In much the same way as a busy executive needs to unwind in a hot bath in order to successfully face another day, so work hardened metals need tempering to alleviate the internal stresses that forming, bending or rolling has created within. Tempering steel is a fascinating process, proven over time to change its internal structure and ensure a hard-wearing and usable metal.
What is Tempering?
Tempering is a heat-treating process used on iron-based alloys. The process allows the metal to be heated to a predefined temperature for a specific period of time (depending on the alloy itself and its desired use) and subsequently cooled. Metals tempered at a lower temperature are done so to promote hardness, while higher temperatures allow for more ductility.
The process of tempering changes the properties of the steel to improve formability, machinability and to reduce cracking or brittleness.
Why is Tempering Necessary?
Creating a hardwearing tool out of steel requires that it is strong and hard. However, when creating a spring, you would need the steel to be tough (higher resistance to fracture) with the ability to bend.
Tempering allows for these various requirements to be available from the same metal, albeit in slightly different alloys.
This piece on the specific meanings of the descriptions used in metallurgy help us to understand the various requirements of steel and why these properties are so important:
- Strength: also called rigidity, this is resistance to permanent deformationand tearing. Strength, in metallurgy, is still a rather vague term, so is usually divided into yield strength(strength beyond which deformation becomes permanent), tensile strength (the ultimate tearing strength), shear strength (resistance to transverse, or cutting forces), and compressive strength (resistance to elastic shortening under a load).
- Toughness: Resistance to fracture, as measured by the Charpy test. Toughness often increases as strength decreases, because a material that bends is less likely to break.
- Hardness: Hardness is often used to describe strength or rigidity but, in metallurgy, the term is usually used to describe a surface's resistance to scratching, abrasion, or indentation. In conventional metal alloys, there is a linear relation between indentation hardnessand tensile strength, which eases the measurement of the latter.
- Brittleness: Brittleness describes a material's tendency to break before bending or deforming either elastically or plastically. Brittleness increases with decreased toughness, but is greatly affected by internal stresses as well.
- Plasticity: The ability to mold, bend or deform in a manner that does not spontaneously return to its original shape. This is proportional to the ductilityor malleability of the substance.
- Elasticity: Also called flexibility, this is the ability to deform, bend, compress, or stretch and return to the original shape once the external stress is removed. Elasticity is inversely related to the Young's modulusof the material.
- Impact resistance: Usually synonymous with high-strength toughness, it is the ability to resist shock-loading with minimal deformation.
- Wear resistance: Usually synonymous with hardness, this is resistance to erosion, ablation, spalling, or galling.
- Structural integrity: The ability to withstand a maximum-rated load while resisting fracture, resisting fatigue, and producing a minimal amount of flexing or deflection, to provide a maximum service life.
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