Continuous magnetic method for quantitative monitoring of martensitic transformation in steels containing metastable austenite

doi:10.1016/j.scriptamat.2004.10.034

Scripta Materialia

Volume 52, Issue 6, March 2005, Pages 525-530

https://doi.org/10.1016/j.scriptamat.2004.10.034 Get rights and content

Abstract

A fast and accurate in situ magnetic method has been developed. This technique is suitable for obtaining the continuous kinetics of the martensitic transformation that occurs during deformation in steels containing metastable austenite, like TRIP steels; it could also be very helpful in studying the austenite stability when designing/optimizing such steels. The results are successfully compared to those of a static magnetic measurement method, which also allows the investigation of the effects of test temperature and stress triaxiality on the phase transformation kinetics.

Introduction

The martensitic transformation that occurs during plastic straining contributes to the enhancement of the deformation process of iron-based alloys such as fully austenitic steels [1] or ultra high-strength steels containing dispersed retained austenite islands [2]. For these materials, the composite deformation behavior, together with a transformation induced plasticity (TRIP) effect, improves the work-hardening rate and delays the onset of necking during a tensile test [3], [4].

The stability of austenite controls its transformation rate during deformation and has a strong impact on the mechanical behavior [5]. Some important parameters influencing the stability of austenite are the chemical composition, carbon enrichment and grain size of austenite, the stress state of the surrounding matrix and the temperature. Based on generalized models describing the kinetics of martensitic transformation in fully austenitic steels [6], [7], some authors [8], [9] have developed physically-based models suitable for predicting the macroscopic behavior of TRIP-aided carbon steels. These models give good predictions of the evolution of strain-induced martensite volume fraction in uniaxial tension, but the influence of a multiaxial stress state on the transformation rate mechanism is not well documented or fully understood.

Since the martensitic transformation and the stability of austenite strongly influence the formability and the mechanical behavior, an accurate determination of the volume fraction of retained austenite as a function of plastic strain is necessary to improve the design of metallurgical processes. A critical comparison between the currently used methods for measuring the amount of austenite, such as X-ray diffraction, Mössbauer spectroscopy, neutron diffraction and a static magnetic method is given by Zhao et al. [10]. Among these techniques, the magnetic method is the most readily adaptable to in-situ measurements during tensile tests. The aim of the paper is to present such an experimental setup and its validation.

Continuous in situ magnetic measurements of the transformation kinetics have already been realized for fully austenitic steels (see e.g., papers by Lecroisey and Pineau [11], and Abrassart [12]). When dealing with TRIP steels, the small amount of retained austenite (<15%) makes these measurements particularly difficult. In the first part of this study, the principle and operating conditions of the static magnetic method are recalled. In the second part, a continuous in situ magnetic method for measuring the amount of austenite is described and validated on a TRIP-aided steel. Finally, the effect of temperature and macroscopic multiaxial stress state on the retained austenite stability is underlined.

Section snippets

Material and sample preparation

TRIP-aided ferrous alloys have been developed for the automotive industry as a response to new demands for increased formability, improved crash behavior and weight reduction of vehicles. Their multiphase microstructure, obtained after specific thermal treatments [13], is composed of ferrite (F), bainite (B), martensite (M), and metastable austenite (A), and confers on them excellent mechanical properties [14].

The microstructure of the 0.19C–1.54Mn–1.58Si–0.04Al (wt%) TRIP steel sheet (1.35 mm

Design of an in situ magnetic measurement facility

The retained austenite transforms during mechanical loading into martensite. This corresponds to a transformation paramagnetic phase (γ) → ferromagnetic phase (α′) measurable with an appropriate magnetic device.

The principle of both static and continuous magnetic methods is shown in Fig. 2(a) and (b), respectively. A stabilized power supply (min. 15 A) generates a current into electromagnets (1) which create a strong constant magnetic field (H ∼ 2 T) between the pole tips of cores. A saturation state

Use of the magnetic method to assess temperature and stress state effects on the stability of austenite

The influence of macroscopic multiaxial stress state on the transformation kinetics is underlined by some authors for Fe–Ni–C alloys [16] and for initially fully austenitic steels [17]. They all showed that higher stress triaxialities promote the martensitic transformation. This is essentially due to the volume change associated with the phase transformation. For TRIP-assisted multiphase steels, Pyshmintsev et al. [18] showed that an increase of 100 MPa of the compressive hydrostatic stress

Conclusions

A newly designed in situ magnetic method enabled the stability of retained austenite when deforming a low alloy TRIP steel to be quantified. The strong effects of temperature and macroscopic stress state on the transformation rate were underlined by static magnetic measurements. in situ tensile tests were shown to be an accurate method for continuous measurements of the amount of transformed austenite.

More generally, the continuous magnetic method can be utilized to provide fast measurements of

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Continuous magnetic method for quantitative monitoring of martensitic transformation in steels containing metastable austenite

Abstract

Introduction

Section snippets

Material and sample preparation

Design of an in situ magnetic measurement facility

Use of the magnetic method to assess temperature and stress state effects on the stability of austenite

Conclusions

Scripta Metall. Mater.

Int. J. Plast.

Mater. Sci. Eng.

Mater. Sci. Eng.

Mater. Sci. Eng.

Scripta Metall.

Acta Metall.

Acta Metall. Mater.

Mater. Sci. Eng.

Trans. ASM