CONCLUSIONS AND SCOPE FOR FUTURE WORK

Transcription

1 CHAPTER 9 CONCLUSIONS AND SCOPE FOR FUTURE WORK From the present study conducted on the effects of cryogenic treatment and its mechanism on AISI M2 HSS and tungsten carbide cutting tools, the following conclusions can be drawn from the analysis of the results: 9.1 MECHANICAL AND METALLURGICAL CHARACTERIZATIONS AISI M2 HSS 1. Shallow cryogenic treatment of AISI M2 HSS manifestly reduces the amount of soft retained austenite by transforming it in to relatively hard martensite (from to 1.64 %). However, no further significant reduction in retained austenite was noticed by treating the material at deep cryogenic temperature. Hence, the cryogenic treatment cannot completely transfer the retained austenite in to martensite. 2. Cryogenic treatment accelerates the precipitation of tiny secondary carbides, increases their volume fraction and promotes uniform distribution of the carbides in entire bulk of material as compared to conventionally treated AISI M2 HSS. The volume fraction of carbides can be further enhanced by subjecting the AISI M2 HSS to deep cryogenic temperature as compare to shallow cryogenic temperature. 3. The hardness of AISI M2 HSS increases by 7.76% after subjecting the material to shallow cryogenic treatment, but no further significant improve in hardness (1.47%) was reported by further reducing the treating temperature to deep cryogenic temperature. Hence, there exists an optimum cryogenic treatment temperature for AISI M2 HSS which may ensure the maximum possible enhancement in hardness. Identification of such temperature can lead to wider applications of cryogenic treatments. 203

2 4. The primary reason that can be attributed for enhanced hardness of AISI M2 HSS is conversion of soft retained austenite in to relatively hard martensite. Refinement and evenly distribution of secondary carbides also contribute in increasing hardness but by significantly less extent. 5. The hardness values of cryogenically treated AISI M2 HSS do not vary significantly throughout the entire bulk of material. It proves that cryogenic treatment is not a surface modification technique rather it affects the bulk material almost uniformly. 6. Both types of cryogenic treatments substantially decrease the wear rate of the AISI M2 HSS compared to the conventional treated ones. However, the improvement in wear rate by deep cryogenic treatment is significantly higher than that achieved by shallow cryogenic treatment. 7. The significant improvement in wear resistance of shallow and deep cryogenically treated AISI M2 HSS is attributed to the decrease in the retained austenite content along with the increasing amount of tiny secondary carbides and their uniform distribution. 8. The degree of improvement in wear rate depends on the wear test conditions, which govern the wear mode and mechanism. Rigorous mode of wear is identified as extensive plastic deformation for conventionally treated AISI M2 HSS, whereas mild mode of wear is identified as combined effects of formation of oxides and pull-out of carbides for shallow and deep cryogenically treated AISI M2 HSS. 9. The maximum wear resistance was attained by subjecting the AISI M2 HSS to deep cryogenic treatment, and this observation is in good agreement with variations in the amount, population density and size of the secondary carbides. 10. Cryogenic treatment can be a better option as compared to surface coatings especially in case of cutting tools as the cryogenically treated AISI M2 HSS tools always retain their improved mechanical properties even after sharpening of cutting edges. 204

