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

Characterization of the Surface Integrity induced by Hard Turning of Bainitic and Martensitic AISI 52100 Steel

Depending on the process parameters and the tool condition, hard turned surfaces can consist of a “white” and a “dark” etching layer having other mechanical properties compared to the bulk material. X-ray diffraction measurements revealed that tensileresidual stresses accompanied with higher volume fraction of retained austenite are present in the thermally induced white layer. While compressive residual stresses and decreased retained austenite content was found in the plastically created white layer. The surface temperature was estimated to be ~1200 C during white layer formation by hard turning

Microstructure of surface zones subjected to high-velocity parting-off

A hydraulic high-velocity pressing machine with a parting-off tool was used for adiabatic cutting with impact velocities ranging from 5 to 10 m/s. In this study the associated fracture mechanisms and microstructures of three different materials (100Cr6, 100CrMn6 and C56) in the form of wire or bar were investigated. It was concluded that the parting-off is initiated through a shearing effect resulting in ductile shear fracture being responsible for the cutting. In all of the samples microcracks were found in the severely deformed region around the cut, which became larger with increasing sample diameter. Evidence of heating was not observed in the cut zone of samples having 6 mm diameter. However, for samples with a diameter of 70 mm and above, a white-etching band could be found, indicating that the temperature had increased considerably in this region. Analysis of the fracture surfaces using scanning optical microscopy showed that the fracture mode had mostly been ductile shear, with exception of the largest samples where some evidence of tensile fracture could be observed

A Methodology for Temperature Correction When Using Two-Color Pyrometers - Compensation for Surface Topography and Material

In this investigation, the applicability of the twocolorpyrometer technique for temperature measurements indry hard turning of AISI 52100 steel was studied, where bothmachined surfaces as well as cutting tools were considered.The impacts of differing hard turned surface topography onthe two-color pyrometer readings was studied by conductingtemperaturemeasurements on reference samples created usingcutting tools with different degrees of tool flank wear. In order to conduct measurements in a controlled environment, a specially designed furnace was developed in which the samples were heated step-wise up to 1,000 \ub0C in a protective atmosphere. At each testing temperature, the temperatures measured by the two-color pyrometer were compared with temperatures recorded by thermocouples. For all materials and surfaces as studied here, the two-color pyrometer generally recorded significantly lower temperatures than the thermocouples;for the hard turned surfaces, depending on the surface topography, the temperatures were as much as ~20 % lower and for the CBN cutting tools, ~13 % lower. To be able to use the two-color pyrometer technique for temperature measurements in hard turning of AISI 52100 steel, a linear approximation function was determined resulting in three unique equations, one for each of the studied materials and surfaces.By using the developed approximation function, the measuredcutting temperatures can be adjusted to compensate for differing materials or surface topographies for comparable machining conditions. Even though the proposed equations areunique for the hard turning conditions as studied here, theproposed methodology can be applied to determine the temperature compensation required for other surface topographies, as well as other materials

A Methodology for Temperature Correction When Using Two-Color Pyrometers - Compensation for Surface Topography and Material

In this investigation, the applicability of the twocolorpyrometer technique for temperature measurements indry hard turning of AISI 52100 steel was studied, where bothmachined surfaces as well as cutting tools were considered.The impacts of differing hard turned surface topography onthe two-color pyrometer readings was studied by conductingtemperaturemeasurements on reference samples created usingcutting tools with different degrees of tool flank wear. In order to conduct measurements in a controlled environment, a specially designed furnace was developed in which the samples were heated step-wise up to 1,000 \ub0C in a protective atmosphere. At each testing temperature, the temperatures measured by the two-color pyrometer were compared with temperatures recorded by thermocouples. For all materials and surfaces as studied here, the two-color pyrometer generally recorded significantly lower temperatures than the thermocouples;for the hard turned surfaces, depending on the surface topography, the temperatures were as much as ~20 % lower and for the CBN cutting tools, ~13 % lower. To be able to use the two-color pyrometer technique for temperature measurements in hard turning of AISI 52100 steel, a linear approximation function was determined resulting in three unique equations, one for each of the studied materials and surfaces.By using the developed approximation function, the measuredcutting temperatures can be adjusted to compensate for differing materials or surface topographies for comparable machining conditions. Even though the proposed equations areunique for the hard turning conditions as studied here, theproposed methodology can be applied to determine the temperature compensation required for other surface topographies, as well as other materials