Heat treatment of carbon steel

Steel is an important engineering material because,

although cheap, it can be given a wide range of

mechanical properties by heat treatment. Heat

treatment can both change the size and shape of

the grains, and alter the microconstituents. The

shape of the grains can be altered by heating the

steel to a temperature above that of recrystallization.

The size of the grains can be controlled by the

temperature and the duration of the heating, and

the speed at which the steel is cooled after the

heating; the microconstituents can be altered by

heating the steel to a temperature that is sufficiently

high to produce the solid solution austenite

so that the carbon is dispersed, and then cooling it

at a rate which will produce the desired structure.

The micrograin structure of carbon steel has the

following constituents:

Ferrite Pure iron.

Cementite Carbon and iron mixed.

Pearlite A sandwich layer structure of ferrite and

cementite.

All low-carbon steels of less than 0.83 per cent

carbon content consist of a combination of ferrite

and pearlite. Carbon steel containing 0.83 per cent

carbon is called eutectoid and consists of pure

pearlite structure. Steels over 0.83 per cent carbon

up to 1.2 per cent carbon are a mixture of cementite

and pearlite structures. If a piece of carbon

steel is heated steadily its temperature will rise at a

uniform rate until it reaches 700 °C. At this point,

even though the heating is continued, the temperature

of the steel will first remain constant for a

short period and then continue to rise at a slower

rate until it reaches 775 °C. The pause in the temperature

rise and the slowing down of the rate indicate

that energy is being absorbed to bring about a

Metal forming processes and machines 165

chemical and structural change in the steel. The

carbon in the steel is changing into a solid solution

with the iron and forming what is known as

austenite. The temperature at which this change in

the structure of the steel starts is 700 °C, which is

known as the lower critical point; the temperature

at which the change ends is known as the upper

critical point. The difference between these points

is termed the critical range. The lower critical

point is the same for all steels, but the upper critical

point varies with the carbon content as shown

in Figure 5.1. Briefly, steels undergo a chemical

and structural change, forming austenite, when

heated to a temperature above the upper critical

point; if allowed to cool naturally they return to

their normal composition.

Steel can be heat treated by normalizing, hardening,

tempering and case hardening as well as by

annealing, which has already been described in

Section 5.2.1.

Normalizing

Normalizing is a process used to refine the grain

structure of steel after it has been subjected to prolonged

heating above the critical range (as in the

case of forging) and to remove internal stresses

caused by cold working. The process may appear

to differ little from annealing, but as its name suggests

the effect of normalizing is to bring the steel

back to its normal condition and no attempt is

made to soften the steel for further working.

Normalizing is effected by slowly heating the steel

to just above its upper critical range for just sufficient

time to ensure that it is uniformly heated, and

then allowing it to cool in still air.

Hardening

It has already been said that if a piece of steel is

allowed to cool naturally after heating to above its

upper critical point, it will change from austenite

back to its original composition. If, however, the

temperature of the heated steel is suddenly lowered

by quenching it in clean cold water or oil, this

change back from austenite does not take place,

and instead of pearlite, a new, extremely hard and

brittle constituent is formed, called martensite.

This process makes steels containing 0.3 per cent

or more carbon extremely hard, but steels having a

carbon content of less than 0.3 per cent cannot be

hardened in this way because the small amount of

carbon produces too little martensite to have any

noticeable hardening effect. The steel to be hardened

should be quenched immediately it is uniformly

heated to a temperature just above the

upper critical point. It is also important not to overheat

the steel and to allow it to cool to the quenching

temperature. Whether water or oil is used for

quenching depends upon the use to which the steel

is to be put. Water quenching produces an

extremely hard steel but is liable to cause cracks

and distortion. Oil quenching is less liable to cause

these defects but produces a slightly softer steel.

A more rapid and more even rate of cooling can be

obtained if the steel is moved about in the cooling

liquid, but only that part of the steel which is to

be hardened should be moved up and down in the

liquid in order to avoid a sharp boundary between

the soft and hard portions.

A workshop method of hardening carbon tool

steel is to heat the steel, using the forge or oxyacetylene

blowtorch, to a dull red colour (see Table 5.3)

and then quench it in water or oil. This would

harden the article ready for tempering.

Tempering

Hardened steel is too brittle for most purposes, and

the process of tempering is carried out to allow the

steel to regain some of its normal toughness and

ductility. This is done by heating the steel to a temperature

below the lower critical point, usually

Figure 5.1Changes in structure of carbon steel

with temperature and carbon content

166Repair of Vehicle Bodies

between 200 and 300 °C, thereby changing some

of the martensite back to pearlite. The exact temperature

will depend on the purpose for which the

steel is intended; the higher the temperature, the

softer and less brittle the tempered steel becomes.

Methods used for controlling the tempering

process depend upon the size and class of the article

to be tempered. One method is to heat the hardened

steel in a bath of molten lead and tin, the

melting points of various combinations of these

two metals being used as an indication of the temperature.

Another method of tempering small articles

is to polish one face or edge and heat it with a

flame. This polished surface will be seen to change

colour as the heat is absorbed. The colour changes

are caused by the formation at different temperatures

of thin films of oxide, called tempering

colours (Table 5.4). After tempering, the steel is

either quenched or allowed to cool naturally.

Case hardening

Although mild steels having a carbon content of

less than 0.3 per cent cannot be hardened, the surface

of the mild steel can be changed to a highcarbon

steel. Case hardening of mild steel can be

divided into three main processes:

1 Carburizing

2 Refining and toughening the core

3 Hardening and tempering the outer case.

A method called pack carburizing is often used in

small workshops. After thoroughly cleaning them,

the steel parts to be carburized are packed in an

iron box so that each part is entirely surrounded by

2.5 cm of carburizing compound. The box is sealed

with fireclay, heated in a furnace to 950 °C and

kept at that temperature for two to twelve hours,

during which time the carbon in the compound is

absorbed by the surface of the steel parts. The steel

parts are then allowed to cool slowly in the box. At

this stage their cores will have a coarse grain structure

due to prolonged heating, and the grain is

refined by heating the steel parts to 900 °C and

quenching them in oil. Next the casing is hardened

by reheating the steel to just under 800 °C and then

quenching it in water or oil. Tempering of the casing

may then be carried out in the normal way.

Small mild steel parts can be given a very thin

casing by heating them in a forge to a bright red

heat and coating them with carburizing powder.

The parts are then returned to the forge and kept

at a bright red heat for a short period to allow the

carbon in the compound to penetrate the surface of

the metal. Finally the parts are quenched and ready

for use.