‫ السنة االولى‬/ ‫قسم الهندسة الميكانيكية‬
‫ المحاضرة الثامنة‬/  ‫مادة هندسة المعادن‬
‫م اسراء فيصل غازي‬. ‫م‬
Heat-Treatment of Steels:
C. Hardening (quenching):
Steel is hardening by the following procedure ;heating steel blow eutectoid to
temperature above A3 by amount (30-50)C whilst above eutectoid the heating
temperature equal to(A1+ 50) and then quenched in some medium . The medium
used will depended upon the composition of steel and the ultimate properties
required.
The result steel called martensite is formed when austenitized iron–carbon alloys are
rapidly cooled (or quenched) to a relatively low temperature. Martensite is occurs in
such a way that the FCC austenite experiences a polymorphic transformation to a
body-centered tetragonal (BCT) martensite .Martensite is a magnetic and its grains
take on a plate-like or needle-like appearance.
The successful heat treating of steels to produce a predominantly martensitic
microstructure throughout the cross section depends mainly on three factors:
(1) the composition of the alloy, (2) the type and character of the quenching medium,
and (3) the size and shape of the specimen.
The martensitic transformation occurs when the quenching rate is rapid enough to
prevent carbon diffusion. Any diffusion whatsoever will result in the formation of
ferrite and cementite phases.
Martensite is the hardest and strongest and, in addition, the most brittle; it has, in fact,
negligible ductility. Its hardness is dependent on the carbon content, up to about 0.6
wt%
Influence of Quenching Medium, Specimen Size, and Geometry
The cooling rate of a specimen depends on the rate of heat energy extraction, which
is a function of the characteristics of the quenching medium in contact with the
specimen surface, as well as the specimen size and geometry.
Severity of quench” is a term often used to indicate the rate of cooling; the more
rapid the quench, the more severe the quench. Of the three most common quenching
media—water, oil, and air—water produces the most severe quench, followed by oil,
which is more effective than air
As far as specimen shape is concerned, since the heat energy is dissipated to the
quenching medium at the specimen surface, the rate of cooling for a particular
quenching treatment depends on the ratio of surface area to the mass of the specimen.
During the quenching of a steel specimen, heat energy must be transported to the
surface before it can be dissipated into the quenching medium. As a consequence, the
cooling rate within and throughout the interior of a steel structure varies with position
and depends on the geometry and size
The larger this ratio, the more rapid will be the cooling rate and, consequently, the
deeper the hardening effect. Irregular shapes with edges and corners have larger
surface-to-mass ratios than regular and rounded shapes (e.g., spheres and cylinders)
and are thus more amenable to hardening by quenching.
Hardenability:
The influence of alloy composition on the ability of a steel alloy to transform to
martensite for a particular quenching treatment is related to a parameter called
hardenability. For every different steel alloy there is a specific relationship between
the mechanical properties and the cooling rate. “Hardenability” is a term that is used
to describe the ability of an alloy to be hardened by the formation of martensite as a
result of a given heat treatment. Hardenability is not “hardness,” which is the
resistance to indentation; rather, hardenability is a qualitative measure of the rate at
which hardness drops off with distance into the interior of a specimen as a result of
diminished martensite content. A steel alloy that has a high hardenability is one that
hardens, or forms martensite, not only at the surface but to a large degree throughout
the entire interior.
D.Tempering:
In the as-quenched state, martensite, in addition to being very hard, is so brittle that
it cannot be used for most applications; also, any internal stresses that may have
been introduced during quenching have a weakening effect. The ductility and
toughness of martensite may be enhanced and these internal stresses relieved by a
heat treatment known as tempering.
Tempering is accomplished by reheating a quenched component (martensitic steel)
to a temperature below the eutectoid for a specified time period. Normally,
tempering is carried out at temperatures between 250 and 650 C; internal stresses,
however, may be relieved at temperatures as low as (200C) the cooling rate after
tempering don’t important it often done by use water .
Tempering steel is much tougher but some what softer than martensite making it
more useful in cases where strength and reliability are more important than extreme
hardness. Tempered martensite may be nearly as hard and strong as martensite, but
with substantially enhanced ductility and toughness.
The microstructure of tempered martensite consists of extremely small and
uniformly dispersed cementite particles embedded within a continuous ferrite
matrix. This is similar to the microstructure of spheroidite except that the cementite
particles are much, much smaller.
8.1