Mechanical Properties of Steel

This alloy is just indispensable. Steel comprises carbon (0.2%-2.1%) and iron. However, other alloying elements like manganese, tungsten, chromium, and vanadium are also used enhance the properties of the steel.

The mechanical properties of steel decide its utility. Made from iron alloy, steel is found in three types - carbon steel, alloy steel, and stainless steel.

The physical properties are

• Tensile strength
• Hardness
• Toughness
• Elasticity and Plasticity
• Brittleness
• Malleability
• Ductility

These properties help in determining how an alloy or a metal would behave under load.

Tensile strength is a property that refers to the amount of stretching a material can withstand. It is the maximum amount of tensile stress a particular material can stand before breaking. It is measured by dividing the cross section of the steel by stress exerted on the area.

For steel, the measurement unit is pounds per square inch. It has a tensile strength of 40,000 pounds of force per square inch (PSI), also known as 40 KSI (K being a denominator for thousand pounds).

Apart from this steel also has a unique property where its compressive strength is almost equivalent to its tensile strength. It implies, that it responds the same way to a crushing force as to a stretching force.

This property of steel determines the resistance of the alloy to deformation, penetration, scratching, machining, and abrasion. The common tests used for calculating the hardness of this alloy are Brinell, Vickers, and Rockwell.

All these tests have an indenter placed on the surface of the alloy to be tested. This indenter is then subjected to an intense load and once the test is over, the indentation size is calculated. Based on this indentation the hardness of the steel is calculated.

For instance, Brinell hardness formula is:

P/[pi (d/2) {D-√(D² - d²)}],

where, P is the load, D is the diameter of the steel ball, d is the diameter of the indentation on the ball surface. Similarly, Vickers test calculates the hardness using the formula: 1.854P/L2, where, L is the diagonal length of the indentation and P is the load applied.

Rockwell hardness testing is not done for structured steel, and is basically performed to assess the depth made by the indenter on the steel surface. Using this parameter, one can also calculate other properties like elasticity, plasticity, and tensile strength.

The ability of an alloy to regain its normal shape after being subjected to an external force temporarily is called elasticity. It implies, solids with low elasticity either break or are deformed after the external force is applied.

It is usually measured as Young's modulus and is calculated as Tensile stress/Tensile strain = (F/A)/(∆L/L₀), where, F is the force applied, A is the cross-sectional area of steel subjected to force, ∆L is the change in the length of the steel object, and L₀ is the original length of steel object. The elasticity of steel (SAE 950) is 30 x 10⁶ psi.

This property is further influenced by the rate of loading (strain), temperature, and the notch effect (concerns the distribution of stress). This property is inversely proportional to rate of loading and directly proportional to the temperature change.

For instance, steel with moderate levels of carbon, when subjected to stress and strain at high temperature is comparatively tougher than steel containing low and high amounts of carbon.

Ductility and malleability are mechanical properties that determine the utility and application of steel. Ductility determines the extent to which the alloy can be deformed plastically (rather stretched) without fracture.

Malleability determines whether the alloy can be compressed to an extreme level without cracking. It defines the ability of a material to be beaten into thin sheets. Based on these properties, it can be decided whether the alloy is suitable for forging or rolling purposes.

Stainless steel, like normal steel contains iron and steel, but apart from this it also has chromium (11%). That makes it corrosion resistant. At room temperature, there are four types of stainless steel, based on the earlier-stated mechanical properties stated for steel.

• Martensitic
• Ferritic-martensitic
• Ferritic
• Ferritic-austenitic
• Austenitic

To assess the toughness of stainless steel, impact testing is done through the method of rapid loading and temperature variation. Austenitic steel has been found to have an excellent toughness at all temperatures.

Another interesting feature of this stainless steel is the yield strength, which indicates the maximum stress developed on an alloy without causing any plastic deformation. This property can be derived from the relation of stress and strain.