3 9.1.2 Tungsten Carbide 1. Hard phase particles of tungsten carbide are refined into their most stable form via the phenomenon of spheroidization after shallow and deep cryogenic treatment. It also aligns the hard phase carbide particulate structure into a durable, stress free configuration. 2. The cryogenic treatment relieves the internal stresses of tungsten carbide material resulting in rearrangement of cobalt binder crystals in their relatively stable close packed hexagonal shape. 3. It is not wise to generalise that the precipitation of η phase carbides is responsible for enhancing the wear resistance of tungsten carbide. Only M 12 C carbides of η phase are helpful whereas M 6 C η phase carbides makes the structure brittle therefore reducing the strength of the material. 4. Precipitation of η phase carbides is not the only reason for enhanced durability of tungsten carbide material. It is established that the cryogenic treatment causes the crystal structure changes in both hard and soft binder phase of tungsten carbide material which may be responsible for the enhanced hardness and wear resistance properties along with precipitation of η phase carbides. 5. The hardness of tungsten carbide material increases by 4.75% after subjecting the material to shallow cryogenic treatment, but no further significant improve in hardness (0.21%) was reported by further reducing the treating temperature to deep cryogenic temperature. 6. The primary reason that can be attributed for enhanced hardness of tungsten carbide material is the reduced amount of soft binder contents after cryogenic treatment. 7. The hardness values of cryogenically treated tungsten carbide material do not vary significantly at different locations of the specimen. It is thought that 205

4 cryogenic treatment is not a surface modification technique rather it affects the bulk material uniformly. 8. Both types of cryogenic treatments substantially decrease the wear rate of the tungsten carbide material compared to the untreated ones. However, the improvement in wear rate by deep cryogenic treatment is significantly higher than that achieved by shallow cryogenic treatment though the reason for this is yet to be crystallized. 9. The significant improvement in wear resistance of shallow and deep cryogenically treated tungsten carbide material is attributed to the refinement and alignment of hard α phase particles, decreased amount of cobalt binder β phase and precipitation of fine eta (η)-carbides. 10. The degree of improvement in wear rate depends on the wear test conditions, which govern the wear mode and mechanism. Rigorous mode of wear is identified as extensive plastic deformation for untreated tungsten carbide material, whereas mild mode of wear is identified as abrasion for shallow and deep cryogenically treated tungsten carbide material. 11. It is inferred that the time and temperature of cryogenic treatment of tungsten carbide material are critical parameters for obtaining the best combination of desired microstructure and mechanical properties. Cryogenic treatment can be a better option as compared to surface coatings especially in case of cutting tools as the cryogenically treated tungsten carbide inserts always retain their improved mechanical properties even after removal of coating. 9.2 TURNING PERFORMANCE AISI M2 HSS Tools 1. The shallow cryogenic and deep cryogenic treatment can significantly enhance the service life of M2 HSS turning tools, however the tools subjected to deep cryogenic treatment stand to gain relatively more as compared with shallow cryogenically treated tools. The recorded maximum tool life enhancement 206

5 over traditionally heat treated tools in the present study is approximately 35% for shallow cryogenically treated tools and 50% for deep cryogenically treated tools. 2. Deep cryogenically treated turning tools of M2 HSS perform more consistently as compared to shallow cryogenically treated as well as traditionally heat treated tools. This might result in smaller cutting forces on the cutting tools and lesser vibrations during turning. 3. Through SEM images of tool flank wear and chemical analysis of the chips, diffusion wear was identified as the dominated wear mechanism in case of M2 HSS turning tools. 4. Transformation of retained austenite to martensite is not the only reason for wear resistance improvement in M2 HSS turning tools where diffusion wear mechanism is dominant. It is established that cryogenic treatment accelerate the formation of secondary fine carbides in martensite matrix of M2 HSS tool material which causes improvement in wear resistance by suppressing diffusion mechanism. 5. Cryogenic treatment causes morphological changes in the entire cross-section of the M2 HSS turning tools; hence, the same tool lives can be anticipated even after any number of resharpenings. Evidently, it is better to all type of coatings as the coatings become futile even after single resharpening of tool Uncoated Tungsten Carbide Tools 1. The shallow cryogenic and deep cryogenic treatment can significantly enhance the cutting life of tungsten carbide turning inserts; however the inserts subjected to deep cryogenic treatment stand to gain relatively more as compared with shallow cryogenically treated inserts. The recorded maximum tool life enhancement over non-treated inserts in the present study is approximately 27% for shallow cryogenically treated and 36% for deep cryogenically treated inserts. 207

