ME 207 Material Science I

Transcription

1 ME 207 Material Science I Chapter 5 Hardness and Hardness Testing Dr. İbrahim H. Yılmaz Automotive Engineering Adana Science and Technology University

2 Introduction Hardness is defined as the resistance of a material to permanent deformation such as indentation, wear, abrasion, or scratch. It is a mechanical property related to: wear resistance of material ability of material to abrade or indent another material resistance of material to permanent(plastic) deformation There are many hardness tests for various materials. Selection of the appropriate hardness test depends upon relative hardness of material to be tested and amount of damage to be tolerated on material surface. Hardness testers are based on arbitrary definitions as: 1. Resistance to permanent indentation(indentationtest) 2. Resistance to scratching(scratchtest) 3. Energy absorption under impact loads(dynamictest) 4. Rebound of a falling weight(reboundtest) Hardness Tests: Indentation Tests + Macro Level Brinell Rockwell Vickers + Micro Level Vickers Diamond Knoop Diamond Scratch Test Shore Scleroscope Ultrasonic Test Hot Hardness Test Durometer 1

3 Hardness Testing by Indentation This is the most employed method in which a hard indenter (a small sphere, pyramid, or cone) is pressed onto the surface being tested under a specific load for a definite time interval, and size or depth of the corresponding indentation is measured. Shape and size of the indenter and magnitude of the load are selected in accordance with purpose of the test, structural properties of the material, state of the surface beingtested,andsizeofthepart. Indentation type hardness testers are classified based on the level of destruction of the surface being tested: Macrohardnesstests (load 1 kg):brinell,rockwell,vickers Microhardnesstests (load < 1 kg):vickersdiamond,knoopdiamond In microhardness tests, the indentation size is very small that a powerful microscope is required for the measurements. This method is more tedious than macrohardness testing, and used for testing very thin materials(down to mm), extremely small parts, thin superficially hardened parts, plated surfaces, and research purposes. 2

4 Brinell test is carried out by indenting the surface with F Figure 1 BHN Brinell Test 2 2 steel ball (usually Ø10 mm) ( )( πd 2 D D d ) under the load of 3000 kg for s (Fig. 1). For soft materials, load is reduced to 1500 or 500 kg for avoiding too deep indentations. Brinell Hardness Number (BHN, HB) is determined based on the extent of indentation. Any combination of applied load and ball diameter can be used as long as the ratio of F/D 2 is constant (e.g. 3000/10 2 Ød and 187.5/2.5 2 give the same ratio of 30). Brinell tester (invented by Dr. J. A. Brinell) employs hydraulic system to apply the required load with dead weights (Fig. 2). Loads of187.5,250,500,750, 1000,1500,2000,2500,3000kg can be used using balls with diameters of2.5,5,10mm. = F : applied load (kg) D : diameter of ball indenter (mm) d : diameter of impression (mm) Figure 2 F

5 For obtaining reliable test results, the followings must be observed: Brinell Test 1. Loading speed: The rate of loading should not exceed 500 kg/s. Applying the load too rapidly will add extra loading to the nominal load due to inertia of loading system. 2. Loading time: It is s for iron and steel whereas at least 30 s for other metals. An error would result from allowing insufficient time for plastic flow to take place. 3. Measurement of impression: Division of scale of measuring device must permit direct measurement of the indentation diameter down to 0.1 mm. 4. Thickness: Part thickness (t) must be 10 times greater than depth of indentation (h) so that no bulge or other marks should appear on the other side: t > 10h 5. Spacing of indentations: Distance from indentation center to edge of part or other indentation (L) should be 2.5 times greater than diameter of indentation (d): L > 2.5d 6. Radius of curvature: For indenting on a curved surface, the minimum radius of curvature (R min ) should not be less than 25 mm for using Ø10 mm ball: R min > 25 mm 7. Selection of load: The load should be selected such that the ratio of diameter of indentation to diameter of the ball (d/d) is kept within certain limits: 0.25 < d/d < 0.60

6 Brinell Test The most suitable F/D 2 ratio depends on the average hardness of material to be tested. The recommended ratios for various materials are given in table (recommended after Meyer analysis). Approximate HB F/D 2 Representative Material above Steels, cast iron 200 to Copper and copper alloys, aluminum alloys 15 to Aluminum 4 to 6 1 Lead, tin and tin alloys HB without any suffix denotes the hardness number obtained using ball of Ø10 mm and load of 3000 kg with duration of seconds (e.g. 350 HB ). On the other hand, 75 HB/5 500/30 indicates the hardness value of 75 measured with a ball of Ø5 mm and a load of 500 kg applied for 30 seconds. In order to standardize the test results, standard method of testing has been issued by various institutions such as TS139 (TSE) and E10-73 (ASTM): Deviation in diameter of Ø10 mm ball is limited to mm (10 ± mm). Use of steel ball is limited to materials with the maximum hardness of 450 HB. For harder materials, it is essential to use a carbide ball. Under all circumstances, Brinell method is limited up to 630 HB. 5

