Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK's advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650uC and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650uC. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650uC in conjunction with the materials properties that were determined from the uniaxial test data. This paper is part of a special issue on Creep-Fatigue Crack Development Nomenclature a 0 Crack depth used to define crack initiation in a specimen or structure a i Crack depth of 0?02 mm which is used to define crack nucleation in a specimen or structure A 1 , P 1 , m 1 , n 1 Parameters in the stress modified ductility exhaustion model, equations (14) and (15) b, B0 Parameters in the stress relaxation equation (2) d c Creep damage per cycle d f Fatigue damage per cycle D c Total creep damage per test D f Total fatigue damage per test E Young's modulus N Cycle number N 5% Number of cycles to a 5% decrease in maximum stress N 25% Number of cycles to a 25% decrease in maximum stress N 0 Continuous cycling fatigue endurance to create a crack of depth a 0 N f Number of cycles failure as defined by complete separation N9 g Number of cycles to grow a fatigue crack from the nucleation size of 0?02 mm to a 0 N i Number of cycles to nucleate a fatigue crack of size of 0?02 mm N pred Predicted number of cycles to the initiation of a creep fatigue crack N obs Observed number of cycles to the chosen failure criterion R p0?2% 0?2% proof stress R p1% 1% proof stress R m Tensile strength t Time t f Time at failure t h Dwell time T Temperature Z Elastic follow-up factor Ds Stress drop during the creep dwell s 0 2s e c Creep strain that accumulates during a creep dwell _ e e c Instantaneous creep strain rate e f Uniaxial creep ductility De T Total strain range e U Upper shelf total inelastic strain at failure s Instantaneous stress

Improved creep‐fatigue and remnant tensile/creep properties of 321 stainless steel by self‐healing coating

“…As mentioned before, an awareness of remnant creep and tensile properties is essential for the readiness against service failures and remnant life assessment 3,4 . Hence, Figure 6 compares the remnant creep properties of the uncoated, aluminized, and self‐healing 321 steel samples with different prior C‐F lifetime fraction.…”

“…As mentioned before, an awareness of remnant creep and tensile properties is essential for the readiness against service failures and remnant life assessment. 3,4 Hence, Figure 6 C-F exposure of these samples was selected as 30 s due to the consideration of good combination of fatiguedominant damage and creep-dominant damage. 39 Detailed reasons can refer to our previous work.…”

“…Because the density of the additional generated TiO 2 phase (4.26 g/cm 3 ) is lower than that of TiC phase (4.93 g/cm 3 ), there would be volume increment around the area where the above oxygen reaction takes place. As a result, the pores and/or cracks around these areas would be filled by such volume increment, showing the occurrence of self-healing behavior.…”

“…Meanwhile, the mechanical properties of 321 steel would continuously alter due to the dynamic microstructural evolution occurring during service. Consequently, understanding the remnant mechanical properties of 321 steel after C‐F exposure is essential for its remnant life assessment and readiness against service failure 3,4 . In this regard, the remnant creep and tensile properties following sole creep/fatigue or C‐F interaction loading are generally recognized as crucial indicators of the remnant service life of 321 steel.…”