99 Diseases of Pressure Equipment: Temper Embrittlement

Temper embrittlement is another form of metallurgical degradation resulting from exposure of susceptible low alloy steels to higher temperature ranges, usually in service, but can occur to some extent even during heat treatment. And, once again, if significant temper embrittlement has occurred, the equipment may be susceptible to catastrophic brittle fracture. The low alloy steel most susceptible is the 2.25Cr- 1Mo steel, from which so much of our industry’s heavy wall hydroprocess equipment is fabricated. It’s now fairly well established that certain low levels of “tramp” elements, i.e. contaminating elements, enhance the potential for temper embrittlement. These contaminants enter into steel making process and fabrication, i.e. welding, without the steel- makers or fabricators intent. Those include: phosphorous, tin, antimony and arsenic. The level of these elements, plus the % of manganese and silicon determine the overall susceptibility to temper embrittlement. 

Newer steels are much less susceptible, because the phenomena has been well studied and control of the notorious “tramp” elements is much better than in older steels. An excellent reference for specifying materials and fabrication practices for new hydroprocess equipment is API RP 934(2). But of course, most of the industry hydroprocess equipment was fabricated before the early 70’s when the newer steels became available, so the industry is going to need to pay attention to the temper embrittlement issue as long as the older equipment remains in service.

Like most other metallurgical degradation phenomena, temper embrittlement cannot be detected by normal inspection methods, and once again, toughness testing is the only conclusive way of determining the extent of embrittlement. And since there is no practical NDE method for toughness testing, users are mostly forced to assume their steel is potentially temper embrittled and take precautions to avoid critical defects that might propagate brittlely in temperature ranges where the steel exhibits low toughness. The primary precaution is to use controlled start-up and cool-down (pressure and temperature combinations) of heavy wall equipment in temperature ranges below about 350F.

Do you have an effective management system for controlled start-ups and shutdowns for heavy wall hydroprocess equipment that may be susceptible to temper embrittlement, and therefore susceptible to brittle fracture?