ABSTRACT
Many metal components require a combination of mechanical properties which at first sight seems impossible to attain. Thus, bearing metals must be both hard and, at the same time, ductile, whilst many steel components, like cams and gears, need to be strong and shock-resistant, yet also hard and wear-resistant. Alloy steels used for case-hardening contain up to 4.0" nickel, since this increases the strength of the core and retards grain-growth during the carburising process. Chromium is sometimes added to increase hardness and wear-resistance of the case, but it must be present only in small quantities, as it tends to promote grain growth. Nitriding and case-hardening have one factor in common - both processes involve heating the steel for a considerable time in the hardening-medium, but, whilst in case-hardening the medium contains carbon, in nitriding it contains gaseous nitrogen. Carbonitriding is a surface-hardening process which makes use of a mixture of hydrocarbons and ammonia.
