Question1 Pick three of the stressstrain curves shown below

Question1

Pick three of the stress-strain curves shown below and describe the tensile properties of the corresponding ferrous alloys. Explain how the tensile properties of these alloys are related to their microstructures, which in turn depend on composition and heat treatment.

Stress o, MPa 1400 LA Spring steel 200 Tempered 1000 steel 800 Struct. Steel ine grain 600(ine gran 400 200I (f ) Cast ron Austenitic steel Struct. Steel 0 10 20 0 10 20 30 40 0 10 2C Strain ,% Rig. Examples of stress-strain curves of some metallic materials.

Solution

1.) Structural Steel: The stress strain curve is just like stress strain curve of medium carbon steel. Stress strain curve suggests that tensile yield strength of steel is 400 MPa and has more ductility than any other steel. This steel has microstructure containing mostly ferrite grains and some pearlitic grains. As ferrite is soft it is providing ductility and pearlite structure which is lamellar plates of ferrite and cementite providing strength to this steel. So, due to both ductility and strength this steel is tough and better for structures. And ferrite and pearlite grains can be produced in medium carbon steel (0.35% < Carbon <0.76%) from austenite (carbon = 2% at Temperature > 910 degree celcius) by heat treatment that is cooling it very slowly in furance that is known as Annealing.

2.) Structural steel (Fine grains): The stress strain curve is just like stress strain curve of medium carbon steel having fine grains. Stress strain curve suggests that tensile yield strength of steel is 600 MPa and has more ductility than any other steel but less than Annealed medium carbon steel. This steel has microstructure containing mostly fine ferrite grains and some fine pearlitic grains. As ferrite is soft it is providing ductility and pearlite structure which is lamellar plates of ferrite and cementite providing strength to this steel. So, due to both ductility and strength this steel also is tough and better for structures where we need high yield strength and less ductiltiy. In this steel yield strength is high and ductility is low due to fine grain structure because in fine grain structure grain boundaries are more and grain boundaries acts as obstacles in movement of dislocations and resistance to deformation increases which in turn decreases ductilty and increase stress values. And fine ferrite and fine pearlite grains can be produced in medium carbon steel (0.35% < Carbon < 0.76%) from austenite by heat treatment that is cooling it in air that is known as Normalizing.

3.) Tempered steel: The stress strain curve is just like stress strain curve of Tempered medium carbon steel. Stress strain curve suggests that tensile yield strength of tempered steel is 800 MPa and has less ductility than Annealed and normalized medium carbon steel. This steel has microstructure just like tempered martensite containing mostly needle like structure having body centered tetragonal structure and can have carbide precipitate due to high temperature tempering. . And tempered martensite can be produced in medium carbon steel (0.35% < Carbon < 0.76%) from austenite by heat treatment that is cooling it very fast in water not allowing the carbon to diffuse from autenite to cementite that is known as Quenching which produces martensite of body tetragonal structure by shear deformations producing large number of dislocations which is very hard and brittle and does not possess any industrial application. So after quenching this steel is heated again to certain temperature known as Tempering to relieve residual stresses in martensite and make it less brittle and machinable for industrial application. Therefore due to formation of fine needles, precipitates and residual stresses, this steel possesses High tensile strength, hardness and low ductility.