How does heating the metal affects its properties?
Heat can affect the electrical, magnetic, and structural properties of metals. Since metal has a wide range of uses, various conditions emphasize different attributes. Toughness is required in industrial applications, whereas low electrical density is significant in electronic applications.
There are many methods of heating metal that are widely used to change these properties. To obtain the desired result, the temperature at which the metal is heated and the rate at which it cools is closely regulated.
The following are the most critical ways that metals are converted by heat:
Magnetism
Electrical Resistance
Thermal Expansion
Magnetism
Iron, nickel, and cobalt are the three metals that have magnetic properties. It is referred to as ferromagnetic metal. Heating these metals reduces their magnetism to the point that magnetism is no longer there. The Curie temperature is the temperature at which this happens. This temperature is 626 ° Fahrenheit for nickel, 2,012 degrees Fahrenheit for cobalt, and 1,418 ° Fahrenheit for iron.
Electrical Resistance
The electrical resistance of a metal is an indicator of how deeply it obstructs the flow of electrical current. Electrons scatter when they collide with the metallic structure as they flow through the metal. Electrons consume more energy and travel faster while the metal is heated. This causes further scattering, which raises the sum of the resistance. Thermometers typically calculate temperature by measuring the difference of electrical resistance in a piece of wire.
Thermal Expansion
When heated, metal expands. Temperature causes an increase in length, surface area, and thickness. Thermal expansion is the technical name for this. The degree of thermal expansion varies depending on the metal. Thermal expansion happens as a result of heat increasing the motions of the metal’s atoms. When building metallic structures, it is critical to account for thermal expansion. A simple example is the construction of household pipes, which must allow for expansion and contraction as the seasons shift.
Heat Treatment on metals
Heat treatment is a method of altering the characteristics of metal in order to make it more suitable for its desired applications. The following are the most common methods of heat treatment:
Annealing
Normalising
Hardening
Tempering
Annealing
Materials such as iron, steel, copper, brass, and silver are commonly softened by annealing. The procedure entails heating the material to a certain degree and then allow to cool slowly and steadily. Annealing changes the metal’s physical and chemical characteristics to make it more ductile and less rigid. This makes for easier carving, stamping, and formation methods, as well as easier cutting of the metal. Electrical conductivity is also improved by annealing.
Normalizing
Normalizing also known as normalization is a process used to achieve uniformity of grain size and composition in alloys. The metal is heated to a certain degree before being cool by air. The resulting metal is free of impurities and has increased strength and hardness. Normalizing is often used to manufacture harder and tougher steel, but it is less ductile than annealing. Since the procedure improves this attribute, the normalizing process is typically done on products that may be exposed to machining.
Hardening
Steel and other alloys are hardened to enhance their mechanical property. During hardening, the metal is heated to a high temperature and kept there until a proportion of the carbon has been melted. The metal is then put out, which means it is quickly cooled in oil or water. Hardening results in an alloy with high strength and wear-resistant. Hardening, on the other hand, makes it more brittle and is thus unsuitable for industrial application. Surface hardening is used where the surface of a part has to be hard enough to prevent wear and degradation while preserving ductility and resilience to withstand impact and shock loading.
Tempering
Tempering is used to increase the ductility of steel. Untempered steel is very strong, but it is too porous for the majority of practical applications. Tempering is a low-temperature heat treatment technique used to achieve a desired hardness/toughness ratio after hardening (neutral hardening, double hardening, ambient carburizing, carbonitriding, or induction hardening). To reduce some of the excess hardness, steel is heated to a lower temperature. After that, the metal is able to cool in still air, resulting in a harder and less brittle material.
It is not the heating that changes the properties — it is the cooling
This is the point almost every explanation skips. Heating a steel to 850 °C and cooling it slowly, or to the same 850 °C and quenching it, gives two materials with completely different hardness. The temperature decides what phase you start from; the cooling rate decides what you end up with.
Above the critical temperature (roughly 720–900 °C depending on the carbon content) the structure becomes austenite. How fast that austenite is cooled then determines which of four structures forms at room temperature — and each one has a different hardness, toughness and use.
What each cooling rate produces
| Cooling | Structure | Hardness | Character |
|---|---|---|---|
| Furnace, very slow | Coarse pearlite + ferrite | 150–200 HB | Softest, easiest to machine, lowest strength |
| Air | Fine pearlite | 200–280 HB | Normalised condition, good balance for structural work |
| Oil or polymer | Bainite or martensite | 35–55 HRC | The compromise: hard, less distortion than water |
| Water or brine | Martensite | 55–65 HRC | Hardest and most brittle; highest cracking risk |
The same steel, three different products. This is why a heat treatment specification that says only “harden and temper” is not a specification — the quench medium and the tempering temperature are the two variables that actually decide the result.
Temperature does a second job: grain size
Holding steel above the critical temperature also grows the austenite grains. Small grains give a tougher steel at the same hardness; coarse grains give a brittle one. That is why austenitising temperatures are held as low as the process allows, and why a part left in the furnace too long at too high a temperature comes out with a coarse structure that cannot be recovered by tempering.
Carry it far enough and the steel is “burnt”: the grain boundaries actually melt, oxidise and cannot be re-dissolved. Burnt steel is scrap. Overheated steel, where the grains are merely coarse, can usually be recovered by a normalising cycle.
Then tempering reverses part of it
Quenched martensite is too brittle to use as it is. Tempering reheats it to 150–650 °C, allowing carbon to precipitate and the structure to relax. Every 50 °C of tempering temperature trades hardness for toughness, which is why the tempering temperature — not the quench — is how a heat treater dials in the final property.
Why the same grade behaves differently in two workshops
Two heat treaters given the same 42CrMo4 and the same “28–32 HRC” instruction can produce parts that perform differently, because hardness is only one output. Grain size, retained austenite, decarburisation depth and residual stress all depend on the detail of the cycle — and none of them appear on a hardness test report.
You get chemical composition, mechanical properties and a price — with inspection certificates to EN 10204 3.1 issued against the heat number.