6 2. Long tool life can be expected by using deep cryogenically treated inserts for high speed continuous machining however cryogenic treatment may not give any significant improvements in tool life for short duration low speed machining. Thus, it can be inferred that cryogenically treated tungsten carbide inserts are more suitable for a particular set of cutting conditions. 3. Deep cryogenically treated tungsten carbide turning inserts perform more consistently as compared to shallow cryogenically treated as well as nontreated inserts. This might result in smaller cutting forces on the cutting insert and lesser vibrations during turning. 4. Through SEM images and optical micrographs of insert flank wear, it was found that cryogenic treatment effectively enhanced the cutting life of inserts by resisting the chipping, notch and plastic deformation wear significantly. 5. Surface finish of turned work pieces improved mainly due to reduction of tool flank wear by subjecting the tungsten carbide inserts to cryogenic treatment. These reductions in tool flank wear lead to improvement in cutting life of turning tools and enhancement of productivity TiAlN Coated Tungsten Carbide Tools 1. The shallow cryogenic treatment can significantly enhance the cutting life of TiAlN coated tungsten carbide turning inserts. The recorded maximum tool life enhancement over untreated inserts in the present study is % for shallow cryogenically treated inserts. The percentage improvement in tool life of shallow cryogenically treated TiAlN coated insert decreases with increasing the cutting speed. 2. The deep cryogenic treatment has destructive effect on the performance of TiAlN coated tungsten carbide inserts especially at lower cutting speeds. However, at higher cutting speeds, marginal gain in tool life can be obtained. Overall, deep cryogenic treatment is not recommended for TiAlN coated tungsten carbide inserts as the benefit gained is not significant. 208

7 3. Long tool life can be expected by using shallow cryogenically treated inserts for low speed continuous machining however cryogenic treatment may not give any significant improvements in tool life for short duration low speed machining. Thus, it can be inferred that cryogenically treated tungsten carbide inserts are more suitable for a particular set of cutting conditions. 4. Shallow cryogenically treated tungsten carbide turning inserts perform more consistently as compared to deep cryogenically treated as well as untreated inserts. This might result in smaller cutting forces on the cutting insert and lesser vibrations during turning. 5. Through SEM images and optical micrographs of insert flank wear, it was found that shallow and deep cryogenic treatment effectively enhanced the cutting life of inserts by resisting the notch, chipping and plastic deformation wear significantly. Also, deep cryogenic treatment adversely affected the TiAlN coated inserts by weakening coating-substrate interfacial adhesion bonding. 6. Application of VDI-3198 indentation test confirms the reduced adhesion strength of TiAlN coating on tungsten carbide substrate for deep cryogenically treated TiAlN coated inserts. 7. Precipitation of η phase carbides is not the only reason for enhanced durability of cutting inserts. It is thought that the cryogenic treatment causes the crystal structure changes in both hard and soft binder phase of tungsten carbide which may be responsible for the enhanced cutting life of cutting inserts along with precipitation of η phase carbides. 8. Surface finish of turned work pieces improved mainly due to reduction of tool flank wear by subjecting the tungsten carbide inserts to cryogenic treatment. These reductions in tool flank wear lead to improvement in cutting life of turning tools and enhancement of productivity. 9. On the whole the shallow cryogenic treatment has constructive influences on the performance of all the TiAlN coated tungsten carbide inserts tested. 209

