Thermography Provides Insights into Cost-Effective Steel Production for Press Hardening
As part of the IGF project “Manufacture of high-strength, ductile components from low-alloy quenched and tempered steel using novel quenching & partitioning press hardening strategies”, infrared thermography was used for process analysis and material characterization. The work focused on thermomechanical interactions during forming. Thermographic measurements were taken both during conventional heating processes (for example, furnace heating) and during innovative strategies such as contact heating.
How “Quenching and Partitioning” improves steel properties
To produce low-alloy quenched and tempered steels, the heat-treatment strategy quenching and partitioning (Q&P) is often employed to achieve exceptional combinations of properties. During “quenching,” the material is rapidly and selectively cooled to form a microstructure consisting of martensite and retained austenite. During “partitioning,” the retained austenite is then stabilized by redistributing carbon.
In Q&P steels, costly alloying elements can generally be omitted; only carbon and a sufficient silicon content are required to achieve the desired material properties. The process thus offers enormous potential for the cost-effective production of complex formed parts made from ultra-high-strength steels.
Synchronized measurement of temperature distributions and strain fields
In the experiments, a VarioCAM® HD head 980 infrared camera from InfraTec was used, and its temperature measurement data were combined with digital image correlation (DIC). A trigger box, which connected the infrared camera to the DIC system’s controller, enabled controlled, synchronized recording of the strain fields and temperature distributions throughout the entire experiment at a frame rate of 30 Hz.
These thermographic measurements revealed significant differences between the heating strategies. For example, contact heating was found to have a significantly higher heating rate (>300 K/s) compared to furnace heating, resulting in a shorter heating time and thus potentially higher process productivity.
Thermography as the key to robust Press-hardening processes
Thermal imaging also played a key role in experiments involving simultaneous forming and quenching, as well as in temperature monitoring during press-based process trials. Using realistic press partitioning strategies, it was demonstrated that the component temperature could be successfully maintained above the martensite finish temperature – a fundamental prerequisite for successful partitioning, i.e., carbon redistribution between different microstructures in the steel.
In addition, the high-resolution thermographic images provided important insights for adjusting the die temperature control and transfer times. Of particular note was the use of thermography in optimizing the thermal process window, which is crucial for achieving the desired microstructure (martensite and retained austenite) and the targeted combination of properties.