Microstructure study of ferrous and non ferrous alloys under various compositions and heat treatment conditions

ABSTRACT

This experiment was carried out in order to make students understand the differences between ferrous and non ferrous alloys from metallurgical point of view. Student will also be able to understand the phase diagram of iron-carbon and non ferrous alloys system that enables for heat treating and procedures in heat treatment involved.Furthermore, students will also be able to describe the principle engineering properties and industrial application of ferrous and non ferrous alloys.

INTRODUCTION

The properties of metals can be changed or controlled by these different processes; strain hardening or cold – working, alloying process and heat treatment. All three processes are influenced by and dependent on the crystalline nature of metals. Engineering metals are commonly categorized into two main groups; ferrous and non ferrous.

Steels are essentially alloys of carbon containing up to 1.5% carbon. By varying the manner in which carbon steels are heated and cooled, different combinations of mechanical properties for steel can be obtained. Heat treatment process is a process of ability to change the properties by applying heat. Such treatment modifies microstructures, producing a variety of mechanical properties that are important in manufacturing, such as improve formability and machinability.

Copper and aluminium are categorized as non ferrous metal which have been used in engineering either as in its pure state or as an alloy. The applications of copper and aluminium have been very wide in the electrical conductors as well as in corrosive environment. Heat treatments of these metals have in many ways improved their properties for specific or specialized applications. The properties of copper and aluminium either in their pure state or as in an alloy can be improved by heat treatment. These changes in properties are the results in the microstructures in these materials through heat treatment. Thus microstructures transformation has influenced the properties of these materials.

THEORY

Ferrous Alloy

Ferrous alloys are which iron is the prime constituents that are produced in larger quantities that any other metal type. There are especially important as engineering construction materials. Their widespread use is accounted for by three factors:

1. Iron containing compounds exist in abundant quantities within the earth's crust.
2. Metallic iron and steel alloys may be produced using relatively economical extraction, refining, alloying and fabrication techniques.
3. Ferrous alloys are extremely versatile

In that they may be tailored to have a wide range of mechanical and physical properties. The principle disadvantage of many ferrous alloys is their susceptibility to corrosion. This section discusses compositions, microstructures and properties of a number of different classes of steels and cast irons.

Steels

Steels are iron-carbon alloys that may contain appreciable concentrations of other alloying elements. There are thousands of alloys that have different composition and heat treatments. The mechanical properties are sensitive to the content of carbon, which is normally less than 1.0 wt%. Some of the more common steels are classified according to carbon concentration which is:

1. Low-carbon type
2. Medium-carbon type
3. High carbon type

Subclasses also exist within each group according to the concentration of other alloying elements. Plain carbon steels contain only residual concentrations of impurities other than carbon and a little manganese. For alloy steels, more alloying elements are intentionally added in specific concentration.

• Low-Carbon Steels

Of all the different steels, those produced in the greatest quantities fall within the low-carbon classification. These generally contain less than about 0.25 wt% C and are unresponsive to heat. Microstructure consists of ferrite and pearlite constituents.

• Medium-Carbon Steels

The medium carbon steels have carbon concentration between about 0.25 wt% and 0.60 wt%. These alloys may be heat treated by austenitizing, quenching, and then tempering to improve their mechanical properties. They are most often utilized in the tempered condition, having microstructures of tempered martensite.

• High-Carbon Steels

The high carbon steels, normally having carbon contents between 0.60 wt% and 1.4 wt%, are the hardest, strongest and yet least ductile of the carbon steels. They are almost always used in a hardened and tempered condition and as such are especially wear resistant and capable of holding a sharp cutting edge.

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:

1. Martensitic
2. Ferritic
3. Austenitic

Cast Iron

Generally, cast irons are a class of ferrous alloys with carbon contents above 2.14 wt%. However, most cast irons contain between 3.0 wt% and 4.5 wt% C and in addition of other alloying elements. A re-examination of the iron-iron carbide phase diagram reveals that alloys within this composition range become completely liquid at temperatures between approximately 1150'C and 1300'C, which considerably lower than for steels. Thus, they are easily melted and amenable to casting. Furthermore, some cast irons are very brittle and casting is the most convenient fabrication technique.

Gray Iron

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. For most of these cast irons, the graphite exists in the form of flakes, which are normally surrounded by an α-ferrite or pearlite matrix.

