Isothermal martensite transformation in a 1.80 Wt Pct C steel
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I.
INTRODUCTION
I T is well known that the isothermal martensite transformation has occurred in some ferrous and nonferrous alloys. Isothermal martensites of ferrous alloys have been extensively studied in Fe-Ni-Mn ~-4 and Fe-Mn-C 3'5 alloys. As for plain carbon steels, several investigations on isothermal transformations 6'7'8 at temperatures near Ms were carded out, but they did not make sure that the product was either isothermal martensite or lower bainite. In 1970, Kennon and Edwards, 9 however, found isothermal martensites in a 1.44 wt pct C steel at temperatures near Ms (366 K). They indicated that the morphology of isothermal martensites was of a plate with 0.5 to 4/xm width and the habit plane was near {225}i*, which was the same as the *Subscripts b, f, and e refer to the bct martensite, the fcc austenite, and the epsilon carbide, respectively.
athermal one. Recently, the present authors have found the isothermal products formed during austempering for more than 2 d at 373 K in a 1.80 wt pct C steel. These were the same as the "black line product" in a 1.78 wt pct C steel found by Greninger and Troiano t~ in 1940. At that time, however, they did not describe them as either martensite or bainite. No attempt has been made to investigate them so far. The purposes of this paper are to provide substructures and crystallographic data of the isothermal products formed at 373 K in a 1.80 wt pct C steel by means of transmission electron microscopy and to see whether the product is martensite or bainite.
II.
EXPERIMENTAL P R O C E D U R E S
Square rods (15 x 15 x 50 mm) were cut from a hot forged bar of a vacuum melted high purity 1.80 wt pct C Table I.
Element wt pct
steel, which was supplied by Hitachi Metals Ltd. The chemical composition and Ms temperature of the steel used are given in Table I. They were homogenized at 1453 K for 20 hours in evacuated quartz capsules. Specimens for optical and electron microscopies were cut to sizes of 7 x 5 • 1 and 15 • 12 x 0.3 mm, respectively, from the homogenized rods. After austenitizing for 30 minutes at 1423 K under a dynamically evacuated condition, these specimens were quenched into an oil bath kept at 373 K, and were austempered there for various holding times. They were subsequently quenched into water to suppress the transformation. Partially transformed specimens for electron microscopy were chemically thinned to 0.05 mm in a H202-HF-H20 solution. Electro-polishing in a chromicacetic acid solution was done for foil preparation. Electron microscope used was JEM-200A operated at 200 kV.
III.
RESULTS AND DISCUSSION
A. Metallography Figure 1(a) is an optical micrograph of typical athermal martensites formed by quenching from 1423 K to water at room temperature. Figures l(b) through (d) are optical micrographs showing black line products in the specimens austempered at 373 K for 2, 10, and 30 d, respectively. Black line products (BLP's) with 20/xm length and 0.5/~m width could be seen in 2 d, as shown in Figure l(b) and their volume increased with time, Figu
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