What You Should Know About Metal Hardening Processes and its Basic Objectives
The hardness of the metal is its ability to withstand impact forces. A hard metal will possess high resistance to deformation and other vital properties.
Objectives of Metal Hardening
The main objectives of hardening metals are:
To increase the hardness of the metal so that it can resist wear.
To enable it to cut other metals, that is, to make it suitable for cutting tools.
Metal Hardening Process and Reactions Involve
The process of hardening of the metal consists of heating up to a temperature of 30 degree-Celsius to 50 degree-Celsius above the upper critical temperature for the hypo-eutectoid steels and by the same temperature above the lower critical point for hyper-eutectoid steels.
The metal is held at this temperature for a considerable time depending upon the thickness and then quenched in a suitable cooling medium.
The hardness obtained from a given treatment depends upon the rate of cooling, the carbon content, and the work size. A very rapid cooling is necessary to harden low and medium carbon steels.
The quenching in a water or brine solution is a method of rapid cooling which is commonly used as the quenching medium because its action is not so severe to that of water. Certain alloy steels can be hardened by air cooling.
But for ordinary steels, such a cooling rate is too slow to give an appreciable hardening effect. Large parts are usually quenched in an oil bath.
The temperature of the quenched medium must be kept uniform so as to obtain uniform results. Any quenching bath used in production should be provided with some means for cooling.
A rapid cooling from the hardening temperature causes the austenite to be transformed into another constituent called martensite, which is very hard and brittle.
The hardening of steel depends entirely on the formation of martensite. It may be noted that the low carbon steels may not be hardened appreciably because of the presence of ferrite which is soft and cannot be hardened by heat treatment.
As the carbon content keeps increasing, the possibility of hardness also increases.
Four Ways To Carry Out Metal Hardening
There are four ways or processes of hardening a metal. They are:
Work hardening
Age hardening
Air hardening
Hardening by heating and quenching.
WORK HARDENING: it is a process of hardening a metal while working on it. This process can be done either consciously or unconsciously. Steady working on a steel material can cause hardness property of the steel to increase even without enough knowledge from the worker.
AGE HARDENING: this method is also known as precipitation hardening. In it, a metal is allowed to obtain hardness by allowing it to remain or age after heat treatment. It is mostly applicable to non-ferrous metals such as alloys of aluminum, magnesium, and nickel.
However, the effect of age hardening shows a partial increase in strength and hardness for duralumin.
The duralumin is an alloy of aluminum containing 4% copper and small quantities of other alloying elements. The process of age hardening consists of two steps namely: solution treatment and precipitation treatment.
In the solution treatment, the alloy is heated into the single-phase region, and held long enough to dissolve all the existing soluble particles, and is then rapidly quenched into the two-phase region. This produces a supersaturated solid solution.
In the precipitation treatment, the alloy is allowed to age at or above the room temperature for a specific time. This produces a very fine precipitates particle which increases the strength and hardness of the alloy.
AIR HARDENING: it is a process of hardening a metal when it is cooled slowly in air blast. The effect of air hardening is usually seen in high-speed steels and some of the tungsten alloys.
HARDENING BY HEATING & QUENCHING: it is the most common process of hardening just as the name indicates.
The process is generally employed for iron-based alloys having a low carbon content.
The simplest form of the process is to heat the metal to a certain temperature beyond its critical temperature so as to alter the crystal structure to soft and small size, leading to martensite steel, then throwing it inside water or oil to quench it. It is a well-known common way of hardening any iron-based metal.
Conclusion
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