October 201733

eat Treatment is a method of Hcontrolled heating and cooling of metals to alter their mechanical and physical properties without changing the product shape. It is also applied to castings produced in the foundries. By heating or quenching (rapid cooling), the desired result, such as - hardening or softening of a metal is achieved. Some of the common techniques of heat treatment include annealing, case hardening, precipitation strengthening, tempering and quenching. Normally, the term heat treatment is applied only to processes where the heating and cooling is done for the basic objective of altering the properties intentionally. Heat treatment can also be used to improve machining, formability and to restore ductility after cold working operation. The current trends in the Heat Treatment practices include use of Bogie

hearth furnace (car bottom furnace) in place of conventional muffle furnace. These furnaces find wide use in annealing, normalizing processes for castings. In addition, two cars are used for the same furnace in place of one car. This saves the furnace downtime. Large capacity furnaces are also replacing the small size furnaces. Continuous furnaces in place of batch type furnaces are also gaining wide acceptance in view of large output which can be obtained. Among the operational changes, use of polymers quenching media is gaining popularity in place of quenching oils. In general, the use of ceramic fibre insulation is also very common as compared to conventional insulating refractory bricks used in the past. In this article basic concepts of common heat treatment processes and emerging trends in the field of heat treatment practices are highlighted. Basic Heat Treatment processes – Four methods which are commonly applied to metals and castings are Annealing, Normalizing, Hardening

and Tempering. These are briefly described below. Annealing - Generally, annealing is the opposite of hardening. Metals are annealed to relieve internal stresses, to soften them, to make them more ductile, and to refine their grain structures. Annealing comprises of heating a metal to a particular temperature, holding it at that temperature for a definite time, and then cooling the metal to room temperature. The cooling process depends on the metal and the characteristics desired. Some metals are furnace-cooled, while others can be cooled by burying them in ashes, lime, or other insulating materials. Normalizing - A type of heat treatment applicable to ferrous metals only, normalizing differs from annealing in that the metal is heated to a desired temperature, and then removed from the furnace for air cooling. The basic objective of normalizing is to remove the internal stresses caused by heat treating, welding, casting, forging, forming, or machining. If not controlled, stress can result in metal failure; hence, before

- Dhirajlal K. ChauhanConsultant

Email : chauhan_dhirajlal@yahoo.com

Changing Trends inHeat Treatment in Foundries

Changing Trends inChanging Trends inHeat Treatment in Foundries Heat Treatment in Foundries

Changing Trends inHeat Treatment in Foundries

In-sight

October 201734

hardening the steel, it should be first normalized to get the desired results. Usually, the castings are annealed, instead of normalizing; however, some castings require normalizing. Normalized steels/castings are stronger and harder than annealed ones. Castings which require maximum toughness in impact and resistance to external stress are generally normalized. Hardening - Metal hardening treatment constitutes heating the steel/casting to a definite temperature and then quenching it (rapid cooling) into oil, brine, or water. Majority of steels require rapid cooling (quenching) for hardening, however a few can be air-cooled with the same results. Although hardening enhances the hardness and strength of the workpiece, it makes it less ductile. Usually, the harder the steel, the more brittle it becomes. To remove some of the brittleness, the steel/casting should be tempered after hardening. Tempering - After hardening, steel/casting is generally harder than required and is too brittle for most practical applications. In addition, internal stresses are developed during the speedy cooling from the hardening temperature. To relieve the internal stresses and to reduce brittleness, the steel is tempered after it hardening. Tempering process comprises of heating the workpiece to a particular temperature (which is below the lower critical point of the steel and castings) holding it at that temperature for a definite time, and then cooling it, generally in still air. The final hardness,

strength, and ductility depend on the temperature to which the steel is heated during tempering. Besides reducing the brittleness, tempering decreases the hardness of steel and castings. The extent, to which hardness is decreased, depends on the temperature to which the workpiece is heated to during the process of tempering. Present trends in Heat Treatment processes for metals and foundry products. These trends include changes in the heat treatment practices to improve productivity, enhance quality of heat treated products, improvements in the efficiency and working of heat treatment equipments, use of alternate sources of heat, elimination of accidents and so on. Some of these trends are briefly discussed below.

1) Globalization of heat treatment standards, procedures, processes and furnace designs. These developments lead to uniformity in quality of heat treated products. With standardization of furnace design, uniform quality with very narrow variations is possible irrespective of the furnace manufacturer.

2) Increasing size of furnaces - This is a noticeable trend particularly in case Sealed Quench furnaces. One of the Korean car company estimated that the production of 1 million-car production requires 100 small size (750 kg capacity) sealed quench furnaces. Today majority of sealed quench furnaces manufactured are 1500 kg and 3500 kg load output. Increasing loading capacity of the furnace decreases the cost of operation too.

3) Replacement of quenching mediums – Nanoquenchants, oil mist, polymers and vegetable oil are replacing conventional quenching oils for many applications. These alternate quenching media increase efficiency of quenching process, uniformity of cooling of the workpiece. Better life of quenching medium is also possible.

4) New Sources of Heat – Rotational Magnetic, Plasma, gas fire for low temperature vacuum furnaces, Microwaves, Laser and Infrared Heating are being used for some applications. Such alternate heating sources increase rate of heating, prevent oxidation of workpiece and increase output of heat treating equipment.

5) Furnaces with fully automated “line of operation” system are used due to increasing labour cost and labour shortage in some countries (e.g. Japan, Korea). Continuous furnaces are typical examples of automatic operations. These furnaces provide high output, consistent quality and have lower cost of production. However, the initial cost of these furnaces is quite high as compared to batch type furnaces. Therefore, the use of these furnaces is justified only when high production is desired.

6) The most significant trend in manufacturing is to create total control of manufacturing processes, quality (utilisation of standards) and production streamlining (to lower cost of operation and avoidance of overproduction). Furnace control systems have changed drastically in the last 10 years and the progress is continuing. In order to increase the efficiency of the heat treatment process, furnaces are integrated into the manufacturing production flow. For example, induction hardening unit can be installed along side of other machines in the same plant. This will result in saving of time and labour for movement of work pieces from one section of plant to another.

7) Opportunities - Fast-growing Asian economies create market opportunities for heat treatment services and equipment, particularly in India, Thailand, Indonesia and in time, Vietnam and the Philippines. According to demographic trends the highest growth will be in Asia and Africa in the next 10 to 15 years.

In-sight