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Hardenability 

Materials 

New- Better Definition

The hardenability of steel is defined as that property which determines the depth and distri­bution of hardness induced by quenching. Hardcnability is an inherent characteristic of steel largely determined by the per­centage of alloying elements, austcnitic grain size, time and temperature during the austcniz-ing, and prior structure.

Hardness is often confused with hardenability. The mere sur­face hardness of a steel part is dependent upon the carbon con­tent and the cooling rate. The depth to which a certain hardness level exists is hardenability. The hardenability of a certain analy­sis is a constant when determined by standard procedures. Hard­ness will vary with the cooling rate.

The cooling rate is dependent upon certain factors: (1) the amount of heat present in the material being cooled, (2) the ratio of the surface area to vol­ume of the object, (3) the effi­ciency of the heat transfer between the quenching media and the object, and (4) the cool­ing capacity of the quenching media. The amount of heat pres­ent in a part depends upon the heat capacity and temperature of the part. The cooling capacity of the quenching media is controlled by the volume, temperature, spe­cific heat, viscosity, and degree of agitation.

(21st Centuty Manufacturing, 1994, DIANE Publishing Company, P64)

Previous:

Hardenability is the ability of the Fe-C alloy to be hardened by forming martensite . Hardenability is not “hardness” . It is a qulitatice measure of the rate at which hardness decreases with distance form the surface because of decreased martensite content .High hardenability means the ability of the alloy to porduce a high martensite content throughout the volume of specimen.

The hardenability of a steel is broadly defined as the property which determines the depth adn distribution of hardness induced by quenching . Hardenability is a characteristic determined by the following factors.

Chemical composition

Austenite grain size

Structure of alloy before quenching

The hardenability is the depth and evenness of hardness of a steel upon quenching form austenitc.

(A Textbook of Engineering Materials and Metallurgy ; A.Alavdeen, N. Venkateshwaran ; pg: 232 , 2006 )

Thermoplastic Polymers

 Material

Polymers can be classified according to the structural shape of the polymer molecules. They can be either linear, branched, or cross-linked. Linear polymers have repeat­ing units linked together in a continuous length. When branches protrude from the main polymer chain at irregular intervals, the polymer is termed a branched polymer. Both linear and branched polymers are ther­moplastics. They can be softened and hard­ened by heating and cooling. Fabricating processes like injection molding, casting and blowing take advantage of this feature to shape thermoplastic resins. The rigidity of thermoplastic resins at low temperatures is attributed tothe existence of secondary bond forces between the polymer chains. These bonds lose strength or are destroyed athigher temperatures, thereby causing fluidity of the resin.

Polymers used as textile fibers are linear. However, they must be of high molecular weight and have a permanent orientation of the molecules parallel to the fiber axis. The chain orientation necessary to develop suffi­cient strength by crystallization is achieved in the melt during spinning and then by a process known as cold drawing, in which the initially formed filaments are drawn (stretched and further oriented) at a tempera­ture above the glass transition temperature, Tg. (Tg is the temperature at which sufficient energy is available to the molecular seg­ments to cause them to begin to rotate and materials become soft and rubbery.)

Elastomeric materials, Iike thermoplastic resins and fibers, are essentially linear polymers. Certain distinctive features in their molecu­lar structures make them rubbery. Because their Tg*s are below room temperature, they can be stretched to twice their original length and then can return essentially to their origi­nal shape when tension is relaxed.

(New Materials Society, Challenges and Opportunities, 1993, DIANE Publishing Company, P8.8)

There is no previous definition.

Yield Strength

Materials

New- Better Definition

The yield strength is the stress at which the material begins to plastically deform. It is

usually measured as the 0.2% offset value, which is the point where the stress-strain curve for the

material intersects a line that is parallel to the straight-line portion of the curve but offset from it by

0.2%.

(Fundamentals of Modern Manufacturing, 2010, MPGroover, P3.1)

Previous:

A stress value obtained graphically that describes no more than a specifed amount of deformation (usually 0.002).Also known as offset Yield strength.

(Essentials of Materials Science and Engineering: SI Edition, Donald R. Askeland,Pradeep P. Fulay, p.183)

In machines and structures the metal must not deform permanetly when subjected to a load. For this reason, yhe yield strength is often reported. The yield strengt is the stress to which am metal can be subjected without permanent deformation. In ductile metarials the yield strength is at lower stress level than the ultimate tensile strength.

(Pipe Welding Procedures, Rampaul H., p.173)