MICROSTRUCTURE STUDY OF FERROUS AND NON FERROUS ALLOYS UNDER VARIOUS COMPOSITIONS AND HEAT TREATMENT CONDITIONS
ABSTRACT This experiment in material science laboratory was carried out in order to teach the differences between ferrous and non ferrous alloys from metallurgical aspect. Student will be able understand the phase diagram of iron-carbon and non ferrous alloys system that enables for heat treating and procedures in heat treatment. Moreover, student will be able to describe the principle of engineering properties in material science and industrial application of ferrous and non ferrous alloys. OBJECTIVES Upon completion of this experiment, students should be able to; 1. Understand the differences between ferrous and non-ferrous alloys from the metallurgical point of view. 2. Understand the phase diagram of iron-carbon and non ferrous alloys systems that enables for heat treating and procedures in heat treatment involved 3. Describe the principal engineering properties and industrial application of ferrous and nonferrous alloys. 4. INTRODUCTION Metal have the certain properties that can be changed or controlled by different processes such as ; strain hardening or cold – working, alloying process and heat treatment. This process related with the crystalline nature of metals. Metallurgy is subdivided into ferrous metallurgy (sometimes also known as black metallurgy) and non-ferrous metallurgy or colored metallurgy. Ferrous metallurgy involves processes and alloys based on iron while non-ferrous metallurgy involves processes and alloys based on other metals. The production of ferrous metals accounts for 95 percent of world metal production. Steels are alloys of iron and other elements, primarily carbon, widely used in construction and other applications because of their high tensile strengths and low costs. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocations that otherwise occur in the crystal lattices of iron atoms. Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (grain size and composition) is one of the most effective factors that can determine the overall mechanical 1
behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling the rate of diffusion and the rate of cooling within the microstructure. Heat treating is often used to alter the mechanical properties of a metallic alloy, manipulating properties such as the hardness, strength, toughness, ductility, and elasticity. In metallurgy, a non-ferrous metal is a metal which is not ferrous, including alloys, that does not contain iron in appreciable amounts. Generally more expensive than ferrous metals, non-ferrous metals are used because of desirable properties such as low weight (aluminiums), higher conductivity (copper), non-magnetic property or resistance to corrosion (zinc). Some nonferrous materials are also used in the iron and steel industries. For example, bauxite is used as flux for blast furnaces, while others such as wolframite, pyrolusite and chromites are used in making ferrous alloys.
THEORY The characteristics of ferrous metals Ferrous metals include mild steel, carbon steel, stainless steel, cast iron, and wrought iron. These metals are primarily used for their tensile strength and durability, especially mild steel which helps hold up the tallest skyscrapers and the longest bridges in the world. Ferrous metals can also find in housing construction, industrial containers, large-scale piping, automobiles, rails for railroad and transportation, most of tools and hardware such the knives and other utensils at home. Due to the high amounts of carbon used when creating them, most ferrous metals and alloys are vulnerable to rust when exposed to the elements. Most ferrous metals also have magnetic properties, which makes them very useful in the creation of large motors and electrical appliances. Most importantly, ferrous metals make up the most recycled materials in the world. In 2008 alone, 1.3 billion tons of steel were produced, and 500 million tons of that was made from scrap materials. Steels The carbon content of steel is between 0.002% and 2.1% by weight for plain iron-carbon alloys. These values vary depending on alloying elements such as manganese, chromium, nickel, iron, tungsten, carbon and so on. Basically, steel is an iron-carbon alloy that does not undergo eutectic reaction. In contrast, cast iron does undergo eutectic reaction, suddenly solidifying into solid phases at exactly the same temperature. Too little carbon content leaves (pure) iron quite soft, ductile, and weak. Carbon contents higher than those of steel make an alloy, commonly called pig iron that is brittle (not malleable). While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium. Additional elements are also important in steel: phosphorus, sulfur, silicon, and traces of oxygen, nitrogen, and copper. 2
Alloys with a higher than 2.1% carbon content, depending on other element content and possibly on processing, are known as cast iron. Cast iron is not malleable even when hot, but it can be formed by casting as it has a lower melting point than steel and good cast ability properties. Certain compositions of cast iron, while retaining the economies of melting and casting, can be heat treated after casting to make malleable iron or ductile iron objects. Steel is also distinguishable from wrought iron (now largely obsolete), which may contain a small amount of carbon but large amounts of slag.
