Modern high-strength steels (HSS), microalloyed and produced via controlled thermomechanical processing, find their superior mechanical properties compromised by the high peak temperatures and cooling rates of fusion welding. This work demonstrates, for a particular modern HSS, that friction stir welding (FSW) can join HSS via thermomechanical processing at peak temperatures below A1 ≈ 727 ºC , minimizing detrimental microstructural degradation and resulting in joint properties that can match, or even surpass, those of the base material. The microstructure and the mechanical properties of sound FSW joints, depend on the relationship between the peak temperature and the intercritical temperature range [A1, A3] of the HSS. Thus, proper control of peak temperatures and cooling rates in the stirred material is required to ensure process stability, repeatability and so that the microstructure and mechanical properties of welded joints can be tailored by FSW. There are currently no known methods for direct measurements of non-uniform and transient temperature fields inside solid materials, such as steel plates. This thesis was developed on the scope of the implementation of a new online control strategy for FSW of ferromagnetic low-alloy steels based on the relationship between the magnetic transformation of steel, at the Curie temperature TCurie ≈ 740 ºC , and the [A1, A3] range. The monitoring concept, supporting the control strategy, should be applicable to other ferromagnetic steels but not to austenitic-based steels. The study includes comprehensive mechanical testing supported by microstructural analysis. The real critical temperatures TCurie, AC1 and AC3 were measured via DSC. Contactless magnetic-based monitoring systems, using eddy currents and Hall-effect sensors, were developed and tested on thermal and thermomechanical processing cycles. The magnetic-based monitoring accurately detects the TCurie inside the processed volume and, above TCurie, the signal is sensitive to temperature gradients inherent to the volume change of paramagnetic material. The application of this magnetic-based monitoring can be extended to other thermal or thermomechanical solid-state domain processing of steel to produce raw material and components.
|Translated title of the contribution||Nykyaikaisten suurlujuusterästen kitkatappihitsaus: liitoksen karakterisointi ja magneettisen prosessimonitorointimenetelmän kehitys|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- high-strength steels
- friction stir welding
- magnetic permeability
- mechanical properties