STEEL

        STEEL

1.Introduction:-

Steel is a fundamentally an alloy of iron and carbon, with carbon content up to 1.5%.   The carbon is distributed through out the mass of the metal as a compound with iron.  If the carbon % increases above 1.5%, a stage soon arrives when no more carbon can be contain in the combined state and any excess must be present as free-carbon.  It is at this stage that the metal merges into the group termed cast iron.

     

2  Classification of steels :-

Steels may be classified according to 

(a)   Carbon content

·         Low carbon steel      (0.008% to 0.3% Carbon)

·         Medium carbon steel (0.31% to 0.6% Carbon)

·         High carbon steel (0.61% to 2% Carbon)

(b)   Purpose and use

·                 Tool Steels

·                 Structural steels

·                 Boiler steels

·                 Electric steels

(c)   Method of manufacture

·         Open hearth steel

·         Bessemer steel

·         LD steel

(d)   Amount of Deoxidation

·         Rimmed steel

·         Killed steel

·         Semi-killed steel

(e)   Amount of Alloy content

·         Low alloy steel (Alloying elements <5 o:p="">

·         High alloy steel (Alloying elements >5%)

10.2    Plain carbon steels-Properties and Uses

1.Low Carbon Steel

Properties: - They are soft, ductile, malleable, tough, machinable, weldable but non hardenable, they are good for cold working and fabrication purpose.

Uses:-

Dead mild steel                                   used in making steel wires, sheets, rivets,

(C 0.05 to 0.15%)                                Screws, pipes, nails, and chain.                                              

Mild steel                                             used for making camshafts, sheets and strips, for

(C 0.15 to 0.20%)                                Fan blades, forging, etc.

           

Mild steel                                             used for making crankshafts, Connecting rods,

(C 0.20 to 0.30%)                                railway axles, fishplates, small forging etc.

2. Medium carbon steel    

Properties: - These steels have intermediate properties to those of low carbon steels and high carbon steels. They are medium hard, not so ductile and malleable, medium tough, slightly difficult to machine, weld and hardened. They are difficult to cold work and hence hot work. They are also called as “machinery steels “.                                                                  

Steels with                                           used for making gear shafts, shift and brake

C 0.30 to 0.70%                                  levers, wires and rods, small and medium  

 forging.

Steel with                                            used for making parts, which are to be subjected 
(C 0.45 to 0.55%)                                to Shock and heavy reversal of stresses such as   

                                                            railway coach axles, crank pins on heavy

                                                            machines, spline shafts, axles etc.

Steel with                                            used for drop forging dies, die block, clutch disks,

(C 0.45 to 0.70%)                                self taking screws, valve springs, thrust washers

                                                            etc.

3.High carbon steel

Properties: - They are hard, wear resistant, brittle, difficult to machine and weld but can be hardened. Hardenability is high. They cannot be cold worked and hence hot worked; they are also called as “Tool Steels”.

Steel with C 0.70 to 0.80%                 used in cold chisels, wrenches shear blades, hack

saws, pneumatic drill bit.

Steel with C 0.80 to 0.90%                 used in machine chisels, leaf springs, punch and

                                                            dies etc.

Steel with C 0.90 to 0.10%                 used in keys, shear blades, pins, speed disk, and

dies etc.

Steel with C 1. 02 to 1.10%              used railways springs, mandrels, machine tools,

 taps etc.

Steel with C 1.10 to 1.20%                 used in twist drills, knifes, thread metal dies, taps

 etc.

Steel with 1.20 to 1.30%                     Used in files, reamers, metal cutting tools etc.

Steel with  1.30 to 1.50%                    used in wire drawing dies, metal cutting saws etc.

10.3    ALLOY STEEL

         

                  Steel is considered to be alloy steel when the maximum range given for the alloying elements exceeds one are more of the following limits.

Mn 1.65%                                  Si 0.60%                           Cu 0.60%

Given below is the composition of typical alloy steel.

C 0.2  - 0.4%                                                   Mn 0.5   -1.0%

Si 0.3  - 0.6%                                                  Ni 0.4    - 0.7%

Cr 0.4  - 0.6%                                                  Mo0.15  - 0.3%

10.4 Advantages and Disadvantages of Alloy Steel

Advantages                                                     

1. Greater hardenability.                          
2. Less distortion and cracking.               
3. Greater ductility at high strength.                                                           
4. Greater stress relief for a given hardness.
5. Higher elastic ratio and endurance strength.
6. More strength, hardness and toughness at low and high temperatures.
7. Better resistance to wear and abrasion due to the presence of alloy carbides.
8. More uniform properties throughout the cross-section.
9. High resistance to tempering.
10. More resistance to corrosion and oxidation resistance.

Disadvantages

1. Cost.
2. Special handling.
3. Temper brittleness in certain grades.

Some of the prominent Alloy Steels 

1.         Silicon steel                                 7.        Chrome – Vanadium Steel

2.         Silicon Mn steel                           8.        Molybdenum Steel   

3.         Nickel Steel                                 9.        Chrome Molybdenum Steel                

4.         Chrome Nickel Steel                   10.       Chrome Steel   

5.         Tungsten Steel                            11.       Manganese Steel           

6.         Vanadium                                    12        Cobalt Steel