what is 885°F or 475°C Embrittlement?


885°F or 475°C Embrittlement

It is a loss of ductility and fracture toughness of the material, due to a metallurgical change, that can occur in stainless steels, containing a ferrite phase, as the result of exposure in the temperature range 600 °F to 1000 °F or 315 °C to 540 °C. This type of embrittlement can lead to cracking failure.

Watch this YouTube video for full classroom training on API 571 Damage Mechanism- 885°F or 475°C Embrittlement.

When the stainless steels are heated into the range 400 – 550°C, although the effect is most pronounced at 475°C, a dramatic fall in toughness is observed after extended exposure. This is due to the formation of chromium-rich domains and precipitates within the iron-rich matrix by spinodal decomposition of ferrite at these temperatures. This effect becomes more pronounced as chromium content increases.

This phenomenon is termed 475°C embrittlement as the rate of embrittlement is highest at 475°C. It is also called thermal aging embrittlement.

Which materials are being affected by the 885°F Embrittlement

 400 series SS (for example 405, 409, 410, 410 S, 430, and 446).

Duplex stainless steels such as Alloys 2 2 05, 2 3 04, and 2 5 07.

Austenitic (300 series) stainless steel weld metals, which normally contain up to about 10 % ferrite phase to prevent hot cracking during welding.

Critical Factors for the 885°F Embrittlement

a.  Alloy composition, particularly chromium content, amount of   ferrite phase, and Operating temperature.

b. The lower-chromium alloys, (For example SS4 0 5, SS4 0 9, SS 4 1 0, and 410 S) are less susceptible to embrittlement. The higher chromium ferritic stainless steels [for example SS 4 3 0, which contains 16 % to 18 % Chromium) and SS 4 4 6 containing 23 % to 27 % chromium and duplex stainless steels (22 % to 25 % Cr) are much more susceptible. In 300 series, alloy SS308 and SS347H have found evidence of 885 °F embrittlement in Charpy impact testing. These alloys weld metals were aged in approx. 850 to 885 °F.

  • Increasing amounts of ferrite phase in duplex stainless steels increase susceptibility to damage when operating in the high-temperature range of concern. A dramatic increase in the ductile-to-brittle transition temperature will occur. Duplex stainless steels also need to be cooled rapidly after welding to avoid formation of embrittling phases.
  • High-temperature exposure is required for embrittlement. A primary consideration is operating time at temperature within the critical temperature range. Damage is cumulative and results from the formation of an embrittling ordered metallic phase or alpha prime phase that occurs most readily at approximately 885 °F. Additional time is required to reach maximum embrittlement at temperatures above or below 885 °F. For example, many thousands of hours may be required to cause embrittlement at 600 °F or 315 °C.
  • The effect on toughness is not pronounced at the operating temperature but is significant at lower temperatures experienced during plant shutdowns, start-up, or upsets.
  • Embrittlement can also result from heat treatment if the material is held within or cooled slowly through the embrittlement range.

Affected Units or Equipment’s in Refinery

  • 885 °F embrittlement can be found in any unit where susceptible alloys are exposed to the embrittling temperature range. Most refineries limit the use of ferritic steel to non-pressure boundary applications because of this damage mechanism.
  • Common examples include fractionator trays and internals in high-temperature vessels used in crude, vacuum, fluid catalytic cracker (FCC), and coker units. Typical failures include cracking when attempting to weld or to straighten bent, upset tower trays made of Type 4 0 9 and 4 1 0 SS. (This occurs often with vacuum tower trays of this material.)
  • Other examples include duplex stainless-steel heat exchanger tubes and other components exposed to temperatures above 600 °F or 315 °C for extended time periods. Duplex stainless steels are normally limited to a maximum service temperature of 600 °F.

Appearance or Morphology of Damage

  • 885 °F embrittlement is a metallurgical change that is not readily apparent with metallography.
  • The existence of 885 °F embrittlement can possibly be identified by an increase in hardness in affected areas. Failure during bend testing or impact testing of samples removed from service is the most positive indicator of embrittlement.
  • Most cases of embrittlement are found in the form of cracking during turnarounds or during start-up or shutdown when the material is at lower temperature where the effects of embrittlement are most detrimental. Embrittled 4 1 0 SS has been shown to require a temperature of about 350 °F or 175 °C before adequate toughness has been restored.

Prevention / Mitigation of Graphitization

  • The best practice to prevent 885 °F embrittlement is to avoid exposing the susceptible material to the embrittling range or to use a non-susceptible material.
  • Cracking of embrittled material can often be avoided through temperature controls during start-up and shutdown.
  • 885 °F embrittlement is reversible by heat treatment followed by rapid cooling. The de-embrittling heat treatment temperature is typically 1100 °F  or 595 °C or higher and may not be practical for many equipment items. If the de-embrittled component is exposed to the same service conditions, it will re-embrittle faster than it did initially.

Inspection and Monitoring

  • This damage mechanism is very difficult to find prior to equipment failure. It is also time dependent and may take a while to develop in service. Online inspection is not applicable.
  • The most effective method of detecting or confirming embrittlement is removing and impact or bend testing a sample of the suspect material. A failed bend test, confirm the presence of embrittlement.
  • Visual inspection to seek cracking presence is also one indicator.
  • Field hardness testing may distinguish embrittled from non-embrittled material, but hardness testing alone is generally not definitive. Also, the hardness test itself may produce cracking, depending on the degree of embrittlement.
  • Hammer testing (“field impact testing”) is considered a destructive test. Tapping a suspect component with a hammer may crack the component, depending on the degree of embrittlement. Hammer testing might confirm that a component is not badly embrittled, if it does not crack, or that it is embrittled, if it does crack.

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