Engineering materials used in design, Lecture notes of Machine Design

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Prepared by: Dr. Gagandeep Bhardwaj, Assistant Professor, MED Email: gagandeep.med@thapar.edu Contact No. 8954388548

ENGINEERING MATERIALS

STRESS-STRAIN DIAGRAM

The following information can be obtained from a tension test: (i) Proportional limit; (ii) Elastic limit; (iii) Modulus of elasticity; (iv) Yield strength; (v) Ultimate tension strength; (vi) Modulus of resilience; (vii) Modulus of toughness; (viii) Percentage elongation; (ix) Percentage reduction in area The shape and dimensions of this specimen are standardised. They should conform to IS 1608.

MECHANICAL PROPERTIES OF

ENGINEERING MATERIALS

The mechanical properties of Engineering materials are: ( 1 ) Strength; ( 2 ) Elasticity; ( 3 ) Plasticity; ( 4 ) Stiffness; ( 5 ) Resilience; ( 6 ) Toughness; ( 7 ) Malleability; ( 8 ) Ductility; ( 9 ) Brittleness; ( 10 ) Hardness Strength is defined as the ability of the material to resist, without rupture, external forces causing various types of stresses. Resilience is defined as the ability of the material to absorb energy when deformed elastically and to release this energy when unloaded. Malleability is defined as the ability of a material to deform to a greater extent before the sign of crack, when it is subjected to compressive force.

CAST IRON

• Cast iron is a generic term, which refers to a family of materials that differ widely in their mechanical properties.
• Cast iron is an alloy of iron and carbon, containing more than 2 % of carbon.
• In addition to carbon, cast iron contains other elements like Silicon, Manganese, Sulphur and Phosphorus.
• There is a basic difference between steels and cast iron. Steels usually contain less than 1 % carbon while Cast iron normally contains 2 to 4 % carbon.
• Typical composition of ordinary cast iron is as follows: Silicon: It increases strength and hardness without lowering the ductility. Sulphur : It makes the cast iron hard and brittle. it should be kept well below 0. 1 % for most foundry purposes. Manganese : It makes the cast iron white and hard. It is often kept below 0. 75 %. Phosphorus : It aids fusibility and fluidity in cast iron, but induces brittleness.

DRAWBACKS OF CAST IRON

 It has a poor tensile strength compared to steel.  Cast iron parts are section-sensitive. Even with the same chemical composition, the tensile strength of a cast iron part decreases as the thickness of the section increases. This is due to the low cooling rate of thick sections.  For thin sections, the cooling rate is high, resulting in increased hardness and strength.  Cast iron does not offer any plastic deformation before failure and exhibits no yield point. The failure of cast iron parts is sudden and total. Cast iron parts are, therefore, not suitable for applications where permanent deformation is preferred over fracture.  Cast iron is brittle and has poor impact resistance.  The machinability of cast iron parts is poor compared to the parts made of steel.

MECHANICAL PROPERTIES OF CAST IRON

PLAIN CARBON STEEL

The designation of plain carbon steel consists of the following three quantities:  a figure indicating 100 times the average percentage of carbon;  a letter C;  a figure indicating 10 times the average percentage of manganese. As an example, 55C4 indicates a plain carbon steel with 0.55% carbon and 0.4% manganese. A steel with 0.35–0.45% carbon and 0.7–0.9% manganese is designated as 40C8.

FREE CUTTING STEELS

The designation of unalloyed free cutting steels consists of the following quantities:  A figure indicating 100 times the average percentage of carbon;  A letter C;  A figure indicating 10 times the average percentage of manganese;  A symbol ‘S’(Sulphur), ‘Se’ (Selenium), ‘Te’ (Tellurium) or ‘Pb’ (Lead) depending upon the element that is present and which makes the steel free cutting;  A figure indicating 100 times the average percentage of the above element that makes the steel free cutting. For example: 25 C 12 S 14 indicates a free cutting steel with 0. 25 % carbon, 1. 2 % manganese and 0. 14 % sulphur. Similarly, a free cutting steel with an average of 0. 20 % carbon, 1. 2 % manganese and 0. 15 % lead is designated as 20 C 12 Pb 15.

