Demonstrating the Effect of Precipitation on the Mechanical Stability of Fine-Grained Austenite in Reversion-Treated 301LN Stainless Steel

Antti Järvenpää,Timo Juuti,Matias Jaskari,Pentti Karjalainen

doi:10.3390/met7090344

Abstract

According to recent investigations, a huge difference exists in the mechanical stability of austenite between the grain-refined structure states obtained in reversion annealing at 800–700 °C or at 900 °C, in a 301LN type austenitic stainless steel. Precipitation of chromium nitride occurring at these lower temperatures has been argued to be the factor reducing the stability. To prove this argument, a fine-grained, very stable austenitic structure was created at 900 °C in 1 s, and subsequently annealed at lower temperatures between 850 and 750 °C, up to 1000 s. It was found that the subsequent annealing at 750 and 800 °C resulted in prominent gradual decrease of the mechanical stability under tensile straining, detectable after 10 s annealing duration and continued until 1000 s. Only minimal grain growth occurred, which decreased the stability very marginally. The degree of the stability drop followed the predicted kinetics of the Cr2N precipitation with regards as its dependence on annealing duration and temperature. Further, the tensile yield strength of the fine-grained structure increased slightly due to the annealing. The presence of nano-sized Cr2N particles was verified after 1000 s holding at 750 °C. These observations and predictions yield firm evidence for the imperative contribution of precipitation to the highly reduced mechanical stability of grain-refined austenite in this steel.

Highlights

Deformation-induced martensite transformation (DIM) plays a significant role in metastable austenitic steels with regards to their static and dynamic mechanical properties, e.g., [1,2,3,4]
The work was aimed at verifying that nitride precipitation, instead of a submicron grain size, is the factor for strongly reducing the mechanical stability of grain-refined austenitic structures obtained by reversion annealing at temperatures of 700–800 ◦ C in 301LN steel
In order to confirm this, the influence of subsequent low-temperature annealing on the high stability of the fine-grained austenitic structure, obtained by reversion annealing at 900 ◦ C in 1 s, was investigated under tensile straining

Summary

Introduction

Deformation-induced martensite transformation (DIM) plays a significant role in metastable austenitic steels with regards to their static and dynamic mechanical properties, e.g., [1,2,3,4]. Simmons [9] has presented an overview of high-nitrogen steels, and more recently Lo et al [3] and Machado et al [10] have reviewed the role of nitrogen in iron-based alloys. It has been well known for decades that nitrogen alloying increases the mechanical stability of austenitic stainless steels, the stability being equal to [11,12] or stronger [13] than that of carbon

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Results

Conclusion