Mechinopedia For Mechanical Engineers

Free cutting steels (also free machining Steels) are extensively applied for machining parts subject to comparatively light loads (bolts, nuts, screws, etc.) and produced on automatic screw machines or multiple spindle automatics. These steels  are intended for uses where easy machining is the primary equipment.  The distinguishing features of free cutting steels i.e., high machinability and high quality surface finish after machining., are due to the higher Sulphur and Phosphorous content. Sulphur exists in the free cutting steel in the form of manganese sulphide which forms inclusions stretched out in the direction of rolling. These inclusions promote the formation of short brittle chips, reduce the friction on the surface being machined, and enable a satisfactory surface finish to be obtained at high cutting speed. Phosphorus dissolves in the ferrite (pure iron) and increases its brittleness. This also makes the chip more brittle and enables a smooth bright surface to be obtained i

  Bright steels are produced by various processes of cold working therefore, they may be said as cold - worked steels. They are made in very wide range of sizes, e . g., in rounds , squares , hexagons , flats and special sections. Bright steel bars are characterized by a clean, smooth finish of close dimensional tolerance. The advantage of cold working in bright steels is that the machinability of steels of the lower carbon grades is improved. The ferrite constituent which is the main one in low carbon steels is work hardened by cold working and this leads to crisper machining, less tendency to drag , better finish and smoother threading.  

The properties of steel are dependent primarily on the carbon content and to a large extent upon silicon, manganese, Sulphur and Phosphorus. Therefore, an understanding of the effect of each of the chemical elements on the properties of steel is necessary in the selection of steel for definite purposes. Carbon Increases strength, elasticity (determined by yield point) and hardness, and lowers the ductility (characterized by elongation) and Impact strength. An Important fact is that 0.3 per cent carbon , the steel can be hardened by heating and quenching in water or oil. Silicon in the finished steel usually ranges from 0.05 to 0.30 per cent. Silicon is put in carbon steels to prevent them from becoming porous. It acts as a very good deoxidizer and removes the gases and oxides, prevents blowholes, and thereby makes the steel tougher and harder.  Manganese also serves as a valuable deoxidizing and purifying agent. Manganese also combines with Sulphur and thereby decreases the harmful

  According to IS : 7598 - 1974 steels shall be classified as : 1. Unalloyed steels , commonly called plain carbon steels, and  2. Alloy Steels. UNALLOYED STEELS OR PLAIN CARBON STEELS : The principal factors affecting the properties of plain carbon steels are the carbon content and the microstructure. Carbon is the principal determinant of many performance properties. It has a strengthening and hardening effect. At the same time, it lowers ductility , as evidence by decrease in elongation and reduction of area,. In addition, a rise in carbon content lowers machinability and decreases weldability. The amount of carbon present also affects physical properties and corrosion resistance. With an increase in carbon content, thermal and electrical conductivity decline, magnetic permeability decreases drastically , and corrosion resistance is lowered.  The microstructure is determined by the composition of steel , i.e., carbon manganese, silicon, phosphorus and Sulphur which are always presen

 Combination methods of steel making known as duplex processes are carried out with two steel making units. The following combinations are usually done : 1. Basic and acid open - hearth furnaces . 2. A basic open - hearth furnace and a basic electric furnace. 3. A Bessemer converter and a basic open - hearth furnace.           

The principle of an induction furnace, resembles that of a transformer. It has a primary coil about which an alternating magnetic field is set up with magnetic lines of force of a definite density when an alternating current is passed through the coil. The magnetic field induces alternating eddy currents in the secondary circuits which comprises a crucible containing the metal charge. The eddy currents heat up and melt the metal.  An induction crucible furnace comprises a refractory crucible and a coil or inductor. The latter is made or copper tubing through which cooling water circulates and is arranged inside the refractory crucible. An insulating lining is provided between the coil and the crucible. The metal to be melted is charged into the crucible where it is melted down by the heavy secondary currents induced by the magnetic flux of the primary coil. The crucible can be tilted on horizontal trunnions to pour the molten metal.  Induction furnaces usually operate, on an alternatin

The direct arc furnace consists of a steel shell lined with refractory bricks and a removable roof through which carbon or graphite electrodes ( about 2m long ) pass. Graphite electrodes offer less resistance to current and are considered to be more durable at high temperature but they are more expensive than the carbon electrodes. The number of electrodes correspond to the number of phases. The electrodes are lowered into the furnace and the current is switched on. The heat generated by a powerful spark between the electrodes and the metallic charge on the hearth melts the charge. The charge usually consists of steel scrap and iron oxide in the form of iron oxide ore. Pig iron is not directly treated in electric furnaces, though sometimes it is partly purified in an open hearth furnace and then transferred to electric furnaces for final treatment and alloying. When the furnace is in operation, the electrodes are consumed and they become shorter. So a definite distance must be maintain

 Within recent years, the melting of steel by the electric furnaces have developed rapidly for the availability of cheap electric power. Electricity is used solely for the production of heat and does not impart any special properties to the steel. Nevertheless, The electric furnace has the following advantageous features. 1. It generates extremely high temperature, about 2000*C , in the melting chamber without introducing oxygen or nitrogen from the air or impurities from the fuel. This facilitates the removal of the harmful impurities such as oxygen, Sulphur, and Phosphorus , and also non metallic inclusions. 2. The temperature at all times may be easily controlled and regulated. 3. It permits the additions of expensive alloying elements such as chromium , nickel , tungsten , etc. without loss by oxidation.   4. A great variety of steels, differing on carbon content and with any content of alloying elements , can be manufactured.  On account of the above considerations, high grade all

 In The crucible process of  making steel, mixtures of wrought iron, steel scrap and ferro manganese are melted down with charcoal in the air tight crucible. Other ferro alloys may be added when alloy steel is produced by this process. In this crucible process, Carbon is added to the iron as the carbon content of the wrought iron is low. Necessary carbon is taken up by the metal from charcoal during melting. After the materials are melted and thoroughly alloyed, the crucibles are taken from the furnace known as regenerative furnace which is heated by gaseous fuel as in the open - hearth furnace , and finally the steel is poured into ingot mold. The length of time between placing the crucible in the furnace and withdrawing the crucible is approximately four hours. The process is primarily a melting and alloying process and no refining of the metal takes place in the crucible. 

