Abstract
The author made various heat-treatments on Ni-Cr steel (0·3% C, 3·3% Ni and 0·8% Cr), and investigated the influence of heat-treatment on the form of stress-strain diagram in the vicinity of yielding point and the degree of work-hardening (or the difference between the tensile strength and the yielding point) by accurately measuring the stress-strain diagram up to 5% strain. The results of the investigation are outlined below:-
A. Investigation by quenching and tempering
(1) When the tempering temperature is raised to about 40°C below Ac1 (715°C), carbide begins to aggregate in crystallographic planes, and simultaneously the form of the stress-strain diagram varies and the yielding point lowers; contrarily the degree of work-hardening suddenly increases.
(2) When the tempering temperature approaches Ac1, the aggregated carbide makes beautiful laminar layers forming a special structure which is named "laminar sorbite" by the author, and the yielding point steadily lowers, whilst the degree of work-hardening considerably increases.
(3) In accordance with the reduction of the cooling speed in quenching, the yielding point lowers, but the degree of work-hardening contrarily increases, whilst the cooling speed in tempering has no effect.
B. Investigation by double quenching.
(1) Double quenching is a heat treatment, in which the secondary quenching is carried out between the stages of the primary quenching and the tempering. In accordance with the temperature of the secondary quenching, two kinds of completely different mechanical properties are given to the material.
(2) When the secondary quenching temperature is in the vicinity of Ac1, i.e., 680°-730°C, the structure is of laminar sorbite with a low yielding point and a condiderably high degree of workhardening.
(3) When the secondary quenching temperature is below Ac3 (775°C), i.e., 740°-770°C, a special structure named "granular pearlite" by the author appears, and the yielding continues over 2% and the yielding point is comparatively high but the degree of work-hardening is extremely small.
