UNDERSTANDING OXIDATION AND DECARBURIZATION

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. Decarburization

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.

DECARBURIZATION

Decarburization 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 DECARBURIZATION ON METALS

Oxidation leads to loss of dimensions and material as extra material allowance needs to be kept to provide for scaling, decarburization and their subsequent machining. Often, surface quality is deteriorated due to pitting. Metallurgical transformation during austenitizing and subsequent quenching may be non-uniform due to layer of scale on metal that acts as a barrier to quenchants. 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.

METHODS OF PREVENTING OXIDATION / SCALING AND DECARBURIZATION

Prevention of oxidation and decarburization is not only better than cure, it is profitable too. There are several ways to address problems caused by the two harmful reactions. Decarburized surface removal by machining operations after heat treatment, copper plating of thickness upto 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. Fluidized 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 decarburization. Use of ESPON protective anti-scale coating has proven to be a logical solution to the problem of scaling and decarburization in heat treatment, hot forging and hot rolling.

INSIGHTS INTO CHARACTERISTICS OF ESPON PROTECTIVE ANTI SCALE COATING AND ITS USE IN PREVENTING SCALING AND DECARBURIZATION:

Use of protective coating has been found beneficial and cost-effective. ESPON protective anti-scale coating is applied on components or billets to be heated before charging them into furnace. It 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. ESPON coating ensures prevention of scaling and decarburization. For exceptionally long heat treatment cycles of 10 to 15 hours, the extent of scaling and decarburization mechanism is substantially reduced. ESPON anti-scale coating also reduces decarburization on billets and ingots during hot forging and hot rolling operations. Heat transfer from heating media to metal is not affected due to use of ESPON anti-scale coating.

No reaction with steel surface, no release of toxic fumes during use or heat treatment or storage, non-hazardous nature and economical implementation are other characteristics of ESPON anti scale coating. Coated tools and components are able to be heat treated in air using a box type or bogie hearth; electric, gas or oil fired furnace.

BENEFITS OF ESPON PROTECTIVE 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 decarburization, uniform surface hardness is achieved. Rejected components can be salvaged (Fig. 1). Large savings are possible when plates of expensive alloy steel can be re-heat treated by using the anti-scale coating.

(Fig. 2) Due to prevention of oxidation even in ordinary oil fired furnace by use of ESPON protective anti-scale coating, pickling time could be reduced by 75% in case of hot forming and solution annealing of stainless steel tubes and stainless steel pipe fittings. Buffing could be eliminated or minimized in many cases as scaling was substantially reduced. In manufacturing process of shearing blades of expensive high carbon, high chromium grade steel, grinding allowance is substantially reduced when ESPON anti scale coating is applied prior to heat treatment. Some other distinct case studies are enlisted below.

1. PREVENTION OF QUENCH CRACKS:

Forgings like knuckle joints and crank shafts 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 sections of forgings that are thinner compared to the rest of the forging and due to differential quench severity at different areas. Chrome-moly grades of steel are most susceptible to quench cracks, which usually occur in the gear-end portion of the crankshaft. By coating the gear-end with ESPON protective anti-scale coating, the quench cracking is effectively prevented. (Image 1). Reputed forgers of largest crank shafts in India use this technique. (Endorsement 1).

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 operations like shot blasting, grinding, acid pickling, etc. can be substantially reduced if ESPON protective anti scale coating is applied on components before heat treatment (Endorsement 1). Aesthetic appeal of components is automatically enhanced without much effort as scaling is either prevented or reduced by the use of ESPON anti oxidation protective 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 operation. In such cases, even small amount of scaling can render components to be scrapped. Use of ESPON protective anti-scale coating ensures prevention of oxidation during re-heat treatment, hence preventing huge losses. Fully machined components can be successfully re-heat treated without the problem of oxidation or scaling by coating them with ESPON before re-heat treatment. Aesthetic appeal of components is retained (Image 2). Coating itself can be removed after heat treatment by cleaning the forgings with diesel, amery paper brushing or light wire brushing (Endorsement 2).

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. Scaling on threaded portions will cause leakages and prove dangerous during use of such equipments. Protection of critical areas of pressure vessels is achieved by use of ESPON anti scale coating being applied only on areas where oxidation / 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 forgings getting scrapped / rejected due to excessive scaling (Image 4). ESPON 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 DECARBURIZATION DURING HOT FORGING & HOT ROLLING:

During hot rolling of special grades of steel where decarburization 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 decarburization. In both cases, applying ESPON anti-scale coating on billets before charging them into the re-heating furnace ensures that billets are protected from decarburization. (Endorsements 3 & 4). Reduced decarburization on automobile leaf springs leads to increased fatigue strength of the leaf springs and greater reliability. Leaf springs heated after applying ESPON protective coating show substantially reduced decarburization and scaling.

7. IMPROVING SURFACE FINISH OF HOT ROLLED COMPONENTS AND PREVENTING WELDING OF BILLETS DURING HEATING:

During hot rolling of special grades of steel Nickel (Ni), mill scale is substantially increased. Surface finish of hot rolled product may be compromised due to pit marks caused by mill scale. Subsequent grinding operations are increased due to excessive mill scale. Applying ESPON anti-scale coating on billets before charging them into the re-heating furnace ensures reduced oxidation and mill scale. Surface finish of rolled product is improved due to reduced mill scale. During re-heating of billets for hot rolling and also during induction heating of billets for hot forging, sometimes, billets are welded together. This leads to unproductive process of separating the welded billets and downtime. Welding of billets during hot rolling and hot forging can be prevented by applying ESPON protective coating on billets before charging them into the furnace.

SUMMARY

1. Use of ESPON protective coatings are established as an effective technique of preventing oxidation / scaling and decarburization 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 ESPON coating process has simplified and accelerated many metallurgical heat treatment operations, saving a fortune in capital investment, reducing costs and improving quality.

Table-1 Efficacy of Protective Coating.