Waygate Technologies MIC 10 User Manual

FAQ's – Hardness Testing

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FAQ 1: How do Rebound testers such as the DynaMIC and DynaPOCKET work?..........................................2

FAQ 2: How does the MIC 10 use ultrasonics to measure hardness?................................................................3

FAQ 3: What is the proper technique for carrying out a hardness measurement?..............................................5

FAQ 4: What model of your portable hardness testers will best solve my application? .................................... 6

FAQ 5: How can my parts mass and thickness affect the results and what are the minimum requirements?.....7

FAQ 6: What surface finish is needed and how can I properly prepare it?........................................................9

FAQ 7: How do I select the best MIC 10 probe for my application?...............................................................10

FAQ 8: How do I select the right DynaMIC impact device for my application?.............................................11

FAQ 9: Does gravity affect the results of the instrument? ...............................................................................12

FAQ 10: Does your portable hardness test equipment meet ASTM standards?...............................................13

FAQ 11: When is it necessary to calibrate the instrument and how is this accomplished?..............................14

FAQ 12: What is the accuracy I can expect and how is the equipment’s performance verified?..................... 16

FAQ 13: How thin a coating or surface treatment can I measure using the MIC 10? ......................................17

FAQ 14: How do the sizes of the indentations produced by the various portable hardness testers compare?.18

FAQ 15: What is an effective method to measure the HAZ on welded parts?................................................. 19

FAQ 1: How do Rebound testers such as the DynaMIC and DynaPOCKET work?

The rebound method indirectly measures the loss of energy of a so-called Impact Body. A spring projects it towards the test piece and its spherical indenter str ikes the object ’s surface at a defined speed. The indentation created absorbs a portion of the energy thereby reducing its original speed. The sof ter the material t he larger the indent ation and the hig her the loss of energy. The velocities before and after t he impact are each measur ed in a non-cont act mode. T his is accomplished by a small permanent magnet within the impact body ( see f igure) that induces a voltage during its passage throug h a coil. The voltage created is proportional to the speed as shown in the figure below.

Cross-cut of a typical impact device Voltage signal generated by the impact body

travelling through the coil. The signal is shown before and after the impact.

The inventor of this method, D. Leeb, defined his own hardness value, the Leeb hardness value. The Leeb hardness value, HL, is calculated from t he ratio of the im pact and rebound speed according to: HL = 1000 B/A with A, B = speed before / after t he im pact Who uses the Leeb value? The fact is that although the HL is the actual physical measurement value behind this method rarely does a user indicate the Leeb value in his specifications or test report s. Normally he reports a converted har dness value (HV, HB, HS,

HRC, HRB, N/mm

). Therefore conversion tables for various material groups are stored

within the instrument. The graphic below illustrat es such a conversion t able.

80,0 70,0 60,0

HRC

50,0 40,0 30,0 20,0 10,0

500 600 700 800 900

Conversion of Hardness Leeb, HL, into HRC as a typical example for conversion tables stored in rebound hardness testers. These curves are experimentally generated by material samples of different hardness measured b

rebound and Rockwell test.

FAQ 2: How does the MIC 10 use ultrasonics to measure hardness?

Conventional Vickers or Brinell hardness testing requires optical evaluation of the area of an indentation produced by its indentor under a specified load. Testing using the UCI (Ultrasonic Contact Impedance) met hod the diagonals of the test indentation, which have to be known in order to determine the Vickers Hardness value, are not evaluated optically as usual, but the indentation area is electronically detected by measuring the shift of an ultrasonic frequency.

A UCI probe consists of a Vickers diam ond attached to the end of a metal rod. This rod is excited into a longitudinal oscillation of approximately 70 kHz by Piezoelectric transducers. Imagine the rod as a large spiral spr ing held at one end and f ree to oscillate at the resonant frequency at the other end. At tached to the f ree end is a contact plate, t he Vick ers diamond. Now picture the surface of the material to be comprised of a system of smaller spiral spring s positioned vertically to the surface with the quantity of these springs r epresenting the elastic properties of the material (Refer t o FAQ 11 for more information reg arding how to properly calibrate the instrument f or a m aterials elastic modulus).

The diamond’s penetration depth into the ma terial is determined by the m aterial’s hardness with a very hard material having a shallow indentation allowing only a few of these "at omic springs" to contact the diamond r esulting in a slight frequency shift. On the other hand if a softer part is tested, the diamond penetration is deeper and the frequency shift is more significantly as additional "springs" are touched. This is the secret of UCI hardness testing: the frequency shift is proportional to the size of the t est indentation produced by the Vickers diamond.

