Heat treatment is a controlled process that changes the microstructure of metals and alloys like steel and aluminum to impart properties that improve a component’s working life, such as increased surface hardness, temperature resistance, ductility, and strength.
When attempting to change the properties of metals, the first factor to consider is heat treatment. This mechanical process involves heating metal at a specific temperature with a standard cooling method to obtain the desired properties. Well, in this reading, we’ll explore what heat treatment is, its methods, and how it’s done.
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What is the Heat Treatment?
Heat treatment is a crucial process for obtaining desired properties in metals. Various methods are used to heat and cool metals, ensuring their specific properties. Metallurgists continuously improve the outcomes and cost-efficiency of these processes by developing new schedules or cycles. These methods produce metals of various standards with unique physical and chemical properties.
Heat treatment is the process of heating metal without letting it reach its molten, or melting, stage, and then cooling the metal in a controlled manner to get the desired mechanical properties. Heat treatment is used to either make metal stronger or more malleable, more resistant to abrasion, or more ductile.
In the engineering field, heat treatment is a metalworking process that involves changing a material’s physical and sometimes chemical properties. It is also said to be a way in which the physical or mechanical properties of a metal are changed by heat and cooling methods without changing the shape of the working metal.
Theory of Heat Treatment
Heat treatment can differ depending on the metal material, but there are three main differences in the process: the heating temperatures, the cooling rates, and the quenching types. Tweaking this process will get you the properties you desire. Well, heat treatments are often performed on metals containing iron, which are known as ferrous metals. Annealing, hardening, normalizing, and tempering are done on such kinds of metals.
Proper heat-treating equipment will be required so one can accurately control the factors around heating, cooling, and quenching. For instance, the furnace must be the proper size and type to control temperature, including the gas mixture in the heating chamber, and you need the appropriate quenching media to cool metal correctly.
Purpose of Heat Treatment
The following are the purposes of heat-treating metals:
- To improve ductility and toughness
- Increase the wear and corrosion resistance of a metal.
- Improve electrical and mechanical properties.
- Homogenize the structure to remove coring.
- Spheroidize tiny particles.
- Improve machinability and toughness.
- Grain size refinement.
- Relieving internal stresses.
The Three Stages of Heat Treatment
Heat treatment serves several purposes when it comes to metal, and these can be achieved in three stages. These three stages of heat treatment include heating, soaking, and cooling.
1. Heating
Heating is the first stage in a heat-treating process. It is done to change the structure of alloys when heated to a specific temperature. The alloy is said to be at room temperature either as a solid solution, a mechanical mixture, or a combination of both.
In a solid solution, two or more metals are used on each other to form a solution without showing their elements, even when checked with a microscope. In the mechanical mixture, the elements and compounds are visible and compacted by a matrix of base metals.
2. Soaking
Soaking is the stage at which the complete part of the heated metal completely changes its structure. The mass of the metal will determine the time it will be soaked. In other words, soaking is when a part of metal evenly turns red due to being subjected to heat for some time.
3. Cooling
The third stage of heat treatment is cooling. It also changes the structure of the soaked metal from one chemical composition to another due to the way it is been cooled, either by quenching or by still air.
None of the three stages of heat treatment can be left aside. The heating stage helps in changing the structure of the metal from room temperature until it completely reaches the soaking stage, which is when the metal evenly turns red. The cooling stage is the stage at which the metal completely changes to its new properties due to the cooling process.
How is Heat Treatment Done?
Heat treatment is a process that involves heating metal, holding it at a specific temperature, and then cooling it back. This process alters the metal’s mechanical properties due to the high temperature, which affects its microstructure. The final outcome depends on factors such as the time of heating, the temperature of the metal part, the rate of cooling, and surrounding conditions.
The properties of the metal, such as electrical resistance, magnetism, hardness, toughness, ductility, brittleness, and corrosion resistance, change during heat treatment.
An alloy may exist in three different states during heat treatment: a mechanical mixture, a solid solution, or a combination of both. The holding stage, or soaking stage, keeps the metal at the desired temperature, with the duration depending on the requirements. The holding time depends on the material type and part size.
After the soaking stage, the metal must be cooled in a prescribed manner, causing structural changes. Different media, such as brine, water, oil, or forced air, control the rate of cooling. Brine absorbs heat fastest, while air is the slowest. Furnaces can also be used in the cooling process for high precision when slow cooling is necessary.
Methods of Heat Treatment
Common heat-treating methods include annealing, hardening, quenching, and stress-relieving. Each method has a unique process to produce specific results.
Annealing
Annealing heat treatment is used to alter the microstructure of a metal. This enhances its ductility while reducing internal stress and overall hardness. This makes the shaping process of the material easy, preventing it from cracking. Annealing has to do with heating metal to a point where the crystalline structure becomes fluid while the metal remains solid.
This temperature is maintained for the metal so that any flaws in the material can heal themselves. After that, the metal is gradually allowed to return to room temperature to form a more ductile crystalline structure.
Hardening
Hardening is a heat treatment process for enhancing a surface metal’s hardness by heating and rapid cooling. It has to do with heating the material in a hardening furnace to a point that transforms its internal structure without melting it. Then the metal is held for one hour per inch of thickness at the temperature, following rapid cooling. There’s a more hard and stable crystalline structure because of the rapid cooling.
Quenching
Quenching heat treatment is a process of rapidly cooling the metal to get the physical or mechanical properties needed. Oil is most commonly used for cooling the heated material. However, water, brine, and air can be used based on the material and the qualities you want.
The quenching heat-treating process sets it apart from others because the metal is heated to a point below the melting point at which the crystalline structure is fluid. The properties you want will determine the period they will be held for. Afterward, it is quenched in one of the media to reduce the temperature of the material and yield the necessary internal structure.
Stress Relieving
This process has to do with heating the material above the point where the internal structure transforms and then air-cooling it at a particular rate. It allows for a more stable structure, reducing internal stress and enhancing the strength and hardness of the metal. It is particularly useful for metals that have been subjected to stress-inducing forming processes, such as machining, straightening, and rolling.
Aging
Precipitation hardening or aging is a heat treatment method used to increase the yield strength of malleable metals by producing uniformly dispersed particles within the grain structure. This process, which occurs after another heat treatment, reaches medium temperatures.
Carburization
During this heat treatment procedure, the metal is heated in the presence of an element that, when dissolved, creates carbon. The metal’s surface absorbs the carbon that has been released. The surface becomes harder than the inner core due to an increase in carbon concentration.
Tempering
Tempering reduces excess hardness and brittleness during hardening, relieves internal stresses, and makes metals suitable for various applications. Temperatures are typically lower than hardening temperatures, with higher temperatures resulting in softer final workpieces. Cooling rate doesn’t affect structure.