Part XII – December 1969 – Papers - Tempering of Low-Carbon Martensite

The distribution of carbon and the type of substructure in iron-carbon martensites containing 0.02 to 0.57pct C has been studied in the as-quenched condition and after tempering at 25" to 700°C by using electrical resistivity, internal friction, hardness, and light and electron microscope techniques. in marten-sites containing less than 0.2 pct C, almost 90 pct of the carbon segregates to dislocations and to lath boundaries during quenching; in martensites containing greater than 0.20 pct C, appreciable amounts of carbon enter normal interstitial positions located far from defects. Tempering martensites with carbon contents below 0.20 pct at temperatures below 150°C results in additional carbon segregation to dislocations and to lath boundaries but no carbide precipitation whereas -carbide precipitation occurs in martensites with carbon contents exceeding 0.2 pct. Above 150°C, a rod-shaped carbide (either Fe3C or Hagg) is precipitated in all cases. At 400°C, spheroidal Fe3C precipitates at lath boundaries and at former aus-tenite grain boundaries. At 400" to 600"C, recovery of the martensite defect structure occurs. At 600" to 700°C, recrystallization of the martensite and Ost-waW ripening of the Fe3C occur. The effects of the carbon segregation that occurs during quenching and the subsequent substructural changes that occur during tempering on martensite tetragonality, hardness, and precipitation behavior are discussed. A mathematical analysis of carbon segregation during quenching is presented. RECENT studies of the strength of low-carbon martensitel-4 emphasize the importance of carbon segregation to the martensite lath boundaries and to the dislocations contained between them during quenching. Unfortunately, very few studies of the tempering of low-carbon martensites have been conducted, so the exact nature of this segregation is poorly understood. In fact, most early tempering studies5,6 were restricted to carbon contents greater than 0.20 pct. Moreover, these studies did not determine the amount of carbon segregated to the martensite substructure during quenching so that the initial state of the martensite was not established. Aborn7 studied the precipitation of carbide in low-carbon martensite during quenching but did not establish whether carbon segregation occurs prior to carbide precipitation, nor did he study the subsequent tempering sequence in detail. In the present work we have used electrical resistance and internal friction measurements, supplemented by electron transmission microscopy to establish the carbon distribution in as-quenched specimens. Specimens thin enough to avoid carbide precipitation (but not carbon segregation) were employed. The redistribution of carbon on subsequent tempering below 250°C was followed by measurements of elec- trical resistance. Additional studies were made on specimens tempered at 250" to 700°C to elucidate the overall tempering behavior of low-carbon martensites, including the formation of cementite and recrystalli-zation of the martensite. EXPERIMENTAL PROCEDURE Eight iron-carbon alloys with 0.026, 0.057, 0.097, 0.18, 0.20, 0.29, 0.39, and 0.57 wt pct C were prepared as 8-lb ingots by vacuum melting. Typical impurities in wt ppm were 40 Si, 20 Mn, 30 S, 10 P, and 10 N. These alloys were hot rolled to 3 in. plate at 1095°C) (2000°F). The hot-rolled plates were surface ground to remove scale and the decarburized layer, then cold rolled to 0.010 in. sheet. Specimens cut from the sheet were austenitized for 30 min at 1000°C (1830°F) in a vacuum tube furnace in which the pressure did not exceed 2 x 10-3 torr. Chemical analysis of specimens after austenitization indicated no decarburization at this pressure. Immediately before quenching, the furnace was filled with prepurified helium. The specimen was then pushed rapidly through an aluminum foil gasket, which sealed the bottom of the furnace, into an iced-brine bath (10 pct NaC1, 2 pct NaOH). The quenching rate at the M, temperature is about 104'c per sec for 0.010 in thick specimens, as calculated from Newton's law of heat flow2 using a heat transfer coefficient of 25 ft-'. This quenching rate is sufficiently high so that all the alloys transformed completely to martensite throughout the entire 0.010 in thickness and no carbide precipitation occurred in the martensite. All specimens were immediately transferred to liquid nitrogen after quenching and stored there until needed. Tempering below 250°C (480°F) was done in silicone oil baths thermostatically controlled to *;"C. Tempering above 250°C was done in circulating air furnaces or lead pots with the specimens contained in evacuated silica capsules. Electrical resistance was determined by measurement of the potential drop across both a standard resistance and the specimen, connected in series. All resistance measurements were made in liquid nitrogen (77K, -196°C) to minimize thermal scattering of electrons and thus maximize the contribution of impurity scattering to the resistance. Specimen dimensions were 5.10 by 0.19 by 0.025 cm. Although the precision in the electrical resistance measurements was +0.1 pct, the electrical resistivities could only be measured with an accuracy of +5 pct because of uncertainty in the specimen dimensions. Internal friction measurements were performed in an inverted pendulum apparatus at vibration frequencies of either 1.9 or 66 Hz. The specimen dimensions were 5.10 by 0.375 by 0.025 cm. Hardness measurements were made with a Leitz-Wetzlar microhardness machine with loads of 100 g. Specimens were examined by light microscopy after etching in 2 pct Nital and by electron transmission microscopy after preparation of thin sections by electrolytic thinning in a chromic-acetic acid solution.