Galling of high strength steels and stainless steels in sheet metal forming as evaluated using pin-on-disc testing

Sammanfattning: The thesis focuses on the problem galling in sheet metal forming operations. Galling occurs when sheet metal transfers from the sheet surface onto the tool surface and causes surface damages to following sheets in the forming operation. When this happens the tool must be replaced causing production stops and additional expenses.The increasing use of high strength steel grades combined with the desire to use more environmentally friendly or no lubricants exposes the tools used for sheet metal forming for tougher forming conditions with higher contact pressures and thus an increasing risk for material pick-up and galling. To prevent this new tool steels, lubricants and coatings have been developed. In this thesis a number of tool steels, high strength steels, stainless steels, coatings and lubricants have been investigated with the test method pin-on-disc testing in order to explain how different premises, such as chemical composition, mechanical properties and surface topography influence on the galling tendency in sheet metal forming.The results show that there is a big difference in performance between a conventional cast tool steel and a powder metallurgy tool steel with respect to the galling tendency. The results also show that the surface topography of both the tool steels and the sheet materials influence on the performance of the lubricants and the galling tendency.   When coating a tool steel, the substrate hardness is of greatest importance for the performance and wear of the coating. For the different coatings investigated, the performance of the coatings depends on whether the sheet material is zinc coated or not. However, the CrC/C coating investigated show a good performance with respect to the galling tendency in contact with all of the investigated high strength steel grades.