(57) A method for hardening steel in which the steel is from an austenitic state cooled
through, or held at, that temperature range in which the transformation of the steel
to ferrite, pearlite or bainite occurs the most rapidly, and in which after the transformation
has taken place to a sufficient degree, rapid heating to hardening temperature is
performed with a brief holding time at that temperature before the steel is hardened
in the conventional manner in a suitable hardening medium.
[0001] The invention is for a method of hardening steel. Hardening is generally employed
to enhance the strength of the steel, by which is understood also tensile strength
and yield strength, and its hardness.
[0002] A general problem in connection with employment of hardened steel components is that
the steel's toughness generally decreases when the strength and hardness are increased
by various hardening methods.
[0003] The problem is particularly prominent when hardened steel components are employed
directly in a hardered condition without the hardening being followed by a tempering
process. Tempering will generally enhance the toughness of the steel at the expense
of its strength and hardness.
[0004] For many applications of hardened and hardened/ tempered steel respectively it is,
however, necessary that the steel has maximum strength and hardness combined with
maximum toughness. Examples of such applications are the wear components of agricultural
implements and digging and loading machines.
[0005] It is known in the art to increase steel toughness by making the granular structure
as fine-grained as possible. This principle is enployed in various methods of producing
and treating steel, for example in controlled rolling. The same beneficial effect
of reduced grain size also applies to hardened and hardened/tempered grain structures
in steel.
[0006] The grain size of the steel after hardening is chiefly determined by the grain size
in a heated condition at the hardening temperature before hardening. In this condition
the grain structure of the steel is known to practitioners of the art as austenite.
[0007] Hardening creates a new structure, transformed front austenite, called martensite.
The martensite structure is a series of fine needles or plates formed within the individual
austenite grain, in such a manner that the martensite grain size is determined by
the austenite grain size before hardening.
[0008] The austenite grain size varies greatly according to the steel type and the holding
temperature/ holding times at which the steel is at high temperature and has an austenitic
structure. A particularly coarse austenitic structure is obtained if the steel is
heated above the temperature that produces granulation. This generally happens in
various forms of heat treatment, for example forging, and it often directly limits
the ability to harden steel direct from the forging temperature, in that the steel
becomes too brittle after hardening, and there is also a greater risk of the steel
cracking during the hardening itself. In order to avoid this the steel has in the
prior art been allowed to cool to room temperature followed by a new heating and hardening,
that is, heating to hardening temperature and sudden cooling. This, however, involves
heavy energy expenditure in that the steel must be reheated. The grain structure of
the steel, moreover, does not become particularly fine as a result of the afore-mentioned
procedure, as the austenite grain structure is formed by the transformation of a relatively
coarse structure formed by the slow cooling to room temperature.
[0009] The purpose of the invention is to supply a method for hardening of steel in which
the grain size of the steel is reduced and which, when the method is employed together
with the heat treatment of steel, will enable energy savings to be made.
[0010] The purpose is achieved according to the invention by causing a transformation of
the steel in a temperature range between the hardening temperature and the critical
temperature for formation of martensite.
[0011] The transformation is achieved by cooling the steel from an austenitic state through
or holding it at that temperature at which its transformation to ferrite, pearlite
or bainite occurs most rapidly. After the transformation has taken place to a sufficient
degree, a rapid heating to hardening temperature is performed, with a brief holding
at that temperature before the steel is hardened in the conventional manner in a suitable
hardening medium. Heating and hardening times are made short to avoid new unwanted
grain growth.
A method for hardening steel, characterised in that steel is from an austenitic state
cooled through, or held at, that temperature range in which the transformation of
the steel to ferrite, pearlite or bainite occurs the most rapidly, and that after
the transformation has taken place to a sufficient degree, rapid heating to hardening
temperature is performed with a brief holding time at that temperature before the
steel is hardened in the conventional manner in a suitable hardening medium.
