Inspectioneering Journal

99 Diseases of Pressure Equipment: Strain-Aging

By John Reynolds, Principal Consultant at Intertek. This article appears in the September/October 2005 issue of Inspectioneering Journal.

Strain-aging problems are another form of metallurgical degradation and thankfully are not very common and becoming less so; but since strain-aging does still occasionally occur, it still makes the list of one of the “99 diseases of pressure equipment”. It occurs mostly in older vintage steels (carbon and low alloy) that were more common back when I went to school 100 years ago, or so it seems. But since most refineries are even older than I am, refineries likely still have some of equipment manufactured with the open-hearth or Bessemer processes, and therefore could be susceptible to strain-aging, and thereby susceptible to potential brittle fracture. Modern steels that are made with the basic oxygen furnace process, and fully killed with aluminum are not susceptible. What happens to the older steels is that they can undergo a form of precipitation-hardening which increases their strength a bit, but more importantly results in lower toughness. The lower toughness in turn could lead to brittle fracture when associated with a critical defect of some sort.

Unfortunately there’s no reasonable way to detect strain- aging before it leads to brittle cracks. Only microstructural examination will reliably detect the problem, though hardness testing may detect the increase in strength associated with strain-aging. It’s just another reason why we need to take extra care with older equipment to assure that we don’t have any critical size defects after welding repairs or modifications are conducted, especially those pieces of equipment that have been cold-worked without stress relief. The greater the thicknesses (above about 0.750 inches) the more susceptible the equipment may be to effects of strain-aging. The good news is that PWHT will usually restore strain-aged material and thereby reduce the potential for brittle fracture.

Have you identified all your older, thicker equipment, so that special precautions, PWHT and/or QA/QC can be applied when welded repairs/ modifications are conducted if the material is susceptible to strainaging?

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