Pounds per square inch (psi)

Pounds per square inch is a very common unit of pressure.  It belongs to the Imperial System of measurement.  
It is widely used in industries, tyres and other applications.  The unit indicates that the pressure is equivalent to a certain number of pounds applied to a square inch of area.  
There are two variations of the unit, the absolute and the gauge versions. 
psia (pounds per square inch absolute) indicates that the pressure value is made vacuum as the reference while psig (pounds per square inch gauge) indicates that the atmospheric pressure is the reference.

Welding Slag

Welding flux is used to protect the weld area from contamination.  The flux forms a protective layer over the surface.  In certain cases, it is left as a residue over the weld area.  This is called welding slag.

Welding Slag has to be removed in order to be able to view the welded area for inspection as well for aesthetics and visual appearance.  Welding slag can be chipped away with a pointed hammer.  Sometimes, it is also removed by grinding.

Shielded metal arc welding

Shielded metal arc welding is one of most popular methods of welding.  Shielded metal arc is simpler than other forms of welding.  It requires an welding transformer kit and an electrode.  Shielded metal arc welding works by creating an electric arc between the work piece and the electrode.

The welding transformer can produce high currents.  Current from the welding transformer passes through a cable and then through the electrode into the work piece which is earthed or connected directly to the transformer. 

An arc is generated by touching the work piece with the electrode and then breaking the contact.  The high current which is interrupted now forms an arc as it tries to jump across the gap between the electrode and the work piece. 

Shielded metal arc welding is widely used in almost all industries.  It is also used in construction.  Shielded metal arc welding can be used to weld both ferrous and non-ferrous metals.

Welding Flux

Welding flux is a substance used during the welding process to shield the weld area from atmospheric contamination.  In metal arc welding, the flux is part of the welding rods. 

When the arc is initiated, the high temperature melts the work piece and the welding rod.  The flux coating on the rod also melts and forms a protective shield over the weld poor, protecting it from the atmospheric gases. 

Porosity in Welding

Porosity in welding is the formation of air pockets or bubbles in the weld area.  Porosity makes the weld weak and can cause failure of the weld area. 

Some of the causes of porosity are

  • Improper shielding
  • Presence of water or moisture on the weld surface
  • Presence of paint, which can get vaporised during the weld process.
  • Improper gas flow setting in gas welding

Shielding Gas

Shielding gases are gases used in gas arc welding.  Shielding gases serve to insulate the weld are from gases such as oxygen and water vapour.  This is necessary to prevent oxidation at the weld area at high area and porosity(formation of bubbles in the weld). 

Inert and semi-inert gases are used for shielding.  Examples are Helium, Argon, Carbondioxide, etc.  Shielding gases are generally denser than air (an exception is helium).  They also have good thermal conductivity. 

Rockwell Scale

The Rockwell scale is a scale to measure hardness.  The scale is based on the depth of the indentation produced on a material.  The test works by using an indentor to makes an indentation.

The Rockwell test is a non-destructive testing.  Its setup is easy to install.

The indentation hardness is linearly related to tensile strength.  Thiss permits the quick and reliable testing of bulk materials. 

The test
A minor load is placed on the specimen.  The depth of the indentation formed is noted.  This is the zero point.  A bigger load, called the major load, is now placed upon the minor load.  The depth of the indentation with reference to the zeropoint is noted. 

The depth of indentation and the hardness are inversely related.  A hard material will have lesser depth of indentation while a softer material will have a greater depth of indentation. 

The hardness of a material can be directly calculated from the formula

HR= N- d/s

where N is the Rockwell scale used and s is the scale factor

High speed steels

Hi Speed steels are steels which have high hardness even at high temperatures.  They also have good wear resistance. These steels have molybdenum, vanadium, chromium and tunsten as their constituents.  These elements generally constitute about 7% of the material.  These elements form carbides.

The name High Speed Steels was used as they cut faster than other types of steels.

High speed steels are categorized after the name of their constituents, such as Molybdenum high speed steels, Vanadium high speed steels, Chromium high speed steels and so on.

High Speed steels are used in cutting equipments, such as blades and saws. They are also used in tool bits and in dies. 

Quenching

Quenching is process of strengthening and hardening steels and alloys.  Quenching is the quick cooling of a hot metal, such that the phase transformation do not occur. The hardness is increased as the crystal grain size is increased. 

Quenching is done in oil or in water.  The medium is chosen depending on the level of quenching desired.  Quenching in water can sometimes cause too much hardness that the material may crack.  In such situations, oil may be chosen.  Salt baths and special polymers are also used in special applications. 

Quenching Rate
Quenching rate is the rate at which the drop in temperature has to occur.  Oil has a quenching rate lesser than water.  It means that the temperature will fall less faster in oil than in water. 

Tempering

Tempering is a heat treatment used to reduce the hardness of a metal and to increase its toughness and ductility. The internal stresses in the material are relieved during the process. 

It is used particularly in iron alloys, such as steel.  It is generally used after quenching.  Quenching causes the material to become hard and brittle.  The metal, in that state, may not be suitable for many applications.

Tempering is done by heating the material and then gradually cooling it.  The heating should be very gradual to prevent the formation of cracks.  The temperature to which the metal must be heated is based on the desired toughness and hardness.   

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