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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D ecarburization is detrimental to the wear life and fatigue life of steel heat-treated components. This article explores some factors that cause decarburization while concentrat- ing on its measurement. In most produc- tion tests, light microscopes are used to scan the surface of a polished and etched cross-section to find what appears to be the greatest depth of total carbon loss (free-ferrite depth, or FFD) and the great- est depth of combined FFD and partial loss of carbon to determine the maxi- mum afected depth (MAD). In some cases, there is no free ferrite at the surface. In research stud- ies, this may be supplemented with a Knoop hardness traverse to determine the depth where hardness becomes con- stant. The Knoop-determined MAD is of- ten somewhat deeper than the visually determined MAD, as variations in the mi- crostructure of carbon contents close to the core may be dificult to discern. The MAD determined by hardness traverse may be slightly shallower than that de- termined by quantitative carbon analysis with the electron microprobe. This is es- pecially true when the bulk carbon con- tent exceeds about 0.45 wt%, as the rela- tionship between carbon in the austenite before quenching to form martensite and the as-quenched hardness loses its linear nature above this carbon level. Decarburization basics Decarburization occurs when car- bon atoms at the steel surface interact with the furnace atmosphere and are removed from the steel as a gaseous phase [1-8] . Carbon from the interior difus- es towards the surface, moving from high to low concentration and continues until the maximum depth of decarburization is established. Because the carbon dif- fusion rate increases with temperature when the structure is fully austenitic, MAD also increases as temperature rises above the Ac 3 . For temperatures in the two-phase region, between the Ac 1 and Ac 3 , the process is more complex. Carbon difusion rates in ferrite and austenite are diferent, and are influenced by both temperature and composition. Decarburization is a serious prob- lem because surface properties are infe- rior to core properties, resulting in poor wear resistance and low fatigue life. To understand the extent of the problem, two characteristics that may be present at a decarburized steel's surface can be measured: Free-ferrite layer depth (FFD, when present) and partial decarburiza- tion depth (PDD, when free-ferrite is unDerstanDinG anD MeasurinG Decarburization Understanding the forces behind decarburization is the frst step toward minimizing its detrimental efects. George F. Vander Voort, FASM*, Struers Inc. (Consultant), Cleveland *Life Member of ASM International Fig. 1 — Decarburized surface of as-rolled, eutectoid carbon steel (Fe-0.8% C-0.21% Mn-0.22% Si) at two diferent locations around the periphery show a substantial variation in the amount and depth of ferrite at the surface. The matrix should be nearly all pearlite (4% picral etch, 500×). Fig. 2 — Erratic depth of free ferrite at the surface of a bar of 440A martensitic stainless steel afer quenching from 2000°F (1093°C); etched with Vilella's reagent. 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 2

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