Band 75, Heft 1

In the field of massive forged components the mechanical engineering industry searches for processes with increasing energy and resource efficiency. The new generation bainitic steels are promising for such application because of the high strength, toughness and fatigue properties. In order to achieve the desired mechanical properties, the development of the bainitic microstructure depending on the parameters of the thermomechanical process and on the cooling procedure must be well-known. In the present work diverse experimental techniques were applied for the investigation of the microstructural development during thermomechanical treatment and subsequent continuous cooling through the bainitic transformation range. The thermomechanical processes were simulated using dilatometers and at the same time, the specimens were analyzed using an eddy current sensor or using in-situ X-ray diffraction measurements at synchrotron (DESY). The results show that the eddy current sensor is suitable for the monitoring of the microstructural development during cooling and during deformation. From the investigations suitable process parameters were deduced for achieving a possibly fine bainitic microstructure. The main factors are a relatively low deformation temperature in austenitic range, a fast cooling (> 2 K/s) into the bainitic range, bainitic transformation and/or a short deformation in the lower bainite range, and finally a slower cooling until room temperature.

The heat treatment of aluminum (Solution Heat Treatment, Quenching and Aging) are critical processes to insure that the desired mechanical and corrosion properties are achieved. Of these steps, quenching is perhaps the most critical of all the operations. If quenching is too fast, properties are met, but the part may have excessive distortion or residual stresses. This can result in shortened life due to residual stresses, or result in additional non-value added straightening of the component and this increases cost and cycle time. In this paper, the mechanism for distortion of aluminum is explained, and methods (racking and quenchants) are shown that can reduce distortion and residual stresses in heat treated components.

The process chain, typical for additive laser beam powder bed fusion manufacturing of TiAl6V4 components in the aerospace industry, includes hot isostatic pressing (HIP) to reduce the initial porosity. The comparably high temperatures and pressures required by this process, are likely to affect the microstructure and thus also the material properties. Specific variations of the HIP parameters are examined by means of controlled HIP cycles. The parameter variations were kept within conventional boundaries of aeronautics specifications. Specific tests designed to more clearly point out any interdependencies and potential benefits to HIP were also conducted using parameters deviating from the conventional ones. The influence of the HIP parameters was examined using hardness measurements, tensile tests and optical microscopy. Some samples were also examined under a scanning electron microscope. The examinations show that, especially as HIP temperatures rise, the α-lamellae in typical microstructures created by this manufacturing process will grow, reducing strength and simultaneously increasing ductility. It could also be shown that average cooling rates of 75–3000 K/h did not measurably change the microstructure or static properties. As opposed to this, specific quenching at rates of 18 000 K/h during hot isostatic pressing and subsequent ageing for 20 h at 500 °C will create secondary structures comprising α-needles of < 100 nm width. The resultant local microstructure is significantly finer and will increase strength without decreasing ductility.

The high demands on wear resistant tools have led to the development of wear resistant claddings on a substrate, which can be a low alloyed steel with higher ductility than the cladding to improve the resistance of the tool against fracture. In this study, the post heat treatment of sinter-cladded X245VCrMo9-4 steel coating on X120Mn12 steel substrate was investigated, as it is expected that the substrate remained austenitic while the coating possessed a tough martensitic matrix with uniform dispersion of carbide precipitates. Samples were prepared by sintering at 1250 °C in a vacuum furnace under a nitrogen atmosphere at 80 kPa and a heating rate of 10 K/min, and was allowed to cool in the furnace after a dwell of 30 min at sintering temperature. These samples were subjected to heat treatment by austenitisation, oil quenching and tempering. The effect of heat treatment procedures deployed on the samples was examined using optical microscopy, scanning electron microscopy, X-ray diffraction and hardness. Experimental results were supported by computational thermodynamic calculations. The results indicated that the optimised heat treatment, through which the hardness of the steel coating is significantly enhanced while the substrate microstructure remained austenitic, is by austenitising at 950 °C, quenching and low temperature tempering at 150 °C. Quenching temperature was significant to the hardness of the steel coating, as quenching from higher temperature led to a lower hardness of the matrix when compared to quenching at lower austenitisation temperature owing to a high fraction of retained austenite.