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What is Post Weld Heat Treatment (PWHT) ?
Welding is the process of joining metals or thermoplastics by using high heat or pressure and fusing the parent material with weld metal. Welding has many applications in industries like automotive, shipping, construction, mechanical, and many more. The major drawback of welding is the residual stress formed by the rapid cooling of the weld material. The sudden shrinking of the joint material weakens the equipment, thus impacting the metal's quality and structure. For dense materials, these stresses can even exceed the specified design stress.
Post Weld Heat Treatment (PWHT)/Stress Relief helps retain the material strength of the part subjected to welding by reducing and redistributing the residual stress. PWHT not only reduces residual stress but also controls the material's hardness and enhances mechanical strength. This treatment can be performed after any welding process, but it has a specific meaning for few industries. Current design codes such as ASME and BSI Pressure vessels and Piping codes specify that PWHT is required to ensure safe and desired design with optimum mechanical and metallurgic properties.
What happens in Post Weld Heat Treatment (PWHT) ?
Post Weld Heat Treatment (PWHT) reheats the welded material using a controlled ramp-soak profile with a temperature below its initial transformation temperature. The temperature ranges from 595˚C to 775˚C depending upon the properties of the metal/material. The heating rate is determined and controlled by the type of material thickness. The holding temperature is the one that is high enough to relieve stress and reduce hardness, but it is lower than the initial transformation temperature. Uneven or quick heating or cooling can form cracks in the component. Heating and cooling rate, holding temperature, and soak duration must be controlled and monitored precisely to avoid any detrimental effects such as reheat cracking, distortion, decrease in the toughness, and over softening to obtain the full benefit of the process.
Advantages of Post Weld Heat Treatment (PWHT) ?
PWHT uses a technique that reduces or redistributes the Residual Stress formed by the welding Process by involving 4-step processes like Pre-Heating, Heating, Soaking & Cooling the Welded Surface to controlled temperatures for a specified amount of time. This enhances the properties of the welded surface.
Helps to improve the Metallurgical Structure.
Helps to improve the Ductility of the Material.
Reduces the risk of Brittle Fracture as the Ductility increases.
Relaxed Thermal Stress due to the redistribution of Residual Stresses.
Safety and Precautions of Post Weld Heat Treatment (PWHT) .
PWHT must be performed by certified experts with experience in this field.
The portion exposed to PWHT must be covered properly to protect it from rain or wind.
A hardness test must be performed to check if Post weld heat treatment is successfully performed.
Some metals and alloys can experience cracks during PWHT. Therefore, the alloy system must be taken into account.
Thermocouples are used for monitoring PWHT to assure heating rates, cooling rates, and hold temperature is within standard specifications. Along with thermocouples computer software is used to provide documentation about PWHT.
Why Choose Us?
We have a team of highly skilled and experienced PWHT technicians that will ensure that all machined surfaces or weldments that are to be treated meet the required welding code requirements. That way, we can know if they are fit for the required process.
The biggest benefit of partnering with us or hiring our services is the fact that we have a global reach. Our non-destructive inspections are mandatory after PHWT, which includes magnetic particle inspection, hardness testing, and stress radiography. Plus, we have metallurgical laboratories and engineers with a lot of experience and knowledge, allowing us to execute all types of pre weld heat treatment procedures while ensuring complete Quality Assurance.
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