AISI 304 steel: anomalous evolution of martensitic phase following heat treatments at 400 °C
Introduction
Following heat treatments at a temperature of about 400 °C an increase of martensite content takes place in austenitic stainless steels with biphasic structure (martensite α′+ austenite γ) producing an increase of yield stress and ultimate tensile strength [1], [2]. Controversial results made difficult to explain the anomalous phenomenon, which occurs in materials where martensite is induced by both cooling and deformation [3]. Smith and West [4] ascribed “the magnetic peak at 400 °C” to the formation of a ferrite α as an intermediate stage of the α′ → γ reversion. Magnetic measurements, performed directly at the annealing temperature, showed no evidence of variations of α′ phase volume fraction [5]. On the basis of this result, Harries [6] argued that the heat treatment induces a fine carbide precipitation, which lowers the content of alloying elements in solid solution with consequent increase of the martensite start (Ms) temperature: new α′ phase can form during cooling to room temperature.
One of us investigated AISI 304 samples with different initial martensite volume fractions (up to 84%) [7]. The samples were submitted to annealing at 400 °C for increasing times and the amount of α′ phase was determined by X-ray diffraction (XRD) at room temperature. For all the samples the amount of α′ phase plotted versus annealing time showed a overall decreasing trend, due to α′ → γ reversion. Superimposed to this trend two relative maxima after ≈4 × 103 s and 3.5 × 104 s were observed. The first maximum was attributed to the growth of martensitic zones when microstresses relax in the surrounding austenitic phase due to recovery of defective structures (dislocations and stacking faults) following the treatment. This interpretation was confirmed by further results, in particular by Mössbauer spectroscopy (MS) data [8].
The explanation of the second maximum was given assuming operative the mechanism described by Harries [6], which is supported by MS experiments evidencing that long time of annealing at 400 °C produces a lowering of alloying elements in martensite without a corresponding composition change in austenite [8] and by acoustic emission (AE) detected during cooling [9].
This work has been carried out to achieve further information on the origin of the second maximum by means of high-temperature XRD and dynamic modulus measurements.
Section snippets
Material and experimental
The AISI 304 steel has the following nominal chemical composition (wt.%): C 0.06, Ni 8.60, Cr 18.25, Mn 0.82, Mo 0.16, S 0.015, P 0.021, Si 0.56, Ti, Cu, V traces, Fe to balance.
The samples were preliminary heated for 1.8 × 103 s at 1050 °C and then water quenched. After this treatment, the structure is fully austenitic (γ phase), martensite (α′ phase) was induced by cold rolling at room temperature. The samples examined here had an initial martensite content of 94%, higher than those investigated
Results
In Fig. 1 the XRD spectra collected at room temperature before and after the isothermal measurement cycle are displayed. After annealing (2.0 × 104 s at 400 °C) the {1 1 1}, {2 0 0}, {2 2 0}, {3 1 1} and {4 2 0} γ reflections are detected whereas only the {2 2 0} γ reflection was present before. The austenite content, which was 6% in the as-deformed sample, increases to 37% after the heat treatment.
Since the diffraction patterns of deformed samples show only one γ reflection, the structure evolution during
Discussion
High-temperature XRD clearly shows that the α′ → γ transformation is the prevailing phenomenon occurring during annealing at 400 °C. After the measurement cycle (2.0 × 104 s) the relative amounts of the two phases has changed: austenite content, which was 6% in the as-deformed sample, increases to 37%. However, when the steel is cooled down to room temperature a certain amount of austenite transforms to martensite (Fig. 3). Since the MS temperature of AISI 304 steel is far below room temperature, the
Conclusions
For increasing time of annealing at 400 °C the amount of α′ phase shows a decreasing trend, due to α′ → γ reversion, with two maxima after ≈4 × 103 s and 3.5 × 104 s [7]. The present work was carried out to investigate the second maximum by high-temperature XRD and dynamic modulus measurements. The main results can be summarized as follows:
- (i)
the α′ → γ transformation is the prevailing phenomenon occurring during annealing;
- (ii)
some martensite forms during cooling;
- (iii)
the martensitic transformation takes place in the
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