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Lamellar Tearing

Lamellar Tearing. “Form of brittle fracture occurring in planes essentially parallel to the rolled surface of a plate under high through thickness loading”. Lamellar Tearing. Highly restrained welded joints susceptible Localized strains due to weld metal shrinkage are very high (> yield)

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Lamellar Tearing

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  1. Lamellar Tearing “Form of brittle fracture occurring in planes essentially parallel to the rolled surface of a plate under high through thickness loading”

  2. Lamellar Tearing • Highly restrained welded joints susceptible • Localized strains due to weld metal shrinkage are very high (> yield) • Service load stresses are usually to small to produce • Thick plates more susceptible • Concentration of impurities from rolling • Related to cooling of the ingot • Not as common in modern steels

  3. Lamellar Tearing(Fort Duquesne Bridge)

  4. Lamellar Tearing(Fort Duquesne Bridge)

  5. Banding

  6. Continuous CasterSegregation

  7. Rolling Can Concentrate Segregation Occurs in thin plates unlike lamellar tearing

  8. Cracked VMS Support Structure

  9. Support Structure at Lab

  10. Photos of Cracks

  11. Macro View of Exposed Fracture Surface

  12. Cross Section

  13. What can we tell? View of Secondary Cracks – 100x

  14. Banding at CenterlineAllowed Fracture to propagate parallel to plate

  15. Conclusions • Likely caused by liquid metal embrittlement (LME) during galvanizing of the towers. • Not hydrogen embrittlement, pure lamellar tearing, fabrication defects, material deficiencies, strain age embrittlement, or fatigue. • The physical and mechanical properties in conformance with ASTM A572 Gr. 50. • No correlation between fillet weld reinforcement size and tendency for cracking. • The combination of high weld metal strength, generally large weld reinforcements, and highly restrained geometry of the strut connection allowed the development of high weld residual stresses, • An important factor in the occurrence of LME.

  16. High Performance Steel

  17. What make it High Performance”? • The main differences compared to conventional 70 ksi steels (or 50 or 100) • Improved weldability • Improved toughness • Other properties such as corrosion resistance and ductility will be essentially the same

  18. Weldability • Weldability is a property that is somewhat difficult to define. • Conventional 485-MPa steels typically require • preheating of plates • control of temperature between weld passes • controlled handling of welding consumables • precisely controlled energy input • post-weld heat treatment in some cases • When all of these operations are performed correctly, it is usually possible to produce high-quality welds in conventional high-strength steel. • Difficulties can arise, when one or more of these operations deviate from prescribed procedures.

  19. Weldability • Minor differences in procedure and quality control are the norm for bridge construction, • Many different fabricators in different parts of the country work under different climates and conditions • The result is that conventional high-strength steels have experienced a higher percentage of weld problems compared to lower strength steels. • In particular the control of temperature adds significantly to the cost and time required for welding

  20. HPS Metallurgy • Many approaches were tried to develop a steel with high performance and 70 ksi strength. • Both processing methods and alloy composition were varied until the optimum combination for HPS-70W was selected. • The optimum alloy a modified version of the existing A709 grade 485W Q/T steel • The big difference is that the carbon level was greatly reduced, thereby providing the large boosts in weldability and toughness.

  21. Definitions • Quenching • Most commonly used to harden steel by introducing martensite • Cooling is very quick and austenite can be formed

  22. Definitions • Tempering • Process involving slow and moderate heating to increase the hardness and toughness of metals that have undergone previous heat treatment • Metals are usually hardened by being heated to high temperatures and quenched rapidly

  23. Thermo-mechanical Controlled Processing (TMCP) • Highly controlled Process of temperature and reduction due to rolling • Hence the name ->Thermo-mechanical • Not Q/T Steel • Generally up to 2 inches thick • However, can roll much longer than Q/T • Q/T < 50 ft • TMCP >150 ft

  24. HPS Metallurgy • Compared to a conventional A709-70 ksi steel carbon is reduced from a maximum of 0.19 percent to about 0.10 percent • Other alloy adjustments, micro-alloy additions, and processing changes enable strength to be maintained • The low carbon level is the primary reason for the great improvements in weldability and toughness

  25. HPS 70 W Fy vs. Thickness

  26. HPS Improves Structural Resistance to Fracture • Increased crack tolerance a.k.a. Fracture Initiation Resistance • Yield on net section • Better chance of catching cracks in inspection • High tolerance of bending and cold forming • Higher dynamic crack arrest capability • Backup for unforeseen problems

  27. Variability in Properties

  28. HPS 70 W Fy vs. Thickness

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