Austenitic stainless steels have for some time been used as superheater tubes in the electricity generation industries in harsh environments with temperatures as high as 650°C at pressures of some 200 atm; they are expected to provide reliable service for 30 years or more. Their detailed mechanical properties are dependent on the stability of the microstructure, particularly the formation, dissolution, and coarsening of precipitates. Although the precipitation processes have been studied extensively, there remain important discrepancies. It is known that small changes in the chemical composition or thermomechanical processing can profoundly influence the evolution of the microstructure. This review focuses on precipitation in creep resistant austenitic stainless steels, in particular wrought heat resistant grades containing niobium and titanium additions. Conventional alloys such as 18–8 and 16–10 are included together with the new NF709 (20–25) and other recent variants. Precipitates forming in a...

Precipitation in creep resistant austenitic stainless steels

Formation and stability of radiation-induced phases in neutron-irradiated austenitic and ferritic steels

The literature which describes the microstructural development during neutron irradiation of austenitic stainless steels is reviewed, with emphasis on solution annealed AISI 316 stainless steel The main difference in primary damage caused by fission and fusion neutron irradiation is the considerably larger amounts of transmutation gases (hydrogen and helium) produced by fusion neutrons The secondary damage consists of dislocation loops, cavities, and radiation-affected phases The microstructural changes produced by secondary damage is largely determined by radiation-induced segregation due to coupled migration of impurity atom/point defect complexes, and by the amount of helium produced by transmutation

Microstructural evolution in annealed austenitic steels during neutron irradiation

Void-swelling and precipitation in a neutron-irradiated, niobium-stabilised austenitic stainless steel

When polycrystalline metals and their alloys are used at high temperature, creep deformation leads to changes in their internal state. The change in internal state manifests itself in many ways, but the two ways that concern us in this review are (i) the creation of internal stress arising from the strain incompatibility between grains and/or the formation of cell/sub-grain structures and (ii) a change in the material resistance. This review aims to provide a clear separation of these two concepts by exploring the origin of each term and how it is associated with the creep deformation mechanism. Experimental techniques used to measure the internal stress and internal resistance over different length-scales are critically reviewed. It is demonstrated that the interpretation of the measured values requires knowledge of the dominant creep deformation mechanism. Finally, the concluding comments provide a summary of the key messages delivered in this review and highlight the challenges that remain to b...

A review of the changes of internal state related to high temperature creep of polycrystalline metals and alloys

The effects of long-term thermal ageing on the microstructure of AlSl type 316 stainless steel are described. The microstructure of the aged steel is related to the reported embrittlement of the steel. The precipitation found is complex and the embrittlement appears to result, in part, from specimen preparation techniques interacting with the microstructure. The dominant creep deformation mechanism under service conditions appears to be diffusion creep.

L. P. Stoter

Papers

Thermal ageing effects in AISI type 316 stainless steel