Rockwell Hardnes Testing

Rockwell Hardness Testing

• Test Principle:

Rockwell hardness Number (RHN) based on an inverse relationship to the measurements of the additional depth

Which an indenter is forced by a heavy (major) load beyond the depth resulting from a previously applied (minor) load.

• Indenter:

• 120 degree sphero-conicall diamond indenter is used for hard material.

• Hardened steel ball indenter with diameter of 1/16,1/8,1/4.1/2 inch.

Types of Test

• Two types of test based on load

Load Rockwell Superfical

Minor 10Kg 3 Kg

Major 60,100,150 Kg 15,30,45 Kg

Rockwell Scales

• There are 30 different scales ,depending on the combination of the indenter and the load.

• 64 HRC means RHN of 64 on C- scale, (diamond indenter and 150 Kg major load)

• 100 HRB means RHN 100 on B-scale (1/16 inch steel ball and major load 100Kg )

• 81 HR 30 N means SHN 81 on 30 N-scale (30 N diamond indenter and major load 30 Kg)

Dial Gauge• One revolution of the large pointer equals

1mm

• There are 100 divisions to a revolution

• Therefore ,1 division=0.01 mm

• Dial gauge is connected to a plunger system in the head of a tester

• By means of the index lever having 5:1

• Therefore depth for 1 division = 0.01x0.2=0.002

Indentation depth caused by major load

HRC= 100 - --------------------------------------------------

0.002

indentation depth caused by major load HRB= 130 - ------------------------------------------------ 0.002

Rockwell indentation

One dimension on the dial gage - a depth of 0.002mm.Zero reference line –100 for C scale and-130 for B scale

Depth of indentation

• Indentation depth from minor to major load: in case of Rockwell test

(60HRC). (100-60)x0.002mm= 0.08mm. (130-Ball indentation reading)x0.002mm. Superficial test (regardless of the indenter used) 80HR30N (100-80)x0.001mm=0.02mm

Factors that control Scale selection

• -Type of material,

• -Specimen thickness ,

• -Test location ,

• -Scale limitation.

Type of Materials

• Hard materials such as steel or tungsten carbide diamond indenter would be used.

• This limits to one of six Rockwell scale i.e C,A,D,45N,30N,15N.

• Out of the above scales now determine which scale will guarantee accuracy, sensitivity and repeatability.

Specimen Thickness

• The material around the indentation is cold worked.

• The extent of cold work area depends on the type of material and previous history of the testing material

• The depth of material affects has been found by extensive experimentation to be the order of ten times the depth of indentation.

Test location

• If an indentation is placed with in the cold worked area (previous indentation).

• The reading usually will be higher than that obtained had it been placed out side this area.

• As a precaution three diameter from the centre of one indentation to another is sufficient for most material.

• The distance from edge should be minimum of 2.5 diameter of indentation.

Scale limitation• No upper limit to the hardness of a

material that can be tested with a diamond indenter.

• However C scale should not be used on tungsten carbide (material may fracture or indenter may damage). A scale is acceptable in this case.

• Diamond indenter are not calibrated below values of 20.

Anvils

• The work piece must be supported by the proper anvil so that,

• It can not slide, roll or sag during the test.

• If not supported properly then accurate reading is obtained.

• Or the indenter may damage.

Superficial hardness

Rockwell hardness

Case hardened and Decarburization Parts

• Using several different scales case characteristics can be determined. e.g

• parts shows 91 HR15 N and 62 HRC. This means same hardness at an appreciable depth .

• 91HR15N and 55 HRC. This means indenter has broken through a relatively thin case.

• Decarburization can be detected by reversing the techniques described above.

Rockwell Testing at Elevated Temperature

• Tester in a furnace with provision of controlled atmosphere .

• Facilities to move the specimen in the furnace.

Rockwell testing Machines

• Test load can be applied either dead weight or through springs.

• Measuring device - a dial or digital meter.

• Microprocessor to control the test process.

• Bench type testing machine.

• Production type.

• Portable type machine