In the following, the presentation of the results will concentrate on the stroke itself, which takes only a few tenths of a second. At first, measurements were taken with a low energy setting for the forging press. The goal was to test the functionality of the sensors. Therefore, six measurements were taken with 10% press energy and 32.5 mm end stop height (Fig. 11). Sensors 1, 3 and 12 do not have contact with the part during the forging process, therefore, they measure only the die temperature and not the part-die-contact temperature. In the case of sensor 3 a slight dip is observed. This is not originated by an actual temperature change but rather attributed to be caused by pressure and consequent deformation of the die and thereby leading to a decreased resistance of the chromium sensor which has a negative pressure coefficient. The same can be seen for sensor 10, although the temperature increase overlays this for most of the time. This sensor also perfectly shows the effect of manual insertion and positioning of the billet: While in stroke #3 a maximum temperature of ≈450 °C is measured, in stroke #6 there seems to be no contact with the part, only the effect of deformation. Sensors 4 and 6 have a recurring temperature profile for all six strokes because of their central position. Maximum temperatures vary between 400–425 °C and 350–380 °C, respectively. The only difference between the strokes is when the press is opening up. In some cases, the temperature decreases rapidly by more than 100 °C in about 0.15 s (stroke #6), caused by the heat dissipating into the colder die. In other cases, the temperature drops only 10–20 °C (stroke #1), which could be explained by the hot forged part being stuck to the die. For sensor 2, as was the case during heating-up, spontaneous jumps can be observed again. They are attributed to a defective contact which is why its calculated values should not be considered.
To investigate the effects of die temperature increase due to continuous forging, five parts were forged consecutively (Fig. 12). Sensor 12, which is placed with a distance of more than 10 mm from the forging area, shows a continuous increase of temperature. All other sensors have a periodic behaviour while the temperature before the stroke increases with every stroke. The maximum temperature during the stroke, which is not dissolved in this diagram, does not increase significantly over the series. Sensor 6 and in some cases sensor 4 as well have a defect leading to a loss of signal. When looking at the data more closely and with a higher time resolution it can be seen that during the strokes, temperatures can be temporarily measured with both sensors though. The reason is suspected to originate from wear of the Al2O3 layer at the curved areas of the die leading to temporary shorts or disruptions that are temporarily reversed by the stroke. The measurement also reveals at what time the forged part is taken out of the die. At this time, the temperatures of sensor 2, 10 and, 4, when sensor 4 works as planned, drop abruptly. Sensor 3 shows a slight delay due to its distance.
In the following, the press energy and the height of the mechanical end stops were modified (stroke #12–#17). Table 3 shows the changes that were made to the setup with the intention to find out the maximum workload the sensors can measure at.
In Fig. 13 four strokes were chosen representatively to demonstrate the effect of the setup changes. Unfortunately, sensors 4 and 6 showed further damage in stroke #12 and afterwards were out of order. Sensor 10 experiences the highest temperature of all sensors if in contact with the part. These are even higher than before due to the higher energy and the reduced end stop height with a maximum of about 550 °C. It can also be seen that sensor 3 takes a slightly larger dump with increased energy. After two strokes were performed with the final setup, all sensors, except the two lying on top of the die (3 and 12), were no longer working due to increased wear.
The effect of increased energy and reduced end stop height can be analysed in more detail when looking at the temperature profile of sensor 1 (see Fig. 14). All strokes with changed parameters are depicted in the diagram. While the energy increase from 10 to 28% has no effect due to the part not coming in contact with the sensor, this is the case when the end stop height is reduced from 32.5 to 31.4 mm. In two out of three strokes performed with these parameters a modest temperature rise can be observed. The deviation of the third stroke is again attributed to manual positioning of the billet. Another energy change to 35% leads to even higher temperatures of up to 330 °C, indicating that the sensor is at the border up to where the part is forged and therefore being a good indicator for the measure of form filling within the forging die.