hardness testing hand out

8/2/2019 Hardness Testing Hand Out

1/25


2/25

Hardness is the property of a material to resist plastic deformation against penetration.

Measurement of hardness is called Hardness Testing.

Types of Hardness Testing:Hardness Testing

(1) (2) (3)Approximate Accurate and Productive Super Accurate

Hardness Type Hardness Hardness

Testing Testing Testing

(1)Approximate Hardness Testing Methods:a) Very rough idea By Soundb) Approximate Idea File Hardness Test for Hardened metals and case

hardened

items

c) Nearly Accurate Poldi Hardness Testing for items having no morehardness

Than 450 HB

d) Nearly Accurate Ball rebound Hardness Tester for items having highhardness.

Poldi Hardness TestingParts of the instrument:

a) Body (steel body) with plunger, magnifying glass (Lens) and scale fittedb) Standard Test Pieces (with known hardness)c) Other items: hammer 1.5 or 2 Kg., piece for testing (Smooth ground, at least some

area in one plane)

Procedure: Testing piece is being placed on any rigid support and standard test piece isbeing fixed in the groove made for it. Plunger of the tester is struck by the hammer in an

exactly vertical direction. The steel ball will leave its impression on the standard piece

and also on the testpiece.

Let the hardness of the testpiece (already known) be = Hs.the dia of the impression made by the ball on the

Standard test piece be =Ds and

the dia. Of impression made by the ball on

the actual testpiece be = Da.


3/25

28


4/25

Then the hardness of the actual test piece Ha = Hs x (Ds/Da)2.

n actual practice, some calculated charts are provided alongwiht the instrument. With the

help of such charts the hardness value can be noted by measuring only the diameter of the

mpressions made by the ball on the standard and the actual test pieces.

With these instruments we can check big casting and other objects which can not be adjusted

roperly on the testingtable of other harnesstesters.

2) Accurate Hardness Testing called Rockwell Hardness TestingThis is a very quick and productive type method of hardness testing and has wide ranges

f scales according to the different values of loads used for different purposes.

n this method, the following type of scales are used.

HRA, HRB, HRD, HRE, HRF, HRG, HRH, HRK, HRN and HRTOut of which only some are more popular.

n this method a dial is directly calibrated in terms of hardness.

.002 mm depth of penetration will be equal to a unit of hardness in that particular scale (for

articular loads).

29


5/25


6/25

Hah3 = Elastic DeformationH3h1 = Plastic DeformationHardness = K- (h

3h1)

C

K = Constant

For bralle (diamond core, K = 0.2

For ball 1/16 K = 0.26C = Value of scale Division = 0.002

Thus hardness = K- (h3-h1)

0.002

Depth of penetration is inversely proportional to the hardness of the materials.

(a) Accurate Hardness Testing of the Soft MaterialsHardness in Rockwell B Scale (HRB)

In this method 1/16 steel ball is being used as a penetrator. 10 kg. minor load is applied and 90 kg

additional load is being used. Hardness can be directly measured in dial indicator. This is thelimitation that by this method only those materials can be checked which have hardness below 20

HRC.

For very thin and soft materials we can use T scales in which minor load is 5 kg. and additional loa

are 10,25,40 kg.

Accurate Hardness Testing of materials which have Hardness Rockwell Hardness testing C Scale

By this method, we can check the hardness of the materials accurately which have hardness of morthan 20 HRC. Materials having hardness less than 20 HRC are not advisable to check by this metho

In this method, 1200

angled brale is being used as a penetrator. At the tip of brale 0.2 mm radius

diamond is fixed.


7/25


8/25

PENETRATOR FOR ROCKWELL HARDNESS TESTIN.

For testing for hardened tool materials and deeply case hardened materials 10 Kg minor

and 140 kg additional load is applied. For thin sheets or thin casehardened materials ascale is being used in which minor load is 10 Kg. and additional load is 50 Kg. for very

much thinner hard sheets and thinner case carburized materials. We can use N scales, in

which minor load is 5 Kg and additional loads are 10, 25 and 40 Kg respectively.

A, B and C scales are widely used. Below 50 HRC 1.5 HRC and above 50 HRC 2 HRC

is admissible. Correction table is available for round items as per dia.

