Metallurgical Features of Steel

By Dr Thoguluva Raghavan Vijayaram PhD

Senior Lecturer
Faculty of Manufacturing Engineering, UTeM
Universiti Teknikal Malaysia Melaka
Ayer Keroh, 75450 Melaka Malaysia
Email: vijayaram1@gmail.com

Steel is an alloy of iron and carbon. According to the Fe-C Phase diagram, it contains carbon content not more than 2%. Steel containing carbon content less than 0.8 % is called as hypo eutectoid steel and if it lies between 0.8% to 2% often called as hyper eutectoid steel. Steel containing carbon percentage exactly of 0.8% is called as eutectoid steel. It is called as mild steel if it contains carbon content, equal to 0.2% and otherwise called as structural steel.

Plain carbon steels are divided into the following groups according to their carbon contents: low carbon steels with 0.10 to 0.25% Carbon, medium carbon steels with 0.25% to 0.55% Carbon and high carbon steels with 0.55 to 1.00 % Carbon. In steel, carbon is present in a combined form as Cementite, called as iron carbide or Pearlite, a mixture of ferrite and cementite. In addition to carbon, plain carbon steels contain the following other elements: Manganese up to 1.0 percent, sulphur up to 0.05 percent, phosphorus up to 0.04 percent, and silicon up to 0.30 percent. Plain carbon steels are the most important group of engineering alloys with wide range of properties make them of prime importance to use as key engineering materials. The applications of plain carbon steels are innumerable. Some of the major product forms of plain carbon steels are sheet, strip, bar, wire, tubular products, structural shapes, forgings, plate, and castings.

In order to produce the finished steel product of desired strength, it is sometimes necessary to heat treat the steel. It allows a certain degree of control on the structure and properties of the steel. Rimmed steel is hot rolled at as high a temperature as possible to produce a refined grain structure for subsequent cold rolling and annealing. Semi killed steels are those contain only a slight amount of blowholes, so that the volume contraction due to solidification can be compensated for. Fully killed steels evolve no gas and form a pipe cavity at the top of the ingot, since the addition of aluminium or silicon to the molten steel in the ladle or mold stops the gas reaction. Aluminium killed steels are widely used for cold rolled sheet that will be used for severe forming or deep drawing and also for sheet that will be stored for long periods before being used. These steels show minimum strain aging and have a fine grain size. The composition of the killed steels is more uniform than rimmed steels because there is no gas reaction.

After primary rolling, steel ingots will be in the shape of different steel products as slabs, blooms, billets, and sheets. Low carbon steel is used in large tonnages primarily for consumer products such as automobile body stock, tin plate, and sheet steel for porcelain enamelling. These mass produced materials, which are relatively low in cost, have special properties, such as ease of fabrication due to excellent formability and weldability, sufficient strength after fabrication, attractive appearance before and after fabrication, and compatibility with other materials and for various coatings. In order to produce low carbon sheet steel which meets some or all of the above requirements, the chemical composition, fabrication practices, and heat treatment procedures are varied as is necessary. It is produced from the ingots of rimmed, capped, semi killed, or killed steel. Steels are subjected to various heat treatment processes in order to enhance its mechanical properties.

Plain carbon steels are classified by several different systems, depending on the type of the steel and its application. There is thus no one classification system that applies to all plain carbon steels. The two most commonly use systems are the AISI-SAE system and ASTM classification. AISI-SAE system is applied to hot rolled and cold finished bars, wire, rod, and seamless tubing, and semi finished products for forging. Since the carbon content of plain carbon steels essentially determines their strength, this system uses the percent carbon to designate the different steels. In the ASTM system, standards are written for various alloys to meet special requirements. In addition to establishing chemical compositions, the ASTM standards also set mechanical property levels and often specify fabrication procedures and heat treatments. For example, plate steels, are mainly classified according to ASTM standards. Special standards are often set for special products. For example, many low carbon steel products such as tin plate and special automotive sheet are produced according to special specifications, and so there is no general numbering system for these steels.

Low carbon steels have increased strength and hardness and reduced cold formability compared to non heat treatable 0.06 to 0.10% Carbon. Medium carbon steels are usually strengthened by quenching and tempering because of their higher carbon content. These grades are normally produced as killed steels. Many parts of automobiles are made from medium carbon steels, such as engine parts, transmissions, suspensions, and steering. High carbon steels are more restricted in application than the medium carbon steels since they are more costly to make, and have poor formability and weldability.

Steel containing alloying elements are called as alloy steels. They have been developed which, although they cost more, are more economical for many uses. Further, it is classified as low alloy steel or high alloy steel depending on the alloying element percentage. In some applications, alloy steels are the only materials that are able to meet engineering requirements. The principal elements that are added to make alloy steels are nickel, chromium, molybdenum, manganese, silicon, and vanadium. Other elements sometimes added are cobalt, copper, and lead.

In recent years, Micro alloyed steels are produced for special applications. The micro alloying of plain carbon steels with small amounts, rarely exceeding about 0.1 % weight, of strong carbide and nitride forming elements such as niobium, titanium, and vanadium has achieved a great improvement in their mechanical properties. The addition of small amounts of niobium, titanium, and vanadium in conjunction with controlled rolling practices has produced low carbon, 0.05 to 0.1% Carbon, plain carbon steels at low cost with yield stresses of 50 to 80 ksi and good toughness qualities. Micro alloyed steels are strengthened by a combination of grain refinement, subgrain formation, and precipitation hardening.

Dual Phase Steels are a new class of high strength low alloy steels, HSLA, characterized by a microstructure consisting of a mixture of about 20 percent hard martensite particles in a soft, ductile ferrite matrix. The term “dual phase” refers to the existence of essentially two phases, ferrite and martensite, in the microstructure even though small amounts of bainite, pearlite, and retained austenite may also be present. Dual phase steels have relatively high tensile strengths, continuous yielding behaviour, low 0.2 percent offset yield strengths, and a higher total elongation than other high strength low alloy steels of similar strength. They are used in automobiles for applications requiring high strength and good formability such as bumpers and reinforcing posts.

Maraging steels are a class of high strength steels which are characterized by very low carbon contents and the use of substitutional elements to produce age hardening in iron-nickel martensites. The name maraging was coined from a combination of martensite and age hardening. It contains 18% nickel along with cobalt, molybdenum, titanium, and aluminium, often called as ultrahigh strength structural steels. The nominal yield strengths of these steels in the fully age hardened condition ranges from 200 to 350 ksi.

Hadfield’s manganese steel, was developed in 1882 and was one of the first high alloy steels. Low alloy chromium steels are produced to improve its hardenability, strength, and wear resistance. It is used to make ball and roller bearings. Molybdenum steels are extensively used for rear-axle gears and automatic transmission components.

Stainless steels are selected as engineering materials mainly because of their excellent corrosion resistance, which is principally due to their high chromium contents. On the basis of compositional and structural differences, wrought stainless steels are classified as Ferritic stainless steels, martensitic stainless steels, austenitic stainless steels, and precipitation hardening stainless steels. The duplex stainless steels are a separate class of steels intermediate between the ferritic and austenitic stainless steels. They are more resistant to stress corrosion but not quite as resistant as the ferritic steels and their toughness is superior to that of the ferritic steels. Thus for some engineering designs the duplex steels offer the optimum materials selection.

About the author
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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