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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SiC abrasive paper can be used for grind- ing, resin-bonded diamond discs such as MD-Piano provide excellent flatness and long life. Napless cloths are used for di- amond polishing while low-nap clothes are used to polish with alumina or colloi- dal silica abrasives. In most cases, a ret- icule is used to make the measurement. Alternatively, many image-capture soft- ware programs allow operators to make point-to-point distance measurements. However, these systems must be proper- ly calibrated. Hot working temperatures can produce ferrite at the surface with the amount and size of the ferrite grains growing as carbon loss increases at the surface. The upper critical tempera- ture, Ac 3 , of these grains could be above 1600°F, as alloy composition and residu- al elements influence the Ac 3 of the steel grade. Spring steels are used as an ex- ample. If a decarburized specimen is in- duction hardened, the heating rate to the austenitization temperature is extremely fast. To put all of the carbon in solution (assuming that the steel has a carbon content of 0.60-0.65%), it is heated to roughly 1700°-1750°F. As the holding time is short, perhaps no more than 10 s, there is little time for appreciable carbon diffusion and the decarburization depth after heat treatment is a function of the as-rolled mill decarburization depth. Fig. 4 — Frequency histograms of decarburization measurements made around the periphery of 5160 modified bars afer heat treatment. a) 1600°F specimen with a mill scaled surface has an average FFD of 0.08 mm and average MAD of 0.266 mm for 132 measurements. Note the narrow, peaked distribution of the FFD measurements and broad distribution of MAD measurements. b) 1600°F specimen with a descaled surface does not exhibit any free ferrite. No decarburization was observed for almost 19.5% of the measurements and distribution of MAD values is broad. Fig. 5 — Decarburized surfaces of 5160 Mod austenitized at 1600°F for 80 min. and oil quenched. Free ferrite on the scale covered specimen, top. No free ferrite present on the specimen that was descaled before being austenitized (2% nital, 200×), bottom. Fig. 6 — Rockwell C hardness tests on the surface of 5160H, 5160M, and 9260M (afer glass bead blasting) is a simple screening test to determine if the surface is decarburized or free of decarburization. The correlation is much better when there are no free-ferrite grains at the surface (blue data points) than when free ferrite (red data points) is present. 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 4

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