Piezo Transducer Piezo Receiver

Oscillating Rod

Vickers Diamond

Material

Schematic description of the UCI probe UCI principle in an imaginary experiment:

an oscillating spring in contact with material. The large spring represents the oscillating rod, the contact plate represents the diamond, the smaller springs represent the material and its elastic constants.

Fixture

Spring

Contact Plate Material S

rings

The equation below describes this basic relation in comparison to the definition of the Vickers hardness value.

AEf

⋅≈∆

elast

HV =

The graphic below illustrates the relationship of frequency shift to har dness.

900

700

500

300

100

2 2,5 3 3,5 4 4,5 5

Frequency shift [kHz]

Vickers Hardness value versus frequency shift of the oscillating rod.

FAQ 3: What is the proper technique for carrying out a hardness measurement?

DynaMIC Series Including the DynaPOCKET Pressing the Loading Tube of the impact device of t he DynaPOCKET and DynaMIC grasps and suspends the impact body. Upon pressing the Release Button a spring propels it towards the part and hardness value is updated.

Aligning the impact device within 3° of being perpendicular to the surface is required. The standard support rings provided with each Dyna D and Dyna E can be used to test convex or concave radii greater than 30 mm (1.2 in.). The larger diameter of the Dyna G standard support ring requires the r adius to be greater than 50 mm (2. 0 in. ).

Support rings are offered as accessories that can be used with the Dyna D and Dyna E impact devices. They are available to cover the range of 10-30 mm (0.4 to 1.2 in.) f or t esting the ID’s or OD’s of cylindrical and spherical shaped parts (see Dyna 41 and Dyna 42). Custom support rings are also available on request.

MIC 10 Series The recommended technique for using a handheld probe is to use one hand to steady the probe at the bottom while the other hand applies the load in a slow and controlled manner. Force is continuously applied until the end of the probe sleeve contacts the material at which time the instrument displays the updated hardness value.

Caution: To prevent damage to the diamond care must be taken not to twist the

probe while the diamond is in contact with the test material.

To carry out a reading, the probe must be aligned perpendicular to within 5° of part’s surface. Of course, the m ore precise t he alig nment t he mor e consist ent the result s. T o aid in alignment the protection sleeve can be removed and replaced with various probe shoes. For example a probe shoe with a V groove base is handy when testing cylindrical parts having a radius of 3 - 75mm (.12 - 3.0 in.) And a flat pr obe shoe, although designed primarily to test flat surfaces, also can be used in test ing radii greater than 75 mm (3.0 in.).

FAQ 4: What model of your portable hardness testers will best solve my application?

Several factors enter in the decision as to what method and portable instrument packag e is best suited for a particular applicat ion.

As with conventional hardness testers the size of the indentation produced by the portable equipment is extremely important in det ermining its suitability for a hardness application. To obtain accurate and repeatable readings the indentation must cover several grains of the materials microstructure; must be proportionally larger than the surface roughness; and if testing coatings or surf ace hardened components their thickness must be at least 10 times larger than the indentation depth so as not t o be affected by the softer subst rate material.

The UCI method is recommended for testing fine grained material having any shape and size. It is especially used where material properties are to be processed with narrow tolerances, e.g. f or det ermination of strain hardening on dr op forged parts.

Rebound hardness testing is carried out on large, coarse grained mater ials, f or ged par ts and all types of cast materials because the spherical tip of the impact device produces a r ather larger indent than the Vickers diamond and therefore processes the characteristics of the casting structure better.

With the small indent of the Microdur UCI probes, determination of the hardness can be made on welded parts in the critical area of the weld, the heat affected zone (HAZ).

The Leeb and UCI methods can be influence by the mass and thickness of the part to be tested. Therefore bot h of these factors must be considered in deter mining the best method (refer to FAQ 5 for additional information) .

A number of probes and impact devices having diff erent test loads provide a large rang e of applications.

Application UCI testing Rebound testing

Solid parts

Coarse grain materials

Steel and aluminium cast alloys

HAZ with welds Tubes: wall thickness > 20 mm Tubes: wall thickness < 20 mm

Inhomogeneous surfaces

Thin layers

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-++

o++ ++ ++ ++ ++ -

++ -

Difficult to access positions

(++ especially suited / + well suited / o suited sometimes / - not recommended)

Applications for UCI and rebound hardness testing.

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