Four basic types of heat
treatment are used today. They are annealing, normalizing, hardening, and tempering. The
techniques used in each process and how they relate to steelworkers are given in the
In general, annealing is the
opposite of hardening, You anneal metals to relieve internal stresses, soften them, make
them more ductile, and refine their grain structures. Annealing consists of heating a
metal to a specific temperature, holding it at that temperature for a set length of time,
and then cooling the metal to room temperature. The cooling method depends on the metal
and the properties desired. Some metals are furnace-cooled, and others are cooled by
burying them in ashes, lime, or other insulating materials.
Welding produces areas that
have molten metal next to other areas that are at room temperature. As the weld cools,
internal stresses occur along with hard spots and brittleness. Welding can actually weaken
the metal. Annealing is just one of the methods for correcting these problems.
To produce the maximum softness in steel, you heat the metal to its proper
temperature, soak it, and then let it cool very slowly. The cooling is done by
burying the hot part in an insulating material or by shutting off the furnace
and allowing the furnace and the part to cool together. The soaking period depends
on both the mass of the part and the type of metal. The approximate soaking periods
for annealing steel are given in table 2-2.
Steel with an extremely
low-carbon content requires the highest annealing temperature. As the carbon content
increases, the annealing temperatures decrease.
Copper becomes hard and brittle when mechanically worked; however, it can be made soft
again by annealing. The annealing temperature for copper is be-tween 700°F and 900°F.
Copper maybe cooled rapidly or slowly since the cooling rate has no effect on the heat
treatment. The one drawback experienced in annealing copper is the phenomenon called
hot shortness. At about 900°F, copper loses its tensile strength, and if not
properly supported, it could fracture.
Aluminum reacts similar to
copper when heat treating. It also has the characteristic of hot shortness. A
number of aluminum alloys exist and each requires special heat treatment to produce their
Normalizing is a type of heat
treatment applicable to ferrous metals only. It differs from annealing in that the metal
is heated to a higher temperature and then removed from the furnace for air cooling.
The purpose of normalizing is
to remove the internal stresses induced by heat treating, welding, casting, forg-ing,
forming, or machining. Stress, if not controlled, leads to metal failure; therefore,
before hardening steel, you should normalize it first to ensure the maximum desired
results. Usually, low-carbon steels do not re-quire normalizing; however, if these steels
are normal-ized, no harmful effects result. Castings are usually annealed, rather than
normalized; however, some cast-ings require the normalizing treatment. Table
2-2 shows the approximate soaking periods for normalizing steel. Note that the soaking
time varies with the thickness of the metal.
Normalized steels are harder
and stronger than an-nealed steels. In the normalized condition, steel is much tougher
than in any other structural condition. Parts subjected to impact and those that require
maximum toughness with resistance to external stress are usually normalized. In
normalizing, the mass of metal has an influence on the cooling rate and on the resulting
structure. Thin pieces cool faster and are harder after normal-izing than thick ones. In
annealing (furnace cooling), the hardness of the two are about the same.
The hardening treatment for
most steels consists of heating the steel to a set temperature and then cooling it rapidly
by plunging it into oil, water, or brine. Most steels require rapid cooling (quenching)
for hardening but a few can be air-cooled with the same results. Hardening increases the
hardness and strength of the steel, but makes it less ductile. Generally, the harder the
steel, the more brittle it becomes. To remove some of the brittleness, you should temper
the steel after hardening.
Many nonferrous metals can be
hardened and their strength increased by controlled heating and rapid cooling. In this
case, the process is called heat treatment, rather than hardening.
To harden steel, you cool the
metal rapidly after thoroughly soaking it at a temperature slightly above its upper
critical point. The approximate soaking periods for hardening steel are listed in table
2-2. The addition of alloys to steel decreases the cooling rate required to produce
hardness. A decrease in the cooling rate is an advantage, since it lessens the danger of
cracking and warping.
Pure iron, wrought iron, and
extremely low-carbon steels have very little hardening properties and are dif-ficult to
harden by heat treatment. Cast iron has limited capabilities for hardening. When you cool
cast iron rapidly, it forms white iron, which is hard and brittle. And when you cool it
slowly, it forms gray iron, which is soft but brittle under impact.
