Study Notes - Power Engineering 4th Class - Basic Properties of Engineering Materials

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Engineering Materials

Unit 10Chapter 46

Learning Outcome:When you complete this chapter you will be able to…

Describe the mechanical properties of ferrous and non-ferrous engineering materials, plus the effects and purposes of various alloys.

Learning Objectives:Here is what you will be able to do when you complete each objective

1. Discuss the mechanical properties of materials.

2. Describe the various types of ferrous materials with respect to their mechanical properties.

3. Describe the various types of non-ferrous materials with respect to their mechanical properties.


Lecture Outline

1. Introduction2. Mechanical Properties of

MetalsI. Hardness

a. The Brinnel Testerb. The Rockwell Tester

II. BrittlenessIII. DuctilityIV. PlasticityV. ElasticityVI. MalleabilityVII.Toughness

3. Types of Engineering MaterialsI. IronII. Cast IronIII. Wrought IronIV. Steel

a. Carbon SteelsV. Alloy Steels

4. Nonferrous Metals

I. CopperII. AluminumIII.White Metal


IntroductionThe properties of a metal are the characteristics by which it can be accurately identified, or by which its range of usefulness can be determined. Power engineers are concerned with the applications and properties of two main groups of metals; ferrous and nonferrous.

The word ferrous is derived from Latin, meaning iron. Ferrous metals include pure iron, as well as alloys of iron. Alloys are a metals which are made by combining two or more elements (such as iron and nickel). Alloys are created to result in a metal which has greater strength, resistance to corrosion, or to achieve other properties not possible by a “pure” non-alloy metal.

Nonferrous metals contain no iron, or only trace amounts. Examples of nonferrous metals are copper, lead, aluminum, zinc, nickel, antimony, tin and magnesium Nonferrous metals can also be combined into alloys, such as brass (copper + zinc), and bronze (copper + tin)

Serviceability and safety are the ultimate criteria in choosing metals. Knowledge of types, their performance, and preservation are absolutely essential. By knowing to what extent each property exists in a metal, the metal can be used with the assurance that it will meet requirements for a specific application.

Mechanical Properties of MetalsHardness:

Hardness is the ability to resist wear, abrasion, cutting, and indentation. This may be a property which is uniform throughout the metal, or it may be a surface condition of a metal, such as when a metal is subjected to case-hardening. Case-hardening is a process which increases the hardness of the exterior face of a metal, while leaving its interior at its natural softness - this is often done to allow the metal object to absorb stresses. Increasing hardness throughout would also increase its brittleness.

The hardness of a metal is often measured using its resistance to indentation. This may be done by a hardened steel or diamond ball, cone or pyramid pressed into a metal sample to be tested. Another common test is to strike the sample with the indenter, and measure how much the indenter rebounds (a softer metal will experience a greater indentation, absorbing kinetic energy from the indenter, resulting in a smaller rebound).

Two common tests to measure hardness are the Brinnel, and the Rockwell tests.

Mechanical Properties of MetalsHardness:

The Brinnel test can be performed on materials with a more coarse structure than the Rockwell test is capable of, and is often used to test the hardness of large metal parts.

In the USA and Canada, Brinnel tests are typically performed on ferrous metals using 10 mm carbide balls as the indenter, with a standard force of 3,000 kg. Tests on softer, nonferrous metals are performed with a 5 or 10 mm carbide ball, with a pressure of 500 kg. In Europe, Brinnel tests are performed with a much wider range of ball sizes and pressures.

The Brinnel Test

Test Method:

The carbide ball (indenter) is pressed into the metal sample at the pressure of 3,000 kg (for ferrous metals) or 500 kg (for nonferrous) for a dwell time of 10-15 seconds.

A low-powered microscope is then used to measure the diameter of the indentation made, and then the surface area of the indentation is calculated from its diameter, or a chart is referenced. The hardness value is calculated with this formula:

Mechanical Properties of MetalsHardness: The Brinnel Test

In the Rockwell test, a steel ball, or diamond cone is placed on a metal sample under a minor load (typically 10 kg). The position of the indenter at this point is set as the zero, reference point. A major load (typically a total of 100 kg for a steel ball, or 150 kg for very hard materials being tested with a diamond cone) is then placed onto the indenter for a dwell time.

The difference in depth between the indenter while under the minor load to its depth under the major load is measured and used as it’s hardness value. Readings made with a steel ball are referred to as a Rockwell B measurement, whereas readings made with a diamond cone are Rockwell C measurements.

Mechanical Properties of MetalsHardness: The Rockwell Test

Mechanical Properties of MetalsBrittleness:

Brittleness is a property of a metal which permits no permanent deformationbefore breaking. Brittle materials generally break instantly, without any intermediate stage of bending (lack of plasticity). An example of a brittle material is cast iron.

The opposite of brittleness is ductility and plasticity.

Mechanical Properties of MetalsDuctility:

Ductility is a material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire.

Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized by the material's ability to form a thin sheet by hammering or rolling.

Both of these mechanical properties are aspects of plasticity.

