TWI Industrial Member Report Summary 839/2005
By G Hopkin
Sulphide stress corrosion cracking of steel in sour service is prevented by ensuring that the materials used are not susceptible, i.e. that the maximum hardness of the steel does not exceed 250HV, as specified in standards such as ISO 15156.
Grades of steel used in pressure plant and/or process piping often need to be of high strength. The combination of alloying required for through-hardening and microalloying to limit loss of strength on long term tempering, also can lead to the formation of hard martensite in the heat affected zone (HAZ), which does not soften easily on post-weld heat treatment (PWHT).
Post-weld heat treatment will affect the weldment in two distinct ways. Firstly, the microstructure of the HAZ will be altered, and in particular, martensitic and bainitic microstructures will be tempered. These altered microstructures may be more cracking resistant than un-tempered and as-welded microstructures, as has previously been shown for parent steels. Secondly, the overall residual stresses, both micro-stresses and macrostresses, will be reduced by the PWHT due to thermal relaxation.
The purpose of this study was to investigate if the differences between post-weld heat treated and as-welded weldments described above affect the sulphide stress cracking performance. Currently the ISO 15156 limit for HAZ hardness is 250HV, and this work sets out to see if this limit may be relaxed for weldments that have undergone a post-weld heat treatment, and therefore have different microstructures and levels of residual stress to as-welded components of similar hardness.
To assess whether the 250HV hardness limit for sulphide stress cracking of steels in sour service may be relaxed for welds that have undergone a post-weld heat treatment.