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2015_MST15_PPT_MIC Final
- 1. Sometimes, Even the Very Best Welds Will Fail Robert P. O’Shea, Jr., P.E. Dr. Kent L. Johnson, P.E., FASM Applied Materials Technologies Inc.
- 2. A Recent Example of a Very Good Welded Structure That Failed At an Advanced Recycled Water Treatment Facility, the welding was executed almost flawlessly on both a 2,250,000 gallon Product Water Storage Tank and a 220,000 gallon Inter-Process Storage Tank fabricated from 304L stainless steel. The final repairs, inspections, and testing were completed, and the two tanks successfully passed hydro-testing, and a yet a failure occurred.
- 3. Overall View of One of the Stainless Steel Tanks Attacked by MIC The use of contaminated water led to a severe microbiological induced corrosion (MIC) failure of the inner surfaces on these two new stainless steel tanks (mainly at the weld locations), as well as, in the facility’s piping and other equipment.
- 4. Close Up View of One of the MIC Attacked Weld in These Tanks After hydrostatic testing with customer supplied water, MIC pitting and crevice corrosion were discovered. This was found in both stainless steel tanks, while mainly at the welds, it was also found at some locations at a distance from the welds. •Delete text box and insert picture here
- 5. MIC Failure Facts In a classic review of MIC failures in stainless steel for water used in cooling or hydrostatic testing, (NiDI Technical Publication No. 10085, September 1998), the following was concluded: •Both 304L and 316L can suffer from MIC when active bacteria were present. •There is a wide range of bacteria that can cause MIC of stainless steels. •The 300 series of stainless steels are very susceptible to MIC and may also suffer crevice corrosion at chloride concentrations below the threshold that applies without bacteria.
- 6. MIC Failure Results • These aforementioned conditions were found to be present in this case, including an unusual, wet- dry-wet situation, that accentuated the MIC corrosion. • The project was significantly delayed due to the repairs and component replacements, as well as the decontamination of the overall system. • The project experienced a very significant cost over-run, and when coupled with the lengthy delays, led to contentious relationships.
- 7. A Concept for a Presentation on the Reasons Why Good Weldments Fail for Causes Other Than Weld Quality • This particular MIC Investigation triggered a recollection of many other such failures that we have examined in the past. • While most of these “good weld” failures have unique and sometimes challenging aspects, the welding quality or workmanship was not the issue in the failure. • Many factors are involved in causing a “good weld” component or structure to fail.
- 8. LIFE DETERIORIATION MECHANISMS THAT AFFECT GOOD WELDS
- 9. Improper Design/Fabrication Failures • Design Failures/Fabrication Failures, (e.g., wrong stress levels used in design/improper size of length of welds either specified or provided). • Improper control of welding parameters leading to deterioration from undesirable weldment microstructures. • Lack of heat treatment after welding, providing desirable properties in weldment. • Improper weldment processing, (i.e., forging, casting, heat treatment, grinding, machining, coating, etc.) • Failure to consider other factors that can lead to weldment failure.
- 10. Hyatt Regency Skywalk Collapse
- 11. Hyatt Regency Skywalk Collapse Change in Design of the Walkway Beam Supports to Allow the Two Walkways on the Fourth and Second Floors to be Placed One Over the Other. Note the Increase in Load (2P) on the Outer Nut In the Design Modification Change.
- 12. Hyatt Regency Skywalk Collapse The Outer Nuts on the Fourth Floor Walkway Beams Pulled Through the Box Beams, Dropping the Fourth Floor and Second Floor Walkways to the Atrium Floor.
- 13. Reduction in Effective Section Size • Corrosion/Wastage – All Forms • Wear/Abrasion • Erosion/Cavitation • Microbiologically induced corrosion (MIC) • High Temperature Oxidation and Scaling • Top of the Line Corrosion (TLC) in piping
- 14. Typical MIC Round & Spherical Tunneling Presentation A Cross Section Revealing Severe Subsurface Tunneling, Typical of the Presence of MIC
- 15. Environmentally Induced Cracking • Stress Corrosion Cracking All Forms • Hydrogen Induced Cracking All Forms – Including SOHIC • Liquid Metal Embrittlement • Stress Relaxation Cracking • Sulfide Stress Cracking • Hydrogen Blistering
- 16. Union Oil Amine Absorber Explosion
- 17. Union Oil Amine Absorber Explosion The heat affected zone of the welded replacement course contained a hard and high strength microstructure that was susceptible to hydrogen induced cracking. The Term SOHIC, stress-orientated hydrogen induce cracking was used to describe this failure mechanism.
