Hardness Test

Definition: Hardness is a very important but hard todefine property of materials.

“Hardness is the resistance offered by a materialto indentation.”

IES-1992Hardness as a function of carbon content is shown in

(a) Fig-A

(b) Fig-B

(c) Fig-C

(d) Fig-D

IAS-2001

With the increase of percentage of carbon in thesteel, which one of the following properties doesincrease?

(a) Modulus of elasticity (b) Ductility

(c) Toughness (d) Hardness

Moh’s Scale of HardnessMaterial Moh’s hardness

Talc 1

Gypsum 2

Calcite 3

Fluorite 4

Apatite 5

Orthoclase 6

Quartz 7

Topaz 8

Corundum 9

Diamond 10

Brinell Hardness test Most commonly used tests.

A 10 mm tungsten carbide or hardened steel ballintended.

Gradually applied load.

Load applied at right angle to the specimen surface

Indentation diameter made on the specimen ismeasured.

Contd…

2

2 2/

2

;

Load PBHN kgf m m

S urface Area of Indentation D D dD

P Kgf D and d in m m

It cannot be used on very hard or very soft material.

It is best if the thickness of the material is at least 10 times the depth.

Tensile strength = k×BHN MPa

k = 3.296 for alloy steel

= 3.342 for Plain Carbon steel.

For steel, flexural endurance limit e

1.75 BHN MPae

VHN BHN

BHNRc

x

10

IES 2010The hardness of lathe bed material should be measured by

(a) Rockwell tester

(b) Brinell hardness tester

(c) Shore Scleroscope

(d) Vickers hardness tester

IES-1992A carbon steel having Brinell Hardnessnumber 100 should ultimate tensilestrength closer it

(a) 100 N/mm2 (b) 200 N/mm2

(c) 350 N/mm2 (d) 1000 N/mm2

Rockwell Hardness test A sphere – conical diamond cone of 120º angle and a

spherical apex of radius 0.2 mm is used

Depth of indentation t is measured.

Gradually applied load,

Load is applied at right angle to the specimen surface.

R = 100 – 500 × t

Depending on the load used a scale A, B, C, etc. used.

Rockwell test should not be used on thin materials(generally less than 0.16 mm, on rough surface or onmaterials that are not homogeneous).

IAS-1999A measure of Rockwell hardness is the

(a) Depth of penetration of indenter

(b) Surface area of indentation

(c) Projected area of indentation

(d) Height of rebound

Vickers hardness test(i) A square based diamond pyramid having 136ºbetween the opposite faces is used.

(ii) Average Diagonal of the indention (D) ismeasured.

(iii) Even a light load will produce plastic deformationthat so why we may use VHT on very hard materiallikes Tungsten Carbide.

pVHN

D

2

p in kgf1.854,

D in mm.

The Scleroscope(i) Based on the rebound height of a ball from thespecimen.

(ii) Harder the material, higher is the height to whichthe ball rebounds.

(iii) Scleroscope hardness numbers are comparableonly among similar materials. A comparison betweensteel and rubber would not be valid.

Knoop Test (Micro Hardness Test) Various micro hardness tests have been developed for

application where it is necessary to determine thehardness of a very precise area of material, or wherethe material or surface layer is exceptionally thin.

Special machine have been developed for this purpose.The location for this test is select in the highmagnification.

A small diamond penetrator is then loaded with apredetermined load ranging fm 25g to 3000g.

IES-1992Which of the following would you prefer for checking the hardness of very thin sections?

(a) Hebert cloud burst test

(b) Shore's Scleroscope

(c) Knoop hardness test

(d) Vickers hardness test

Durometer

When testing soft, elastic materials, such asrubbers and non rigid plastics, a durometer isuse

Herbert Cloudburst hardness test:

It is used to know the uniformity of hardnessover a surface.

IES-1992Herbert cloudburst Hardness test is conducted toknow

(a) Uniformity of hardness over a surface

(b) Softness of non-metallic components

(c) Hardness of non-metallic components

(d) Hardness at specified depth inside the surface.

Charpy and Izod test Toughness of a material is measured by means of impact

tests.

Notched bar prepared as per standard from the testmaterial, is held in a vice and a weight is allowed to swingfrom a known height in such a way that it hits the notchedbar in its path and breaks it.

Since the material has absorbed some amount of energyduring its fracture, the swinging mass loses part of itsenergy and therefore will not be able to reach the sameheight from where it started.

The loss in height (h) multiplied by the weight representsthe energy absorbed by the specimen during fracture,which can be directly measured from the indicator on thetester. Contd…

In the Charpy impact test, the specimen is held betweentwo grips whereas in the Izod impact test the specimen isheld at one end like a cantilever .

The standard test specimens used in the izod and Charpyimpact tests are made of bars with a square cross section of10 mm.

The impact resistance is dependent upon the materialcomposition as well as the heat treatment process given toit.

The annealed materials normally would have bettertoughness than the corresponding normalized orquenched specimens. Coarse-grained structure would tendto have higher ductility compared to fine grain structureand consequently better toughness.

Sheet Formability Tests During the various sheet metal forming processes, the

stresses and strains developed in the material are quitecomplex. So, any measure of the formability of thesheet metal on the basis of material properties derivedfrom a simple tension test( for example, % elongationas an index of ductility of the material) is not veryaccurate.

So cupping tests have been developed to evaluate theformability of the sheet metals.

GATE-2006Match the items in Column I and Column II.

Column I Column II

P. Charpy test 1. Fluidity

Q. Knoop test 2. Micro hardness

R. Spiral test 3. Formability

S. Cupping test 4. Toughness

5. Permeability

(a) P - 4, Q - 5, R - 3, S – 2 (b) P - 3, Q - 5, R - 1, S - 4

(c) P - 2, Q - 4, R - 3, S – 5 (d) P- 4, Q - 2, R - 1, S – 3

IES-2002Assertion (A): The hardness test is a slow, expensivemethod of assessing the mechanical properties ofmaterials.

Reason (R): The hardness is a function of yield stressand the work hardening rate of material.

(a) Both A and R are individually true and R is the correctexplanation of A

(b) Both A and R are individually true but R is not thecorrect explanation of A

(c) A is true but R is false

(d) A is false but R is true

The End