Coating eliminates oxidation and decarburisation

S P Shenoy, chief executive officer of Steel Plant Specialities, has described a novel coating that enables any grade of steel to be heat treated without oxidation and decarburisation problems.

Heat treatment is an important operation in the manufacturing process of engineering components, machine parts and tools.

When steel is heated in an electric furnace or oil-fired furnace, in the presence of air or products of combustion, oxidation and decarburisation of steel take place.

Protection against them 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 small heat-treatment shops cannot afford them, and yet they are under mounting pressure to prevent oxidation and decarburisation.

Steel Plant Specialities, an Indian firm, has developed a coating that enables any grade of steel to be heat treated without the basic problems of oxidation and decarburisation.

The coating process can be adopted by both small- and large-scale units.

Some of the benefits claimed are : the ability to use existing furnace, box type or bogie hearth - electric, oil or gas; the elimination of the need of salt baths or expensive controlled atmospheres or even vacuum furnaces in many cases; considerable savings in capital investment and operating costs; improved productivity by as high as 66 per cent; the elimination or minimisation of post-heat treatment operations like grinding, shot blasting or acid pickling.

Heat treatment is a combination of heating and cooling operations in a prescribed manner (with respect to time, temperature, rate of heating and cooling) to induce desired properties in metals and alloys in the solid state.

It is an important operation in the manufacturing process of engineering components, machine parts, tools and castings.

When steel is heated in an open furnace in the presence of air or products of combustion, two surface phenomena take place: oxidation and decarburisation.

The basic reactions involved are explained below: O2 + 2Fe = 2FeO O2 + 4FeO = 2Fe2O3 CO2 + Fe = CO + FeO CO2 + 3FeO = Fe3O4 + CO Oxidation of steel is caused by oxygen, carbon dioxide and/or water-vapour.

The general reactions are given below.

Oxidation of steel may range from a tight, adherent, straw-coloured film that forms at a temperature of about 180C to a loose, blue-black oxide scale that forms at temperature above about 450C with resultant loss of metal.

Decarburisation or depletion of surface carbon content takes place when steel is heated to temperatures above 650C.

It progresses as a function of time, temperature and furnace atmosphere.

Typical reactions involved are: O2 + C = CO2 O2 + Fe3C = 3Fe + CO2 CO2 + C = 2CO CO2 + Fe3C = 2CO + 3Fe H2O + Fe3C = CO + H2 + 3Fe These reactions are reversible.

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.

The harmful effects of oxidation and decarburisation include: loss of dimensions; deterioration of surface quality; non-uniform metallurgical transformation during austenitising and subsequent quenching; lowered surface hardness and strength; reduced fatigue strength of heat-treated product.

Prevention of oxidation and decarburisation is not only better than cure, it is also profitable.

There are several ways to prevent or minimise the two harmful reactions: removal of decarburised surface by machining operations after heat treatment; copper plating prior to heat treatment, up to a thickness of 0.025 mm; use of molten salt bath as heating media; use of protective atmospheres (liquid hydrocarbon atmosphere; dissociated ammonia; exothermic gas; nitrogen; endothermic gas; use of fluidised bed furnaces); use of vacuum furnace; switching over to grades that do not require heat treatment; use of protective coatings.

The first two methods are not practical and work out to be expensive.

Molten salt baths provide a means of rapid heat transfer for austenitising and tempering.

The composition of the 'neutral' bath should be regularly controlled.

Limitations are: the size of available salt baths; possible corrosion of heat-treated parts caused by ineffective salt removal; difficulty of quenching parts with blind holes.

Protective atmospheres are widely used as a result of years of basic research on chemical equilibria of steel in combination with various gases at elevated temperatures, together with the development of modern furnaces of gas-tight construction for effective utilisation of furnace atmospheres without infiltration of air.

Fluidised bed furnaces provide flexibility of carrying out several types of heat treatment in a single furnace.

Vacuum furnaces have been found very useful in heat treatment of tools.

Switching over to boron steel enables to eliminate heat treatment.

Considerable savings in capital investment and operating costs have been possible.

The greatest progress in cold-heading quality steel grades in the last century is the discovery of boron steel.

Unfortunately, there are not many grades that do not require heat treatment.

Salt bath, protective atmospheres, fluidised bed and vacuum furnaces demand heavy capital investment, highly skilled personnel and special safety precautions.

Many small heat treatment shops cannot afford them, and yet they are under mounting pressure to prevent oxidation and decarburisation.

Steel Plant Specialities has studied the above problems in depth and has developed a range of protective coatings that can prevent scaling and decarburisation.

The characteristics of coating compound include: does not react with steel surface; does not give any toxic fume during use or during heating the coated job; non-hazardous; economical.

The benefits of protective coatings include: coated tools and components can be heat treated in air using a box type or bogie hearth; electric, gas or oil fired furnace; coating eliminates need of salt bath or controlled-atmosphere equipment in many cases; considerable savings in capital investment and operating costs; due to prevention of decarburisation, uniform surface hardness is achieved; ability to salvage the rejected components - ground plates of maraging quality steel (10 x 1 x 0.1m) could be satisfactorily re-heat-treated, with large savings; elimination or minimisation of post-heat treatment operations like acid pickling, grinding, shot blasting, and so on.

Due to prevention of oxidation even in an ordinary oil fired furnace, pickling time could be reduced by 75 per cent.

Buffing is a time-consuming operation in stainless steel processing.

It can be eliminated or minimised in many cases.

In the manufacturing process of shearing blades of high carbon, high chromium grade steel, grinding allowance of 0.8mm is kept.

It can be brought down to 0.4mm when the protective coating is used during heat treatment.

Gas nitriding is a case-hardening process by which the surface hardness of certain alloy steel components is increased by heating in a nitrogenous gas.

This process improves wear resistance, seizing resistance, corrosion resistance and fatigue life.

The retorts and fixtures are made of inconel, which is expensive.

A special coating gives the following benefits: prevention of ammonia dissociation by the retort walls and work fixtures and hence reduction in ammonia gas consumption up to 20 per cent.

Inconel is the most satisfactory metallic material for nitriding retorts and fixtures.

After protracted use, however, this alloy also begins to dissociate ammonia at an excessive rate.

This problem is eliminated if the retorts and the fixtures are coated.

Plain carbon or low-alloy steels are not considered suitable for retorts and fixtures because a surface which rapidly dissociates ammonia is soon formed.

The coating prevents this and makes plain carbon or low-alloy steels suitable for retorts and fixtures.

The use of protective coatings has established itself as an effective technique of preventing oxidation and decarburisation during heat treatment.

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