Influence of tool steel microstructure on origin of galling initiation and wear mechanisms under dry sliding against a carbon steel sheet

The drive to reduce lubrication in sheet metal forming (SMF) operations has increased the interest to study the contact between polished tool materials sliding over relatively rough sheet surfaces. Commonly, transfer of sheet material to the tool surface occurs by adhesive wear. In the present work, a study of wear showed that the origination of the damage was complex and several simultaneous wear mechanisms were operative, even though friction did not change remarkably. A low strength carbon steel sheet was tested under dry sliding conditions against conventionally ingot cast and powder metallurgy cold work tool steels. The sheet was sputtered with a thin gold layer, which acted as a marker helping to reveal details of the wear mechanisms. Initial sliding was characterized by local adhesive wear with transfer of sheet material, predominantly, to the metallic matrix of the tool. Seemingly, the carbides have less adhesion to the sheet material than the steel matrix. Subsequently, materials transfer resulted in gradual coverage of carbides with formation of a semi-continuous thin layer of sheet material on the tools surface. Further sliding led to initiation of local microscratching of the sheet surface due to formation of lumps of sheet material adhered to the tool. Comparison of the two different tool steels revealed that the amount of adhered sheet material depended on amount, size and distribution of carbides and was higher for the ingot cast steel with coarser carbide phase. The advantage of the powder metallurgy steel was associated to removal of adhered material from the tool due to a higher amount and a more homogeneous distribution of finer carbides.