Mechanical Properties of Steel

Be it on commercial or medical front, be it for science or culinary purposes, the one alloy that is indisputably used in making equipment to suit the purpose, is steel. Even your cutlery are made of stainless steel. Or let me put it this way, this alloy is just indispensable. But what does this alloy contain, after all? Steel comprises carbon (0.2% - 2.1%) and iron usually. However, other alloying elements like manganese, tungsten, chromium and vanadium are also used.

Mechanical Properties

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 of steel are listed, as under

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

The above listed properties help in determining how an alloy or a metal would behave under load.

Tensile strength
Tensile strength is a property that refers to the amount of stretching a material to check how much it can withstand. 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, also known as 40 KSI. Steel has a unique property where its compressive strength is almost equivalent to its tensile strength. It implies, that this alloy responds the same way to a crushing force as to a stretching force.

Hardness
This property of steel determines the resistance of the alloy to deformation, penetration, scratching, machining, yielding and abrasion. The common tests used for calculating steel hardness are Brinell, Vickers and Rockwell. All these tests have an indentor placed on the surface of the alloy to be tested. This indentor is further subjected to an intense load. Then, once the test is over, the indentation size is calculated and on basis of this procedure, hardness 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 done to assess the depth made by the penetration of the indentor in the steel surface. Using this parameter, one can also calculate other properties like elasticity, plasticity and tensile strength.

Elasticity
The ability of alloy to regain its normal shape after having subjected to external force for a while and then removed. It implies, solids with low elasticity either break or are deformed after the test. It is usually measured as Young's modulus and is calculated as Tensile stress/Tensile strain = (F/A)/(∆L/L₀), where F: force applied, A: cross sectional area of steel subjected to force, ∆L: change in the length of the steel object and L₀: The original length of steel object. The elasticity of steel (SAE 950) is 30 x 10⁶ psi.

Toughness
Toughness is a property of an alloyed metal to deform plastically and absorbing energy in the process. This property is further influenced by the rate of loading (strain), temperature and the notch effect (concerns the distribution of stress). This property is inversly proportional to rate of loading and directly proportional to the temperature change. For instance, steel with moderate levels of carbon, subjected to stress and strain at high temperature is comparatively tougher than steel containing low and high amounts of carbon.

Ductility and Malleability
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. Based on these properties, it can be decided whether the alloy is suitable for forging or rolling purposes.

Mechanical Properties of Stainless Steel

Stainless steel, like normal steel contains iron and steel, but has additional mix of chromium (11%). That makes it corrosion resistant. Now at room temperature, there are four types of stainless steel, based on the above 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 maximum stress developed on an alloy without causing any plastic deformation. This property can be derived from the relation of stress and strain. Other properties are same as the above mentioned mechanical properties of steel.

Steel finds a versatile use in our everyday life, pertaining to the suitable mechanical and physical properties of steel. Hope the mechanical properties of steel, helped you understand the reasons for its extensive application in industrial and commercial purposes.