February 2020 Volume 2

FORGING RESEARCH

Background One of the goals of this project was to identify a testingmethodology that could quantitatively distinguish between the various coatings and test conditions. The ring compression test was chosen (6-9) , as it is an easy test to perform, it simulates metal deformation conditions present in commercial forging applications better than other test procedures (such as pin-on-disk, for example), and suitable test equipment is already available at Mines. The test involves the compression of thin metallic rings with controlled dimensions (typically OD:ID:thickness in the ratio of 6:3:1), and, as shown schematically in Figure 1, the coefficient of friction can easily be estimated based on measurements of the ring dimensions after forging (change in height and change in ID). The friction factor can be plotted on the calibration curves shown in Figure 2.

Figure 2: Calibration curve for upset ring test with OD:ID:thickness in the ratio of 6:3:1 (6,7) The interface friction factor, m, is defined as the interface shear strength divided by the material yield stress in shear, and is applicable for conditions with high interface pressure where deformation of the forged component occurs at or near the interface to accommodate relative motion (6) .

Figure 1: Schematic drawing of the cross-section of a sample with original OD:ID:thickness in the ratio of 6:3:1 and then upset ring tested (forged), showing the impact of friction conditions on the inside and outside diameter for a given change in height. The equipment currently available atMines is shown in Figure 3, and consists of a set of hardened steel forging dies loaded onto a 100 kip hydraulic forging press. This equipment was modified as described in the experimental procedure below, to allow an evaluation of the effect of various coatings on friction during forging. Experimental Procedures The ring test equipment at Mines shown in Figure 3 was modified to allow the use of steel inserts that could be easily coated. Objectives of the modified test equipment included the capability to fit the test equipment onto the 100 kip hydraulic press at Mines, the ability to quickly switch die faces, the ability to pre-heat the die inserts, and to measure both load and displacement during testing. The test die is shown in Figures 4 and 5, and involved the use of steel inserts that could be easily coated, and quickly inserted into

Figure 3: Left, ring forging tooling on a 100kip hydraulic mechanical testing frame at Mines. Right, industrial application of the forging ring test, showing the applicability of the test in an industrial setting (8) .

die holders previously installed on the hydraulic press. The holders were fabricated from 4140 steel, and five pairs (upper and lower) of inserts were fabricated from H13 steel hardened to about 42 HRC. The dimensions of the inserts are shown in Figure 6. Prior to testing, the faces of the steel inserts were polished to a consistent finish. This involved grinding to a 1200 grit finish, followed by polishing using 6 μ m, 3 μ m and 1 μ m diamond paste. The steel inserts were tested in both un-coated and coated conditions. A complete list of the coatings evaluated in this study is provided in Appendix 1, and these coatings were produced by commercial coating companies.

FIA MAGAZINE | FEBRUARY 2020 55