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.
The condition observed was not the result of classic non-metallic inclusions that cause defects in drawn parts. It is my opinion that if small non-metallic inclusions are causing this condition, then they would be well below the normal levels that producing mills would be willing to work to on a routine basis (ASTM E45 Method A – 2.0 maximum; A, B, C and D type inclusions for Type 305).
I would suggest that we explore obtaining ESR (Electro Slag Re-melt) quality material for this application. This is a double melted product that reduces the dirt levels in the steel by an order of magnitude. Material produced via this method is used in critical applications such as aerospace and medical. The material is initially melted via a traditional air melt technique (electric furnace – AOD – cast into electrodes). The electrodes are then arc re-melted under controlled conditions using a protective slag blanket to remove addition non-metallic’s from the material. The electrode is re-melted into a slab that is processed by normal strip manufacturing techniques. One of the domestic mills have manufactured and supplied this product. It is usually quoted in quantities of a re-melt slab (5,000 to 8,00 lbs) and has long lead times and a significant price premium over conventional Type 305.