The solid-solution hardening of carbon has a major effect on the strength of martensite, but ductility can only be obtained at low carbon levels. Although alloying elements affect hardenability, they have a minor effect on hardness except to reduce it at high carbon levels by causing austenite to be retained.
The solid-solution hardening of carbon has a major effect on the
strength of martensite, but ductility can only be obtained at low carbon
levels. Although alloying elements affect hardenability, they have a minor
effect on hardness except to reduce it at high carbon levels by causing
austenite to be retained.
Alternative ways of improving the strength of alloy steels are:
(1) Grain refinement, which increases strength and
ductility. This can be developed by severely curtailing the time
after the cessation of forging at some low temperature of austenite
stability or by rapid heating, coupled with a short austenitising period.
Fine grain is produced in 9% Ni steel by tempering fine lath
(2) Precipitation hardening by carbide, nitride or
(a) By secondary hardening, e.g. 12% Cr steel
(b) Age hardening a low carbon Fe-Ni lath martensite
supersaturated with substitutional elements, e.g. maraging.
hardening of austenite, e.g. stainless steels. Phosphorus and titanium are
common additions. Stacking faults are often associated with fine carbide
precipitates, and strength can be raised by increasing the number of
stacking faults (i.e. lower fault energy).
transformation 18/8 austenite steels in which transformation to martensite
is induced by refrigeration or by strain.
(3) Thermomechanical treatments which may be classified into
three main groups:
(a) Deformation of austenite prior to the
Ausforming consists of steel deforming
in a metastable austenitic condition between Ac1 and Ms (e.g. 500°C called
LT) followed by transformation to martensite and light tempering (Fig. 1).
This results in increased dislocation density in the martensite and a
finer carbon precipitation on tempering. Strengths up to 1800 MPa can be
obtained without impairing the ductility (~6 % deformation). Steels must
possess a TTT-curve with a large bay of stable austenite, e.g. 826 M40.
Typical application is for leaf springs.
Figure 1. Methods of thermomechanical treatment
Deformation of stable austenite just above Ac3
before cooling (called HT). The properties are somewhat inferior to those
produced by ausforming.
Deformation induced transformation
originally used in Hadfield 13% Mn steel, but can be adapted to metastable
austenitic stainless steels. The fully austenitic steel is severely
warm-worked above the lowest temperature at which martensite is produced
during the straining. The distinctive property is the high rate of
straining hardening, which increases ductility.
(b) Deformation of austenite during the
Isoforming is the deformation of a steel
(e.g. 1% Cr) during the isothermal transformation to pearlite, which
refines the structure and improves fracture toughness (Fig. 1). A somewhat
similar thermomechanical process can be used in the bainitic region,
producing bainite and martensite.
Zerolling consists in
forming martensite by deformation at subzero temperatures to strengthen
18/8 austenitic steels. The amount of martensite is influenced by alloy
composition and increased with deformation and lowering of the
(c) Deformation after the transformation of austenite
Marforming consists of deforming the maraging steel in the soft
martensitic condition, generally cold. There is a pronounced increase in
strength of the subsequent maraged product. With other steels,
considerable increases in strength can be obtained by straining martensite
(~3 %) either in the untempered or tempered condition. A strengthening
effect also occurs on re-tempering, probably due to the resolution and
reprecipitation of the carbides in a more finely dispersed form.
Strain tempering and dynamic strain ageing
involve about 5% deformation at the room temperature between two stages of
tempering -- strain tempering -- while in dynamic strain ageing
deformation is concurrent with tempering.