MECHANICAL TESTING OF METALLIC MATERIALS

Transcription

1 MECHANICAL TESTING OF METALLIC MATERIALS

2 Outline Hardness Testing Tensile Testing Impact Testing Fatigue Testing Creep Testing Wear Testing 22-Jan

3 MATERIALS PROPERTIES Design Engineer - Materials properties Materials Properties CHEMICAL Composition Crystal structure Bonding MECHANICAL Hardness Strength-ductility Impact Fatigue Creep wear PHYSICAL Density Thermal Electrical Magnetic DIMENSIONAL Size Shape Microtography SERVICE LIFE 22-Jan LT/HT/RT

4 HARDNESS TESING Hardness is resistance of material to plastic deformation caused by indentation Deeper or larger impression Softer materials most plastics brasses Al alloys easy to machine steels file hard cutting tools nitrided steels diamond increasing hardness 22-Jan

5 HARDNESS Tribologist : high hardness desirable to reduce plastic deformation and wear in bearings Machinist : - low hardness preferable for easy and faster machining Fracture mechanics : hard material brittle engineer undesirable under impact loads Metal forming : low hardness preferable for engineer cold rolling 22-Jan

6 BASIC PRINCIPLE Principle of hardness test method : - forcing an indenter into the sample surface followed by measuring dimensions of the indentation (depth or actual surface area of the indentation) 22-Jan

7 TYPES OF HARDNESS MEASUREMENTS There are three general types of hardness measurements 1) Scratch hardness The ability of material to scratch on one another Important to mineralogists, using Mohs scale 1= talc, 10 = diamond Not suited for metal annealed copper = 3, martensite = 7. 2) Indentation hardness Major important engineering interest for metals. Different types : Brinell, Meyer, Vickers, Rockwell hardness tests. 3) Rebound or dynamic hardness The indentor is dropped onto the metal surface and the hardness is expressed as the energy of impact 22-Jan

8 CLASSIFICATION HARDNESS TESING -Depending on the loading force value & the indentation dimensions Macro-hardness tests Micro-hardness tests Nano-hardness test 22-Jan

9 HARDNESS TESING Macro-hardness tests : - the most widely used methods for rapid routine hardness measurements - indenting forces in macro-hardness tests are in the range of 50N to 30000N Brinell hardness test Rockwell hardness test Vickers hardness test 22-Jan

10 BRINELL HARDNESS TEST - One of the oldest tests Static test that involves pressing a hardened steel ball (10mm) into a test specimen while under a load of 3000 kg load for hard metals, 1500 kg load for intermediate hardness metals 500 kg load for soft materials BALL PRESSED INTO THE SURFACE INDENTATION PRODUCED IN 22-Jan-11 SURFACE

11 BRINELL HARDNESS TEST If the BHN value is higher, then the material is said to be harder where BHN = the Brinell hardness number P = the imposed load in kg D = the diameter of the spherical indenter in mm d = diameter of the resulting indenter impression in mm 22-Jan

12 BRINELL HARDNESS TEST Rules of thumb 3000 kg load should be used for a BHN of 150 and above 1500 kg load should be used for a BHN between 75 and kg load should be used for a BHN less than 100 The material s thickness should not be less than 10 times the depth of the indentation 22-Jan

13 BRINELL HARDNESS TEST Brinell Test can not be used : very soft or very hard materials (Indentation becomes equal to the diameter of the ball or No or little Indentation) Accurate measurement : -impression should be a diameter between 0.25D and 0.50D 22-Jan

14 BRINELL HARDNESS TEST Limitation of Brinell Hardness Test : Materials with hardness upto 450HBN with steel ball Materials with hardness upto 600 HBN with tungsten carbide ball Brinell numbers greater than 650 should not be trusted because the diameter of the indentation is too small to be measured accurately and the ball penetrator may flatten out. 22-Jan

15 Brinell Test Method Advantages Well known throughout industry with well accepted results Tests are run quickly (within 2 minutes) Test inexpensive to run once the machine is purchased Insensitive to imperfections (hard spot or crater) in the material Limitations Not well adapted for very hard materials, wherein the ball deforms excessively Not well adapted for thin pieces Not well adapted for case-hardened materials Heavy and more expensive than other tests 22-Jan

16 ROCKWELL HARDNESS TESTING Rockwell Hardness Test : - widely used method (accuracy, simplicity and rapidity) -there are two types of Rockwell tests (i) Rockwell: - the minor load is 10 kgf, the major load is 60, 100, or 150 kgf. (ii) Superficial Rockwell: -the minor load is 3 kgf and major loads are 15, 30, or 45 kgf. In both tests, the indenter may be either a diamond cone or steel ball, depending upon the characteristics of the material being tested. 22-Jan

17 ROCKWELL HARDNESS TESTING (ASTM18) Principal of the Rockwell Test 1. The indenter moves down into position on the part surface 2. A minor load is applied and a zero reference position is established (10kg) 3. The major load is applied for a specified time period (dwell time) beyond zero 4. The major load is released leaving the minor load applied - The Rockwell number represents the difference in depth from the zero reference position as a result of the applied major load. Deeper indentation Softer material 22-Jan

18 ROCKWELL HARDNESS TESTING Rockwell Indenters: hardened steel ball Diamond cone or Rockwell hardness numbers are always quoted with a scale symbol representing the indenter and forces used. The hardness number is followed by the symbol HR and the scale designation. Ex; 64 HRC= Rockwell hardness number of 64 on Rockwell C scale. 81 HR30N = Rockwell superficial hardness number of 81 on Rockwell 30N scale. 22-Jan

19 Superficial hardness test : Initial force : 29.4 N 22-Jan

20 ROCKWELL HARDNESS SCALES Superficial hardness test : Initial force : 10kg Note: The diameter of the balls arise from standard sizes in inches, 1.588mm being 1/16in, 3.175mm being 1/8in, 6.350mm being 1/4in, and 12.70mm being 1/2in 22-Jan

21 ROCKWELL SUPERFICIAL HARDNESS SCALES Superficial hardness test : Initial force : 29.4 N Note: N scales are used for materials that if thick enough would have been tested 22-Jan-11 on the C scale, the T scales for 11111those on the B scale 21