Elsevier

Materials Letters

Volume 179, 15 September 2016, Pages 65-68
Materials Letters

Role of grain boundary ferrite layer in dynamic recrystallization of semi-solid processed type 304L austenitic stainless steel

https://doi.org/10.1016/j.matlet.2016.05.049Get rights and content

Highlights

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    Study of semi-solid processed type 304L austenitic stainless steel

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    Comparison with conventional steel with ferrite stringers across the austenite grains.

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    Understand role of grain boundary ferrite layer in dynamic recrystallization pattern

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    Presence of delta-ferrite layer around austenite-globules delays DRX grain nucleation.

Abstract

A semi-solid processed steel with well arranged delta-ferrite layer around the austenite-solid globules is found to show different dynamic recrystallization (DRX) pattern compared to its conventionally processed counterpart with ferrite stringers across the austenite grains. Analysis of experimental results indicates that the presence of delta-ferrite layer around austenite-globules delays the nucleation of new DRX grains. On the other hand, with increase in temperature, the delta-ferrite layer contributes to increase in DRX fraction by restricting the grain growth of the austenite matrix prior to deformation.

Introduction

In recent years, semi-solid processing of steel has drawn significant research attention as it reduces intermediate steps for the manufacturing of near net-shape products. To study the feasibility of semi-solid processing of austenitic stainless steel (SS), a type 304L SS was processed in semi-solid state. The chemical composition (in wt%) of the steel is: 0.012 C 18.1Cr, 8.3Ni, 1.5Mn, 0.3Mo, 0.43Si, 0.04P, 0.01S and 0.4Cu. The microstructure obtained by semi-solid processing of 304L SS is shown in Fig. 1(a). Compared to its hot-rolled and subsequently annealed counterpart (Fig. 1(b)), the semi-solid processed (SSP) steel shows larger grain size, and well arranged delta (δ) ferrite layer around the solid austenite (γ) globules. Presence of such continuous layer of second phase at grain boundaries is classically known as grain boundary wetting by a solid phase [1]. This unique microstructure obtained by semi-solid processing influences behavior of the alloy. In many cases, SSP materials are subjected to further thermo-mechanical treatment (TMT) to obtain desired mechanical properties [2]. TMT is usually carried out in different temperature domains depending on the properties needed in the steel for the end use. TMT in hot-working domain is known to change the microstructure of the steel by dynamic recrystallization (DRX). A change in DRX behavior of 304L SS due to semi-solid processing can be expected due to grain boundary wetting by δ-ferrite phase and their evolution with temperature. However, to the best of our knowledge, no such studies have been reported so far. Hence, the present study has been carried out with an objective to understand the role of grain boundary ferrite layer, grain size and grain growth in DRX pattern in SSP steel.

Section snippets

Experimental

Hot isothermal uniaxial compression tests were carried out on a hot-rolled and subsequently annealed (AR) steel and SSP steel specimens of 10 mm diameter and 15 mm height in the temperature range 1273–1473 K in steps of 100 K at true strain rate of 0.1 s−1 up to 50% nominal strain using Gleeble 3800®. To indentify the effect of temperature on δ-ferrite evolution and γ grain size, some specimens were soaked at 1273, 1373 and 1473 K for 5 min without any deformation. With the help of integral quenching

Result and discussion

Initial microstructures of the SSP and AR steel specimens are given in Fig. 1(a) and (b), respectively. It is evident from this figure that the microstructure of AR steel shows δ-ferrite stringers, with different lengths oriented in the rolling direction across equiaxed γ-grains while the microstructure of SSP steel shows wetting of grain boundaries by δ-ferrite. Selected microstructures of thermally treated specimens, given in Fig. 2, show that at higher temperatures in both AR and SSP steel,

Conclusions

The main finding from this work is that at lower hot working temperatures presence of δ-ferrite layer around γ-globules reduces the nucleation sites, i. e. the γ/γ interface, for DRX; hence, initiation of DRX gets delayed in SSP steel. As the temperature increases, the depletion of δ-ferrite layer due to spherodization and dissolution increases the γ/γ interface, and DRX nucleation becomes easier. With increase in temperature, the depleting layer of δ-ferrite positively contributes to

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