Creep resistance of Fe–Ni–Cr heat resistant alloys for reformer tube applications

Abstract

Relations between microstructure, phase transformations and creep resistance of austenitic Fe–Ni–Cr alloys are investigated. As-cast alloys with different silicon contents and an ex-service tube are submitted to laboratory agings to trigger specific phase transformations, and subsequently creep-tested at 950°C under stresses of 24–48 MPa. As-cast microstructures contain interdendritic chromium-rich M₇C₃ carbides with niobium-rich MC carbides. After aging at 950°C, primary M₇C₃ carbides transform into chromium-rich M₂₃C₆ carbides, associated to a loss in creep strength. The G phase present in the ex-service alloy is reversed into MC carbides by a heat treatment at 1100°C, associated to a slight decrease in creep resistance. Besides, the addition of silicon is highly detrimental to creep strength. Results can be used for alloy design.

KEYWORDS:

• Stainless steel
• Fe–Ni–Cr alloy
• Cr₇C₃
• Cr₂₃C₆
• NbC
• HP alloys
• refractory steel
• precipitate
• precipitation

Acknowledgements

The authors would like to acknowledge the contribution to this work of late Professor René Le Gall.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1 For brevity, in the presentation of results the details of phase identification will not be given since the methods have been comprehensively described elsewhere [18].