抄録
The cause of the hard tempered martepsite brittleness which has come to be known as "500°F embrittlement" has not been explained until today. The phenomenon was much relatd to the deoxidation, nitrogen-fixation, and austenite grain size of steel. The tempering characteristics of the steel plays a major role in the development of embrittlement. This investigation consists of a stuay of the effect of Al, Ti and B addition on the impact-resistance and an electron micrgscopic study of the manganese-chromium case hardening steel when tempered between 120° and 420°C. The results obtained are as follows:
(1) The Al-Ti (0.05-0.10%) addition markedly improves the impact values during tempering at these temperatures, especially with Ti addition the benefibal effect in the ternperature range of embrittlement is distinct.
(2) The tempering embrittlement temperqture is not changed with the Al or Al-Ti addition and it occures at 300°C to 360°C; however in the case of Al-Ti-B addition it is the temperature range at 300°C to 420°C
(3) The impact minimum occurs in 20 to 30 minutes at the tempering embrittlement temperatures and the value is decreased with keeping time at 300°C, it is almost constant up to 10h at 360°C.
(4) The transition temperature is lowered by the Al, Ti and Al-Ti-B additiop; with the Al (0.04%), Ti(0.10%) and Al-Ti (0.06%)-B addition it falls down to 0°C-10°C.
(5) It is obeerved that the embrittlement arises from a precipitation of elongated thin cementite network along ferrite grain boundaries, with the B addition the formation is obserVed at the higher tempering temperature.