In plain carbon steel, the
maximum hardness ob-tained by heat treatment depends almost entirely on the carbon content
of the steel. As the carbon content in-creases, the hardening ability of the steel
increases; however, this capability of hardening with an increase in carbon content
continues only to a certain point. In practice, 0.80 percent carbon is required for
maximum hardness. When you increase the carbon content beyond 0.80 percent, there is no
increase in hardness, but there is an increase in wear resistance. This increase in wear
resistance is due to the formation of a substance called hard cementite.
When you alloy steel to
increase its hardness, the alloys make the carbon more effective in increasing hardness
and strength. Because of this, the carbon content required to produce maximum hardness is
lower than it is for plain carbon steels. Usually, alloy steels are superior to carbon
Carbon steels are usually
quenched in brine or water, and alloy steels are generally quenched in oil. When hardening
carbon steel, remember that you must cool the steel to below 1000°F in less than 1
second. When you add alloys to steel, the time limit for the temperature to drop below
1000°F increases above the l-second limit, and a slower quenching medium can produce the
Quenching produces extremely
high internal stresses in steel, and to relieve them, you can temper the steel just before
it becomes cold. The part is removed from the quenching bath at a temperature of about
200°F and allowed to air-cool. The temperature range from 200°F down to room temperature
is called the cracking range and you do not want the steel to pass through it.
Case hardening produces a hard, wear-resistant surface or case over a strong, tough
core. The principal forms of casehardening are carburizing, cyaniding, and nitriding. Only
ferrous metals are case-hardened.
Case hardening is ideal for
parts that require a wear-resistant surface and must be tough enough internally to
withstand heavy loading. The steels best suited for case hardening are the low-carbon and
low-alloy series. When high-carbon steels are case-hardened, the hardness penetrates the
core and causes brittleness. In case hardening, you change the surface of the metal
chemically by introducing a high carbide or nitride content. The core remains chemically
unaffected. When heat-treated, the high-carbon surface responds to hard-ening, and the
is a case-harden-ing process by which carbon is added to the surface of low-carbon steel.
This results in a carburized steel that has a high-carbon surface and a low-carbon
When the carburized steel is
heat-treated, the case be-comes hardened and the core remains soft and tough.
Two methods are used for
carburizing steel. One method consists of heating the steel in a furnace con-taining a
carbon monoxide atmosphere. The other method has the steel placed in a container packed
with charcoal or some other carbon-rich material and then heated in a furnace. To cool the
parts, you can leave the container in the furnace to cool or remove it and let it air
cool. In both cases, the parts become annealed during the slow cooling. The depth of the
carbon penetration depends on the length of the soaking period. With to-days
methods, carburizing is almost exclusively done by gas atmospheres.
process is a type of case hardening that is fast and efficient. Preheated steel is dipped
into a heated cyanide bath and allowed to soak. Upon removal, it is quenched and then
rinsed to remove any residual cyanide. This process produces a thin, hard shell that is
harder than the one produced by carburizing and can be completed in 20 to 30 minutes vice
several hours. The major drawback is that cyanide salts are a deadly poison.
case-hardening method pro-duces the hardest surface of any of the hardening processes. It
differs from the other methods in that the individual parts have been heat-treated and
tempered before nitriding. The parts are then heated in a furnace that has an ammonia gas
atmosphere. No quenching is required so there is no worry about warping or other types of
distortion. This process is used to case harden items, such as gears, cylinder sleeves,
camshafts and other engine parts, that need to be wear resistant and operate in high-heat
Flame hardening is another procedure that is used to harden the surface of metal parts.
When you use an oxyacetylene flame, a thin layer at the surface of the part is rapidly
heated to its critical temperature and then immediately quenched by a combination of a
water spray and the cold base metal. This process produces a thin, hardened surface, and
at the same time, the internal parts retain their original properties. Whether the process
is manual or mechanical, a close watch must be maintained, since the torches heat the
metal rapidly and the temperatures are usually determined visually.
|Flame hardening may be either
manual or automatic. Automatic equipment produces uniform results and is more desirable.
Most automatic machines have vari-able travel speeds and can be adapted to parts of
various sizes and shapes. The size and shape of the torch de-pends on the part. The torch
consists of a mixing head, straight extension tube, 90-degree extension head, an
adjustable yoke, and a water-cooled tip. Practically any shape or size flame-hardening tip
is available (fig. 2-1). Tips are produced that can be used for hardening flats, rounds,
gears, cams, cylinders, and other regular or irregular shapes.
