TWI Industrial Member Report Summary 962/2010
by V C M Beaugrand, L S Smith and M G Gittos
Joints employing dissimilar buttering layers are an integral part of subsea oil and gas systems for hubs and tees. The joints of interest incorporate low-alloy steel forgings and nickel-alloy buttering layers. The latter are employed to allow field welds to be made without postweld heat treatment (PWHT). Thus, a typical procedure involves depositing a buttering layer and applying the PWHT appropriate to the forging material. When the field weld is made to a pipeline steel, the HAZ of the weld on the forging side of the joint lies within the nickel-alloy buttering and, therefore, does not require further PWHT. Typical materials are 8630M 4130 or F22 forgings, alloy 625 filler metal and X65 or X70 microalloyed steel pipes. The buttering is conventionally applied by the hot-wire TIG process.
A narrow, partially mixed zone (PMZ) is observed at the dissimilar interface, where the composition is graded between that of the parent forging and the 'bulk' chemistry of the first layer buttering runs. In order to prevent corrosion of piping systems subsea, cathodic protection (CP) is applied. Although this does inhibit corrosion, it can result in a certain amount of hydrogen charging. Whilst the vast majority of subsea joints have given successful service, a small number has failed with significant consequence, warranting the present study.
The microstructure and failure modes in commercially produced, hot-wire TIG buttered, 8630M dissimilar joints, have been detailed in a previous report (Beaugrand and Smith, 2008). The work conducted previously at TWI demonstrated the importance of the PMZ chemistry and microstructure on environmental performance. Modifying the process or procedure might be a way of improving the performance of 8630M buttered with alloy 625, by either reducing the degree of penetration into the parent metal to generate a Ni-rich PMZ or minimise the formation of swirls of steel-rich material which penetrate into the weld.
The present work aimed to determine the effect of fabrication parameters on the microstructure, chemistry and metallurgy of dissimilar interfaces. Test variables included forging material, buttering process and procedure and PWHT. Their impact on environmental performance was also investigated.
- Effect of fabrication parameters on the microstructure and chemistry and thus the performance of dissimilar combinations.
- Influence of parent material on environmental performance.
- Effect of PWHT on environmental performance.