| Cemented carbides and high speed steel are two types of materials most widely used for cutting tools.In recent years,with the emergence of various difficult-to-machine materials and the need for high-speed cutting,a higher standard has been required of cutting-tool materials.On the other hand,deep cryogenic treatment,which is environment friendly,can significantly improve the mechanical properties of cemented carbides and high speed steel and very well to the trend of green manufacturing technology in 21st century.Currently,due to the insufficient understanding about deep cryogenic strengthening mechanism and those studies oftentimes reached markedly different,lead to deep cryogenic treatment has not been wildly applied so far.Therefore,to these two kinds of materials,it is necessary to go one step further and carry out in-depth and detailed research on the strengthening mechanism.Ultrafine-grained cemented carbides are the trend of cemented carbides used as cutting tool material.Therefore,in this thesis,the preparation technology of ultrafine-grained WC-lOCo cemented carbides was firstly studied by low-pressure sintering.In view of deep cryogenic treatment can promote the transition of a-Co(fcc)??-Co(hcp),therefore,WC-12Co ultrafine-grained cemented carbide,which has higher Co content and easier to observe the phase transition,was selected as the research object in this study,so that revealing the strengthening mechanism of deep cryogenic treatment about ultrafine-grained cemented carbides.In addition,the thermal stability of binding phase Co in cemented carbides were analyzed,and the temperature range for the use of the cutting tools has been proposed in theory and the cutting effects of cutting tools were tested.In the thesis,the effects of heat treatment and deep cryogenic treatment on microstructure and mechanical properties of M2 Al high speed steel were also studied,reveals the deep cryogenic treatment strengthening mechanism and secondary hardening mechanism of high speed steel.The main study work and results are as follows:(1)Low-pressure sintering was used to study the effects of different grain growth inhibitors and TiC on the grain refining and mechanical properties of ultrafine-grained WC-10Co cemented carbides.Results show that,for the three grain inhibitors—Cr3C2,VC,and TaC,the uniform or 0.4%Cr3C2+0.2%VC adding achieves better result;adding TiC can increase hardness,but it also causes a reduction in bending strength significantly.(2)Three sets of comparative processes-"slow cooling vs.direct immersion into liquid nitrogen","quenching + deep cryogenic vs.air cooling + deep cryogenic," and "deep cryogenic + medium-temperature tempering vs.deep cryogenic + high-temperature tempering" ? are adopted to study the effect of deep cryogenic treatment on ultrafine-grained WC-12Co cemented carbides mechanical properties,micro-structure and residual stress,in an effort to explore the strengthening mechanism of such deep cryogenic treatment.Results show that deep cryogenic treatment has the most significant effect on bending strength and toughness,and that toughness and bending strength are positively correlated,but the treatment does not have a significant impact on hardness.The effect of deep cryogenic treatment on bending strength is mainly attribute to the change of stress state and the transformation of?-Co to ?-Co,and the effect of change in stress state on bending strength is the most pronounced;when the tensile stress on s-Co decreases and the ?-Co content increases,the bending strength increases,and vice versa;phase transition of ?-Co to ?-Co can fix defects of crystal,and further to enhance the mechanical properties of cemented carbides.In addition,the thermal stability of binder-phase Co in cemented carbides was analyzed on and validates in theory the applicability of the deep cryogenic treatment process in the making of metal cutting tools.Studies show that,when reheating the phase-transition of Co in cemented carbides happens in the range from 700 to 1000 ? due to the pinning of WC/Co interface.Therefore,when the temperature of the cutting tool is under 700 ?,the phase-transition strengthening effect of deep cryogenic treatment will not be affected.(3)Since the manufacturing processes for the cutting tools available in the market are unknown,the milling cutters and lathe tools made from the WC-lOCo ultrafine-grained cemented carbides prepared earlier in the research are used as the research objects.Slow cooling is used as the method of deep cryogenic treatment for the two types of cutting tools and the treated tools are then tested for the effectiveness in cutting TC4 titanium alloy.Results show that deep cryogenic treatment can substantially lengthen the service life of milling cutters,but it has no significant effect on lathe tools.(4)Cutting tools made from high speed steel need to go through heat treatment before using.Deep cryogenic treatment is the extension of heat treatment technology and the strengthening effect of deep cryogenic treatment process can have a full play only when such process is organically integrated with the heat treatment process.Therefore,study is done on the impact of the quenching-tempering process on the mechanical properties,microstructure,and residual stress of the M2A1 high speed steel and the mechanism for the secondary hardening of high speed steel has been revealed.Results show that after quenching,there is a large amount of residual austenite in M2A1 high speed steel and the residual austenite content is an important factor affecting the hardness of quenched steel;tempering once can eliminate a majority of the residual austenite,tempering twice can eliminate all residual austenite.After both quenching and tempering,the matrix is in a compressive stress state,and tempering has a more pronounced impact on the micro-stress than on the macro-stress;though tempering can weaken the lattice distortion of the martensite,after three times of tempering,the lattice distortion is still serious and the hardness remains high.Mechanism for the secondary hardening of the M2A1 high speed steel is largely attributed to the increase in martensite as a result of the residual austenite transformation.(5)Studies are done on the effect of deep cryogenic treatment on the microstructure,mechanical properties,residual stress,and residual austenite ageing effect of quenched M2A1 steel,and the strengthening mechanism of deep cryogenic treatment of M2 A1 steel is revealed by several of deep cryogenic treatment processes.Results show that,the strengthening mechanism of deep cryogenic treatment of the M2A1 high speed steel is that the transformation of residual austenite leads to an increase in martensite,however,when the quenching temperature is low,the treated M2A1 steel can easily be softened in the tempering process.In every deep cryogenic treatment processes,deep cryogenic treatment does not have a significant impact on the stress state and it only reduces the residual austenite content,and hardness increases with decrease in the residual austenite content.The A1 element in the alloy does not play any special roles in the strengthening mechanism of deep cryogenic treatment,and it simply exists in the matrix in the form of a solid solution.It is found that in the experiment,a short time of deep cryogenic insulation(0.5?1 h)after quenching will eliminate a lot of residual austenite in M2A1 and thereby increase the hardness and bending strength,but a longer time of deep cryogenic insulation does not have an more obvious effect;after quenching,a long holding time of high speed steel M2A1 in room temperature will bring out ageing effect on residual austenite,but the effect is not obvious,and a large amount of residual austenite can still be transformed by deep cryogenic treatment;but a tempering process after quenching produces a strong ageing effect on the residual austenite and,when such happens,deep cryogenic treatment cannot promote the transformation of residual austenite,and the minimum tempering temperature of M2A1 steel to happen is various with quenching temperature. |
PDF Full Text Request