What is the heat treatment process?
Heat treatment of a metal involves a group of thermal, industrial, or metalworking processes followed to alter the physical and sometimes chemical properties of metal to harden or smoothen, its shape to meet the desired requirements. Heat treatment involves exposing any metal like steel to extreme temperatures to alter its shape as needed, altering its properties while maintaining its composition.
Typically, heat treatment process involves annealing, normalizing, hardening, and tempering the metal to modify its properties to enhance its utility to meet specific projects or suit demands.
Here’s briefly discussed various types of heat treatment process.
Annealing Steel
The idea of annealing steel is to relieve stress, soften the metal, increase ductility, and better its grain structures.
Annealing is one useful way to fix common problems of welding and relieve internal stresses. To anneal steels or other ferrous metals to get the highest level of ductility, one must heat the metal slowly and gradually at an appropriate temperature, soak it, allowing it to cool slowly by either burying it in a good insulating material or by turning off the furnace and letting both the furnace and the steel part cool slowly together.
Normalizing
The main purpose of normalizing is to remove any sort of internal stresses infusing from heat treatment, forging, machining, forming, welding, or casting. Sometimes, metal failure can also result from uncontrolled stress, so normalizing steel before any hardening ensures successful completion of the task!
Hardening
The prime goal of hardening is not only to harden the steel but also to strengthen it. While hardening increases strength, it also decreases ductility, making it more brittle. After hardening, one might need to temper the metal to remove its brittleness.
Three stages of heat treatment include - slow temperature heat followed by soaking the metal at a specified time to a uniform temperature and the third stage. During hardening, metals cool by plunging them into oil, water, or brine. After heating, steel requires rapid cooling, known as quenching, to be hardened. However, there are few that can be air-cooled.
As alloys are added to steel, the rate of cooling required to harden the steel decreases. There's more to this: the slower cooling rate lessens the risk of warping or cracking. The hardness of carbon steel is proportional to its carbon content, i.e., up to .80% carbon, the ability to harden increases with the carbon content. Beyond .80%, you can only increase wear resistance owing to the hard cementite formation. But the harness can't be increased.
When the steel is hardened, it must be cooled to under 1000°F in less than one second. But, once alloys are added to the steel, and the effectiveness of the carbon is increased, we increase that time limit beyond one second. That allows us to select a slower quenching medium to get that required hardness.
Tempering
Once the metal is hardened and introduced to internal stresses after the rapid cooling inherent in the process, steel is found to be both harder than needed or too brittle. Thus arises the need to temper the steel to reduce that brittleness and remove the internal stresses.
The process of tempering involves:
- Heating steel to a specific temperature below its hardening temperature
- Hold the steel at that set temperature for a planned time
- Cool the steel, typically in still air
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