7 Rockwell Test Invented by S. P. Rockwell, Rockwell test is consisting of measuring additional depth to which a steel ball or a Brale diamond penetrator is forced by heavy (major) load beyond depth of a previously applied light (minor) load (Fig. 3). This concept aims elimination of measurement errors due to surface imperfections around the periphery of indentation. Figure 3 As the result of application of minor load, an initial indentation of depth ( m ) is made on the surface, which also serves as the datum line before the major load is applied. The major load is applied without removing the minor load, and the penetrator is forced beyond the depth of previously applied load by the depth ( M ). The major load is removed after certain time, and the depth of permanent indentation is measured. 6

8 Rockwell Test Fig. 4 illustrates measurement of Rockwell hardness (HR) based on different scales. The datum line is specified by the initial depth due to minor load ( m ). Incremental depth ( M ) is due to major load while the minor load is still in position. After the major load is applied and removed, the reading on dial gauge is the hardness value. Removal of the addiditonal load allows a partial recovery, reducing the depth of penetration. Permanent increase in depth of penetration (e) due to applying and removing the major load is used to deduce the hardness number,asgivenbelow. HR = K HR : Rockwell hardness number e : increase in depth of penetration 1 unit / mm for brale penetrator 1 unit / mm for ball penetrator e K : constant depending on scale 100 for brale penetrator 130 for ball penetrator Figure 4 For example, after a Rockwell hardness test,anadditionaldepthof0.08mm anadditionaldepthof008mm means: (0.08 mm) x (1 unit / mm) = 40 units (for brale penetrator) HR = = 60 (for regular testers) (0.08 mm) x (1 unit / mm) = 80 units (for ball penetrator) HR = = 50 (for superficial testers) 7

9 Types of penetrators used in Rockwell test: Diamond sphero-conical (Brale) penetrator (having angle of 120 with spherical tip of 0.2 mm radius) is used for hardened steels and cemented carbides. Steel ball penetrator (having diameter of 1/16, 1/8, 1/4, 1/2 inches) is used for steels, copper alloys, aluminum, plastics, and likewise. Rockwell testing falls into two categories: Regular testing: The minor load is always 10 kg. Major load depends upon type of penetrator. Rockwell Test Regular (Normal) Test Scale Penetrator Load Letter Type (kgf) B 1/16" ball 100 C Brale 150 A Brale 60 D Brale 100 E 1/8" ball 100 F 1/16" ball 60 G 1/16" ball 150 Superficial Test Scale Penetrator Load Letter Type (kgf) 15N N Brale 15 30N N Brale 30 45N N Brale 45 15T 1/16" ball 15 30T 1/16" ball 30 45T 1/16" ball 45 15W 1/8" ball 15 Superficial testing: Used for shallow indentations H 1/8" ball 60 30W 1/8" ball 30 (due to smaller loads and more sensitive depth K 1/8" ball W 1/8" ball 45 measuring). The minor load is always 3 kg. L 1/4" ball 60 15X 1/4" ball 15 Hardness is shown by scale letter and number: C60 or 60RC means Rockwell hardness of 60 on scale C under load of 150 kg with brale penetrator. 30N80 indicates superficial hardness of 80 on scale 30N under load of 30 kg with brale penetrator. M 1/4" ball 100 P 1/4" ball 150 R 1/2" ball 60 S 1/2" ball 100 V 1/2" ball X 1/4" ball 30 45X 1/4" ball 45 15Y 1/2" ball 15 30Y 1/2" ball 30 45Y 1/2" ball 45 8

10 Rockwell Test The most suitable Rockwell scale is chosen according to following factors: 1. Material type: Table shows the listing (ASTM E18), providing a valuable source of reference for regular scales for typical materials with applicable scales. Superficial scales are: N scale is for materials similar to those on C & D scales with thinner gauge or case depth. T scale is for materials similar to those on B, F, G scales. W, X, Y scales are for soft materials (the smallest ball is recommended). Scale Load (kgf) Application B 100 Copper alloys, soft steels, aluminum alloys, malleable iron, etc. C 150 Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel, other materials harder than B100 A 60 Cemented carbides, thin steel, shallow case-hardened steel D 100 Thin steel, medium case-hardened steel, pearlitic malleable iron E 100 Cast iron, aluminum and magnesium alloys, bearing materials F 60 Annealed copper alloys, thin soft metals G 150 Malleable irons, copper-nickel-zinc and cupro-nickel alloys (upper limit G92 to avoid possible flattening of ball) H 60 Aluminum, zinc, lead OTHER Bearing materials and other very soft or thin materials (smallest ball and heaviest load must be used whenever possible) 9

