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January 2014
Article
Steel Plant Specialities
Cost reduction in heat treatment
by using protective coatings
By SP Shenoy, M Tech (Met Engg), CEO, Steel Plant Specialities, India
Introduction
Heat treatment is an important operation in the manufacturing
process of engineering components, machine parts and tools.
Oxidation and decarburisation of steel take place when steel
is heated in an electric furnace or oil fired furnace, in the
presence of air or products of combustion.
Oxidation leads to numerous problems like scale pit marks, loss
of dimensions, bad quality surface finish of metal, rejections,
quench cracking and increased expensive operations like shot
blasting, machining and acid pickling.
Protection against scaling and decarburisation is achieved
by heating in molten salts, fluidised bed furnaces, protective
gaseous media or vacuum. These measures demand heavy
capital investment, highly skilled personnel and special safety
precautions. Many companies cannot afford them, and yet
they are under mounting pressure to prevent oxidation and
decarburisation.
This article introduces a practical technique pioneered
by an experienced metallurgist from the Indian Institute
of Technology (IIT). The technique enables any kind of
steel to be heated without basic problems of oxidation and
decarburisation. The discussed technique, established in a
number of hot forging units, heat treatment shops and hot
rolling mills, can be adopted by both small and large scale
units.
Understanding oxidation and
decarburisation
When steel is heated in an open furnace in the presence of
air or products of combustion, two surface phenomena take
place: 1, oxidation and 2, decarburisation.
This article introduces a practical technique pioneered by
a metallurgist from the Indian Institute of Technology. It
enables the prevention of oxidation and decarburisation
during heat treatment using open air atmosphere.
It has simplified and accelerated many metallurgical
heat treatment operations, saving a fortune in capital
investment, reducing costs and improving quality.
Oxidation
Oxidation of steel is caused by oxygen, carbon dioxide and/or
water vapour. The general reactions are given below:
O
2
O
2
CO
2
CO
2
+
+
+
+
2 Fe
4 FeO
Fe
3 FeO
⇌
⇌
⇌
⇌
2 FeO
2 Fe
2
O
3
CO + FeO
Fe
3
O
4
+ CO
Oxidation of steel may range from a tight, adherent straw-
coloured film that forms at a temperature of about 180°C to
a loose, blue-black oxide scale that forms at temperatures
above about 450°C with resultant loss of metal.
Decarburisation
Decarburisation or depletion of surface carbon content takes
place when steel is heated to temperatures above 650°C. It
progresses as a function of time, temperature and furnace
atmosphere.
Typical reactions involved are :
O
2
+
C
⇌
CO
2
O
2
+
Fe
3
C
⇌
3 Fe + CO
2
CO
2
+
C
⇌
2 CO
CO
2
+
Fe
3
C
⇌
2 CO + 3 Fe
H
2
O +
Fe
3
C
⇌
CO + H
2
+ 3 Fe
The equilibrium relationship depends on the ratio of carbon
dioxide to carbon monoxide. It is neutral to a given carbon
content at a given temperature.
Harmful effects of oxidation
and decarburisation
Oxidation leads to loss of dimensions and material as extra
material allowance needs to be kept for scaling. Often,