Microstructural characterization and transformation kintetiks of austempered bearing steels.=Austenperizatutako errodamendutarako altzairuen mikroegituraren karakterizazioa eta eraldaketa zinetikak.
Palabras clave : 
Microbeams.
Dilatometria.
Microzutabeak.
Errodamenduak.
Bainite.
High carbon steels.
Bearings.
Fecha de publicación: 
2015
Fecha de la defensa: 
4-dic-2015
Editorial : 
Servicio de Publicaciones. Univesidad de Navarra
Cita: 
ARAMBURU, Ane.""Microstructural characterization and transformation kinetiks of austempered bearing steels= Austenperizatutako errodamendutarako altzairuen mikroegituraren karakterizazioa eta eraldaketa zinetikak"" Sánchez-Moreno, José Manuel. Tesis doctoral. Universidad de Navarra. 2015
Resumen
Tesi honek, errodamenduetan gehien erabiltzen diren 100Cr6, 100CrMnSi6-4 eta 100CrMo7-3 altzairuetan, austenpering tratamendu ezberdinen eragina aztertzen du. Erreferentziatzat 100Cr6 altzairua hartu da eta beste biak, duten tenplagarritasun hobeagatik aukeratu dira, uztailen pareta 30mm baino lodiago den kasuetarako erabilgarriagoak direlako. Nahiz eta tenplaketa eta iraoketa izan altzairu hauetan ohiko gogortze prozedura, distortsioak direla eta, mekanizagarritasun kosteak handitzen direnerako, austenpering tratamenduak egokiagoak dira. Austenizazioa, bereizmen altuko dilatometria bidez ikertu da, altzairu hauen transformazio zinetikak ulertzeko. Hozte jarraiko tratamenduak ere burutu dira, Ms tenperatura egokiro determinatzeko, bereziki 100CrMnSi6-4 eta 100CrMo7-3 altzairuetan. Fase desberdinen frakzio bolumetrikoen eboluzioa, modelo ezberdinak erabiliz kalkulatu da bai austenizazioko berotze arrapalan eta baita austenizazio tenperaturako mantentzean. Lortutako emaitzek, bat egiten dute neurketa metalografikoekin. 100Cr6 altzairuaren transformazio bainitikoak azkar egiten du aurrera, 230ºC-tako tratamendu isotermikoko lehenengo orduan. Hala ere, prozesua nabarmen moteltzen da puntu hortatik aurrera. Behaketa honen harira, bi pausutako tratamenduak diseinatu dira, non temperatura 20ºC igo den azken 5 minutuetarako. Prozedura honekin lortutako bainita kantitateak 230ºC-tan 6 orduz lortutakoen oso antzekoak dira. Aleazio altuagoko altzairuetan (adib. 100CrMnSi6-4 eta 100CrMo7-3) 2 pausutako austenpering tratamenduak ez dira hain eraginkorrak. Transformatu gabeko hondar austenita kantitate esanguratsu bat gelditzen da laginetan tenplaketaren ondoren, eta honek, dimentsio kontrola eta gogortasuna mugatzen ditu. EBSD analisiek, altzairu hipereutektoide hauen mikroegitura, 1 mikra inguruko ferrita aleek osatzen dutela ikusi da; hauek paraleloki 5 mikrako pakete morfologikoak osatzen dituztelarik. Ondoz-ondoko aleen arteko desorientazioa 55ºC-koa da eta txandakako aleen artekoa < 15º. Aldaera hauen hautaketa, transformazio energiaren gutxitzearekin dago zihurrenik erlazionatuta. Martensita orratzen kasuarekiko guztiz bestelakoak dira, kasu honetan aleen arteko desorientazioa nagusiki 60º izanik. Globulizazio ondorengo zementita partikulen tamaina kritikoa da 100Cr6 altzairuan transformazio bainitikoa azkartzeko, perlita hondarrak edo globulizatutako karburo oso finak dituzten materialetan, ez bait da transformazio bainitikoa guztiz gauzatzen. Hau, austenizazioan zehar karburo hauen disolbagarritasun altua dela eta, austenitak duen karbono kantitatea handitzen delako gertatzen da. 100CrMnSi6-4 altzairuan, bainitaren nukleazioa, motelago gertatzen da. Kasu honetan, austenizazioaren ondoren, zementita gutxien duten laginetan, bainita gehiago aurkitu da. Beraz, bainitaren hazkuntzan eragin handiagoa du hazten ari diren bainitaren eta zementita partikulen arteko elkarrekintzak, austenitaren egonkortzeak baino. Errodamendutarako osagaien diseinurako, eskala ezberdinetarako fidagarriak diren emaitzak behar dira. Uztailen eta elementu gurpildunen arteko kontaktuan, mikratako tamaina aldaketekin sortutako aldaketak nabarmenak dira errodamenduen jardunean. Gainera, ukipen errodadura nekearen bidez eragindako kalteak aztertuz, hauek tentsio kontzentrazio altuak dituzten inklusioen inguruan, oso material bolumen txikian gertatzen direla esan daiteke. Horregatik, tesi honen amaieran, mikrozutabeetan flexio esperimentuak burutu dira austenperizatutako laginetan topatutako talderik txkienak (adib. ferrita aleak) deformazio gradientepean entsegatuz eta hauen, propietate mekanikoak neurtuz. Horretarako, enpotratutako zutabe oso txikiak (1,5x1,5x5 mikra) mekanizatu dira fokalizatutako ioi emisio bidez (FIB) ehotuz. Metodo honen bidez neurtutako isurpen tentsioen balioak eskala makroskopikoko trakzio entseguetan lortutakoen bikoitzak dira. Hau, zutabearen enpotramenduan eragindako deformazio gradiente handiekin azaldu daiteke, zeinak estatistikoki, pakete morfologikoan dauden dislokazio iturri kantitatea murrizten duten.
