LAB 2 HARDNESS TEST OBJECTIVES (1) Make Brinell, Vickers and Rockwell hardness measurements on different materials specimens. (2) Compare the measurements in hardness with each other. INTRODUCTION Hardness can be defined as resistance to penetration, resistance to abrasion, resistance to scratching, or resistance to cutting. These are related in that they all require plastic flow of the material. A variety of tests for hardness are in use, depending on which of the above situations is of most interest. Material properties, such as hardness can be altered to desired levels by various heat treatment procedures. Heat treatments affect the microstructure of the metal. Detailed descriptions of the changes to the microstructure can be found in material science (1) lecture notes. This experiment will include Brinell, Vickers and Rockwell tests which measure resistance to penetration. The amount of deformation that occurs when a small, hard steel ball or diamond point is pressed into a material surface at some designated load is the measure of the hardness of the material. BRINELL HARDNESS TEST The Brinell test for this experiment uses steel ball with different diameters which is pressed into a specimen by a different load that is maintained for 15 to 30 seconds. The Brinell hardness number (BHN) is calculated by: Where: P is the test load [kg] D is the diameter of the ball [mm] d is the average impression diameter of indentation [mm] Since the Brinell number is based on the area of indentation, the diameter of the indentation must be measured. This is done with a microscope. The scale seen through Hardness Test 1/5
the microscope is in millimeters. The larger diameter indentation corresponds to a softer material and lower Brinell number. VICKERS HARDNESS TEST The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation. P is the load and it can be varied from 1 to 1000 grams d is the average diagonal diameter of indentation [mm] When the mean diagonal of the indentation has been determined the Vickers hardness may be calculated from the above formula. The Vickers hardness should be reported like 800 HV/10, which means a Vickers hardness of 800, was obtained using a 10 kgf force. The advantages of the Vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is used for all types of metals and surface treatments. ROCKWELL HARDNESS TEST The penetrators for the Rockwell hardness tester range from 1/2-inch diameter steel balls to very small diamond (brale) tips (points). The smaller points are used for harder materials that have a greater resistance to indentation. There are various force scales used for various materials. The Rockwell B and Rockwell C scales will be used for this experiment. The Rockwell B scale is suitable for soft engineering metals, and the Rockwell C scale is appropriate for hard engineering metals. Each scale requires a specified tip and load. The B scale uses a 1/16- inch diameter hard steel ball and a 100- kg load. The C scale uses a conical diamond point and a 150-kg load. Hardness Test 2/5
To perform the Rockwell tests, the penetrator is pressed against the specimen with an initial 10-kg preload to properly seat the penetrator. The remaining load is applied gradually after the dial on the hardness tester has been zeroed. After the penetrator has stopped moving into the specimen, the final position of the dial pointer indicates the Rockwell hardness number that is related to the depth of penetration. There are numerous scales in Rockwell test, which are made possible by the combination of different loads and indenters. The scales are identified by letters and the common scales range from A through K, with I and J left out, as given in the following table Hardness Test 3/5
MATERIALS TO BE TESTED Different materials specimens will be tested in this laboratory experiment namely: aluminum alloy, carbon steel, stainless steel, Al-alloy, commercial pure copper and brass. The hardness of mild-steel washer will also be tested to check the validity of Brinell hardness test. EQUIPMENT TO BE USED: Zwick/ZHU187.5 - Universal hardness tester up to 187.5 kg STUDENT WORK Selected students will be invited to perform Brinell and Rockwell hardness test, instructor will do perform Vickers test in order to avoid the diamond pyramid damaging that could be taken place. For Brinell experiment, student has to calculate the BHN and depth of impression (h) through the following formulas for each material tested: For Vickers experiment, student has to calculate the VHN through the following formula for each material tested: For Rockwell experiment, student has to calculate the depth due to major load (t) through the following formulas for each material tested: Hardness Test 4/5
Rockwell Hardness Test Vickers Hardness Test Brinell Hardness Test Results: Exp. Type Material Mild steel Stainless steel Al alloy Pure copper Brass Steel washer d 1 [mm] d 2 [mm] d=(d1+d2)/2 [mm] P [kg] D [mm] H [mm] [mm] d/d H/h d 1 [mm] d 2 [mm] d=(d 1 +d 2 )/2 [mm] P [kg] R C (diamond & 140 kg) R B (1/16 & 90 kg) R E (1/8 & 90 kg) R H (1/8 & 50 kg) Comment: Hardness Test 5/5