In hardening localized areas,
you should heat the metal with a standard hand-held welding torch. Adjust the torch flame
to neutral for normal heating; however, in corners and grooves, use a slightly oxidizing
flame to keep the torch from sputtering. You also should particularly guard against
overheating in comers and grooves. If dark streaks appear on the metal surface, this is a
sign of overheating, and you need to increase the distance between the flame and the
For the best heating results,
hold the torch with the tip of the inner cone about an eighth of an inch from the surface
and direct the flame at right angles to the metal. Sometimes it is necessary to change
this angle to obtain better results; however, you rarely find a deviation of more than 30
degrees. Regulate the speed of torch travel according to the type of metal, the mass and
shape of the part, and the depth of hardness desired.
In addition, you must select
the steel according to the properties desired. Select carbon steel when surface hardness
is the primary factor and alloy steel when the physical properties of the core are also
factors. Plain carbon steels should contain more than 0.35% carbon for good results
inflame hardening. For water quench-ing, the effective carbon range is from 0.40% to
0.70%. Parts with a carbon content of more than 0.70% are likely to surface crack unless
the heating and quenching rate are carefully controlled.
The surface hardness of a
flame-hardened section is equal to a section that was hardened by furnace heating and
quenching. The decrease in hardness between the case and the core is gradual. Since the
core is not affected by flame hardening, there is little danger of spalling or flaking
while the part is in use. Thus flame hardening produces a hard case that is highly
resistant to wear and a core that retains its original properties.
Flame hardening can be
divided into five general methods: stationary, circular band progressive, straight-line
progressive, spiral band progressive, and circular band spinning.
STATIONARY METHOD. In
this method the torch and the metal part are both held stationary.
CIRCULAR BAND PROGRESSIVE
METHOD. This method is used for hardening outside surfaces of round sections.
Usually, the object is rotated in front of a stationary torch at a surface speed of from 3
to 12 inches per minute. The heating and quenching are done progressively, as the part
rotates; therefore, when the part has completed one rotation, a hardened band encircles
the part. The width of the hardened band depends upon the width of the torch tip. To
harden the full length of a long section, you can move the torch and repeat the process
over and over until the part is completely hardened. Each pass or path of the torch should
overlap the previous one to prevent soft spots.
METHOD. With the straight-line progressive method, the torch travels along the
surface, treating a strip that is about the same width as the torch tip. To harden wider
areas, you move the torch and repeat the process. Figure 2-2 is an example of progressive
SPIRAL BAND PROGRESSIVE
METHOD. For this technique a cylindrical part is mounted between lathe centers,
and a torch with an adjustable holder is mounted on the lathe carriage. As the part
rotates, the torch moves parallel to the surface of the part. This travel is synchronized
with the parts rotary motion to produce a continuous band of hardness. Heating and
quenching occur at the same time. The number of torches required depends on the diameter
of the part, but seldom are more than two torches used.
CIRCULAR BAND SPINNING
METHOD. The circular band spinning method provides the best results for
hardening cylindrical parts of small or me-dium diameters. The part is mounted between
lathe centers and turned at a high rate of speed pasta station-ary torch. Enough torches
are placed side by side to heat the entire part. The part can be quenched by water flowing
from the torch tips or in a separate operation.
When you perform heating and
quenching as sepa-rate operations, the tips are water-cooled internally, but no water
sprays onto the surface of the part.
In flame hardening, you
should follow the same safety precautions that apply to welding. In particular, guard
against holding the flame too close to the surface and overheating the metal. In judging
the temperature of the metal, remember that the flame makes the metal appear colder than
it actually is.
After the hardening treatment
is applied, steel is often harder than needed and is too brittle for most practical uses.
Also, severe internal stresses are set up during the rapid cooling from the hardening
temperature. To relieve the internal stresses and reduce brittle-ness, you should temper
the steel after it is hardened. Tempering consists of heating the steel to a specific
temperature (below its hardening temperature), holding it at that temperature for the
required length of time, and then cooling it, usually instill air. The resultant strength,
hardness, and ductility depend on the temperature to which the steel is heated during the
The purpose of tempering is
to reduce the brittleness imparted by hardening and to produce definite physical
properties within the steel. Tempering always follows, never precedes, the hardening
operation. Besides reducing brittleness, tempering softens the steel. That is unavoidable,
and the amount of hardness that is lost depends on the temperature that the steel is
heated to during the tempering process. That is true of all steels except high-speed
steel. Tempering increases the hardness of high-speed steel.
