Properties of Metals Hardness. Prepared By: John Cawley

Transcription

1 Properties of Metals Hardness Prepared By: John Cawley

2 Presentation Objectives Explain what hardness is. Describe how the hardness of metal is found. Compare different hardness testing methods. Examine how each hardness testing method works. Convert between hardness scales.

3 What is Hardness Defined as a measure of resistance to deformation or its resistance to another material penetrating its surface. Hardness is proportional to the size of penetration. The harder the material, the smaller the penetration.

4 Hardness Testing Methods Brinell Hardness Vickers Microhardness Knoop Microhardness Rockwell Hardness Rockwell Superficial Hardness Shore Scleroscope Hardness Sonodur Hardness Mohs Hardness File Hardness

5 Brinell Hardness Test Dr. J. A. Brinell invented the Brinell test in Sweden in The oldest of the hardness test methods in common use today, the Brinell test is frequently used to determine the hardness of forgings and castings that have a grain structure too course for Rockwell or Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying the test force and ball size, nearly all metals can be tested using a Brinell test. Brinell values are considered test force independent as long as the ball size/test force relationship is the same.

6 Brinell Hardness Testing Uses a 10mm diameter tungsten carbide sphere. The ball is pressed into the test piece under loads ranging from 500kg to 3000kg. The load is applied for 10 to 15 seconds. A microscope with a calibrated lens is used to measure the diameter of the dent.

7 Brinell Test Strengths One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same. A wide range of test forces and ball sizes to suit every application. Nondestructive, sample can normally be reused. Used on softer alloyed material such as Aluminum or Iron and Steel castings.

8 Vickers Hardness Test The Vickers (HV) test was developed in England is 1925 and was formally known as the Diamond Pyramid Hardness (DPH) test. The Vickers test has two distinct force ranges, micro (10g to 1000g) and macro (1kg to 100kg), to cover all testing requirements. The indenter is the same for both ranges therefore Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV1000).

9 Vickers Microhardness Testing Uses a 136 degree squarebased diamond cone penetrator. A 50kg maximum force is applied to the test piece. The length of the indentation diagonal can be measured using a microscope, calibrated micrometer barrel, digital readout, or computer software which can display the image.

10 Vickers Test Strengths One scale covers the entire hardness range. A wide range of test forces to suit every application. Nondestructive, sample can normally be used. Test can be used on harder materials because the pointed penetrator can probe into a hard surface more easily than a ball penetrator. Used on smaller areas of a part. Requires a smaller load.

11 Knoop Hardness Test Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests.

12 Knoop Microhardness Testing Uses a rhombic -shaped penetrator having angles of 130 and degrees. A load of less than 4kg is applied to the test piece. A load of 25g can be used for extremely small areas. The length of the indentation diagonal can be measured using a microscope, calibrated micrometer barrel, digital readout, or computer software which can display the image.

13 Knoop Test Strengths The elongated diamond indenter and low test forces allows testing very small parts or material features not capable if being tested any other way. One scale covers the entire hardness range. Test essentially does no damage to the specimen. Can be used on very thin material. Can be used on very small surface areas. A wide range of test forces to suit every application.

14 Brinell, Vickers & Knoop Test Weaknesses The main drawback of the Knoop test ingeneral, is the need to optically measure the indent size. This requires that the test point be highly polished to be able to see the indent well enough to make an accurate measurement. Slower Process - Testing can take 30 seconds not counting the sample preparation time.

15 Rockwell Hardness Test Stanley P. Rockwell invented the Rockwell hardness test. He was a metallurgist for a large ball bearing company and he wanted a fast nondestructive way to determine if the heat treatment process they were doing on the bearing races was successful. The only hardness tests he had available at time were Vickers, Brinell and Scleroscope. The Vickers test was too time consuming, Brinell indents were too big for his parts and the Scleroscope was difficult to use, especially on his small parts.

16 Rockwell Hardness Testing Uses either a 1/16 or 1/8 diameter tungsten carbide ball otherwise it uses a conical-shaped diamond point penetrator. Loads of 60kg, 100kg, or 150kg are used. Hardness values are read directly from a dial gauge or digital readout.

17 Rockwell Superficial Hardness Testing Uses a 1/16 diameter tungsten carbide ball otherwise it uses a conical-shaped diamond point penetrator. Loads of 15kg, 30kg, or 45kg are used. Hardness values are read directly from a dial gauge or digital readout.

18 Rockwell Test Strengths & Weaknesses Rapid test, usually less than 10 seconds Direct readout, no questionable optical measurements required. Non-destructive, part normally can be used. Thin materials can be tested. Multiple test scales (30) needed to cover the full range of metal hardness. Conversions between scales can be material dependant. Samples must be clean and have a smooth test point to get good results.

19 Scleroscope Hardness Test The Scleroscope is a very old hardness testing instrument, originating in the early 1900's. It is a dynamic indentation hardness test that drops a diamond tipped hammer vertically from a fixed height onto the surface of the material under test. The height of the rebound of the hammer is a measure of the hardness of the material. Scleroscopes are no longer produced in this country, however many are still in use.

20 Shore Scleroscope Hardness Testing Uses a small metal ball or diamond tipped hammer. The 3g hammer ball is dropped in a graduated glass tube from approximately 10 inches. The hammer rebounds off the test piece and a number is read on the glass tube, the higher the bounce the harder the material.

21 Sonodur Hardness Testing Uses a diamond tipped magnetostrictive rod 0.75mm in diameter. The rod is pressed against the test piece and an electrical coil is used to vibrate the rod. The frequency at which it vibrates is measured. The harder the material, the higher the frequency.

22 Mohs Hardness Test The Mohs hardness scale was introduced in 1822 by the German mineralogist Friedrich Mohs, and is still used as a classification for some minerals. It simply consists of 10 minerals arranged in order from 1 to 10. Diamond is rated the highest and is indexed as 10. Talc is the softest with an index number of 1. Each mineral in the scale will scratch those with a lower number. A completely arbitrary scale, the Mohs hardness has little value to an engineer. Copper would be in the 2 to 3 range, iron would be between 3 to 4 and hardened steels are in the 7 to 8 range.

23 Mohs Scale Hardness Testing Uses ten stones of varying hardness. The hardest stone has a value of ten while the softest stone has a value of one. If the test sample is scratched by the stone it is softer than the stone. No longer used except in the field of geology. The difference between #9 and #10 is about as great as the difference between #1 and #9.

24 Basic Tools for Scratch Hardness Mohs Testing Finger Nail 2.5 Penny (old) 3.5 Knife Blade 5.5 Steel File 6.5

25 Conversion Scales Since hardness can be measured in many different units, using different machines and scales. They must be able to be converted from one method to the next. This is accomplished through the use of conversion charts. Use your textbook as a guide to cross-reference readings between methods.

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