Precipitation behavior in a nitride-strengthened martensitic heat resistant steel during hot deformation

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Abstract

The stress relaxation curves for three different hot deformation processes in the temperature range of 750–1000 °C were studied to develop an understanding of the precipitation behavior in a nitride-strengthened martensitic heat resistant steel. The first sharp jump in the stress relaxation plots was assumed to be indicative of the commencement of precipitation in this paper, according to the abrupt change in σ/t curve and the microstructure observation of the samples. The precipitation revealed by the stress jumps in the stress relaxation plots was further studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and energy dispersive X-ray analysis (EDAX). The results confirmed that, under the deformation situation of “Group C”, almost all the Nb atoms would precipitate as Nb(C,N) particles at its optimum precipitation temperature of 940 °C; Nearly all the C+N atoms would be consumed in forming M23C6 particle at its nose precipitation temperature of 800 °C; only 60% V atoms would deposit as V(C,N) particles at its fastest nucleation temperature of 750 °C.

Introduction

Precipitation hardening is crucial to the creep strength of heat resistant steels. Since M23C6-type (M=Cr and Fe) carbides are believed to grow fast and lose their benefit of pinning the movement of dislocations during creep exposure [1], the studied nitride-strengthened heat resistant steel is aimed to suppress the formation of M23C6 by reducing carbon content to a very low level. It is expected that the nitride-strengthened heat-resistant steel would mainly employ MX (M=Nb or V, and X=C, N) type carbonitrides to obtain precipitation hardening. Thus, the effects of M23C6 are ignored. Thus, it is important to study the precipitation behavior in nitride-strengthened martensitic heat resistant steel so that hot deformation can be controlled to obtain the desired precipitates.

However, it is known that during hot deformation strain-induced precipitation, strain-induced phase transformation, and dynamic recrystallization (DRX) can happen. The precipitates can also interact with each other. For example, the strain-induced precipitation can efficiently retard austenite recrystallization [1]. The precipitation kinetics in the austenite phase of Nb-microalloyed steels has been well studied [1], [2], [3]. Some researchers [4] modeled the precipitation kinetics of NbC in alpha-iron by Monte Carlo simulations. It was also reported that V particles precipitated in a manner similar to Nb, although they preferred to precipitate at a relatively lower temperature [5], [6]. The above reported work was mainly focused on microalloyed steels. Unlike the microalloyed steels, the present steel [6] contains 9 wt% Cr, which would highly effect the diffusion of Nb atoms during stress relaxation. The highly alloyed heat resistant steels cannot be considered as a dilute solution and majority of the kinetics equations are not suitable for calculations. The accurate precipitation behavior can only be obtained by experiments.

On the other hand, the present steel experienced dynamic strain-induced transformation (DSIT) during deformation [7], which makes the precipitation kinetics more complicated. Thus, in the present study we attempt to DSIT during hot deformation by controlling the temperature and plastic reduction during deformation, and then explore the precipitation behavior in austenite phase in nitride-strengthened heat resistant steel.

Section snippets

Experimental procedure

The steel was melted in a 25 kg vacuum induction melting furnace. The chemical composition of nitride-strengthened (NS) heat resistant steel is listed in Table 1. The content of all the elements, except N, S, and O that were obtained from the gas analysis, were tested by chemical analysis. Bars for the compression test were cut from the ingot and machined into specimens with diameter of 8 mm and gage length of 12 mm.

The stress relaxation [8] and the conventional double-compression methods were

Microstructure during deformation

As illustrated in Fig. 2, the content of strain-induced ferrite was reduced with decrease in deformation temperature, which is consistent with the previous work [9], [10], [11]. Since DSIT preferred to occur at higher temperature, the DSIT ferrite rarely existed at temperatures below 940 °C, as seen in Fig. 2(b). On giving reduction of 30% at temperatures less than 1000 °C, DSIT did not influence the stress relaxation plot. On the other hand, as mentioned in the previous work [9], the bainite

DIST and DRX

When austenite is transformed to ferrite in steels, the lattice volume is increased and causes the stress to increase during the stress relaxation, which may also produce a stress sharp jump. Thus, it is necessary to define what is the jump on the stress relaxation plots. It is obvious from Fig. 2 that the strain-induced ferrite was formed, when the steel was deformed at 1000 °C. From the previous work, it is assumed that DSIT prefers to occur on deformation at high temperature. Thus, the

Conclusions

1. The DSIT ferrite was rarely present in the NS steel, when deformed at temperatures below 940 °C, leading to a single phase austenite microstructure.

2. Two kinds of stress serrations, including stress wave and sharp stress jump, were observed in the stress relaxation plots of NS steel. The former was related to the diffusion of substitutional atoms and the latter represented the beginning of precipitation.

3. Increase of deformation reduction prompted precipitation. Almost all the Nb atoms

Acknowledgments

This work was financially supported by National Basic Research Program of China (No. 2010B630800) and National Natural Science Foundation of China, China (No. 51001102). Some of the data analysis was also sponsored by Natural Science Foundation of Guilin University Aerospace of Technology (No. YJ1405) and the Dr. Start-up fund from Guilin University Aerospace of Technology.

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