ME 215 Engineering Materials I

Transcription

1 ME 215 Engineering Materials I Chapter 5 Hardness and Hardness Testing (Part II) Mechanical Engineering University of Gaziantep Dr. A. Tolga Bozdana

2 Shore Scleroscope Test Invented by A. F. Shore, it is done by dropping a diamond tipped hammer by its own weight from a fixed height and reading rebound height (Fig. 8). Figure 8 Higher the rebound, harder the metal. There are two models (scales) having certain height & weight of hammer: 1. Model C (251.2 mm & 23 g) 2. Model D (17.9 mm & 36 g) Hardness number is indicated based on ASTM A448 as follows: HSc XX (Model C), HSd XX (Model D). Duetoitsportability, the testers can be used for various size of parts, including small parts (e.g. thin sheets) and large parts (e.g. steel rolls). Table shows the minimum material thickness. Surface finish of the part is important for reliable measurement. Material t (mm) Hard steel 0.15 Cold-rolled steel 0.25 Semi-hard brass strip 0.25 Annealed brass strip 0.38 Tests should not be made more than once on the same spot due to cold working occurring around that spot.thus, the indentations must be at least 0.51 mm apart from each other.

3 Durometer This is an instrument for measuring indentation hardness of elastomers, rubbers, polymers (ASTM D ). Figure 9 Shore durometer has Models A & D for testing soft and hard materials, respectively. They differ in sharpness of indenters and magnitude of load applied by calibrated spring (Fig. 9). Spring force (in grams) is H A (for Model A) and H D (for Model D) where H A and H D are the hardness readings taken within 1 second of pressing. Similar to Rockwell testing, the hardness is a measure of the depth of indentation. The indenter is pressed against the surface, and the indentation depth is measured by an indicating device. The hardness reading is 100 when depth is zero. Reading is zero when the indenter meets no resistance (i.e. when the depth is h = 2.5 ± 0.04 mm). 2

4 Scratch Test In scratch test, a hard indenter pressed onto part surface and drawn steadily in horizontal direction. Thus, the resistance of a material against wear due to grinding and scratching is measured. Moh s scale defines the hardness of minerals numbered consecutively from 1 to 10 based on their relative hardness (e.g. calcite can scratch talc). The extended version of this scaling is available by Ridgway. Both scales are not quite objective. Thus, other scratch methods have been proposed as alternative. Moh s Scale Extended Scale by Ridgway No. Reference Material No. Reference Material 1 Talc 1 Talc 2 Gypsum 2 Gypsum 3 Calcite 3 Calcite 4 Fluorite 4 Fluorite 5 Apatite 5 Apatite 6 Feldspar 6 Orthoclase (feldspar) 7 Vitreous pure silica 7 Quartz 8 Quartz 8 Topaz 9 Topaz 10 Garnet 11 Fuser zirconia 9 Sapphire or corundum 12 Fuser alumina 13 Silicon carbide 14 Boron carbide 10 Diamond 15 Diamond

5 Scratch Test Bierbaum s test has been the most widely-known test, but discontinued due to lack of reproducibility. A diamond indenter is drawn under a vertical load steadily along surface (Fig. 10), and Bierhaum Scratch Hardness (BSH) is defined as: F Figure 10 BSH F w 2 F : applied load (kg) w : measured width of scratch (mm) specimen w The shape of indenter can be adopted to particular requirements. It can be conical with angles of 90, 120, 150 (similar to Brale indenter in Rockwell test) or pyramidal with an interfacial angle of 120 (similar to diamond indenter in Vickers test). The scratch produced by moving the indenter is compared with a reference scratch. Marten s scratch hardness is the required load to produce scratch of 0.01 mm. There are also reference scratches for soft or thin materials. 4

6 Special Purpose Hardness Tests 1. Ultrasonic Hardness Test: A magnetostrictive diamondtipped rod vibrating at ultrasonic frequencies (> 20kHz)is in contact with the surface at a load of 3.3 kg. The change in vibration frequency is read on a scale in terms of Rockwell or Vickers number. The tester is portable, and suitable for superficial testing (only for steel) due to shallow indentation (< mm). 2. Hot-hardness Test: It is used for measuring the hardness of metals at elevated temperatures. Some degree of success has been obtained in correlating hothardness with strength. Dedicated testers as well as some Rockwell testers (models 6JR, 6JS, 6JT) can be used. There are difficulties with this test: Both indenter and specimen must be at the same temperature. A cold indenter will cause a local cooling, causing similar effect of quenching and resulting in errors. Test must be carried out in vacuum or inert atmosphere due to oxidation of surface. 5

