For applications requiring higher tensile strengths and toughness than the carbon steels there is a range of low alloy steels. These are categorised as high tensile or constructional steels and case hardening steels. The high tensile strength steels have sufficient alloying additions enabling through hardening (by quench and temper treatment) according to their alloying additions.

Case Hardening (carburising) Steels

Case hardening steels are a group of low carbon steels in which a high hardness surface zone (hence the term ‘case hardened’) is developed during heat treatment by absorption and diffusion of carbon. The high hardness zone is supported by the unaffected underlying core zone, which is lower hardness and higher toughness.

Plain carbon steels that can be used for case hardening are restricted. Where plain carbon steels are used, the rapid quenching necessary to develop satisfactory hardness within the case can cause distortion and the strength that can be developed in the core is very limited. Alloy case hardening steels allow the flexibility of slower quenching methods to minimise distortion and high core strengths can be developed.

Nitriding steels can have higher surface hardness developed by absorption of nitrogen, when exposed to a nitriding atmosphere at temperatures in the range of 510-530ºC, after hardening and tempering.

High tensile steels suitable for nitriding are: 4130, 4140, X4150, 4340, En25, 8660, En26.

Chemical Composition (% by weight - nominal values) 

Grade	C	Si	Mn	Cr	Mo	S	Ni
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