LOW AND MEDIUM ALLOY STEELS

An example, 25 Cr 4 Mo 2 is an alloy steel having average 0. 25 % of carbon, 1 % chromium and

1. 2 % molybdenum. Similarly, 40 Ni 8 Cr 8 V 2 is an alloy steel containing average 0. 4 % of carbon, 2 % nickel, 2 % chromium and 0. 2 % vanadium. Consider an alloy steel with the following composition: Carbon = 0. 12 – 0. 18 %; Silicon = 0. 15 – 0. 35 %; Manganese = 0. 40 – 0. 60 %; Chromium = 0. 50 – 0. 80 % The average percentage of carbon is 0. 15 %, which is denoted by the number ( 0. 15 x 100 ) or 15. The percentage content of silicon and manganese is negligible and, as such, they are deleted from the designation. The significant element is chromium and its average percentage is 0. 65. The multiplying factor for chromium is 4 and ( 0. 65 x 4 ) is 2. 6 , which is rounded to 3. Therefore, the complete designation of steel is 15 Cr 3.

LOW AND MEDIUM ALLOY STEELS

Consider a steel with the following chemical composition: Carbon = 0. 12 – 0. 20 %; Silicon = 0. 15 – 0. 35 %; Manganese = 0. 60 – 1. 00 %; Nickel =

1. 60 – 1. 00 %; Chromium = 0. 40 – 0. 80 % The average percentage of carbon is 0. 16 % and multiplying this value by 100 , the first figure in the designation of steel is 16. The average percentage of silicon and manganese is very small and, as such, the symbols Si and Mn are deleted. Average percentages of nickel and chromium are 0. 8 and 0. 6 , respectively, and the multiplying factor for both elements is 4. Therefore, nickel: 0. 8 x 4 = 3. 2 rounded to 3 or Ni 3 Chromium: 0. 6 x 4 = 2. 4 rounded to 2 or Cr 2. The complete designation of steel is 16 Ni 3 Cr 2.

PLAIN CARBON STEELS

Low Carbon Steel: Low carbon steel contains less than 0. 3 % carbon. It is popular as ‘ mild steel ’. Low carbon steels are soft and very ductile. However, due to low carbon content, they are unresponsive to heat treatment. Medium Carbon Steel: Medium carbon steel has a carbon content in the range of

1. 3 % to 0. 5 %. It is popular as machinery steel. Medium carbon steel is easily hardened by heat treatment. High Carbon Steel: High carbon steel contains more than 0. 5 % carbon. They are called hard steels or tool steels. High carbon steels respond readily to heat treatments. When heat treated, high carbon steels have very high strength combined with hardness. They do not have much ductility as compared with low and medium carbon steels.

MECHANICAL PROPERTIES of PLAIN CARBON STEELS

05/02/2019 Dr. Gagandeep Bhardwaj, AP MED, TIET 19

HEAT TREATMENT OF STEEL

The heat treatment process consists of controlled heating and cooling of components made of either plain carbon steel or alloy steel, for the purpose of changing their structure in order to obtain certain desirable properties like hardness, strength or ductility.

• Annealing: consists of heating the component to a temperature slightly above the critical temperature, followed by slow cooling. It reduces hardness and increases ductility.
• Normalising: is similar to annealing, except that the component is slowly cooled in air. It is used to remove the effects of the previous heat treatment processes.
• Quenching consists of heating the component to the critical temperature and cooling it rapidly in water or air. It increases hardness and wear resistance. However, during the process, the component becomes brittle and ductility is reduced.
• Tempering consists of reheating the quenched component to a temperature below the transformation range, followed by cooling at a desired rate. It restores the ductility and reduces the brittleness due to quenching.

MATERIAL PROPOERTY

CHART (ASHBY CHART)