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Module 8COMIMSA
Module 8
Welding Metallurgy for theWelding Inspector
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Module 8COMIMSA
1. Introduction
Metallurgy
Is the science that deals with the internal structure of metals and the
relationship between those structures and the properties exhibited by
metals.
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Basic Metal Structures
Solids vs Liquids
Solids Liquids
Energy (-) Energy (+)
Atoms in a fixed position Free to move
Each atom has a specific “home”held in place by the attracting andrepelling forces
The atomic configuration determinestheir physical, mechanical, andelectrical properties
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The atoms are not however stationary in this positions. In reality they tend
to vibrate about an equilibrium position to maintain a balanced spacing.
Any attempt to force to force the atoms closer together will be
counteracted by repulsive forces which increase as the atoms are pushedcloser together.
Similary, any attempt to pull the atoms further apart will result in acounteracting attractive force. These attractive forces, however, tend to
decrease as the atoms are pulled further apart
Basic Metal Structures
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Module 8COMIMSA
The atoms exhibit a very specific spacing at given temperture.
The internal energy of a metal is increased when its temperature is raised.The atoms to vibrate more which increases their interatomic spacing
The metal to expand,,, if heat is elevated the vibration and spacingcontinue increase,, the solid metal then transforms into a liquid
Basic Metal Structures
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Module 8COMIMSA
Begins to cool and shrink
Residual stresses
The portion heated expands and is restrained
by the portion no heated, the bar tend to bend
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Module 8COMIMSA
Basic Metal Structures
Crystal Structures
The smallest number of atoms that can completely describe their orderly
arrangement is referred as a “ unit celd”
When a metal solidifies, it always does so in a crystalline pattern. The most
common crystal structures, or phases are:
1) Body Centered Cubic (bcc) -
iron, carbon steels, Cr, Mo, W
2) Face Centered Cubic (fcc) -
Al, Cu, Ni, austenitic SS.
3) Hexagonal close packed (hcp) -
Zn, Cd, Mg
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Basic Metal Structures
Solidification of metals
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Basic Metal Structures
Alloying
The proprieties of metallic elements can be altered by the addition of other
elements, wich may be or not metallic
Example; metallic zinc + metal cooper = the alloy brass
Nonmetal carbon is one of the alloying elements added to iron to form the
alloy steel
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Basic Metal Structures
Alloying
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Basic Metal Structures
Microestructural Constituents
of Carbon Steel
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Steel exist in several phases, typically Austenite, ferrite, perlite, bainite,
and martensite. See, Figures 7.5 – 7.7.
Welds Under the Microscope
Basic Metal Structures
Microestructural Constituents of Carbon Steel
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Module 8COMIMSA
Welds Under the Microscope
By altering the cooling rate
from the austenite range
we can affect the phases
of steel
Basic Metal Structures
Microestructural Constituents of Carbon Steel
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Metallurgical Considerations for Welding
The critical cooling
rate is governed by the
carbon content, and
for alloy steels, by
their additional
chemical composition.
M d l 8
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Module 8COMIMSA
Cooling Rate from austenizing temperature
(-) (+)
FurnaceAnneal
NormalizeOil
QuenchWater
QuenchBrine
Quench
Quenching the steel results in a martensitic structure.
Faster cooling forms Bainite
Slow cooling forms Ferrite and Perlite.
Heat treatments
Metallurgical Considerations for Welding
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Heat treatments
Steels quenched to form martensite usually require a “tempering” heat
treatment to lower their hardness and strength, and improve ductility andtoughness.
Metallurgical Considerations for Welding
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Cooling Rate
Slow Fast
Hardness (+)
Strength (+)
Ductility (-)
Toughness (-)
Susceptibility
to crack (+)
Metallurgical Considerations for Welding
Heat treatments
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Module 8COMIMSA
Welding Chemistry of Specific Base Metals
When the carbon content increases, weldability decreases
0.15 to 0.30 %C - Easily Weldable
About above 0.30 %C - More difficult to weld and may require:
Preheat
Interpass temperature control
PWHT
Weldability also decreases with alloying elements such as Cr, Mo,
Ni, may require the use of:
Metallurgical Considerations for Welding
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Carbon Equivalent Calculations (CE)
CE= %C + %Mn + %Si + %Cr + %Mo + %Ni +%Cu
6 5 15
CE >0.40 Preheat 200 – 400 º F (93 – 204 ºC)
Low Hydrogen Electrodes
CE >0.60 Preheat 400 – 700 º F (204 - 370 ºC)
Low Hydrogen Electrodes
There are many different CE formulas
Metallurgical Considerations for Welding
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Module 8COMIMSA
Heat Affected Zone (HAZ)
Factors that affect the HAZ properties:
Preheat (Figure 8.15)
Heat Input
Heat Input is the amount of energy supplied by the welding arc to heat the
base metal
Heat Input, Joules/in = Welding current x Welding Voltage x 60
travel speed, in/min
As the heat input increase the cooling rate decreases.
