Guocai Chai

Study of crack initiation or damage in very high cycle fatigue using ultrasonic fatigue test and microstructure analysis

Fatigue damage behaviors of four metal materials in the very high cycle fatigue (VHCF) regime have been studied using ultrasonic fatigue test and microstructure analysis. The results show that the fatigue crack initiation in VHCF regime could occur at subsurface non-defect fatigue crack origin (SNDFCO), where the accumulated cyclic strains or damage in the specimens were highly localized, especially in the materials with some softer phase, where the local maximum strain can be eight times higher than the average strain value in the specimen. This high strain localization can cause a local plasticity exhaustion that leads to a stress concentration and consequently fatigue crack initiation, and finally the formation of SNDFCO. For pure single phase austenitic material, strain localization can also occur due to dislocation accumulation at or near grain boundaries, which can become fatigue crack initiation origin in the VHCF regime. The results in this study show that fatigue damage and crack initiation mechanisms in the VHCF regime can be different in different metals due to the mechanisms for local plasticity exhaustion.

Creep behavior of the newly developed advanced heat resistant austenitic stainless steel grade UNS S31035

The UNS S31035 austenitic stainless steel grade is a newly developed advanced heat resistant material for use in coal fired boilers at material temperatures up to about 700°C. This new grade has go ...

In Situ X-Ray Diffraction Study of Load Partitioning and Microyielding for the Super Duplex Stainless Steel SAF2507 (UNS S32750)

The deformation behaviour of the super duplex stainless steel SAF2507 (UNS S32750) under successive uniaxial tensile loading-unloading was investigated with respect to load sharing and inter-phase interactions. The steel consists of 58% austenite and 42% ferrite in volume. By insitu X-ray diffraction experiment the evolution of phase-specific stresses with applied load was monitored for three successive loading-unloading cycles with the maximum total strains being 0.34%, 0.75% and 1.63%, respectively. It was found that yielding occurred earlier in the austenitic phase than in the ferritic phase during the first loading cycle. In the followed loading cycles, both phases yielded under larger but similar applied stresses. Due to a similar behavior of the phases in the elasto-plastic regime inter-phase interactions were relatively weak. Low microstresses induced by the plastic straining resulted in somewhat larger stresses in the ferritic phase.

Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel (AISI 316L) and one nickel-base alloy (Alloy 617) have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromechanisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650°C.

Advanced Heat Resistant Austenitic Stainless Steels

UNS S31035 is a newly developed austenitic stainless steel with the highest creep strength among the commercial available heat resistant grades for the next generation of coal fired power plants. Alloy 800HT is a well developed material that has been recommended as a candidate material for generation IV nuclear power plants. In this chapter, several advanced heat resistant austenitic stainless steels are reviewed, but the focus will be on these two materials. The influences of composition on the structural stability and on the creep behavior are discussed. The creep mechanisms at different temperatures and loading conditions have been identified. The interaction between dislocations and precipitates and their contribution to the creep rupture strength are discussed. Different models have been used to evaluate the long-term creep behavior of the grades. Finally, highly alloyed composite tube products for different corrosive steam boiler applications are introduced.

On the Development of Grain-Orientation-Dependent and Inter-Phase Stresses in a Super Duplex Stainless Steel under Uniaxial Loading

Microstresses due to intergranular and inter-phase interactions in an austenitic-ferritic super duplex steel (SAF 2507) under uniaxial compressive deformation have been studied by in-situ neutron diffraction experiments. Lattice strains of several hkl planes of austenite respective ferrite were mapped as a function of sample direction at a number of load levels during loading into the plastic regime and unloading. The analysis of the experimental results has shown that during loading both grain-orientation-dependent and inter-phase stresses were generated under plastic deformation that was inhomogeneous at the microstructural level. Residual stresses depending on the grain-orientation and phase have been found after unloading. The results also indicate stronger intergranular interactions among the studied hkl planes of austenite than those of ferrite.

Characterization of Austenitic Stainless Steels Deformed at Elevated Temperature

Highly alloyed austenitic stainless steels are promising candidates to replace more expensive nickel-based alloys within the energy-producing industry. The present study investigates the deformation mechanisms by microstructural characterization, mechanical properties and stress–strain response of three commercial austenitic stainless steels and two commercial nickel-based alloys using uniaxial tensile tests at elevated temperatures from 673 K (400

Mechanisms of Hydrogen Induced Stress Crack Initiation and Propagation in Super Duplex Stainless Steels

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