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Questions and Answers
What is the primary requirement for a substance to be classified as an alloy?
What is the main purpose of adding alloying elements to steels?
What is the typical range of carbon content in ferrous alloys classified as steels?
What is the primary difference between ferrous alloys and non-ferrous alloys?
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What is the effect of adding alloying elements to improve the toughness of a material?
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What is the purpose of alloying elements in improving the wear resistance of a material?
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What is the primary reason for the limited hardenability of medium carbon steels?
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Which type of steel is characterized by a high strength, hardness, and limited ductility?
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What is the primary role of carbon in the microstructure of steels?
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What is the minimum percentage of Cr required to make a steel highly corrosion resistant?
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Which type of stainless steel is strengthened and hardened by cold work because it is not heat treatable?
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What is the primary advantage of adding Cr, V, Mo, and W to high carbon steels?
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Which type of steel is commonly used in applications such as cutlery, razor blades, and surgical knives?
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What is the primary reason for the relatively low electrical and thermal conductivities of ferrous alloys?
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Study Notes
Alloy Theory
- An alloy is composed of two or more chemical elements, at least one of which must be a metal.
- Alloys are crystalline substances obtained by mixing with other metals and also metals with non-metals.
- Examples of alloys include iron and carbon for steels, and brass, which is a copper-zinc alloy.
Purpose of Alloying
- Alloying elements are added to steels for many purposes, including:
- Increasing hardenability
- Improving strength at ordinary temperature
- Improving mechanical properties at either high or low temperature
- Increasing wear resistance
- Increasing corrosion resistance
- Improving toughness at any minimum hardness or strength
- Improving magnetic properties
Types of Metal Alloys
- Ferrous Alloys
- Alloys in which iron is the major or principal component
- They are produced in large quantities than any other metallic materials
- In ferrous materials, the main alloying element is carbon (C)
- Non-ferrous Alloys
Ferrous Alloys
- Disadvantages of ferrous alloys include:
- Susceptibility to corrosion (environmental degradation)
- Relatively high density and comparatively low electrical and thermal conductivities
Steel
- Steels are alloys of iron and carbon plus other alloying elements
- In steels, carbon is present in atomic form, and occupies interstitial sites of the Fe microstructure
- Mechanical properties of steels are very sensitive to carbon content
Classification of Steels
- Low Carbon Steel
- Carbon present in these alloys is limited, and is not enough to strengthen these materials by heat treatment, except by cold work
- Their microstructure consists of ferrite and pearlite
- These alloys are thus relatively soft, ductile combined with high toughness
- Typical applications include: structural shapes, automobile body components, sheets used in buildings, bridges, and tin cans
- Medium Carbon Steel
- These are stronger than low carbon steels
- However, these are less ductile than low carbon steels
- As hardenability of these alloys is low, only thin sections can be heat treated
- Ni, Cr, and Mo alloying additions improve their hardenability
- Typical applications include: railway tracks & wheels, gears, other machine parts which may require good combination of strength and toughness
- High Carbon Steel
- These are the strongest and hardest of carbon steels, and of course their ductility is very limited
- These are heat treatable, and mostly used in hardened and tempered conditions
- They possess very high wear resistance
- Used for tool application such as knives, razors, hacksaw blades, etc.
- Addition of alloying element like Cr, V, Mo, W which forms hard carbides by reacting with carbon present, wear resistance of high carbon steels can be improved considerably
- Stainless Steel
- They are highly resistant to corrosion
- Steels are made highly corrosion resistant by addition of special alloying elements, especially a minimum of 12% Cr along with Ni and Mo
- Stainless steels are mainly three kinds: ferritic, austenitic, and martensitic steels
- Typical applications include: cutlery, razor blades, surgical knives, etc.
- Ferritic Stainless Steel
- They are principally Fe-Cr-C alloys with 12-14% Cr
- They are magnetic
- They are strengthened and hardened by cold work because they are not heat treatable
- Austenitic Stainless Steel
- They contain 18% Cr and 8% Ni in addition to other minor alloying elements
- They are the most corrosion resistant because of the high chromium content
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Description
Learn about alloys, their composition, properties, and purposes of alloying elements in engineering applications. Understand how alloys are formed and their uses.
