Structure and properties of the carburised surface layer on 35CrSiMn5-5-4 steel after nanostructurization treatment

Abstract

The aim of the paper was to investigate the structure and properties of the carburized surface layer of the 35CrSiMn5-5-4 steel after the nanostructurisation with the austempering heat treatment. During vacuum carburizing the surface layer of the steel was enriched with carbon above w = 0.6 %. Steel samples were subsequently austenitized, quenched at two different temperatures, 260°C and 320°C, and annealed at these temperatures for the time necessary for the completion of the bainitic transformation. For comparison, one set of carbonized samples was subjected to the conventional heat treatment: martensitic quenching and low tempering. The microstructural characterisation of the steel after different heat treatments was performed using scanning (SEM) and transmission (TEM) electron microscopes. It was shown that both austempering treatments led to a carbide-free nano-bainitic structure composed of nanometric ferrite plates separated by thin layers of retained austenite. The microhardness and wear resistance of three kinds of steel samples were investigated: the two subjected to the austempering treatment at different temperatures and the one subjected to the conventional treatment. It was shown that the nano-bainitic structure containing an increased amount of retained austenite displays a higher wear resistance than the tempered martensite. The results confirm that austempering can be a competitive method of thermal treatment, in comparison to the conventional heat treatment, for the steels after the carburizing process. Keywords: vacuum carburizing treatment, carburized surface layer, carbide-free bainite, austempering, wear resistance Namen tega dela je preiskati strukturo in lastnosti naoglji~ene povr{ine jekla 35CrSiMn5-5-4 po nanostrukturiranju s toplotno obdelavo austempranja. Med naoglji~enjem v vakuumu se je povr{ina jekla obogatila z ogljikom nad masnim dele`em w = 0,6 %. Vzorci jekla so bili segreti v avstenitno podro~je, ga{eni na dve razli~ni temperaturi: 260°C in 320°C, in zadr`ani na teh temperaturah, potrebnih za popolno bainitno pretvorbo. Za primerjavo je bila serija naoglji~enih vzorcev toplotno obdelana po navadni metodi: ga{enje v martenzit in popu{~ano pri nizki temperaturi. Karakterizacija mikrostrukture jekla po razli~nih toplotnih obdelavah je bila izvr{ena z vrsti~nim (SEM) in presevnim (TEM) elektronskim mikroskopom. Izkazalo se je, da obe toplotni obdelavi austempranja povzro~ita nastanek nanobainitne strukture brez karbidov in sestavljene iz nanometrskih feritnih plo{~ic, lo~enih s tanko plastjo zaostalega avstenita. Pri treh razli~nih vzorcih je bila izmerjena mikrotrdota in odpornost proti obrabi; pri dveh z austempranjem pri razli~nih temperaturah in enem z navadno toplotno obdelavo. Izkazalo se je, da ima nanobainitna struktura s pove~anim dele`em zaostalega avstenita ve~jo odpornost proti obrabi kot pa popu{~eni martenzit. Rezultati so potrdili, da je toplotna obdelava z austempranjem bolj{a metoda toplotne obdelave kot navadna toplotna obdelava za jekla po postopku naoglji~enja...

Effect of hot deformation and isothermal holding temperature on retained austenite characteristics in 3-5% Mn multiphase steels

“…As the temperature of isothermal holding increased for the same steel grade, the carbon content in the austenite of the tested steels decreased. It is due to the need to redistribute C to a larger volume of the  phase [44][45][46]. Moreover, at the highest temperature of isothermal holding, the precipitation process of carbides may already begin, despite the increased concentration of Al in the tested steels [47,48].…”

Recent Progress of Low and Medium-Carbon Advanced Martensitic Steels

“…Skołek et al [151] report that DIN-35CrSiMn5-5-4 steel subjected to the DQ-P (or Q&T) process after the VC treatment (without FPP or shot-peening) possesses low surface ) subjected to fine particle peening (FPP) treatment after heat-treatment (the DQ-P process, T p = 180 • C) and vacuum carburization (VC) followed by the DQ-P process (T p = 180 • C) and then fine particle peening (FPP) treatment (VC+FPP) [147]. (b) Carbon potential (C p ) dependence of σ w , notch-fatigue limit (σ wn ) and notch sensitivity (q) in the TM steel subjected to VC+FPP treatment [148].…”

“…Skołek et al [151] report that DIN-35CrSiMn5-5-4 steel subjected to the DQ-P (or Q&T) process after the VC treatment (without FPP or shot-peening) possesses low surface wear resistance compared to the steel with nano-bainitic structure subjected to the IT process above M s . They say that the low wear resistance of DQ-P process steel is caused by low retained austenite fraction.…”

Desarrollo de aceros fundidos para ser tratados térmicamente mediante carbo-austemperado

“…Los aceros bainíticos libres de carburos (CFB, por sus siglas en inglés) han demostrado una mejor resistencia al desgaste y a la fatiga que los aceros convencionales, cuando se logra favorecer la formación de estructuras bainíticas en la capa carburada en vez de estructuras martensíticas. Se han reportado valores de dureza aproximada de 59HRC, resistencia a la tensión entre 1.1GPa y 2.2GPa, resistencia a la compresión del orden de 3GPa, límite de cedencia cercano a 1.2GPa, y ductilidad entre 5% y 20% con resistencia a la fractura (KIC) del orden de 45-90MPa-m 1/2 [1][2][3][4][5][6][7]. Esta combinación de propiedades excepcionales hace a los aceros carbo-austemperados superiores a los aceros cementados, con costos de producción equiparable o menores, con gran potencial de aplicación en las industrias del transporte, construcción, agrícola y militar, entre otras.…”