Metallurgical Considerations for Welding
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Module 8COMIMSA
Carbono equivalente
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Thermal Expansion
It is often necessary to remove these residual stresses by a PWHT
referred to as stress relief
There are three methods of removing weld stresses:
2) Peening
3) Vibratory Stress Relief
1) Thermal Treatment Approved by Code
Metallurgical Considerations for Welding
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Thermal Expansion
1) Thermal Stress Relief
The part is heated uniformly
Temperature below its transformation temperature
Held for a prescribed time period
Relax residual stress because the materials strength is reduced
Slow uniform cooling to room temperature
Metallurgical Considerations for Welding
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Thermal Expansion
2) Peening
Mechanical distortion of the weld
bead trough mechanical means
Usually when the metal is stillhot
Should not be done on the root
pass (crack) nor final pass of a
weld (interfere with later VI)
Only on the intermediate layers
Metallurgical Considerations for Welding
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Thermal Expansion
3) Vibratory Stress Relief
Imparts high vibratory vibrational energy into the part
Prevents the buildup of stresses in the weldment while welding or
Removes the stresses after welding
Metallurgical Considerations for Welding
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Thermal Expansion
One technique that may be used to reduce the need for PWHT is preheat
Slows cooling rate
May eliminate the need for PWHT
More ductile structure with lower residual stresses
Preheat
Reduce or eliminate hot cracking
Aids in removing moisture
Helps to remove Hydrogen
Retards the formation of Martensite
Metallurgical Considerations for Welding
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Diffusion
Under certain conditions, even atoms in the solid state can change
positions.These changes of atom position in the solid position in the solid
state are referred as diffusion.
Example Pb and Au
Example Hydrogen - underbead or delayed cracking
Metallurgical Considerations for Welding
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Metallurgical Considerations for Welding
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Welding Metallurgy of Commonly Used Materials
Stainless Steel
In severe corrosion environments, many of the SS corrode at very high rates
SS are defined as having at least 12% Cr.
The four main classes of SS are:
Ferritic
Martensitic
Austenitic
Precipitation Hardening (PH)
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Austenitic Grades
Very Weldable with available filler metal composition.
Very Weldable with available filler metal composition.Can be subject to
short cracking which occurs when metal is very hot this problem is
solved by controlling the composition of the base and filler metal to
promote the formation of “delta ferrite” phase.
Typically cracking will be avoided by selecting filler metals with a delta
ferrite percent of 4 – 10%. This percentage is often referred as ferrite
number and can be measured using the magnetic gauge.
Welding Metallurgy of Commonly Used Materials
Stainless Steel
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Austenitic Grades
One of the common problems to be encountered when welding
austenitic grades is referred to as carbide precipitation, or
sensitization .
800 – 1600 º F (427 – 870 º C) form Chromium carbides.
Most severe temperature for their formation is about 1250 º F (677 º C)
This carbides are typically found along the grain boundaries of the
structure
Welding Metallurgy of Commonly Used Materials
Stainless Steel
Module 8COMIMSA
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Austenitic Grades
Reduction of Chromium content within the grain Chromium depletion .
In certain corrosive enviroments, the edges of the grain corrode at a highrate Intergranular attack .
Welding Metallurgy of Commonly Used Materials
Stainless Steel
Module 8COMIMSA
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odu e 8COMIMSA
Austenitic Grades
Sensitization can be attacked by several methods:
Reheat Treating
Addition of stabilizers to the base and filler metals Ti (321) and Nb
(347).
Reduction of carbon content in the base and filler metals - “L” %C
as less as 0.03.
Welding Metallurgy of Commonly Used Materials
Stainless Steel
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COMIMSA
Ferritic Grades
Weldable with the proper filler metals
Martensitic Grades
More difficult to weld and often require special preheating and PWHT.
PH Stainless Steels
Weldable, but attention must be given to the changes in mechanical
properties caused by welding.
Welding Metallurgy of Commonly Used Materials
Stainless Steel
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COMIMSA
Welding Chemistry of Aluminum Alloys
Very tenacious oxide film on their surfaces – protect against corrosion.
The same oxide interfere with the joining process.
Alternating current is used.
Reformation of oxide film is avoided by shielding with He, or Ar gas.
Welding Metallurgy of Commonly Used Materials
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COMIMSA
Welding Chemistry of Cooper Alloys
Unlike steel, pure cooper and many of its alloys can not be hardened byquench and temper by heat treatment.
Usually hardened by “cold work”
Welding softens the cold worked material
One of the major problems when welding cooper and its alloys is due totheir relative low melt point and very high metal conductivity.
Considerable heat must be applied to the metal to overcome its loss
through conductivity, and the relatively low melting point often results in
the metal melting earlier than expected and flowing out of the weld joint.
Welding Metallurgy of Commonly Used Materials