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Post-weld heat treatment, or PWHT for short, reduces and redistributes the residual stresses introduced by welding, while also providing for tempering, precipitation, and ageing processes to take place in certain materials.

These changes in the material’s chemical makeup help reduce residual stress and enhance its strength, ultimately relieving tension during welding processes and materials that would otherwise exert unnecessary tension on them. It is imperative to perform PWHT correctly in accordance with welding processes and material strengths in order to provide relief and relieve tension during this process.

Residual Stress Reduction

Post Weld Heat Treatment, or PWHT, helps to alleviate residual stresses caused by welding by increasing its temperature for an appropriate duration (usually 1 hour per inch of thickness).

Residual stress levels combined with load stresses can exceed a material’s design limits, increasing its susceptibility to brittle fracture. To protect against this outcome, PWHT is often employed prior to subjecting welded parts to fatigue loading conditions.

Studies comparing the fatigue crack propagation behavior of Ti-6Al-4V specimens welded by linear friction welding with and without PWHT have demonstrated that shear components of internal residual stress, particularly within the weld center zone (WCZ), contribute to fatigue crack deflection from desired paths; however, when treated at 750 degC PWHT significantly lowers these stress components for better stress relaxation and relaxation.

Stress Corrosion Cracking (SCC) Reduction

SCC differs from conventional corrosion in that it attacks metal’s microstructure instead of just wearing away material, leaving fine cracks that quickly propagate to cause catastrophic failure. Crack formation is often caused by tension coupled with exposure to highly corrosive environments like H2S or chlorides.

Manufacturing processes such as welding, cold working and roll forming can expose materials to internal tensile stresses that approach their yield strength, leading to potential stress corrosion cracking (SCC) development in an aggressive corrosive environment. pwht can help mitigate SCC by relieving internal stress concentration points such as sharp corners or notches which act as crack initiation sites – and by relieving these internal stresses.

SCC can also be reduced through careful structural design, which favors uniform load distribution to eliminate stress hotspots and minimize fluctuations in load conditions – both factors which help limit stress-induced crack initiation.

Mechanical Properties Improvement

Apart from relieving and redistributing residual stress, proper PWHT can also significantly enhance mechanical properties like ductility and toughness. This is largely due to tempering, precipitation or ageing effects which reduce hardness in material which increases ductility while making it more resistant to cracking.

PWHT technology can assist in decreasing fracture toughness by encouraging the formation of acicular martensite instead of grain boundary ferrite, and in doing so significantly reducing plasticity within the weld metal and increasing fatigue resistance of structures.

By following best practices based on the material and strength of materials being welded, PWHT processes can be tailored to maximize stress relief. This involves choosing an effective heat treatment method with appropriate heating/cooling rates and using suitable equipment/facilities; monitoring temperature; maintaining an acceptable weld temperature during welding processes; as well as instituting quality control measures to maintain quality standards.

Increased Durability

No matter your project–pressure vessels, pipelines, or aerospace components–PWHT is a proven technique for increasing weld joint durability by relieving residual stresses and improving mechanical properties. In corrosive environments it also reduces stress corrosion cracking risks by relaxing residual stresses before redistributing them thereby preventing trans-granular cracking from happening. When done properly it also relaxes residual stresses to help prevent stress corrosion cracking from occurring and ultimately stress corrosion cracking occurs less often.

PWHT allows tempering, precipitation or ageing effects to take place which can reduce hardness while increasing ductility and fracture resistance of as-welded materials. Fractography analysis on NiTi wires show that heat treatment results in smoother fracture surfaces than as-welded wires – without river patterns.

Be mindful that incorrect heat treatment application can significantly lower tensile strength, creep strength and notch toughness of welds as well as increase their susceptibility to hydrogen cracking. Therefore, PWHT must only be applied according to an established procedure with defined heating/cooling rates, temperature ranges and durations.