Low temperature improvement of the mechanical properties of 4340 type ultrahigh strength steel with heat treating techni
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The present work is concerned with low temperature improvement of the mechanical properties of 4340 type ultrahigh strength steel with the heat treating techniques using an interrupted quenching method. The steel used in this investigation was a commercial 4340 type hot rolled bar stock. The chemical composition is given with the Ms point in Table I. The steel was received as 130 mmq5 bars and subsequently hot-forged to 13 mm thick plates. Specimens used for mechanical tests were cut from the plates with their length axes in the longitudinal direction and were fully annealed. The heat treatments studied are given in Table II. All the specimens were first austenitized in an argon atmosphere tube furnace with a flat zone accuracy of -+0.5 K. A rapid austenitization in the 3 / ~ a ' repetitive heat treatment was conducted in a lead bath. The first quenching stage, which decreases the temperature from 573 to 553 K, and the intermediate tempering step at 673 K were both performed in lead-tin baths. These baths had a thermal capacity sufficient to avoid appreciable temperature change during the immersing, quenching, or tempering operation. The final tempering at 473 K was done in an oil bath. The subzero treatment was performed in liquid nitrogen. This was done to avoid the effect of thermally transformed austenite, i.e., untempered martensite formed at the low test temperature studied. Mechanical properties were determined through the smooth and notch tensile test. Smooth tensile specimens with a gage length of 25 mm and a gage section of 1.5 x 4.0 mm and notch tensile test pieces with a 2 mm V-notch in both sides and an effective cross section of 1.5 x 4.0 mm were used. Tensile tests were performed on a 245 kN Instron machine from an ambient temperature (287 K) to 123 K at a constant rate of 3.35 x 10 -4 per second using an auto-strain pacer. Tests at 203 K and below were conducted in a stainless chamber cooled with liquid nitrogen gas which allowed controlling the desired temperature to within -+ 1 K. The microstructure was categorized using optical metallography. Quantitative metallographic determination of the volume fraction of the second phase was performed by the standard point counting technique. ~,~9 To begin with, in order to determine the optimum interrupted quenching temperature, the effect of the interrupted quenching temperature on the mechanical properties of 4340 type steels treated by the interrupted quenching techniques after austenitization at 1133 K (IQT steels) or the IQT coupled with four cyclic 3' ~- a ' repetitive heat treatments (7 ~- c~' RHT plus 1QT steels) was investigated at test temperature of 287 and 203 K. The results obtained are summarized as follows: (1) At 287 and 203 K, the optimum interrupted quenching temperature for providing a better combination of strength and ductility and for improving notch tensile strength (NTS) was 573 K (just below the Ms point) for both IQT and y ~ a ' RHT plus IQT steels. The microstructure of these steels consists of mixed areas of about 10 vol pct high
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