Dissolution of iron-chromium carbides during white layer formation induced by hard turning of AISI 52100 steel

The (Fe, Cr)3C carbide morphology in the surface region of hard turned bainitic AISI 52100 steel was investigated using both experimental techniques and simulations, where microstructural analysis was correlated with analytical studies of the carbide dissolution kinetics using DICTRA1. The experimental results showed that for both predominantly thermally and mechanically induced white layers no significant carbide dissolution took place down to a depth of 20 μm below the machined surfaces. This was confirmed by the analytical results from DICTRA, which showed that no significant carbide dissolution should take place during hard turning given the short contact times. Within the hard turned surfaces up to ∼12% of the carbides were elongated, indicating plastic deformation of the carbides during machining

White-etching matter in bearing steel. Part II: Distinguishing cause and effect in bearing steel failure

The premature failure of large bearings of the type used in wind turbines, possibly through a mechanism called “white-structure flaking”, has triggered many studies of microstructural damage associated with “white-etching areas” created during rolling contact fatigue, although whether they are symptoms or causes of failure is less clear. Therefore, some special experiments have been conducted to prove that white-etching areas are the consequence, and not the cause, of damage. By artificially introducing a fine dispersion of microcracks in the steel through heat treatment and then subjecting the sample to rolling contact fatigue, manifestations of hard white-etching matter have been created to a much greater extent than samples similarly tested without initial cracks. A wide variety of characterization tools has been used to corroborate that the white areas thus created have the same properties as reported observations on real bearings. Evidence suggests that the formation mechanism of the white-etching regions involves the rubbing and beating of the free surfaces of cracks, debonded inclusions, and voids under repeated rolling contact. It follows that the focus in avoiding early failure should be in enhancing the toughness of the bearing steel in order to avoid the initial microscopic feature event.Funding by CONACyT, the Cambridge Overseas Trust, and the Roberto Rocca Education Programme is highly appreciated and acknowledged.This is the accepted manuscript version. The final published version is available from Springer at http://link.springer.com/article/10.1007%2Fs11661-014-2431-x

Formation mechanisms of white layers induced by hard turning of AISI 52100 steel

This paper concerns the formation mechanisms of white layers (WLs) induced by machining of through-hardened (martensitic and bainitic) AISI 52100 steel. The microstructures of different types of WLs were investigated using transmission electron microscopy; those that had been predominantly mechanically induced (M-WL) and those that had been predominantly thermally induced (T-WL). Independent of the process parameters and the starting microstructure, the WLs consisted of a randomly oriented nano- and submicron-sized microstructure with an average grain size in the order of several tens of nanometres. The M-WLs were characterised as bcc-(α) ferrite and orthorhombic-(θ) cementite where the initial martensite/bainite platelets had been reoriented along the shear direction and broken-down into elongated sub-grains through dynamic recovery. The T-WLs were shown to consist of fcc-(γ) austenite, bcc-(α) martensite, and orthorhombic-(θ) cementite. Here the elongated sub-structure was found to coexist with equiaxed grains, meaning that the formation was initiated by dynamic recovery, which advanced to dynamic recrystallisation at the increased temperatures caused by the higher cutting speeds. Although times and temperatures are considered to be insufficient to dissolve the secondary carbides, carbide refinement was observed independently on the type of WL. The carbide refinement was controlled by (i) deformation and fragmentation of the carbides via dislocation movement along the {1 1 0} and {1 0 0}, (ii) precipitation of nano-sized carbides and (iii) diffusion-based carbide refinement by carbon depletion via dislocations in the nano-sized grains and grain boundaries, which act as high diffusivity paths

Cutting temperatures during hard turning—Measurements and effects on white layer formation in AISI 52100

This paper concerns the temperature evolution during white layer formation induced by hard turningof martensitic and bainitic hardened AISI 52100 steel, as well as the effects of cutting temperaturesand surface cooling rates on the microstructure and properties of the induced white layers. The cuttingtemperatures were measured using a high speed two-colour pyrometer, equipped with an optical fibre allowing for temperature measurements at the cutting edge. Depending on the machining conditions,white layers were shown to have formed both above and well below the parent austenitic transformationtemperature, Ac1, of about 750◦C. Thus at least two different mechanisms, phase transformation abovethe Ac1(thermally) and severe plastic deformation below the Ac1(mechanically), have been active during white layer formation. In the case of the predominantly thermally induced white layers, the cutting temperatures were above 900◦C, while for the predominantly mechanically induced white layers thecutting temperatures were approximately 550◦C. The surface cooling rates during hard turning wereshown to be as high as 104–105 ◦C/s for cutting speeds between 30 and 260 m/min independent of whetherthe studied microstructure was martensitic or bainitic. Adding the results from the cutting temperaturemeasurements to previous results on the retained austenite contents and residual stresses of the whitelayers, it can be summarised that thermally induced white layers contain significantly higher amountsof retained austenite compared to the unaffected material and display high tensile residual stresses. On the contrary, in the case of white layers formed mainly due to severe plastic deformation, no retainedaustenite could be measured and the surface and subsurface residual stresses were compressive