November AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 9 6 3 S everal tools and advanced techniques are available at the Advanced Steel Processing and Products Re- search Center (ASPPRC), Colorado School of Mines, to investigate microstructures and mechanical property limita- tions with the goal of increasing performance in steel com- ponents. One such limitation is that hardness greater than 53 HRC (565 HV) achieved using conventional furnace quench and temper heat treatment leads to brittle inter- granular fracture and limits maximum tensile strength [1] . The change from ductile to brittle behavior is illustrated in Fig. 1 where peak strength is about 2000 MPa, then decreases with increasing hardness. Similar behavior has been reported for torsional fatigue properties of induction-hardened polished specimens, where low-cycle fatigue strength increases with increasing hardness up to about 58 HRC (650 HV), but frac- ture becomes intergranular at higher hardness, and there is METALLURGICAL STRATEGIES FOR HIGHER STRENGTH INDUCTION HARDENED PARTS Induction hardening parts with small grain size achieves higher fracture strengths, but close control of thermal cycles is required to prevent grain growth. Robert Cryderman,* Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden *Member of ASM International 14 Fig. 1 — (a) Maximum tensile strength of furnace austenitized, quenched, and tempered steel is limited by brittle fracture in tensile specimens at hardness levels >53 HRC, (b) dimpled ductile fracture in specimen at 51 HRC hardness, and (c) brittle intergranular fracture in specimen at 55 HRC hardness. (a) (b) (c) 13