Effect Of Cryogenic And QPQ Compound Treatment On The Cutting Life Of HSS M2 Drill

This thesis is a research on the effect of cryogenic and QPQ compound treatment on HSS(M2) drill ofΦ8. In the first chapter, the history of the development of high-speed steel is introduced briefly, and then the experiment's research goal and significance are proposed based on analyzing the problem existed in the development of high-speed steel at present in our country, and a brief outline of QPQ and the deep cold technology is introduced. In the second, third chapters, starting from the phase diagram, the basic principle of nitriding is introduced, and from the theories of thermodynamics, kinetics and mechanisms, the formation of nitrided layer is specified and chemical reactions in nitriding medium and the mechanism of cryogenic treatment are compared in detail. The introduction of these principles is used to establish the feasibility of cryogenic and QPQ compound treatment on HSS (M2) drill to enhance the drill bit's working durability. Then my opinions on the stress distribution and operations arrangement are proposed, and a specific test method is developed. In the fourth chapter, according to my test method, technological parameters such as nitrided time, cryogenic time, nitrided temperature are adjusted and different arrangements of procedures such as nitriding ,deep cold, tempering etc. are tried, which enables drill bit's working durability to have a large scale enhancement. Then changes on the nitrided layer and the base of the drill bit are analyzed by several methods, such as optics metallographic microscope, the scanning electronic microscope (SEM), EDS, X-ray diffraction and micro hardness.The experimental results demonstrate: (1) Under 550-560℃, 32% CNO- density, the best nitrided time of QPQ treatment is 15min. (2) The best processing orders in the experiments are"cryogenic + tempering after a long time of natural aging process + nitrogen"and"nitrogen + cryogenic + tempering after a long time of natural aging process". These two kinds of procedure combination can both effectively improve the drill bit's working durability.(3) The best nitride layer depths in the experiments are 20-23μm, and cryogenic treatment can improve the diffusion rate of active nitrogen atoms in the layer. (4) After QPQ processing, the surface hardness is enhanced up to 1050Hv-1330Hv, at least 300Hv higher than the matrix hardness, which is the main reason of abrasion performance improvement. (5) The average hardness of samples after cryogenic processing is 1.2HRC higher than without cryogenic processing, and the uniformity of hardness is improved. (6) After quenching and three times tempering, the transformation of martensite has been almost completely and the over-saturation of carbon is also greatly reduced, so the capacity of carbide precipitation has been very limited. The transformation of residual austenite to martensitie is very little, so it will not cause too much internal stress and the influence to the stress of surface nitrided layer is very little. The questions in the experiment are discussed in the fifth chapter, and suggestions to further research are also proposed.