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Distinguishing Microstructures in TRIP Steel using EBSD And Optical Imaging

Distinguishing Microstructures in TRIP Steel using EBSD And Optical Imaging. Julie Smith Washington State University Materials Science and Engineering Department. TRIP Steels. Transformed Induced Plasticity Ret. Austenite to Martensite High in Strength and Elongation Automotive Industry

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Distinguishing Microstructures in TRIP Steel using EBSD And Optical Imaging

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  1. Distinguishing Microstructures in TRIP Steel using EBSD And Optical Imaging • Julie Smith • Washington State University • Materials Science and Engineering Department

  2. TRIP Steels • Transformed Induced Plasticity • Ret. Austenite to Martensite • High in Strength and Elongation • Automotive Industry • Cooling Problems

  3. The Problem • FCC- Austenite • BCC- Ferrite, Martensite and Bainite • Advanced Methods Can’t be Used • Limitations in Analysis and Advancements

  4. Optical Microscopy-Color Etchants

  5. AFM • Topography Measurements • Martensite and Retained Austenite= Same Height • Heat Treatment • Small Scans

  6. SEM - EBSD • Image Quality • Distinguish Between Bainite and Ferrite • Operator Variation • No Strain or Stress • Image Quality and Confidence Index • Ret. Austenite, Bainite and Ferrite • High Magnification and Multiple Scans

  7. STILL IN NEED OF A SOLUTION!

  8. Techniques • EBSD/OIM and Optical • Other Property Analysis • Ability to Reprogram OIM Software • Medium Size Scans • Easy Sample Prep

  9. Proposed EBSD • Compare OIM phase with Etched Sample • Austenite and Ferrite- same color with etchant • Austenite and Ferrite- Different Crystal Structure • Misorientations between Grains (future) • Relationship Between Grains (future)

  10. Procedure • Trial 1 • Diamond Polish then Vibro Polish • Make Markings • OIM • Plasma Etch • 2-step etch (4% Picral + HCl followed by 10% aqueous metabisulfite) • Optical Imaging • Compare - Phase and Optical to get all 4 microstructures

  11. Optical Imaging Results • Bainite: Blue • Martensite: Gold • Ferrite and Austenite: Off- White

  12. EBSD/ OIM Results

  13. Procedure Cont. • Problems with Trial 1 • Plasma Etch appears to remove material in addition to hydrocarbons. • Etchant removes too much material.

  14. Procedure Cont. • Trial 2 • Diamond Polish then Vibro Polish • Make Markings • 2- step Etch (4% Picral + HCl followed by 10% aqueous metabisulfite) • 8 sec. Picral and 15-18 sec aq. metabisulfite • Optical Imaging • Vibro Polish (.02μm Colloidal Silica) • OIM • Compare - Phase and Optical to get all 4 microstructures

  15. Etching • Etchant Time • 2- step Etch (4% Picral + HCl followed by 10% aqueous metabisulfite) • 1st part- Etches • Too long- over-etch. Too Short- no grains • 8 sec. Optimum • 2nd Part - Colors • Too Short- not enough color, Too long- darkens • 15-18 sec Optimum

  16. Vibro Polishing • Vibro Polishing Time (.02μm Colloidal Silica) 0 Hours 1.5 Hours 2.5 Hours 4 Hours No Kikuchi Patterns No Kikuchi Patterns

  17. Vibro Polishing- 4 Hours • Vibro Polishing Time • 4 Hours.... Too Long

  18. Vibro Polishing- 4 Hours • Vibro Polishing Time • 4 Hours After- No Fish Before- Fish Grain

  19. Vibro Polishing- 2.5 Hours • Vibro Polishing Time • 2. 5 Hours

  20. Vibro Polishing- 2.5 Hours

  21. Conclusions • Optical etch shows all phases except it cant distinguish between austenite and ferrite. • EBSD distinguishes between austenite and ferrite, martensite, bainite. • Method unsuccessful due to high material removal rate in comparison to the grain size. • vibro polishing (.02 Colloidal Silica) in combination with 2-part etchant (4% Picral + HCl followed by 10% aqueous metabisulfite)

  22. STILL IN NEED OF A SOLUTION!

  23. Future Work • Etchant/ Optical • Try a new etchant with same procedure. • Program OIM to differentiate Relationships Between Grains (distinguish martensite) • AFM- MFM Technique

  24. EBSD Background • Transformation to Martensite- Orientation • Kurdjumov-Sachs Relationship • (110)bcc//(111)fcc, [1-11]bcc//[0-11]fcc • Study: Misorientation Relationships-Martensite

  25. Distinguishing Martensite

  26. Thank You!!Questions??????

  27. AFM and MFM • All Phases Shown to have some Magnetism • AFM • Cantilever Tip • Contact- Repulsion • Non-contact- Attraction • MFM • Special Tip • Resolution 25 to 50 nm • Promising Solution

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