Question

Explain why mild steel is used extensively in marine machinery outlining the properties influencing its choice.

State the stresses allowable in crankshafts, piston rods, boilers, and pressure vessels and discuss the type of failures that may occur.

Answer.

Mild steels are those which contain 0.05 to 0.30% carbon. There are five grades of mild steels that are used in ship construction and in marine machinery namely grades A,B,C,D & E. the major difference in the properties is the resistance to brittle fracture, with Grade E being the most resistant having a least carbon content of 0.18%. compared to Grade A having a maximum carbon content of 0.23%

Carbon increases strength and hardness and lowers the ductility and impact strength. In mild steel (low carbon steel), the carbon is mainly present in ferrite form, used mainly for all. structural purposes due to its softness, ductility and impact strength and good resistance against brittle fracture. Silicon in mild steel acts as a deoxidiser during the process and removes the gases, oxides, prevents blowholes and thereby making steel Rougher and harder. Impurities like sulphur phosphorus and aluminium which can all reduce ductility and impact strength are kept to a minimum (less than 0.05%). However, manganese increases strength and toughness and neutralizes the harmful effect of sulphur, and its content in GradeE steel is kept between. 0.7 to 1.5%.

The stress levels for different applications are as under :-

The specified minimum tensile strength of castings and forgings for crankshafts is to be selected with in the following general limits -A, carbon and carbon - manganese steel castings -400 to 550 n/mm2

B, carbon and carbon - manganese steel forgings - 400 to 600 n/mm2(normalized and tempered)

C, carbon and carbon - manganese steel forgings - not exceeding 700 n/mm2(quenched and tempered)

D, alloy steel castings - not exceeding 700 n/mm2

E, alloy steel forgings - not exceeding 1000 n/mm2

F, spheroidal or nodular graphite iron castings - 370to 800 n/mm2 Crankshafts is subjected to torsional stress, bending stress and direct shear. It should have also adequate fatigue strength due to alternating stresses, stress raisers and surface irregularities.

Piston rod is subjected to compressive stress and for carbon and carbon-manganese steel forgings, the specified stress is between 400 to 600n/mm2.

The specified minimum tensile strength for seamless class 1 and class 2/1 fusion welded boiler and pressure vessels are to be selected within 340 to 520 n/mm2 (carbon & carbon-mahganese steel). For class 2/2 and class 3 fusion welded pressure vessels - 340 to 430 n/mm2.

Boiler plate is subjected to-hoop stress due to internal pressure and thermal stress due to temperature variation.