11-05-2011, 10:04 AM
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Welding Processes
EN358 – Ship Structures
A Brief History of Welding
Late 19th Century
Scientists/engineers apply advances in electricity to heat and/or join metals (Le Chatelier, Joule, etc.)
Early 20th Century
Prior to WWI welding was not trusted as a method to join two metals due to crack issues
1930’s and 40’s
Industrial welding gains acceptance and is used extensively in the war effort to build tanks, aircraft, ships, etc.
Modern Welding
the nuclear/space age helps bring welding from an art to a science
Types of Welding
Weldability of a Metal
Metallurgical Capacity
Parent metal will join with the weld metal without formation of deleterious constituents or alloys
Mechanical Soundness
Joint will be free from discontinuities, gas porosity, shrinkage, slag, or cracks
Serviceability
Weld is able to perform under varying conditions or service (e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.)
Fusion Welding Principles
Base metal is melted
Filler metal may be added
Heat is supplied by various means
Oxyacetylene gas
Electric Arc
Plasma Arc
Laser
Fusion Welding
Weld Metal Protection
During fusion welding, the molten metal in the weld “puddle” is susceptible to oxidation
Must protect weld puddle (arc pool) from the atmosphere
Methods
Weld Fluxes
Inert Gases
Vacuum
Weld Fluxes
Typical fluxes
SiO2, TiO2, FeO, MgO, Al2O3
Produces a gaseous shield to prevent contamination
Act as scavengers to reduce oxides
Add alloying elements to the weld
Influence shape of weld bead during solidification
Inert Gases
Argon, helium, nitrogen, and carbon dioxide
Form a protective envelope around the weld area
Used in
MIG
TIG
Shield Metal Arc
Vacuum
Produce high-quality welds
Used in electron beam welding
Nuclear/special metal applications
Zr, Hf, Ti
Reduces impurities by a factor of 20 versus other methods
Expensive and time-consuming
Types of Fusion Welding
Oxyacetylene Cutting/Welding
Shielded Metal Arc (“Stick”)
Metal Inert Gas (MIG)
Tungsten Inert Gas (TIG)
Oxyacetylene Welding
Flame formed by burning a mix of acetylene (C2H2) and oxygen
Fusion of metal is achieved by passing the inner cone of the flame over the metal
Oxyacetylene can also be used for cutting metals
Shielded Metal Arc (Stick)
An electric arc is generated between a coated electrode and the parent metal
The coated electrode carries the electric current to form the arc, produces a gas to control the atmosphere and provides filler metal for the weld bead
Electric current may be AC or DC. If the current is DC, the polarity will affect the weld size and application
Shielded Metal Arc (con’t)
Process:
Intense heat at the arc melts the tip of the electrode
Tiny drops of metal enter the arc stream and are deposited on the parent metal
As molten metal is deposited, a slag forms over the bead which serves as an insulation against air contaminants during cooling
After a weld ‘pass’ is allowed the cool, the oxide layer is removed by a chipping hammer and then cleaned with a wirebrush before the next pass.
Inert Gas Welding
For materials such as Al or Ti which quickly form oxide layers, a method to place an inert atmosphere around the weld puddle had to be developed
Metal Inert Gas (MIG)
Tungsten Inert Gas (MIG)
Welding Positions
Weld Defects
Undercuts/Overlaps
Grain Growth
A wide T will exist between base metal and HAZ. Preheating and cooling methods will affect the brittleness of the metal in this region
Blowholes
Are cavities caused by gas entrapment during the solidification of the weld puddle. Prevented by proper weld technique (even temperature and speed)
Weld Defects
Inclusions
Impurities or foreign substances which are forced into the weld puddle during the welding process. Has the same effect as a crack. Prevented by proper technique/cleanliness.
Segregation
Condition where some regions of the metal are enriched with an alloy ingredient and others aren’t. Can be prevented by proper heat treatment and cooling.
Porosity
The formation of tiny pinholes generated by atmospheric contamination. Prevented by keeping a protective shield over the molten weld puddle.
Residual Stresses
Rapid heating and cooling results in thermal stresses detrimental to joint strength.
Prevention
Edge Preparation/Alignment – beveled edges and space between components to allow movement
Control of heat input – skip or intermittent weld technique
Preheating – reduces expansion/contraction forces (alloys) and removes moisture from the surface
Peening – help metal stretch as it cools by hitting with a hammer. Use with care since it may work harden the metal
Heat Treatment – “soak” the metal at a high temperature to relieve stresses
Jigs and Fixtures – prevent distortion by holding metal fixed
Number of Passes – the fewer the better.
