Grain size of high-speed tool steels
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T H E final (hardening) heat treatment of high speed steel tools consists of austenitizing, quenching, and tempering. The size of the austenite grains formed during the hardening treatment is generally regarded as an important factor in the m i c r o s t r u c t u r a l c h a r a c terization of high speed steels. While fine austenite grain size is considered to be advantageous, published evidence of its beneficial effect on relevant properties, particularly on cutting tool performance, is quite limited. However, indications of a toughness advantage resulting from grain refinement have been presented. F o r example, Gill reported that austenite gram refinement was responsible for a marked toughness improvement of T1 high speed steel m torsion testing. ~ Also, Bungardt and Mfilders showed that p r o g r e s s i v e austenite grain refinement led to a continuous, substantial increase of the bend f r a c t u r e strength of M2 high speed steel.2 The present paper 1) considers factors involved in controlling the austenite grain size in high speed steels, 2) presents test results which show the effect of grain size on cutting tool performance, and 3) p r o poses a mechanism to explain the observed effects. I) MATERIALS
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EXPERIMENTAL
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The materials included in this study were AISI M1 and M2 high speed steels of essentially nominal compositions. Both particle-metallurgy (atomized, consolidated, and worked) and conventionally produced (cast to ingot and worked) materials were studied. The sizes of products from which specimens were pre1 pared ranged from -~ in. (12.5 ram) to 4 in. (I00 ram) diam bar. Identification of steels used in specific tests is given in the Appendix. Initially, the materials were m the annealed condition: either transformation annealed or temper annealed. Transformation annealing involved reheating of the hot-worked product to 1600~ (871~ equalizing, cooling to 1400~ (760~ and holding for 6 hr, followed by cooling to room temperature. Microstrucrurally, this process leads to austenite formation and partial carbide solutionmg followed by transformation T. A. NEUMEYERand A. KASAKare Research Metallurgist and Technical Director, respectively, Colt Industries, Crucible Materials Research Center, Pittsburgh, Pa. Manuscript submitted May 6, 1971. METALLURGICAL TRANSACTIONS
of the austenite to a relatively coarse aggregate of carbides in ferrite. Temper annealing consisted of reheating the hot-worked product (which had been allowed to substantially t r a n s f o r m to martensite during cooling) to about 1450~ (788~ holding for 16 hr, and cooling to room temperature. This treatment results in tempering of the initial martensite at a high t e m p e r ature, just below the A1 temperature of the steel, and produces a relatively fine aggregate of carbides m ferrite.* *Hardness of the temper-annealed product averages about 4 points R b higher than that of lhe transformation-annealed product.
Generally, specimens for tests and microscopic studies were given the following hardening treatment: aus
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