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It is well known that cobalt additions can substantially increase the secondary hardening response in a number of alloys. For steels where secondary hardening is due to the precipitation of Mo2C or (Mo, Cr)2C particles, cobalt additions result in a finer dispersion of alloy carbides and the enhanced secondary hardening is attributed to this refinement of alloy carbide size and spacing. The finer dispersion of alloy carbides is believed to result from an increased alloy carbide nucleation rate. Chandok et al. concluded that cobalt increases the nucleation rate because cobalt increases the activity of carbon in ferrite and thus increases the driving force for carbide precipitation. Speich et al. observed that cobalt retards dislocation recovery and this would also lead to a higher nucleation rate and a finer dispersion of alloy carbides. Silicon additions also appear to enhance the secondary response in some steels. The purpose of this work has been to compare the effects of four elements on the secondary hardening response of a base steel, a 4.4 wt% chromium, 0.4 wt% carbon hot work die steel. This was done primarily to directly compare the effects of cobalt and silicon. Possible effects of aluminum and nickel additions on the tempering behavior of this base steel have also been investigated. The strong effects of aluminum additions on the tempering response of low alloy steels are similar to those exerted by silicon additions to the same steel, which suggests aluminum may also influence the precipitation of alloy carbides and enhance secondary hardening. In this work cobalt additions of 4 wt% and 8 wt%, silicon additions of 2 wt%, aluminum additions of 2 wt% and nickel additions of 4 wt% were made to a base hot-work die steel, having a nominal composition (in wt%) of 0.4C/4.5Cr/2Mo/0.5W/0.5V. This base steel was selected on the basis of its excellent toughness after secondary hardening. The compositions of the six heats utilized are given in Table 1. These alloys were melted using pure starting materials in a vacuum induction furnace and then cast in copper chill molds as 9 kg cylindrical ingots 6.4 cm in diameter. The ingots were upset and cross forged at 1 200°C to bars 7 cm wide and 1.75 cm thick. The forged bars were sand blasted and then homogenized at 1 200°C in vacuum for 24 h and then furnace cooled. Specimen blanks were cut from the homogenized bars after they had been softened by tempering at 650°C for 2 h. Specimen blanks 7 cm 1.75 cm 2.5 cm were austenitized for 1 h in an argon atmosphere in a vertical tube furnace and quenched in oil. Austenitizing temperatures of 1 000°C, 1 050°C, 1 100°C and 1 200°C were used. These blanks were then tempered for 1 h in neutral salt baths and then water quenched. The tempering temperatures used were 200°C, 300°C, 400°C, 500°C, 550°C, 600°C and 650°C. The Rockwell C scale (Rc) hardness values were taken from specimens cut from the tempered blanks using an abrasive saw run at a slow speed under conditions of flood cooling. The reported hardness values are the average of five hardness measurements. The hardness for each alloy is plotted for the asquenched condition and as a function of tempering temperatures from 200 to 600°C for the four austenitizing temperatures in Fig. 1(a) through 1(d). In discussing these results the primary interests are the influence of the alloying additions on the peak secondary hardness, the influence of austenitizing temperature on the peak secondary hardnessess and how the alloying additions influence softening on tempering at temperatures above that associated with the peak secondary hardness. To compare the influence of the alloying additions on the peak secondary hardness it seems most reasonable to make the comparison after austenitizing at 1 200°C where the existence of primary carbides is least likely. These data are shown in Fig. 1(d) and for all alloys the peak secondary hardness occurs on tempering at 550°C. The peak secondary hardness of the base steel is 52.4 Rc. As expected the cobalt additions increase the peak secondary hardness ISIJ International, Vol. 46 (2006), No. 5, pp. 782–784

A Comparison of the Effects of Cobalt, Silicon, Nickel and Aluminum on the Tempering Response of a Medium Chromium Secondary Hardening Steel