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,...

Gray Iron

Gray Iron Gray iron (also called grey iron) is a type of cast iron that has a graphitic microstructure. It is named after the grey color of the fracture it forms, which is due to the presence of graphite. It is the most common cast iron and the most widely used cast material. Gray iron is one of the oldest cast ferrous products. In spite of competition from newer materials and their energetic promotion, gray iron is still used for those applications where its properties have proved it to be the most suitable material available. MacKenziein his 1944 Howe memorial lecture referred to gray cast iron as ‘steel plus graphite’. Although this simple definition still applies, the properties of gray iron are affected by the amount of graphite present as well as the shape, size, and distribution of the graphite flakes. Composition and effect of composition on properties  Gray iron is commercially produced over a wide range of compositions. The range of compositions which one may find in gray iron castings is given below. Carbon (C) – 2.75 % to 4.00 % Silicon (Si) – 0.75 % to 3.00 % Manganese (Mn) – 0.25 % to 1.50 % Sulfur (S) – 0.02 % to 0.20 % Phosphorus (P) – 0.02 % to 0.75 % One or more of the alloying elements namely molybdenum, copper, nickel, vanadium, titanium, tin, antimony, and chromium may be present in varying amounts. Nitrogen is generally present in the range of 20 to 92 ppm. Si is important for the gray iron since it is a graphite stabilizing element in cast iron, which means it helps the iron to produce graphite instead of iron carbides. Another factor affecting graphitization is the solidification rate. The slower is the rate, the greater is the tendency for graphite...

TMCP Steels

TMCP Steels TMCP stands for Thermo Mechanical Control Process and TMCP steels are those steels which are produced by this process. TMCP is a microstructural control technique combining controlled rolling (thermo mechanical rolling) and controlled cooling (accelerated cooling). These steels are sometimes microalloyed. Thermo mechanical control process is normally used to obtain excellent properties for steel plates such as high strength, excellent toughness along with excellent weldability through maximizing of grain refinement. These steels have almost same formability and weldability compared with mild steels. The superior mechanical properties introduced to the steel through this processing route are virtually equivalent to those obtained by heat treating conventionally rolled or forged steel and hence Thermo mechanical control process is used as a substitute for heat treatments that require additional material handling and furnace facilities. TMCP technology was developed early in 1980s and major Japanese plate rolling mills have started to produce TMCP steels by middle of 1980s. TMCP steels production When TMCP is chosen as the process route, the input material (i.e. the slab) is heated to a temperature regularly used for hot rolling operations (around 1200 deg C). The roughing operation during rolling in the rolling mill is carried out in a normal way, but the finish rolling is carried out at a lower temperature (around 750 deg C to 800 deg C) than the temperatures used in a normal rolling process. Plastic deformation at this lower temperature promotes fine grain sizes and retards precipitation. The final hot working may continue down to temperatures below the critical temperature of transformation from austenite to ferrite. This requires heavy rolling equipment capable of deforming the steel at low hot working temperatures. The optimum precipitate size and dispersion is obtained when the finish rolling temperature is around 775...