steel  Definition  Types and its properties.  Manufacturing of steel  Defects In steel  Heat treatment of steel  Market forms of steel

definition: Steel  steel is an alloy of iron with carbon.  Steel is a hard, strong grey or bluish grey ferrous material containing a maximum of 1.5% of carbon in its composition.

Types: Based on carbon content,steel is classified into:  low carbon steel – less than 0.15%  Mild steel – 0.15 to 0.3  medium carbon steel – 0.3 to0.8  High carbon steel – 0.8 to 1.5  Hard steel – more than 1%

Properties of steel: properties Mild steel Hard steel Melting point Specific gravity Compressi ve strength Shear strength Tensile strength 1400degree KN/cm KN/cm degrees kN/cm2 110KN/cm KN/Cm2

Properties of steel mild steel  tougher than wrought iron  It can be readily welded  It cannot be easily hardened  It has fibrous structure  It corrodes easily  Used for all structural work. hard steel  Tougher than mild steel  It cannot be welded  Ti can be hardened easily  It has granulose structure  It rusts easily  Used for finest cutlery edge tools.

Manufacturing of steel: bessemer process Open hearth process Crucible process Duplex process Electric smelting process Lintz and donawitz process Cementation process

Bessemer process: The mass production of steel from molten pig iron before the development of open hearth furnace. Principle: the removable of impurities from the iron by oxidation with air. process:  acidic process  Basic process

Basic bessemer process: it is suitable for pig iron containing Impurities. Bessemer converter is wide at bottom and narrow at top. It can be tilted by 2 horizontal trunioins. The air is allowed to pass from air duct to pig iron through tuyers.

Pig iron

Bessemer converter

Steps involved:  Through tuyers hot air is passed into converter.  The air passing through molten pig iron oxidixes the impurities within 15minutes  Molten pig iron from the blast furnace is charged into converter.  A reddish yellow flame is seen at the nose of the converter.  Required quantity of ferromanganese, spiege-leisen are added to make steel of desired properties.  Again air is blasted for few minutes, and molten metal is taken out in a container.  The molten metal is poured into large rectangular moulds for solidification and the material is called ingot.  The ingot futher treated to form steel of commercial pattern.

Inner view of bessemer converter

Open hearth process: A process of making steel from pig iron in a reverbatory furnace equipped with a regenerator. This process is also called as siemens martin process. `

Steps involved: The hearth is filled with molten pig iron from blast furnace. The mixture of coal gas and preheated air is allowed to pass over the hearth. The mixture oxidizes the impurities present in the molten metal. When impurities are removed suitable materials are added to make the steel of required property. The molten metal is formed as ignots and then treated futhur to form steel of commercial pattern.

Comparison Bessemer process  Time consuming for oxidation is 10to 20min  Low output(70to80%)  Operations are difficult  Scrap iron cannot be used  Superior quality cannot be made  Low capital investment  Steel is less homogeneous. Open hearth process  oxidation finishes within 10 minutes  High output(80to95%)  Simple operations  Scrap iron can be used  Superior quality can be made  High capital investment  Steel produced is more homogeneous.

3.Duplex process: combined process of bessmer and open hearth process. It takes advantages of both process and neglects disadvantages. Steps involved: o Molten pigiron is proccesed in bessemer process to eliminate impurities of Si,Mn,C. o Then heated in open hearth,in this S,P are eliminated.

4.Electric process: 5.Lintz and donawitz process: o modified process of bessemer process. o Oxygen is used instead of air. o Suitable for indian pig irons. 6.Crucible process: Used for production of small quantity of high carbon steel The fragments of blister steel is mixed with charcoal. The mixture is placed in crucible and molted. The molten liquid is pourede in suitabled moulds to get steel. the electricity is used to heat and melt the pig iron. The other process are same as open hearth and bessemer process.

Defects in steel: Checks Segregation Blow holes Cold shortness

1.checks:  Small cracks running traverse to the longitudinal axis.  Due to the presence of irons with high sulphur content.

2.segregation:  Separation of solid steels from main mass.  C,Si segregate that interior portion of steel remains white while exterior remains grey.

Blow holes:  Due to improper venting of mould.  Due to presence of sulphur.

Cold shortness: It means get brittle when it’s cold. It is the tendancy of brittleness to appear with a reduction in temperature.