Mechanical Properties of MetalsPlasticity:

A material is said to exhibit plasticity, or to be plastic, if it is very soft and easilydeformed. Examples of plastic materials include wax, lead, and babbitt. Plasticmaterials have very little elasticity; that is, they do not return to their original shape after the deforming force has been removed.

Materials with high plasticity are not brittle.

Mechanical Properties of MetalsElasticity:

Elasticity is the ability of a material to return to its original shape after any force acting on it has been removed. Elasticity is one of the most important properties for power engineers to be concerned with, as pipes, pressure vessels, and valves must reliably return to their original shape and position after being altered slightly from pressures or changes in heat.

Materials which are tough and ductile, such as wrought iron, typically have high elasticity.

Materials which are hard and brittle, such as cast iron, have very little elasticity.

Mechanical Properties of MetalsMalleability:

Malleability is that property which allows a material to be hammered or rolled into other sizes and shapes. Copper is an example of metal which is very malleable.

Brittle materials are not malleable,

The malleability of most materials will increase significantly when the material is heated, such as when iron or steel are heated before forging.

Mechanical Properties of MetalsToughness:

Toughness is the property that determines whether or not a material will break under a sudden impact. This property is also referred to as “impact strength”, and impact tests are used to determine the toughness of a material.

A common test used to determine toughness is the Izod Impact Test, in which a sample is notched, secured at one end, and a swinging pendulum is struck against the other end. The amount of joules expended by the pendulum swing necessary to break the sample is used to determine the material’s toughness.

Types of Ferrous Engineering Materials

Iron:

To produce iron, iron ore is placed in a blast furnace. Coke and limestone are added to the iron ore and the furnace’s contents are heated to melt the pure iron out of the ore.

Coke is a fuel (often made from coal) with very few impurities. It also produces carbon monoxide during combustion, which prevents much of the iron from forming into iron oxides.

Iron:

Limestone combines with the impurities in the iron ore to form slag, which floats on top of the molten iron, and is removed and discarded. The purified molten iron is then ladled off into molds to cool and form pig iron.

Pig iron is very brittle and hard, and not very machinable at cold or hot temperatures, and is virtually useless in any real world application. For this reason, pig iron is refined further into two classes of materials: steel and cast iron


Cast Iron:

Cast iron is produced by melting pig iron together with some scrap iron in a cupola furnace, which is similar to a small blast furnace. The resulting molten iron contains 2%-4% carbon.

If most of the carbon is combined chemically with the iron, performed by rapidly cooling it within molds, white cast iron is produced. White cast iron is very hard and brittle, and is used for machinery parts which are subjected to excessive wear, such as crusher jaws and grinding mill balls and liners.

White Cast Iron:


Cast Iron:

If the molten iron is cooled slowly, it causes the carbon to disassociate from the iron and form into graphite. This mechanically combined product (as opposed to being chemically combined as above), is known as grey cast iron.

Grey cast iron is softer, with good compressive strength, and is widely used for machinery bases and supports.

Grey Cast Iron


Cast Iron:

If the molten metal is annealed (a process of heating and cooling at a controlled rate), malleable cast iron is produced. This form of cast iron has increased toughness and ductility, and is used for some farming implements, automobile parts, pipe fittings and tools.

Malleable Cast Iron


Wrought Iron:

Wrought iron is produced by a process known as a puddling. By melting pig iron, and allowing the carbon and other impurities to oxidize while it melts, which is then removed, the remaining iron forms a plastic mass which is formed into a ball by use of a puddling bar.

The ball is then removed from the furnace and squeezed and rolled to remove most of the slag. The result is wrought iron.

Wrought iron is very ductile, and resistant to corrosion. At one time, it was used extensively for boiler tubes, piping and bolts, however steel has largely replaced wrought iron for these purposes.


Steel:

Steels are alloys of iron and carbon containing less than 2% carbon. If the carbon content is greater than 2%, then the alloy is cast iron. Steels are divided into plain carbon and alloy steels. Plain carbon steels are alloys of iron and carbon only.

Approximately 90% of the steel manufactured in North America is produced by the open hearth method, with most of the remainder produced by the electric furnace method.


Steel:

The open hearth furnace is charged with scrap iron, pig iron, and ore, and the heat is supplied by the combustion gas or oil with preheated air. The very hot gases pass from the furnace through a brickwork checker chamber at one end of the furnace, while the air for combustion enters through another brickwork checker chamber at the other end of the furnace. Every fifteen minutes, the gas and air flows are reversed with the air being heated as it passes through the checker chamber, which in the previous fifteen minutes had been exposed to the hot gases.

As the charge in the furnace is heated and melted, the carbon content is reduced to the proper point by oxidation, and in this way carbon steel is produced. If alloy steel is desired, then the required alloying materials are added to the molten carbon steel.


Steel:

Special high alloy steel is frequently produced in electric furnaces where the heat is furnished by electrical arcs.


Steel:

Carbon steels are grouped according to their carbon content

low, medium, high, and very high carbon.

• Hardness, strength and often brittleness increase with increasing carbon content.

• Impurities such as phosphorus or sulphur will lower the ductility, malleability, and welding qualities of a steel.

