ABSTRACT

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.

INTRODUCTION

Sigma phase is a non-magnetic intermetallic phase composed mainly of iron and chromium which forms in ferritic and austenitic stainless steels during exposure at 560º-980ºC (1,050º-1,800ºF). It causes loss of ductility, toughness and is generally strain intolerant at temperatures under 120º-150ºC (250º-300ºF) but it is believed it has little effect on properties in the temperature range where it forms. If this is so it would appear that there should be little consequence as long as the affected components continuously operate at the elevated temperature. However, cracking could occur if the components were impact loaded or excessively stressed during maintenance work.

Austenitic stainless steel Type 304H (TP 304H SS) has traditionally been used in FCC regenerator internals, associated equipment, and piping involving temperatures about 650º-760ºC (1,200º-1,400ºF). This stainless steel is chosen as it meets a cost effective solution for a material with the necessary oxidation resistance, strength, and creep properties for this service. Over time, however, as the ductility and toughness decrease because of the presence of sigma phase, the question is often asked as to replacement timing or criteria for replacement. The effect of sigma phase on the degradation of creep properties has also been a concern but this issue was not Sigma Phase Embrittlement of Stainless Steel in FCC Service addressed in this paper. To assess the effect of sigma phase on reducing creep properties and ductility, creep testing of samples removed from service may be necessary.

After having been involved in several studies in different FCC and other oil refining process units, the opportunity was taken to select some relevant findings and testing results obtained when examining sigmatized austenitic stainless steels. This study is intended to complement the information available in previous publications on the same subject of sigma phase embrittlement and to propose a criterion to assist in the decision making process.

EXPERIMENTAL PROCEDURE

The presence of sigma phase in TP 304H SS is determined by metallography, either using samples removed from service or field metallography replication (FMR or in-situ metallography). This steel is delivered in the solution-annealed condition when it contains all its alloying elements in solid solution and hence its microstructure appears as a fairly homogeneous single phase alloy. This microstructure changes upon aging. F

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