Strengthening via the Formation of Strain-Induced Martensite and the Effects of Laser Marking on the Microstructure of A
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AISI 304 austenitic stainless steel is an important engineering material. The hardness that results from the low-temperature formation of strain-induced martensite in austenitic stainless steel is above 460 HV10, with the martensite acting as an elastic reinforcing phase with a higher stress than the austenite tensile strength, which deforms plastically. Such a material has many potential applications in architecture, chemical processing plants, and as a magnetic–non-magnetic substrate for an encoder scale,[1] etc. The aim of the previous work on this topic was to identify the compositions that are the most effective for achieving high strength levels and a fundamental understanding of the martensite induced by the concomitant plastic strain.[2–5] A little martensite forms in austenitic stainless steel during cooling, even to 4 K ( 269 C). Also, it appears that plastic strain and the creation of new nucleation sites play a fundamental role in the martensite transformation in this system.[6,7] Studies[3,5] show that the transformation sequence with plastic straining is fcc fi hcp fi bcc, with the phases being denoted as c fi e fi a¢, respectively. The intersections of the stacking faults or the hcp bands act as nuclei for the a¢ and if their formation is accelerated, the rate of a¢ nucleation is also accelerated. The hcp phase may not act as an intermediate phase if the composition is changed and the stacking-fault energy is increased; VOJTEH LESKOVSˇEK and MATJAZˇ GODEC, Researchers, are with the Institute of Metals and Technology, Lepi Pot 11, 1000 Ljubljana, Slovenia. Contact e-mail: [email protected] PETER KOGEJ, Researcher, is with the RLS Merilna tehnika d.o.o., Poslovna cona Zˇeje pri Komendi, Pod vrbami 2, 1218 Komenda, Slovenia. Manuscript submitted August 2, 2013. Article published online February 11, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
however, this sequence was found to be applicable in this work. It has also been shown[8] that the stackingfault energy in austenitic stainless steel decreases with decreasing temperature. The formation of strain-induced martensite occurs also at 77 K ( 196 C), but it requires large strains at room temperature to increase the thermodynamic driving force. The objective of this work was to explore the evolution of the martensite structure in terms of an imposed deep-cryogenic treatment and plastic strain and to reveal how the c–a¢ composite formed by the deformation acts as an enhancer of the magnetic and strength levels, i.e., to examine the role on the magnetic properties and the hardness of the a¢ phase produced by the deformation. Namely, the combination of a deepcryogenic treatment, plasticity, and phase transitions in this order increases the amount of formed magnetic a¢ phase, thereby improving the magnetic contrast when applying a non-magnetic c marking on the substrate by laser surface quenching. Laser processing is an advanced and highly efficient technology with very many applications in aeronautics, mechanical engineering, and the defence industr
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