An experimental study of carbide/austenite equilibria in the high-speed steel alloy system
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INTRODUCTION
H I G H - s p e e d steels are Fe-base alloys containing usually 4 pct Cr, different amounts of Mo, W, and V, some Si, and matching amounts of C. Some grades also contain Co. A typical feature is the high hardening temperature (around 1200 ~ at which carbides are dissolved in the austenitic matrix. After qurnching to room temperature, temperings are made at 550 ~ to 570 ~ at which secondary hardening takes place by precipitation of very fine carbides. This gives these steels a high hot hardness and their ability to cut at "high speed." Also, at the high hardening temperature, significant amounts of carbides remain undissolved, giving the steels additional wear resistance. In the literature, there is some information about the composition of the individual phases in high-speed steels but mostly on one or a few alloys. It can also be doubted whether these are equilibrium compositions, since they are made on ingot-solidified alloys without long holding times. The purpose of this investigation was to obtain more systematic information of the equilibrium compositions and the carbon activity in this alloy system. Such information is valuable for the design of alloys and for controlling the processes that take place during production and heat treatment. II.
EXPERIMENTS AND R E S U L T S
A. Alloy Preparation Alloys were prepared by melting pure iron and alloying elements in a 50-ks induction furnace. The melts were atomized with high-pressure nitrogen jets to powder with a mean size of about 200 /xm and maximum size of 1 mm. The powders were poured into tubes of plain carbon steel (0.2 pct C) (45-mm inner diameter), welded together, and compacted to 100 pct density at 1000 atm pressure and 1150 ~ The obtained bars were machined to 30-mm round to remove the tube material. The bars were then heat-treated in a slight overpressure
of argon at 1200 ~ --- 5 ~ for 240 to 270 hours. Parts of these specimens from some of the alloys were also given an additional heat treatment at 1100 ~ --- 5 ~ for 300 hours. Due to the fast solidification at atomization, the alloys got a very homogeneous chemical composition. The long holding time at 1200 ~ was needed to let the carbides grow to sufficient size to assure reliable microprobe measurements. The additional holding time at 1100 ~ for some of the alloys was used to achieve equilibrium at this temperature. The chemical compositions of the alloys are listed in Table I.
B. Carbon Activity In order to measure the carbon activity, a 3-mm hole was drilled in a 60-mm-long specimen from each alloy. A 3-ram round rod of pure iron was put into the hole. This specimen was put into a steel tube with powder of the same alloy as the specimen (Figure 1). The tube was sealed by welding, and the package was compacted at 1000 atm and 1150 ~ This encapsulation was done in order to have the iron rod wholly in contact with the alloy in which the carbon activity was to be measured. The package was heat-treated for 5 to 6 hours at 1200 ~ +- 2 ~ in a salt bath, in order to let carbon diffuse i
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