Strain Rate Sensitivity of Pre‐Strained AISI 301LN2B Metastable Austenitic Stainless Steel

Abstract

The dynamic behavior of AISI 301LN2B (EN 1.4318) metastable austenitic steel grade has been investigated at 296 K by means of servohydraulic tensile and split Hopkinson bar testing in the strain rate range 0.005–1000 s−1. As delivered, as well as 10% uniaxial, biaxial, and plane strain pre‐strained conditions, without subsequent heat treatment have been tested. A negative strain rate sensitivity is observed in the low strain rate range between 10−4 and 1–10 s−1. Pre‐straining reduces the magnitude of the adiabatic tensile strength softening, especially in the plane strain condition with higher triaxility. The thermal activation related dynamic flow stress increase is not dependent on pre‐straining. The γ → α′ induced additional flow stress increase, however, is highly strain rate and pre‐straining sensitive. The amount of pre‐straining determines the overall ductility at fracture, and therefore the adiabatic temperature increase. The pre‐straining stress state influences the amount of α′‐martensite formed before dynamic testing, and consequently the maximum intensity of the TRIP induced flow stress increase by subsequent dynamic testing.

The TRIP Effect and Its Application in Cold Formable Sheet Steels

“…It is observed that high strain rates promote the formation of stacking faults and ϵ ‐martensite, however suppress α ′‐martensite in 1.4301 . Figure reveals a systematic investigation on the mechanical behavior of steel grade 1.4318 at the strain rates between 6.0 × 10 −4 s −1 and 920 s −1 . As shown in Figure a, in the low strain rate region from 1.6 × 10 −4 s −1 to 0.8 s −1 , the negative strain rate sensitivity at high deformation strain is very pronounced.…”

“…Meta‐stable ASS show a negative strain rate sensitivity, which means the tensile strength declines as the strain rate increases . The main reason is that adiabatic heating at high strain rates reduces the chemical driving force for martensitic transformation .…”

“…Influence of strain rate on the flow curves of a meta‐stable ASS grade 1.4318: a) Flow curves at different strain rates; b) The contribution of dynamic stress offset Δ σ dynamic and transformation stress offset Δ σ TRIP on the macroscopic flow stress. The dashed line indicates the quasi–static flow stress without phase transformation …”

Adiabatic Heating of Austenitic Stainless Steels at Different Strain Rates

“…At the quasi-static strain rate of 2.5 × 10 −4 s −1 , the flow stress of the material increases significantly at around 0.1 true strain, and the strain-hardening rate increases until it reaches a maximum at around 0.25 true strain, after which the hardening rate starts decreasing. The increase in the strain-hardening rate can be related to the phase transformation of austenite to martensite, i.e., the transformation of a soft phase into a much stronger phase [22,23]. Increasing the strain rate decreases the phase transformation rate, and consequently decreases the strain-hardening rate of the material [24].…”

Shape Memory Effect Induced by Stress-induced α′ Martensite in a Metastable Fe–Cr–Ni Austenitic Stainless Steel

“…Type 304 austenitic stainless steels are extensively used in various industries, such as petroleum, chemistry, power generation, and nuclear industries, due to their good combination of mechanical properties, corrosion resistance, and weldability [1][2][3][4]. In addition, many work clearly showed that the austenitic phase (c) in the 304 stainless steels is metastable.…”