The function of preheating is that preheating can reduce the cooling rate after welding. For a given steel composition, the microstructure and properties of weld and heat affected zone depend on the cooling rate. For steel with high hardening tendency, preheating can reduce the hardening degree and prevent welding cracks. In addition, preheating can reduce the temperature difference in the heat affected zone and obtain a more uniform temperature distribution in a larger range, which is helpful to reduce the welding stress caused by temperature difference. Therefore, preheating measures are often taken for steel with hard tendency. For Cr-Ni austenitic steel, preheating increases the residence time of heat affected zone in dangerous temperature zone, thus increasing the corrosion tendency. Therefore, preheating is not allowed when welding Cr-Ni austenitic stainless steel.

The selection of preheating temperature should be determined by weldability test according to the composition of weldment, structural stiffness, welding method and other factors. Preheating is generally carried out in the range of about 80mm on both sides of the groove, and the heating width should be greater than 5 times the plate thickness. Commonly used are flame heating, power frequency induction heating and infrared heating. After welding, heat preservation and slow cooling can slow down the cooling rate of weld and heat affected zone, which plays a similar role in preheating.

Second, control the interlayer temperature and post-heating temperature.

Interlayer temperature refers to the lowest temperature at which the next layer (pass) weld is connected with the upper layer (pass) weld during multi-layer and multi-pass welding, but the interlayer temperature in preheating welding should generally be equal to or slightly higher than the preheating temperature. The interlayer temperature is also controlled to reduce the cooling rate, the escape of facilitated diffusion hydrogen and help to prevent cracks. But for some steels, such as austenitic steel, in order to prevent intergranular corrosion, the interlayer temperature should not be controlled above 60℃.

Role of post-heating: After welding, heat preservation and slow cooling can slow down the cooling rate of weld and heat affected zone, and play a similar role of preheating. For materials such as low-alloy high-strength steel with large cold crack orientation, there is also a special post-heat treatment, also known as hydrogen treatment, that is, immediately after welding, the weldment is heated to the temperature range of 250 ~ 350℃, and the temperature is kept for 2 ~ 6 hours before air cooling. The main purpose of heat treatment is to accelerate the escape of diffused hydrogen in weld metal, reduce the hydrogen content in weld and heat affected zone, and prevent cold cracks. For weldments that need heat treatment after welding, it is not necessary to remove hydrogen, because it can be realized during heat treatment. However, if heat treatment cannot be carried out immediately after welding, hydrogen should be removed in time, otherwise cracks may appear in the weldment before heat treatment.

The method of post-heating is the same as that of preheating, but the heating temperature is different from the selected temperature of preheating.

Thirdly, heat treatment after welding.

Post-weld heat treatment is a kind of treatment method that the whole or part of the weldment is heated and insulated, and then cooled by furnace or air. Post-heat treatment can reduce welding residual stress, soften hardened parts, improve the microstructure and properties of weld and heat affected zone, improve the plasticity and toughness of welded joints and stabilize the microstructure size.

The most commonly used post-weld heat treatment is stress relief annealing in the range of 600-650℃ and high temperature tempering below Ac 1 temperature. In addition, there are stabilization treatments to improve the corrosion resistance of Cr-Ni austenitic stainless steel.

Post-weld heat treatment needs to be considered in the following cases:

① Ordinary low-alloy steel has a higher strength grade of the base metal and a greater tendency of delayed cracking. ② Pressure vessels and other welded structures working at low temperature, especially those used below ductile-brittle transition temperature. (3) Parts that work under alternating load and have fatigue strength requirements. ④ Large pressure vessels. ⑤ Welded structure with stress corrosion and stable geometric dimensions after welding.