Grain-boundary segregation and precipitation of boron in 0.2 percent carbon steels

The segregation and precipitation of boron have been studied in two 0.2C−0.6Mn0.−5Mo steels containing (nominally) 10 and 50 ppm B. After heating to 1260 δC, samples were air-cooled to 870 °C and then held for times between 0 and 5000 seconds. Additional samples were heated to 1260 °C, air-cooled to 900 °C, reduced in thickness by 50 pct, and then held for various times at 870 °C, as above. The distributions of boron under these various conditions were investigated qualitatively by an autoradiographic technique. In both steels, segregation and/or precipitation of boron at austenite grain boundaries was detected under all conditions examined. Precipitation of iron borocarbide particles occurred along austenite grain boundaries in the 50 ppm B steel during cooling to the holding temperature of 870 °C, while in the 10 ppm B steel, such precipitation occurred only after long times at 870 °C. Mechanical properties of single-pass-rolled samples were measured after tempering to assess the effects of borocarbide precipitation on notch toughness. Such precipitation lowered the Charpy upper shelf energy and increased the transition temperature.