NEW REAL-TIME TEST FOR PREDICTION OF ZINC PHOSPHATE STEARATE COATINGS BREAKDOWN - OPTIMAL STEARATE SETTLING PARAMETERS FOR STEEL BILLETS IN COLD FORGING

The test discussed here quickly reproduces interface pick-up occurrence and degradation of phosphate coatings which have been dipped in stearate solutions. It involves a rotating cylindrical specimen which is directly cropped from bar stock of forging plants and phosphate/stearate processed. A constant load is applied on the specimen coating by means of a profiled indenter. Tangential friction force values are measured throughout the rotation time. The stearate settling parameters are analysed in terms of pick-up sensitivity. It is shown that low friction stress and phosphate/stearate coating reliability are mainly due to zinc stearate formation and stearate settling conditions. Secure rules for stearate settling are proposed from the described reference experimental data. The test is found to be a good support for real-time management of phosphate/stearate baths.