Case-hardening

There are many characteristics of hard cases that make their development desirable. One is wear resistance. Usually, the process is designed to develop high compressive residual stresses in the surface which counteract tensile stresses induced by the loading condition during use of the component (1) (Fig. lb). 

Machine components ate commonly subjected to loads, and hence stresses, which vary over time. The response of materials to such loading is usually examined by a fatigue test. The cylinder, loaded elastically to a level below that for plastic deformation, is rotated. Thus the axial stress at all locations on the surface alternates between a maximum tensile value and a maximum compressive value. The cylinder is rotated until fracture occurs, or until a large number of cycles is attained, eg, lO. The test is then repeated at a different maximum stress level. The results ate presented as a plot of maximum stress, C, versus number of cycles to fracture. For many steels, there is a maximum stress level below which fracture does not occur called the 

Steel Surface hardness, HRC Method of hardening Number of shafts tested Cycles to failure x 10  

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Decomposition of Austenite. In heat-treating steels, the initial step is usually to heat the steel into the austenite region ( 723° C) and then control the cooling process to produce the desired stmeture. The phase diagram (Fig. 2) shows that austenite decomposes into the two phases d and Fe C 

133 SOURCE REDUCTION FOR OTHER TYPES OF METAL SURFACE TREATMENT 7JJ.1 Case Hardening 

The salt baths in some nitriding and cyaniding processes also constitute hazardous wastes when spent. Nitriding and cyaniding baths for example, contain sodium and potassium cyanide and cyanate. Implementation of gas nitriding eliminates this waste stream. Gas nitriding employs ammonia gas to supply the nitrogen, and produces no hazardous waste stream. 

Recycling and resource recovery includes technologies that either directly use waste from one process as raw material for another process or recover valuable materials from a waste stream before the waste is disposed of. Some of the spent chemical process baths and much of the rinsewater can be reused for other plant processes. Also, process chemicals can be recovered from rinsewaters and sold or returned to the process baths. This section describes some of the recycling and resource recovery technologies available to the metal finishing industry. 

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In case hardening C and N are introduced into the iron by heating in a Nj —NH3 atmosphere or by immersing the steel in molten NaCN/Na2C03 at about 900"C. 

Case-based reasoning CA. See Cyanuric acid Case-hardened steels Case hardening... 

Fig. 1. (a) Schematic diagram showing definition of case hardening and of case depth where (—) is the diameter (b) residual stress across the radius of a... 

Table 1. Improvement in Material Life as a Result of Case Hardening ...

Because the time at high temperature is much less, austenite is produced, which is chemically inhomogeneous especially with undissolved carbides, and has a fine grain crystal size. The formation of the hard martensite requites more rapid cooling than for conventional hardening. Thus case hardening by heat treatment intrinsically requites that the surface region to be hardened be relatively thin and cooled rapidly. 

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