Question

How do heat treatments affect the hardness and impact properties (microstructure)?

Answer 1

Heat treatment is a combination of timed heating and cooling applied to a particular metal or alloy in the solid state in such ways as to produce certain microstructure and desired mechanical properties (hardness, toughness, yield strength, ultimate tensile strength, Young’s modulus, percentage elongation and percentage reduction). Annealing, normalising, hardening and tempering are the most important heat treatments often used to modify the microstructure and mechanical properties of engineering materials particularly steels.

The effect of heat treatment (annealing, normalising, hardening, and tempering) on the mechanical properties (ultimate tensile strength, hardness, toughness, percentage elongation, and percentage reduction) varies from one material to another material due to different microstructures and physical, alloying percentages and nature of metal used.

Tensile strength, yield strength and hardness will be increased with plastic deformation while ductility and impact strength decreased due to strain hardening effect. Normalization treatment results in higher tensile strength and hardness than annealing.

The alloying elements in steels affect both the thermodynamic and kinetic properties of the steels. Carbon is the main alloying element in steel, and is present for its strengthening effect. Manganese and silicon are common elements in modern steelmaking, and can in most steels be considered remnants of the production process. Nickel will generally enhance the properties of the steel, especially at low temperatures by increasing its toughness, as well as by stabilizing the oxide layer at the steel surface, making the steel less susceptible to corrosion. Molybdenum is also known for its ability to stabilize oxide layers. The hardness of the alloy will be decreased with increasing normalizing and tempering temperatures and times. The decrease in hardness was attributed to increase in the grain size, martensite lath size, and decrease in dislocation density and precipitate coarsening.

The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Ordinarily, annealing is carried out to (1) relieve stresses; (2) increase softness, ductility, and toughness; and/or (3) produce a specific microstructure. A variety of annealing heat treatments are possible; they are characterized by the changes that are induced, which many times are microstructural and are responsible for the alteration of the mechanical properties.