Hardness And Impact Tests (1975)

e [Music] the term hardness has a specific meaning in engineering it is the resistance to permanent penetration a steel ball pressed into a piece of lead produces a spherical indentation a specific amount of force is used resistance to penetration is measured either by the depth of penetration or by the residual area of the indentation such measurements combined with the magnitude of the contact force can be used to calculate a hardness number two common test for quantitatively evaluating resistance to penetration will be examined next the Brunell hardness tester uses a specified load to force part of the lower surface of the 10 mm diameter hardened steel ball into the test material the penetrator is brought into contact with the specimen the load is applied hydraulically held for 30 seconds and then released a spherical indentation results a special microscope is used to measure the diameter across the top of the indentation the Brunel hardness number is then calculated by dividing the applied load through 3,000 kg in the standard Brunell test by the spherical area of the residual indentation this area can be determined from the diameter of the indentation small D and the diameter of the steel ball capital D commonly however the area is not calculated instead charts are used to go directly from a measured diameter to the corresponding Brunel hardness number Brunel hardness numbers have the same units as stress load divided by area for Metals the Brunel hardness number ranges from about 100 to 600 the conventional Rockwell hardness tester indicates resistance to penetration directly on a dial gauge which measures the depth of penetration of a specially shaped penetrator when pressed into the test material under a particular load depending on the resistance of the material to be tested forces of 60 and 100 kg are used with a 11/6 in diameter hardened steel ball penetrator affording Rockwell B readings and for harder materials 150 kg of load with a diamond tipped conical penetrator affording Rock well C readings we shall determine the Rockwell hardness of certain Steels the diamond tipped cone penetrator is inserted into the machine a small cylindrical specimen of ASM 1045 steel with a carbon content of 45% will be tested first the specimen is brought into to contact with the penetrator a 10 kg minor load is applied to hold the material and penetrator in contact this is accomplished by tightening a hand wheel until the dial indicator reaches the set Mark the major load of 150 kg is applied when the load lever is tripped the dial gate is marked to subtract the depth of penetration from an arbitrary value so that a softer material yields a lower reading than a harder one when the load is removed the dial gives the reading directly in this case C3 the specimen has been indented by the cone the indentation is quite small a similar test is performed on as TM 1095 steel which contains 95% carbon here the hardness number is C30 for a softer material the diamond cone would penetrate beyond the reading capability of the instrument hence the 11/16 inch diameter ball tipped penetrator is used analogously with a 100 kg load the hardness of this ASM 1020 steel specimen with a 2% carbon content will be read on the B scale its Rockwell hardness number is b80 we see from these tests that higher carbon content is associated with greater hardness the tensil strengths of these materials have also been observed to increase with carbon content berell and Rockwell readings may be compared on a chart these hardness numbers are different and are not linearly related readings on the Rockwell B and C scales may also be compared to each other it is seen that a hardness of 100 on the B scale would correspond to a hardness number of 20 on the C scale materials with a greater tensil strength also exhibit greater hardness Domer has found that the tensil strength of materials is approximately 500 times their Brunel hardness number hence a hardness measurement can be used as a rough estimate of tensil strength there are several other hardness testing methods among them the Mo's hardness test for minerals is based on a scratch test the harder material in this case diamond is able to scratch a softer glass plate though glass is a comparatively hard material on the Mo's hardness scale the softest material is tal number one and the hardest diamond is number 10 the mo scale may also be compared to the other measurements the scratch test measures abrasive hardness in addition to indentation hardness and hence another nonlinear relation between hardness numbers results another mechanical property frequently measured is ductility which is defined as percent elongation to fracture some materials are ductal when pulled slowly but brittle under a dynamic load furthermore a material that is ductal at one temperature may become brittle when it is cooled to the temperature of liquid nitrogen in a static tension test toughness was measured by the modulus of toughness the area under the stress strain curve it indicated the amount of energy absorbed in deformation and fracture for materials of comparable strength toughness and ductility are closely related Dynamic toughness is measured in Impact testing machines a heavy pendulum has potential energy proportional to the height to which it is raised when allowed to swing potential energy changes to kinetic and back to potential energy if the pendulum Rises to its initial height there is no loss of potential energy energy loss is measured on this dial when a piece of Steel is inserted into the path of the pendulum the energy required to break it will be registered as a loss of potential energy and the Pendulum will rise to a diminished height this energy loss as indicated by the dial is measured directly in foot pounds the energy loss like the modulus of toughness is related to ductility some materials such as this SAE 4140 steel do not break under a single impact in an impact test the total energy absorbed to fracture is desired but this specimen did not break specimens are therefore notched in addition to producing a stress razor the notch concentrates the energy at a narrow region a notched 4140 specimen does break bending impact specimens may be either simply supported as in the Sharpie test or canal levered in isod machines the size of the specimen as well as notch geometry are standardized a notched SAE 4140 steel specimen when tested in the Sharpie configuration breaks the energy absorbed to fracture is 8 ftlb the fracture surface is rough and relatively flat typical of brittle failures steel is a material whose toughness may be improved with certain types of heat treatment the fracture surface of the heat treated 4140 steel exhibits sheer lips on the sides and a rough horizontal fracture region in the middle sheer lips are a sign of high ductility ductility and toughness are temperature dependent properties we shall retest our heat treated 4140 material at various temperatures a thermocouple is attached to the specimen in order to measure its temperature after a liquid nitrogen immersion the specimen is inserted into the impact testing machine and its temperature is allowed to rise when the temperature reaches a specified level minus 170° F in this case the pendulum is released and the energy absorbed 20 foot- PBS is recorded atus 250° F the energy absorbed is still lower 12tb the absorbed energy can be plotted against the test temperature it is low for low temperatures but eventually a sudden increase is observed Beyond which the curve flattens out again the behavior of the material has changed from brittle low energy absorption to ductal high energy absorption the temperature at which this change takes place is called the ductal brittle transition temperature hardness and impact test because of their Simplicity and because specimens for them are relatively inexpensive are frequently used for the purposes of quick comparison of materials and for quality control procedures [Music] [Applause] [Music]