Heat Treatment of Steel

Published on March 2017 | Categories: Documents | Downloads: 38 | Comments: 0 | Views: 239
of 3
Download PDF   Embed   Report

Comments

Content


Heat Treatment of Steel
Various types of heat treatment processes are used to change the following
properties or conditions of the steel:
Improve the toughness
Increase the hardness
Increase the ductility
Improve the machinability
Refine the grain structure
Remove the residual stresses
Improve the wear resistance
The following are the general reasons for heat treatment:
Hardening (Steels can be heat treated to high hardness and strength
levels. The reasons for doing this are obvious. Structural components
subected to high operating stress need the high strength of a
hardened structure. Similarly! tools such as dies! "nives! cutting
devices! and forming devices need a hardened structure to resist wear
and deformation.#
Tempering ($s%&uenched hardened steels are so brittle that even
slight impacts may cause fracture. Tempering is a heat treatment that
reduces the brittleness of a steel without significantly lowering its
hardness and strength. $ll hardened steels must be tempered before
use.#
Softening a Hardened Structure (Hardening is reversible. If a
hardened tool needs to be remachined! it may be softened by heat
treatment to return it to its machinable condition. 'ost steels weld
better in their soft state than in their hardened state( softening may be
used to aid weldability.#
Recrystalli)ation (If a metal is cold wor"ed! grains or crystals
deform! become elongated! and in doing so harden and strengthen a
metal. There is a limiting amount of cold wor" that a particular metal
can be subected to. In rolling of steel into thin sheets! you can only
reduce the cross%sectional area so much before it gets too hard to roll.
$t this point it would be desirable to return the grains to their original
shape. Heat treatment can accomplish this. The transformation of
cold%wor"ed grains to an undistorted shape is called recrystallization.
Very large coarse grains can also be refined by recrystalli)ation.This
type of heat treatment is essential if a steel is to be subected to severe
cold wor"ing in rolling! drawing! etc.#
Stress Relief (*ne of the most fre&uent reasons for heat treatment is
to remove internal stress from a metal that has been subected to cold
wor"ing or welding. Stress relieving is a heat treatment used to
remove internal strains without significantly lowering the strength. It
is used where close dimensional control is needed on weldments!
forgings! castings! etc.#
Hot%+or"ing *perations ('ost metal shapes produced by steel
mills are at least rough shaped at elevated temperatures. Heat treating
is re&uired to bring the rough metal shapes to the proper temperature
for hot%forming operations.,orging! hot rolling! roll welding! and the
li"e are all performed at temperatures of sufficient magnitude as to
prevent the formation of distorted grains that will harden the metals.
Hot%wor"ing operations re&uire dynamic recrystalli)ation which is
achieved by wor"ing at the proper hot%wor" temperatures.#
-iffusion of $lloying .lements (*ne of the criteria for hardening a
steel is that it have sufficient carbon content. /ow carbon steels can be
hardened! at least on the surface! by heat treating at an elevated
temperature in an atmosphere containing an alloying element that will
diffuse into the steel and allow surface hardening on &uenching.
0arbon is fre&uently diffused into the surface of soft steels for surface
hardening. 1sing this same principle! elements such as chromium!
boron! nitrogen! and silicon can be diffused in the surface of a steel for
special purposes.#
,igure 2 shows maor types of heat treatment processes.

Sponsor Documents

Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close