 In the Open hearth process for producing steel, pig iron, steel scrap, and iron oxide in the form of iron ore or scale are melted in a Siemens - Martin open hearth furnace, so called because the molten metal lies in a comparative shallow pool on the furnace bottom or hearth. The hearth is surrounded by the roof and walls of refractory bricks. The charge is fed through a charging door and heated to 1600*C to 1650*C mainly by radiation of heat from the burning of gaseous fuels above it. It is not the amount the heat but rather the high temperature heat that is essential for the purpose.  In practice, a considerable quantity of steel scrap is previously charged and heated in the furnace and a partly purified molten iron known as blown metal from the Bessemer converter is added to this. Thus the impurities in the Pig Iron are "Diluted" and the refining process does not take so long period as if  the entire charge were pig iron. Very often more than 60 per cent of charge consist

 The latest development in steel making process is the L-D Process. The name of this process comes from the initials of two separate plants in Austria, at Linz and Donawitz. The local Austrian ore is two low in Phosphorous to enable the air-blown basic Bessemer method to be used. Since air , a mixture of nitrogen and oxygen , is used , the resulting steel contains nitrogen which makes the steel liable to brittleness under certain conditions. Further the bulk of nitrogen which is not dissolved carries away so much heat that only a metal at high phosphorous content will generate enough heat to give the required temperature of the liquid steel. The remedy has been to replace the air blast by oxygen or a gas mixture containing no nitrogen. The L-D process consists of blowing a jet of almost pure oxygen about 99.5 per cent at high pressure and travelling at supersonic speed through a water cooled lance on to the surface of molten iron held in a converter lined with basic refractories such a

 The Bessemer Process consists in blowing air through molten pig iron contained in a special furnace known as converter which shaped like a huge concrete mixer. The converter is made of steel plates lined inside with a refractory material. The type of refractory lining used depends upon the character of the steel - making process, i.e; upon the acid process or basic process.  In the acid process, the converter is lined with silica brick which is known in the refractory trade as "acid". The acid process does not eliminate phosphorus or Sulphur from the metal. In the basic process, the converter is lined with dolomite, which is known as "basic". It removes Phosphorus and to some extent Sulphur. Three clearly distinctive stages may be noted in the conversion of normal pig iron steel. These are : (1) The slag Formation or Blowing period ,  (2) The Brilliant flame Blowing Period , (3) The reddish Smoke Period.  The first stage commences as soon as the blast is put on u

Steel is the fundamentally an alloy of iron and carbon, with the carbon content varying up to 1.5 percent. The carbon is distributed throughout the mass of the metal, not as elemental or free carbon but as a compound (chemical combination) with Iron. If  however, the carbon is increased above 1.5 percent , a stage soon arrives when no more carbon can be combined state and any excess must be present as free carbon ( graphite ). It is at this stage that the metal merges into the group termed cast iron. Therefore, for a material to be classed as steel there must be no free carbon in its composition ; Immediately free graphite that occurs passes into the category of cast Iron. Besides carbon, there are other elements present in the steel , e. g ; Sulphur, silicon, Phosphorus , manganese, e t c ; but carbon is by for the most important modifying element . Iron forms the mass of the alloy: It is the quantity partner while on carbon falls the duty of determining the quality of the steel to me

The various elements that affect iron are : 1.) Carbon  2.) Silicon 3.) Sulphur  4.) Manganese 5.) Phosphorus  6.) Nickel  7.) Chromium 8.) Molybdenum 9.) Copper 10.) Vanadium 

Vanadium amounts from 0.1 to 0.50 percent. This is to increase strength, hardness, and machinability.

  Copper promotes formation of graphite. It usually presents in cast iron from 0.02 to 2.5 percent. 

Molybdenum ranges from 0.25 to 1.5 percent. This is added along or with other elements to improve tensile strength, hardness, and shock resistance of castings. The presence of molybdenum in cast iron produces uniformity. This improves toughness , fatigue strength , machinability , hardenability , and high temperature strength. 

Chromium acts as a carbide stabilizer in cast iron. Thus it intensifies chilling of cast Iron , Increases strength , hardness and wear - resistance and is conductive to fine grain structure. Most additions of chromium range from 0.15 to 0.90 percent with or without other alloying elements. Chromium of 1.0 percent or more make castings hard to machine. With 3.0 percent chromium, white cast iron is formed. Special Irons to resist corrosion and high temperatures contain as much as 35 percent chromium.

Nickel acts as a graphitizer but is only half as effective as silicon. Small amounts help to refine the sizes of grains and graphite flakes. Most additions are from 0.25 to 2.0 percent. Larger amounts from 14 to 38 percent are added in the grey Irons to resist heat and corrosion and have low expansivity. 

 In wrought Iron,  The presence of only a very small amount of phosphorus is very injurious ; only 0.1 percent is sufficient to make the iron cold-short, that is , the metal is brittle and liable to crack when cold, but may be malleable and easily worked at red heat.