1. Welding Inspection and Metallurgy

2. Welding Inspection and Metallurgy
Solid metals are crystalline in nature and all have a structure
in which the atoms of each crystal are arranged in a specific
geometric 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.

3. Welding Inspection and Metallurgy
Metals in molten or liquid states have no orderly arrangement
to the atoms contained in the melt. As the melt cools, a
temperature is reached at which clusters of atoms bond with
each other and start to solidify developing into solid crystals
within the melt. The individual crystals of pure metal are
identical 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 irregular
transition layer of atoms is formed, which is called the grain
boundary. Eventually the entire melt solidifies, interlocking
the grains into a solid metallic structure called a casting.

4. Welding Inspection and Metallurgy
Knowledge of cast structures is important since the welding
process 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.

5. Welding Inspection and 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.

6. Welding Inspection and Metallurgy
Welding Metallurgy
Some Important Points
Most typical weld metals are rapidly solidified and, like the
structure of a casting described earlier, usually solidify in the
same manner as a casting and have a fine grain dendritic
microstructure.
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 and
is that portion of the base metal that has not been melted, but
whose mechanical properties or microstructure have been
altered by the preheating temperature and the heat of welding.

7. Welding Inspection and Metallurgy
Welding Metallurgy
Some Important Points
There will typically be a change in grain size or grain structure
and 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 to
greater than 1350°F (700°C). Each weld pass applied will
have 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.

8. Welding Inspection and Metallurgy
Physical Properties
Some 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.

9. Welding Inspection and Metallurgy
Melting Temperature
Some Important Points
The melting temperature of different metals is important to
know because the higher the melting point, the greater the
amount of heat that is needed to melt a given volume of metal.
A pure metal has a definite melting temperature that is just
above its solidification temperature. However, complete melting
of alloyed materials occurs over a range of temperatures.
Alloyed metals start to melt at a temperature, which is just
above its solidus temperature, and, because they may contain
different metallurgical phases, melting continues as the temperature increases until it reaches its liquidus temperature.

10. Welding Inspection and Metallurgy
Thermal Conductivity
Some Important Points
The thermal conductivity of a material is the rate at which
heat is transmitted through a material by conduction or thermal
transmittance. 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.

11. Welding Inspection and Metallurgy
Thermal Conductivity
Some Important Points
Aluminum is a good conductor of heat and has the ability to
transfer heat very efficiently. This ability of aluminum to
transfer heat so efficiently also makes it more difficult to weld
with low temperature heat sources.

12. Welding Inspection and Metallurgy
Electrical Conductivity
Some Important Points
The electrical conductivity of a material is a measure of its
efficiency 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 much
less 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.

13. Welding Inspection and Metallurgy
Coefficient of Thermal Expansion
Some Important Points
Metals with a high coefficient of thermal expansion are
much more susceptible to warping and distortion problems
during 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.

14. Welding Inspection and Metallurgy
Coefficient of Thermal Expansion
Some 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.

15. Welding Inspection and Metallurgy
Density
Some Important Points
The density of a material is defined as its mass per unit volume.
Castings, and therefore welds, are usually less dense
than 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.

16. Welding Inspection and Metallurgy
Mechanical Properties: Tensile Strength
Some 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

17. Welding Inspection and Metallurgy
Mechanical Properties: Ductility
Some 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.

18. Welding Inspection and Metallurgy
Hardness
Some Important Points
The hardness of a material is defined as the resistance to
plastic deformation by indentation.
Indentation hardness may be measured by various hardness tests, such as Brinell, Rockwell, Knoop and Vickers.

19. Welding Inspection and Metallurgy
Toughness
Some Important Points
The toughness is the ability of a metal to absorb energy and
deform plastically before fracturing. An important material
property to tank and pressure vessel designers is the “fracture
toughness” of a metal which is defined as the ability to resist
fracture 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