Initiation of Dynamic Recrystallization of As-Cast N08028 Alloy

Iñaki Hurtado,Aitor Navarro,Elena Silveira,Elena Mora,Cecilia Poletti,Joseba Mendiguren

doi:10.25518/esaform21.896

Abstract

The use of high nickel content austenitic stainless steels (SASS) has significantly increased in the last decade. The corrosion and high fatigue resistance of these materials make them suitable for manufacturing oil country tubular goods (OCTG). SASS are processing by forging from casting conditions. Dynamic recovery (DRV) and recrystallization (DRX) of as-cast super austenitic stainless steel, N08028 Alloy, is investigated to study the refining effect from the as-cast grain structure to fully recrystallized austenite due to hot deformation. Both the critical stress and strain for the initiation of DRX are determined using the flow curves. To perform this analysis, hot compression tests are performed at temperatures between 900°C and 1250°C, and strain rates between 0.1 s-1 and 10 s-1, up to 0,8 final strain using a Gleeble®3800 thermomechanical simulator. Subsequently, the Johnson-Avrami-Mehl-Kolmogorow (JMAK) model is used to numerically fit the flow curves and consequently determine the critical strain. No critical points are seen for temperatures under 1100°C. Above this temperature, the JMAK model proves to be valid in all studied strain rates.

Highlights

N08028 Alloy is widely used in chemical and petrochemical industry due to its excellent corrosion resistance and its large yield strength [1]
The flow curves obtained at different temperatures and strain rates are shown in Fig. 1, where the flow curves are dependent on the temperature and strain rate
The hot deformation behavior of the N08028 Alloy coming from an initial microstructure of rough equiaxed grains with a large number of twin boundaries, consist of three stages [8]. (I) Work Hardening (WH) and Dynamic recovery (DRV) before the critical strain

Summary

Introduction

N08028 Alloy is widely used in chemical and petrochemical industry due to its excellent corrosion resistance and its large yield strength [1]. During thermos-mechanical treatments dynamic recovery (DRV) and dynamic recrystallization (DRX) are the most important restoration mechanisms They affect the final microstructure and, the mechanical properties. Significant differences exist between as-cast ingots and as-forged billets These include the grain sizes, morphologies, and solidification textures. These factors, strongly affect the hot-working ability, microstructural evolution, and mechanical properties of the final material [5]. It is important to investigate the critical conditions of DRX initiation of as-cast SSAS alloys during hot working. The critical conditions for initiation of DRX of as-cast Alloy N08028 are investigated by using hot compression test at different strain rate and temperature conditions. The critical strain is calculated using the work hardening rate versus the flow stress curves, and validated using the JMAK model in all studied conditions. Electron backscattered diffraction (EBSD) analysis are used to identify the recrystallization state after hot deformation

Experimental

Stress-strain curves

Predicting the critical strain and peak strain

Summary and Conclusions