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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 5 164 gradually decreased with increasing grain size. For a grain size of ASTM 12.5 (5 μm), achieved by simulated induction heating at 850°C for two seconds, peak bending load was 20,000 N, or almost three times the load for grain sizes larger than ASTM 7. Fracture surfaces for test-steel W from specimens aus- tenitized at 850°C for two and 1000 seconds are compared Fig. 3 — (a) Notched bend test (three point bending) of simulated induction-hardened specimen, and (b) areas evaluated on the fractured specimen. Fig. 4 — Effect of austenitizing temperature and time on austenite grain size for test-steel Mo [5] . (a) (b) Fig. 5 — Relationship of peak bending load to austenite grain size for test-steel W; austenite grain sizes smaller than 25 μm result in higher fracture strength and transgranular fractures [6] . in Fig. 6. For the 1000-second austenitizing time, the austen- ite grain size was 53 μm (ASTM 6.2) and the hardness was 61 HRC (711 HV). After austenitizing for two seconds, the grain size was 4.4 μm (ASTM 12.8) and hardness was 59 HRC (666 HV). Fracture was intergranular for the 1000-second time compared with transgranular for the two-second aus- tenitizing time. Achieving the benefits of grain refinement for induction hardened parts requires careful design of the starting micro- structure and the induction hardening cycle. Figure 4 shows that the peak induction-heating surface temperatures must be held below about 900°C to achieve small austenite grain size. Many components such as axles and half shafts are produced from as hot-rolled AISI 1045 steel with a fer- rite-pearlite microstructure (Fig. 7a and c). During induction heating of a ferrite-pearlite microstructure, transformation to austenite begins in the pearlite areas. As heating contin- ues, ferrite transforms to austenite, and carbon must diffuse from previous higher carbon pearlite regions to previous low-carbon ferrite regions. If the temperature is not high enough, the carbon distribution remains non-homogenous, and on subsequent quenching the lower carbon regions can transform to lower strength bainite rather than martensite. Fig. 7(c) shows that the size of the ferrite requires carbon to diffuse over a distance up to about 10 μm. Non-martensitic transformation products that form during quenching are easily observed in the microstructure, so the tendency in induction hardening operations is to increase peak heating temperatures up to 1050°C or higher, resulting in larger aus- tenite grain sizes. In contrast, with the pre-quenched and tempered mi- crostructures (Fig. 7b), small cementite particles are pres- ent uniformly throughout the microstructure (Fig. 7d), the distance required for carbon diffusion is on the order of only 0.5 μm, and lower induction heating temperatures are 15