WHY HEAT TREATMENT IS REQUIRED ? CHECK OUT THE ANSWER !!

Heat treatment is the process of heating (but never allowing the metal to reach the molten state) and cooling a metal in a series of specific operations which changes or restores its mechanical properties.

WHY HEAT TREATMENT IS REQUIRED?

Welding, cutting, or even grinding on metal produces heat, which in turn has an effect on the structure of the metal. These change in structures affect the properties of the material. Heat treatment makes the metal more useful by making it stronger and more resistant to impact, or alternatively, making it more malleable and ductile.

However, no heat-treating procedure can produce all of these characteristics in one operation; some properties are improved at the expense of others. For example, hardening a metal may make it brittle, or annealing it may make it too soft.

STAGES OF HEAT TREATMENT :

You accomplish heat treatment in three major stages:

• Stage l — Heat the metal slowly to ensure uniform temperature.
• Stage 2 — Soak (hold) the metal at a given temperature for a given time.
• Stage 3 — Cool the metal to room temperature.

VARIOUS TYPES OF HEAT TREATMENT PROCESSES :

All heat-treating processes are similar because they all involve the heating and cooling of metals. However, there are differences in the methods used, such as the heating temperatures, cooling rates, and quenching media necessary to achieve the desired properties.

The heat treatment of ferrous metals (metals with iron) usually consists of annealing, normalizing, hardening, and/or tempering. Most nonferrous metals can be annealed, but never tempered, normalized, or case hardened.

1.)ANNEALING: Anneal metals to relieve internal stresses, soften them, make them more ductile, and refine their grain structures. The process includes all three stages of heat treatment already covered (heat the metal to a specific temperature, hold it at a temperature for a set length of time, cool it to room temperature), but the cooling method will depend on the metal and the properties desired.

2.)NORMALIZING: The intent of normalizing is to remove internal stresses that may have been induced by heat treating, welding, casting, forging, forming, or machining. Uncontrolled stress leads to metal failure; therefore, you should normalize steel before hardening it to ensure maximum results. Normalizing applies to ferrous metals only, and it differs from annealing; the metal is heated to a higher temperature, but then it is removed from the furnace for air cooling.

3.)HARDENING: The purpose of hardening is not only to harden steel as the name implies, but also to increase its strength. However, there is a trade-off; while a hardening heat treatment does increase the hardness and strength of the steel, it also makes it less ductile, and brittleness increases as hardness increases. To remove some of the brittleness, you should temper the steel after hardening.

4.)TEMPERING: After hardening by either case or flame, steel is often harder than needed and too brittle for most practical uses, containing severe internal stresses that were set during the rapid cooling of the process. Following hardening, you need to temper the steel to relieve the internal stresses and reduce brittleness.

Tempering consists of:

• Heating the steel to a specific temperature (below its hardening temperature)
• Holding it at that temperature for the required length of time
• Cooling it, usually in still air.

The difference from other heat treatment process is in the temperatures used for tempering, which will affect the resultant strength, hardness, and ductility.

5.)QUENCHING: Quenching is a process of hardening a metal substance beyond its natural hardness level. In this process, a metal/alloy object is heated to a temperature till it becomes RED hot i.e. sub-boiling temperature and then is suddenly sunk into a fluid of room temperature or lesser eg, Water.

The main objective behind this process is to increase the hardness of the material. This takes place by making the micro grain structure homogenous and improving the strength of the metal. This process makes the metal brittle. The tensile strength of the material is reduced heavily.