8 Optimization of the cryogenic treatment parameters for TiAlN coated tungsten carbide insert material can further enhance the tool life Turning Under Wet And Dry Conditions 1. The cryogenically treated tungsten carbide inserts performed better during wet machining both in continuous and interrupted machining mode. Evidently, the application of coolant reduces the tool chip interface temperature thus enabling the tungsten carbide inserts to retain superior properties induced by cryogenic treatment for relatively longer span of time. 2. The cryogenically treated tungsten carbide inserts used under wet cutting conditions stand to gain relatively more at higher cutting speeds for both continuous as well as interrupted machining mode. Thus, in general, coolant is more effective at higher cutting speeds. 3. In totality, cryogenically treated tungsten carbide inserts performed better in interrupted machining mode as compared with continuous machining mode in both dry and wet cutting conditions. The extent of advantage gained was even better at higher cutting speeds. 4. Cryogenically treated tungsten carbide inserts perform more consistently under interrupted machining mode as compared with continuous machining mode. 9.3 TAGUCHI OPTIMIZATION OF CRYOGENIC TREATMENT CYCLE 1. Soaking temperature is the most significant factor and the maximum percentage contribution of soaking temperature on the flank wear resistance of AISI M2 HSS and tungsten carbide tools was and 70.42% respectively. The best soaking temperature in the possible range is 196 C. 2. The second significant factor is the soaking period and it contributes and 23.17% respectively for AISI M2 HSS and tungsten carbide tools for the improvement of flank wear resistance. The best level for this factor is 38 h. 210

9 3. The third significant factor is the cooling rate. The maximum percentage contribution of cooling rate on the flank wear resistance of AISI M2 HSS and tungsten carbide tools was 9.54 and 9.82% respectively. The best level was determined as 1 C/min. 4. The tempering temperature shows only little significance and its contribution on the flank wear resistance was 2.70% only for both types of tools. The optimum level of tempering temperature was arrived at, as 150 C. 5. The confirmation test results are found to be within the confidence interval with 95% confidence. Flank wear test was conducted on untreated tool samples also and the results were compared with that of the confirmation test results. This shows that the cryogenically treated AISI M2 HSS and tungsten carbide tool samples as per the arrived optimum conditions improve in wear resistance by about 47 and 33% respectively. 9.4 ANFIS MODELING OF FLANK WEAR 1. The present work has set out to apply the ANFIS to predict tool flank wear of AISI M2 HSS and tungsten carbide tools in turning. MISO ANFIS model is developed and validated with experimental results for given conditions. It has been found that results generated by the designed ANFIS model are close to the experimental results with and 98.76% accuracy for AISI M2 HSS and tungsten carbide tools respectively. 2. With this much accuracy, model can be used by the practicing engineer who would like to get quick answers for on-line intelligent control and/or optimization. 3. ANFIS system was found to be very flexible and easy to comprehend and hence can act as an alternative to the conventional modeling techniques. Present study support that the ANFIS technique can be introduced as a viable alternative to carry out analysis without conducting actual experiments. Fuzzy logic allowed the modeling and on-line control problem to be treated simultaneously. 211

10 9.5 SCOPE OF FUTURE WORK Even with the numerous findings that were made in this study, there are still some areas in which further investigation could lead to a greater understanding of the effect of cryogenic treatment on AISI M2 HSS and cemented tungsten carbide. Analysis of the results obtained from the current work suggests several feasible extensions to the research. Some of them are listed below: 1. It is very clear that minimum possible soaking temperature is always beneficial to improve the properties of AISI M2 HSS and tungsten carbide. Application of other cooling media like hydrogen and helium which can further reduce the soaking temperature also needs to be investigated. 2. Combined effect of surface coatings and cryogenic treatment on AISI M2 HSS tools may further enhance the tool life. 3. Coatings other than TiAlN and their combination can be tested on tungsten carbide substrate. Also the optimized thickness of the surface coating coupled with cryogenic treatment can improve turning performance. 4. Further studies of the microstructure of cryogenically treated tungsten carbide at higher resolutions can be done. Microstructural analyses made in this study were limited to the magnification provided by the scanning electron microscope. Microstructural studies at higher magnifications, which can be done by using equipment such as transmission electron microscopes, could be done to provide insights into changes in the microstructure at an intra-granular level. Much of the arguments re phase contents could be supported by use of magnetic measurements like coercivity or moment. 212