• High and very high carbon steels respond well to heat treatments.

• Most of these materials may, in the annealed state, be readily machined.

Carbon Steels


Steel

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to alter its mechanical properties.

Some of the alloying elements combine with the carbon to form compounds; other elements do not form compounds but remain in solution in the iron. "In solution" means that the alloy and carbon elements do not combine chemically with each other, but are held suspended as crystals in the ferrite.

The main advantages of alloy steels are the ability to respond to heat treatment, improved corrosion resistance, improved properties at high and low temperatures, and combination of high strength with good ductility.

Most alloy steels may be welded, provided the carbon content is within welding range. Generally, these steels require heating before, during, and after welding in order to avoid residual stresses.

Alloy Steels


Steel

An example of an alloy steel is Specification SA-335-P22 which is a chrome molybdenum steel used for high temperature steam piping. This material is suitable for severe service because of its high creep strength and resistance to oxidation and corrosion at high temperatures (above 500°C).

Creep is slow, permanent stretching of a material under stress, at high temperatures.

Alloy Steels


Steel

Some of the more important elements which are added to steel to produce alloy steel, with their effect on the properties of the steel, are the following:

• Nickel

Nickel is a tough, silvery element of about the same density as copper. It has excellent resistance to corrosion and oxidation even at high temperatures. It improves toughness, and prevents brittleness at low temperatures.

Nickel steels are especially suitable for the case hardening process for such applications as roller bearings and gears. These steels provide strong, tough cases that are resistant to wear and fatigue.

Alloy Steels


Steel


• Chromium

Chromium resists oxidation caused by hot gases, maintains high strength at elevated temperatures, and increases hardness and abrasion resistance. When chromium is present in amounts in excess of 4.0%, corrosion resistance is greatly increased. With a minimum of 12% chromium, the steel is called stainless steel.

• Molybdenum

Molybdenum increases hardness and endurance limits of steel, and decreases the tendency towards creep. It also increases the steel’s resistance to corrosion.

Alloy Steels


Steel


• Vanadium

Vanadium produces a fine grain structure during heat treatment, promotes hardening ability, and increases ductility.

• Copper

Copper readily combines with many other elements and improves the atmospheric corrosion resistance qualities of the steel.

• Lead

Lead improves machinability.

Alloy Steels


Steel


• Manganese

Manganese increases strength and hardness, promotes high impact strength, and offers excellent resistance to wear by abrasion.

• Tungsten

Tungsten produces a fine grain structure. The alloy retains hardness and strength at high temperatures.

Alloy Steels


Copper

• Obtained from copper ore which is smelted and then further refined by electrolysis.

• Commonly made into castings, wire, bars, sheets, plates, tubes, etc.

• The properties which make copper desirable as an engineering material are its:• high electrical conductivity• high heat conductivity• high corrosion resistance• high ductility and toughness.

In a power plant, it is used primarily for electrical equipment, and as an alloy in the materials used for heat exchanger tubes, valves, and fittings.

Types of Nonferrous Engineering Materials

Copper

Copper alloys are stronger, easier to machine, and have better corrosion resistance. The most commonly used copper alloys are various brasses and bronzes. Brasses and bronzes find use as condenser tubes, piping, valves, fittings, and bearing shells.

• Brass

An alloy of copper and zinc (up to 40%). Frequently, small amounts of other metals such as lead, tin, nickel, aluminum, and manganese are also included in the mixture.

• Bronze

An alloy of copper and tin, sometimes also containing zinc to ensure non-porous castings, and lead to improve machining qualities. Additions of up to 1% phosphorus produces bronzes, or bearing bronzes, which are hard but not abrasive. Bronze is approximately equal to pure copper in its resistance to corrosion.

Copper Alloys


Aluminum

Produced by electrolysis of bauxite ore. Being only 1/3 as heavy as an iron or steel, its low density is one of the most valuable properties of aluminum. It is also an important material because of its:

• Good conduction of electricy• Excellent conduction of heat• High resistance to corrosion

A disadvantage in its pure form is its low tensile strength. Aluminum is usually alloyed with other materials such as copper, silicon, manganese, zinc, nickel, magnesium, and chromium in order to improve its properties.

Aluminum alloys are used for internal combustion engine parts, aircraft parts, tubing, water jackets, etc.


White Metal

White metal is the name given to alloys made up primarily of lead and tin. It often contains small amounts of other elements, such as antimony, bismuth, silver, or zinc.

For a power engineer, white metal is chiefly found in bearing materials, because of the following properties:

• Easily melted and cast into the bearing shell• Sufficient strength and ductility not to crack or squeeze out under

heavy loads• Soft enough to wear to conform to the shape of the shaft• Good thermal conductivity to carry heat away from the bearing

surface

Different combinations of the alloying elements are used to produce white metals suitable for various applications. Babbit, a common tin-based white metal, is composed of 89% tin, 7.5% lead, and 3.5% copper, and is used for high speed and light load applications.

A lead-based white metal used for slower speeds and heavier loads is composed of 10% tin, 15% antimony, and 75% lead.


Study Notes - Power Engineering 4th Class - Basic Properties of Engineering Materials

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