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UNDERSTANDING and MEASURING DECARBURIZATION Understanding the Forces Behind Decarburization Is the First Step Toward Minimizing Its Detrimental Effects

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UNDERSTANDING and MEASURING DECARBURIZATION Understanding the Forces Behind Decarburization Is the First Step Toward Minimizing Its Detrimental Effects 22 UNDERSTANDING AND MEASURING DECARBURIZATION Understanding the forces behind decarburization is the first step toward minimizing its detrimental effects. George F. Vander Voort, FASM*, Struers Inc. (Consultant), Cleveland ecarburization is detrimental to crostructure of carbon contents close to the maximum depth of decarburization the wear life and fatigue life of the core may be difficult to discern. The is established. Because the carbon dif- steel heat-treated components. MAD determined by hardness traverse fusion rate increases with temperature ADVANCED MATERIALS & PROCESSES | FEBRUARY 2015 2015 FEBRUARY | & PROCESSES MATERIALS ADVANCED D This article explores some factors that may be slightly shallower than that de- when the structure is fully austenitic, cause decarburization while concentrat- termined by quantitative carbon analysis MAD also increases as temperature rises ing on its measurement. In most produc- with the electron microprobe. This is es- above the Ac3. For temperatures in the tion tests, light microscopes are used to pecially true when the bulk carbon con- two-phase region, between the Ac1 and scan the surface of a polished and etched tent exceeds about 0.45 wt%, as the rela- Ac3, the process is more complex. Carbon cross-section to find what appears to be tionship between carbon in the austenite diffusion rates in ferrite and austenite the greatest depth of total carbon loss before quenching to form martensite and are different, and are influenced by both (free-ferrite depth, or FFD) and the great- the as-quenched hardness loses its linear temperature and composition. est depth of combined FFD and partial nature above this carbon level. Decarburization is a serious prob- loss of carbon to determine the maxi- lem because surface properties are infe- mum affected depth (MAD). rior to core properties, resulting in poor In some cases, there is no free DECARBURIZATION BASICS wear resistance and low fatigue life. To ferrite at the surface. In research stud- Decarburization occurs when car- understand the extent of the problem, ies, this may be supplemented with a bon atoms at the steel surface interact two characteristics that may be present Knoop hardness traverse to determine with the furnace atmosphere and are at a decarburized steel’s surface can be the depth where hardness becomes con- removed from the steel as a gaseous measured: Free-ferrite layer depth (FFD, stant. The Knoop-determined MAD is of- phase[1-8]. Carbon from the interior diffus- when present) and partial decarburiza- ten somewhat deeper than the visually es towards the surface, moving from high tion depth (PDD, when free-ferrite is determined MAD, as variations in the mi- to low concentration and continues until Fig. 1 — Decarburized surface of as-rolled, eutectoid carbon steel (Fe-0.8% C-0.21% Mn-0.22% Si) at two different locations around the periphery show a substantial variation in the amount Fig. 2 — Erratic depth of free ferrite at and depth of ferrite at the surface. The matrix should be nearly all pearlite (4% picral etch, 500×). the surface of a bar of 440A martensitic stainless steel after quenching from 2000°F *Life Member of ASM International (1093°C); etched with Vilella’s reagent. 0215 AMP features.indd 22 2/10/2015 4:24:25 PM 23 ADVANCED MATERIALS & PROCESSES | FEBRUARY 2015 FEBRUARY | & PROCESSES MATERIALS ADVANCED not present). If free ferrite is present, the free-ferrite layer’s maximum depth (often variable) plus the depth of partial decarburization to the unaffected core is measured. This total—FFD + PDD—is called maximum affected depth (MAD). These depths are not uniform and can vary substantially, leading to measure- ments of average FFD, PDD, and MAD, as well as maximum values for each. ASTM E1077 covers decarburization measurement. In practice, decarburization should be evaluated on a plane transverse to the Fig. 3 — Decarburization of 5160 modified spring steel defined by surface hardness and hot working axis, as depth variation is incremental turnings analyzed chemically for carbon content as a function of