Low Carbon Steel – Composition of 0.05%-0.25% carbon and up to 0.4% manganese. Also known as mild steel, it is a low-cost material that is easy to shape. While not as hard as higher-carbon steels, carburizing can increase its surface hardness. Medium Carbon Steel – Composition of 0.29%-0.54% carbon, with 0.60%-1.65% manganese. Medium carbon steel is ductile and strong, with long-wearing properties. High Carbon Steel – Composition of 0.55%-0.95% carbon, with 0.30%-0.90% manganese. It is very strong and holds shape memory well, making it ideal for springs and wire. Very High Carbon Steel - Composition of 0.96%-2.1% carbon. Its high carbon content makes it an extremely strong material. Due to its brittleness, this grade requires special handling.
Stainless Steel The stainless steels are highly resistant to corrosion in a variety of environments, especially the ambient atmosphere. Their predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required. Corrosion resistance may be enhanced by nickel and molybdenum additions. Stainless steels are divided into three classes which is:
Martensitic Ferritic Austenitic
Cast Iron Cast iron is a group of iron-carbon alloys with carbon content greater than 2%. The alloy constituents affect its color when fractured: white cast iron has carbide impurities which allow cracks to pass straight through. Gray Iron 3
Gray iron is a hard brittle material with excellent damping characteristics and good mach inability. This is due to graphite flakes which precipitate into the iron during solidification. The carbon and silicon contents of gray cast irons vary between 2.5 wt% and 4.0 wt% and 1.0 wt% and 3.0 wt%, respectively. A great thermal conductor with great wear resistance, gray iron is the engineering alloy. Ductile Iron Adding a small amount of magnesium and cerium to the gray iron before casting produces a distinctly different microstructure and set of mechanical properties Ductile Iron is a unique engineering alloy that is similar to gray iron except that it is not brittle. The material is able to flex and has more fatigue resistance than gray iron. These properties are a result of creating graphite nodules instead of graphite flakes during solidification. Ductile iron boasts more strength and flexibility than gray iron while also featuring impact resistance.
White Iron and Malleable Iron For low-silicon cast irons which contains less than 1.0 wt% Si and rapid cooling rates, most of the carbon exists as cementite instead of graphite. A fracture surface of this alloy has a white appearance, and thus it is termed white cast iron. Generally, white iron is used as an intermediary in the production of yet another cast iron, malleable iron. Compacted Graphite Iron A relatively recent addition to the family of cast irons is compacted graphite iron. As a gray, ductile and malleable irons, carbon exists as graphite which formation is promoted by the presence of silicon. Silicon content ranges between 1.7 wt% and 3.0 wt%, whereas carbon concentration is normally between 3.1 wt% and 4.0 wt%. The characteristics of non-ferrous metals Non-ferrous metals include aluminum, brass, copper, nickel, tin, lead, and zinc, as well as precious metals like gold and silver. While non-ferrous metals can provide strength, they are primarily used where their differences from ferrous metals can provide an advantage. For instance, non-ferrous metals are much more malleable than ferrous metals. Non-ferrous metals are also much lighter, making them well-suited for use where strength is needed, but weight is a factor, such as in the aircraft or canning industries. Because they contain no iron, non-ferrous metals have a higher resistance to rust and corrosion, which is why you’ll find these materials in use for gutters, water pipes, roofing, and road signs. Finally, they are also nonmagnetic, which makes them perfect for use in small electronics and as electrical wiring.
