Cu-Ni-Cr-Mo-B low alloy steels whose total boron content exceeds a certain limit show superior hardenability when they are austenitized at lower temperatures (≤900°C), although their hardenability decreases with higher austenitizing temperatures and longer holding times. As M23 (C, B) 6 in these steels is partly undissolvable at lower austenitizing temperatures, B content in solid solution is not determined uniquely from Fe-B-Al-N equilibrium state but rather regulated by the solubility product of M23 (C, B) 6.
At lower temperatures (≤900°C) boron content in solid solution which balances with M23 (C, B) 6 is considered appropriate for hardenability. At higher temperatures (>900°C), boron content in solution which equilibriates with M23 (C, B) 6 increases and exceeds the limit over which the hardenability of boron steels decreases. Heating below Ac1 for a long period, while M23 (C, B) 6 is enriched with C, Mn, and Mo atoms, austenitizing condition appropriate for the hardenability is transferred to higher temperatures and longer periods. This is because the solubility product of M23 (C, B) 6 is lessened and the particles grow coarsed.
Boron content in solid solution which balances with M23 (C, B) 6 is determined approximately from the relation among the decrease of hardenability, quenching temperature, and boron content in solution calculated from the Fe-Al-B-N system. The result is as follows: 880°C-3ppm, 900°C-6ppm and 930°C-8ppm.
These values are considered to depend strongly on Cr, Mn, and Mo contents and prior heat treating history. In excess boron containing steels, boron constituents are observed on the prior austenite grain boundaries at distances longer than 15mm from the quenched end of Jominy end quench test specimens, when they are quenched from 930°C.