Microstructural characterization and transformation kintetiks of austempered bearing steels.

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.