Tempering is always conducted
at temperatures be-low the low-critical point of the steel. In this respect, tempering
differs from annealing, normalizing, and hardening in which the temperatures are above the
upper critical point. When hardened steel is reheated, temper-ing begins at 212°F and
continues as the temperature increases toward the low-critical point. By selecting a
definite tempering temperature, you can predetermine the resulting hardness and strength.
The minimum temperature time for tempering should be 1 hour. If the part is more than 1
inch thick, increase the time by 1 hour for each additional inch of thickness.
Normally, the rate of cooling
from the tempering temperature has no effect on the steel. Steel parts are usually cooled
in still air after being removed from the tempering furnace; however, there are a few
types of steel that must be quenched from the tempering tem-perature to prevent
brittleness. These blue brittle steels can become brittle if heated in certain temperature
ranges and allowed to cool slowly. Some of the nickel chromium steels are subject to this
Steel may be tempered after
being normalized, pro-viding there is any hardness to temper. Annealed steel is impossible
to temper. Tempering relieves quenching stresses and reduces hardness and brittleness.
Actually, the tensile strength of a hardened steel may increase as the steel is tempered
up to a temperature of about 450°F. Above this temperature it starts to decrease.
Tempering increases softness, ductility, malleability, and impact resistance. Again,
high-speed steel is an exception to the rule. High-speed steel increases in hardness on
temper-ing, provided it is tempered at a high temperature (about 1550°F). Remember, all
steel should be removed from the quenching bath and tempered before it is complete] y
cold. Failure to temper correctly results in a quick failure of the hardened part.
Permanent steel magnets are
made of special alloys and are heat-treated by hardening and tempering. Hard-ness and
stability are the most important properties in permanent magnets. Magnets are tempered at
the mini-mum tempering temperature of 212°F by placing them in boiling water for 2 to 4
hours. Because of this low-tempering temperature, magnets are very hard.
Case-hardened parts should
not be tempered at too high a temperature or they may loose some of their hardness.
Usually, a temperature range from 212°F to 400°F is high enough to relieve quenching
stresses. Some metals require no tempering. The design of the part helps determine the
Color tempering is based on
the oxide colors that appear on the surface of steel, as it is heated. When you slowly
heat a piece of polished hardened steel, you can see the surface turn various colors as
the temperature changes. These colors indicate structural changes are taking place within
the metal. Once the proper color appears, the part is rapidly quenched to prevent further
structural change. In color tempering, the surface of the steel must be smooth and free of
oil. The part may be heated by a torch, in a furnace, over a hot plate, or by radiation.
Cold chisels and similar
tools must have hard cut- cutting edge. When you have completed the above de-ting edges
and softer bodies and heads. The head must be tough enough to prevent shattering when
struck with shammer.The cutting edge must be more than twice as hard as the head, and the
zone separating the two must be carefully blended to prevent a lineof demarcation. A
method of color tempering frequently used for chisels and similar tools is one in which
the cutting end is heated by the residual heat of the opposite end of the same tool.
To harden and tempera cold
chisel by this method, you heat the tool to the proper hardening temperature and then
quench the cutting end only. Bob the chisel up and down in the bath, always keeping the
cutting edge below the surface. This method air-cools the head while rapidly quenching the
cutting edge. The result is a tough head, fully hardened cutting edge, and a properly
When the cutting end has
cooled, remove the chisel from the bath and quickly polish the cutting end with a buff
stick (emery). Watch the polished surface, as the heat from the opposite end feeds back
into the quenched end. As the temperature of the hardened end increases, oxide colors
appear. These oxide colors progress from pale yellow, to a straw color, and end in blue
colors. As soon as the correct shade of blue appears, quench the entire chisel to prevent
further softening of the cutting edge. The metal is tempered as soon as the proper oxide
color appears and quenching merely prevents further tempering by freezing the process.
This final quench has no effect on the body and the head of the chisel, because their
temperature will have dropped below the critical point by the time the proper oxide color
appears on the scribed process, the chisel will be hardened and tem-pered and only needs
|During the tempering, the oxide
color at which you quench the steel varies with the properties desired in the part. Table 2-3 lists the different colors and their
corre-sponding temperatures. To see the colors clearly, you must turn the part from side
to side and have good lighting. While hand tempering produces the same result as furnace
tempering, there is a greater possibility for error. The slower the operation is
performed, the more accurate are the results obtained.