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ing it increasingly corrosive . Crevice corrosion is similar to pitting but occurs specifically in crevices where chlorides pool out of sight . Common problem areas can include under clamps , between adjacent tubing , or underneath deposits that sit uncleaned .
What Is a Crack ?
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technology can pinpoint underlying defects , which might have gone unnoticed with other non-destructive testing methods , including minuscule stress points that are harder to read . ECA testing can ensure the proper identification of stress cracking , corrosion , inclusions , and other types of degradation . |
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Cracking is another possibility for stainless steel that needs to be anticipated and prevented . When undue stress is placed on a metal , damaging the atomic bond strength and separating the material , cracking occurs . While the threshold at which cracking occurs for a given metal can generally be anticipated by the atomic bond strength , ‘ artifacts ’ in the fabric of the metal , like inclusions or bubbles , can amplify the experienced stress at a specific region .
Another type of cracking is SCC ( Stress Corrosion Cracking ) which arises when a corroding component is stressed by a load , or by residual welding stress . SCC can be observed as fine clusters of parallel cracks , sometimes eventually forming pinholes . This indicates that SCC has pierced through the thickness of the metal . Taking into consideration that cracks can result in crevice corrosion , it becomes clear that cracks and corrosion are compounding risks ; the presence of either increases the risk of the other .
The Broader Importance of Testing and Inspection
Before looking at the range of methods for testing and inspecting stainless steel , it would be helpful to situate and motivate this topic within a broader project . Testing and inspecting stainless steel for imperfections , cracks , and corrosion is not merely a question of prudent maintenance ; it is a contribution towards the longevity of building materials and sustainable construction practices . Stainless steel itself brings a lot of possible solutions to the question of greener industry : it is a durable material that does not need to be regularly replaced ; it is one hundred percent recyclable ; it is a lighter metal requiring less fuel to transport ; and it reflects heat well , so that when used in construction it can reduce the need for air-conditioning . To be a true green solution , however , two other things are needed : innovation in green milling , and diligent inspection practices .
Environmental Considerations
More sustainable production practices are sorely needed because the traditional practices are carbon intensive ; for every kilogram of stainless steel produced , seven kilograms of CO2 are emitted . Luckily , efforts are underway to change the way things are done . Electric Arc Furnaces ( EAF ) and Submerged Arc Furnaces are facilitating the switch away from coke-fueled blast furnaces . While the electrification of mills is not new , integrated mills are now turning to EAFs as , alongside being an inherently
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less carbon intensive process , they make more efficient use of energy , and can be quickly turned on and off , saving time and money .
Where electrification is yet to be implemented , carbon capture and utilization methods are also finding usage . At the cutting edge , Hydrogen Reduction ( H-DRI ) is an innovation from Europe that can cut emissions by about 95 %. Instead of CO 2
, the waste product of this process is water . However , H-DRI facilities are expensive to set up and currently reliant on public subsidy . In fact , much of the effort to achieve “ green steel ” is ultimately reliant on the presence and resilience of renewable energy infrastructure . Government policy is the major variable in the sustainability of steel .
Testing and Inspection
But what can a technician or operations team do to provide diligent care ? What are some possible methods that offer to test and inspect stainless steel ?
1 ) The first step towards effective inspection is chemical composition analysis , allowing the technician to assess whether the metal meets the standard . Many downstream issues can be anticipated in this step .
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2 ) Macroscopic Inspection involves using the naked eye or a magnifying glass no greater than x10 to identify defects . This is an essential early step that can guide subsequent inspection and testing . This will immediately locate more obvious pitting corrosion .
3 ) Moving to more intensive analysis , Metallographic Structure Inspection looks at the internal structure of the steel and pinpoints defects . It can determine grain size , inspect inclusions , inspect depth of decarburization layer , and the segregation of chemical composition .
4 ) Testing of hardness is an important next step . The two most common approaches are the Brinell and Rockwell methods , but the Vickers , Shore , and microhardness methods are also effective .
5 ) Tensile testing measures both the strength index and plastic index of the metal . This is an essential step to match the material with the design and engineering needs . At normal temperature , strength indicators include yield point and tensile
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strength . At high temperatures , strength indicators include creep strength , enduring strength , high temp . specified non-proportional elongation stress .
6 ) Impact testing measures the materials ability to absorb impact energy – the higher the absorbed energy , the higher the resistance .
7 ) Non-destructive testing ( NDT ) includes other methods that can be utilized without requiring down-time . Ultrasonic testing ( UT ) involves a transducer emitting ultrasonic beams to the metal surface . The amplitude of the reflected beam is then compared to identify the type , sizing , and location of the flaw . The ability to vary frequency levels makes the inspection of components with varying wall thickness possible and the identification of a wide range of flaws easier . Eddy Current Array ( ECA )
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Final Words
The reliability of stainless steel is dependant upon comprehensive testing and inspection practices . From its chemical composition to pain points and failures , testing not only extends the lifecycle of the metal , but it contributes to safe and sustainable applications across the board . Emerging technologies aiding in inspection methods continue to grow with the global push towards emissions abatement while ensuring the integrity of stainless steel goes beyond maintenance .
References :
- https :// ssw-americas . com / a-simple-overview-of-cracking /
- https :// www . hermessteel . net / news / inspection-of-stainlesssteel
- https :// www . zetec . com / blog / stainless-steel-testing-usingultrasonic-and-eddy-current-ndt /#:~: text = During % 20 stainless % 20steel % 20testing % 2C % 20technicians , the % 20 heat % 2Daffected % 20welded % 20joints .
- https :// ssw-americas . com / green-and-circular-the-futureof-stainless-steel-production /
- https :// www . swagelok . com / en / blog / pittingcorrosion-crevice-corrosion-identifying-thedifferences #:~: text = Crevice % 20corrosion % 2C % 20like % 20 pitting % 20corrosion , wide % 20and % 20relatively % 20 shallow % 20pits .
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