Metallurgical Principles in the Heat Treatment of Steels Nov04

Metallurgical Principles in the Heat Treatment of Steels...

Metallurgical Principles in the Heat Treatment of Steels Heat treatment of steels is carried out for achieving the desired changes in the metallurgical structure properties of the steels. By heat treatment, steels undergo intense changes in the properties. Normally very stable steel structures are obtained when steel is heated to the high temperature austenitic state and then slowly cooled under near equilibrium conditions. This type of heat treatment, normally known as annealing or normalizing, produces a structure which has a low level of the residual stresses locked within the steel, and the structures can be predicted from the Fe (iron)- C (carbon) equilibrium diagram. However, the properties which are mostly required in the steels are high strength and hardness and these are generally accompanied by high levels of residual stresses. These are due to the metastable structures produced by non-equilibrium cooling or quenching from the austenitic state. Crystal structure and phases The crystal structure of pure Fe in the solid state is known to exist in two allotropic states. From the ambient temperature and up to 910 deg C, Fe possesses a body centered cubic (bcc) lattice and is called alpha-Fe.  At 910 deg C, alpha-Fe crystals turn into gamma-Fe crystals possessing a face-centered cubic (fcc) lattice. The gamma crystals retain stability up to temperature of 1400 deg C.  Above this temperature they again acquire a bcc lattice which is known as delta crystals. The delta crystals differ from alpha crystals only in the temperature region of their existence. Fe has two lattice constants namely (i) 0.286 nm for bcc lattices (alpha-Fe, delta-Fe), and (ii) 0.364 nm for fcc lattices (gamma- Fe). At low temperatures, alpha-Fe shows strong ferromagnetic characteristic. This disappears when it is heated to around 770 deg C, since the lattice...

Corrosion of Cast Steels...

Corrosion of Cast Steels Cast steels are generally classified into the categories of (i) carbon (C) steels, (ii) low alloy steels, (iii) corrosion resistant steels, and (iv) heat resistant steels, depending on the alloy content and the planned usage. Steel castings are categorized as corrosion resistant if they are capable of sustained operation when exposed to attack by corrosive agents at operating temperatures which are generally below 300 deg C. The high alloy iron base compositions are generally given the name ‘stainless steels’, though this name is not recognized universally. Actually, these steels are widely referred to as cast stainless steels. Some of the high alloy steels (e.g. 12 % chromium steel) show many of the familiar physical characteristics of C steels and low alloy steels, and some of their mechanical properties, such as hardness and tensile strength (TS), can be altered by suitable heat treatment. The alloy steels of higher chromium (Cr) content (20 % to 30 % Cr), Cr-Ni (nickel)  steels and Ni-Cr steels do not show the changes in phase observed in ordinary C steel when heated or cooled in the range from room temperature to the melting point. Consequently, these steels are non hardenable, and their mechanical properties depend on the composition instead of heat treatment. The high alloy steels (stainless steels) differ from C steels and low alloy steels in other respects, such as their production and properties. Special attention is required to be given to each grade with regard to casting design and casting practice in the foundry. For example, such elements as Cr, Ni, C, N2 (nitrogen), Si (silicon), Mo (molybdenum), and Nb (niobium) can exert a deep impact on the ultimate structure of these complex steels. Hence, balancing of the alloy compositions is normally required to...

Carbon Steels and the Iron-Carbon Phase Diagram...

Carbon Steels and the Iron-Carbon Phase Diagram Steels are alloys having elements of iron (Fe) and carbon (C). C gets dissolved in Fe during the production of steels. Pure Fe melts at a temperature of 1540 deg C, and at this temperature, C readily dissolves into the liquid iron, generating a liquid solution. When this liquid solution solidifies, it generates a solid solution, in which the C atoms are dissolved into the solid iron. The individual C atoms lie in the holes between the Fe atoms of the crystalline grains of austenite (at high temperatures) or ferrite (at low temperatures). Austenite has a face centred cubic (fcc) structure while the ferrite has a body centred cubic (bcc) structure (Fig 1). If the amount of C dissolved in the liquid iron is kept below 2.1 %, the product is steel, but if it is above this value, then the product is cast iron. Although liquid iron can dissolve C at levels well above 2.1 % C, solid iron cannot. This leads to a different solid structure for cast irons (iron with total C greater 2.1 %). In addition to C, all the types of steels contain the element manganese (Mn) and low levels of the impurity atoms of phosphorus (P) and sulphur (S). Hence, steels can be considered as alloys of three or more elements. These elements are Fe, C, other element/elements additions, and impurities. It is normal to classify steel compositions into two categories namely (i) plain C steels, and (ii) alloy steels. In plain C steels, other elements consist only of Mn, P, and S, whereas in alloy steels, one or more additional alloying elements are added. Solid solutions are similar to the liquid solution; that is, after the solid substance is dissolved,...

Pure Iron

Pure Iron Iron is a chemical element with symbol Fe (from Latin word Ferrum). Its atomic number is 26 and atomic mass is 55.85. It has a melting point of 1538 deg C and boiling point of 2862 deg C. It is a metal in the first transition series. It is by mass the most common element on the earth, forming much of earth’s outer and inner core. It is the fourth most common element and the second most common metal in the earth crust. Steels contain over 95 % Fe. Pure iron is a common metal but it is mostly confused with other metals such as steel and wrought iron. All these metals vary in composition. The carbon content of pure iron makes it unique and different from the other metals and ferrous alloys. The carbon content in pure iron is always less than 0.008 %. Wrought iron has a higher carbon content of up to 0.5 %. This shows how less the impurities are in the pure iron. Pure iron is silvery white colored metal and is extremely lustrous. Its most important property is that it is very soft. It is easy to work and shape and it is just soft enough to cut through (with quite a bit of difficulty) using a knife. Pure iron can be hammered into sheets and drawn into wires. It conducts heat and electricity and is very easy to magnetize. Its other properties include easy corrosion in the presence of moist air and high temperatures. Pure iron has got valencies +2 and +3. Compounds of iron with valency +2 are known as ferrous compounds while the compounds of iron with valency +3 are known as ferric compounds. Metallurgy of pure iron The metallurgical nature of solid pure iron can be studied from...

Cast irons and their Classification...

Cast irons and their Classification  The term ‘cast iron’ represents a large family of ferrous alloys. Cast irons are multi-component ferrous alloys, which solidify with a eutectic. The major elements of cast irons are iron, carbon (2 % or more), silicon (1 % to 3 %), minor elements (less than 0.1 %), and often alloying elements (less than 0.1%). Cast iron has higher carbon and silicon contents than steel. The structure of cast iron displays a richer carbon phase than that of steel because of its higher carbon content. Cast iron can solidify according to the thermodynamically metastable Fe-Fe3C (iron carbide) system or the stable iron-graphite system depending principally on composition, cooling rate, and melt treatment. Cast iron in its basic form is a brittle material which has a very little impact strength. It has a little or practically no toughness when compared to low carbon steels.  It has a fraction of the tensile strength of low carbon steels.  When a cast iron piece fails it does not deform in a noticeable way and appears to snap apart or break in a manner consistent with a snap.  There is no early warning of a failure. The graphite phase which is pure carbon acts as a natural defect in the material.  The iron is so saturated with carbon that graphite forms (free carbon) and causes the cast iron to be weaker.  Much smaller amounts of carbon is combined with iron (Fe) in the form of iron carbide (Fe3C, cementite) which is hard and brittle. During the solidification process, when the metastable route is followed, the rich carbon phase in the eutectic is the iron carbide and when the stable solidification route is followed, the rich carbon phase is graphite. Referring only to the binary Fe-Fe3C or...