730 U. S. BUREAU OF STANDARDS NOTES. [J. F. I.

RELATION OF THE HIGH-TEMPERATURE TREATMENT OF HIGH-SPEED STEEL TO SECONDARY HARDENING

AND RED HARDNESS.’

By Howard Scott.

[ABSTRACT.]

THERE has, been more or less mystery attached to certain well- known phenomena connected with the heat treatment of high- speed steel. Anticipating a disappearance of the mystery with the acquirement of sufficient pertinent experimental data, the physical characteristics and the microstructure of a standard type of high- speed steel a,s affected by quenching temperature, tempering tem- perature, and treatment below ordinary temperatures were studied.

The experimental results obtained appear to warrant the fol- lowing conclus’ions. : ( I ) Secon,dary hardening necessitates prior partial austenitization produced by the high-temperature treat- ment ; (2) the variations with heat treatment in physical proper- ties and microstructure of high-speed s;teel are analogous to those of hypereutectoid carbon steel with the exception of the micro- structure of hardened but untempered steel ; (3) red hardness is largely a function of the composition of the matrix and is only colmparatively slightly affected by the amount of dissolved car- bide. Three reasons are given for the establis’hed advantages derived fro&m th,e use of the high-temperature heat treatment, namely, increased red hardness, increased initial hardness, and reduced brittleness.

THERMAL AND PHYSICAL ‘CHANGES ACCOMPANYING THE HEATING OF HARDENED CARBO’N STEELS.6

By Howard Scott and H. Gretchen Movius.

[ABSTRACT.]

THERMAL analysis was used here for determining the: thermal ch‘aracteristics of hardened carbon steels below A,. A definite heat evolution, called Act, was observed for a normal rate of heating starting at about 165” C., reaching a maximum at 270~ C. and ending at 290’ C.; the effect on it of the variables-rate of heating, tempering, carbon content, and quenching temperature- was studied. -

4 Scientific Paper No. 395. ‘Scientific Paper No. 396.