The growth behavior of austenite grain in the heating process of 300M steel

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Abstract

The 300M steel was heated in an electric furnace at the temperatures of 850, 900, 950, 1000, 1050 °C, and holding time of 5, 10, 30, 60, 90, 120 min. The grain size of austenite was measured by using the linear intercept method on Olympus PMG3 metallographic microscope. The experimental results show that grain size of austenite increases with the increasing of heating temperature and holding time, and the grain growth model of austenite in 300M steel is in the following: d = 4.04 × 106t0.17exp(  (1.32 × 105/RT)).

Highlights

► A model for grain size in heating process of 300M steel is established. ► The growth model is derived considering the effect of heating temperature and holding time. ► The model can be used to predict the austenite grain size in heating process of 300M steel.

Introduction

The grain growth of austenite in heating process of steel has been focused by many researchers for many years. Sha and Sun [1] investigated the grain growth behavior of austenite in Nb–V–Ti microalloyed steel, and found that the grains of austenite grow up with the increasing of heating temperature; the Ti-rich carbonitrides assembling in the grain boundaries of austenite restrain the grain growth of austenite at the temperature below 1250 °C. Yu and Sun [2] pointed out that the critical growth temperature of austenite grains in 0.015% Nb steel is 1240 °C; the grains of austenite in 0.015% Nb steel are finer than that in Nb free steel in the range of 1150–1230 °C. The austenite grain size in 16MnNi4 HSLA steel increases rapidly by heating at 1200 °C for 5–30 min or 1150 °C for 60 min, which was obtained by Fernández et al. [3]. Therefore, the major factors affecting the grain size of austenite are the heating temperature, holding time and alloying elements.

The commercial ultrahigh-strength low alloying steel 300M (40CrNi2Si2MoV), has been modified by adding the alloying elements of silicon and vanadium in 4340 steel. Because of the high strength, good fracture toughness and fatigue behavior, 300M steel is widely used to manufacture the central spindle, wheel gear, aerofoil fastener and so on. It becomes one of the best steels for manufacturing the aircraft landing system. Much more investigation was carried out for the microstructure and mechanical properties of 300M steel [4], [5], [6], [7], [8]. As the coarse grains are generally observed in the manufacturing process of 300M steel forging, many researchers have paid attention to investigate the mechanism of low plasticity and ductility caused by the coarse grains. Zhang et al. [9] pointed out that the 300M steel being fabricated by vacuum induction melting and vacuum arc remelting has higher purity and stronger overheating sensitivity, in which the growth tendency of austenite grain increased and it would result in the occurrence of coarse grains as adopting unsuitable forging method. In terms of the experimental results, two heat treatment processes of 300M steel were suggested so as to eliminate the coarse grains as follows [10]: (1) high temperature normalizing in the temperature range of 970–980 °C, and (2) heated to 700 °C for 60 min and air-cooling, then heated to 930 °C rapidly with holding time of 60 min, and finally cooled in air. However, the growth behavior of austenite grain in the heating process of 300M steel was reported in a few literatures. In this article, the effect of the heating temperature and holding time on the morphology and grain size of austenite in as received 300M steel is investigated. Based on the experimental results, the growth model of austenite grain in 300M steel is presented.

Section snippets

Experimental procedures

The chemical composition of as received 300M steel is shown in Table 1, and the morphology of austenite grains is shown in Fig. 1. The as received 300M steel bar is of 22.0 mm in diameter, its Ac1 temperature and Ac3 temperature are 748 °C and 802 °C, respectively. The dimensions of experimental specimens machined from as received 300M steel are 10.0 mm × 8.0 mm × 5.0 mm. The heating treatment was carried out in a type SX-5-12 electric furnace at the heating temperatures of 850, 900, 950, 1000, 1050 °C

Effect of heating temperature on the morphology and grain size of austenite

Fig. 2 shows the morphology of austenite grain in 300M steel at different heating temperatures and a holding time of 5 min. As seen from Fig. 2, the fine austenite grains distribute inhomogeneously as new austenite grains contact with each other, the grain boundary is bend, and the undissolved particles (carbides) occur as shown in Fig. 2a. Adding Cr, Mo and V as alloying elements in 300M steel, the carbides having high melting point and stability occur in 300M steel. Those carbide particles

The growth model of austenite grain

According to the metallographic principles, the grain size of austenite in 300M steel can be expressed as the following empirical equation:d=Ktnwhere d is the average grain size (μm), t is the holding time (s), K and n are the material constants depending on material and temperature, n is less than 0.5 for most of materials and it is 0.3 generally.

The K value is depending on material and temperature, the relation between austenite grain growth and austenitizing condition has been explained by

Conclusions

With the increasing of heating temperature and holding time, the grain size of austenite in 300M steel increases, while the grain boundaries of austenite reduce and become flat being close to an angle of 120°.

The growth of austenite grain is affected by the heating temperature and holding time. The growth model of austenite grain in 300M steel is described as follows: d = 4.04 × 106t0.17exp(  (1.32 × 105/RT)).

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