Microstructure changes occurring in heat-resistant steels during severe plastic deformation and subsequent creep

P Kral,M Svobodova,J Dvorak,V Sklenicka,Z Horita,K Kucharova,M Kvapilova,Y Tang,Y Takizawa

doi:10.1088/1757-899x/1178/1/012033

Abstract

This work investigates creep and microstructure characteristics in P92 martensitic and 304L austenitic stainless steel after the imposing of large value of plastic strain. The as-received coarse-grained steels were strongly deformed at room temperature (RT) before tensile creep testing. The flat specimens were tested at 873 K and 923 K under different applied stresses. Microstructures were analysed by means of scanning electron microscope with EBSD (electron back-scatter diffraction) camera and transmission electron microscope. It was observed that application of severe plastic deformation (SPD) techniques at RT significantly reduces grain size and changes phase structure of 304L austenitic steel. The microstructure investigations revealed that the growth of new phases is enhanced in plastically deformed P92 martensitic and 304L austenitic stainless steels compared to their undeformed states. Creep results showed that the mean grain size of SPD-processed steels is still near ultrafine-grained region even after long-term creep testing and the coarsening of microstructure is predominantly influenced by creep time and strain.

Highlights

The methods of allowing to impose the large deformation into the microstructure provide a unique opportunity to study creep processes in ultrafine-grained or even nano materials [1]
Microstructure of severe plastic deformation (SPD)-processed steels: a) P92 processed by high-pressure torsion (HPT), b) P92 deformed by high-pressure sliding (HPS) with εeq ~ 2.6, c) 304L processed by HPT
Even if the grain coarsen during creep testing of UFG P92 and 304L steel, the mean grain size of both steels is finer than predicted quasi-stationary subgrain size wqs [5] for their coarse-grained counterparts

Summary

Introduction

The methods of allowing to impose the large deformation into the microstructure provide a unique opportunity to study creep processes in ultrafine-grained or even nano materials [1]. The grain size of these materials is comparable or even finer than stationary subgrain size in their coarse-grained counterparts For this reason creep mechanisms related to the high-angle grain boundaries (HAGBs) may significantly affect the mechanical properties of materials at high temperatures [2,3]. The martensitic and austenitic chromium steels are very often used for the high-temperature components of steam power plants Their creep resistance is significantly influenced by the density of free dislocations and low-energy boundaries, such as low-angle grain boundaries and martensitic lath boundaries whose movement is restricted by precipitates [6,7]. The aim of the present investigation is to evaluate the influence of severe plastic deformation at room temperature (RT) on microstructure and creep in P92 martensitic and 304 austenitic steels

Experimental Materials and Methods

Microstructure processed by HPT

Microstructure at the beginning of the creep testing

Conclusions