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Inspectioneering Journal

99 Diseases of Pressure Equipment: Hydrogen Stress Cracking

By John Reynolds at Intertek. This article appears in the January/February 2005 issue of Inspectioneering Journal

Hydrogen stress cracking occurs when corrosion from acids like wet hydrogen sulfide or hydrofluoric acid (HF) cause atomic hydrogen to penetrate hardened or higher strength steels and cause stress cracking. Steel hardness, strength and stresses present are the critical factors determining susceptibility. When hydrogen stress cracking is a threat, equipment is often preheated and/or post weld heat treated (PWHT) to reduce hardness and residual stress levels. Typically, a maximum of 200HB is specified for normal carbon steels in hydrogen stress cracking environments; and hardness should be checked after PWHT. Keeping the carbon equivalent below 0.43 is also helpful. Bolting materials such as B7M, which is softer than the standard B7 bolting material, is also more resistant to hydrogen stress cracking and is especially important for internal bolting materials, like those on floating heads of exchangers in sour service.

Zones of high hardness, which are susceptible to hydrogen stress cracking, can sometimes be found in the cover pass of welds, because they are not tempered by subsequent weldpasses. Another cause of cracking occurs when metals or welds which contain higher amounts of residual elements (that can increase HAZ hardness) are exposed to hydrogen stress cracking environments. Because hydrogen stress cracking is nearly always a surface phenomena, most any typical surface NDE is sufficient to find such cracking, and it’s not unusual for the cracks to be clearly visible to the naked eye, especially those that are transverse across a weld cap. Hardness testing can find welds and bolting material that will likely be more susceptible to hydrogen stress cracking.

Those sites processing wet hydrogen sulfide and/or HF are familiar with the problem and usually take steps to avoid hydrogen stress cracking. But the problem tends to recur when the specified preheating or PWHT are not carried out at sufficiently high temperatures to reduce hardnesses and the corresponding strength of weldments; or where steels and weldments with residual hardening elements creep into fabricated or repaired equipment.

Do you have a sufficiently robust maintenance and construction QA/QC program to preclude the fabrication of equipment with high hardnesses or high strength bolting for services where hydrogen stress cracking is a potential threat?


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