The student Andrea Sánchez Fernández obtained an OUTSTANDING CUM LAUDE qualification with 'INTERNATIONAL DOCTORATE' mention
The student Andrea Sánchez Fernández obtained an OUTSTANDING CUM LAUDE qualification with 'INTERNATIONAL DOCTORATE' mention
The student Andrea Sánchez Fernández obtained an OUTSTANDING CUM LAUDE qualification with 'INTERNATIONAL DOCTORATE' mention
Thesis title: Microstructural evolution analysis during the Near Solidus Forming process: the case of AISI 316
Court:
- Chairmanship: Platon Kapranos (University of Sheffield)
- Vocal: Zigor Azpilgain Balerdi (Mondragon Unibertsitatea)
- Vocal: Pello Jimbert Lacha (UPV/EHU)
- Vocal: Sumit Kumar Hazra (University of Warwick)
- Secretary: Gurutze Arruebarrena Lizarralde (Mondragon Unibertsitatea)
Abstract:
The current trend in manufacturing processes aims for the reduction of steps and the decrease of raw material used. In order to achieve this with high melting point alloys such as steels, a new process was defined under the umbrella of semi-solid called Near Solidus Forming (NSF). Complex steel parts have been manufactured by using this technique, obtaining similar mechanical properties to forged material, reducing the number of steps to one and decreasing both energy and raw material consumption. In this work, the process was analysed for AISI 316 a widely used steel in industry.
Despite the advances made in this field, the reasons which explain the behaviour are still unknown, thus defining the scope of this work. First, the material was thermomechanically characterized under the conditions representative of the NSF process as a noticeable gap was observed in the literature, proving that material behave our of austenitic stainless steel changes at temperatures close to the solidus. The solidus temperature was determined using DSC analysis to be 1435C, the results being in agreement with those reported through FactSage simulation. For instance, although the activation energy was close to the one reported in the literature, it was set to be around 20% higher due to delta ferrite formation, meaning that recrystallization was hampered. In addition, flow stress behaviour according to the Hansel-Spittel equation was optimized taking into account NSF conditions, reducing error in the predictions by more than half.
Also, recrystallization was proven to occur allowing the obtention of parts with smaller grain sizes despite delta ferrite formation. Although the DSC analysis reported that delta ferrite tended to appear at temperatures around 1410C for this alloy, under no equilibrium conditions, it was demonstrated that delta ferrite appeared at temperatures over 1300C, influencing the mechanical properties as was proven through microhardness measurements.
Regarding the simulation of the process, it was demonstrated that extrapolation using the existing laws in the literature up to NSF conditions tended to fail as material behaviour changes at temperatures close to the solidus. In contrast to what might have been expected, recrystallized grain sizes remained almost constant with temperature under commercial conditions, as delta ferrite had enough time to nucleate along austenite grain boundaries.
Finally, the capability of the NSF process in manufacturing complex stainless steel parts was established by manufacturing a lifting gear component. However, delta ferrite is known to be detrimental to the properties of the final part, that is why two different temperatures were tested. It was demonstrated that not only the temperature, but the heating rate are also important factors in obtaining the desired microstructure in the final part when manufacturing stainless steel parts using the NSF process.