Answer to Question #287393 in Material Science Engineering for jay

Question

B.                 What is heat treatment?

C.                 Define and explain the processes of the following heat treatment process.

i.       Hardening ii. Tempering iii. Case hardening iv. Age hardening

D.                List the mechanical properties of the following materials and give TWO (2) engineering applications for each.

i.       Glass

Wood iii. Metal

Polymers

E.                 What is heat treatment and list TI--IREE (3) quenching media.

Answer 1

B. Heat treatment is the process of heating metal without letting it reach its molten, or melting, stage, and then cooling the metal in a controlled way to select desired mechanical properties. Heat treatment is used to either make metal stronger or more malleable, more resistant to abrasion or more ductile.

C. i. The main aim of the hardening process is to make steel hard tough. In this process, steel is heated 30° – 40°C above the upper critical temperature and then followed by continues cooling to room temperature by quenching in water or oil. 

ii. When the hardening process hardens a steel specimen, it becomes brittle and has high residual stress. It is an operation used to modify the properties of steel hardened by quenching for the purpose of increasing its usefulness.

Tempering or draw results in a reduction of brittleness and removal of internal strains caused during hardening. Steel must be tempered after the hardening process.

The tempering is divided into three categories according to the usefulness of steel required.

• Low-temperature tempering.
• Medium temperature tempering.
• High-temperature tempering.

The steel after being quenched in the hardening process is reheated to a temperature slightly above the temperature range at which it is to be used, but below the lower critical temperature. The temperature here varies from 100°C to 700°C.

The reheating is done in a bath of oil or molten lead or molten salt. The specimen is held in the bath for a period of time till attains the temperature evenly, the time depends on the composition and desired quality of steel. Now the specimen is removed from the bath and allow to cool slowly in still air.

iii. Case hardening is a material processing method that is used to increase the hardness of the outer surface of a metal. Case hardening results in a very thin layer of metal that is notably harder than the larger volume of metal underneath of the hardened layer. Case hardening almost always requires elevated temperatures to perform. Through heating, the hardening can be caused by altering the crystal structure of a metal or adding new elements to the composition of the exterior surface of a metal. Since hardening processes reduce formability and machinability, case hardening is typically done once most other fabrication processes have been completed.

iv. Age hardening is a heat-treatment process used to strengthen metal alloys. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours, or "aged," to allow precipitation to take place.

Age hardening creates changes in physical and mechanical properties by producing fine particles of a precipitate phase, which impede the movement of dislocations, or defects in a crystal's lattice. Dislocations serve to harden the material.

D. i.

Density

The density of glass is 2.5, which gives flat glass a mass of 2.5kg per m2 per mm of thickness, or 2500 kg per m3.

Compressive strength

The compressive strength of glass is extremely high: 1000 N/mm2 = 1000 MPa. This means that to shatter a 1cm cube of glass, it requires a load of some 10 tonnes.

Tensile strength

When glass is deflected, it has one face under compression and the other in tension. Whilst the resistance of glass to compressive stress is extremely high, its resistance to tensile stress is significantly lower.

The resistance to breakage on deflection is in the order of:

- 40 MPa (N/mm²) for annealed glass

- 120 to 200 MPa for toughened glass (depending on thickness, edgework, holes, notches etc).

The increased strength of SGG SECURIT toughened glass is the result of the toughening process putting both faces under high compression.

Elasticity

Glass is a perfectly elastic material: it does not exhibit permanent deformation, until breakage. However it is fragile, and will break without warning if subjected to excessive stress.

Young’s modulus, E

This modulus expresses the tensile force that would theoretically have to be applied to a glass sample to stretch it by an amount equal to its original length.

It is expressed as a force per unit area.For glass, in accordance with European standards :

E = 7 x 1010 Pa = 70 GPa

Besides the window-type applications, glass fibers are uses in insulation, sound deadening, as fillers in plastics, and as reinforcement in plastic laminates and structural shapes.

ii.

The mechanical properties or strength, properties of wood are measures of its ability to resist applied forces that might tend to change its shape and size. Resistance to such forces depends on their magnitude and manner of application and to various characteristics of the wood such as moisture content and density. It is important to note that wood has drastically different strength properties parallel to the grain (i.e., in the axial direction) than it does across the grain (in the transverse direction).

The mechanical properties of wood include strength in tension and compression (as measured in axial and transverse directions), shear, cleavage, hardness, static bending, and shock (impact bending and toughness). Respective tests determine stresses per unit of loaded area (at the elastic limit and maximum load) and other criteria of strength, such as the modulus of elasticity (a criterion of stiffness), the modulus of rupture (bending strength), and toughness. Tests are normally conducted with small, clear specimens, usually 2 × 2 cm or 2 × 2 inches in cross section. Laboratory data are analyzed to produce working values of stresses, which are made available for use by engineers and architects in designing wooden structures. Tests are sometimes conducted with structural components of actual size. Individual cells (tracheids and fibres) also are subject to testing, since their strength relates to the strength of products—paper, for example. (The testing of materials to ascertain their mechanical, thermal, electrical, and other properties is discussed in the article materials testing.)

