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Porosity in Welds - Porosity is a condition in which gas pockets or voids occur in a metal as a result of contamination or poor protection of the molten solidifying metal. It is generally accepted that porosity in weld metals is formed by the entrapment of evolved gases in the solidifying weld metal.
It has been suggested that bubbles are first formed at the solid liquid interface and remain there until they have grown to a sufficient size to float up through the molten weld metal and escape to the atmosphere.
If the rate of detachment and flotation is less than the rate of advance of the solidification front, the bubbles ate trapped and the resultant weld is porous. In carbon steel welds porosity can result from the following reactions that occur either singly or in combination.
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Hydrogen, oxygen and nitrogen are gases dissolved in the metal. Pores in weld metal are, in general, spherical in shape, but pores of other shapes can occur weld metal porosity (black) and heat affected zone (dark portion around the weld bead).
A form of pores, which is most prevalent, apart from spheres, is a type known as worm hole porosity. This pore is roughly elliptical in shape and has its major axis normal to the advancing solidification wave. Where irregular shaped voids are present or where cavities occur in groups, these are perhaps due to mechanically entrapped gas caused by arc instability.
In welds, both scattered porosity and piping* can be found. Scattered porosity consists of spherical pores scattered through the volume of the weld metal. Piping implies roughly cylindrical cavities running in line with the
longitudinal axis of the weld and can be seen in submerged arc welds.
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Physical factors such as the position in the weld pool where the gas bubbles nucleate, also playa part in the incidence of porosity. If nucleation occurs away from the weld pool boundary, there is a possibility that the bubbles will all escape, leaving a rim of degassed metal to solidify without pores. On the other hand, if nucleation persists up to the weld boundary, then bubbles will almost entirely be trapped.
Porosity due to oxygen is normally indirect, in that troublesome gas is usually a reaction product (e.g. CO formed by oxygen and carbon). Porosity due to hydrogen is typically associated with the welding of Al Cu, Ti, Mg etc. Moisture and organic compounds form the main sources of hydrogen.
In steel welds, porosity is less troublesome than cracking, which occurs because excess hydrogen diffuses to the heat affected zone where the microstructure tends to be brittle.
The main source of nitrogen is air and porosity seems to be particularly associated with the MIG welding of carbon steel, stainless steel and nickel. Porosity in aluminium occurs due almost entirely to rejection of hydrogen.
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Porosity is most severe with metallic arc welding using coated electrodes and least with TIG welding. Porosity appears most frequently or severely at the start of a weld run and particularly in overhead or horizontal vertical joints. Porosity reduces the strength of the weld metal. The common sources of porosity in various materials and the deoxidants to prevent porosity.
1. Mild steel and low alloy steels
2. Stainless steels
3. Copper and copper alloys
4. Aluminium
5. Magnesium
6. Titanium
Oxygen
Nitrogen
Hydrogen, Oxygen, Nitrogen Hydrogen
Hydrogen
Oxygen, Argon
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