Heat treatment, also known as heat treating, is a heated metalworking process that brings metal alloys to a melting point and cools to achieve certain properties. In this article, you will learn the purpose of heat treatment, the different types of heat treatment, and the furnace types that perform heat treatment.
Purpose of Heat Treatment
Heat treatment is a vital part of processing and altering different types of metals to achieve designed mechanical characteristics. Heat treatment alters metal properties by heating the metal to a molten stage and controlling the cooling process of the alloy to produce a desirable change in characteristics. The characteristics affected may be hardness, strength, toughness, ductility, or elasticity. Changes to these characteristics occur because heat treatment alters alloy microstructures by heating and cooling to a desired microstructure. The most commonly heat-treated material is steel as it has a vast variety of industrial applications. When steel undergoes heat treatment, microstructure orientation shifts from a softer pearlite microstructure to a much harder martensite microstructure steel. Heat treatment consists of three main stages: heating, soaking, and cooling. Alloy composition and practical application of the phase diagram determine expected results.
The heating stage of heat treatment increases a metal alloy’s temperature to a specified melting point or molten stage. Heating typically occurs in large industrial-scale furnaces. Once the metal reaches the desired temperature, it is subsequently held at that temperature to achieve an even temperature throughout the entirety of the metal.
Upon achieving the desired temperature, the metal soaks. Soaking holds a metal at the specified temperature for a specified amount of time. Soaking ensures that heat evenly distributes to the inner core of the metal. Depending on the type of treatment that is being done, the soaking time varies.
After completion of the soaking phase, the metal cools. In this stage, the final desired property of the metal is solidified. Methods of cooling depend on the desired cooling rate. For example, processes requiring rapid cooling require quenching the metal in water or brine. Air cooling or lightly dousing of the metal with coolant results in slower cooling.
Type of Heat Treatment
The rate at which heat, soaking, and cooling apply to the metal determine the type of heat treatment process. There are four basic types of heat treatment: hardening, annealing, normalizing, and tempering.
Hardening occurs when a metal heats and quickly cools through quenching. This process increases the strength and hardness of an alloy such as steel. In the hardening process, the metal is heated to a temperature above its critical temperature and soaked there for a time-dependent on the thickness of the specimen. The quenching shocks the alloy and traps the carbon molecules in higher stress microstructure configurations leading to a much harder specimen.
Annealing reduces the internal microstructure stress of alloy specimens. This process is performed when a metal heats and soaks at a temperature above the specimen’s critical point and cools in a furnace at a specified and controlled rate to dictate the ductility of the specimen. The rate of cooling is much slower than that of the hardening process as the desired outcome is a softer and refined grain structure to promote ductility. The slower the cooling, the more ductile the metal becomes.
Normalizing creates a more ductile and tough specimen from an already hardened alloy. This process shares characteristics with annealing as it is heat raises the temperature above a critical rate and allowed to cool. The major difference between normalizing and annealing is that normalizing relies on air cooling at room temperature while annealing is a controlled cool that occurs in a furnace.
Once an alloy has been hardened they may also become extremely brittle as a result of internal stress from quenching. Tempering is a process that reduces that internal stress as it reheats the hardened metal to a specified temprature that is lower than the critical temprature is heated to for hardening. Once the specimen has been heated to the lower temprature, the alloy lacing molecule such as carbon in steel is allowed to relieve some internal microstructure stress and is then and air cooled at a slower pace than quenching. This process may decrease the strength and hardness of an alloy and increase its ductility.
Types of Furnaces
Proper heating application allows for the alteration of metal microstructure. The naturally high melting point of metal requires heavy-duty furnaces. This section covers the different types of furnaces in industrial heat treatment.
Batch furnaces carry one load at a time. They are extremely cost-effective as they are typically smaller than higher load types of furnaces. Heating sources may be electric or gas-fired and are the basis from which many other furnaces are designed. Batch furnaces are grouped into two main types: pit furnaces and box furnaces.
Pit furnaces, known as top loading furnaces, are a type of batch furnace. Due to their single batch and top-loading nature, they offer cost-effectiveness and ease of use. Like most other batch furnaces, gas or electric heat provides the required temperature rise.
Box furnaces are also a type of batch furnace. They feature a swing or lift door at the front and heat electrically or via gas. Because of their larger design, they are ideal for larger, heavier, and hard to load specimens.
Car Bottom Furnace
Car bottom furnaces are among the largest and most expensive furnaces and load with a forklift or crane. The bottom of the furnace slides out for ease of loading. Their design optimizes for larger loads but are easy to load especially when dealing with extremely large specimen. They are typically heated through various combustion systems as they are much larger and require much more heat to evenly regulate the furnace.
Bell furnaces are another type of batch operated furnace. Bell furnaces heat objects under a movable dome and are ideal in heat treating narrow strips or sheets which lack thickness. They are useful in much smaller scale industrial heat treating applications and are usually gas fired.
Salt Bath Furnace
Salt bath furnaces are heated by immersing the subject in a salt fused solution which is then heated by gas fire or electrically. They are ideal for much smaller applications of heat treatment and are among the smallest of industrial grade furnaces.