Hot-Deformation Behavior of High-Nitrogen Austenitic Stainless Steel under Continuous Cooling: Physical Simulation of Surface Microstructure Evolution of Superheavy Forgings during Hot Forging.

Zhenhua Wang,Yong Wang

doi:10.3390/ma12071175

Abstract

Superheavy forgings are increasingly used in the nuclear industry. The strain rate is extremely low during hot forging due to the huge size of the superheavy forging; in fact, the surface temperature of the forging decreases obviously during each deformation step. Hot-deformation behavior differs from that of isothermal deformation. In this study, 18Mn18Cr0.6N steel was selected as a model material. Hot-compression tests were conducted using a Gleeble 3800 simulator at a strain rate of 10−4 s−1 and continuous cooling rates of 0.0125 Ks−1 and 0.025 Ks−1. The microstructure was observed using electron backscatter diffraction analysis and transmission electron microscopy. The flow stress increased with increasing strain: the higher the cooling rate, the higher was the hardening rate. Continuous cooling inhibited dynamic recrystallization by delaying its nucleation. The subgrain/cell size increased linearly with increasing final temperature of deformation in the temperature range 1273 to 1448 K. An intense <001> texture formed in 0.8-strained specimens and the matrix exhibited a low Taylor factor orientation. Most dislocations were separately distributed in subgrains and did not entangle with each other or with the subgrain boundary. Dislocation arrays transferred easily through boundaries and dislocation accumulation at boundaries was weak. This study contributes to understanding the hot-forging process of superheavy forgings.

Highlights

Temperature affects the deformation, dynamic recovery (DRV), and dynamic recrystallization (DRX) mechanisms of metals during hot deformation, as reviewed by Sakai et al [1]
Systematic descriptions on the hot-deformation behavior, such as the hot-deformation equation [4], hot-processing maps [5], Zener–Hollomon parameter [6], and DRX grain size model [7] are established under isothermal conditions
After solution heat treatment at 1473 K for 5 h, a small slab was cut from the ingot and hot-rolled at 1371 K to induce a fully recrystallized microstructure

Summary

Introduction

Temperature affects the deformation, dynamic recovery (DRV), and dynamic recrystallization (DRX) mechanisms of metals during hot deformation, as reviewed by Sakai et al [1]. Hot-forming processes, such as rolling, extrusion, drawing, and mechanical press forging, are conducted at a high strain rate, usually above 1 s−1 , as shown by Dieter et al [2]. Because of the high strain rate, each deformation step can be considered as an isothermal forming process. Systematic descriptions on the hot-deformation behavior, such as the hot-deformation equation [4], hot-processing maps [5], Zener–Hollomon parameter [6], and DRX grain size model [7] are established under isothermal conditions

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Methods

Results