Material properties

Prudent material selection can literally make or break an otherwise solid design. As engineers, we are the ones responsible for material choices. The companies we go to work for will rarely hire a materials expert; they trust that our education and experience equip us to make good material choices. In other words, you are their materials expert. With that in mind, we are going to investigate several material properties that are relevant as we choose materials for our designs.

Most of this material is taken from an excellent online resource called the Engineers Edge. The most important material properties this course / project are underlined.

Alloying Elements. Chemical elements added for improving the properties of the finished materials. Some alloying elements are: nickel, chromium, manganese, molybdenum, vanadium, silicon & copper.

Annealing. The process of putting material in its softest condition for further processing. Annealing is normally done by heating material to a certain temperature, then cooling it under controlled conditions.

Brinell Hardness. A measurement of a metal’s hardness (or the ability to resist penetration). A ball is pressed into a sample under a 3000 kilogram load. The diameter of the depression is measured, and the hardness is the ratio of the load to the spherical area of the impression.

Case-Hardening. A process of hardening a ferrous alloy so that the surface layer, or case, is made substantially harder than the interior or core. Typical case-hardening processes are carburizing and quenching, cyaniding, carbonitriding, induction hardening and flame hardening.

Cold Rolling (Cold Finishing). A forming process in which metal is rolled or drawn through dies, usually at room temperature. This produces a product with certain advantages over hot rolled steel, such as tighter tolerances, improved material strength, improved finish and straightness.

Cold Working. Plastic deformation which is carried out in a temperature region and over a time interval such that the strain hardening is not relieved. Considerable knowledge on the structure of the cold-worked state has been obtained. In the early stages of plastic deformation, slip is essentially on primary glide planes and the dislocations form coplanar arrays. As deformation proceeds, cross slip takes place. The cold-worked structure forms high dislocation density regions that soon develop into networks. The grain size decreases with strain at low deformation but soon reaches a fixed size. Cold working will decrease ductility but will make the material stronger.

Cost. This is a property often neglected when selecting materials for a particular design. Be sure to consider not only the raw material cost, but also the manufacturing cost associated with that particular material. For example, a material’s machineability is inversely proportional to its strength and hardness; therefore manufacturing cost is also inversely proportional to its strength and hardness. With this in mind, as engineers we should select the material that is strong enough to satisfy the mechanical properties of the particular application, but not much stronger, as that will only increase the price. I will use a motor mount bracket from class as an example. These mounts can easily be manufactured from 3/16” thick aluminum flat bar. However, they can also be made from mild steel. The aluminum is sufficiently strong and about three times more machinable than the mild steel. Therefore, the cost of the aluminum part will be lower than that of the same part made of mild steel.

Density. The ratio of a material’s mass per unit volume. Note that density is an intrinsic property, meaning its value does not depend on the quantity of material under consideration. When discussing general material properties, always refer intrinsic properties. For example, saying steel is heavy or aluminum is light makes us sound foolish and uneducated, because a steel part weighing 10 grams is NOT heavy compared to an aluminum part weighing 2000 kg.

Ductility. The property that permits permanent deformation before fracture by tensile stress. Stated differently, ductility is the ability of a material to deform easily upon the application of a tensile force.

Elastic Deformation. A temporary change in shape of a metallic body as the result of forces acting on it. Upon reduction or removal of the external forces, the body returns to its original shape.

Elongation. The change in length of a tensile specimen expressed as a percent of the varying loads.

Fatigue Resistance. The ability of a metal to withstand repeated and varying loads.

Finish. In the steel and aluminum industry, finish refers to the type of surface condition desired or existing in the finished product. For cut plates, finish refers to the quality of an edge or surface required for the part to be acceptable.

Forging. A hot working operation generally involving plastic deformation of metal into desired shapes with compressive force.

Formability. The ability of a material to be shaped by plastic deformation.

Free Cutting or Free Machining (Improved machining). A term to describe a type of material that has been modified to increase its machineability.

Galvanizing. The process of applying a coating of zinc to cold-reduced sheet, bar, structural, or to fabricated parts made from steel. The coating is applied by hot dipping or electrolytic deposition and is applied to make product more corrosive resistant.

Hardenability. The ability of a ferrous metal to harden deeply upon quenching.

Heat Treatment. Any process involving heating metal to an elevated temperature to obtain change in properties or metallurgical structure.

Hot Rolled. Hot rolled products are those products that are rolled to finish at temperatures above the recrystallization temperature.

Hot Working. Hot working refers to the process where metals are deformed above their recrystallization temperature and strain hardening does not occur. Hot working is usually performed at elevated temperatures. Lead, however, is hot-worked at room temperature because of its low melting temperature. At the other extreme, molybdenum is cold-worked when deformed even when red hot because of its high recrystallization temperature.

Machinability. The ability of a material to be machined to particular tolerance and surface finish requirements.

Malleability. Where ductility is the ability of a material to deform easily upon the application of a tensile force, malleability is the ability of a metal to exhibit large deformation or plastic response when being subjected to compressive force.

Pickling. Chemical or electrochemical removal of surface oxides (surface scale). Pickled steels must be oiled or they will rust rapidly.

Plastic Deformation. A permanent change in shape of a metallic body as the result of forces acting on its surface.

Quenching. A process of rapid cooling from an elevated temperature by contact with liquids, gases or solids.

Rockwell Hardness. A method of measuring the hardness of materials (resistance to penetration). Rockwell measures the hardness by pressing an indenter into the surface of the steel with a specific load, then measuring how far the indenter was able to penetrate. There are a number of Rockwell tests; the most common is Rockwell B.

Scale. An oxide of iron which forms on he surface of hot rolled material.

Stiffness. The resistance of an elastic body to deformation by an applied force.

Strength. The ability of a material to withstand an applied stress without failure.

Stress Relieving. A process of reducing residual stresses in material by heating to a suitable temperature and holding for a sufficient time. This treatment may be applied to relieve stresses inducted by casting, quenching, normalizing, machining, cold working or welding.

Temper. A condition produced in a metal or alloy by mechanical or thermal treatment and having characteristic structure and mechanical properties.

Toughness. Toughness describes the way a material reacts under sudden impacts. It is defined as the work required to deform one cubic inch of metal until it fractures. Toughness is measured by the Charpy test or the Izod test. Both of these tests use a notched sample. The location and shape of the notch are standard. The points of support of the sample, as well as the impact of the hammer, must bear a constant relationship to the location of the notch. The tests are conducted by mounting the samples to a test stand and allowing a pendulum of a known weight to fall from a set height. The maximum energy developed by the hammer is 120 ft-lb in the Izod test and 240 ft-lb in the Charpy test. By properly calibrating the machine, the energy absorbed by the specimen is measured from the upward swing of the pendulum after it has fractured the material specimen. The greater the amount of energy absorbed by the specimen, the smaller the upward swing of the pendulum will be and the tougher the material is.

Weldability. The ability of a material to be permanently joined to a separate piece of identical material using the welding process.

Work Hardening. Increase in resistance to deformation (hardness) produced by cold working.