Cryogenic machining as an alternative turning process of normalized and hardened AISI 52100 bearing steel

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Abstract

Environmental and health friendly technologies with economic justification have nowadays an increasing importance in global industrial trends. Idea of global sustainable development dictates cleaner and less health hazardous machining processes. With these limitations in mind, the main issue is now to change the way mechanical components are being machined and move to alternative technologies that could moreover increase the machining performance. Cryogenic machining is one possibility to reach this goal. It consists of a system for cutting (turning, milling, etc.) assisted by liquid nitrogen, which enables a clean process with possible lower production costs and higher productivity. This article presents the results of turning hardened and normalized bearing steel AISI 52100 (DIN 100Cr6), comparing conventional flood and dry with cryogenic machining. Turning results show drastic improvements in tool lifetime (up to 370%) for cryogenic machining of normalized bearing steel 100Cr6 and reduction of thermal residual stress inducements in case of hardened bearing steel 100Cr6, while tool life is also extended.

Highlights

► Cryogenic machining has been applied on machining of hardened bearing steel AISI 52100. ► Cryogenic machining can considerably prolong tool lifetime and increase productivity. ► Appropriate cutting tool geometry and edge preparation, dedicated to cryogenic application, has to be defined. ► Cryogenic machining has a beneficial impact on surface integrity and so part fatigue life and quality. ► Cryogenic machining represents an alternative process for machining of hardened bearing steels.

Introduction

Research and design in the field of machine – tool – workpiece interactions, have brought towards innovations, which have increased productivity and also quality of machined surfaces. The momentary trend of high productivity and high speed machining, inevitably induces the presence of high temperatures in cutting zones, especially concerning hard-to-machine (high-temperature) materials. The generated heat mainly remains in the cutting zone and builds up temperature to extreme values, causing softening of the cutting tool material and engage aggressive wear mechanisms such as diffusion. This usually leads to rapid tool-wear and consequently shorter cutting tool-life, with deteriorated machined surface integrity at the end. Industrial machining applications widely utilize conventional cooling lubrication fluids (CLFs) in order to counter the high levels of generated heat. CLFs such as air, oils, and aqueous emulsions are mostly used even though they are (for the exception of air) environmentally unfriendly, hazardous to health and relatively costly. It has been reported that 15% of the total machining costs relates to the use of CLF emulsions, while cost fraction for cutting tools is significantly lower; around 4% (Klocke and Eisenblatter, 1997, Klocke et al., 1980). By simply avoiding CLF usage and applying dry machining alternatives, with new high performance coated cutting tools, there would be a huge process gain from a sustainability point of view (Duzbinski et al., 2004, Hong, 2001). However, some specific materials are still extremely difficult to machine, for instance high-temperature alloys (nickel alloys, titanium alloys, Co–Cr alloys, etc.), used in the aerospace industry due to their ability of maintaining properties even at high operational temperatures (Ezugwu, 2005, Ezugwu and Bonney, 2004, Ezugwu et al., 2005a, Ezugwu et al., 2005b), as well as hardened steels employed in bearings and molding industry (Benga and Abrao, 2003, Yallese et al., 2009). The extension of the global idea about sustainable manufacturing has set conventional CLFs to an unenviable spot of unsustainable elements that need further research and development towards alternative CLF methods/mechanisms. There are several different ways to improve machining, while one of the most efficient ways is to reduce the temperature in the cutting zone (Ezugwu, 2005, Kramar et al., 2010, Pusavec et al., 2009). Cryogenic machining appears as an efficient solution to reach this objective by injecting liquid nitrogen into the cutting zone (Pusavec et al., 2010). Using this technology, cutting process reaches temperatures, which are much lower than the softening temperatures of the cutting tool. As a consequence cutting tool lifetime is improved/prolonged in comparison to conventional flood cooling. Due to the absence of oil based CLFs, cryogenic cooling is notorious as an environmentally friendly sustainable technology.

This article compares machining performances of conventional flood (using 7% emulsion based on oil Primol OLMA 3000), conventional dry and cryogenic cooling conditions. Machining performance tests were carried out regarding maximum productivity, acceptable chip formation (chips that do not interfere with the work or machine tool and cause no problems of disposal. Unacceptable chips interrupt regular manufacturing operation, as they tend to entangle the tool, workpiece and safety problems to operators, Shetty et al., 2008), induced constant specific force (less than 10% deviation from average value) and mean arithmetic roughness value (Ra) below 0.8 μm, which corresponds to fine finish machining. Surface integrity has been covered by the investigation of micro-hardness (HV 0.5), metallurgical analysis, residual stress (RS) measurement and measurement of surface roughness. Experimental setup for performing experiments is presented in Fig. 1.

Section snippets

Material and methods

Productivity rate and quality of the machining process are reflected by the machining performance characteristics of the material. Machining performance of a material is a technical term, which is used to index machining difficulty of a certain material to a desired shape in interaction with the cutting tool and machining procedure. Within the range of cutting technologies, machining performance is mostly referred to cutting tool-wear or tool lifetime. Materials with bad machining performance

Tool lifetime results

Main goal of this work is to compare conventional flood and dry to cryogenic machining. More precisely, it shows the trend with which they are affecting tool-life and surface integrity for turning normalized and hardened bearing steel AISI 52100. In this way, Fig. 12 shows results of wear trend (VBmax) relative to the cut volume (material removed volume – MRV) for both cooling media, while turning is performed with optimal parameters for individual cooling/lubrication type.

Wear trends show that

Discussion and conclusion

This work represents the necessity and possibility to introduce sustainable principles in machining processes. One of those presents implementation of cryogenic over conventional dry or flood machining process that was in this work analyzed in machining of hardened and normalized bearing steel AISI 52100. Results reveal that cryogenic machining considerably prolongs tool lifetime of cutting inserts and increases productivity. Cryogenic machining of normalized bearing steel AISI 52100 prolongs

Acknowledgments

Authors wish to express their gratitude to Ascometal S.A., France, the manufacturer of special steels, for all the support with providing tools, work materials and their help in analyses of residual stresses. Additionally authors would like to thank the French Ministry of Foreign and European Affairs (MAEE) and French Ministry of Higher Education and Research (MESR) for their financial support.

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