Effect of Austenitizing Temperature on the Hardenability and Tensile Properties of Boron Steels

The hardenability of boron steel specimens with different molybdenum and chromium contents was investigated using dilatometry and microstructural observations, and then was quantitatively measured at a critical cooling rate corresponding to 90 % martensite hardness obtained from a hardness distribution plotted as a function of cooling rate. Based on the results, the effect of an austenitizing temperature on the hardenability and tensile properties was discussed in terms of segregation and precipitation behavior of boron atoms at austenite grain boundaries. The molybdenum addition completely suppressed the formation of pro-eutectoid ferrite even at the slowest cooling rate of 0.2 degrees C/s, while the chromium addition did at the cooling rates above 3 degrees C/s. On the other hand, the hardenability of the molybdenum-added boron steel specimens decreased with an increasing austenitizing temperature. This is associated with the preferred precipitation of boron atoms since a considerable number of boron atoms could be concentrated along austenite grain boundaries by a non-equilibrium segregation mechanism. The secondary ion mass spectroscopy results showed that boron atoms were mostly segregated at austenite grain boundaries without noticeable precipitation at higher austenitization temperatures, while they formed as precipitates at lower austenitization temperatures, particularly in the molybdenum-added boron steel specimens.