Case study – What are the consequences of excessive PWHT time for 12Cr1MoVG header tanks?
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A header made of 12Cr1MoVG, with a thickness of 16 mm; the heat treatment was carried out at a constant temperature of 670 degrees Celsius for 0.8 hours. The actual heat treatment is 670 degrees Celsius for 3 hours. Everyone, what impact does this have on materials and products?
It is a boiler component designed for a temperature of 450 degrees Celsius. Design pressure: 4 MPa.
It is recommended to create a welding process verification specimen; if there are no issues, then it’s fine.
There shouldn’t be too many problems, just my personal opinion. I hope someone knowledgeable can answer this.
1 The welding process generates welding stress or an increase in hardening elements within the welding material, which leads to an increase in hardness at the weld joint and in the heat-affected zone, thereby increasing the tendency for cracks to form. The heat treatment process that combines heating temperature and time is aimed at reducing peak stress; the testing method involves conducting a hardness test after heat treatment. For the increase in hardness caused by welding stress, heat treatment is quite effective, but its effectiveness is reduced when the increase in hardness is due to hardening elements. The temperature and the duration of maintaining that temperature can be adjusted within a certain range: increasing the temperature reduces the required time, while decreasing the temperature increases it. When the temperature remains constant, an increase in the heat treatment time leads to a more significant reduction in stress. 5 As for whether it is allowed, it depends on to what extent the hardness is reduced. It is permissible if the hardness of the welded joint corresponds to a strength that is still higher than the strength of the base material, or higher than the calculated strength value. 6 For the same welded joint, multiple heat cycles are permitted; the number of such cycles is a key factor affecting strength
First, check and test whether the actual heat-treated specimens meet the requirements, with regard to key mechanical and physical properties, and records and reports should be kept. Then, the welding engineer checks whether it complies with the existing welding procedure qualification; if not, or if it does not match the current welding procedure, welding procedure qualification specimens that meet the actual heat treatment requirements must be added. If these qualification specimens pass the tests as well, then it is acceptable.