The isothermal high temperature pneumoforming process to form tubes at constant elevated temperatures by means of internal pressure is investigated. Two materials, a ferritic (X2CrTiNb18) and a martensitic stainless steel (X12Cr13) are used for the investigations. The required material characterization is performed at the temperature and strain rate of the actual process. A new method for quantifying thermal softening via the time-dependent decrease in static yield stress is presented. At a temperature of 1000 °C, the static yield stress decreases by 50% within 100 s for both materials. The numerical models are validated on the basis of the formed geometry and used to study the influence of maximum internal pressure, axial feed, holding time under load and die edge length on the final part geometry. It was observed, that with higher internal pressures and longer holding times smaller corner radii are formed for both materials. In contrast, a superimposed axial feed as well as the effective friction coefficient have a negligible influence on the formed geometry. With an increasing die edge length, smaller radii are formed with the ferritic stainless steel numerically and experimentally. By contrast, for the martensitic stainless steel, larger radii are observed numerically. Experimentally, the limited formability of these tubes weld seam becomes apparent. Based on the findings, process windows depending on the process parameters internal pressure and die edge length were derived. Numerically, forming limit curves of tubular semi-finished products under comparable conditions serve as a failure criterion. Good agreement with experiments was observed.

研究了通过内部压力在恒定高温下形成管材的等温高温气力成型工艺。研究使用了两种材料：铁素体 (X2CrTiNb18) 和马氏体不锈钢 (X12Cr13)。所需的材料表征是在实际工艺的温度和应变速率下进行的。提出了一种通过静态屈服应力随时间的变化来量化热软化的新方法。在 1000 °C 的温度下，两种材料的静态屈服应力在 100 秒内降低了 50%。数值模型在成形几何形状的基础上进行了验证，并用于研究最大内压、轴向进给、负载下的保持时间和模具边缘长度对最终零件几何形状的影响。它被观察到，较高的内部压力和较长的保持时间，两种材料都会形成较小的角半径。相比之下，叠加的轴向进给以及有效摩擦系数对成形几何形状的影响可以忽略不计。随着模具边缘长度的增加，铁素体不锈钢在数值和实验上形成了更小的半径。相比之下，对于马氏体不锈钢，从数值上观察到更大的半径。通过实验，这些管焊缝的有限成形性变得显而易见。根据研究结果，得出了取决于工艺参数内部压力和模具边缘长度的工艺窗口。从数值上看，管状半成品在可比条件下的成形极限曲线可作为失效准则。观察到与实验的良好一致性。