The excerpt is from the tech notes of Mr. S.P. Shenoy, CEO Steel Plant Specialties, India released in the August issue of Steel 360. The attempt of Mr. Shenoy is to familiarize audience with the new Anti-Coating scaling technique.
A new technique of Anti-Scale Coating has been pioneered by a metallurgist from the Indian Institute of Technology, Mumbai, India. Steel Plant Specialties’ which is engaged in manufacturing innovative products in heat treatment, hot forging, hot rolling, die casting, metal working, to increase productivity of metallurgical industries since 1985 is using the new technique in order to avoid harmful effects being occurred because of heating treatment of steel.
Steel is melted through a heating process to give it another shape or form. Heat treatment is very expensive for the steel industries. In line it also includes with it problems like oxidation and decarburization. Oxidation and decarburization take place when steel components are heated in the presence of air or products of combustion.
Harmful effects of Heat Treatment:
Unwanted effects of oxidation and decarburization include dimensional and material loss. 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 o scaling. The fatigue strength of heat-treated products is reduced, especially in the case of automobile leaf springs.
Protective coatings and their characteristics:
An anti-scale coating called ESPON is applied on components or Billets to be heated before charging them into the furnace. This anti-scale coating acts as a barrier to the basic reactions of oxidation and decarburization. The coating reduces decarburization on Billets and Ingots during hot-forging and hot-rolling operations, does not react with the steel surface or releases toxic fumes and it is non-hazardous.
Various companies such as 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 given below:
- Prevention of Quench Cracks: Quench cracks usually occur in the gear-end portion of crankshafts made of Chrome-moly grade of steel. By coating the gear-end with an anti-scale coating, quench cracking is prevented.
- Reduction in Shot-Blasting Time after Heat Treatment: Operations like shot blasting, grinding and pickling are necessary to remove scaling from components. These operations can be substantially reduced if a coating is applied to components before heat treatment.
- Salvaging Machined Rejected Components by Re-Heat Treatment: Often, machined forgings need to be thermally re-treated for metallurgical reasons. The use of a coating prevents scaling during re-heat treatment. The coating can be removed after heat treatment by cleaning with light wire brushing.
- Heat Treatment of Pressure Vessels: Valve areas of pressure vessels are critical and can be protected from scaling during thermal cleaning and heat treatment by the use of an anti-scale coating.
- Salvaging Rejected Forgings by re-working: Re-working of forgings is sometimes required due to under filling and other reasons. For parts with stringent dimensional tolerances however, components with excessive scaling could be scrapped for dimensional reasons. The anti-scale coating, when applied to forgings before reheating, ensures minimal or no scaling and eliminates the risk of scrapping due to dimensional intolerance.
- Reducing Decarburization during Hot Forging and Hot Rolling: In 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. 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.
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.