Abnormal Austenite-Ferrite Transformation Kinetics of Ultra-Low-Nitrogen Fe-N Alloy

Abstract

The kinetics of the isochronal austenite (c) fi ferrite (a) transformation of ultra-low-nitrogen Fe-N alloy were investigated for cooling rates in the range of 5 to 15 Kmin -1 by high-resolution dilatometry. The explored c fi a transformation takes place in the single a-phase region, and the onset temperature of the transformation decreases with increasing applied cooling rate. According to the variation of the ferrite formation rate data, an abnormal transformation mechanism, i.e., multiply peaked nature with rate maxima appearing at fixed transformed fractions, was recognized. For the later (main) part of the transformation, a phase-transformation model, incorporating site saturation, interface-controlled growth, and an appropriate impingement correction, has been employed to extract the migration velocity of the c/a interface from the measured data. The thus determined interface migration velocity is approximately constant for all applied cooling rates. The first abnormal part of the transformation was discussed in comparison to the recently observed abnormal phenomena for the c fi a transformation in pure iron and ultra-low-carbon Fe-C alloy. On the basis of the thermodynamic and kinetic results, it is shown that autocatalytic nucleation occurs in the first part of the transformation and that interface-controlled growth prevails for the entire transformation.

Kinetics of anomalous multi-step formation of lath martensite in steel

“…Finally, in the light of the interpretation obtained for the abnormal massive formation of ferrite [31][32][33][34] and for the multi-step formation of bainite [35], the present conclusion suggests a generalization of the kinetics of the fcc to bcc transformations in iron-based alloys.…”

“…[11,30], anomalous observations were reported where the steps appear smoothed into a train of retardations and accelerations of the transformation, such that the kinetics of the transformation was characterized by a series of transformation rate maxima. Specifically, multiple retardations and accelerations of the transformation during continuous cooling have been reported several times for the transformation of face-centred-cubic (fcc) austenite into body-centred-cubic (bcc) microstructure constituents in steels, as ferrite [31][32][33][34], bainite [35] and martensite with lath [36][37][38] and lenticular morphology [11,30,[39][40][41], respectively.…”

A JMAK-like approach for isochronal austenite–ferrite transformation kinetics in Fe–0·055 wt-%N alloy

“…The obtained T onset for all applied cooling rates are located in the two phase (azc) region, which is different from that reported in Ref. 25 (T onset for the cRa transformation in the Fe-0?005 at.-%N alloy located in single a phase region). The T onset is lower than T 0 for three applied cooling rates.…”

Study of plasma nitriding and nitrocarburizing for higher corrosion resistance and hardness of 2205 duplex stainless steel

“…3(d-f). One of the possible reasons could be that the diffusion coefficient of carbon in the austenite phase is higher than that of nitrogen making the plasma nitrocarburized layer to grow faster in the austenite phase [27]. The WDS analysis summarized in Table 3 show that the austenite contains much more carbon than in the ferrite phase.…”