Carbon steel surface hardening

Whether decarburization will be an issue for internal combustion engines burning H2 is difficult to predict from existing information. Low-alloy carbon steels begin to decarburize at temperatures around the operating temperature of exhaust valves, but exhaust valves and valve seats are made from high-alloy steels, austenitic alloys, and superalloys where the carbon is much more stable than low-alloy carbon steels. The hardenable martensitic valve stems of exhaust valves may experience decarburization over extended periods, and this would lead to accelerated wear because of the softened surface that results from decarburization. 

Low-carbon steel is heated at 850-930°C in contact with gaseous, liquid, or solid carbon-containing substances for several hours. The high-carbon steel surface produced is then hardened by quenching... 

When a component at an austenitizing temperature is placed in a quenchant, eg, water or oil, the surface cools faster than the center. The formation of martensite is more favored for the surface. A main function of alloying elements, eg, Ni, Cr, and Mo, in steels is to retard the rate of decomposition of austenite to the relatively soft products. Whereas use of less expensive plain carbon steels is preferred, alloy steels may be requited for deep hardening. 

An iatermediate treatment that adds both carbon and nitrogen to steel surfaces can be obtained by exposiag the parts to a bath of molten cyanide at just above the critical temperature of the core for about one hour followed by direct quenching. The hardened area is about 0.25-mm deep. 

Figure 12-7. Piston rods. Precision-manufactured rolled threads and induction hardening provide high fatigue strength and long life in heavy duty service. Standard rod material is AISI 4142 carbon steel other materials are available as required. Tungsten carbide coatings are also available for maximum surface hardness. Pistons are locked securely onto the rods. For higher pressure, smaller bore cylinders, the piston may be integral with the rod. (Used by permission Bui. 85084, 1992. Dresser-Rand Company.)...

Fig. 12.23 shows the variation of hardness with distance from surface for a chromised high-carbon steel (1% C). The full line represents the material as treated , the broken line corresponds to conditions after full-hardening and tempering. The apparent greater depth of the hard zone in the heat-treated material is due to the effect of small concentrations of chromium... 

Rockwell s hardness -carbonized steel [METAL SURFACE TREATMENTS - CASE HARDENING] (Vol 16)... 

CARBONITR1DING. A surface hardening process for steels involving ihe introduction of carbon and nitrogen into steels by healing in a suitable almosphere containing various combinations or hydrocarbons, ammonia, and carbon monoxide followed by a quenching to harden the case. 

Cementation process. This process, now little used, consists of heating wrought iron or low-carbon steel in powdered charcoal or leather dust for 6 to 11 days in a closed boxlike furnace at 650 to 700°C. At these temperatures, carbon diffuses slowly into the surface of the steel, thus producing a thin coat of high-carbon steel over a core of low-carbon steel. This is essentially a case-hardening procedure, and steel produced in this manner is used largely in the manufacture of tools. 

Acid etching can be used to treat types 301 and 302 of stainless. These processes result in a heavy black smut formation on the surface. This material must be removed if maximum adhesion is to be obtained. The acid etch process produces bonds with high peel and shear strengths. The 400 series of straight chromium stainless steels should be handled in the same manner as the plain carbon steels. The various types of precipitation hardening (PH) stainless steels each present an individual problem. Processes must be adopted or developed for each type. 

The measurements have also so far been confined to soft metals and alloys in the annealed condition and it is difficult at present to predict from these results the likely depths in harder alloys it can only be assumed as a rough approximation that the depth will be inversely proportional to hardness (36,37). It may even be doubted by some whether there can be such a thing as a plastically-deformed layer in very hard metals, but there is good reason to believe that this can be so. An extensive zone of this nature has, in fact, been identified beneathindentationhardness impressions in fully-hardened high-carbon steels (35,55). It is again necessary to remember that the surface deformation occurs under very special conditions. 

The surface hardening of a mild steel component by the diffusion of carbon molecules is a transient mass diffusion process. 

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