(3)Super Accurate Hardness Testing(4)uper Accurate Hardness Testing

To measure the hardness by this method we have to get the surface area of the

impression made by indentors, which can be measured very accurately. That is why this

method of hardness testing is called Super Accurate Hardness Testing.

(a)Hardness Testing of materials having low or medium hardness (Brinell HardnessTesting)

The test is more suitable for the materials having hardness less than 450 BHN (just like

for castings, hotrolled, normalized and annealed steel, temperhardened steel, Cu, Aland other alloys.)


9/25

31


10/25

In this method, steel balls of different diameters are being used as indentors. Generally

10,5 and 2.5 mm balls are used which depends upon the thickness of the material.

Selection of load is dependent upon the materials:

30 d2

for steel and cast iron

10 d2

for copper and its alloys

5 d2

for a1 and its alloys

2.5d2

for soft (white) metals, bimetals.

After applying the above calculated weights the ball will make its impression on the

testing materials. Harness of the ball should be 1.7 times than the hardness of the materials

which are to be tested.

After removing the load, we can calculated the value of hardness:

Hardness of the material (BHN) = Applied load in kg

Area of the impression in mm2

= Load (P) D (D-D2-d2)

= 2 load

D (D-1 (D2-d2)Where D = diameter of the ball used as a penetrator

d = diameter of the impression made by the ball.

In usual practice, a calculated chart is being provided along with the hardness tester.With the help of that of that chart we can get the hardness by simple nothing the diameter

of impression made by the ball used the items for testing; there is no need for calculation.


11/25


12/25

(B) Superaccurate Hardness Testing of materials having higher hardnessVickers Hardness Testing

This type of testing is used for materials which have high hardness. A correct hardnesstesting is that in which within the limit, maximum possible load is applied. This is the main

advantage in this type of testing that we can use a wide range of loads ( 5, 10,20,30,50,100, or

120 kg) as per the requirement. Selection of load can be done by the following formula.

F = H.T.2

1.8544

Where H = approx. hardness of the test piece.

T = thickness of the material or casedepth.

In this test 1360

angle pyramid shaped, diamond tipped indentor is used. There is norequirement of any initial load, the abovementioned load can be used as per the requirement.

Testingpiece should be fine polished. After applying selected load indentor will make itsimpression on the testing item. By measuring the diagonals of the impression, we can calculate

the hardness of the testing item by the following formula:

Where P = Load in kg.

= angle between two faces of the impression.

= 1260

D = d1+d2

( d and d2

are diagonal lengths of the impression)

Thus HV = 1.8544 x P

D2

By this method we can check the hardness of the hardened tools, casehardened items andother metals having high hardness.


13/25

33


14/25

(4) Micro Hardness TestingHardness testing under very low load is micro hardnesstesting in this testing very lowload series is being used as a force. By this method we can measure the hardness of very

fine and thin items like cutting edge of cutters, wires, thin sheets. Even hardness can be

checked at different phases and grains of micro structures. 10 gram to 10 kg weight is used

for testing.

To measure the accurate diameter of the impression, it is magnified 100, 200, 400, and 800

times as per requirement. Since this is a very accurate measurement, the thickness of the

calibration line is also taken into account. The diameter of the impression is measured in

such way that the thickness of the calibration line gets eliminated. This is also useful for

measuring the effective case depth of case hardened materials. By using the followingformula, we can calculate the hardness number:

HV = 1.8544 X p/d2

Where p = load in kg

D = diameter of the impression in mm

Utmost care is taken at the time of the preparation of the sample for testing;

The testing surface of the sample should be parallel to the base surface and that should be

polished mirror like without any heating and surface distorting. If the sample requirescutting, then it should be cut without heating so that every property and structure remains

the same.

Distortion

Even after taking all precautions during heat treatment of steel items, distortion can notbe fully avoided. The tendency for getting more distortion depends mainly ona) Type of steelb) Dimensionc) Fixtures availabled) Other precautions.


15/25

34


16/25

a) Type of SteelThe most critical point for getting distortion during heat-treatment is by the quenching

process, because of stresses developing in the quenched items by cooling down so fast

Especially by items with complicated cross-sections, by which the cooling proceeds no

process to get maximum hardness, the greater the extent of distortion (non-uniform

steels) that develops in the heat treated job. Since unalloyed steels need a very fas

quenching (water) to achieve maximum hardness, the tendency for distortion is ver

high. Alloyed steels do not require such fast cooling (by hardening ) to achiev

maximum hardness can be seen in TTT- diagram.