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. Graphite still forms but as a nodules or sphere-like particles instead of flakes. The resulting alloy is called nodular or ductile iron.

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%.

Nonferrous Alloy

Steel and other ferrous alloys are consumed in exceedingly large quantities because they have such a wide range of mechanical properties, may be fabricated with relative ease and are economical to produce. However, they have some distinct limitations chiefly:

1. A relatively high density
2. A comparatively low electrical conductivity
3. An inherent susceptibility to corrosion in some common environment

Thus, for many applications it is advantageous or even necessary to utilize other alloys having more suitable property combinations. Alloy systems are classified either according to the base metal or according to some specific characteristic that a group of alloys share.

Copper and Its Alloy

Copper and copper-based alloys, possessing a desirable combination of physical properties, have been utilized in quite a variety of applications since antiquity. 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. The mechanical and corrosion-resistance properties of copper may be improved by alloying. Most copper alloys cannot be hardened or strengthened by heat-treating procedures; consequently, cold working and solid-solution alloying must be utilized to improved these mechanical properties.

Aluminium and Its Alloys

Aluminium and its 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 and Its Alloys

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 and Its Alloys

Titanium and its alloys are relatively new engineering materials that posses an extraordinary combination of properties. The pure metal has a relatively low density, a high melting point, and an elastic modulus of 107GPa. Titanium alloys are extremely strong.

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.

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)	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

DISCUSSION
Ferrous Alloy
Specimen 1 (X17)

Specimen 1 (X17) which is containing 0.8% carbon steel, rolled bar was heated for 1 hour at 800'C. It then undergoes annealed process which is furnace cooled to room temperature. Annealing is a term that often used to define heat treatment process that produces some softening of the structure. True annealing involves heating the steel to austenite and holding for some time to create stable structure. The structure is then cooled very slowly to room temperature. This will produces a very soft structure, but also creates very large grains, which are seldom desirable because of poor toughness.

When Specimen 1 (X17) undergoes annealing process, it will produced ferrite and pearlite. The white areas are a solid solution known as ferrite. The dark areas are actually a composite called pearlite.

Specimen 2 (X18)

Specimen 2 (X18) which is containing 0.8% carbon steel, rolled bar was heated for 1 hour at 800'C. It then undergoes normalized process which is cooled in still air. Normalizing is as term of returning the structure back to normal. The steel is heated until it just starts to form austenite, It is then cooled in air. This moderately rapid transformation creates relatively fine grains with uniform pearlite.

When Specimen 2 (X18) undergoes normalizing, it will produced fine pearlite with excess of ferrite or cementite. The resulting material is soft and the degree of softness depends on the actual ambient conditions of cooling. Normalizing is more commonly used than annealing, as it is considerably cheaper that full annealing since there is not the added cost of controlled furnace cooling.

Specimen 3 (X19)

Specimen 3 (X19) which is containing 0.35% carbon steel bar. It then undergoes spheroidizing which is furnace cooled from 870'C. According to the percentage of carbon steel, specimen 3 can be classified as medium-carbon steel. When this type of carbon steel undergoes spheroidizing, it will developed the spheroidite structure. Spheroidited steels have a maximum softness and ductility and easily machined or deformed. The carbon steels will produced ferrite, cementite and also bainite microstructure.

Specimen 4 (X20)

Specimen 4 (X20) which is containing 1.3% carbon steel bar. It the undergoes spheroidizing which is furnace cooled from 970%. According to the percentage of carbon steel, specimen 3 can be classified as high-carbon steel. During cooling process of this steel, from the austenite field, the first phase to form is cementite on the austenite grain boundaries. This partitions iron and at the eutectic composition pearlite is formed from the remaining enriched austenite.

Nonferrous Alloy
Specimen 5 (X12)
Specimen 5 (X12) which is containing 58% Copper (Cu) or 42% Zinc (Zn). It then reheated to 800^oC for 1 hour. The specimen is then was furnace cooled to 600^oC and then undergoes water quenched. Hardenability means the influenced for alloy composition on the ability of steel alloy to transform to martensite for particular quenching treatment. After quenching, specimen 5 will form microstructure of martensite

Specimen 6 (X13)

Specimen 6 (X13) which is containing 58% Copper and 42% Zinc. It then reheated to 800^oC for 1 hour. The specimen is then was cooled to room temperature. The specimen undergoes hardenability just like specimen 5. The microstructure of the grains of this specimen expands.

