SREE CHAITANYA COLLEGE OF ENGINEERING: KARIMNAGAR

DEPARTMENT OF MECHANICAL ENGINEERING

EXPERIMENT NO – 7

DETERMINATION OF HARDNESS OF VARIOUS HEAT TREATED

AND UNHEAT TREATED STEELS

AIM: To determine the hardness of various heat treated and un heat treated steels.

APPARATUS & MATERIALS:

Muffle furnace, Brinell hardness tester, tongs, metallurgical microscope, heat treated steels (EN36), un heat treated steels(EN36).

THEORY:

Heat treatment may be defined as “A combination of heating and cooling operations, timed and applied to a metal or alloy in the solid state in a way that will produce desired properties”. Almost in all the heat treatment processes for steel, there is either transformation of austenite or decomposition of austenite. In almost all types of steels, austenite production is the first step in heat treatment.

Objectives of heat treatment:

  1. To relieve internal stresses, which are set up in the metal due to cold or hot working.
  2. To soften the metal.
  3. To improve hardness of the metal surface.
  4. To improve machinability
  5. To improve mechanical properties like tensile strength, ductility and shock resistance.
  6. To increase the resistance to wear, tear, heat and corrosion.

Types of heat treatment process:

There are many types of heat treatment processes, yet the following are important:

  1. Annealing
  2. Normalizing
  3. Hardening
  4. Tempering
  5. Carburizing
  6. Nitriding
  7. Induction hardening
  8. Flame hardening

Annealing: Annealing is the process of heating the steel to an elevated temperature, holding it at that temperature for some time, and then cooling slowly in the furnace. The slow cooling is continued to low temperature. The steel is held at high temperature, for uniform heating throughout. Normally annealing is carried out to (i)relieve the internal stresses,(ii) increases softness, ductility and toughness,(iii)refine the crystal structure and produce specific microstructure.

The various annealing operations are:

(i)full annealing,

(ii)process annealing,

(iii)spheroidise annealing

(iv)diffusion annealing

Full annealing: this process consists of heating hypo-eutectoid steels, 300-500C above the upper critical point (i.e A3 line) and holding it for sufficient amount of time at this temperature. This ensures that metal is heated up and phase transformation has taken place throughout the whole volume. The material is then cooled slowly in the furnace.

Normalizing: normalizing consists of heating the steel to 500C above the upper critical temperature (A3 or Acm line) and holding it at this temperature for some time (soaking), so that complete transformation can take place. The steel is than cooled in air. During air cooling the rate if cooling is faster than furnace cooling and austenite transforms into finer and more abundant pearlite structure as compare to annealing. Because of line pearlite structure, the normalized steel becomes harder. Normalizing temperature is higher than annealing temperature. Properties obtain by normalizing depend upon the size and composition of steel piece. Normalizing is used for grain refinement in cast steel and welding assemblies.

Hardening: The process of hardening consists of heating the metal up to a temperature ranging from 300C to 500C above the upper critical temperature for the hypo-eutectoid steels and by the same temperature above the lower critical point for the hyper eutectoid steels. The metal is held at this temperature for a considerable time and then cooled suddenly in an oil or water medium. The hardness obtained from a given treatment depends upon the rate of cooling. A very rapid cooling is necessary to harden medium carbon steels. For high carbon and alloy steels, mineral oil is generally used as the quenching medium. A rapid cooling from the hardening temperature causes the austenite to be transformed into another constituent called martensite, which is very hard and brittle. The hardening of steel depends entirely upon the formation of martensite, because austenite is comparatively soft and ductile.

The main objectives of hardening are:

i)To increase the hardness of the metal, so, that it can resist wear.

ii)To enable it to cut other metals, i.e., to make it suitable for cutting tools.

In this experiment all the above three processes are adopted.

DESCRIPTION: The various equipment used in this experiment is described below:

Muffle furnace: A muffle furnace is a heating chamber called muffle that separates the combustion space from the workspace, the size of the muffle is 225 x 100 x 100 mm, it is provided with a digital temperature indicator and energy regulator. The maximum temperature is about 10000C. The advantage of the muffle furnace is it prevents the direct contact of the sample to be heated with the combustion products and therefore reduces the contamination of the sample.

Brinell hardness tester: hardness is the property exhibited by a material .it can be defined as the property of a material by virtue of which it resists scratches, wear, abrasion or indentation. Brinell hardness testing machine uses a hardened steel ball as the indenter. Balls of different diameter 5mm to 10mm are employed with suitable loads depending upon the type of specimen material to be tested .this method consists of indenting the surface of the metal by a hardened steel ball of specified diameter‘d’. The diameter of the impression is measured with the help of a Brinell microscope fitted with a scale. The Brinell hardness HB is defined as the quotient of the applied force F divided by the spherical area of the impression.

HB=2F/ [πD (D-√D2-d2)]

PROCEDURE: The following is procedure is adopted for both heat treated and un heat treated steels.

  1. Clean the surface of the specimen free from any dirt and oxide scales.
  2. Select the proper size of the ball and load w suit the material under test for ferrousmaterial select 10 mm ball indenter & apply 30000 N load
  3. Keep the operating lever in horizontal position and the specimen on the testing table or anvil
  4. Turn hand wheel in clockwise direction, so thatthe specimen touches the indenter
  5. Lift the operating lever in horizontal position upwards slightly, after which it rotates automatically to apply the load and wait for some time.
  6. Release the load by shifting the lever.
  7. Take out the specimen and measure the diameter of indentation by means of the Brinell microscope. The diameter of each indentation should be measure in two directions at right angles and the mean valve of the two readings is used for the direction at right angles and the mean value of the two reading is used for the purpose of determining the hardness number.
  8. Repeat the experiment at other position of the test piece.
  9. Calculate the average value of Brinell hardness number (HB).

METALLURGICAL MICROSCOPE

MUFFLE FURNACE

BRINELL HARDNESS TESTER

METALLURGY LAB