For sour service conditions there are stringent weldment hardness requirements that must be satisfied in order to minimise the risk of sulfide stress corrosion cracking (SSC) in service. Currently in standards such as DNV OS-F101:2000 and BS4515:2000 the hardness threshold requirements remain unaltered for welds that are strained, for example, as can occur during pipe reeling and laying operations. Work has been carried out to investigate the SSC behaviour of samples, taken from a pipe girth weld, that were strained to conditions that simulated those of a typical pipe reeling operation and conventional hardness limits were shown to be unsafe. Samples were taken from a C-Mn steel girth welded pipe, manufactured to API 5L X65, and were subjected to tensile and compressive strain to simulate the pipe installation. Four point bend sulfide stress corrosion cracking tests of 30 days duration were performed on specimens manufactured from the strained and unstrained material in NACE TM0177 solution A. The results showed that the application of such prestrain to the specimens led to failure by SSC despite the HAZ hardness remaining below the allowable limits specified in the relevant codes (e.g. ISO15156, DNV OS-F101 and BS4515).
For sour service conditions there are stringent weldment hardness requirements that must be satisfied in order to minimise the risk of sulfide stress corrosion cracking (SSC) in service. Hardness testing is routinely carried out as part of the weld procedure qualification testing (as specified for example in BS4515:2000(1) and DNV OS-F101:2000(2) to ensure that a maximum limit (250HV10 in BS4515) is not exceeded. What is generally not taken into account is the effect of the significant level of strain that can be induced in a weldment during, for example, a pipe reeling and laying operation. Certain codes, such as DNV OS-F101, do require that the mechanical properties (hardness, strength and toughness) are verified in both the as-welded and strain aged conditions, to simulate the strain induced during standard installation. However, even if the effect of strain hardening has been taken into account, and the weldment hardness after pipe laying does not therefore exceed the allowable hardness limits, no account is taken of the possibility that the threshold hardness for SSC may in fact be different for strained material than for unstrained material.
This paper describes work performed under a short study programme into the sulfide stress corrosion cracking behaviour of samples taken from a pipe girth weld that were strained to conditions that simulated those of a typical pipe reeling and laying operation.
