Microstructural characterization and transformation kintetiks of austempered bearing steels.=Austenperizatutako errodamendutarako altzairuen mikroegituraren karakterizazioa eta eraldaketa zinetikak
Authors:
Directors:
Keywords:
Microbeams
Dilatometria
Microzutabeak
Errodamenduak
Bainite
High carbon steels
Bearings
Dilatometria
Microzutabeak
Errodamenduak
Bainite
High carbon steels
Bearings
Issue Date:
2015
Defense Date:
4-Dec-2015
Publisher:
Servicio de Publicaciones. Univesidad de Navarra
Citation:
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
Abstract
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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