Welding Inspection and Metallurgy. Metallurgy Solid metals are crystalline in nature and all have a...

Welding Inspection and MetallurgyWelding Inspection and Metallurgy

Metallurgy

Solid metals are crystalline in nature and all have a structurein which the atoms of each crystal are arranged in a specificgeometric pattern. The physical properties of metallic materials including strength, ductility and toughness can be attributed to the chemical make-up and orderly arrangement of these atoms.

Welding Inspection and Metallurgy

Metallurgy

• Metals in molten or liquid states have no orderly arrangementto the atoms contained in the melt. As the melt cools, atemperature is reached at which clusters of atoms bond witheach other and start to solidify developing into solid crystalswithin the melt. The individual crystals of pure metal areidentical except for their orientation and are called grains.

• As the temperature is reduced further, these crystals change in form eventually touch and where the grains touch an irregulartransition layer of atoms is formed, which is called the grainboundary. Eventually the entire melt solidifies, interlockingthe grains into a solid metallic structure called a casting.


Metallurgy

• Knowledge of cast structures is important since the weldingprocess is somewhat akin to making a casting in a foundry.

• Because of the similarity in the shape of its grains, a weld can be considered a small casting.

• A solidified weld may have a structure that looks very much like that of a cast piece of equipment. However, the thermal conditions that are experienced during welding produce a cast structure with characteristics unique to welding.


Metallurgy

Welding Metallurgy

We will begin our discussion here with Welding Metallurgy. Previous sections about the structures of metals are very general in nature, reading these a time or two should be sufficient preparation.


Welding MetallurgySome Important Points

• Most typical weld metals are rapidly solidified and, like thestructure of a casting described earlier, usually solidify in thesame manner as a casting and have a fine grain dendriticmicrostructure.

• The solidified weld metal is a mixture of melted base metal and deposited weld filler metal, if used.

• The heat-affected zone (HAZ) is adjacent to the weld andis that portion of the base metal that has not been melted, butwhose mechanical properties or microstructure have beenaltered by the preheating temperature and the heat of welding.


Welding MetallurgySome Important Points

• There will typically be a change in grain size or grain structureand hardness in the HAZ of steel.

• The size or width of the HAZ is dependent on the heat input used during welding.

• For carbon steels, the HAZ includes those regions heated togreater than 1350°F (700°C). Each weld pass applied willhave its own HAZ and the overlapping heat affected zones will extend through the full thickness of the plate or part welded.

• The third component in a welded joint is the base metal.


Physical PropertiesSome Important Points

• The physical properties of base metals, filler metals and alloys being joined can have an influence on the efficiency and applicability of a welding process.

• Examples of physical properties of a metal are the melting temperature, the thermal conductivity, electrical conductivity, the coefficient of thermal expansion, and density.


Melting TemperatureSome Important Points

• The melting temperature of different metals is important toknow because the higher the melting point, the greater theamount of heat that is needed to melt a given volume of metal.

• A pure metal has a definite melting temperature that is justabove its solidification temperature. However, complete meltingof alloyed materials occurs over a range of temperatures.

• Alloyed metals start to melt at a temperature, which is justabove its solidus temperature, and, because they may containdifferent metallurgical phases, melting continues as the temperature increases until it reaches its liquidus temperature.


Thermal ConductivitySome Important Points

• The thermal conductivity of a material is the rate at whichheat is transmitted through a material by conduction or thermaltransmittance. In general, metals with high electrical conductivity also have high thermal conductivity.

• Materials with high thermal conductivity require higher heat inputs to weld than those with lower thermal conductivity and may require a pre-heat.

• Steel is a poor conductor of heat as compared with aluminum or copper. As a result it takes less heat to melt steel.


Thermal ConductivitySome Important Points

• Aluminum is a good conductor of heat and has the ability totransfer heat very efficiently. This ability of aluminum totransfer heat so efficiently also makes it more difficult to weldwith low temperature heat sources.


Electrical ConductivitySome Important Points

• The electrical conductivity of a material is a measure of itsefficiency in conducting electrical current. Aluminum and copper have high electrical conductivity as compared to iron and steel.

• The ability of steel to carry an electrical current is muchless efficient and more heat is produced by its high electrical resistance.

• Steel can be heated with lower heat inputs than necessary for aluminum or copper because of its lower electrical conductivity and higher electrical resistance.


Coefficient of Thermal ExpansionSome Important Points

• Metals with a high coefficient of thermal expansion aremuch more susceptible to warping and distortion problemsduring welding.

• The increases in length and shrinkage that accompany the heating and cooling during welding should be anticipated, and procedures established which would assure that proper tolerances are used to minimize the effects of thermal conditions.


Coefficient of Thermal ExpansionSome Important Points

• The joining of metals in which their coefficients of thermal expansion differ greatly can also contribute to thermal fatigue conditions, and result in a premature failure of the component.

• Welding procedures are often employed, which specify special filler metals that minimize the adverse effects caused by inherent differences between the metals being joined.


DensitySome Important Points

• The density of a material is defined as its mass per unit volume.

• Castings, and therefore welds, are usually less densethan the wrought material of similar composition.

• Castings and welds contain porosity and inclusions that produce a metal of lower density.• This is an important factor employed during RT of welded joints.


Mechanical Properties: Tensile StrengthSome Important Points

Tensile testing is used to determine mechanical properties of metals;

• Ultimate Strength (point of failure)

• Yield Strength (place where it will not return to original shape)

• Elongation ( after yielding the value of increase in length)

• Strain is defined as the amount of deformation under load or change of shape


Mechanical Properties: DuctilitySome Important Points

In tensile testing, ductility is defined as the ability of a material to deform plastically without fracturing, measured by elongation or reduction of area.


HardnessSome Important Points

The hardness of a material is defined as the resistance toplastic deformation by indentation.

• Indentation hardness may be measured by various hardness tests, such as Brinell, Rockwell, Knoop and Vickers.


ToughnessSome Important Points

• The toughness is the ability of a metal to absorb energy anddeform plastically before fracturing. An important materialproperty to tank and pressure vessel designers is the “fracturetoughness” of a metal which is defined as the ability to resistfracture or crack propagation under stress. It is usually measured by the energy absorbed in a notch impact test.

• The primary test method is the Charpy Impact Test


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