The study focused on investigating the effect of Deep Cryogenic Treatment(DCT)on the mechanical and ballistic properties of AA7075-T6 aluminum alloy.The microstructure,microhardness,tensile strength,and impact strengt...The study focused on investigating the effect of Deep Cryogenic Treatment(DCT)on the mechanical and ballistic properties of AA7075-T6 aluminum alloy.The microstructure,microhardness,tensile strength,and impact strength of the Base Material(BM)and DCT-treated 7075 samples were analyzed through metallographic analysis and mechanical tests.The microstructure of the DCT-treated 7075 samples revealed fine grains and a distribution of secondary phase particles.The tensile strength,impact strength,and microhardness of DCT-treated samples increased by 7.41%,4%,and 9.68%,respectively,compared to the BM samples.The fractography analysis of the tensile samples showed cleavage facets,microvoids,and dimples in both the samples.The ballistic behavior of the BM and DCT target plates were studied by impacting hard steel core projectiles at a velocity of 750±10 m/s.The target plates failed due to petaling and ductile hole enlargement,and the depth of penetration(DOP)of the DCT target was less than that of the BM target,indicating a higher ballistic resistance.The post-ballistic microstructure examination of the target plates showed the formation of an Adiabatic Shear Band(ASB)without any cracks.It was concluded that the DCT treatment improved the mechanical and ballistic properties of the aluminum alloy due to grain refinement and high dislocation density.展开更多
The effects of deep cryogenic treatment on the microstructure and properties of WC-11 Co cemented carbides with various carbon contents were investigated.The results show that after deep cryogenic treatment,WC grains ...The effects of deep cryogenic treatment on the microstructure and properties of WC-11 Co cemented carbides with various carbon contents were investigated.The results show that after deep cryogenic treatment,WC grains are refined into triangular prism with sound edges via the process of spheroidization,but WC grain size has no evident change compared with that of untreated alloys.The phase transformation of Co phase from α-Co（FCC） to ε-Co（HCP） is observed in the cryogenically treated alloys,which is attributed to the decrease of W solubility in the binder（Co）.Deep cryogenic treatment enhances the hardness and bending strength of the alloys,while it has no significant effects on the density and cobalt magnetic performance.展开更多
Combining with the low temperature material properties and the boiling heat transfer coefficient of specimen immersed in the liquid nitrogen, a numerical model based on metallo-thermo-mechanical couple theory was esta...Combining with the low temperature material properties and the boiling heat transfer coefficient of specimen immersed in the liquid nitrogen, a numerical model based on metallo-thermo-mechanical couple theory was established to reproduce the deep cryogenic treatment （DCT） process of a newly developed cold work die steel Cr8Mo2SiV （SDC99）. Moreover, an experimental setup for rapid temperature measurement was designed to validate the simulation results. The investigation suggests that the differences in temperature and cooling rate between the surface and core of specimen are very significant. However, it should be emphasized that the acute temperature and cooling rate changes during DCT are mainly concentrated on the specimen surface region about 1/3 of the sample thickness. Subjected to DCT, the retained austenite of quenched specimen continues to transform to martensite and finally its phase volume fraction reduces to 2.3%. The predicted results are coincident well with the experimental data, which demonstrates that the numerical model employed in this study can accurately capture the variation characteristics of temperature and microstructure fields during DCT and provide a theoretical guidance for making the reasonable DCT procedure.展开更多
