Effects of Nitrided and Chrome Plated Die Surface Roughness on Friction in Bending Under Tension

Different die surface polish conditions result in a noticeable effect on material flow in stamping, which can lead to splitting, wrinkling, or other surface stretching issues associated with different friction conditions. These occurrences are not only limited to the non-coated dies, but also nitrided and chrome plated dies. To ensure quality control of the stamped parts, the die conditions corresponding to different polishing procedures need to be developed based on measurable parameters such as surface roughness (Ra). The intent of this study is to investigate the effects of nitrided and chrome plated die surface roughness on friction. The Bending-Under-Tension (BUT) test was conducted to simulate the stamping process due to the test’s versatility and flexibility in changing test parameters. The test involves moving sheet metal across a 3/8-inch diameter pin, which substitutes for a die surface. The pin can be modified by material, heat treatment, coating, and surface roughness. Pins were made of D6510 steel and heat treated to 54-58 HRC. Two different die polishing procedures were developed. The first is the progressive polishing procedure, which is applied to the pins after heat treatment but before coating. The second polishing procedure added a second stage single polishing procedure after final die surface treatments were completed. Half of the pins were nitrided and the other half were chrome plated. The BUT test used hot-dipped galvanized (HDGI) bake-hardenable 210 MPa tensile strength steel (BH210). Results indicate the 600 grit# polish condition generates the lowest average friction coefficient among all polish conditions regardless of the die surface treatments. Additionally, 3.4 μin and 5 μin is the optimal die surface roughness (Ra) for the chrome plated dies and nitrided dies, respectively, in forming HDGI BH210 steel.