The influence of process variables on the gas forming and press hardening of steel tubes

In this paper, a study on the innovative process of hot tube metal gas forming and hot press hardening (Hot Metal Gas Press Hardening - HMG-PH) is presented. The aim is to provide new insights into the influence of process variables (gas pressure, tool temperature, tube pre-heating temperature, etc.) on the hardening phenomenon and on the quality of the produced tubular components (calibration radii, minimum wall thickness, etc.). Several experiments are described on two different kinds of steel, using a reference die geometry, specifically designed for investigating the typical critical issues of the process. The study demonstrates that the hardening phenomenon is strongly local not only because the hardness distribution is non-uniform over the final part, but especially because the hardening of different regions depends on different process parameters: in regions that need calibration, hardening is governed more by the pressure vs. time curve, i.e. by its rate and its maximum value and less by the tool temperature; in regions that rapidly go in contact to the die, hardening is governed more by the tool temperature and less by the pressure vs. time curve. Another relevant conclusion is that an optimal value of pressurization rate can be found that maximizes formability. Finally, the study proves that, on the formed tubes, obtaining small calibration radii and obtaining high values of hardness are conflicting objectives. The physical mechanisms behind these behaviours are discussed.