Steel Hardening by Quenching and Tempering...

 Steel Hardening by Quenching and Tempering Hardening is carried out by quenching steel, which consists of cooling it rapidly from a temperature above the transformation temperature (A?).  The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite (pearlite), and to cause a partial decomposition at such a low temperature to produce the new phase called martensite. To achieve this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements. In such case hardening of steel occurs with mild quenching. Martensite is a supersaturated metastable phase and has body centered tetragonal lattice (bct) instead of bcc. After steel is quenched, it is usually very hard and strong but brittle. Martensite looks needle like under microscope due to its fine lamellar structure. Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels. Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the work piece. Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidize and form sludge with consequent lowering of efficiency. The quenching velocity of oil is much less than water. To minimize distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated. Steel can be hardened by the simple expedient of heating to above the A? transformation temperature, holding long enough to insure the attainment of uniform temperature and solution of carbon in the austenite, and then cooling rapidly (quenching). Complete hardening depends...

Heat Resistant Steels...

Heat Resistant Steels The properties of steel and its yield strength considerably decrease as the steel absorbs heat when exposed to high temperatures. Heat resistance means that the steel is resistant to scaling at temperatures higher than 500 deg C.  Heat resistant steels are meant for use at temperatures higher than 500 deg C since they have got good strength at this temperature and are particularly resistant to short and long term exposure to hot gases and combustion products at temperature higher than 500 deg C. These steels are solid solution strengthened alloy steels. As these steels are used over a certain broad temperature ranges, these steels are usually strengthened by hard mechanism of heat treatment, solid solution and precipitation. All the heat resistant steels are composed of several alloying elements for the purpose of achieving the desired properties and are used in applications where resistance to increased temperatures is critical. The level of the heat resistance of the heat resistant steels depends on the environment conditions in which they operate and cannot be characterized by a single testing method. Maximum service temperatures which can be extended to 1150 deg C depending on the alloy content can be severely reduced by the presence of some compounds such as sulphurous compounds, water vapour or ash. Resistance to molten metal and slag is also limited in these steels. In heat resistant steels, the two most important elements are chromium for oxidation resistance and nickel for strength and ductility. Other elements are added to improve these high temperature properties. The effect of various alloying elements is described below. Chromium – Chromium is the one element which is present in all the heat resistant steels. Besides imparting oxidation resistance, chromium adds to high temperature strength and carburization resistance. Chromium...

The Iron-Carbon Phase Diagram Mar11

The Iron-Carbon Phase Diagram...

The Iron-Carbon Phase Diagram In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The study of the constitution and structure of iron and steel start with the iron-carbon phase diagram. It is also the basis understanding of the heat treatment of steels. The Iron Carbon diagram is shown in Fig. 1. Fig 1 Iron Carbon phase diagram The diagram shown in Fig 1 actually shows two diagrams i) the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop specially in the low temperature and low carbon range hence the metastable diagram is of more interest. Many of the basic features of this irpn carbon system also influence the behavior of alloy steels. For example, the phases available in the simple binary Fe-C system are also available in the alloy steels, but it is essential to examine the effects of the alloying elements on the formation and properties of these phases. The iron-carbon diagram provides a solid base on which to build the knowledge of both plain carbon and alloy steels. There are some important metallurgical phases and micro constituents in thr iron carbon system. At the low-carbon end is the ferrite (?-iron) and austenite (?-iron). Ferrite can at most dissolve 0.028 wt% C at 727 deg C and austenite (?-iron) can dissolve 2.11 wt% C at 1148 deg C. At the carbon-rich side there is cementite (Fe3C). Between the single-phase fields are found regions with mixtures of two phases, such as ferrite & cementite, austenite & cementite, and ferrite & austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid & austenite, liquid & cementite, and liquid...