Flame Hardening of Steels,Flame Hardening of Steels,Flame Hardening of Steels,Flame Hardening of Steels
Introduction
FLAME HARDENING is a heat-treating process in which a thin surface shell of a steel part is heated rapidly to a
temperature above the critical point of the steel. After the grain structure of the shell has become austenitic (austenitized),
the part is quickly quenched, transforming the austenite to martensite while leaving the core of the part in its original
state. In contrast, slow cooling causes transformation, as the temperature passes through the corresponding ranges, to
pearlite, bainite, and martensite, with the final structure being a combination of the three. The result is a relatively soft
and ductile steel. To achieve hardness, therefore, the steel must be cooled rapidly so that it bypasses the first two
transformation phases and transforms directly from austenite to martensite.
Flame hardening employs direct impingement of a high-temperature flame or high-velocity combustion product gases.
The part is then cooled at a rate that will produce the desired levels of hardness and other properties. The hightemperature
flame is obtained by combustion of a mixture of fuel gas with oxygen or air; flame heads are used for burning
the mixture. Depths of hardening from about 0.8 to 6.4 mm ( 1
32
to 1
4
in.) or more can be obtained, depending on the fuels
used, the design of the flame head, the duration of heating, the hardenability of the work material, and the quenching
medium and method of quenching used. The process can be used for the through hardening of work 75 mm (3 in.) or less
in cross section, depending on the hardenability of the steel.

Hardening by flame differs from true case hardening because the hardenability necessary to attain high levels of hardness
is already contained in the steel, and hardening is obtained by localized heating. Although flame hardening is mainly used
to develop high levels of hardness for wear resistance, the process also improves bending and torsional strength and
fatigue life. One of the major advantages of flame hardening is the ability to satisfy stringent engineering requirements
with carbon steels.
Scope and Application
Flame hardening is applied to a wide diversity of workpieces and ferrous materials for one or more reasons. This process
is used because:
· Parts are so large that conventional furnace heating and quenching are impracticable or uneconomical.
Typical examples include large gears, machineways, dies, and rolls
· Only a small segment, section, or area of a part requires heat treatment, or because heat treating all over
would be detrimental to the function of the part. Typical examples include the ends of valve stems and
pushrods and the wearing surfaces of cams and levers
· The dimensional accuracy of a part is impracticable or difficult to attain or control by furnace heating
and quenching. A typical example is a large gear of complex design for which flame hardening of the
teeth would not disturb the dimensions of the gear
· The use of flame hardening permits a part to be made from a less costly material, thereby effecting an
overall cost saving in comparison with other technically acceptable methods. The process gives
inexpensive steels the wear properties of alloyed steels, and parts can be hardened without scaling or
decarburization, thereby eliminating costly cleaning operations. For example, a large steel part might be
made at a lower cost if produced from a flame-hardened plain carbon steel rather than from a carburized
low-carbon alloy steel
For a detailed discussion of materials suitable for flame hardening and for a comparison of flame hardening with other
methods used to attain similar results, see the sections "Selection of Process" and "Selection of Material" near the end of
this article.

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