How does the shape of graphite in cast iron affect its prope

How does the shape of graphite in cast iron affect its properties? (make a list of possible shapes versus properties) List the advantages of each of the four types of heat-treating furnaces: batch, continuous, gas-fired, and electric. It was stated in this chapter that, in parts design, sharp corners should be avoided in order to reduce the tendency toward cracking during heat treatment. If it is essential for a part to have sharp corners for functional purposes, and it still requires heat treatment, what method would you recommend for manufacturing this part?

Solution

Solution:

(12) Graphite refers to the free carbon present in the cast iron microstructure. Based on the shape of the graphite inclusions in the microstructure, cast iron exhibits varied properties.

(a) No graphite: Cast iron without graphite is called white cast-iron. It is vary hard and brittle, highly wear resistamt (hence used in liners of mixers, ball mill, extrusion nozzles). It is non-ductile. non-malleable and difficult to machine.

(b) Graphite Flakes: Cast iron with flalky graphite is referred to as gray cast-iron. Flakes are usually sharp needle like graphite structures. They cause localized rise in stresses under loading, therefore gray cast iron has low tensile strength and lacks ductility but it has very high compressive strength. It is low on costs and very effective for damping of vibrations.

(c) Dark graphite rosettes: Such graphite shapes are seen in malleable cast iron. This provides cast iron with good machinability, good magnetic properties and wear resistance.

(d) Dark nodular graphite: Here cast iron has spheroidal nodule like graphite structures. This makes cast iron highly ductile, increased machinability, high corrosion resistance and creep resistance.

(13) (a) Batch heat furnace: Batch furnaces are designed to handle one work load at a time where the parts are heat-treated in groups or batches. This provides a very good balance between production rate, flexibility and equipment cost. They can be built of variable sizes based on the requirement with compact designs suitable for small labs to production plants. Temperature can be varied based on the treatment time. This flexibility gives high production performance and constant reproducable quality of heat treatment.

(b) Continuous furnaces: As the name suggets, such furnaces are used for continuous production of heat treated components in large quantities. For example, pipes, bars, tubes, plates, wires or any components in large numbers. All processes of heating, quenching or cooling are performed in the same furnace. Such furnaces are usually very large and are used in large production industries. It has rollers, conveyor belts or chains that act as automated feed devices enabling it to continuously feed components and move them through the heat treatment process. There are no production or cycle limitations and the length of such furnaces can be designed to suit the product to be heat treated. One of the major advantages of these furnaces is lower production cost per unit components that is heat treated. Everything is mostly automated. For large quantities to be treated in a given time, continuous furnace would take very less space as compared to batch furnace, as multiple batch furnaces would be required.

(c) Gas-fired furnace: These furnaces use gas as the source of energy. They are generally compact used for batch-type of heat treatment. The most important advantage of these furnaces is that they have high heating efficiency and the fuel (gas) is generally very cheap. Such furnaces can be installed anywhere that has gas supply. These furnaces have very little maintanence costs. The furnaces are are quiet and clean.

(d) Electric furnace: These furnaces use electricity powerd heating coils to product heat. They too are very compact and portable. Ideal for single part or batch type of heat treatment. They can be installed anywhere with very little space. It has low installations costs. It is comparitively safe as they is no risk of gas leaks. They too are quiet and clean in their operation. They cost less comparitively to other furnaces.

(14) Sharp corners are points where usually stress concentrations occur in any structural component. During heat treatment of such components, especially, liquid quenching, cracks can initiate at these places causing failure. If these corners cannot be avoided, it is recomended to atleast make sure that the largest possible fillet radii is used at these corners to relieve the stresses. If this fails to prevent the crack, instead of liquid quenching, air-quenching can be used. Some grades of steel are especially tolerant to sharp edge residual stresses when air-quenched. If this too is not possible then \'relief notches\' should be provided at the sharp corners to avoid cracking during heat-treatment.

Hope this helps. Do let me know if there are any questions.

Cheers!