[ Pobierz całość w formacie PDF ]
.Copper melts suddenly and solidifies instantly.Copper alloys,containing small amounts of other metals, melt quicker and solidifyslower.m.Lead.Lead melts at a very low temperature and the molten metalbecomes covered with a thin, dull slag.n.Magnesium.Magnesium oxidizes rapidly when heated in the air toits melting point; because of this and as a safety precaution, thismetal is melted in an atmosphere free from oxygen.When heated inthe open air, it produces an oxide film which is highly refractoryand insoluble in the liquid metal.o.Monel Metal.Monel flows clearly without any sparkle.A heavyblack scale forms on cooling.p.White Metal Die Castings.The melting points are low and themetal will boil under the torch.7.Hardness Testinga.General.The quality of hardness is a complex one whichdetailed study has shown to be a combination of a number of physicalproperties.It is most often defined in terms of the method used forits measurement and usually means the resistance of a substance toindentation.Hardness may also be defined in terms of resistance toscratching and, thus, is related to wear resistance.The word"hardness" is sometimes used to refer to the stiffness or temper ofwrought products because the indentation hardness of a metal isclosely related to its tensile strength.The cutting characteristicof metal, when used as a tool, is sometimes called its hardness, but39METAL PROPERTIES, CHAR, USES, AND CODES - OD1643 - LESSON 1/TASK 2reflection will show that these various indications of hardness arenot the same.The following subparagraphs describe the processes forthe performance of various hardness tests.b.Brinell Hardness Test.One of the most common methods ofmeasuring the hardness of a metal is to determine its resistance tothe penetration of a nondeformable steel ball.This is done bydetermining the depth to which such a ball will sink into metal undera given load.This test is made by forcing the hardened steel ballinto the test material by the weight of a known load.The ball isusually 10 millimeters in diameter and has an applied pressure of 500kilograms for soft materials such as copper and brass, and 3000kilograms for materials such as iron and steel.Once the load hasbeen applied, the diameter of the resulting impression is measuredwith a small microscope.The hardness number of the metal is foundby dividing the load applied by the area of the impression andcomparing the results of the division with a standard hardnessconversion table, which provides the hardness number of the metal.c.Rockwell Tester.This test is based upon the difference betweenthe depth to which a test point is driven into a metal by a lightload and the depth to which it is driven by a heavy load.The lightload is applied first and then, without moving the piece beingtested, the heavy load is applied.The hardness number isautomatically indicated on a dial.In this test, a 120� diamond conefor hard metals or a 1/16 inch steel ball for softer materials isimpressed into the surface to be tested by a deadweight actingthrough a series of levers.The hardness is indicated on a dial gagegraduated in the Rockwell "B" and "C" scales.The harder the piece,the higher the Rockwell number will be.For example, machinablesteel should not show a reading of more than 30 to 35 on the Rockwell"C" scale, while a hardened high-speed cutter would show a reading of63 to 65.When testing hard steel, the diamond point should be usedand should be read on the "C" scale.For nonferrous metals, thesteel ball should be used and read on the  B scale.d.Vickers Hardness Test.The Vickers hardness testing method isvery similar to the Brinell method.The penetrator used in theVickers machine is a diamond pyramid rather than the round steel40METAL PROPERTIES, CHAR, USES, AND CODES - OD1643 - LESSON 1/TASK 2ball of the Brinell.The impression made by this penetrator is adark square on a light background.This type of impression is easierto measure than the circular impression.Another advantage lies inthe fact that the diamond point does not deform as is possible withthe steel ball.e.Scleroscope Test.With this process, the hardness is measuredby the height of rebound of a diamond-pointed hammer after it hasbeen dropped through a guiding glass tube onto the test piece and therebound checked on a scale.The harder the material used, thegreater the rebound of the hammer because the rebound is directlyproportional to the resilience or springiness of the test piece.Theheight of the rebound is recorded on a gage.Since the scleroscopeis portable, it can be carried to the work enabling tests to beperformed on a large section of metal too heavy to be carried to thework bench.The indentations made by this test are very slight.f.File Test.The file test is a method of determining thehardness of a piece of material by trying to cut into it with thecorner edge of a file.The hardness is indicated by the bite thatthe file will take.This is the oldest and one of the simplestmethods of checking hardness; it will give results ranging from quitesoft to glass hardness.The principal objection to the use of thefile test is that no accurate record of results can be maintained asnumerical data.8.Other Systems for Identification of Metalsa.Numerical Index System [ Pobierz całość w formacie PDF ]