It should be known: unalloyed steel items will get always more distortion by H.T than

alloyed steel items.

To reduce distortion to an absolute minimum, non-distortion steels are available. Th

speciality of non-deforming steels (p.ex.OHNS = 90MnV8)are:

- low austenitizing temperature required- low cooling rate4 (quenching) sufficient to achieve hig

hardness.

Non-distortion (non-deforming)steels are recommended for following components:

- if no or very little grinding allowance is given (and H.T irequired)

- if items are of very thin or long siz(p.Ex.5mmx10mmx150mm)

- if non-uniform quenching is expected because ocomplicated cross-section on the item to be heattreated.

- If straightening would be impossible or could be manageonly with great difficulty (after H.T.).


17/25

35


18/25

Usually components with thicker cross-sections (diameter above 50mm) have

lesser tendency to get distorted/deformed during heat-treatment. Therefore als

other low-alloyed steels (en-18, En-19, En-24) are suitable to keep distortion t

minimum. Also highly alloyed steels have lesser tendency to get distorted durin

heat- treatment, but they are too costly to be used only for the purpose of getting le

distortion and no other high properties are required for the component.

It should be known that of distortion would be present after heat-treatment in item

of alloyed steels (including High x Carbon Steels), straightening would be very

difficult. To maintain H.T. jobs distortion free, attention should be given to otheprecautions (d) to be made by all heat-treatment processes.

By using non-alloyed steels. Always distortion will be present after heat-treatin

them for maximum hardness(above HRC 35). It should be avoided to use this stee

for manufacturing of titems with less grinding allowance or to put unde

consideration that straightening has to be done.

b) DimensionsVariations in the cross-section of one item (holes, thick and thin diameters)always lead

to

- No uniform relieving of stress during heating because ofaster heating smaller section

(=extension ).

- Non-uniform developing of stress during quenchingbecause of faster cooling of smaller sections (=shrinking).

To keep these stresses to a minimum, preventive measures have to be taken adescribed under type of steels (=selection of steels )And other precautions(normalizing , stress-relieving, pre-heating, alternativquenching).


19/25

36


20/25

c) Fixtures availableFixtures help to keep items to be heat-treated in a proper position (=vertical o

uniform supported )in the furnace during quenching.


21/25

37


22/25

d) Other precautionsNormalizing (of raw materials)

Items which have gotten their rough shape due to a previous cold or warm-formin

process(forging,

rolling etc.) have a non-uniform structure. Transformation of structures during heating

up for

austenitizing will lead also to change of volume (different atomic configuration )

To this transformation os also added transformation of grain sizes, if these are no

uniform (as after forming processes), resulting in more deformations.

Stress-relieving

Items getting mechanical stresses during aching, especially during tough machinin

(shaping, honing ) with heavy cut and large feed.

Theses tresses get relieved fast by heating up item above transformationline, For thslow-relieving of these stresses without deformation, a process called stress-relirein

(450 to 650 C for minimum 3 hours) is required after rough-achining. (By finamachining, items will absorb only minor stresses, which are not worth mentioning in

the context of precautions against distortion).

Pre-heating

It provides more uniformity by heating up of the items, because temperature gaps arnot so large. It also relives some stresses. Therefore highly alloyed steel items need

more pre-heating steps because of higher final maddening temperature (=austenitizin

temperature)


23/25


24/25

Also because of reaching the final temperature more uniformly (thinner to thicke

sections), it prevents thinner cross-sections against over-timing.

Austenitizing Temperature

For high distortion sensitive items, lowest possible austenitizing temperaturshould be selected, because of temperature gradiation.

Alternative quenching (if possible )

To minimize stresses which develop during quenching:

Saltwater .. Water oil emulsion.Oil .. heated oil (70/80 C )

Quenching salt bath air.

Or interrupted hardening =suppose for ites to be normally water-quen

ched=quenched for very fiw seconds in water (depending on dia.) and continuin

with oil quenching.

********


25/25

39

hardness testing hand out

Documents