Hardness

Hardness is the resistance of a material to localized deformation. The term can apply to deformation from indentation, scratching, cutting or bending. In metals, ceramics and most polymers, the deformation considered is plastic deformation of the surface. For elastomers and some polymers, hardness is defined at the resistance to elastic deformation of the surface. The lack of a fundamental definition indicates that hardness is not be a basic property of a material, but rather a composite one with contributions from the yield strength, work hardening, true tensile strength, modulus, and others factors. Hardness measurements are widely used for the quality control of materials because they are quick and considered to be nondestructive tests when the marks or indentations produced by the test are in low stress areas.

 

There are a large variety of methods used for determining the hardness of a substance. A few of the more common methods are introduced below.

AIR HARDENING STEEL :- An alloy steel which does not require quenching from a high temperature to harden but which is hardened by simply cooling in air from above its critical temperature range.

ANNEALING :- Applies normally to softening by changing the microstructure and is a term used to describe the heating and cooling cycle of metals in the solid state. The term annealing usually implies relatively slow cooling in carbon and alloy steels. The more important purposes for which steel is annealed are as follows: To remove stresses; to induce softness; to alter ductility, toughness, or electric, magnetic or other physical and mechanical properties; to change the crystalline structure; and to produce a definite microstructure.

BRINELL HARDNESS :- A hardness number determined by applying a 3000 kilogram load to the surface of the material to be tested through a hardened steel ball of 10mm. The diameter of the depression is measured and the hardness is the ratio of load to spherical area of the impression. Tables of numbers have been prepared, and the hardness is read from the table from the diameter of the depression.

CARBURIZING :- Adding carbon to the surface of steel by heating the metal below its melting point in contact with carbonaceous solids, liquids, or gases.

CASE HARDENING :- A heat treatment or a combination of heat treatments of surface hardening involving a change in the composition of the outer layer of an iron-base alloy in which the surface is made substantially harder by inward diffusion of a gas or liquid followed by appropriate thermal treatment. Typical hardening processes are carburizing, cyaniding, carbo-nitriding and nitriding.

DECARBURIZATION :- When steel is subjected to high temperatures, such as are used in hot rolling, forging, and heat treating in a media containing air, oxygen, or hydrogen there is a loss of carbon at the surface which is known as decarburization. This resultant loss of carbon or chemistry change at the surface of the steel part reduces the strength of the part by reducing the size of the section and produces a softer surface hardness than the core of the part.

INDUCTION HARDING :- Exposing a steel work-piece to an electro-magnetic field produces a heating effect in the surface of the work-piece, by the phenomenon known as induction. This surface heating produced by the induced electro-magnetic current in the work-piece, can be used for softening, hardening or metal joining operations, depending upon the steel, the work-piece environment and particular temperature changes employed. The most common application of induction treatment is in the hardening of steel components with carbon contents of between 0.4 and 0.5%. A copper induction coil is made to surround the work-piece and the surface temperature is raised to above the upper critical temperature in a few seconds. For most applications high current frequencies of 200kHz or above are used or alternatively medium frequencies of up to 10kHz. A quenching spray of a suitable solution follows the inductor as it traverses the work-piece, providing rapid cooling to produce the full hardening transformation of the heated zone.   

QUENCHING AND TEMPERING :- In this operation the procedure consists of heating the material to the proper austenitizing temperature, holding at that temperature for a sufficient time to effect the desired change in crystalline structure, and quenching in a suitable medium – water, oil or air depending on the chemical composition. After quenching, the material is reheated to a predetermined temperature below the critical range and then cooled under suitable temperatures (tempering).

ROCKWELL HARDNESS :- In a Rockwell hardness test, a penetrator makes an indentation in the metal under two constant loads, a “minor” load (generally 10 kilograms) and then a “major” load. The difference in penetration depth between the two loads provides the measure of the hardness, usually read from a gauge on the testing machine. There are several Rockwell scales for different ranges of hardness. The most common are the B scale (RHB), for which a steel ball is used as the penetrator, and the C scale (RHC), for which a cone-shaped diamond is used. The B scale is appropriate for soft metals, the C scale for hard metals. Rockwell hardness numbers are not proportional to Brinell hardness readings.

SURFACE HARDENING OF STEELS :- Surface hardening a process which includes a wide variety of techniques is used to improve the wear resistance of parts without affecting the softer, tough interior of the part. This combination of hard surface and resistance and breakage upon impact is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear, along with a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steels has an advantage over through hardening because less expensive low-carbon and medium-carbon steels can be surface hardened without the problems of distortion and cracking associated with the through hardening of thick sections.

TEMPERING :- Also termed drawing. Reheating hardened, usually quenched, steel to some temperature below the lower critical temperature followed by any desired rate of cooling after the steel has been thoroughly soaked at temperature. Usual tempering temperatures are 300” to 1100” F.

 WATER HARDENING :- High carbon grades of tool steel, straight carbon steels and low alloy steels that are hardened by quenching in water during the heat treating operation.

INDUCTION HARDING :- Exposing a steel work-piece to an electro-magnetic field produces a heating effect in the surface of the work-piece, by the phenomenon known as induction. This surface heating produced by the induced electro-magnetic current in the work-piece, can be used for softening, hardening or metal joining operations, depending upon the steel, the work-piece environment and particular temperature changes employed. The most common application of induction treatment is in the hardening of steel components with carbon contents of between 0.4 and 0.5%. A copper induction coil is made to surround the work-piece and the surface temperature is raised to above the upper critical temperature in a few seconds. For most applications high current frequencies of 200kHz or above are used or alternatively medium frequencies of up to 10kHz. A quenching spray of a suitable solution follows the inductor as it traverses the work-piece, providing rapid cooling to produce the full hardening transformation of the heated zone.