- 18. Union Oil Amine Absorber Explosion An Example of the SOHIC Failure Mechanism in a Carbon Steel Component
- 19. Stress Corrosion Cracking An example of Caustic Stress Corrosion Cracking (SCC) in a Carbon Steel. This type of failure is all to often found in petrochemical equipment.
- 20. Material Microstructure Deterioration • Grain Growth • Carburization/Decarburization • Graphitization • 885°F Embrittlement • Temper Embrittlement • Tempered Martensite Embrittlement • High-Temperature Hydrogen Attack (HTHA) • Sigma Phase Embrittlement • Liquid Metal Embrittlement
- 21. High-Temperature Hydrogen Attack HTHA A metallographic sample showing the linkup of microfissures to form continuous cracks in the final stage of a HTHA failure. Also note that this damage is accompanied by a significant amount of decarburization.
- 22. Mechanically Induced Deterioration • Over-Pressurization • Ductile Overload • Brittle Fracture • Mechanical Damage • Mechanical Fatigue • Corrosion Fatigue • Thermal Fatigue • Creep/ Stress Rupture • Abuse/Improper Loading • Insufficient Inspection/Maintenance
- 23. Creep and Stress Rupture At elevated temperatures, metal can slowly and continuously deform under load below the yield stress. This time dependent deformation is known as creep, and it leads to damage that creates cracking, which leads to eventual rupture.
- 24. Weld Related Life Reductions Unfortunately there are also a significant number of welding quality related failures. Too Many. • Improper base metal or consumable selection • Improper welding procedures or processes. • Improper workmanship and technique • Intentionally not depositing the proper amount of weld • Insufficient or Inadequate QA/QC to timely correct problems • Failure to properly train and equip the welding workforce, supervisory and QA/QC staff. • We have not touched on the problems of weld safety.
- 25. Sometimes, Even the Very Best Welds Will Fail Robert P. O’Shea, Jr., P.E. Dr. Kent L. Johnson, P.E., FASM Applied Materials Technologies Inc. Thank You.
Editor's Notes
- During many of our root cause, metallurgical failure analyses, we have found that not all welded component failures happen as a direct result of the quality and/or workmanship of the welding that was performed. However, in many such instances, and all to often, the welding is wrongly identified as the problem.
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- Bob – talk about the wet – dry – wet condition with the Conservation Vent allowing humid air to enter the tanks.
- This MIC evaluation certainly did have many unique conditions, including a wet-dry-wet type of MIC continuation in causing the crack-like degradation of the stainless steel tanks. Additionally, the Code of Practice (API 650) also played a role, as well as, the hydrostatic testing of the tanks themselves. The API Code clearly states that the Owner is responsible for the quality of the water that is supplied to perform the hydrostatic testing of API 650 tanks.
- Here is a brief review of those factors that can cause good welds to look bad. A few of examples are given to illustrate that welded structures and components are sometimes at the mercy of other factors.
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- On July 17, 1981, during a very large tea party dance in the atrium of the Hyatt Regency hotel in Kansas City, two walkways that spanned the atrium at the fourth and second floor, one above the other, collapsed onto the crowded atrium floor and pancaked together. A total of 114 persons were killed, and over 200 were injured. As a result of a massive and fast acting rescue response, 29 live victims were removed from under the very heavy debris
- This original design called for a single, very long, rod to hold the two walkways. Such a very long, threaded rod, simply could not be fabricated. While the two different designs look quite the same, the loading on the outer nuts on the fourth floor walkway has been doubled by this design change. Investigators found that this change, without appropriate changes to the design of walkway’s box beams to handle such increased loading, was the cause of this failure.
