Surface hardening is a generic term covering a number of
processes used to improve the wear resistance of ferrous parts without
affecting the more soft, tough interior of the part. The combination of a hard
surface and softer interior, made possible, for example, by case hardening
methods, is of inestimable value in modern engineering practice.
By the use of
high-quality alloy steels, great strength and toughness in the core can be combined with extreme surface hardness,
resulting in a composite structure capable of withstanding
certain kinds of stress to a high degree. For less exacting requirements, there
are many applications where low or moderate core properties, together with a
high degree of surface hardness, can be obtained with cheaply fabricated, low-priced
carbon steel.
Three of the major types of treatments that
are conventionally used to enhance surface
wear resistance are classified as thermochemical, thermal, and coating or
plating.
Examples of thermochemical treatments include carburizing, nitriding,
and carbonitriding (case hardening
methods). Examples of thermal or applied energy treatments include flame and
induction hardening. Coating or plating processes include hard chromium
plating, electroless nickel plating, and various hard facing methods. These long-established
surface hardening technologies are continually being improved and remain among
the most widely used.
However, today completely different surface hardening technologies are being applied to and
developed for steels. The objective remains the same, that is, enhanced surface
performance, but technologies that incorporate high-energy beams, plasmas,
magnetic and electrical fields, and vacuums are being applied. Some of these
technologies have been used for some time in the electronics industry to
fabricate thin film devices and circuits and therefore cannot be considered new
technologies. Their application to steel,
however, is relatively new, and they elevate the level of sophistication and
control of surface hardening.
Surface Hardening of
Steels: Understanding the Basics is a practical selection guide to help engineers and technicians choose the optimum surface
hardening treatment for a given application from an ever-increasing number of options. Emphasis is placed on characteristics
such as processing temperature, case/coating thickness, bond strength, and
hardness level obtained.
The advantages and limitations of the various surface modification technologies are compared. Recent
developments in the understanding of the relationships between microstructure
and fatigue and wear performance are reviewed, as are more recently introduced
surface hardening processes such as vacuum/plasma-related technologies, laser
processing, chemical vapor deposition/physical vapor deposition, and ion implantation.
Methods for evaluating hardness
patterns and depths of hardness for quality control and failure analysis are
described. Metallurgical comparisons are
made between those processes that offer rapid heating and rapid cooling
(self-quenching) characteristics—for example, induction hardening—and
conventional furnace hardening.
Metallurgical characteristics and properties obtained by the atmosphere and vacuum carburizing are also
compared. Wear and corrosion data are also provided to demonstrate the benefits
of each process.
The successful completion of this book would
not have been possible without the generous assistance of the ASM staff. In particular, I would like to thank Scott Henry,
Assistant Director, Technical Publications; Don Baxter, Managing Editor, Advanced Materials & Processes and
Heat Treating Progress; and Eleanor Baldwin from the ASM Library. I called on them often for help, perhaps too often,
but they always came through. Thanks again my fellow colleagues and friends.
Joseph R. Davis
Davis & Associates
Chagrin Falls, Ohio
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