MELTING PRACTICE IN LABORATORY

Aim: To observe the melting of metals to prepare the casting.

Material Required And Apparatus: Oil furnace, Ladle to sir, Metal.

Specifications:

Capacity – 10 kgs.

Crucible – Graphite of Dia 1 ‘ x height 1.5’’

Burner – O Number

Blower – 1 HP 2880 rpm.

Oil tank – 100 lts.

Oil Consumption – 2-4 Hrs.

Insulation – Fire bricks

Outer Casting – Mild steel duly painted.

Melting Procedure For Alluminium Alloys:-

The charge materials, chemicals should be free from moisture, oil, and corrosion powder and should be preheated before charging. The calculation of charge should be done considering the melting loss of each element in the melting furnace for final desired analysis.

1. The furnace crucible should be clean and red hot for charging.

2. Aluminium alloys get readily oxidized and form dross, using proper covering top with flux and chemicals help to reduce this. Different proprietary chemicals are available for different alloys.

3. Melting should be done under steady conditions without agitation. Stirring is done to reduce gas pickup.

4. Once melting is complete, degassing using solid chemicals like hexachloro-ethane which evolves chlorine by purging with nitrogen or argon gas is done to remove the dissolved hydrogen. Hydrogen is evolved from moisture.

3H2O+2Al→Al2O3+6H

Hydrogen absorbed by liquid metal causes serious porosity in casting during solidification. Degassing should be done in the temperature range 7300 Degree Celsius to 7500 Degree Celsius

5. Liquid metal after degassing is treated with sodium containing chemicals to improve mechanical properties.

6. Liquid metal once ready should not be super heated. Agitated or kept long in the furnace which will cause dressing and gas pickup. Dross should be skimmed properly before pouring.

7. Alloys containing magnesium should be melting carefully as it is highly reacting. Special fluxes and chemicals like sulphur are used to inhibit the reactivity and prevent spontaneous ignition, melting loss and dross.

Casting Defects Due To Improper Melting:

1. Improper chemical analysis: 

Incorrect charge, calculations, including wrong estimates of melting losses, metal recovery, excessive losses due to improper fluxing and slogging operations, improper covering of non- Ferrous melt causes this defect.

2. Gassy metal/hydrogen pickup/pinhole porosity: 

unclean melting causes formation and absorption of hydrogen into liquid metal. As casting solidifies, the absorbed hydrogen losses solubility and forms cavities inside casting.

3. Oxygen absorption

Excessive oxygen furnace operations in atmosphere following oxidation during melting cause this defect. It also causes loss of costly metal added in the charge.

4. Slag inclusions

Improper fluxing and slag removal slag particles to be mixed in the metal being poured. Careless pouring, lip pouring for alloys with fluid slag causes slag particles to enter casting.

5. Cold shut, misrun, unfilled castings

Low pouring temp, delay in pouring, due to many folds being poured, loss of heat from lable, due to improper covering failure of ladle opening in the bottom pouring cause premature solidification of metal causing defects.

6. Sand fusion, metal penetration, rough surface

Excessive pouring temp of liquid causes damage to the casting surface by attacking mould surface.

7. Sand erosion sand inclusions

Uncontrolled high pouring rate from ladle into mould leads to erosion of mould/core

Precautions:

1. The furnace crucible should be clean and red hot for charging

2. Charge material should be free from oil, moisture etc.,

3. Melting must be done under steady condition to reduce gas pickup.