Hardening and Tempering of Plain Carbon SteelPlain carbon steel has been valued from early time because of certain properties. this soft silver-gray metal could be converted into a superhard substance that would cut glass and many other substances，including itself when soft. Furthermore，its hardness could be controlled. This converting of carbon steel into a steel of useful hardness is done with different heat treatments，two of the most important of which are hardening and tempering (drawing)，whats the plain carbon steel and which you will investigate in this lesson.Plain Carbon SteelIn simple terms，a plain carbon steel may be said to be an alloy of iron and carbon containing less than 1.7% carbon. In practice，however，these steels rarely contain more than about 1.4% carbon and other elements are also present，either as deliberate additions(e.g. manganese) or as impurities(e.g. sulphur and phosphorus).Plain carbon steels may be classified into three main groups:1. Low carbon steel (mild steel) containing less than 0.3% carbon.2. Medium carbon steel containing 0.3% to 0.6% carbon.3. High carbon steel containing 0.6% to 1.4% carbon.Low carbon steelLow carbon steels may be sub-divided into:1. Dead mild steel containing 0.05% to 0.10% carbonIn the manufacture of this material，complete deoxidation is not carried out and it is made as rimming steel. It is produced as hot and cold worked sheet，strip，rod，wire and tube，and is available in the hot-worked or process annealed condition for the deep drawing of sheet，but the strength is low. This type of steel is used for the manufacture of motorcar bodies，refrigerators，washing machines，office furniture，pressings，rivets and nails.2.Carburising steels containing 0.1% to 0.2% carbonThe strength and hardness of these steels is low，but a hard surface can be obtained by carburizing in order to combine a wear-resistant surface with tough core properties. The ductility makes rapid machining difficult unless the Mn and S content is increased by using a shlphur content of 0.2% to 0.3% with about 1% manganese.3.Constructional mild steels containing 0.2% to 0.3% carbonAs the carbon content is raised above 0.2%，the strength increases into the range required for constructional purposes，but the ductility decreases. The fabrication qualities (working and welding characteristics) of this material are very good. The steel is hot-rolled into plate for making boilers，ships and vehicles，and also into sections (e. g. girders，beams，joists) for use in bridges and buildings. The steel is often used in the hot-rolled condition，but the smaller sized material may be normalized.It has been estimated that about 90% (by weight) of all steel used is of this low carbon classification.Medium Carbon SteelsMedium carbon steels are often quenched-hardened and fully tempered to give good strength with maximum toughness, which are the properties required of components such as shafts, gears and connecting rods. Steels containing 0.4% to 0.5% carbon may also be surface hardened by localized heating and quenching.High Carbon SteelHigh carbon steel are quench hardened and lightly tempered to give high hardness with only limited toughness. Material in this classification is mainly used for making fairly small, relatively inexpensive cutting tools so that the group as a whole is known as carbon tool steel. Steels containing 0.6% to 0.8% carbon are used for making dies , springs , wire ropes and railway types . Many different kinds of tools are made form higher carbon steels as indicated below .Carbon% Type of tool0.81.0 Cold chisels, shear blades, punches, hammers1.01.2 Files, axes, saws, knives1.21.4 Razors, drills, wood-cutting and metal-cutting toolsLimitations of Plain Carbon SteelsPlain carbon steels have many limitations including the following:(1) If reasonable toughness and ductility are required, the maximum tensile strength obtainable is about 700N/.(2) Large sections cannot be effectively hardened, thus restricting their use to relatively thin sections.(3) Water quenching is necessary for full hardening with consequent risk of distortion and cracking.(4) Rapid softening above about 300 C limits their use for high-speed metal cutting.(5) Poor resistance to corrosion and to oxidation at elevated temperatures.To overcome these limitations, additional elements are added to the steel to give alloy steels with specific properties. The main alloying elements include manganese, nickel, chromium, molybdenum, tungsten, vanadium, cobalt and silicon.Hardening of Plain Carbon SteelAs steel is heated above the lowed critical temperature of 1330F(721C)，the carbon that was in the from of layers of iron carbide in pearlite begins to dissolve in the iron and from a solid solution called austenite. When this solution of iron and carbon is suddenly cooled or quenched，a new microstructure is formed. This is called martensite. Martens