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This review deals with the products of carbon case-hardening—that is, the microstructural features—and their influence on the more important material properties. No intentional reference is made to the execution of the carburizing process itself. The review was compiled primarily to assess the current situation in terms of effects and perhaps show where the areas of ignorance or conflict lay. More importantly, it is to convey to the reader that because of the many variables involved in carburizing, hardening, and allied processes, the subject as a whole is complicated.

In its role as a vehicle for information, this review, as it stands, should be of value to students of engineering and of ferrous metallurgy since it covers aspects of a very important industrial process.

The works control metallurgist may more easily put into perspective the significance of some of the microstructural features observed in control test pieces, or may even look more carefully for, or at, structures hitherto ignored. Thus control procedures can be modified where necessary, and quality, perhaps, can be improved. Further, this review should be useful to those involved in service failure analysis.

Heat treatment supervisors who, from component drawings or through company standards, work to readily measured properties such as surface hardness and case-depth requirements, might question some of the official or unofficial methods of attaining these. The result is that methods will be improved where they need to be improved.

For those engaged in carburizing research, this work should provide a useful background or platform for new work.

For the stress engineer or design engineer, an attempt has been made to provide quantitative values for some of the more important properties pertinent to carburized components; these are values that hitherto may have been assumed. Further, in view of the many complex and interrelated aspects involved and covered by this review, the engineer might exhibit more tolerance toward the metallurgist for his occasional inability to give precise answers to apparently simple questions.

The collecting of material for this book commenced in 1974. The timing for a comprehensive work seemed then to be right, not only for the practitioners but also for those engaged in industrial process computing. Now, as the 1980s begin, the stage has been reached when one might soon expect that from a steel’s chemical composition and a component drawing the microsturctures, hardness distributions, residual-stress gradients, and the distortion and growth behavior in the finished part will be predicted with reasonable accuracy. The main problem at the moment is in determining accurate material-properties data to feed into the programs. Such programs will link up with those for carburizing control on one hand and those for life predicitons on the other. The hope is that this review will be of value to those involved in these computer studies.

To compile a review for an intended readership of such widely varying interests has not been easy. There is, perhaps, a bias toward gears, which is not surprising in view of the fact that the carburizing process is extensively used for gears and the author is engaged in the gear industry. The bias is not really important since case hardening is case hardening, whatever the product.

This series of articles was first published in the Wolfson Heat Treatment Centre’s Journal, Heat Treatment of Metals, during 1976–1977. Since then, the output of data regarding carburizing has continued to flow. Some of this information is relevant to the series and appears in the form of additional notes made by the author and appended at the conclusion of the chapters to which the information relates.

Geoffrey Parrish
Yorkshire, England
January 1980

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