Heat treated steels (30NiCrMo12 - 40NiCrMo7)
Construction steels intended for heat treatment (quenching followed by tempering at 600° C) and have the best compromise between mechanical strength and toughness and generally they are used for the construction of mechanical parts subjected to static and dynamic loads.
The carbon types can be supplied in natural rolled state as well as normalised, while alloy types are normally supplied in the annealed state.
The great variety of uses, of size and shape of the mechanical parts which the heat treated steels are intended for, requires a careful choice of steel according to the stresses to which the item will be subjected to.
It is of fundamental importance then to know the hardenability characteristics of steels to correctly optimise the mechanical characteristics required.
The carbon only types, of extremely low hardenability, are still very much in demand and appreciated when used for pieces of small sections and when deformations resulting from water quenching can be tolerated. When certain dimensions are exceeded, the heat treatment does not lead, in these steels, to substantial variations in characteristics such as that of the normalised state.
Passing from the low carbon type to those with higher carbon, there is a considerable increase in resistance, but accompanied by a sharp fall in resilience, and consequently the higher carbon value types are not suitable for parts subject to shock, instead, they are suitable where higher hardness and wear resistance are both required.
Having examined the general characteristics of these types of steel, let's go into detail by examining the types used for our processing in the workshop: The 40NiCrMo7, corresponding to the well- known SAE 4340, shows a high hardenability that makes it suitable for the manufacture of highly stressed high strength pieces, even of large dimensions.
The 30NiCrMo12 with its high hardenability and fatigue resistance is more often recommended for the construction of pieces of larger dimensions which must withstand very difficult working conditions.
The types Ni Cr Mo are almost insensitive to temper embrittlement displaying excellent toughness even in the transverse direction and perform very well at low temperatures.