Hardness Testing on Case- and Induction-hardened Samples as Part of the Heat Treatment Process Technologies

Heat treatment is an essential method to influence work piece characteristics. Many industries
focus on reliable part design, e.g. by reducing material weight and work piece size while still
maintaining high wear resistance on the outside of the material and tenacity in its core to
avoid breaking parts. Many heat treatment shops are specialized in “case hardening” or
“surface hardening” processes. With case hardening work pieces are carburized, hardened and
finally tempered – a hard and wear-resistant surface is produced whilst the core remains
relatively soft with a smooth passage between the two areas. Case hardening is typically used
e.g. for gears of gearboxes. For hardness testing of case-hardened parts, the “CHD” value is
evaluated by setting several hardness test points throughout the cross section of the hardened
part. The CHD value describes the hardening depth in millimeters from the surface where the
hardness changes from hard to soft. Standards like ISO 2639 define the requirements of the
CHD test (test point distances, Vickers test method, etc.). The CHD limit hardness value is
defined as fixed hardness usually at 550 HV.
In contrast to case hardening, the chemical composition of the surface layer is not changed
with surface hardening. The objective is to achieve a fully martensitic structure at the surface
region, usually after induction or laser hardening, whilst the core of the material remains
unaffected by any hardening influence. Surface hardening is frequently used for shafts like
crankshafts or camshafts. SHD hardness testing looks at the limit hardness as a flexible value:
the limit hardness is defined as percentage value from the surface hardness of the part.