Heat treatment is an important step in the manufacturing of engineering components, machine parts and tools. Oxidation and decarburization of steel will 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, poor quality surface finish, quench cracking, an increase in expensive processing (like shot blasting, machining and acid pickling) — and even to rejection of finished products.
Protection against scaling and decarburization is achieved by heating in molten salts, fluidized bed furnaces, protective gaseous media, or vacuum furnaces. These measures demand significant 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 decarburization.
This article introduces a practical technique that enables any kind of steel to be heated without the basic problems of oxidation and decarburization. The 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 Decarburization
When steel is heated in an open furnace in the presence of air or products of combustion, two surface phenomena will take place, oxidation and decarburization. Oxidation of steel is caused by oxygen, carbon dioxide and/or water vapor. The general reactions are given in Table 1.
Oxidation of steel may range from a tight, adherent straw-colored 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 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. The typical reactions involved are shown in Table 2.
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.
The effects of oxidation and decarburization range from physical to economical, and include
• Deteriorated surface quality due to pitting.
• Loss of material and dimensions as extra material allowance needs to be kept for scaling.
• Non-uniform metallurgical transformation during austenitizing and subsequent quenching. Lowered surface hardness and strength due to layer of scaling.
• Reduced fatigue strength of heat treated product (especially true in case of automobile leaf springs.)
• Costly and time consuming operations like shot blasting, pickling and grinding to remove scaling and remove decarburized layer.
Preventing oxidation and decarburization is not only better than any 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 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. Fluidized bed furnaces and vacuum furnaces have also proven to reduce scaling. Switching to grades that do not require heat treatment is possible in rare cases.
However, most of these proposed 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. Using protective anti-scale coating has proven to be a logical solution to the problem of scaling and decarburization.