Page 34 - CITS - Fitter Trade Practical - Volume -2

FITTER - CITS

• Ferrite: It is a solid solution of carbon in alpha a iron (BCC) and occurs without other constituents in the
area GPQ. The maximum amount of carbon content in ferrite of plain carbon steels is 0.025% at 723° C and
drops off to 0.008 % as the temperature decreases at room temperature. Ferrite is nearly pure iron and has
similar properties. After etching it appears white grains under microscope.

• Cementite:It is an intermetallic compound iron carbide having the formula Fe 3 C. It is very hard and
brittle of low tensile strength. Cementite appears white and in eutectoid steels is found mixed with dark
appearing ferrite to form pearlite. In hyper eutectoid steels it may also be present as boundaries around the
grains or as needless within crystals of pearlite.
• Pearlite: Pearlite is a mechanical mixture, in lamellalr form of ferrite and cementite. Pearlite is formed when
plain steels and cast irons are cooled slowly below 723° C.The eutectoid composition yields only pearlite on
cooling down to room temperatures. The tensile strength of pearlite is around 90 kg/mm , approximately three
times that of ferrite, but may be greater or less, depending upon fineness of structure. Pearlite is much harder
than ferrite but is softer than cementite.

• Ledeburite:It is a mechanical mixture of cementite and austenite. It is the eutectic of iron- arbon system.
Ledeburite appears in all compositions of more than 2% carbon and for this reason the figure 2% is used to
separate steels from cast irons.
• Iron - graphite line: This line is horizontal line at 723°C. It represents the true equilibrium conditions as
opposed to the meta-stable conditions represented by A 1, 3 at 723° C. The reasons for the two lines is
the in stability of Fe3, where in this carbide decomposes to graphite and iron causing a shift of line ES to
E S. But this decomposition is very slow so line PSE is only considered confined to meta-stable condition.

Principle of Hardness testing
Hardness is usually defined as the resistance of a material to plastic penetration of its surface.
There are three main types of tests used to determine hardness:

Scratch tests are the simplest form of hardness tests. In this test, various materials are rated on their ability to
scratch one another. Mohs hardness test is of this type. This test is used mainly in mineralogy.
In Dynamic Hardness tests, an object of standard mass and dimensions is bounced back from a surface after
falling by its own weight. The height of the rebound is indicated. Shore hardness is measured by this method.
Static Indentation tests are based on the relation of indentation of the specimen by a penetrator under a given
load. The relationship of total test force to the area or depth of indentation provides a measure of hardness. The
Rockwell, Brinell, Knoop, Vickers, and ultrasonic hardness tests are of this type.
For engineering purposes, mostly the static indentation tests are used.
Background

It is a common practice to test most materials before they are accepted for processing, and before they are put
into service to determine whether or not they meet the specifications required. indentation hardness has become
one of the most reliable controls of the heat treatment and quality of manufactured parts. Rockwell testing is
covered by ASTM test method E 18.

Rockwell hardness test
The penetrators for the Rockwell hardness tester range from 1/2-inch diameter steel balls to very small diamond
(brale) tips (points). The smaller points are used for harder materials that have a greater resistance to indentation.
There are various force scales used for various materials.
Rockwell Hardness Tester
The Rockwell B and Rockwell C scales will be used for this experiment. The Rockwell B scale is suitable for soft
engineering metals, and the Rockwell C scale is appropriate for hard engineering metals.
There are 30 different Rockwell scales, defined by the combination of the indenter and minor and major loads.
The suitable scale is determined due to the type of the material to be tested. The majority of applications are
covered by the Rockwell C and B scales for testing steel, brass, and other materials..
The B scale uses a 1/16- inch diameter hard steel ball and a 100-kg load. The C scale uses a conical diamond
point and a 150-kg load. To perform the Rockwell tests, the penetrator is pressed against the specimen with an
initial 10-kg preload to properly seat the penetrator. The remaining load is applied gradually after the dial on the

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CITS : C G & M - Fitter - Exercise 61