Transcript Pseudobinary Phase Diagram @ 70% Iron AWS Welding Handbook

Pseudobinary
Phase Diagram
@ 70% Iron
AWS Welding Handbook
Prediction of Weld
Metal Solidification
Morphology
Schaeffler
Diagram
WRC
Diagram
AWS Welding Handbook
Hot Cracking
A few % Ferrite Reduces Cracks
But P&S Increase Cracks
AWS Welding Handbook
Spot Welding Austenitic Stainless Steel
Some Solidification Porosity Can Occur:
• As a result of this tendency to Hot Crack when Proper
Percent Ferrite is not Obtained
• Because of higher Contraction on Cooling
Suggestions:
• Maintain Electrode Force until Cooled
• Limit Nugget Diameter to <4 X Thickness of thinner piece
• More small diameter spots preferred to fewer Large Spots
Spot Welding Austenitic Stainless Steel
Some Discoloration May Occur Around Spot Weld
Oxide Formation in HAZ
Nugget
Solutions
•Maintain Electrode Force until weld cooled below oxidizing
Temperature
• Post weld clean with 10% Nitric, 2% Hydrofluoric Acid
(Hydrochloric acid should be avoided due to chloride
ion stress-corrosion cracking and pitting)
Seam Welding Austenitic Stainless Steel
Somewhat more Distortion Noted
Because of Higher Thermal
Contraction
Solution
• Abundant water cooling to remove heat
Knifeline Corrosion Attack in
Austenitic Stainless Steel Seam Welds
Solution
• See Next Slide for more description
Chromium Carbide Precipitation
Kinetics Diagram
1500 °F
Temperature
1500 F
1200 °F
M23C6
Precipitation
800 F
Chromium Oxide
800 °F
Intergranular
Corrosion
Time
M23C6
Chromium-Rich
Carbides
Preventative Measures

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
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
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Short weld times
Low heat input
Lower carbon content in the base material
 304L, 316L
Stabilization of the material with titanium additions
 321 (5xC)
Stabilization with columbium or tantalum additions
 347, 348 (10xC)
Lower nitrogen content (N acts like C)
Projection Welding Austenitic Stainless Steel
Because of the Greater Thermal Expansion and Contraction,
Head Follow-up is critical
Solution
• Press Type machines with low inertia heads
• Air operated for faster action
In Welding Tubes to tube sheets with Ring projections
for leak tight application, electrode set-up is critical
Solution
• Test electrode alignment
Cross Wire Welding Austenitic Stainless Steel
Often used for grates, shelves, baskets, etc.
• Use flat faced electrodes, or
• V-grooved electrodes to hold wires in a fixture
• As many as 40 welds made at one time
Flash Welding Austenitic Stainless Steel
• Current about 15% less than for plain carbon
• Higher upset pressure
• The higher upset requires 40-50% higher clamp force
• Larger upset to extrude oxides out
Super Austenitic
Alloys with composition between standard 300 Austenitic SS
and Ni-base Alloys
• High Ni, High Mo
• Ni & Mo- Improved chloride induced Stress Corrosion
Cracking
Used in
• Sea water application where regular austenitics suffer
pitting, crevice and SCC
AWS Welding Handbook
The Super Austenitic Stainless Steels are susceptible
to copper contamination cracking. RESISTANCE
WELDING NOT NORMALLY PERFORMED
Copper and Copper Alloy Electrodes can cause cracking:
• Flame spray coated electrodes
• Low heat
Nitrogen-Strengthened Austenitic
•High nitrogen levels, combined with
higher manganese content, help to
increase the strength level of the material
•Consider a postweld heat treatment for
an optimum corrosion resistance
Little Weld Data Available
Martensitic
• Contain from 12 to 18 percent chromium and
0.12 to 1.20 percent carbon with low nickel
content
• Combined carbon and chromium content gives
these steels high hardenability
• Magnetic
• Tempering of the low-carbon martensitic
stainless steels should avoid the 440 to 540 °C
temperature range because of a sharp reduction
in notch-impact resistance
Applications:
Some Aircraft & Rocket Applications
Cutlery
Martensitic SS Wrought Alloys are divided into two groups
• 12% Cr, low-carbon engineering grades (top group)
• High Cr, High C Cutlery grades (middle group)
AWS Welding Handbook
From a Metallurgical Standpoint, Martensitic SS
is similar to Plain Carbon
(12% Chromium)
AWS Welding Handbook
Martensitic
