Investigation of surface scratches created during strip steel manufacturing

Abstract: Thin flapper valves are made from a stainless steel strip that contains 13.5 wt. % Chromium.  Sometimes in its processing this strip experiences a surface defect which may affect the end product performance negatively, so they are not desirable. These surface defects  have  been  termed  as  “scratches“  due  to  their  thin  line appearance,  and are hard to detect during final surface inspection. Flapper valves are made from both thinner and thicker steel gauges; of which the issue is more concentrated to thicker gauges. These scratches are created on the material during the process and they are not acceptable for end use applications such as flapper valves.  These scratches can create an initiation point for a fatigue fracture on a flapper valve; which will then cause a breakdown of the unit it is installed in. The scratches also lead to lower production yields because finished material affected by scratch is scrapped. Scratches are often present on the strip and their occurrence has no obvious correlation to any circumstance that would explain it. The steel strip is processed through various methods which improve surface finish, material properties and dimensional requirements. The  scratches  are generated only during the hardening  phase  of production since they are observed only after the hardening furnace  by  inspection personnel. Hence the hardening furnace is seen to be an area of connection for scratches and looked upon as the most likely source, but no proven link has been made in this regard. Therefore this work aims to find out the  possible  root  cause  for  the scratches and suggest future  improvements  based on their findings . This project work was done at Böhler – Uddeholm Precision Steel AB, Munkfors, Sweden. The project investigation started with initial sample study on finished samples and then proceeded towards samples isolated from different heat treatment stages which can be called as process samples. These process samples were later examined through Confocal Microscopy (CM) and the Scanning Electron Microscopy (SEM) to discover the causes behind these scratches. These instrument measurements & findings were supplemented by citations of abrasive and adhesive wear mechanisms which shall be explained further. These mechanisms point towards tribology . Having found the causes and suitable remedial measures,, production trials were performed to observe the practicality and efficiency of our suggested solution.