Deep cryogenic treatment technology of electrodes is put forward to improve electrode life of resistance spot welding of aluminum alloy LF2. Deep cryogenic treatment makes electrode life for spot welding aluminum allo...Deep cryogenic treatment technology of electrodes is put forward to improve electrode life of resistance spot welding of aluminum alloy LF2. Deep cryogenic treatment makes electrode life for spot welding aluminum alloy improve. The specific resistivity of the deep cryogenic treatment electrodes is tested and experimental results show that specific resistivity is decreased sharply. The temperature field and the influence of deep cryogenic treatment on the electrode tip temperature during spot welding aluminium alloy is studied by numerical simulation method with the software ANSYS. The axisymmetric finite element model of mechanical, thermal and electrical coupled analysis of spot welding process is developed. The numerical simulation results show that the influence of deep cryogenic treatment on electrode tip temperature is very large.展开更多
The effects of deep cryogenic treatment on mechanical behavior of a Cu-Cr-Zr alloy for electrodes of spot welding were investigated employing Brinell-hardness testing unit, abrasion examination machine, electronic alm...The effects of deep cryogenic treatment on mechanical behavior of a Cu-Cr-Zr alloy for electrodes of spot welding were investigated employing Brinell-hardness testing unit, abrasion examination machine, electronic almighty testing machine and X-ray stress analyzer. Tensile fracture surfaces of the alloy were characterized by scanning electronic microscope （SEM） with energy dispersive X-ray spectroscopy （EDS）. The results showed that, after deep cryogenic treatment, σb and σ0.2 increased 23 MPa and 21 MPa respectively, the wear rate of the alloy exhibited the trend of decrease with the decreasing temperature and increasing time of deep cryogenic treatment, and the surface residual stress of the alloy was partially eliminated by deep cryogenic treatment.展开更多
Cr-Zr-Cu alloy electrodes for resistance spot welding of aluminium alloy are treated by deep cryogenic treatment processes. The Cr-Zr-Cu alloy electrodes are analyzed by transmission electron microscope（ TEM ） , and...Cr-Zr-Cu alloy electrodes for resistance spot welding of aluminium alloy are treated by deep cryogenic treatment processes. The Cr-Zr-Cu alloy electrodes are analyzed by transmission electron microscope（ TEM ） , and results show that the common dislocation in Cr-Zr-Cu alloy electrodes is changed into the dislocation loop, and twin crystal is found after deep cryogenic treatment. The parallel twin crystal band is observed by selected electron diffraction（SED） and the twin crystal plane is marked as （ 111 ）. The Cr-Zr-Cu alloy electrode is studied by X-ray diffraction（ XRD ） and results show that the intensity of diffraction peak is obviously changed after deep cryogenic treatment, and the grain rotates to preferred orientation. The Cr-Zr- Cu alloy electrode is studied by positron annihilation technique （PAT） and results indicate that the amount of vacancy defects is less than that of Cr-Zr-Cu alloy before deep cryogenic treatment. The main elements in Cr-Zr-Cu alloy are studied with X- ray photoelctron spectroscopy（ XPS ） and the intensity of spectrum peak is increased after deep cryogenic treatment.展开更多
The effect of deep cryogenic treatment(DCT)on the electrochemical behavior of Ti-6Al-4V alloy in Hank's solution was experimentally investigated in the present work.Deep cryogenic treatments at-196℃soaking for 24...The effect of deep cryogenic treatment(DCT)on the electrochemical behavior of Ti-6Al-4V alloy in Hank's solution was experimentally investigated in the present work.Deep cryogenic treatments at-196℃soaking for 24(DCT-24) and 48(DCT-48) hours were conducted on the commercial annealed Ti-6Al-4V alloy.The methods of polarization and electrochemical impedance spectroscopy(EIS) were employed to evaluate the electrochemical behavior and corrosion mechanism.Furthermore,atomic force microscopy(AFM) was used to detect the corrosion surface.The results showed that deep cryogenic treatment shifted the corrosion potentials(E_(corr))to the positive direction.The corrosion current densities(i_(corr))of samples treated by DCT-24 and DCT-48decreased from 153.1 nA·cm^(-2) in the untreated(UT)sample to 86.3 nA·cm^(-2) and 43.3 nA·cm^(-2),respectively.Furthermore,the roughness of corrosion surfaces of samples subjected to DCT was smaller than that of the UT sample,which indeed demonstrated the improvement in corrosion resistance of Ti-6Al-4V alloy in Hank's solution.The results of EIS test indicated that deep cryogenic treatment had no influence on the corrosion mechanism of Ti-6Al-4V alloy in Hank's,while enhanced resistance value of outer porous layer R_p,which makes the surface more efficient to prevent the corrosion.The reduction of β-phase particles,improvement in dislocation density and release of residual stress caused by deep cryogenic treatment had great contribution to the improvement in corrosion resistance of Ti-6Al-4V alloy in Hank's solution.展开更多
