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Stainless Steel

Stainless Steel

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Stainless Steel

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  1. Stainless Steel High Ni & Cr Content Low (Controlled) Interstitials Nitrogen Strengthened Austenitic Austenitic Martensitic Ferritic Super Austenitic Precipitation Hardened Duplex Super Ferritic

  2. Resistance Welding • Learning Activities • View Slides; • Read Notes, • Listen to lecture • Do on-line workbook • Lesson Objectives • When you finish this lesson you will understand: Keywords

  3. AOD Furnace Argon & Oxygen Today, more than 1/2 of the high chromium steels are produced in the AOD Furnace Linnert, Welding Metallurgy AWS, 1994

  4. A=Martensitic Alloys B=Semi-Ferritic C=Ferritic Castro & Cadenet, Welding Metallurgy of Stainless and Heat-resisting Steels Cambridge University Press, 1974

  5. We will look at these properties in next slide! AWS Welding Handbook

  6. Electrical Resistivity Surface & bulk resistance is higher than that for plain-carbon steels Thermal Conductivity About 40 to 50 percent that of plain-carbon steel Melting Temperature Plain-carbon:1480-1540 °C Martensitic: 1400-1530 °C Ferritic: 1400-1530 °C Austenitic: 1370-1450 °C Coefficient of Thermal Expansion Greater coefficient than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Surface Preparation Surface films must be removed prior to welding Spot Spacing Less shunting is observed than plain-carbon steels General Properties of Stainless Steels

  7. Static Resistance Comparison Plain-carbon Steel Electrode Electrode Stainless Steel Higher Bulk Resistance Alloy Effect Workpieces Higher Surface Resistance Chromium Oxide Class 3 Electrode Higher Resistance Resistance Higher Resistances = Lower Currents Required

  8. Electrical Resistivity Surface & bulk resistance is higher than that for plain-carbon steels Thermal Conductivity About 40 to 50 percent that of plain-carbon steel Melting Temperature Plain-carbon:1480-1540 °C Martensitic: 1400-1530 °C Ferritic: 1400-1530 °C Austenitic: 1370-1450 °C Coefficient of Thermal Expansion Greater coefficient than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Surface Preparation Surface films must be removed prior to welding Spot Spacing Less shunting is observed than plain-carbon steels General Properties of Stainless Steels

  9. Conduction in Plain Carbon Conduction in SS Base Metal Base Metal Weld Nugget Only 40 - 50% Heat conduction in SS Less Heat Conducted Away Therefore Lower Current Required Less Time Required (in some cases less than 1/3)

  10. Electrical Resistivity Surface & bulk resistance is higher than that for plain-carbon steels Thermal Conductivity About 40 to 50 percent that of plain-carbon steel Melting Temperature Plain-carbon:1480-1540 °C Martensitic: 1400-1530 °C Ferritic: 1400-1530 °C Austenitic: 1370-1450 °C Coefficient of Thermal Expansion Greater coefficient than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Surface Preparation Surface films must be removed prior to welding Spot Spacing Less shunting is observed than plain-carbon steels General Properties of Stainless Steels

  11. Melting Temp of Plain Carbon Base Metal Base Metal Weld Nugget Melting Temp of SS Melting Temp of SS is lower Nugget Penetrates More Therefore Less Current and Shorter Time Required

  12. Electrical Resistivity Surface & bulk resistance is higher than that for plain-carbon steels Thermal Conductivity About 40 to 50 percent that of plain-carbon steel Melting Temperature Plain-carbon:1480-1540 °C Martensitic: 1400-1530 °C Ferritic: 1400-1530 °C Austenitic: 1370-1450 °C Coefficient of Thermal Expansion Greater coefficient than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Surface Preparation Surface films must be removed prior to welding Spot Spacing Less shunting is observed than plain-carbon steels General Properties of Stainless Steels

  13. Ferritic, Martensitic, Ppt. = 6 - 11% greater expansion Austenitic = 15% greater expansion than Plain Carbon Steel Therefore Warpage occurs especially in Seam Welding Hot Cracking can Occur Dong et al, Finite Element Modeling of Electrode Wear Mechanisms, Auto Steel Partnership, April 10, 1995

