Mechanical Properties of Steels...

Mechanical Properties of Steels The most important properties of steels which account for their widespread use are their mechanical properties. These properties include a combination of very high strength with the ability to bend rather than break. Different tests have been developed to describe the strength and ductility (a measure of bendability) of steels. A number of these tests which are used to describe the mechanical properties of steels are described below. Tensile testing Tensile testing of steel is a kind of a testing done for the evaluation of the strength of steels. A length of the steel material, usually a round cylindrical rod, is pulled apart in a machine that applies a known force, F. The machine has grips which are attached to the ends of the cylindrical steel rod, and the force is applied parallel to the axis of the rod, as shown schematically in Fig 1. As the force increases, the rod gets longer, and the change in length is represented as delta l (? l), where the symbol delta (?) means ‘a change in’ and the l refers to the original length of the rod. If a force of 50 kg is applied to two rods of the same steel material, where one is thin and the other thick then the thin rod elongate more. To compare their mechanical properties independent of rod diameter, the term ‘stress’ is used. Stress is simply the force divided by the cross-sectional area of the rod. When the same stress is applied to the thin and thick rods, they elongate the same amount, because the actual force applied to the thick rod is now larger than that applied to the thin rod by an amount proportional to its larger area. Because stress is force...

Silicon in Steels

Silicon in Steels  Silicon (Si) (atomic number 14 and atomic weight 28.09) has density of 2.34 gm/cc. Melting point of Si is 1412 deg C and boiling point is 2355 deg C. The phase diagram of the Fe-Si binary system is at Fig 1. Fig 1 Fe-Si phase diagram  Si is present in all the types of steels either as an intentional addition or as a residual from the ore, scrap or deoxidizing agents. Available forms  There are many Si containing addition agents which are used in steel making. Ferrosilicon (Fe-Si) and silico manganese (Si- Mn) are by far the most common addition agents. Fe-Si is a ferroalloy of iron (Fe) and Si. Fe – Si contains 65 % to 90 % of Si and minor amounts of Fe, aluminum (Al) and carbon (C).  Fe – Si is usually produced in four grades. These are standard grade, low Al grade, low C grade, and high purity grade having low content of titanium (Ti). The standard grade of Fe- Si contains Al up to 2 % while the low Al grade has Al content of 0.5 % maximum. It is produced by reduction of quartzite (SiO2) with coke in presence of iron ore.  Fe- Si is made in the submerged arc furnace. Si – Mn is a ferroalloy with high content of Mn and Si. It is produced by heating a mixture of oxides of MnO2, SiO2 and Fe2O3 with C in a furnace. These oxides undergo a thermal decomposition reaction. The standard grade contains Mn in the range of 62 % to 68 %, Si in the range of 12 % to 18 % and C in the range of around 2.0 %.  The low C grade of Si – Mn has a C...

Normalizing Process for Steels...

Normalizing Process for Steels Normalizing process for steels is defined as heating the steel to austenite phase and cooling it in the air. It is carried out by heating the steel approximately 50 deg C above the upper critical temperature (AC? for hypoeutectoid steels or Acm in case of hypereutectoid steels, Fig 1) followed by cooling in air to room temperature, or at no greater than 1 bar pressure using nitrogen if the process is being  run in a vacuum furnace. Normalizing temperatures usually vary from 810 deg C to 930 deg C. After reaching the soaking temperature the steel is held at that temperature for soaking. The soaking time depends on the thickness of the work piece and the steel composition. Higher temperatures and longer soaking times are required for alloy steels and larger cross sections. Fig 1 Typical normalizing temperature range for steels In normalizing, steel is uniformly heated to a temperature which causes complete transformation to austenite. Steel is held at this temperature for sufficient time for the formation of homogenous structure throughout its mass. It is then allowed to cool in still air in a uniform manner. Air cooling results into faster cooling rate when compared with the furnace cooling rate. Thus, the cooling time in normalizing is drastically reduced as compared to annealing. Soaking periods for normalizing are usually one hour per 25 mm of thickness of the work piece but not less than 2 hours at the soaking temperature. The mass of the work piece can have a significant influence on the cooling rate and thus on the resulting microstructure. Thin work pieces cool faster and hence are harder after normalizing than the thicker work pieces. This is different than in the case of annealing where the hardness...

Forging quality steels...

Forging quality steels Forging quality steels are those steels which are subjected to the process of forging during its subsequent processing for the production of end use products. The process of forging consists of converting the steel material into designed shape at a higher strain rate. Forging quality steels have the property of forgeability which is the relative ability of the steel to flow under compressive loading without fracturing. Except for resulphurized and rephosphorized grades, most carbon and low alloyed steels are usually considered to have good forgeability. Difference in forging behaviour among the various grades of steel is small enough and hence selection of steel for a forging is seldom affected by the forging behaviour Forging process The forging process can be of the following three types. Hot forging – In this process the forging operation is usually done at a temperature of around 1200 deg C. Warm forging – This process is carried out below the recrystallization temperature of steel normally at temperatures ranging from 650 deg C to 750 deg C. Cold forging – This process is performed at room temperature and the steel material is not heated. During the process of forging, since high strain rates are employed, the qualities needed in forging steels are critical and demanding. Further forging components demand specialized treatments necessary for imparting special properties based on the end application of the forgings. Also since the end use of the forged steel products is of critical nature, a close control over all the stages of steel manufacturing process is required. Selection of steel for forging Selection of a type of steel for a forged component is an integral part of the forging process and accepted performance of the steel after forging is dependent on this choice....