section 08 (nxpowerlite).ppt

Haward Technology Middle East 1Section 8

AWS Certified Welding Inspector (CWI)

Section 8

Welding Metallurgy for the Welding Inspector



Metallurgy

“The science dealing with the internal structure of metals and metal

properties ”



Welding Metallurgy

“Concerned with the changes occurring in metals during welding”



Discussion Topics

Structures

Changes on heating

Changes on cooling

Heat treatments

Alloying

Phases



Atomic Structure

Matter made up of atoms

Atoms are very small

Electrons orbit atomic nuclei

Have a home location in solids

Oscillate around home position



Atomic Structure - Simple Cubic



Home Location



Solids

Minimum internal energy

Atoms fixed in place

Atoms have ‘home’ position

Atoms vibrating

Orbiting electrons

Crystalline structure



Liquids

Higher internal energy

Atoms free to move

Atoms vibrating more

Orbiting electrons

No fixed structure



Solid vs Liquid



Thermal expansion on Heating

Thermal contraction on Cooling



Welding Stresses (Tensile)

Remain after welding is completed (residual stress)

Can cause distortion

• (welding stress exceeds yield strength of material)

Can cause cracking

• (welding stress exceeds tensile strength of material)

Residual stress can be as high as yield strength

Relieved by three methods



Stress Relieving

Thermally

Vibratory

Peening



Crystal Structures

BCC - Body Centered Cubic

FCC - Face Centered Cubic

HCP - Hexagonal Close Packed

BCT - Body Centered Tetragonal



Common Crystal Structures



Metal Solidification



Grain Size Effects

Fine grained materials• Have good tensile strength• Have good ductility• Have good low temperature properties

Coarse grained materials• Have slightly lower strength• Are slightly less ductile• Have good high temperature properties



Alloying

“Adding elements to change mechanical or physical properties”



Interstitial Alloying



Interstitial Alloying -Results in Lattice Distortion



Substitutional Alloying



Substitutional Alloying -Results in Lattice Distortion



Microstructures

Metals composed of grains (crystals)

Very small

Separated by grain boundaries

Different phases within grains



Microstructure of Iron



Iron-Carbon Phase Diagram



Phases in Steel

Ferrite

Cementite

Austenite

Bainite

Pearlite

Martensite



Cooling Rates

Slow - Pearlite - Softest

Moderate - Bainite

Fast - Martensite - Hardest



Lamellar Appearance of Pearlite



Martensite



Tempering

“Restores ductility and toughness and reduces strength and hardness”


AWS Certified Welding Inspector (CWI)Effects of

Tempering Temperatures

Alloy is AISI 4142

Tempering temperature is in degrees F



Effects of Tempering Temperatures

Alloy contains 12% Cr

Note reduction in impact strength at 885 F (475 C)



Continuous Cooling Diagram & Jominy Hardenability Diagram for 8630 steel. Phase formed on cooling varies with cooling rate.



Weld Area & Fe - Fe3C Diagram



Heat Input

Joules/inch = Current x voltage x 60

Travel speed (in./min)



Preheat

Slows cooling rate

Produces less martensite

Reduces risk of cracking

Reduces distortion

Reduces hydrogen



Carbon Equivalent( CE )

CE = % C + % Mn + % Ni + % Cr + % Cu + % Mo

6 15 5 13 4



Examples of Preheating vs C.E.

C.E. Preheat Temperature

< 0.45 Optional

0.45 - 0.60 200 - 400 °F

>0.60 400 - 700 °F



Heat Treatments for Steels

Super-critical - above transformation temperature

• Annealing

• Normalizing

• Quenching (to harden)

Sub-critical - below transformation temperature

• Tempering

• Stress Relieving



Diffusion

“Movement of atoms within a solution, be it a solid, liquid or gas”



Diffusion of Lead and Gold Atoms



SolubilityDissolving Salt into Water Solid into liquid

Diffusion mechanism

Solubility limit

Temperature effects



Solid Solubility

Unique to metals

Solid into solid

Solubility limit

Temperature effects



Examples of Solid Solubility

Gold and lead bars

Carburizing

Nitriding



Stainless Steels

Ferritic

Martensitic

Austenitic

Precipitation Hardening (PH)

Duplex



Sensitization of Austenitic Alloys

“Formation of chromium carbides between 800 - 1600 °F”

(Sensitization reduces the corrosion resistance in many environments)


AWS Certified Welding Inspector (CWI)Corrosion Attack of Austenitic Stainless

Steel



Avoiding Sensitization

Heat treatment ( SAWQ )

Stabilized grades

Low carbon grades



Other Common Alloys

Aluminum

• Pure aluminum

• Aluminum-copper

• Aluminum-magnesium

Copper

• Pure copper

• Brasses

• Bronzes



End of this Section

section 08 (nxpowerlite).ppt

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