Characteristics of Heat Transfer at Elastic Contact between Die and Material in Hot Working

In the hot rolling and the forging processes, the temperature of the material being processed is decreased by contact with the work rolls or dies. In addition, the scale layer, which was thermally insulated, was generated on surface of the material. The temperature in the working process can be crucial to the mechanical property of the product. It is important to evaluate contact heat transfer quantitatively. In this study, the contact heat transfer coefficient was investigated in two materials with different surface roughness and scale layer. The materials used were copper and SUS304 stainless steel. Thermocouples were inserted inside the test pieces to estimate the heat flux through the contacting surface. The contact surfaces of the test pieces were roughened to various Ra from 0.15 µm to 2.0 µm by lapping and lathing. The oxide layer was made of FeO and Al2O3 by thermal spray. In the experiment, the copper test piece was first heated to 500 K, after which it was pressed onto the stainless steel test piece, which was at room temperature at various contact pressures from 0.2 MPa to 80 MPa. As the result of the experiment, the heat transfer coefficient became higher as the contact pressure was increased. Applying a parameter based on a theory of total thermal resistance in a composite plane wall, a proportional relationship was shown to exist between surface roughness over thermal conductivity and the heat transfer coefficient.