Structure of Steel and Modifications in its Behavior due to Heating, Alloying and Hardening - Book Report/Review Example

Structure of Steel and modifications in its behavior due to heating, alloying and hardening Steel is one of the most extensively used metals in consumption for industrial usage as well as household consumption. It can be shaped in many forms and most heavily used in automobile industry. An automobile is usually consists of 60% of steel comprising of body frame, driveshaft, motor brackets, and door impact beams of vehicle. Steel is resistant to oxidization, commonly known as rusting, thus very important in shaping heavy machinery and automotives. However, it needs to be coated with zinc to resist oxidation. Steel does not only absorb energy in case of colliding with other heavy materials, but it also shows a resilient capability towards denting compared to other metals. Steel is also easier to be separated from other materials due to its magnetic ability but also cheaper than aluminum and other metals of same class. For a long time, Steel was recognized as alloy of carbon and iron; however, not every type of steel can be classified as an alloy. Interstitial-free steel and Type 409 ferritic stainless steel do not fall into this category and carbon is only in a very low quantity in these types, considering it as an impurity. Carbon proportion in these metals is as low as few parts per million. Steel must comprised of 50% iron and one or more alloys. The alloys can be carbon, nickel, silicon, manganese, vanadium, chromium, aluminum, niobium, and titanium. Each element, included with iron to make steel, has to play an important role in defining the steel like corrosion, resistance, hardness, magnetic permeability, strength and machinability. As the most of the steel types contain carbon, the effect of carbon on the mechanical structure of steel is vital. Iron is one of the most elements in modern day life as it goes through three stage transformation when temperature is changed, from ferrite to austenite, then austenite to ferrite again; and then ferrite to liquid iron. Each transformation undergoes a change in arrangement of iron atoms or of crystal structure in the crystal lattice that may make it harden, soften or strengthen it. Temperature affects steel properties as well or more specifically carbon and iron in it as they oxide simultaneously, when heated. The strength of steel is based on the number of carbides in it and it decreases as the temperature rises. Steel heat treatment can be of many types, depends on the type of attributes required. The most common form of steel heating is annealing, also known as stress relieving happens below the transformation temperature (Ac1). It is subsequently cooled down to reduce the internal residual stresses in steel. The only way to reduce residual stress is to reduce the yield strength and the tensile strength in steel, which can only happen by increasing temperature. The process of Annealing helps in softening the steel and helps in its attribute of machinability. It also becomes ductile, all internal stresses eliminated, will be good for cost-effective solution. Normalizing is a process of heat treatment, consisting of heating of steel 30–80˚C above the AC3 temperature, and then cooling it in normal air. Usually only high carbon steel requires normalization. The objective of normalizing is to get a uniformly consistent, fine-grained, and ferrite–pearlite structure. Normalized steel has more strength and ductility than annealed steel. Quenching is a process of heating high carbon steel on normalizing temperature and then cooled in brine, water or oil. The process will result in a martensitic structure that is a form of steel, possessing super-saturated carbon in a deformed Body Centered Cubic (BCC) crystalline structure, named as Body-Centered Tetragonal (BCT). The high crystalline structure helps in increasing internal stress, and makes the steel harder than normalized or annealed steel. Usually the high internal stress produces stress cracks on the surface of steel. Quenched steel is too harder for practical purposes and used for ceramics subject to corrosion like steering shaft. Fig. 1. TTT diagram of 0.6% low alloy carbon steel As per academic definition, Critical Cooling Rate is the lowest cooling rate which produces complete Martensite along with minimizing the distortions and internal stresses. The critical cooling rate in above TTT diagram is encircled with the red color. Fig. 2 TTT diagram of with the effect of various heat treatment In the above diagram, the four pointed arrows represent different types of heat treatments. 1) Annealing 2) Normalizing 3) Oil Quench 4) Water Quench As describes above, the first two methods of heat treatment require a large number of time. It requires so much time that it will become useless to find their critical cooling rate. However, in the process of quenching, it is useful and easier to find critical cooling rate. It can be seen that where the cooling lines intersect the level of start of martensite formation, these points are critical cooling rates. The effect of increasing alloy on the 0.6% low alloy carbon steel will result in shifting of both curves to the right side. The shifting of curves to the right side of the above TTT diagram means an increase in hardening of the steel. However, not every element will take part in increasing hardening of steel. Some elements like S, Ti and Co will not give the hardening effect, if use as alloy. Surface hardening is a process of hardening the surface of metal, often low carbon or low alloy steel, by infusing high carbide or nitride elements into the surface. The infused elements form a thin layer of harder alloy. The most commonly used method of surface hardening is Induction Hardening. In this method, electric current is passed through the steel, by alteration of magnetic field, to increase its temperature and then quenched. Quenching will form a layer of hard alloy on the surface of the steel, forming a martensitic structure, which will be harder than the core steel. This method is used for 0.3% to 0.6% carbon steel and widely used in automotive applications. Automotive parts like steering shaft need surface strength and power density at a very high level along with a low cost for large scale production. All the mechanical parts that transmit power into the vehicle including the steering shaft should be corrosion-resistant, durable and tough. Induction hardening is the best solution to achieve this objective. References: Cahn, R.W. and Cahn, R.W. and Haasen, P., 1996. Physical Metallurgy. Elsevier Science. Rosenhain, W., 1914. Metallurgy: an introduction to the study of physical metallurgy. D. Van Nostrand company. Read More