Post weld heat treatment (PWHT) is a process in which a welded component is heated to a specific temperature and then cooled at a controlled rate, with the goal of reducing the residual stresses that are present in the weld and the heat-affected zone (HAZ). PWHT is often used in the manufacturing of pressure vessels, boilers, and other components that are subjected to high stresses and must meet certain strength and toughness requirements.
There are several different methods that can be used for PWHT, including:
- Electric heating: This method involves using electric resistance heaters to heat the component to the required temperature.
- Gas heating: This method involves using gas burners or a gas-fired furnace to heat the component.
- Induction heating: This method involves using an induction coil to generate an alternating current in the component, which creates heat through resistive heating.
There are several types of furnaces used for post weld heat treatment, including:
- Box Furnaces: These are the most common type of furnace used for post weld heat treatment. They are typically made of steel and are insulated to retain heat. They can be heated by gas, electricity, or oil, and are often used for small to medium-sized parts.
- Pit Furnaces: These are similar to box furnaces, but are sunken into the ground. They are used for larger parts and are often used for heat treatment of steel plates and structural steel.
- Continuous Furnaces: These are used for heat treating large quantities of parts. They consist of a conveyor system that moves the parts through the furnace.
- Car-Bottom Furnaces: These are similar to pit furnaces, but are raised off the ground and have a car-bottom design. They are used for heat treating large, heavy parts, such as castings and forgings.
- Atmosphere Controlled Furnace: This type of furnace is used to control the atmosphere inside the furnace during heat treatment. This is useful to prevent oxidation of the parts.
Ceramic mat heaters are used for localized post weld heat treatment to provide a controlled and consistent heat source for specific areas of a welded joint. These heaters are typically made from high-temperature ceramic materials that can withstand extreme temperatures and provide a uniform heat distribution.
During post weld heat treatment, ceramic mat heaters are placed on the area of the weld that needs to be treated. The heaters are then turned on and the temperature is gradually increased until it reaches the desired level. The heat is maintained for a specific period of time, depending on the type of material and the desired outcome of the heat treatment.
One of the main advantages of using ceramic mat heaters for post weld heat treatment is that they can be easily positioned and repositioned to provide heat to specific areas of the weld. This allows for more precise and accurate heat treatment, which can result in a stronger and more durable weld.
Another advantage of using ceramic mat heaters is that they are very energy efficient, as they only heat the specific area that needs to be treated, reducing energy consumption and costs.
HEAT UP AND COOL DOWN RATE
The heat-up and cool-down rates during post-weld heat treatment (PWHT) are important because they affect the microstructure of the welded area, and thus its mechanical properties. If the heat-up rate is too fast, the weld may not fully austenize, leading to a lack of uniformity in the microstructure and potentially reducing the strength of the welded joint. Similarly, if the cool-down rate is too fast, the weld may not fully transform to the desired microstructure, again leading to a lack of uniformity and reduced strength. Therefore, a controlled heat-up and cool-down rate is necessary to ensure the proper microstructure and mechanical properties in the welded area.
The specific PWHT method and parameters (temperature, hold time, cooling rate, etc.) will depend on the material being welded and the desired properties of the finished component.
Hotfoil-EHS can assist you in designing and fabricating the best solutions for your heat treatment requirement. Call them at 609-588-0900 or visit https://hotfoilehs.com.
