TIME TO INITIATION OF TERTIARY CREEP PROPERTY OF GRADE 91 STEELS

Proceedings of the ASME 2016 Pressure Vessels and Piping Conference

PVP2016

July 17-21, 2016, Vancouver, British Columbia, Canada

PVP2016-63356

TIME TO INITIATION OF TERTIARY CREEP PROPERTY OF GRADE 91 STEELS

Kazuhiro Kimura

National Institute for Materials Science

Tsukuba, Ibaraki, Japan

ABSTRACT

Creep deformation property of Grade 91 steels was analyzed on more than 370 creep curves over a wide range of time to rupture from about 10 hours to beyond 100,000 hours, in order to evaluate time to 1 % total strain, time to minimum creep rate and time to initiation of tertiary creep. Time to initiation of tertiary creep was assessed as a 0.2 % offset with a slope of minimum creep rate. It is difficult to determine time to minimum creep rate precisely, which is a basis of 0.2 % offset, however, it has been confirmed that time to initiation of tertiary creep is not sensitive to the time when the creep rate indicates minimum value. Life ratio of 1% total strain time against creep rupture time increases up to about 60% with increase of temperature and decrease of stress. Life ratio of time to initiation of tertiary creep also tends to increase with decrease in stress. However, change of it is in a range of 50 to 60 % of creep rupture life over a wide range of creep rupture life from 10 hours to 100,000 hours, and it is not sensitive to creep test temperature. Over a range of temperatures from 500 to 600°C and up to about 200,000 hours, a temperature and time-dependent stress intensity limit, St is controlled by 67% of minimum stress to rupture. However, a difference between 67% of minimum stress to rupture and 80% of minimum stress to initiation of tertiary creep decreases with increases in temperature and time, and both values approach each other in the long-term beyond about 100,000 hours at 600°C. In the long-term beyond about 10,000 hours at 650°C, St is controlled by 80% of minimum stress to initiation of tertiary. The stable life fraction of time to initiation of tertiary creep establish a reliability of a temperature and time-dependent stress intensity limit value.

INTRODUCTION

After development of Mod.9Cr-1Mo steel as Grade 91, several type of creep strength enhanced ferritic (CSEF) steels have been developed and widely used as high temperature structural components of Ultra Supercritical (USC) power plant and heat recovery steam generator (HRSG) of combined cycle

Kota Sawada

National Institute for Materials Science

Tsukuba, Ibaraki, Japan

power plant. In addition to the application of CSEF steels to modern thermal power plants, research and development have been carried out to apply Mod.9Cr-1Mo steel to high temperature components of demonstration fast reactor which is called as Japan Sodium-cooled Fast Reactor (JSFR) [1]. Design life of JSFR is 60 years, therefore, time-dependent allowable stresses up to 525,000 hours are required. In order to establish temperature and time-dependent stress intensity factor, not only creep rupture strength, but also both time to 1% total strain and time to initiation of tertiary creep must be evaluated [2]. In the previous study on creep equation of Grade 91 steels [3], it has been demonstrated that 80% of stress to time to initiation of tertiary creep tends to be a governing factor to determine temperature and time-dependent stress intensity limit rather than 67% of stress to rupture. A previous study has been conducted on creep deformation property of single heat, and precise evaluation of time to initiation of tertiary creep is important for accurate evaluation of temperature and time-dependent stress intensity limit.

In the present study, time to initiation of tertiary creep and time to 1% total strain have been evaluated on more than 370 creep curves of Grade 91 steels over a wide range of time to rupture from about 10 hours to beyond 100,000 hours. Creep rupture life ratio of times to both initiation of tertiary creep and 1% total strain are investigated. Stress versus time to rupture, time to 1% total strain and time to initiation of tertiary creep have been analyzed by means of region splitting analysis method [4], and temperature and time-dependent stress intensity limit of the steels has been evaluated.

NOMENCLATURE

t time (h)

tR time to rupture (h)

tm time to minimum creep rate (h) ti time to initiation of tertiary creep (h) t1% time to 1% total strain (h) St temperature and time-dependent stress intensity limit n number of the data

1

Copyright ? 2016 by ASME

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