Measurement of the rate of transformation induced plasticity in TRIP steel by the use of Barkhausen noise emission as a function of plastic straining

Highlights

• Barkhausen noise in TRIP steel exhibits steep decrease at low plastic strains.

• Barkhausen noise evolution is sensitive to the rate of austenite decomposition.

• Higher plastic strains increase magnetic hardness of TRIP steel.

Abstract

This paper investigates the rate of transformation induced plasticity in TRIP steel (TRansformation-Induced Plasticity) after plastic straining by the use of Barkhausen noise emission. The samples were subjected to a variable degree of plastic straining and analysed by the use of conventional techniques such SEM, XRD, as well as microhardness in order to investigate residual stress and microstructural alterations initiated by the uniaxial tensile test. Barkhausen noise emission is analysed as a function of plastic straining as well as in the direction of the exerted load and interpreted with respect to the aforementioned microstructure and stress alterations. It was found that Barkhausen noise markedly decreases along with increasing plastic straining, up to 20%, followed by a strain region in which the evolution of Barkhausen noise reaches saturation. Samples after the tensile test exhibited marked magnetic anisotropy since the Barkhausen noise emission in the direction perpendicular to the tensile stress remained less affected. Apart from the effective value of Barkhausen noise, the Barkhausen noise envelopes were also analysed.

Introduction

TRIP steels represent a specific group of high strength steels with improved formability developed for the car industry. The pioneer work was carried out by Zackey and Parker [1]. The authors proposed the heat treatment regime which resulted in a significant amount of retained austenite (metastable) in the matrix. As a result, TRIP steels exhibit low yield strength and delayed plastic instability (prolonged phase of homogeneous plastic straining) [1]. The TRIP mechanism prevents strain localisation via transformation of metastable austenite to martensite [2]. This transformation hardens the region in which the phase transformation takes place. TRIP steels possess unique high energy absorption [3]. For this reason, these steels are proposed for components in which high accumulation of energy during plastic deformation (high strain hardening) is expected. The final microstructure of TRIP steels after heat treatment comprises ferrite, bainitic ferrite, retained austenite and martensite [3], [4]. The austenite and ferrite phases improve the ductility of TRIP steel, whereas bainitic ferrite and martensite enhance its strength. Plastic deformation in TRIP steels is driven by internal stresses in the matrix and the strain induced transformation of austenite to martensite accompanied by stress relaxation [3]. The effectiveness of the TRIP effect is a function of many variables such as carbon enrichment of the austenite phase, the heat treatment regime, the chemical and phases composition, etc. [3], [4], [5], [6], [7], [8], [9]. El-Sherbiny et al. [5] reported about improved mechanical properties and coatability of the different TRIP steels as a result of the replacement of Si by Al and V. Zeng et al. [6] investigated the influence of Nb and Ti micro alloying on yield strength and total elongation of $\delta$-TRIP C-Mn-Al-Si steel. The authors indicated that nano-sized (Nb, Ti)C precipitates improve the strength of this steel. Tan et al. [7] discussed the influence of the variable phase composition on strain partitioning and mechanical properties. A deeper insight into this field was published later [4] when strain, as well as chemical partitioning, was investigated. TRIP steels properties were examined as a function strain rate and temperature [8], [9]. Tilak Kumar et al. [9] found that elevated temperatures affect the thermal stability of austenite; thus, decreasing the yield and ultimate strength, as well as the elongation. Apart from the conventional tensile tests, the influence of stir welding [10], hydrogen embrittlement [11], and repetitive corrugation and straightening [12] on TRIP steel’s microstructure have also been investigated.

Components made of TRIP steels in a variety of applications can undergo plastic deformation of varying degrees which, in turn, results in corresponding matrix alterations expressed in many terms. Non-destructive monitoring of these parts could be beneficial for a fast and reliable assessment of the microstructural state. This information can be employed for the estimation of lifetime and/or evaluation of the components capability to absorb energy during the next loading. TRIP steels are composed of ferromagnetic phases such as ferrite, bainitic ferrite and/or martensite, as well as a certain fraction of non-ferromagnetic austenite. Strain induced phase transformation of austenite to martensite increases the volume fraction of the ferromagnetic phase [13], [14], [15]. For this reason, magnetic methods have been employed for the detection of the strain induced martensite transformation [16], [17], [18]. However, these methods have been mainly adapted for the steels whose initial microstructure was entirely composed of austenite. The main advantage of magnetic methods is driven by the marked contrast in signals (or parameters) between the paramagnetic austenite phase and the ferromagnetic martensite one.

On the other hand, studies in which magnetic Barkhausen noise (MBN) is associated with martensite/austenite partitioning are rare. Tavares et al. [19] correlated MBN with martensite/austenite partitioning in stainless steel. Vértesy et al. [20] employed different magnetic methods for non-destructive characterisation of TRIP steels exposed to variable plastic straining, including the MBN technique. The authors reported that the effective value of MBN decreases steeply, up to 7.5% strain, followed by only a moderate decrease. MBN is a product of irreversible and discontinuous domain walls (DWs) motion initiated by altering the magnetic field or/and stress [21], [22]. Domain walls (DWs) are pinned in their positions and their sudden jumps occur as soon as the magnetic field attains a critical threshold. MBN is a function of the stress state due to the specific alignment of DWs [23], [24], as well as the microstructure. DWs in motion interfere with all lattice defects such as grain boundaries [25], precipitates [25], [26], dislocation cells [27], and non-ferromagnetic phases [28] etc. whose superimposed contribution to the entire MBN is usually difficult to unwrap. On the other hand, MBN signals have been studied as a function of residual stresses [29], microstructural features [30], as well as their superimposed contribution. TRIP steels during plastic straining undergo marked microstructural transformations as well as stress alterations. For this reason, this study investigates the potential of using the MBN technique for the assessment of the microstructural state after different plastic straining under the uniaxial tensile test. The novelty of this study should be found in the deeper insight into the MBN evolution with respect to the austenite decomposition and the superimposing dislocation motion as the main mechanisms contributing to the strain hardening of TRIP steels.