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What is meant by post-heating and how is it applied?

 

Post-heating refers to the maintenance of preheat after the weld has been completed, to allow increased rates of hydrogen evolution from the weld to occur. The post-heat temperature may be the same as, or greater than, the original preheat temperature specified. Post-heating is different to postweld heat treatment (FAQ: What is meant by postweld heat treatment/stress relief heat treatment of ferritic steels?). 

Post-heat is not usually mandatory in standards, but can be specified by clients. There is some guidance (approximate temperature and length of time required) in standards [1] but this is non-specific and further advice may need to be taken. A post-heat requirement in a weld procedure usually implies the need for preheat.

Post-heat requires full control over time and temperature, to be effective in the removal of hydrogen from the weld, and controlled techniques incorporating thermocouples are preferable. Uncontrolled methods such as hand held flame torches monitored by temperature indicating crayons (see FAQ: What is a Tempil stick?) are best avoided.

Hydrogen bake-out is a technique closely related to post-heat. Post-heat is a fabrication hydrogen evolution process. Hydrogen bake-out is usually done during repair or modification of a component which has seen service in a hydrogen environment, to reduce the hydrogen that has been introduced in service, before attempting to weld the component. Hydrogen release is a term often used to refer to both or either of these processes.

Reference

1.BS EN 1011-2: 'Welding - recommendations for welding metallic materials - Part 2 : Arc welding of ferritic steels' British Standards Institution, 2001 (Annex C).

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