How tempering is important for functional steel.
Metal alloys are made of a precise combination of elements, like ingredients in a recipe. The way these elements are put together under heat change the properties of the alloy, just as different cooking techniques change the flavor of foods.
Tempered steel changes the mechanical properties of the metal to make it stronger and more resistant. This makes it a good material for tools, springs, structural steel, and even swords.
Let’s take a look at the basics of tempered steel…and how steel with a temper is more flexible and giving than steel without a temper.
Heat treatments overview
Heat treating metals will alter its physical properties. It can increase its strength, ductility, toughness, hardness, and resistance to corrosion.
There are 3 common heat treatments:
- Annealing: The metal is heated to make it soft, then slowly cooled. Slow freezing of the microstructures makes for big, round grains of metal. This rids the metal of internal stress and make it more likely only dent or bend when it’s hit.
- Quenching: In this process, the metal is cooled quickly (often in a water or oil bath). This freezes the molecules quickly. Many small, jagged grains are created on the surface by the sudden reduction in temperature. The jagged edges of the grains interlace, making it the metal less likely to bend when hit: the surface is harder.
- Tempering: To reduce any excess hardness created through production or quenching, metal can be tempered by heating the metal to a specific temperature for a specific time depending on the properties you are trying to change.
Why temper steel?
Steel is tempered to give it the right material properties for its application. These can be:
- Reducing hardness while increasing toughness (a tough material resists chipping on impact, where a hard material resists indenting and will fracture before bending)
- Increased ductility (allowing it to change form without breaking)
- Increased resistance to wear and tear
- Increased machineability if the steel needs to be worked further
Tempered steel process
Before you temper steel, you will often quench the steel first, to harden it. Then the tempering temperature determines how much hardness you remove from the metal. The higher the temperature, the more hardness is removed. For example, hard tools are tempered at lower temperatures, while flexible springs are tempered at higher temperatures.
Steel is often heated in a gas, electrical resistance, or induction furnace with a vacuum or inert gas to prevent oxidation. Once the steel is heated to the specified temperature, you hold the temperature for a set amount of time depending on the type of steel and the mechanical properties you want to achieve.
What’s the difference between tempering steel and hardening?
Tempering steel and hardening steel give the same alloy different abilities.
Hardening steel makes it more rigid, and less likely to scratch or indent. However, this harder surface is more brittle. It won’t indent if it is hit, but if the impact force is too strong, it will fracture or chip. With tempering some hardness is lost for increased toughness. Toughness is the ability to withstand fracture or chipping, but the trade is that it’s more likely to scratch or indent.
Often you will harden the steel first, then temper the steel after to reach a specific hardness to toughness ratio.
Heat treatments and tempered steel in “modern” applications
Tempered steel is not just for knives and swords. It has real world applications in modern production. Tools are often tempered to be very very hard: quenching is part of the tool steel process for creating a hard working edge that withstands abrasion and indentation. Precision tools often need to keep this hard edge to stay within working tolerance. However, tempering may be necessary afterward for the integrity of the overall tool, to make it less brittle. Springs, structural steel, and other metal pieces that require specific material properties may also undergo heat treatment, either creating a consistent or differential temper depending on the material needs of the application.
Metal, as a crystal with a malleable microstructure, offers the materials scientist many ways to approach solving a problem with a savvy combination of alloy selection and heat treatment.
Metals/Materials
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