Granular product in a high strength, low alloy containing molybdenum and niobium
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I.
INTRODUCTION
RECENTstudies of a high strength, low alloy steel containing molybdenum and niobium showed that there was a granular product "island", one of the transformation products, which is quite different from upper and lower bainite in morphology, produced either by continuous cooling with an appropriate cooling rate, or by isothermal transformation after undercooling to a suitable temperature. This product is similar to those described by some authors using low carbon bainitic steels containing boron, and called the martensiteaustenite (M-A) constituent ~'2 or granular "islands" distributed in the bainitic ferrite matrix. Throughout the paper, the granular product will be referred to as the granular "islands". Investigations demonstrated that the microhardness of the islands (Hm -- 400) is much higher than that of bainitic ferrite matrix (Hm -- 150) and that the steel containing granular islands was found to have lower Charpy fracture energy and fracture toughness as well as higher tensile strength than those of steel without the islands. This paper presents the results of studies concerning the fine structure and the mechanism of formation of granular islands as well as the influence of the islands on mechanical properties of a high strength, low alloy steel containing molybdenum and niobium.
mined metallographically by the point-counting method using photographs. The heating cycle, microstructure, and the corresponding volume fraction of granular islands of each specimen are presented in Table II. Specimens for optical microscopy were mechanically polished and then chemically etched in 2 pct nital. Observations by transmission electron microscopy were performed on a Philips 420 electron microscope operated at 100 kv, and included both replica and thin foil techniques. The thin foil specimens for TEM were chemically polished to a thickness of about 50/xm in a solution of 5 parts hydrofluoric acid and 95 parts hydrogen peroxide and then electropolished in a solution of 15 pct perchloric acid and 85 pct alcohol. Microanalysis was conducted on a type Hs00 EPMA instrument to compare the differences of the chemical compositions of the granular islands and bainitic ferrite matrix. In situ fracture microprocesses were observed on a Stereoscan type 4-10 SEM. J-integral and crack opening displacement were determined by using the multiple specimen method.
III.
RESULTS
A. Microstructure
II.
MATERIALS AND M E T H O D S
The steel used for the present study was made in a 11.4 ton open-hearth furnace. The ingots were press forged into plates of several thicknesses. The chemical composition of the steel is given in Table I. All specimens, of 120 x 25 x 25 ram, which were cut from hot rolled plates, were normalized at various cooling rates after austenitizing at 950 ~ for 10 minutes in order to obtain different volume fractions of granular islands. Cooling rate was measured with a chromel-alumel thermocouple. The volume fraction of granular islands was deterY.B. XU, Associate Professor, and T Y. ZHANG. Engineer,
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