Specimen 7 (X14)

Specimen 7 (X14) which is containing Aluminium (Al) and 4% of Copper (Cu) alloy, sand cast. It then heated at 525^o for 16 hours. It is finally undergoes water quenched. This specimen undergoes age-hardening process. Age hardening was used to designate this precipitation hardening because the strength developed by time or as the alloy ages.

Specimen 8 (X15)

Specimen 8 (X15) which is containing Aluminium (Al) and 4% Copper (Cu) alloy, sand cast. It then heated at 525^oC for 16 hours. It then undergoes water quenched. Finally, the specimen was reheated at 260^oC for 70 hours. Just like specimen 7, specimen 8 also undergoes age-hardening process.

Heat Treatment
Annealing

Applies normally to softening by changing the microstructure and is a term used to describe the heating and cooling cycle of metals in the solid state. The term annealing usually implies relatively slow cooling in carbon and alloy steels. The more important purposes for which steel is annealed are as follow:

1. To remove stresses
2. To induce softness
3. To alter ductility, toughness or electric, magnetic or other physical and mechanical properties.
4. To change the crystalline structure
5. To produce definite microstructure

Normalizing

Normalizing is a heat treatment process for making material softer but does not produce the uniform material properties of annealing. A material can be normalized by heating it to a specific temperature and then letting the material cool to room temperature outside of the oven. This treatment refines the grain size and improves the uniformity of microstructure and properties of hot rolled steel. Normalizing is used in some plate mills, in the production of large forgings such as railroad wheels and axles, some bar products. This process is less expensive that annealing.

Spheroidizing

Spheroidizing is a process of heating and cooling to produce a spherodial or globular form of carbide in steel.

Hardening

Hardening involves heating steel to its normalizing temperature and cooling (quenching) rapidly in a suitable fluid.

Alloys
Ferrous Alloys

Iron alloys containing chromium, manganese, molybdenum, silicon, titanium, tungsten, vanadium and other elements in varying proportions. Ferrous alloys are added to steel during the manufacturing process to achieve the desired degree of corrosion resistance, tensile strength, yield strength and other qualities.

Nonferrous Alloys

Nonferrous alloys are alloys that are the byproducts of non ferrous metals such as aluminium, cobalt, lead, magnesium, titanium and zinc. By definition, a non ferrous alloy is an alloy that does not intentionally contain iron. In general, non ferrous alloys are invested with non metallic properties, have higher melting point and better strength. These properties make them a favoured choice for several commercial and non commercial uses, including automobile and aircraft parts, communication equipment, water valves, musical instruments and the manufacturing of flammables and explosive.

Microstructural
Austenite

Austenite is a solid solution of ferritic carbide or carbon in iron. It cools to form pearlite or martensite.

Ferrite

Ferrite is a solid solution of carbon in body-centered cubic iron. It is a constituent of carbon steels.

Cementite

Cementite is iron carbide and an orthorhombic crystal structure. It is hard, brittle material, essentially a ceramic in its pure form. It forms directly from the melt in the case of white cast iron. In carbon steel, it either forms from austenite during cooling or from martensite during tempering.

Pearlite

Pearlite is an iron alloy phase which is characterized by the formation of distinct bands of ferrite and cementite. This iron alloy phase contains around 88% ferrite and 12% cementite. It only forms under specialized conditions which must be controlled to create this alloy phase rather than another one. Pearlite is known for being tough, thanks to the way in which it forms, and may be used in a variety of applications.

Martensite

Martensite is a solid solution of carbon in alpha-iron that is formed when steel is cooled so rapidly that the change from austenite to pearlite is suppressed; responsible for hardness of quenched steel.

CONCLUSION

From the experiment that has been carried out, we are able to understand the differences between ferrous and non ferrous alloys from the metallurgical point of view. There are differences in the microstructure of the materials. We are also able to understand the phase diagram of iron-carbon and non ferrous systems that enables for heat treating and procedures in heat treatment involved. Besides that, we are also able to describe the principle engineering properties and industrial application of ferrous and non ferrous alloys.

REFERENCES

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