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. 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 2. Decarburisation.

Oxidation

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 180?C to a loose, blue-black oxide scale that forms at temperature 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:

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, surface quality is deteriorated due to pitting. Metallurgical transformation during austenitising and subsequent quenching may be non-uniform. Surface hardness and strength are also lowered due to layer of scaling. Fatigue strength of heat treated product is reduced. This is especially true in case of automobile leaf springs.

Preventing Oxidation and Decarburisation

Prevention of oxidation and decarburisation is not only better than cure, it is profitable too. There are several ways to address problems caused by the two harmful reactions. Decarburised surface removal by machining operations after heat treatment, copper plating of thickness up to 0.025 mm prior to heat treatment or change of heating media to molten salt bath are some ideas.

A number of protective atmospheres may be introduced like liquid hydrocarbon, dissociated ammonia, exothermic gas, nitrogen and endothermic gas. Fluidised bed furnaces and vacuum furnaces have also proven to reduce scaling. Switching over to grades, which do not require heat treatment, is possible in rare cases. However, most of the mentioned solutions pose a number of problems or practical difficulties. Availability of capital and human resource for using high-end furnaces is a major issue. Many small heat treatment shops cannot afford these solutions. Yet they are under mounting pressure to prevent oxidation and decarburisation. Use of protective anti-scale coating has proven to be a logical solution to the problem of scaling and decarburisation.

Insights into Use of Protective Coating and its Characteristics

Use of protective coating has been found beneficial and cost-effective. An anti-scale coating is applied on components or billets to be heated before charging them into furnace. This anti-scale coating acts as a barrier between oxygen and metal. Care is taken to apply a uniform, impervious layer of coating on the component to be heated. Coating ensures prevention of scaling and decarburisation. For exceptionally long heat treatment cycles of 10 to 15 hours, the extent of scaling and decarburisation mechanism is substantially reduced. Anti-scale coating also reduces decarburisation on billets and ingots during hot forging and hot rolling operations. Heat transfer from heating media to metal is not affected due to anti-scale coating.

No reaction with steel surface, no release of toxic fumes during use or heat treatment or storage, non-hazardous and economical implementation are other required characteristics of the coating. Coated tools and components must be able to be heat treated in air using a box type or bogie hearth; electric, gas or oil fired furnace. Benefits of Anti-scale coating; Industrial Case Studies and Success Stories Table-1 shows the efficacy of the coating in an electric furnace. Coating eliminates need of salt bath or controlled - atmosphere equipment in many cases. Considerable savings in capital investment and operating costs are enabled by use of anti-scale coating. Due to prevention of decarburisation, uniform surface hardness is achieved. Rejected components can be salvaged (Figure 1). Large saving is possible when ground plates of maraging quality steel can be satisfactorily re-heat treated by using anti-scale coating. Figure 2 explains the benefits of using coating during hot forming and solution annealing of stainless steel pipe fittings. Due to prevention of oxidation even in an ordinary oil fired furnace, pickling time could be reduced by 75%. Buffing can be eliminated or minimized in many cases. In manufacturing process of shearing blades of expensive high carbon, high chromium grade steel, grinding allowance is substantially reduced when protective coating is used during heat treatment. Some other distinct case studies are enlisted below.

1. Prevention of Quench Cracks

Forgings like knuckle joints and crankshafts when heat treated in furnaces of oxidizing atmosphere are susceptible to quench cracking. Quench cracks appear when stresses generated during quenching are higher than tensile strength of thin sections of forgings and due to differential quench severity at different areas. Chrome-Molly grades of steel are most susceptible to quench cracks. Quench cracks usually occur in stem portion compared to thicker sections of the rest of the crank shaft (Image 1). By coating the stem with anti-scale coating, percentage of cracking could be effectively prevented.

2. Reduction in Shot Blasting Time after Heat Treatment

Operations like shot blasting, grinding, acid pickling, etc., do not add value, are expensive and time consuming procedures. These operations are necessary to remove adherent scaling from components and to enhance aesthetic appeal of forgings. Time required for these operations can be substantially reduced if a coating is applied on components before heat treatment. Aesthetic appeal of components is automatically enhanced without much effort as scaling is either prevented or reduced by using anti scale coating.

3. Salvaging Fully Machined Components by Protecting During Re-heat Treatment

Often, fully machined forgings need to be re-heat treated for metallurgical reasons. However, there is no material allowance left for further scaling to take place and for subsequent machining or shot blasting. In such cases, even small amount of scaling can render components to be scrapped. Use of anti-scale compound ensures prevention of scaling during re-heat treatment. Hence, huge losses can be prevented by salvaging fully machined components. Aesthetic appeal of components is retained (Image 2). Coating itself can be removed after heat treatment by cleaning the forging with diesel, amery paper brushing or light wire brushing.

4. Heat Treatment of Pressure Vessels

Valve areas of pressure vessels are critical and need to be protected from scaling during thermal cleaning and heat treatment. This is achieved by use of anti-scale coating being applied only on areas where scaling needs to be prevented (Image 3). 5. Salvaging of forgings during re-heating for hot forging Re-heating or re-working of forgings is required due to underfill, improper metal formation and similar reasons. However, with stringent dimensional tolerances, there is a risk of components getting scrapped due to excessive scaling (Image 4). Anti-scale coating, when applied on forgings before re-heating for re-working, ensures minimal or no scaling, thereby eliminating risk of scrapping components during re-working (Image 5).

6. Reducing Decarburisation during Hot Forging and Hot Rolling

During hot rolling of special grades of steel where decarburisation needs to be kept in check, unforeseen conditions like mill breakdown and unplanned downtime may arise. Even when the plant is closed for weekly holiday, furnace is shut off abruptly, leaving billets inside the furnace. In these cases, billets or ingots are left in furnace and are subjected to prolonged heating leading to decarburisation. In both cases, applying an anti-scale coating ensures that billets are protected from decarburisation.

Summary

1. Use of protective coating has established itself as an effective technique of preventing oxidation and decarburisation during heat treatment, hot forging and hot rolling.

2. It has unleashed a number of additional benefits like ability to salvage by re-heat treatment, elimination of post-heat treatment operations like grinding, shot blasting, acid pickling, etc.

3. The coating process has simplified and accelerated many metallurgical heat treatment operations, saving a fortune in capital investment, reducing costs and improving quality.

For further information contact: Steel Plant Specialities, Mumbai, Tel: 022-67978060. Fax: 91-22-25552459. Email: info@steelplantspecialities.com