Case Hardening Case hardening can be described as a process of creating a thin layer of a harder alloy on the surface of a material, usually by infusing elements onto the surface or sometimes just by changing the existing metallurgical properties of the surface of the material.
Low carbon, Non-alloy (mild steels) are popularly case hardened (Non alloy steels are preferred, as sometimes the presence of aluminum in the alloy could result in an inferior case hardened material with lower surface hardness and lower case depth). However recent advances in the process has seen the imposition of hard ceramic coatings as a means of case hardening, also methods to case harden gelatine grafted polymers5 by adding hardener to the saturation limit to form a hard shell have been devised.
Case hardening in steels is typically done by increasing the carbon concentration on the surface layers, that gives it the characteristic hardness. The thickness of the hardened layer is known as the ‘case depth’ and it could range from 0.75mm to 7.5mm depending upon the technique used and its duration. It is to be noted that the hardness is not the same through out the case depth, i.e. the layer gets softer as it nears the core of the material. This is why grinding a case hardened material is not a preferable option to get it within the desired dimension. There are various processes that could be adopted to case harden a material, some of these are discussed below. Flame or induction hardening is the process where the metallurgical properties of steel are altered by applying heat to its surface (by a direct flame or by induction heating) followed by rapid cooling (termed ‘quenching’) to form a hardened surface layer of martensite. This is further explained by means of a phase diagram4:
Iron-Carbon phase diagram.
From the diagram it is evident that ferrite at a low temperature cannot dissolve more the 0.035%wt C, however by Flame/Induction heating the austenitic phase is reached in which the solubility of carbon is significantly higher. Following this the metal is quenched not giving carbon enough time to diffuse to form Perlite which is +Fe3C. The carbon atoms are trapped in the iron lattice and don’t nucleate to form Cementite Fe3C, instead they form the hardest structure possible from the austenitic phase called Martensite.
Carburising1 is another a process commonly employed to case harden steels with carbon concentrations of 0.1% to 0.3%wt C. This is a process where atomic diffusion takes place from regions containing high carbon concentration to regions of low concentration (such as low carbon steel) at an elevated temperature which provides the system sufficient energy to enable mobility. At a microscopic level the carbon atoms are forced into the metal’s crystal lattice, where they could dissolve into the lattice or react to form ceramic carbides, both imparting hardness to the surface. The process is carried out by introducing steel to a carbon rich vicinity at an elevated temperature. The source of carbon could come from a solid medium in which case the process is called ‘Pack carburising’ wherein the steel is packed with a solid carbon rich substance and heated until carbon diffusion takes place. The duration of the process determines the case depth6, it typically lasts for a few hours. If the source of carbon is a gas (usually methane) then the process is termed as ‘gas carburising’, this takes place at temperatures of about 900 to 950c. It is the most popular and versatile process as the amount of carbon diffusion could be controlled by the atmospheric constituents, temperature of the chamber. Liquid baths containing carbon sources such as molten metal cyanides could also be used to carburise steels. Pack carburising could accommodate larger steel parts to be case hardened when compared to the other two. However gas and...