TPT 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

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.

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.

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.

Typical reactions involved are :

⇌

O

+

C

CO

2

2

⇌

O

+

Fe

C

3 Fe + CO 2

2

3

⇌

CO

+

C

2 CO

2

⇌

CO

+

Fe

C

2 CO + 3 Fe

2

3

⇌

H

2 O +

Fe

C

CO + H 2

+ 3 Fe

3

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,

100

www.read-tpt.com

January 2014