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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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 6 Additionally, a simple screening test was used to detect decarburized specimens of 5160H, 5160M, and 9260M spring steel lots used for front wheel drive automobile springs. As the de- sign loads on these springs increased throughout the 1970s, no free ferrite and almost no MAD could be permit- ted or spring life would be reduced. Mill processing helped minimize the MAD to less than the amount removed in the final processing step of turning and burnishing. Figure 6 shows data for a number of specimens where the surface scan was removed by glass- bead blasting afer hardening and bulk Rockwell C tests were made on the OD surfaces at a number of locations and averaged. Bars were sectioned, me- tallographically prepared, and rated for maximum free-ferrite depth (when present) and maximum afected depth of decarburization. The plot shows a much better correlation between HRC and MAD when free ferrite was not pres- ent versus when it was present. Examples of the variation in decar- burization ratings by three methods— carbon analysis of incremental turnings, microindentation hardness traverses, and visual qualitative or quantitative es- timates by light microscopy—are shown in Fig. 7. The spheroidize annealed mi- crostructure of W1 carbon tool steel (~1% C), a typical specimen rated by mill metallurgists in plants that make tool steel, is shown in Fig. 7d at 100×. The carbide in the decarburized surface zone exhibits a significantly lower volume fraction than the interior. At the extreme surface, individual carbides can be seen. Note the seemingly unusual carbon dis- tribution at the surface in Fig. 7a. The lowest carbon content is only to about 0.7%, roughly a 30% loss. So, free fer- rite is not present. Examination at 1000× shows that the cementite in the decarbu- rized zone is not well spheroidized but tends to be lamellar. This is because the annealing cycle cannot spheroidize ce- mentite in the lower carbon surface area compared to the bulk carbon content. Note that the hardness at the surface of the decarburized zone is actually greater than in the core, a result that may seem counterintuitive. However, as some tool steel metallurgists are aware, coarse lamellar carbide structure—even with a lower volume fraction than the spheroid- ized core—is harder and less ductile. Carbon analysis of the incremental turnings provides the best estimate of the maximum afected depth. The MAD esti- mate is more accurate using the Knoop traverses than LOM measurements, but is still rather conservative compared to the MAD from actual carbon analysis. How- ever, this is not a major problem because the hardness became essentially con- stant at a shallower depth than shown by the incremental carbon analysis. The qualitative estimates, based on a simple visual estimate going around the bar's periphery, are slightly lower than the Fig. 7 — Decarburization measurements on a spheroidize annealed bar of W1 carbon steel. a) Carbon analysis of incremental turnings reveal a MAD of 0.64 mm. b) A 200 gf Knoop hardness traverse reveals a MAD of 0.51 mm. c) Qualitative and quantitative visual LOM estimates yield MADs of 0.406 and 0.433 mm, respectively. d) LOM image of a typical surface area. (d)

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