Origins of tertiary creep in microalloyed 25Cr–35Ni centrifugally cast alloy tubes

Abstract

Constant stress tensile creep tests have been undertaken in air on as cast 25Cr35Ni austenitic steel at temperatures of 880, 900 and 950°C. The creep curves were found to consist of a relatively short primary stage, a minimum strain rate and an extended tertiary stage. There was no convincing evidence of a period of steady-state creep but the minimum creep rate was found to be highly dependent on applied stress through a power law with a stress exponent of ∼10. The early occurrence of tertiary creep, after strains of only ∼0·01, was shown not to be caused by a loss of section associated with necking, internal void development nor with the formation of surface cracks. It is concluded that the most likely cause of the early onset of tertiary creep in this alloy is the coarsening of strengthening precipitates.

Keywords:

• Austenitic stainless steel
• Tertiary creep
• Voidage
• Section loss
• Threshold stress

Acknowledgements

The paper is published by permission of Doncasters Paralloy. The authors are grateful to Ian Dunkinson, Managing Director, of the company for his support and encouragement during the course of the research.