THE DISSOLUTION AND PRECIPITATION OF BORON CONSTITUENT AND THEIR EFFECTS ON THE PROPERTIES OF 40MnB STEEL

The effects of dissolution and precipitation of M_(23)(C,B)_6 during heating athigh temperature and cooling subsequently upon the properties of medium carbonsteel 40MnB containing different amounts of boron have been investigated by meansof quantitative phase analysis and boron-autoradiography. The volume fraction andgrain size of M_(23)(C,B)_6 precipitate may be increased with the increase of boroncontent and with the decrease of cooling rate. Although the steels contained dif-ferent amounts of boron, a same critical cooling rate may be appeared at whichM_(23)(C,B)_6 precipitate reached to saturation. The dissolution and precipitation ofM_(23)(C,B)_6 are deeply affected the hardenability and toughness of the steels. Underthe same condition, M_(23)(C,B)_6 in steels containing less boron may be precipitatedout in smaller size and easily dissolved. Thus, they exhibited better combinedproperties and less sensitivity to the change of heat history. Suggestion on stabi-lizing the properties of the steels has been made.