7 Highlights of Hardness Tests Hardness tests are relatively nondestructive although this term is very subjective. Even though extent of indentation is rather small, its destructive effect is dependent upon size of the part (i.e. testing of parts with thin sections is particularly difficult). Hardness measurements are strongly affected by surface quality. The general rule is: lighter the test load, higher the degree of surface finish required. Rockwell test is relatively insensitive to surface finish. With accurate definition of indentation diameter, Brinell test is also not highly influenced by scratches and roughness. Good finish is required in Vickers and microhardness testing (metallographic finish). The advantages of hardness tests can be summarized as follows: Hardness tests are relatively simpler and easier than other mechanical tests. Special specimens are not needed (machine parts can be tested directly). Certain tests can be conducted on-site as well as in the production line. Tests can be made quickly (taking about seconds to few minutes). Hardness can be measured on parts of small thickness or in very thin layers. Quality level of similar materials may be graded according to hardness. Hardness can be related to strength (S ut and S uc for ductile and brittle materials). 6

8 Choice of the Testing Method The followings should be considered in selection of the most appropriate method: 1. Degree of accuracy required: This is depending on scope of the test (to determine the properties that are directly or indirectly related with hardness). Hardness of a part following a heat-treatment is a directly related property. Assessment of the tensile strength based on a hardness value is an indirect way. 2. Speed of testing: This involves the time for conducting the test as well as the time required to prepare the surface. This issue is significant when the test is employed for quality control. Special automated high-speed testers are available for this purpose. Order of tests (fastest to slowest): Shore Scleroscope, Rockwell, Brinell, Vickers 3. Size of the part: This is the issue whether the part could be accomodated by tester. Too large/ heavy parts and the miniature parts cannot be tested on standard testers. Shape and size of the indenter and magnitude of the load are chosen according to structure and properties of the material: For materials with heterogeneous structure of large inclusions (suchasgrayironor nonferrous bearing alloys), large-size indenter should be used. For materials with fine homogeneous structure, tests should be made at small spots using smaller indenters in order to represent the bulk of material sufficiently. 7

9 Choice of the Testing Method Ferrous materials: Practically all ferrous materials of suitable thickness can be tested by standard test methods. Among them, Brinell test gives better results with course grained materials because of the averaging provided by Ø10 mm ball. BHN is used to determine approximate tensile strength of steels: 2 Sut HB 0.35 kg mm (for steels with BHN above 175) 2 Sut HB 0.33 kg mm (for steels with BHN below 175) Hardened and tempered steels: Rockwell and Vickers tests produce more accurate results. For thin parts, Rockwell A scale or superficial testing can be employed. Case hardened steels: For such steels, outside layer (case) is harder than inner portion (core). The hardness is concerned with the depth of hardened layer, thereby the indentation should not disturb this layer. Thus, the variation of hardness across the case depth can only be done by microhardness testing. Vickers and Rockwell (with suitable scales depending upon case thickness) can be used. Steel forgings: Brinell test (with Ø10 mm ball and 3000 kg load) gives a good relationship between BHN and tensile strength of forged steels (as explained above).

10 Choice of the Testing Method Cast iron: Brinell test is preferred due to nonuniformity of cast iron. MacKenzie suggested a correlation between BHN and compressive strength of gray cast iron: uc S HB 10 kg mm Plastics: Following ASTM procedures are used for determining hardness of plastics (The relationship between hardness and strength of plastics is not so determinate): D (Durometer test) D (Barcol Impressor) D (Rockwell test with R, L, M, E, K scales) Elastomers: Rubber and rubber-like materials are tested on Durometer. Model A is used with soft rubbers whereas hard rubber products are tested using Model D. Very hard materials: The difficulty is that very high contact pressures are required to produce an indentation of measurable size. Diamond indenters are used, thus selection of inappropriate scales reduce the life of diamond. Knoop scale is usually employed for ceramics whereas Rockwell A scale is accepted for cemented carbides. 9