Spot Welding
• HAZ Structural Changes
• Tempering of hard martensite at BM side
• Quench to hard martensite at WM side
• Likelihood of cracking in HAZ increases with Carbon
• Pre-heat, post-heat, tempering helps
Flash Weld
• Hard HAZ
• Temper in machine
• High Cr Steels get oxide entrapment at interface
• Precise control of flashing & upset
• N or Inert gas shielding
Effect of Tempered Martensite on
Hardness
As Quenched
Hardness
Loss of Hardness and Strength
Hardened Martensite
Tempered Martensite
Fusion
Zone
HAZ
SS with carbon content above
0.15% Carbon (431, 440) are
susceptible to cracking and need
Post Weld Heat Treatment
Distance
Ferritic
• Contain from 11.5 to 27 percent
chromium, with additions of manganese
and silicon, and occasionally nickel,
aluminum, molybdenum or titanium
• Ferritic at all temperatures, no phase
change, large grain sizes
• Non-hardenable by heat treatment
• Magnetic (generally)
Applications:
Water Tanks in Europe
Storage Tanks
AWS Welding Handbook
FERRITIC STAINLESS STEELS
Spot & Seam Welding
Because No Phase Change, Get Grain Growth
large
HAZ
Base
Grain
Size
fine
Strength
Toughness
885 Embrittlement
DISTANCE
(Decomposition of
Iron-Chrome Ferrite)
FERRITIC STAINLESS STEELS
Flash Weld
• Lower Cr can be welded with standard flash weld techniques
• loss of toughness, however
• Higher Cr get oxidation
• Inert gas shield recommended
• long flash time & high upset to expel oxides
Super Ferritic
• Lower than ordinary interstitial (C&N)
• Higher Cr & Mo
• Better corrosion (Cr) & Higher Hot Strength (Mo)
AWS Welding Handbook
Increased Cr & Mo
promotes Embrittlement
• 825F Sigma Phase (FeCr)
precipitation embrittlement
•885F Embrittlement
(decomposition of iron-chromium
ferrite)
• 1560F Chi Phase (Fe36Cr12Mo10)
precipitation embrittlement
Because of the Embrittlement,
Resistance Welding is Usually
Not Done on These Steels
large
HAZ
Base
Grain
Size
fine
Strength
Toughness
885 Embrittlement
DISTANCE
Precipitation-Hardened
• Can produce a matrix structure of
either austenite or martensite
• Heat treated to form CbC, TiC, AlN,
Ni3Al
• Possess very high strength levels
• Can serve at higher temperature
than the martensitic grades
Applications:
High Strength Components in Jet & Rocket Engines
Bombs
AWS Welding Handbook
Martensitic
• Solution heat treat above 1900F
• Cool to form martensite
• Precipitation strengthen
• Fabricated
Semiaustenitic
• Solution heat treat (still contain 5-20% delta ferrite)
• Quench but remain austenitic (Ms below RT)
• Fabricate
• Harden (austenitize, low temp quench, age)
Austenitic
• Remain austinite
• Harden treatment
AC=Air cooled
WQ=Water Quenched
RC=Rapid Cool to RT
SZC= Rapid cool to -100F
AWS Welding Handbook
Effect on Aging on the Nugget Hardness in
Precipitation-Hardened Stainless Steels
Hardness
Aged
When Welded in the Aged Condition
• Higher Electrode Forces
• Post Weld Treatment
Annealed
Weld
Centerline
Distance
Precipitation-Hardened
Spot Welding
• 17-7PH, A-286, PH15-7Mo, AM350 & AM355 have been welded
• Generally welded in aged condition, higher forces needed
• Time as short as possible
Seam Welding
• 17-7PH has been welded
• Increased electrode force
Flash Welding
• Higher upset pressure
• Post weld heat treatment
Duplex
• Low Carbon
• Mixture: {bcc} Ferrite (over 50%) + {fcc} Austenite
• Better SCC and Pitting Resistance than Austenitics
• Yield Strengths twice the 300 Series
Early grades had 75-80% Ferrite (poor weld toughness due to ferrite)
Later grades have 50-50
AWS Welding Handbook
Due to the Ferrite:
• Sensitive to 885F embrittlement
• Sigma Phase embrittlement above 1000F
• High ductile to brittle transition temperatures (low toughness)
• Solidifies as ferrite, subsequent ppt of nitrides, carbides which
reduces corrosion resistance
• Rapid cooling promotes additional ferrite
• Not Hot Crack Sensitive
Resistance Welds generally not recommended
because low toughness and low corrosion resistance
Unless post weld solution anneal and quench.
Some
Applications
Deep Drawing of Plain Carbon Steel
or Stainless Steel
Method of
Making an Ultra
Light Engine
Valve
Stainless Steel Cap
Resistance
Weld
Larson, J & Bonesteel, D “Method of Making an Ultra
Light Engine Valve” US Patent 5,619,796 Apr 15, 1997