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Hardness03/01

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-- AApppplliiccaattiioonn GGuuiiddee --

Dr. Stefan Frank(February 2001)

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Hardness03/01

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1. Introduction ................................................................................................................. 31.1 What is hardness? 41.2 Why hardness testing? 41.3 On-site hardness testing? 4

2. The UCI Method (MIC 10) .......................................................................................... 52.1 Selecting the MIC probe 6

3. The Rebound method................................................................................................. 8

4. Application ................................................................................................................ 104.1 Selecting the method 104.2 Indentation size 114.3 Test piece mass requirements 144.4 Wall thickness requirements 154.5 Surface quality / roughness 164.6 Handling, Alignment and Fixing 174.7 Calibration 184.8 Verifying instrument performance 20

5. Solution of the test task ............................................................................................ 21

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Hardness03/01

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1. Introduction

Mobile hardness testing is on the advance: In these times of cost pressures and higher quality require-ments there is a quick and economical supplement to stationary hardness testing in the modern produc-tion process. The application possibilities are far ranging, this includes large as well as small parts, andespecially applies to positions which are difficult to access.There are two different physical methods which are particularly recognized in the practical field: the staticUCI method and dynamic rebound hardness testing. The decision as to which method is used mainly de-pends on the test task.

Fig. 1: Hardness testing with a UCI instrument on the tooth flanks of a pinion shaft.

Fig. 2: Hardness testing with a rebound tester on the drive wheel of a large hydraulic digger.

Krautkramer offers two series of portable hardness testers, one operating on the UCI Ultrasonic ContactImpedance principle (MIC 10) and the other on the rebound principle (DynaMIC and DynaPOCKET).

This paper explains the basic principles of both test methods and compares, using examples from thepractical field (e.g. hardness testing in the heat affected zone of welds), the application possibilities of bothmethods. In addition to this, the subjects critically discussed are the factors of influence on hardness test-ing, such as surface preparation or the wall thickness of parts to be tested, e.g. pipelines.

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1.1 What is hardness?

With regards to metals, hardness has always been a subject of much discussion among technical people,resulting in a wide range of definitions. Hardness properties include such varied attributes as resistance toabrasives, resistance to plastic deformation, high modulus of elasticity, high yield point, high strength,absence of elastic damping, brittleness or lack of ductility.To a metallurgist, hardness is a materials resistance to penetration. In general, an indenter is pressed intothe surface of the material to be tested under a specific load for a definite time interval, and a measure-ment is made of the size or depth of the indentationHardness is not a fundamental property of a material, but a response to a particular test method. Basicallyhardness values are arbitrary, and there are no absolute standards for hardness. Hardness has no quan-titative value, except in terms of a given load applied in a specific, reproducible manner and with a speci-fied indentor shape.Static indentation tests in which a ball, cone or pyramid penetrates into the surface of the material beingtested are widespread. The relationship of load to the area or depth of indentation is the measure of hard-ness, such as in common bench-top Brinell, Rockwell, Vickers or Knoop hardness testers.The different methods and differently shaped indenters used by e.g. Brinell and Rockwell produce dis-similar responses of the material under test. Tables relating to HRC and HB values are only approxima-tions there exists no mathematical equation to transfer measurements from one scale to another. So-called conversion tables have to be determined empirically by experimental evaluation of a specific mate-rials hardness with the different test methods. To compare the hardness of two different samples, bothmust be measured using the same hardness scale, or a scale must be developed to convert from onemeasurement to the other. Hardness scales are only in relationship to themselves!

1.2 Why hardness testing?

In manufacturing applications, materials are primarily tested for two reasons: either to research the char-acteristics of a new material or as a quality check to ensure that the sample meets a particular specifica-tion.

1.3 On-site hardness testing?

Conventional hardness testers such as Rockwell, Brinell or Vickers machines require the test piece bebrought to the testing device; but this is not always possible. Portable testing devices have been devel-oped that permit in-situ hardness measurements.One popular device measures the frequency shift of a resonating rod with a Vickers-diamond tip, whichoccurs when the diamond penetrates into the test material by applying a specific test load. The frequencyshift is evaluated and electronically converted to hardness value displayed on the LCD. The MICRODUR10 instrument (Krautkramer) works according this method, the so called UCI (Ultrasonic Contact Imped-ance) method.Another well-known principle for portable hardness testers is the rebound method. The DynaMIC orDynaPOCKET (Krautkramer), for example, measures the velocity of a propelled impact body directly be-fore and after the impact onto the test materials surface. The ratio between both velocities indicates thehardness of the material, which can be converted into different scales by using conversion tables stored inthe instrument for different materials.

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2. The UCI Method (MIC 10)

As in standard Vickers or Brinell hardness testing, the question as to the size of the test indentation in thematerial generated by a certain test load also arises in Vickers hardness testing according to the UCI (Ul-trasonic Contact Impedance) method. However, the diagonals of the test indentation, which have to beknown in order to determine the Vickers Hardness value, are not evaluated optically as usual, but theindentation area is electronically detected by measuring the shift of an ultrasonic frequency. This can beillustrated by a small imaginary experiment.A UCI probe typically consists of a Vickers diamond attached to the end of a metal rod (Fig. 3). This rod isexcited into longitudinal oscillation at about 70 kHz by piezoelectric transducers. Imagine instead of themetal rod (we refer to it as oscillation rod) a large spiral spring held at the end and oscillating at a resonantfrequency of 70 kHz at the free end (Fig. 4).At the very top of this spring (free end) there is a contact plate, the Vickers diamond. The test material,with which the Vickers diamond comes into contact, can also be imagined as being a system of smallerspiral springs positioned vertically to the surface - an atomic bonding, two atoms inter-linked via a "spring".If only one of these "atomic springs" is touched by the Vickers diamond in this imaginary experiment - likevery hard material in which the diamond only slightly penetrates and thus produces a small indentation then an additional spring, i.e. mass, is coupled to the large spiral spring. By doing this, the resonant fre-quency shifts due to this additional mass / spring.

Fig 3: Schematic description of the UCI probe Fig 4. UCI principle in an imaginary experiment: an oscillating spring in contact with material. The spring symbolizes the oscillating rod, the contact plate symbolizes the diamond, the material springs symbolize the material and its elastic constants.

This frequency shift will become greater when additional "springs" are touched, that means if the diamondpenetrates deeper into a material of medium hardness, and the test indentation becomes larger. Analo-gously, the largest frequency shift is produced by soft test materials; the diamond penetrates deeper intothe material and leaves a large indentation.This is the secret of UCI hardness testing: the frequency shift is proportional to the size of the test inden-tation produced by the Vickers diamond. Equation (1) describes this basic relation in comparison to thedefinition of the Vickers hardness value.

Piezo Transducer

Vickers Diamond

Oscillating Rod

Piezo Receiver

Material

Fixture

Spring

Material Springs

Contact Plate

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