- The welding was very good, and met the specifications as detailed in the drawings and contract, however the design was off by a factor of 2.0. With a Code design Safety factor of about 1.65, increasing the loading by a factor of 2.0, results in an effective, new safety factor, of only 0.82 (Note: less than 1.0). The welds were good, the design was bad.
- Note, we have already discussed the problem that arise with MIC. With the recent rebuilding efforts in both Iraq and Afghanistan, a large number of the Civil Engineering water projects have fallen prey to the ravages of MIC failure. The project discussed earlier was rather remarkable in that a continuing WET – DRY – WET MIC attack was involved from the humidity in the air.
- All of the environmental induced cracking problem are very serious concerns. They lead to sudden and unexpected catastrophic failures, that can result in a large number of fatalities. In this case we will look at a major event involving hydrogen induced (SOHIC) type cracking.
- On 23 July 1984, the Union Oil Co refinery near Lemont, Illinois, was seriously damaged (Total ~1/2 billion dollar loss) by an explosion and fire. Seventeen members of the refinery’s fire response team were killed as they just arrived at the scene of a major leak in an amine absorber, when the explosion occurred. The explosion was caused by the ignition of a large cloud of flammable gas (a mixture of propane and butane) that had leaked from a cracked amine-absorber circumferential weld location.
- The failure occurred due to the replacement of a section of the vessel by welding in a replacement course into the pressure vessel. This led to the formation of a hard, high strength microstructure in the heat affected zone of the two circumferential welds. This hard, high strength region was susceptible to hydrogen assisted cracking, resulting in growth of a huge crack in the vessel. A new term, Stress Orientated Hydrogen Induced Cracking, (SOHIC), was used to explain this failure mechanism.
- As of result of massive refinery vessel examinations which followed this tragedy, all welded Code compliant components to be used in amine service were required to be post weld heat treated after welding was completed. In this case too, the welding quality was in conformance with the applicable ASME Pressure Vessel Code at the time, and this failure led to changes in that Code to require post weld heat treatment after welding for vessels in amine service.
- As with fatigue failures, and hydrogen embrittlement failures, and brittle failures, Stress Corrosion Cracking is also a very hazardous type of failure. All of these failures normally occur without warning, and are typically catastrophic in nature. Again, you can have the best of weld quality, and still have a terrible failure.
- Note, we have already discussed the problem that can arise with the formation of a hard, high strength region in the HAZ of a weld. Such microstructures can be particularly susceptible to environmentally induced failures. While the weld quality itself is not at fault, the failure to address the cures for such a condition, rests with the overall design of the equipment or component. This is also the case with these other microstructural deterioration mechanisms.
- HTHA mainly affects the Moly and Chrom-Moly steels that are specifically engineered to handle high temperature hydrogen atmospheres. However, sometimes processes get out of control and either overpressures or more importantly, over temperature conditions occur, which significantly reduce the life of the equipment or component. High temperature hydrogen attack results from exposure to hydrogen at elevated temperatures and pressures. The hydrogen reacts with carbides in steel to form methane (CH4) which cannot diffuse through the steel. The loss of carbide causes an overall loss in strength. Methane pressure builds up, forming bubbles or cavities, microfissures and fissures that may combine to form cracks. Finally, failure can occur when the cracks reduce the load carrying ability of the pressure containing part. This is also typically an unexpected, catastrophic failure.
- Perhaps of all the mechanically Induced Damages, when it come to welds, Mechanical Fatigue is one area that must be given special attention. The weld zone itself provides stress risers at changes in sections size, or at the toes of fillet welds, etc. Additionally, welds are simply not as discontinuity clean as are the wrought materials. Hence, the initiation portion of fatigue life is very short, even if it exists at all. Design, should account for this, however, any deviation from the weld specification of the design, can have major repercussions on the expected fatigue life of the weldment.
- Creep damage and failures have been associated with the HAZ of welds, especially in the Power Generating Utilities. As previously stated, creep and stress rupture lives can be designed for, and it is typically the upset, or transient conditions of over pressurization or high temperature excursions that will significantly lower this expected life. In this instance, while weld HAZ can play a role in the expected life of these weldment under creep condition, a good design can provide an adequate creep life.
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