whether or not the greater around the bar on the transverse surface was descaled or was covered by mill scale, and austenitizing at 1600°F for 80 minutes. plane than at a specific constant position along a longitudinal plane. Decarburiza- tion depth can vary substantially around than planar surfaces. Sampling schemes with precision and depth measurement the periphery of a bar, as shown in Fig. 1. for large cross-sections are also illustrat- accuracy suffers. Good edge retention Qualitative measurements can be sub- ed in ASTM E1077. requires mounting of the specimen in a jective and biased. Free-ferrite depth To obtain good data, edge reten- resin, such as DuroFast, that does not ex- can also be erratic, even over a small sur- tion must be perfect—the surface must hibit shrinkage gaps between the mount face area, as shown in Fig. 2. Corners of be perfectly flat to the extreme edge. If and specimen after polymerization. square or rectangular sections normally edges are rounded, the exact location Grinding and polishing procedures must exhibit greater decarburization depths of the outer surface is difficult to define emphasize maintaining flatness. While 0215 AMP features.indd 23 2/10/2015 4:24:25 PM 24 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, Ac3, of these grains could be above 1600°F, as alloy composition and residu- al elements influence the Ac3 of the steel ADVANCED MATERIALS & PROCESSES | FEBRUARY 2015 2015 FEBRUARY | & PROCESSES MATERIALS ADVANCED 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 Fig. 4 — Frequency histograms of decarburization measurements made around the periphery roughly 1700°-1750°F. As the holding of 5160 modified bars after heat treatment. a) 1600°F specimen with a mill scaled surface has an time is short, perhaps no more than 10 s, average FFD of 0.08 mm and average MAD of 0.266 mm for 132 measurements. Note the narrow, there is little time for appreciable carbon peaked distribution of the FFD measurements and broad distribution of MAD measurements. diffusion and the decarburization depth b) 1600°F specimen with a descaled surface does not exhibit any free ferrite. No decarburization after heat treatment is a function of the was observed for almost 19.5% of the measurements and distribution of MAD values is broad. as-rolled mill decarburization depth. 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 Fig. 6 — Rockwell C hardness tests on the surface of 5160H, 5160M, and 9260M (after glass bead ferrite present on the specimen that was blasting) is a simple screening test to determine if the surface is decarburized or free of descaled before being austenitized (2% decarburization. The correlation is much better when there are no free-ferrite grains at the surface nital, 200×), bottom. (blue data points) than when free ferrite (red data points) is present. 0215 AMP features.indd 24 2/10/2015 4:24:26 PM 25 ADVANCED MATERIALS & PROCESSES | FEBRUARY 2015 FEBRUARY | & PROCESSES MATERIALS ADVANCED ly produce more conservative estimates about 50-70% of the MAD determined by EXPERIMENTAL DATA than the incremental carbon analysis incremental carbon analysis or microin- If spring steels (typically ~0.6% procedure or microindentation hard- dentation tests. This depth, however, carbon) are heat treated in gas-fired ness traverses. This is because it is diffi- can be considered an effective depth furnaces, operating conditions can cult to detect the final minor loss in car- where carbon content is usually with- either increase or decrease the as- bon as the unaffected core is reached. in about 10-25% of the matrix carbon rolled depth of decarburization after Color etchants are likely to perform bet- content and responds reasonably well heat treatment, relative to the starting ter for this purpose than black and white to heat treatment. If the maximum ob- point. Austenitizing of these grades is etchants such as nital or picral, but com- served MAD is used as criteria for stock typically performed in the 1600°-1650°F parative tests have not been performed. removal, the surface’s carbon content range and holding times, which depend For annealed microstructures, the visual will be close to the matrix carbon con- upon bar diameter, are usually at least estimate of the average MAD is generally tent after machining. 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 (after scale removal by glass-bead blasting) and the carbon content was determined.
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