This thesis analyses the effects of several austempering treatments on the most commonly used bearing steels: 100Cr6, 100CrMnSi6-4 and 100CrMo7-3. Having 100Cr6 steel as a reference, the other two grades have been selected for their higher hardenability, a property needed for components with wall thickness above 30 mm. Although quenching and tempering is the standard hardening procedure, austempering is preferred when machining costs increase due to distortion issues. Austenitizing has been investigated by means of high resolution dilatometry in order to understand the transformation kinetics in these steels. Continuous cooling experiments have also been carried out to have a precise determination of the Ms temperatures, especially for 100CrMn6-4 and 100CrMo7-3 grades. Different models have been used to calculate the evolution of phase volume fractions during the heating ramp and the dwelling step of the austenitizing cycle. The results obtained show reasonably good correlation with metallographic measurements. The bainitic transformation of 100Cr6 progresses at a high rate during the first hour of isothermal treatment at 230ºC. However, the process slows down dramatically from this point forward. Based on this observation, two step austempering treatments have been designed in which the temperature is increased by 20ºC for the last 5 minutes. Bainite contents obtained by this procedure are very similar to those obtained after 6 hours at 230ºC. In the steels with higher alloying (i.e. 100CrMnSi6-4 and 100CrMo7-3) 2-step austempering treatments are less effective. A large amount of untransformed austenite is left in the samples after quenching which limits the attainable hardness and deteriorates the dimensional control. EBSD analyses show that the microstructure of these hypereutectoid steels consists of parallel plates with thickness below 1 micron organized in morphological packets with sizes around 5 microns. Misorientation between adjacent plates is close to 55º whereas in alternate positions misorientations are very small (< 15º). This variant selection is likely related to the reduction of the transformation strain energy. This is clearly different from martensitic plates in which the most frequent misorientation is 60º. The size of cementite particles after globulization is critical for accelerating the bainitic transformation in 100Cr6 steel. Materials with rests of pearlite or very fine globulized carbides do not complete the bainitic transformation. It is reasonable to assume that the carbon content of austenite increases due to the higher solubility of these types of carbides during austenitisation. The nucleation rate of bainitic plates in 100CrMnSi6-4 steel is much lower. In this case, a higher volume fraction of bainite is obtained for the material with lower fraction of cementite after austenitisation. Therefore, in this case, the progression of the bainitic transformation is not only affected by the stability of the austenite but also to the interaction between growing bainite grains and cementite particles inside austenite. The design of bearing components requires reliable material data at different size scales. Significant changes in the bearing performance are obtained by micron size modifications in the contact profiles between rings and rollers. In addition, the damage induced by rolling contact fatigue is observed to concentrate in very small volumes of material near highly stresses inclusions. In the final part of this thesis, micro beam bending experiments have been carried out in order to investigate the mechanical properties of the smallest microstructural features found in the austempered samples (i.e. a bainitic ferrite plate) under strong strain gradients. For doing so, extremely fine cantilever beams (1.5x1.5x5 microns) have been produced by focused ion beam milling (FIB). Yield stresses measured by this method are approximately two times higher than those measured in macroscopic tensile test specimens. This is explained by the large strain gradient induced at the beam clamping, which statistically reduces the amount of dislocation sources with respect to those present in the morphological packet.

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