Microstructure
The crystal structure introduced in the previous section describes the ideal arrangement of atoms within a solid material and, hence determines several intrinsic material properties. However, lattice imperfections, which destroy the infinite extension of the periodic atomic structure, are decisive for many extrinsic properties. As a result, a real structure containing crystalline areas and a variety of lattice imperfections is called microstructure. Lattice imperfections are created and can be controlled during the processing and manufacturing of materials. They can be classified throughtheir dimensionality as follows.
(Springer Handbook of Mechanical Engineering, Grote, Antonsson (Eds.),part B, p81)
Liquid carburizing
Liquid carburizing is carried out in a container filled with a molten salt, such as sodium cyanide. This bath is heated by electrical immersion elements or by a gas burner and stirring is done to ensure uniform temperature. This process gives a thin hardened layer up to 0.08 mm thickness. Parts which are to be case-hardened are dipped into liquid bath solution containing calcium cyanide and polymerized hydro-cyanide acid or sodium or potassium cyanide along-with some salt. Bath temperature is kept from 815°C to 900°C. The furnace is usually carbon steel case pot which may be by fired by oil, gas or electrically. If only selected portions of the components are to be carburized, then the remaining portions are covered by copper plating. There are some advantages of the liquid bath carburizing which are given as under.
(Introduction to Basic Manufacturing Processes and Workshop Technology,Rajender Singh,p146)
Pack Carburizing
Metals to be carburized such as low carbon steel is placed in cast iron or steel boxes containing a rich material in carbon like charcoal, crushed bones, potassium Ferro-cyanide or charred leather. Such boxes are made of heat resisting steel which are then closed and sealed with clay. Long parts to be carburized are kept vertical in -boxes. The boxes are heated to a temperature 900°C to 950°C according to type of steel for absorbing carbon on the outer surface. The carbon enters the on the metal to form a solid solution with iron and converts the outer surface into high carbon steel. Consequently pack hardened steel pieces have carbon content up to 0.85% in their outer case. After this treatment, the carburized parts are cooled in boxes. Only plane carbon steel is carburized in this process for hardening the outer skin and refining the structure of the core to make it soft and tough. Small gears are case hardened by this process for which they are enclosed in the cast iron or steel box containing a material rich in carbon, such as small piece of charcoal and then heat to a temperature slightly above the critical range. Depth of hardness from 0.8-1.6 mm is attained in three to four hours. The gears are then allowed to cool slowly with-in the box and then removed. The second stage consists of reheating the gears (so obtained) to about 900°C and then quenched in oil so that its structure is refined, brittleness removed and the core becomes soft and tough. The metal is then reheated to about 700°C and quenched in water so that outer surface of gear, which had been rendered soft during the preceding operation, is again hardened.
(Introduction to Basic Manufacturing Processes and Workshop Technology,Rajender Singh,p146)
Silicon steel
Silicon addition improves the electrical properties of steel. It also increases fatigue strength and ductility.
Applications
1. Steel with
Mn = 1 %, Si = 2% and C = 0.4 to 0.6%
has very high elastic limit and is used for springs.
2. Steel containing 5 to 7% silicon retains its hardness and resistance to oxidation at high temperature. It is used for making internal combustion engines.
3. Steel possessing 13% Si has a very high corrosion resistance and it can be used in chemical industrial applications.
4. Steel possessing 1% Si and up to 0.95% Mn is suitable for structural purposes.
(Introduction to Basic Manufacturing Processes and Workshop Technology,Rajender Singh,p68)
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