Effects of cryogenic cooling on surface layer alterations in machining of AISI 52100 steels

Abstract

Microstructural phase transformations, commonly known as white layer formation in hard turned steel components, have in recent times become an interesting research topic in machining as they are related to the surface integrity and functional performance of components. Three main theories have been proposed to justify the mechanisms of white layer formation: (1) rapid heating and quenching; (2) severe plastic deformation; and (3) surface reaction with the environment. Coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on the machined surface alterations during machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride tool inserts with varying initial work material hardness, tool shape, cutting speed and feedrate. Optical and scanning electron microscopes (SEM) were used to analyse the affected layer in the machined subsurface, while X-ray diffraction technique was utilised to investigate the microstructural phase composition. The experimental results prove that the microstructural phase changes are heavily influenced by the cutting process parameters and the use of cryogenic cooling, in some cases leading to the total removal of martensite.

Finite element modelling of microstructural changes in dry and cryogenic machining AISI 52100 steel

“…The numerical model is also able to predict the thickness of the WL varying the cutting speeds and the cooling conditions as reported in the previous works. 16,17 It is important to underline that in the previous work, 16 it has been experimentally demonstrated by X-ray diffraction and chemical analyses that WL formation under dry condition has been generated by thermal and mechanical effects since martensite and retained austenite were found after machining process. In contrast, when cryogenic coolant is applied the mechanical deformation effect becomes dominant and the presence of martensite can be totally avoided when AISI 52100 steel is machined under certain conditions.…”

“…Finally, to predict the serrated chip formation, the Brozzo et al's 22 criterion (based on the effect of a Experimental set-up dedicated to the orthogonal cutting with cryogenic delivery system; b detail of the cutting zone and the nozzle used to deliver the liquid nitrogen 16 Materials Science and Technology 2016 VOL 32 NO 11 hydrostatic stress) is employed. The analytical expression is represented by equation ( 3):…”

“…Furthermore, the influence of the microstructural changes on the material flow stress is also included considering the grain size modifications and hardness variation. Finally, the proposed FE strategy was validated comparing numerical results with those experimentally obtained, [15][16][17] suggesting in function of the cutting temperatures, if the WL formation mechanism is mainly thermally induced through phase transformations or mainly mechanically induced through SPD.…”

Enhanced Surface Integrity From Cryogenic Machining of AZ31B Mg Alloy: A Physics-Based Analysis With Microstructure Prediction

“…Cryogenic machining firstly has been considered as an ecofriendly and pollution-free machining technology of difficult-tomachine materials, in order to significantly increase the tool life and reduce tool wear due to the reduction of tool-tip temperature [9,10]. With further studies of this process, the improved surface integrity has been found in the machined surface of Inconel 718, AISI 4140, and 52100 steel [11][12][13]. During cryogenic machining, an UFG surface layer is often formed in the machined surface with the increase of surface hardness and compressive residual stresses due to the SPD.…”

Evaluation of Process Performance for Sustainable Hard Machining

“…Also, the white layer formation in dry machining is mainly due to a rapid heating and quenching which results in phase transformation (i.e., generation of martensitic structure); in contrast, the use of cryogenic cooling limits or avoids the martensitic phase changes. This is due to the lower temperatures reached during the material removal process (19) . Fig.…”