Density is the best index of the strength of clear wood; higher density indicates greater strength. The strength of wood is also influenced by its moisture content when it fluctuates below the fibre saturation point. Generally, a decrease in moisture content is accompanied by an increase in most strength properties. Temperature and duration of loading also affect strength. In general, strength falls as temperature rises. Wood loaded permanently will support a smaller maximum load than that indicated by short-term laboratory tests. The most important strength-reducing factors are wood defects, such as knots, compression and tension wood, and grain deviations. Their adverse effect depends on the kind and extent of the defects, their position, and the manner in which the wood is loaded.

Defects constitute the basis for rules by which lumber and other wood products are visually graded. These rules set limits on sizes of defects and other wood characteristics that affect strength—for example, rate of growth, which is expressed as rings per centimetre or inch. Also available are nondestructive grading techniques based on vibration, sound transmission, and mechanics. The latter technique makes use of a correlation established between the modulus of rupture and the modulus of elasticity. This relationship allows the strength of a wooden member (e.g., a lumber board) to be determined with fair accuracy simply by passing it through a machine that applies a bending force. The less the deflection, the higher the predicted strength. Use of such machines in industry is still limited, however, and the main method remains the visual inspection of wood by skilled graders. Grading leads to more efficient utilization of wood and is essential in order to achieve adequate standards of safety in wooden structures.

Wood has many advantages as an engineering material. For example, its high toughness is due to the cellulose microfibrils present in a matrix of lignin and hemicellulose. As wood is a fibre composite, its toughness can be analysed in terms of a fibre pull-out mechanism of failure. 

iii. Hardness

A material’s power to resist a permanent change in shape when acted upon by an external force is known as hardness. ² For example, hard metals are used to make drills and files. Heat can decrease the hardness of some metals, and cold can increase it in others. A metal’s hardness can be used to identify its strength and quality of heat treatment. 

Brittleness

Brittleness is the likelihood that a material will fail or fracture under a relatively small shock, force, or impact. Hardness and brittleness have a direct relationship as a metal’s hardness is increased so does its brittleness. A brittle material cracks in a way that it could be put back together without any deformation.

Ductility

When you bend a piece of aluminum foil, it usually remains that way because it is ductile. Ductility is the capability of a metal to be permanently bent, twisted, or otherwise manipulated without breaking or cracking. Soft steel, copper, aluminum, and zinc are all considered ductile metals. 

Toughness

Toughness is the capacity of a metal to not break when a significant force is applied. 

Strength

A metal’s ability to resist deformation is known as its strength. Strength is quantified in four ways:

• Tensile Strength: A metal’s ability to resist forces attempting to pull it apart.
• Compressive Strength: The capability of a metal to withstand being crushed.
• Shear Strength: The capacity of a metal to tolerate forces trying to slice or cut it apart.
• Torsional strength: The ability of a metal to resist forces attempting to twist it. 

Steel is a popular building material thanks to its excellent properties.Tool steel is primarily used in the automotive, shipbuilding, construction, and packaging sectors.

iv. Tensile Strength

The tensile strength is the stress needed to break a sample⁴⁵. It is expressed in Pascals or psi (pounds per square inch).

1 MPa = 145 psi

The tensile strength is an important property for polymers that are going to be stretched. Fibers, for instance, must have good tensile strength.

% Elongation to Break

The elongation-to-break is the strain on a sample when it breaks - this usually is expressed as a percent. The elongation-to-break sometimes is called the ultimate elongation. Fibers have a low elongation-to-break and elastomers have a high elongation-to-break.

Young's Modulus

Young's modulus is the ratio of stress to strain. It also is called the modulus of elasticity or the tensile modulus. Young's modulus is the slope of a stress-strain curve. Stress-strain curves often are not straight-line plots, indicating that the modulus is changing with the amount of strain. In this case the initial slope usually is used as the modulus, as is illustrated in the diagram at the right.

Rigid materials, such as metals, have a high Young's modulus. In general, fibers have high Young's modulus values, elastomers have low values, and plastics lie somewhere in between.

Toughness

The toughness of a material is the area under a stress-strain curve. The stress is proportional to the tensile force on the material and the strain is proportional to its length. The area under the curve then is proportional to the integral of the force over the distance the polymer stretches before breaking.

This integral is the work (energy) required to break the sample. The toughness is a measure of the energy a sample can absorb before it breaks.

A  material that is strong but not tough is said to be brittle. Brittle substances are strong, but cannot deform very much. Polystyrene (PS) is brittle, for example. High impact polystyrene (HIPS), a blend of polystyrene and polybutadiene (a rubbery polymer above its glass transition temperature) is said to be rubber-toughened.

Polymers are incredibly diverse elements that represent such fields of engineering from avionics through biomedical applications, drug delivery system, biosensor devices, tissue engineering, cosmetics etc.

E. The common cooling media are water-oil, water-nitrate, water-air and oil-air. Generally, water is used as quick cooling quenching medium, oil or air is used as slow cooling quenching medium, and air is less used.