Work at TWI has confirmed that hardness limits for pipe welds can be relaxed without increased risk of sulphide stress cracking (SSC) and has paved the way for the wider use of high-productivity welding systems. The work was supported by the Pipeline Research Committee of the American Gas Association (AGA).
In oil and gas pipelines and other plant, exposure to sour (H2S containing) environments can induce premature service failure. Corrosion reactions involving H2S produce hydrogen which enters the base metal and may embrittle it, leading to SSC.
The SSC susceptibility of steels increases with hardness and strength. Avoidance of the problem therefore requires limitations to be imposed on material hardness. This is particularly important at welded joints, where rapid cooling produces hardened microstructures in the heat affected zone (HAZ).
The maximum hardness limit stipulated in NACE MR 01 75 to avoid SSC of the base metal is 22HRC for C Mn and similar steels and currently this limit is applied to all regions of welds in pipelines and other oilfield equipment. Since the corrosive environment is generally on the inside of the pipe (where low hardnesses are achievable through reheating of the root pass by subsequent runs), blanket application of the hardness criterion to cap as well as root pass regions is, in principle, conservative, and may greatly increase the cost of pipeline fabrication.
There are, however, few quantitative data to confirm this view, and accordingly an experimental programme was undertaken at Abington to explore the degree to which the limit could be relaxed for capping pass weld regions.
The work was designed to examine the influence of hardness and stress level on the cracking propensity of MIG welds in pipeline steel and was carried out in two phases. First, the hardness limit to avoiding cracking was determined for internal pipe weld beads (simulating weld root regions) in sour NACE H2S solution while, in Phase 2, external beads on welds (simulating weld cap regions) were assessed, again with the sour environment on the inside of the pipe.