[en] 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 behaviour of austenitic stainless steel changes at temperatures close to the solidus. The solidus temperature was determined using DSC analysis to be 1435 C, 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 1410 C for this alloy, under no equilibrium conditions, it was demonstrated that delta ferrite appeared at temperatures over 1300 C, 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 capacity for 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.

[eu] Fabrikazio-prozesuen gaur egungo joera, piezak fabrikatzeko egin beharreko urratsak murriztea eta erabilitako lehengaiak gutxitzea du helburu. Horretarako, altzairua bezain fusio-puntu altuko aleazioekin prozesu berri bat definitu da Near Solidus Forming (NSF), prozesu erdi-solidoen baitan dagoena. Altzairuzko pieza konplexuak teknika honen bidez fabrikatu dira, forja bidez lortzen diren antzeko propietate mekanikoak lortuz, beharrezko urratsak bakar batera eramatea lortuz eta beraz, energia eta lehengaien kontsumoa murriztuz. Lan honetan, NSF prozesua industrian asko erabiltzen den AISI 316 altzairuarentzat aztertu da. Arlo honetan egindako aurrerapenez gain, oraindik ez dira ezagutzen portaera honen arrazoiak, eta hau da lan honen zergatia. Lehenik eta behin, materiala termomekanikoki karakterizatu da NSF prozesuaren adierazgarri diren baldintzetan. Analisi honek altzairu herdoilgaitz austenitikoaren portaera solidus egoeratik gertu dauden tenperaturetan aldatzen dela frogatu du. Solidus tenperatura DSC analisiaren bidez zehaztu da, hau 1435 C-koa izanik eta FactSage simulazioaren bidez lortutakoarekin bat eginez. Adibidez, aktibazio-energia literaturan agertzen denaren gertu egon arren, delta ferrita formazioaren ondorioz %20 altuagoa izango dela ezarri da, rekristalizazio prozesua oztopatu dela adieraziz. Gainera, NSF baldintzak kontuan hartuta Hansel- Spittel ekuazioaren bidez lortutako tentsio-deformazio portaera optimizatu da, iragarpenen errorea erdira murriztuz. Era berean, rekristalizazioa gertatzen dela frogatu da, ale-tamaina txikiagoko piezak lortzea ahalbidetuz, nahiz eta delta ferrita sortu. DSC analisiek delta ferrita 1410 C-tik aurrera agertzen dela jakinarazten duten arren, orekabaldintzarik gabe, delta ferrita 1300 C-tik gora agertzen dela frogatu da. Honek, piezaren propietate mekanikoetan eragiten du, mikrogogortasun neurketak frogatu duten bezala. Prozesuaren simulazioari dagokionez, literaturan dauden legeak NSF baldintzetara arte estrapolatzeak huts egiteko joera duela frogatu da, materialaren portaera aldatzen baitoa solidus tenperaturara hurbiltzen den heinean. Espero zitekeenaren aurka rekristalizatutako aleen tamainak tenperaturarekin baldintza estandarretan (NSF) ia konstante mantentzen direla ikusi da, delta ferritak nahiko denbora baitu austenita aleen mugetan metatzeko. Bukatzeko, altzairu herdoilgaitzezko pieza konplexuak fabrikatzeko NSF prozesuaren gaitasuna aztertu da. Horretarako osagai jasotzaile bat fabrikatu da solidus tenperaturatik gertu. Jakina denez delta ferrita kaltegarria izan daitekeela amaierako piezaren propietateentzat, bi tenperatura ezberdin frogatu dira. Ondorioz, altzairu herdoilgaitzezko piezak fabrikatzean nahi den amaierako mikroegitura lortzeko NSF prozesua erabiliz, tenperatura ez ezik, berotze abiadura ere faktore garrantzitsua dela egiaztatu du ikerketa honek.