Article Creep-Ductility of High Temperature Steels: A Review

Hi Stuart,

I read your creep-ductility review with interest and would like to acknowledge the prominence you gave my work. However, I do believe there was some misunderstanding of my use of the term intrinsic ductility so I shall try and clarify.

1. I was unaware that the term is used to describe a construction on a creep to failure curve and also a Monkman-Grant analysis. I do not see how these measurements can be intrinsic when the specimen in both cases has to be subjected to a complex deformation path to failure before it can be measured. A term such as “effective ductility” or even “useful ductility” would be more appropriate. Intrinsic surely implies a current state measurement, like a physical property measurement, that makes no change and does no damage, or in the case of the short time stress relaxation test, minimal change to the specimen.

2. You indicate that the claim of detection of a stress-dependent ductility minimum is dubious because of the role played by time-dependent changes on the ductility minimum. My point is that there is a stress-dependent intrinsic ductility minimum in the original state. After exposure with time–dependent changes there is a new intrinsic state. However, in the analysis for T91steel, which you did not reference, which compared data for service exposed and reheat treated conditions, the change in the stress-dependence of the intrinsic ductility minimum was quite small even though the magnitude was significantly altered. Note also that the intrinsic ductility trends were consistent with elongation at failure trends in many heats of this steel (figure 10 in that paper and the text description) (1).

3. In summary, intrinsic ductility to me means an instantaneous or current snap-shot of the ductility capability. After service exposure a similar snap-shot will indicate any change in that capability. The conceptual difference from the traditional approach, including any arbitrary construction on a creep curve, I hope negates your calling the approach “dubious.”

4. Hart and other theoreticians, followed by many experimentalists made a big splash in the USA, notably in the 60s and 70s, and their work was my inspiration. I do not believe the work was followed in Europe. The SRT test as I developed it at high temperatures can be quite difficult to perform in a reproducible and stable way as I believe you had indicated to me. There are a few tests now reported in China and India that look promising.

I hope this helped to get us on the same wavelength,

Best regards,

David

1 David A Woodford, “Comparison of creep strength and intrinsic ductility for serviced and reheat treated T91 steel based on stress relaxation testing,” Mat. at High Temp., 34 (2), p.140-148.