Effect Of Deep Cryogenic Treatment On The Mechanical Properties And Microstructures Of YGl2 And M2Al Alloys

Titanium alloy is one typical difficult-to-machine material, but it has excellent comprehensive mechanical properties, and titanium alloy plays important role in the field of aerospace. So, there is an increasing requirement for the cutting tools of machining titanium alloys. Ultra-fine grained cemented carbide and high-performance high-speed steel are the typical finish and rough machining cutting tools for the titanium alloys, respectively. Deep cryogenic treatment is an environmental-benign manufacturing technology and plays an important role in improving material mechanical performance. The main contents of this thesis deal with the improvement of the mechanical properties of YG12 and M2Al alloys processed by deep cryogenic treatment and their strengthening mechanism.At first, in this thesis, the cryogenic treatment process and mechanism of YG12 ultra-fine grained cemented carbides were analyzed, which included time, number, way for deep cryogenic treatment, and low temperature annealing post-treatment. The results show that deep cryogenic treatment can improve the mechanical properties of YG12 alloy. With increasing the time for deep cryogenic treatment, the mechanical properties of YG12 alloy are continuously improved. Also, repeatedly short-time deep cryogenic treatment can improve the mechanical properties of YG12 material. After three times of deep cryogenic processing, the bending strength of YG12 alloy was reduced and there were micro-cracks near the fractured surfaces. Two ways of deep cryogenic treatments, direct immersing in liquid nitrogen and slow cooling at 3℃/min to -196℃, were compared and the result showed that one deep cryogenic treatment by immersing in the liquid nitrogen will not degrade the performance of YG12 ultra-fine grained cemented carbides. By using deep cryogenic treatment along with annealing, the effect of residual thermal stress was studied on the material mechanical performance. The results show that cryogenic treatment can relax thermal residual stress and the reduced residual stress is beneficial to increasing mechanical properties of the material. The micro defects in materials can be repaired by repeated treatment of deep cryogenic and annealing cycle and thus the mechanical properties of the materials can be largely improved, e.g. the bending strength can be increased by about 40%. Regarding the mechanism of improving mechanical properties after deep cryogenic treatment, it can be attributed to the phase transformation of α-Co to ε-Co and the relaxiation of residual thermal stress in the cryogenic process.Secondly, the cryogenic treatment process and mechanism of M2Al high-speed steel (HSS) were analyzed, which included time, number, way for deep cryogenic treatment, and tempering post-treatment. The results show that deep cryogenic treatment can improve the mechanical properties of M2A1 high-speed steel. With increasing the time for deep cryogenic treatment, the mechanical properties of M2Al are continuously improved and more tiny carbides are precipitated.After more than 20 h for deep cryogenic treatment, the micro structure and properties of M2Al high-speed steel are changed slight. The carbides are Fe3W3C of diamond-like structure after both quenching and deep cryogenic treatment. Twice of deep cryogenic treatment can improve the mechanical properties of M2Al high-speed steel. The bending strength was reduced and there existed micro cracks in the fractured surfaces after three times of deep cryogenic treatment. Two ways of deep cryogenic treatments, direct immersing in liquid nitrogen and slow cooling at 3℃/min to-196℃, were compared and the result showed that one deep cryogenic treatment by immersing in the liquid nitrogen will not degrade the performance of M2Al high-speed steel. Making deep cryogenic treatment before tempering, the stability of austenite is enhanced and the effect of deep cryogenic treatment is reduced. Deep cryogenic treatment after tempering, the bending strength of the materials will be largely improved, e.g. the bending strength can be increased by about 60%. Because many small Fe3W3C have the effect of dispersion strengthened. The secondary hardening temperature of M2Al HSS is reduced to 500℃ and the hardness is 1180 HV after deep cryogenic treatment. Regarding the mechanism of improving mechanical properties after deep cryogenic treatment, it can be attributed to the phase transformation of residual austenite to martensite and precipitation of Fe3W3C of diamond-like structure. The hardness and bending strength of this carbide are very high and diamond-like structure is very stable which is not easy to be destroyed. These carbides have the dispersion strengthened effect.