force. The residual stress after the expansion of the tube will gradually weaken when the temperature increase so that the sealing performance and strength of the connection between the tube and the tube sheet will decrease. Therefore, the strength expansion is suitable for the design pressure is less than or equal to 4MPa, the design temperature is less than or equal to 300℃. The strength expansion should not be used in the case of severe vibration, large temperature difference, or obvious stress corrosion during operation.

When expanding the tube, the hardness of the tube should be lower than that of the tube sheet. The gap between the pipe and the pipe and the smoothness of the pipe affects the quality of the expanding pipe. The rough surface of the pipe hole can produce a large friction force and is not easy to pull off, but it is easy to produce leakage. The surface of the pipe hole is strictly prohibited to have a longitudinal through the groove. The smooth surface of the tube hole is not easy to leak, but easy to pull off. Generally, the surface roughness is required to be less than or equal to 12.5μm. There are two kinds of pipe holes: holes and annular grooving, the former as shown in figure (a) below, and the latter as shown in Figure (b) and (c) below.

After grooving, the steel tubes are squeezed into the grooves when expanding, which can improve the pull-off resistance and enhance the sealing performance. The number of annular slots in the tube hole depends on the thickness of the tube plate. Generally speaking, a slot is opened when the thickness is less than 25mm, and two slots are opened when the thickness is greater than 25mm. When the tube plate is thick or to avoid gap corrosion, the structure shown in the following figure (d) can be used, the composite tube plate and heat exchange tube can also be expanded, when the cladding is greater than or equal to 8mm, should be in the groove on the tube hole, the structure is shown in the following figure (e).

Strength welding refers to ensure the sealing performance and tensile strength of the heat exchange tube and tubesheet connection, is the most widely used tubesheet connection types. Strength welding manufacturing is simple, the tensile ability is strong, such as welding part failure, can be secondary repair welding, more convenient heat exchange tube. The use of strength welding is not limited by pressure and temperature, but it is not suitable for the occasion of large vibration or gap corrosion. The general form of strength welding is shown in figure (a) below. In order to avoid liquid accumulation around the pipe end, the structure as shown in figure (b) below is often used. The structure as shown in figure (c) below is generally used in the situation where the tubesheet is stainless steel.

The sealing performance of the joint between tube and tube plate is required to be high, or there is clearance corrosion, withstand severe vibration and other occasions, single expansion or welding can not meet the requirements, the combination of the two can provide enough strength and good sealing performance. The combination of expansion and welding can be divided into two kinds according to the expansion and welding sequence: expansion and welding after expansion. The general expansion method will inevitably have oil stains in the joint gap, which will be welded after expansion. These oil stains and the air in the gap will reduce the weld quality.

Weld before expansion, will cause damage to the weld. At present, there is no uniform provision for the choice of the two orders. In the actual engineering, such as expansion after welding, before welding should be clean oil; If the first welding after expansion, should be a limit to the expansion position of the tube end, generally to control from the surface of the tube plate 15mm above the scope of expansion. The first expansion and then welding generally adopts the form of strength expansion and seal welding. The strength expansion ensures the sealing performance of the tube and tubesheet, providing enough tensile strength, and the seal welding further ensures the sealing performance of the tube and tubesheet. The structure is shown in the figure (a). Strength welding ensures the sealing performance of the tube and tubesheet, providing sufficient tensile strength, and sticking expansion eliminate the gap between the tube and the tube hole to ensure the sealing performance. The structure is shown in figure (b).

In essence, explosive expansion is also a kind of strength expansion, the latter usually adopts roller expansion, the former uses the explosive in a very short period of time to produce high-pressure gas shock wave to make the pipe firmly attached to the tube hole. High explosive expansion and connection efficiency, no need of lubricating oil, easy to weld after expansion, large tensile strength, small axial elongation and deformation.

Explosive expansion is suitable for thin wall tubes, small diameter tubes and large thickness tube sheet expansion, heat exchange tube end leakage, mechanical expansion is difficult to repair the occasion.