4
As far as recycling goes, aluminum is the third most recycled material in the world. However, many other non-ferrous materials like copper, brass and lead are relatively scarce, and metallurgists rely heavily on scrap material recycling to make new ones.
Copper Alloys The similarity in external appearance of the various alloys, along with the different combinations of elements used when making each alloy, can lead to confusion when categorizing the different compositions. There are as many as 400 different copper and copper-alloy compositions loosely grouped into the categories: copper, high copper alloy, brasses, bronzes, copper nickels, copper– nickel–zinc (nickel silver), leaded copper, and special alloys. The following table lists the principal alloying element for four of the more common types used in modern industry, along with the name for each type. Unalloyed copper is so soft and ductile that it is difficult to machine. It also has an almost unlimited capacity to be cold work. Furthermore, it is highly resistant to corrosion in diverse environments including the ambient atmosphere, sea water and some industrial chemicals. . Most copper alloys cannot be hardened or strengthened by heat-treating procedures; consequently, cold working and solid-solution alloying must be utilized to improve these mechanical properties. Aluminium Alloys Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. Aluminium alloys are characterized by a relatively low density, high electrical and thermal conductivities and a resistance to corrosion in some common environments including the ambient atmosphere. Many of these alloys are easily formed by virtue of high ductility; this is evidenced by the thin aluminium foil sheet into which the relatively pure material may be rolled. Since aluminium has an FCC crystal structure, its ductility is retained even at very low temperature. The chief limitation of aluminium is its low melting temperature, which restricts the maximum temperature at which it can be used. Magnesium Alloys Magnesium alloys are mixtures of magnesium with other metals (called an alloy), often aluminum, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium is the lightest structural metal. Magnesium alloys have a hexagonal lattice structure, which affects the fundamental properties of these alloys. Plastic deformation of the hexagonal lattice is more 5
complicated than in cubic latticed metals like aluminium, copper and steel. Perhaps the most outstanding characteristic of magnesium is its density, 1.7g/cm3, which is the lowest of all the structural metals; therefore, its alloys are used where light weight is an important consideration. Magnesium has an HCP crystal structure, is relatively soft and has a low elastic modulus. At room temperature magnesium and it alloys are difficult to deform. In fact only small degrees of cold work may be imposed without annealing Titanium Alloys Titanium alloys are metals that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures). They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, medical devices, highly stressed components such as connecting rods on expensive sports cars and some premium sports equipment and consumer electronics. EXPERIMENTAL EQUIPMENT Optical microscope
FERROUS ALLOY SPECIMEN 1 (X17) – 0.8% carbon steel, rolled bar, heated for 1 hour at 800ᵒC, furnace cooled (annealed) to room temperature
SPECIMEN 2 (X18) – 0.8% carbon steel, rolled bar, heated for 1 hour at 800ᵒC cooled in still air (normalized)
SPECIMEN 3 (X19) 0.35% carbon steel bar, furnace cooled from 870ᵒC
SPECIMEN 4 (X20) 6
1.3% carbon steel bar, furnace cooled from 970ᵒC
NONFERROUS ALLOY
SPECIMEN 5 (X12) Cu 58% / Zn 42%, reheated to 800ᵒC for 1 hour, furnace cooled to 600ᵒC and then water quenched
SPECIMEN 6 (X13) Cu 58% / Zn 42%, reheated to 800ᵒC for 1 hour, furnace cooled to room temperature
SPECIMEN 7 (X14) Aluminium / 4% copper alloy, sand cast, heated at 525ᵒC for 16 hours and then water quenched
SPECIMEN 8 (X15) Aluminium / 4% copper alloy, sand cast, heated at 525ᵒC for 16 hours and then water quenched, reheated at 260oC for 70 hours
EXPERIMENTAL PROCEDURE Students were provided with 8 specimens, which have been heat treated under the following conditions. Students were required to observe the microstructure under the optical microscope and the data obtained were recorded.
7
