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.
Read more: Using Protective Coatings in Heat Treatment