Using Protective Coatings in Heat Treatment
This article introduces a practical technique pioneered by a metallurgist at the Indian Institute of Technology. The technique enables any kind of steel to be heated without the problems associated with oxidation and decarburization.
Heat treatment is an important operation in the manufacture of engineered metallic components, machine parts and tools. The oxidation and decarburization of steel take place when steel components are heated in the presence of air or products of combustion. Undesired and excessive oxidation can lead to problems such as scale pit marks, dimensional changes, poor surface finish, rejections and quench cracking. Additionally, these problems may lead to the need for expensive operations like shot blasting, machining and acid pickling. Protection against scaling and decarburization is achieved by heating in molten salts, fluidized-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, yet they are under mounting pressure to prevent oxidation and decarburization.
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: oxidation and decarburization. The oxidation of steel is caused by the presence of oxygen, carbon dioxide and/or water vapor. The oxidation may manifest itself in a range from a tight, adherent straw-colored film that forms at a temperature of about 180°C (lower temperatures) to a loose, blue-black oxide scale (higher temperatures) with a resultant loss of metal.
Decarburization, or the 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. The equilibrium relationship depends on the ratio of carbon dioxide to carbon monoxide.
Oxidation and decarburization may lead to a number of unwanted effects on the dimensions, surface quality or metallurgical properties of the component. These include dimensional and material loss, which must be accounted for in the manufacturing process. Often, surface quality is deteriorated due to pitting. Metallurgical transformation during austenitizing and subsequent quenching may be non-uniform. Surface hardness and strength are also lowered due to a layer of scaling. The fatigue strength of heat-treated products is reduced, which is especially true in the case of automobile leaf springs.
Preventing Oxidation and Decarburization
There are several ways to address the problems caused by these harmful reactions. The removal of decarburized surface material by machining after heat treatment, the copper plating of up to 0.025 mm in thickness prior to heat treatment or a change of heating media to molten salt bath will help remedy the problem. A number of protective atmospheres are effective in preventing decarburization. Fluidized-bed and vacuum furnaces are used to reduce scaling. In rare cases, switching to steel grades that do not require heat treatment is possible. However, most of the mentioned solutions pose problems, extra costs or practical difficulties.
The availability of capital and/or human resources to operate high-end furnaces is one major issue. Many small heat-treatment shops cannot afford these solutions.
Protective Coatings and Their Characteristics
In troublesome situations, an anti-scale coating, which we call ESPON, is applied on components or billets to be heated before charging them into the furnace. Care is taken to apply a uniform coating by brushing, dipping or spraying. The coating is then allowed to dry for 30 minutes at ambient temperature of 35°C. This anti-scale coating acts as a barrier to the basic reactions of oxidation and decarburization. To prevent scaling and decarburization, care is taken to apply a uniform coating layer on the component. The coating also reduces decarburization on billets and ingots during hot-forging and hot-rolling operations. Heat transfer from the heating medium to the metal is unaffected by the coating. Additionally, the coating has no reaction with the steel surface and no release of toxic fumes during use, heat treatment or storage. The coating is nonhazardous and economical to use.
Benefits of Anti-Scale Coating
Table 1 shows the efficacy of the coating in an electric furnace. In many cases, the coating eliminates the need for a salt bath or controlled-atmosphere equipment, which can result in considerable cost savings. Due to the prevention of decarburization, uniform surface hardness is achieved. Rejected components can be salvaged (Figure 1). Large savings are possible when plates of expensive alloy steel can be re-heat treated by using the anti-scale coating.
Figure 2 explains the benefits of coating during hot forming and solution annealing of stainless steel pipe fittings. Due to prevention of oxidation, pickling time could be reduced by 75%. Buffing can be eliminated or minimized in many cases. In the manufacturing process for shearing blades of expensive high-carbon, high-chromium steel, the grinding process is substantially reduced when the coating is used during heat treatment.
The coating technique has been used by a number of forging companies in a variety of applications. These companies, which include Bharat Forge, Echjay Forgings, Vardhman Special Steels and others, benefit by the reduced decarburization, scaling, quench cracking and reduced shot-blasting time in their processes. Some case studies are as follows:
- Prevention of Quench Cracks – Forgings such as 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 greater than the tensile strength of thin sections of the forging. Chrome-moly grades of steel are most susceptible to quench cracks, which usually occur in the gear-end portion of the crankshaft (Figure 3). By coating the gear-end with an anti-scale coating, the cracking is prevented.
- Reduction in Shot-Blasting Time After Heat Treatment – Operations like shot blasting, grinding and pickling are expensive and time-consuming procedures. They are necessary to remove scaling from components and to enhance the product’s aesthetic appeal, but they do not add value to the product. These operations can be substantially reduced if a coating is applied to components before heat treatment.
- Salvaging Machined Components During Re-Heat Treatment – Often, machined forgings need to be thermally re-treated for metallurgical reasons. However, there may be no material allowance for additional scaling and subsequent machining or shot blasting. In such cases, even a small amount of scaling can render components out of specification. The use of a coating prevents scaling during re-heat treatment and helps prevent losses from scrapped parts while still retaining the aesthetic appeal of components (Figure 4). The coating can be removed after heat treatment by cleaning with light wire brushing or abrasives.
- 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 the use of an anti-scale coating applied only on areas where scaling needs to be prevented (Figure 5).
- Salvaging Forgings During Reheating for Hot Forging – Reheating or re-working of forgings is sometimes required due to underfill, improper metal formation or other reasons. For parts with stringent dimensional tolerances, however, components with excessive scaling could be scrapped for dimensional reasons (Figure 6). The anti-scale coating, when applied to forgings before reheating, ensures minimal or no scaling and eliminates the risk of scrapping dimensional intolerance (Figure 7).
- Reducing Decarburization During Hot Forging and Hot Rolling – During the hot rolling of special grades of steel in which decarburization needs to be kept in check, unforeseen conditions like mill breakdown and unplanned downtime may arise. Even when the plant is closed for a weekly holiday, the furnace may be shut off abruptly, leaving billets inside. Billets left in the furnace are subjected to prolonged heating, leading to decarburization. Applying an anti-scale coating ensures that billets are protected from decarburization.
The use of a protective coating has been established as an effective technique of preventing oxidation and decarburization during heat treatment, hot forging and hot rolling. Additionally, the use of a coating offers benefits such as the ability to salvage parts by re-heat treatment as well as the elimination of post-heat-treatment operations like grinding, shot blasting, acid pickling, etc. The coating process has simplified and accelerated many metallurgical heat-treatment operations, saving capital investment and reducing operating costs while improving quality.
Author S. P. Shenoy is CEO of Steel Plant Specialities, Mumbai, India. He may be reached at email@example.com. For additional information visit www.steelplantspecialities.com The Author will make a presentation on this subject at Stainless Steel World Americas Conference & Expo to be held on October 17-18, 2012 at The Woodlands Waterway Marriott Hotel, Houston, Texas. For event details, please visit www.ssw-americas.com/events
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