Relation between residual stress and Barkhausen noise in a duplex steel
Introduction
Duplex stainless steels have two-phase ferritic/austenitic microstructure with 30–70 vol% ferrite. The main reason for using duplex stainless steels is their good resistance to oxidation, corrosion and stress corrosion associated with good mechanical properties. Because of the unique combination of properties, they have found widespread use in chemical and petrochemical industry, pulp and paper industry and power generation [1].
Ferritic and austenitic phases have different thermal expansion coefficients and mechanical properties. This is why residual stresses (RS) commonly appear between the phases due to temperature changes or deformation. These residual stresses are called homogeneous microstresses σII. They can be formed between different phases (interphase microstresses) or between grains of different orientation (intergranular microstresses) [2]. Many production steps in component fabrication introduce also macroscopic residual stresses σI [3]. These macrostresses are balanced over distances which are comparable to the dimensions of the component. Residual stresses acting on the atomic scale of the material are called inhomogeneous microstresses or σIII. They are created by, for example, dislocation pile-ups. Fig. 1 shows the size scales associated with the different types of residual stresses. Because all types of residual stresses superpose to each other, as a result, a component may contain a complex RS pattern after fabrication [2], [3].
In duplex steels the interphase microstresses depend on the volume fractions of the phases [4], [5], morphology of the phases [5], [6], monotonic or cyclic deformation [6], [7] or thermal treatments [7], [8]. Microstresses increase with elastic stress [6]. Also after unloading specimens subjected to large plastic strains, the microstresses increase [7], [9]. This increase shows, however, saturation after approximately 5% strain [7], [9]. The reason for the increase of the microstresses is the differences in the yield strength and strain hardening of the respective phases. Cyclic loading affects both macroscopic [10] and microscopic [6], [11] residual stresses.
It is well known that the presence of residual stresses may influence strongly the mechanical properties of the material and, in particular, its fatigue properties. Therefore, the evaluation of the residual stresses is an important quality control method. Nowadays there are several methods for RS measurement: hole-drilling and X-ray diffraction (XRD) being the most widely applied [12], [13], [14], [15]. The hole-drilling method is, however, destructive and both methods are quite time-consuming, especially, in the case of large components. In this study Barkhausen noise (BN) is investigated as a potential method for RS evaluation for duplex stainless steels. BN is already employed in RS evaluation of carbon steels [16]. However, being based on ferromagnetism, it samples in this case the stresses only from the ferrite phase.
Section snippets
Material
Material studied was Avesta 2205 duplex stainless steel. The sample was a rectangle having dimensions of 460×100×54 mm3. The chemical composition of the material is presented in Table 1. The material contained 52±7% ferrite and 48±7% austenite determined by the point counting method. The microstructure consisted of austenitic islands in a ferritic matrix. It had regions where the austenite islands were elongated parallel to the rolling direction as shown in the micrograph in Fig 2. These
X-ray diffraction measurements
Preliminary experiments revealed that d vs. sin2α relation for austenite {311} planes was linear but for ferrite {211} planes there appeared meaningful oscillation. The relation was, however, linear for the ferrite {200} planes measured with Ti radiation as predicted by the Reuss model. Due to the low penetration depth of the Ti radiation, the oxide layer on the specimen surface had to be polished away before measurements. Polishing was conducted electrolytically in order to avoid changing the
Discussion
In XRD measurements of the ferrite phase, the variations in intensities of individual diffraction peaks and oscillations in d vs. sin2α relation indicated the presence of crystallographic texture. This have to be taken into account when calculating RS values. In this case the linearization of the oscillating data from {211} planes yielded in too low absolute values of stress compared with the ones obtained from {200} planes. These stress values had different linear relations under tensile and
Conclusions
- 1.
Linearizing oscillating d vs. sin2α relation results obtained from the ferrite {211} planes led to inaccurate RS values especially under compressive stress.
- 2.
Local heating caused higher macrostresses parallel to the direction of the heated line than perpendicular to it.
- 3.
The rms value of the BN amplitude varied with the total stress (macrostress+homogeneous microstress) in the ferrite phase.
- 4.
The stress induced changes in the rms value were mainly due to changes in the pulse height distribution.
- 5.
It
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