Advanced Materials & Processes

FEB 2015

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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experiMental Data If spring steels (typically ~0.6% carbon) are heat treated in gas-fired furnaces, operating conditions can either increase or decrease the as- rolled depth of decarburization afer heat treatment, relative to the starting point. Austenitizing of these grades is typically performed in the 1600°-1650°F range and holding times, which depend upon bar diameter, are usually at least 20 minutes. In many cases, a protective atmosphere is not employed. An experiment was conducted us- ing round bars of 5160 modified spring steel. Specimens were austenitized ei- ther with the as-rolled mill scale present or removed by sand blasting. Specimens were austenitized at 1600°F (871°C) for 80 minutes, then oil quenched. Part of each bar was incrementally machined (afer scale removal by glass-bead blasting) and the carbon content was determined. Surface hardness readings were also recorded and results are shown in Fig. 3. Note that the specimen austenitized at 1600°F exhibits a large diference be- tween surface carbon content and sur- face hardness, compared to the bar cov- ered with mill scale to the descaled one. Figure 4 shows results of quantita- tive FFD and MAD measurements for the two specimens austenitized at 1600°F— including 132 measurements around the periphery of the scaled bar and 113 mea- surements for the descaled sample. The scaled bar austenitized at 1600°F exhibits a consistent free-ferrite layer around its periphery with an average depth of 0.08 ± 0.002 mm (95% confidence interval). Note that FFD measurement distribution is very narrow, or peaked. The MAD, how- ever, shows an average depth of 0.266 ± 0.006 mm and distribution is broad. In contrast, for the descaled bar, no free ferrite was seen and 19.47% of the 113 measurements indicate no decarburiza- tion was present. The remaining mea- surements exhibit an average depth of 0.073 ± 0.010 mm, slightly lower than the scaled bar's average FFD. The MAD dis- tribution curve appears to be bimodal. Figure 5 shows typical microstructures observed at the specimens' two surfaces. Visual estimates of the maximum af- fected depth of decarburization general- ly produce more conservative estimates than the incremental carbon analysis procedure or microindentation hard- ness traverses. This is because it is difi- cult to detect the final minor loss in car- bon as the unafected core is reached. Color etchants are likely to perform bet- ter for this purpose than black and white etchants such as nital or picral, but com- parative tests have not been performed. For annealed microstructures, the visual estimate of the average MAD is generally about 50-70% of the MAD determined by incremental carbon analysis or microin- dentation tests. This depth, however, can be considered an efective depth where carbon content is usually with- in about 10-25% of the matrix carbon content and responds reasonably well to heat treatment. If the maximum ob- served MAD is used as criteria for stock removal, the surface's carbon content will be close to the matrix carbon con- tent afer machining. 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 | F E B R U A R Y 2 0 1 5 2 5

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