Certification of salt spray performance of stainless steel strip

The salt spray performance of a stainless steel flat rolled product is not solely dependent on chemical composition.  Exposure to contaminants during fabrication of the strip and subsequent processing of the strip into parts can greatly affect the salt spray performance of a finished part.  NKS is not aware of any producing mill source that will guarantee the salt spray performance of their product in the as received condition or after subsequent fabrication into a finished part.  The best way to qualify the salt spray performance of a product is to lock in the raw material supply, fabrication and subsequent cleaning and handling of the parts.  Salt spray testing is then performed on the final part to quality the raw material source and complete fabrication process.  This would be similar to the PPAP qualification of a part.  Since salt spray testing is costly and time consuming it is not practical to test every production lot.  Periodic monitoring testing can be performed to verify continuing salt spray performance.

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ASTM Specification A480 - What is it?

ASTM A480 is the specification that covers the general requirements for flat-rolled stainless and heat-resisting steel plate, sheet and strip.  It does not reference or control the chemical and mechanical property requirements of specific stainless or heat-resistant grades.  The A480 specification is referenced in stainless steel flat-rolled specifications ASTM A167, A176, A240, A666 and A693.  ASTM A480 controls the ordering, processing, heat and product check analysis, finish, inspection and testing, workmanship, packaging and marking and many physical requirements for the product.  Physical requirements include thickness and width tolerances, camber, flatness and out of square requirements for sheets.  These requirements provide control of the manufacture and define physical limits for flat-rolled stainless products.  These limits remain in force except when superseded by customer specifications or purchase order endorsements.

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Substituting JYH21CT for 304

This material represents a concerted effort by Pacific Rim raw material manufactures to develop cookware grades that have no or low Ni content. The impetus for this effort has been driven by volatile and surging Ni prices. The data sheet extensively documents the corrosion, fabrication and performance characteristics of this alloy compared to Type 304 stainless steel. The published data demonstrates corrosion performance similar to Type 304 in standard corrosion tests. The material has some physical property advantages over Type 304 regarding thermal conductivity and is ferro-magnetic which is important for induction heating applications. Forming characteristic have been enhanced to facilitate deep drawing. The writer is sure that some tooling issues would have to be overcome to transition from Type 304 to JYH21CT. This certainly appears to be a grade that is worth exploring as an economical alternative to Type 304. Developing a viable source of supply would be an important consideration. There are no USA or European stainless steel manufacturers that have invested in the development of equivalent low or no nickel grades optimized for cookware applications. Read More

Ductile Failure

Three sample parts from the material identified above were forwarded to my attention for review.  Each part had a minor blemish that was identified with a black indelible marker.  The condition was very faint and did not appear to be the result of non-metallic inclusions.  I performed a visual examination with a 10X eye loop on the three groups of parts (5 parts total) sent for examination.  I would characterize all the highlighted defects as ductile fractures.  Ductile fractures occur when stresses are concentrated in an isolated area which leads to material thinning and ultimate fracture when the section can no longer tolerate the applied load.  The morphology of the defects is not characteristic of sidewall failures associated with the presence of gross laminations or seams in the base material.  Large laminations or seams generate the classic parabolic spill morphology on the part except when the defect is oriented on the exact centerline of the incoming strip.  In this special case, a continuous straight line defect is present.  The cause of the ductile failure could not be determined by visual inspection.  It is possible that small non-metallic inclusions that are always present in the steel are responsible for the initiation of the ductile failures.  They would act as stress risers that accentuate the ductile failure.  The small surface fissures associated with the cold worked grain structure on the side wall of the part may also act as the stress risers.  Since visual observation is not conclusive; extensive metallographic and SEM analysis would be required to even begin to get to the origin of the fractures.  It is the writer’s opinion that there would be a slim chance of discovering conclusive evidence that non-metallic inclusions were responsible for the ductile failures. Read More

Dry Film Lube (DFL) use in stainless steels

Dry-film lube (DFL) is a coating that is applied to the strip surface via a coil coating operation.  They are used in stamping or forming operations as is without the use of any additional drawing oil or compound.  Acrylic DFLs are the most commonly used for severe drawing applications such as stainless steels.  Acrylic DFLs are water based acrylics with extremely low VOC (volatile organic compounds) and no HAPS (hazardous air pollutants) that form a coating on the surface of the strip.  After the coating has dried, it is not soluble in water and provides a level of water and corrosion resistance.  After forming, the DLFs can be removed via an alkaline cleaning process.  Acrylic DFLs exhibit ultra-low coefficient of friction in the 0.04 to 0.09 range.  Switching to DFLs from oil based lubricants can facilitate steel grade reductions, improved formability, increased productivity, reduced scrap,  reduced die maintenance and reduced environmental costs and reporting requirements.

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