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Cracking risk in steel pipelines from external hard zones

Back to Pre-1998 Articles Adhesive technology demonstration centre Aged samples lend added credence to safety codes Arc welding Demonstration Centre shows best practice in action Boiler takes the heat in comfort Conductive adhesive for ultrasound catheter cracking-risk-in-steel-pipelines-from-external-hard-zones Engineering critical assessment of pipeline welds Far East fitness-for-purpose check First UK Research Council contract for TWI Friction stir welding of titanium Help with adhesives training Joining demonstration centres in industry Joining Forces success in Belfast Joints take 10 seconds in the microwave LIVEMAN - Advanced joining processes for lightweight vehicle manufacture Making calculations easier 'Mildly sour' environment project saves half a million pounds New standard for weld fracture toughness testing Novel method joins plastic pipes Plastics fume - new findings released Project saves time and money for Amerada Hess Small firms seize no-cost reviews and low-cost trials South East Asia ACFM course Space Shuttle technology transferred to power generation industry Spot the evidence Structural integrity assured for Liverpool Bay Supporting technology on the Indian sub-continent Technology demonstrators show best practice in action Thermomechanical material processing by friction [friction pillar processing] 'Tubestress' measures residual stresses in parts that other equipment cannot reach TWI enhances on-stream inspection service Underwater welding work expands at TWI North Wider recognition for welding inspectors

Connect, no.81, November/December 1996


Relaxed hardness limits for sour service are given for cap regions of pipeline girth welds in BS4515, but are these appropriate when the pipe is cathodically polarised for external corrosion protection?

Ferritic steels are potentially susceptible to in-service cracking because of hydrogen pick-up from a corrosion reaction. Susceptibility to hydrogen-induced cracking increases with material hardness so there is a risk of weld-joint failure as a result of local formation of hardened structures. It is therefore normal industrial practice to specify maximum hardness limits for steels and weldments.

With pipelines, the sour (H2S) environment is within the pipe and hydrogen concentration will be greatest at the inner surface. However, hydrogen diffuses through the pipe but concentrations are lower on the external surface so cracking risk is reduced and higher hardness levels are acceptable. Results from work in this area done at TWI over the last decade have been incorporated into BS4515.

In service, the external pipeline surface may well be subjected to cathodic protection which could restrict hydrogen leakage from the inner surface. This is one reason why the allowable external hardness in BS4515 is well below experimentally-determined values.

A Group Sponsored Project just starting at TWI aims to provide data on the significance of cathodic protection in restricting hardness levels at welds on the outside of linepipe. It aims to confirm that an appropriate level of conservation has been built-in to the standard - neither unsafe nor too restricting.

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