A NEW DEVICE FOR UPSETTING TESTS OF STEELS AT HIGH-TEMPERATURES

Comprehensive analysis of forming processes requires reliable and rapid determination of stress-strain relationships. For that purpose, the upsetting of steels is achieved on a new device with cylindrical specimens of diameters in the range from 6 mm to 10mm. The upsetting dies are made of two cylinders of alumina on which are placed 3 mm thick carbide plates. The specimen and the dies are inside a transparent and almost airtight box connected to an argon tap. The specimen is heated by a three-spire inductor and the temperature is measured with an infra-red sensor. Two typical procedure are presented: the first involves heating the specimen to the test temperature and then test achievement; the second involves heating to a temperature higher than that of the test and cooling to the test temperature. Upsetting loads, cylinder current-height, and the ratio between equatorial diameter and contact surface diameter are saved via a RS 232 line. Specific software, written in Basic, has been developed for the computation of the flow stress, taking into account friction and the contact conditions between the specimen and dies, assessed from the barrelling of the cylinder. Experiments have been performed on 1015 continuous-casting steel with the heating-only procedure and on 1010 also continuous-cast steel with the heating-cooling procedure. The stress-strain curves are given and the flow-stress equations are defined for typical problems; large effective strain from 0.2 to 0.8 in the first case, moderate effective strain from 0 to 0.15 in the second case. The accuracy is found to be very good: the variation of measurements for the same test conditions never exceed 4%. The testing procedure and the device are very convenient for daily analysis: each test may be carried out in less than three minutes.