AWS Welding Handbook
PseudobinaryPhase Diagram@ 70% Iron
WRCDiagram
SchaefflerDiagram
AWS Welding Handbook
Prediction of WeldMetal SolidificationMorphology
A few % Ferrite Reduces Cracks
But P&S Increase Cracks
AWS Welding Handbook
Hot Cracking
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
Spot Welding Austenitic Stainless Steel
Some Discoloration May Occur Around Spot Weld
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)
Oxide Formation in HAZ
Nugget
Knifeline Corrosion Attack in Austenitic Stainless Steel Seam Welds
Seam Welding Austenitic Stainless Steel
Somewhat more Distortion Noted Because of Higher Thermal
ContractionSolution• Abundant water cooling to remove heat
Solution• See Next Slide for more description
Chromium Carbide Precipitation Kinetics Diagram
Time
Te
mp
era
ture
1500 °F
1200 °F
800 °F
1500 F
800 F
Chromium OxideChromium Oxide
M23C6
Precipitation
M23C6
Chromium-RichCarbides
IntergranularCorrosion
Preventative Measures
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 AusteniticAlloys 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 ApplicationsCutlery
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
AWS Welding Handbook
(12% Chromium)
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
FusionZone HAZ
Distance
Ha
rdne
ss
Hardened MartensiteTempered Martensite
Loss of Hardness and Strength
SS with carbon content above 0.15% Carbon (431, 440) are susceptible to cracking and need Post Weld Heat Treatment
As Quenched
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 EuropeStorage Tanks
AWS Welding Handbook
FERRITIC STAINLESS STEELS
Because No Phase Change, Get Grain Growth
Spot & Seam Welding
GrainSize
Strength
Toughness
HAZ Base
DISTANCE
large
fine
885 Embrittlement (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
AWS Welding Handbook
• Lower than ordinary interstitial (C&N)• Higher Cr & Mo• Better corrosion (Cr) & Higher Hot Strength (Mo)
GrainSize
Strength
Toughness
HAZ Base
DISTANCE
large
fine
885 Embrittlement
• 825F Sigma Phase (FeCr) precipitation embrittlement•885F Embrittlement (decomposition of iron-chromium ferrite)• 1560F Chi Phase (Fe36Cr12Mo10) precipitation embrittlement
Increased Cr & Mo promotes Embrittlement
Because of the Embrittlement,Resistance Welding is Usually Not Done on These Steels
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 EnginesBombs
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
AWS Welding Handbook
AC=Air cooledWQ=Water Quenched
RC=Rapid Cool to RT SZC= Rapid cool to -100F
Effect on Aging on the Nugget Hardness in Precipitation-Hardened Stainless Steels
Distance
Ha
rdne
ss
Aged
Annealed
WeldCenterline
When Welded in the Aged Condition• Higher Electrode Forces• Post Weld Treatment
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 resistanceUnless post weld solution anneal and quench.
Some Applications
Larson, J & Bonesteel, D “Method of Making an Ultra Light Engine Valve” US Patent 5,619,796 Apr 15, 1997
Method of Making an Ultra Light Engine Valve
Deep Drawing of Plain Carbon Steel or Stainless Steel
Resistance Weld
Stainless Steel Cap
Top Related