In the heat treatment of stainless steel plates, tempering is mainly used to soften any hard microstructures that may be formed during the previous heat treatment, improve the ductility and toughness of the material. In addition, the tempering can also form precipitates, and the size of these precipitates can be controlled to provide the required mechanical properties. This is very important for creep resistant Cr Mo steel. Tempering includes heating the steel to a temperature below the critical temperature, which is affected by the type of alloying elements added to the stainless steel plate.
The temper of tool steel can be tempered at a temperature of 150 degrees centigrade, but for the welding engineer, the tempering temperature of the structural steel is usually between 550-760 degrees centigrade, depending on the composition of the steel.
As for post weld heat treatment, this is a specific term which includes both stress relief and tempering, and should not be confused with heat treatment after welding. These treatments can include aging of aluminum alloy, solution treatment of austenitic stainless steel, hydrogen release and so on. When meeting certain standards, post weld heat treatment is mandatory in many standard procedures. By reducing residual stress and improving toughness, it reduces the risk of brittle fracture and reduces the risk of stress corrosion cracking. However, unless the stress is mostly compressed, there is almost no beneficial effect on fatigue performance.
The method of post welding heat treatment depends on a number of factors, such as the available equipment, the size and configuration of the components, the thermal insulation temperature, the equipment capable of providing uniform heating with the required heating rate, and so on. The best way is to use a stove. This may be a permanent fixed furnace or a temporary furnace erected around the components, which is especially useful for bulky large structures or large parts on site. A permanent furnace can carry a wheeled hearth with components on it or a top hat type furnace with fixed grate and removable cover. Typically, a furnace that can heat 150 tons of pressure vessels will have a size of about 20 meters long, 5 x 5 meters, and will consume about 900cu / meter gas per hour.
Natural gas or oil can be heated by resistance or induction heating. If fossil fuels are used, attention should be paid to ensuring that there is no sulphur and other elements that may cause some alloy cracking in the fuel, especially if these alloys are austenitic stainless steel or nickel base corrosion resistant coatings. No matter which fuel is used, the atmosphere in the furnace should be closely controlled so as not to cause excessive oxidation and scaling or carbonization due to the unburned carbon in the furnace atmosphere. If the furnace is gas or fuel, flame contact elements or temperature monitoring thermocouples are not allowed, which will cause partial overheating or PWHT temperature.
Monitoring the temperature of components during post weld heat treatment is critical. Most modern furnaces use zone control with thermocouples to measure and control the temperature in the furnace area, and automatically control temperature through computer software. Area control is particularly useful for controlling the heating rate when assembled with different thickness of steel. However, it is recommended not to use the monitoring furnace temperature to prove that the components have reached the correct temperature. Therefore, the thermocouples are usually attached to the surface of the element at a specific interval, and these elements are used to automatically control the heating and cooling rates and the soaking temperature to achieve a uniform temperature.
As mentioned previously, the yield strength decreases with increasing temperature, and the component may not be able to support its excessive weight distortion at the post weld heat treatment temperature. It is very important for components to be adequately supported in the heat treatment process, and the bracket suitable for components should be regularly placed. The spacing depends on the shape, diameter and thickness of the item. Internal support may be required in the cylinder, such as pressure vessel; if so, the support should be similar material, so that the coefficient of thermal expansion will match.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)