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Cracking risk in steel pipelines from external hard zones

Connect, November/December 1998


Sour service exposure testing of ferritic steel full-pipe section ring test pieces is being undertaken at TWI as part of a recently launched Group Sponsored Project.

This project (GSP 55693) aims to provide data on the significance of cathodic protection in restricting permitted hardness levels at welds on the outside of linepipe. The pre-stressed rings range from 457 to 915mm in diameter with wall thicknesses from 17 to 25mm.

Ferritic steels are susceptible to cracking as a result of hydrogen pick-up particularly in sour (H2S) environments and the cracking risk increases with material hardness. Consequently there is a risk of failure in welded joints because of local formation of hardened structures. As a result it is normal practice to specify maximum hardness limits for steels and weldments operating in such conditions.

In pipelines, the sour environment is contained within the pipe and, although hydrogen does diffuse through the pipe wall, concentrations are lower at the external surface. There is therefore less cracking risk and higher hardness levels are acceptable - as demonstrated by research work at TWI over the last decade and incorporated into BS 4515:1996.

Current exposure tests at TWI simulate service conditions under which the external linepipe surface is given cathodic protection which it is thought will restrict hydrogen evolution, and thus give rise to higher hydrogen concentrations in the steel. Data obtained will evaluate the significance of cathodic protection in affecting safe hardness levels of external welds on the pipeline under test.

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