Controlling Factors of Steel Properties
Steel properties are majorly influenced by its chemical composition and microstructure. Different properties of steel can be majorly classified under three types, i.e. Physical, Chemical and Mechanical. Here we will discuss the controlling factors influencing the steel properties.
Chemical Composition of Steel influences both chemical and mechanical properties of steel. Chemical composition has a major impact on microstructure. Microstructure of the steel can be changed as per requirement by appropriate treatment.
Steel strengthening processes influences the strength of steel by bringing about changes in microstructure. Different strengthening mechanisms are: solid solution strengthening, precipitation/dispersion strengthening, work hardening, grain refinement and heat treatment.
Among them, heat treatment has strongest influence on microstructure of steel. Examples of heat treatments are annealing for softening steel, normalising for grain refinement and adding strength, hardening and tempering to produce martensitic microstructures increasing its strength and toughness.
In case of structural steels, solid solution strengthening is generally used. Example of this is increasing strength by increasing carbon and manganese in low carbon steels. Any element whose atoms can take position in solid crystal lattice of iron can be used. Elements like C, N, Ti whose atoms are smaller than iron atoms form interstitial solution. Elements like Mn, Si, Cr whose atoms are of similar size as of iron atoms form substitutional solution. Strengthening by interstitial elements is more effective.
To select appropriate steel, key is to choose the right chemical composition which can be suitably provided heat treatment to get the appropriate microstructure.
Types of Properties
Physical Properties like Young’s modulus (E), Shear modulus (G), Bulk modulus (K), Poisson’s ratio (K) are majorly dependent on the metal. They do not change with composition or microstructure and are generally assumed to be constant for all steels
Chemical Properties like Corrosion resistance, Oxidation resistance, Weather resistance, etc. are dependent on the chemical composition and not on the microstructure. Example: elements like Cu, Cr and Ni are generally alloyed to make steel corrosion resistant.
Mechanical Properties are influenced by microstructure of the steel. As we know, microstructure is dependent on the chemical composition and heat treatment. Some of the mechanical properties are hardness, tensile strength, yield strength, toughness, elongation, wear resistance, ductility, etc.
Other than the above properties, there are other application related properties like fabrication property, welding property, etc. Fabrication properties is microstructure dependent and welding property is dependent on chemical composition (carbon equivalent CE factor to be precise).