Abstract
In order to harden, without quenching cracks, the small but complex shaped forgings of high carbon-chromium steels containing about 1% C and 0.1-0.4% Cr, the authors studied on the fundamental conditions about the martempering treatment.
The results obtained were as follows:
1) As for the cooling abilities of quenching media, the so-called "severity of quench" H was determined. H of the Pb-Cd-Sn metal bath and the NaNO3-KNO3-NaNO2 salt bath were 0.95 and 0.5-0.7 respectively. But H of the latter was apt to decrease during the use.
2) At the martempering treatment of the steel bars of 1″ diameter which had the larger hardenability than that of 0.4% Cr steel, the state of hardening was not influenced by the heating temperature between 830 and 880°C.
3) Under the above conditions, the state of hardening of martempering treatment was the same as that of quenching in water, so far as the holding time above the Ms point was shorter than the transformation-beginning period.
4) The authors measured the residual stress at the various heat treatment. The result of the air-cooling after martempering was the best, the oil-quenching the next and watercooling after martempering and the water-quenching the nearly same.
The residual stress became maximum when the isothermal treatment was held till the transformation began.