11 Rockwell Test 2. Material thickness: For a given thickness, any hardness greater than that thickness can be tested. For a given hardness, any thickness greater than that hardness can be tested on the indicated scale. Note that X refers to no indicated hardness. Thickness Regular Superficial (mm) A D C 15N 30N 45N X X X X X X X X X X X X 0.81 X X X X 0.86 X X X X 0.91 X X 37 X X X 0.97 X X 28 X X X 1.02 X X 20 X X X Thickness Regular Superficial (mm) F B G 15T 30T 45T X X X X X X 0.89 X X X X 1.02 X X X X 10

12 Rockwell Test 3. Spacing of indentations: In all types of indentation tests, material in the vicinity of indentation area is cold-worked. The test result will be affected if another indentation is placed within this cold-worked area. It is recommended to allow minimum distance of 2.5d from the center of indentation to the edge of part as well as minimum distance of 3d from the center of indentation to the center of adjacent indentation. 4. Scale limitations: In accordance with the values of coefficient K, display of tester is numbered from 0 to 100 units for Brale scales and 0 to 130 units for ball scales by offsetting the corresponding scale by 30 units (i.e. B scale in Fig. 4). Figure 4 11

13 Rockwell Test 5. Radius of curvature of surface: Compared with a flat surface, a convex surface has less lateral resistance to penetrating force and penetrator will sink further into material. Thus, hardness value will be lower on convex surface than on flat surface of the same material. For concave surfaces, the opposite is true. The difference is negligible above diameters of 25 mm. Otherwise, correction is required. The correction tables for curved surfaces (e.g. cylindrical specimens) are given in standards TS140 and ASTM E18. 12

14 Vickers Test A diamond indenter (in the form of a right pyramid with a square base and an angle of 136 between opposite faces) is forced into material under a certain load (Fig. 5). After removing the load, two diagonals (d 1 and d 2 ) of the indentation are measured and their arithmetic mean (d) is calculated. Vickers hardness is denoted by VHN (TS 207) or HV (ASTM E92 or BS 427): VHN ( θ ) 2Fsin 2 = = d ( 2 F d ) F : applied load (kg) d : mean of diagonal impression (mm) : face angle of the pyramid (136 ) Hardness value is followed by a suffix designating load and another suffix indicating time of loading if different than s (e.g. 455 VHN/30/20 refers to Vickers hardness of 455 obtained by load of 30 kg applied for 20 seconds). Figure 5

15 Vickers Test Vickers hardness number is nearly independent of load for homogeneous materials as the ratio between diagonals of indentation remains constant. Loads of 1 to 120 kg are applied. Hardness number is constant when diamond pyramid is used with loads of 5 kg or higher (although it may be load dependent at lower test loads). Vickers test provides better accuracy than Brinell or Rockwell since the diamond pyramid has a large angle and diagonals of indentation (d 1 and d 2 ) are about 7 times larger than depth of indentation (h) especially for high hardness metals. Thus, higher accuracy can be obtained even if indentation depth is small, which makes this test especially suitable for measurement in thin layers and very hard alloys. Vickers hardness test involves the following advantages: Softaswellashardmetalscanbetested. Testsmayalsobeconductedinmicroranges. Vickers macrohardness test is independent of the applied load. The pyramidal impression damages the surface only slightly. Vickers and Brinell tests are similar to each other in principle and hardness values: Both calculate the hardness as load/area of impression. Vickers uses diamond indenter with angle of 136, resembling ball indenter in Brinell. ValuesofHBandHVofthesametestpieceareclosetoeachotherupto HB

16 Microhardness Testing Microhardness refers to indentation tests made Figure 6 Vickers indenter Knoop indenter with loads up to 1 kg using Vickers or Knoop indenters (Fig. 6). It is crucial that surface to be tested should be lapped flat and be free from scratches. After the indentation is made, its dimensions are measured by means of a high-resolution graticule under the microscope (Fig. 7). Knoop Vickers Used for small precison parts, surface layers, thin materials, small radius wires, constituents, near edges, and so on. Metallographic finish is necessary for the loads of 100 grams or less. Figure 7 15

17 Microhardness Testing Vickers Hardness Number (VHN) was calculated as: VHN = ( 2 F d ) Knoop Hardness Number (KHN) is determined by: ( 2 ) ( F L 2 ) KHN = F A = F CL = F : applied load (kg) A : unrecovered projected area of indentation (mm) L : length of the longer diagonal (mm) C : a constant relating A to the square of L As compared to Vickers indenter, Knoop indenter produces about three times greater diagonal length and about half of indentation depth. Thus, Vickers indenter is less sensitive to surface conditions for the same load, but more sensitive to errors in measuring the indentation. In microhardness testing, the hardness number is dependent on the applied load. The effect is particularly significant for Knoop indenter at loads less than 500 g and for Vickers indenter at loads less than 100 g. Such loads during testing must be applied with great care. 16