Hardening of steels

H. E. Boyer, ed.. Case Hardening of Steel, ASM International, Metals Park, Ohio, 1987. 

C. A. Siebert, D. V. Doane, and D. H. Breen, The Hardenability of Steels—Concepts, Metallurgical Influences andindustrialApplications, American Society for Metals, Metals Park, Ohio, 1977. 

Fig. 8. Effect of chromium on hardenability of steel as indicated by hardness distribution across 41 mm rounds of oil-quenched 0.35% C steel (44).

The polymorphism of certain metals, iron the most important, was after centuries of study perceived to be the key to the hardening of steel. In the process of studying iron polymorphism, several decades were devoted to a red herring, as it proved this was the P-iron controversy. P-iron was for a long time regarded as a phase distinct from at-iron (Smith 1965) but eventually found to be merely the ferromagnetic form of ot-iron thus the supposed transition from P to a-iron was simply the Curie temperature, p-iron has disappeared from the iron-carbon phase diagram and all transformations are between a and y. 

The major use of sodium cyanamide is in the production of sodium cyanide, a compound that is used extensively in preparing solutions for the electroplating of metals. Another use for NaCN is in extraction processes employed to separate gold and silver from ores as a result of their forming complexes with CN . Sodium cyanide, an extremely toxic compound, is also used in the process known as case-hardening of steel. In this process, the object to be hardened is heated and allowed to react with the cyanide to form a layer of metal carbide on the surface. 

Aurantin, molecular formula and structure, 5 91t Ausimont, 7 641 Austempering, 23 287 Austenite, 23 272, 273, 275 decomposition of, 76 197-198 grain size of, 23 276-277 in hardening of steels, 76 196-197 phase transformations in, 23 277 transformation rates of, 23 282—283 Austenite phase, in martensite... 

Hard and soft acid and base (HSAB) principle, 16 780 Hard blacks, 21 775 Hard-burned quicklime, 15 28 Hard coals, 6 703 classification, 6 712 Hard copper alloys, 7 723t relief annealed, 7 723t Hard copy systems, 9 513-514 Hard core repulsion, 23 93 Hard-elastic olefin fibers, 11 242 Hardenability, of steel, 23 283—284 Hardened MF resins, analysis of,... 

Experiments have determined (8) that mechanical blocking of dislocation motion can be achieved quite directly by introducing tiny particles into a crystal. This process is responsible for the hardening of steel, for instance, where particles of iron carbide are precipitated into iron. 

The crystallography of martensitic transformations has been widely analyzed [1-9]. We shall mainly follow Wayman s description [5] which is largely based on the original work of Wechsler et al. [1]. The famous f.c.c.—>b.c.t. (body-centered tetragonal) transformation in iron alloys is the basis for the hardening of steel and will be the focus of the discussion. 

HARDENA8ILITY OF STEEL. The hardenability of steel refers to the ease with which it can be hardened rather than the maximum hardness value attainable. For example, a I - inch diameter bar of a certain 0.20l/fi carbon alloy steel can be hardened to 50 Rockwell "C" in the center by quenching in oil. A similar bar of plain carbon steel requires a drastic quench in brine to attain the same hardness, and therefore, has a lower hardenability. Neither bar can be quenched to a greater hardness because 50 Rockwell C" is ihe maximum attainable for a 0.20 carbon steel. A... 

Sodium cyanide, an extremely toxic compound, is also used in the case hardening of steel objects (see Chapter 10). 

Potassium ferrocyanide finds application in commerce in the manufacture of Prussian blue, and also for case-hardening of steel.6... 

Various iron nitrides and carbonitrides are formed during case hardening of steels and alloyed steels. " These iron nitrides are hard, highly wear resistant and usually do not lead to dimensional changes in the case-hardened items. 

The first waste cyanide solutions of any significance were produced in the gold and silver mining industries, where cyanides were used to dissolve the metals from their ores. Since that time the use of cyanides has expanded significantly into the field of electroplating and case hardening of steel parts. 

The process of case-hardening of steel provides an interesting example of the formation of interstitial compounds. In one method both C and N are introduced into the surface of steel either by immersion in a molten mixture of NaCN, Na2C03, and NaCl at about 870°C or by heating in an atmosphere of H2, CO, and N2 to which controlled amounts of NH3 and CH4 are added. By these means both C and N are introduced. Although Fe does not react with molecular N2 certain steels can be case-hardened by the action of ammonia at temperatures around 500°C. The Fe—N phase diagram is complex, and phases formed include ... 

Case hardening of steel occurs when sodium cyanide oxidizes in the presence of a trace of iron or nickel oxide. The cyanide oxidizes first to cyanate and then to carbonate in the following manner ... 

See Humphrey Chem. News, 1914, 110, 271) for the part played by the amorphous phase in the hardening of steels. 

Krafft, J. M., Crack Toughness and Strain Hardening of Steels, Applied Materials Research, 3 (1964), 88-101. 

USE Extracting gold and silver from ores electroplating baths fumigating citrus and other fruit trees, ships, railway cars, warehouses, etc. manuf hydrocyanic acid and many other cyanides case hardening of steel. Caution See Hy. drogen Cyanide. 

Bailey N Proc Symp Hardenability of steels , Derby, TWI, May 1990. 

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