Abstract
It is said that when high speed steel is quenchedin hot bath at 600°, that is, tempering temperature for the steel, both hardening and tempering are effected at the same time and no further tempering is necessary. The writer has experimented on No. 2, 3, and-4 high speed steels specified in the J. E. S. From the hardening temperature of 1300° they were thrown into hot bath at 100_??_600° for different durations of immersion; then they were taken out and cooled in the still air. After examining their hardnesses and structures, it wa's found that high speed steel was scarcely better hardened by hot bath quenching below 600° than by oil quenching, and that to attain the effect of temper hardening, the hot bath quenching had to be followed by a usual tempering at 600° The sames was true for a steel kept in hot bath at 600° for about five days. But when immersed at 600° for more than twenty days, it developed troostite on the boundary of austenite and had its quenched hardness reduced. In the case of the hot bath raised to 650_??_700° it also developed troostite, had its hardening effect greatly reduced and no satis factory temper hardening was secured even if tempered at 600°. In other words, hot bath quenching of high speed steel serves a prevention against quenching crack and deformation rather than temper hardening and the effective hot bath temperature is below 600°. A two-step hot bath quenching was also tried. In this method, steel was immersed from the hardening temperature into a hot bath below 600° (1 st hot bath), and, without being cooled in the'air, was immediately dipped into the tempering hot bath (2 nd hot bath). After being kept there for the adequate duration of time, it was taken out and cooled in the still air. The result showed that at 200_??_600° of the Ist hot bath, the two-step method could no more effect temper hardening than the simple hot bath quenching. In this case stell must be tempered at 600° for any tempering effect; hardness was found to be independent of hot bath temperature. At 100° of the 1 st hot bath, however, temper hardening was effected by the two-step method with the same hardness and structure obtained as by ordinary oil hardening and tempering. Therefore it can be concluded that high speed steel must be cooled down to 200° during its quenching before it can be tempered at 600° with any satisfactory effect: it must be cooled nearly to room temperature before tempering.
