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06578 Sigma Phase Embrittlement of Stainless Steel in FCC Service

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Assessment of sigma phase embrittlement in austenitic stainless steels such as Type 304H, commonly used in fluid catalytic cracking (FCC) units. Other austenitic stainless steels used in other refining process units are also discussed. The detection and measuring of the amount of sigma phase were made using metallography. 

Product Number: 51300-06578-SG
ISBN: 06578 2006 CP
Author: Jorge Hau, Antonio Seijas
Publication Date: 2006
Industry: Materials Selection and Design
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This paper describes assessment of sigma phase embrittlement in austenitic stainless steels such as Type 304H, commonly used in fluid catalytic cracking (FCC) units. Other austenitic stainless steels used in other refining process units are also discussed. The detection and measuring of the amount of sigma phase were made using metallography. It was found that the relationship of the amount of sigma phase with time in refining service has not yet been established and that, rather than the amount, the most important parameter is to assess the degree of embrittlement attained. This depends not only on the amount of sigma phase but also on the size and distribution, as well as the presence, amount, size and distribution of other intermetallic particles that also precipitate during service. Charpy V-notch (CVN) tests indicate the overall effect and contribution of all these factors. It is proposed to use the requirement of meeting 20 J (15 ft-lbf) at 0°C (32°F) with no single value less than 13 J (10 ft-lbf). Although no criterion was given for hot impact testing, it is considered that CVN tests conducted at service temperature provides useful information about the degree of embrittlement that applies when the metal is hot.

Keywords: Sigma phase, embrittlement, austenitic stainless steel, FCC, Charpy Test, degradation mechanisms

This paper describes assessment of sigma phase embrittlement in austenitic stainless steels such as Type 304H, commonly used in fluid catalytic cracking (FCC) units. Other austenitic stainless steels used in other refining process units are also discussed. The detection and measuring of the amount of sigma phase were made using metallography. It was found that the relationship of the amount of sigma phase with time in refining service has not yet been established and that, rather than the amount, the most important parameter is to assess the degree of embrittlement attained. This depends not only on the amount of sigma phase but also on the size and distribution, as well as the presence, amount, size and distribution of other intermetallic particles that also precipitate during service. Charpy V-notch (CVN) tests indicate the overall effect and contribution of all these factors. It is proposed to use the requirement of meeting 20 J (15 ft-lbf) at 0°C (32°F) with no single value less than 13 J (10 ft-lbf). Although no criterion was given for hot impact testing, it is considered that CVN tests conducted at service temperature provides useful information about the degree of embrittlement that applies when the metal is hot.

Keywords: Sigma phase, embrittlement, austenitic stainless steel, FCC, Charpy Test, degradation mechanisms

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Results are described of sigma phase embrittlement assessments performed at three different refineries and mainly involving regenerator cyclones at fluidized catalytic cracking units (FCCU) but also including regenerator cyclones hanger rods and a regenerator flue gas line.
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