885°F (475°C) Embrittlement | Materials And Corrosion Control

Damage Description

Critical factors include:

·         The critical factors are alloy composition, particularly chromium content, amount of ferrite phase, and operating temperature.

·         Increasing amounts of ferrite phase 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.

·         A primary consideration is operating time at temperature within the critical temperature range. Damage is cumulative and results from the precipitation of an embrittling intermetallic 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 (316°C).

·         The effect on toughness is not pronounced at the operating temperature, but is significant at lower temperatures experienced during plant shutdowns, startups or upsets.

·         Embrittlement can result from tempering at higher temperatures or by holding within or cooling through the transformation range.

Materials & Equipment

400 series stainless steels (e.g., 405, 409, 410, 410S, 430, and 446), Duplex stainless steels such as Alloys 2205, 2304, and 2507 and wrought and cast 300 series stainless steels containing ferrite, particularly welds and weld overlays.

Equipment

·         885°F embrittlement can be found in any unit where susceptible alloys are exposed to the embrittling temperature range.

·         Most companies limit the use of ferritic stainless steels to non-pressure boundary applications because of this damage mechanism.

·         Common examples include fractionator trays and internals in high temperature vessels used in FCC, crude, vacuum and coker units. Typical failures include cracking when attempting to weld or to straighten bent, upset tower trays of Type 409 and 410 materials (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 (316°C) for extended time periods.

Control Methodology

·         It is possible to minimize the effects of embrittlement through modifications in the chemical composition of the alloy; however, resistant material may not always be readily available in most commercial forms.

·         It is reversible by heat treatment to dissolve precipitates, followed by rapid cooling. The de-embrittling heat treatment temperature is typically 1100°F (593°C) or higher and may not be practical. If the de-embrittled component is exposed to the same service conditions it will re-embrittle faster than it did initially.

Monitoring Techniques

·         The existence of 885°F embrittlement can be identified by an increase in hardness in affected areas.

·         Most cases of embrittlement are found in the form of cracking during turnarounds or during startup and shutdown when the material is below about 200°F (93°C).

Inspection Frequency

KPIs

Reference