Post-Weld Heat Treatment (PWHT)
In many cases, no post-weld heat treatment is necessary. However, when the material is required to have the utmost reliability under severe corrosive conditions, it is advisable to consider a heat treatment to ensure that the weld and heat-affected zone is fully restored to an optimum corrosion-resistant structure. This also ensures the relief of any residual internal stresses retained after welding which might otherwise initiate stress-corrosion.

A simple stress-relief anneal may be carried out at temperatures as low as 300-350�C for a time dependent on section thickness. The heat treatment needed to develop optimum corrosion resistance will depend on alloy, section thickness(es) and properties required and should therefore be determined from experience after expert advice. As an example, for weld-fabricated continuously cast tubes to be used in severe conditions, one treatment specified is a soak at 700-730�C for 6 hours followed by a cooling rate not exceeding 250�/hour. For thicker cast sections, such as propellers, some specifications call for soaking times of at least 20 minutes per 25mm of section thickness and rates of cooling not exceeding 50�/hour down to 100�C.

Very much improved properties can be obtained in the duplex alloys by a full heat treatment such as a soak at 925-950�C and quench followed by tempering at 650-700�C and slow cool to restore corrosion resistance. This type of treatment is not often called for because of its cost, the tendency for it to cause distortion and the fact that existing properties are frequently adequate.

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