Module 8 Welding Metallurgy for the WI

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Module 8

Welding Metallurgy for theWelding Inspector



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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




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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




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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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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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Solidification of metals



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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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Alloying



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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


Microestructural Constituents of Carbon Steel



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Welds Under the Microscope

By altering the cooling rate

from the austenite range

we can affect the phases

of steel


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.

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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


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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.


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Cooling Rate

Slow Fast

Hardness (+)

Strength (+)

Ductility (-)

Toughness (-)

Susceptibility

to crack (+)


Heat treatments

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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:


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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


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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.


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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


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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


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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


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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


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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


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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


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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.


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


Stainless Steel

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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 .


Stainless Steel

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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.


Stainless Steel

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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.


Stainless Steel

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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.


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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.


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Module 8 Welding Metallurgy for the WI