PSI - Issue 17

Zizhen Zhao et al. / Procedia Structural Integrity 17 (2019) 555–561 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

556

2

1. Introduction

Various creep-fatigue life prediction models have been proposed. The linear damage summation (LDS) rule proposed by Miner (1945) and Robinson (1952) represents the earliest approach. Coffin (1974) introduced a frequency term into fatigue to account for holding period effect at elevated temperature and developed the frequency modified life prediction model. Coffin (1976) later proposed the frequency separation approach to account for unbalanced holding conditions. Manson (1972) and Halford et al. (1972) assumed that plastic flow and creep influenced the fracture behavior of material separately or interactively, and introduced the strain range partitioning (SRP) method. Goswami (2004) introduced a ductility model that considered dynamic viscosity as a damage parameter, which was suitable for plastic strain range dominant loadings. Fournier et al. (2008) put forward a model, which dealt with crack initiation and propagation separately and obtained favorable predictions for 9Cr1Mo martensitic steel. Among these life prediction models, the LDS rule has received considerable attention, and it has been adopted in design procedures such as ASME-NH and RCC-MR. 2.25Cr1MoV steel was developed as a modification of 2.25Cr1Mo steel to overcome the problem of temper brittleness in 1980 ’ s. It has now been widely used in components and pipelines in petrochemical process and power generation industries because of its high strength, better resistance against creep and hydrogen attack at elevated temperatures. Fatigue properties and failure mechanisms of ferrite 2.25Cr1Mo steel with various holding periods have been comprehensively investigated by Brinkman et al. (1976), Challenger et al. (1983), Hecht et al. (1998). Corresponding researches of bainite 2.25Cr1Mo(V) steel were carried by Kschinka et al. (1989) and Tian et al. (2016) and Zhang et al. (2016). Yet, reports on ratcheting-fatigue or creep-ratcheting-fatigue behavior of the steel have not been found. The deformation behavior of bainitic 2.25Cr1MoV forged steel was investigated with various hold periods under asymmetric stress cycling at 455 o C in this work. Attention was paid to the influence of holding periods on stress-strain hysteresis loops, fatigue life and creep and fatigue damages. A fatigue life prediction approach for creep ratcheting-fatigue was proposed in the study.

Nomenclature c h

compressed hold period

D c D f

creep damage

fatigue damage N CRF cycles to failure in CRF tests N RF cycles to failure in RF tests N pf predicted fatigue life R stress ratio Δt holding period in a cycle t h tensile hold period t R time to rupture under creep σ a stress amplitude σ m mean stress σ max peak stress σ min valley stress

2. Material and methods

The tested 2.25Cr1MoV steel was provided as a tempered forged ring with its chemical composition given in Table 1. The microstructure was 100 percent bainite, where bainitic ferrite was supersaturated with uniformly distributed granular carbides, and the grain size was about 20μm.