  14. Electrical Resistivity Surface & bulk resistance is higher than that for plain-carbon steels Thermal Conductivity About 40 to 50 percent that of plain-carbon steel Melting Temperature Plain-carbon:1480-1540 °C Martensitic: 1400-1530 °C Ferritic: 1400-1530 °C Austenitic: 1370-1450 °C Coefficient of Thermal Expansion Greater coefficient than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Surface Preparation Surface films must be removed prior to welding Spot Spacing Less shunting is observed than plain-carbon steels General Properties of Stainless Steels

  15. Force High Strength High Hot Strength • Need Higher Electrode Forces • Need Stronger Electrodes (Class 3, 10 & 14 Sometimes Used)

  16. Electrical Resistivity Surface & bulk resistance is higher than that for plain-carbon steels Thermal Conductivity About 40 to 50 percent that of plain-carbon steel Melting Temperature Plain-carbon:1480-1540 °C Martensitic: 1400-1530 °C Ferritic: 1400-1530 °C Austenitic: 1370-1450 °C Coefficient of Thermal Expansion Greater coefficient than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Surface Preparation Surface films must be removed prior to welding Spot Spacing Less shunting is observed than plain-carbon steels General Properties of Stainless Steels

  17. Oxide from Hot Rolling Oxide Protective Film • Chromium Oxide from Hot Rolling must be removed by Pickle • Ordinary Oxide Protective Film is not a Problem

  18. Electrical Resistivity Surface & bulk resistance is higher than that for plain-carbon steels Thermal Conductivity About 40 to 50 percent that of plain-carbon steel Melting Temperature Plain-carbon:1480-1540 °C Martensitic: 1400-1530 °C Ferritic: 1400-1530 °C Austenitic: 1370-1450 °C Coefficient of Thermal Expansion Greater coefficient than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Surface Preparation Surface films must be removed prior to welding Spot Spacing Less shunting is observed than plain-carbon steels General Properties of Stainless Steels

  19. Look at Each Grade & Its Weldability Austenitic Super Austenitic Nitrogen Strengthened Austenitic Martensitic Ferritic Super Ferritic Precipitation Hardened Duplex

  20. Austenitic • Contain between 16 and 25 percent chromium, plus sufficient amount of nickel, manganese and/or nitrogen • Have a face-centered-cubic (fcc) structure • Nonmagnetic • Good toughness • Spot weldable • Strengthening can be accomplished by cold work or by solid-solution strengthening Applications: Fire Extinguishers, pots & pans, etc.

  21. AWS Welding Handbook

  22. AWS Welding Handbook

  23. Pseudobinary Phase Diagram @ 70% Iron AWS Welding Handbook

  24. Prediction of Weld Metal Solidification Morphology Schaeffler Diagram WRC Diagram AWS Welding Handbook

  25. Hot Cracking P+S A few % Ferrite Reduces Cracks But P&S Increase Cracks AWS Welding Handbook

  26. 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

  27. 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)

  28. 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

  29. Chromium Carbide Precipitation Kinetics Diagram 1500 °F 1500 F M23C6 Precipitation 1200 °F 800 F Temperature Chromium Oxide 800 °F M23C6 Chromium-Rich Carbides Intergranular Corrosion Time

  30. 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)

  31. 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 with Ring projections for leak tight application, electrode set-up is critical • Solution • Test electrode alignment

  32. 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

  33. 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

  34. 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

  35. AWS Welding Handbook

  36. 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

  37. 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

  38. 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

  39. 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

  40. From a Metallurgical Standpoint, Martensitic SS is similar to Plain Carbon AWS Welding Handbook

  41. 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

  42. Effect of Tempered Martensite on Hardness As Quenched Loss of Hardness and Strength Hardened Martensite Tempered Martensite Hardness Fusion Zone SS with carbon content above 0.15% Carbon (431, 440) are susceptible to cracking and need Post Weld Heat Treatment HAZ Distance

  43. 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

  44. AWS Welding Handbook

  45. FERRITIC STAINLESS STEELS Spot & Seam Welding Because No Phase Change, Get Grain Growth

  46. 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

  47. Super Ferritic • Lower than ordinary interstitial (C&N) • Higher Cr & Mo AWS Welding Handbook

  48. 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