Effect of deep cryogenic treatment （DCT） on the microstructures and mechanical behavior of ultrafine-grained WC-12Co cemented carbide was investigated by using XRD, SEM, and DSC. The phase transformations of pure Co...Effect of deep cryogenic treatment （DCT） on the microstructures and mechanical behavior of ultrafine-grained WC-12Co cemented carbide was investigated by using XRD, SEM, and DSC. The phase transformations of pure Co and binder phase Co in cemented carbide were analyzed in detail to correlate the strengthening mechanism with its x -ε, phase transition. The results show that DCT resulted in a slight increase in hardness and bending strength of ultrafine- grained WC-12Co cemented carbide. For the ultrafine-grained cemented carbide after DCT, there is no significant change in the microstructure and the elemental distribution of the cemented carbides, but the fractured morphology shows a feature of plastic deformation. In the cases of pure Co and the binder phase Co in WC-12Co cemented carbide, they exhibit different features of phase transformation. The improvement of mechanical property of cemented carbide can be attributed to the increased amount of ε-Co in WC-12Co composites after DCT.展开更多
The effects of traditional heat treatment(quenching and then tempering)and deep cryogenic treatment on the microstructure and mechanical properties of a low-carbon high-alloy martensitic bearing steel were studied by ...The effects of traditional heat treatment(quenching and then tempering)and deep cryogenic treatment on the microstructure and mechanical properties of a low-carbon high-alloy martensitic bearing steel were studied by Rockwell hardness test,X-ray diffractometry,scanning electron microscopy and transmission electron microscopy.The results show that the deep cryogenic treatment promotes the transformation of the retained austenite to martensite during cooling,which leads to the hardness of the sample after deep cryogenic treatment higher than that at the quenched state.Also,the carbon content in the martensite matrix after different treatments was calculated and the results indicated that deep cryogenic treatment can promote the segregation of carbon atoms in martensite to dislocations.The segregated carbon atoms act as and grow into nuclei for the formation of fine carbide particles during subsequent tempering.And this resulted in the fact that the hardness of the tempered experimental steel after deep cryogenic treatment is higher than that without deep cryogenic treatment.展开更多
The effects of the prior austenite grain size in deep cryogenic treatment on the hardness, the structural change and the wear resistance of D6 tool steel were investigated. The wear resistance of the cryogenically tre...The effects of the prior austenite grain size in deep cryogenic treatment on the hardness, the structural change and the wear resistance of D6 tool steel were investigated. The wear resistance of the cryogenically treated samples was determined using the pin-on-disk wear machine. The microstructural characteristics and phases present in heat treated samples were determined using SEM and XRD techniques. The results showed that the retained austenite is completely transformed to martensite during the cryogenic treatment. Besides, there is an optimum grain size of which the maximum wear resistance and hardness are obtained.展开更多
The microstructure evolution and the pitting corrosion resistance of a supermartensitic stainless steel after deep cryogenic treatment process were clarified through X-ray diffraction,field emission scanning electron ...The microstructure evolution and the pitting corrosion resistance of a supermartensitic stainless steel after deep cryogenic treatment process were clarified through X-ray diffraction,field emission scanning electron microscopy,transmission electron microscopy(TEM)and electrochemical methods.The results showed that the microstructure of supermartensitic stainless steel mainly consisted of reversed austenite,tempered martensite,and M_(23)C_(6)carbides after tempering.The deep cryogenic treatment promoted the refinement of the martensite laths and the precipitation of the carbides in comparison with the traditional process.TEM analysis indicated that the segregation of Si atoms at the boundary was found at the interface between carbide and martensite.The pitting corrosion potential of the specimens subjected to deep cryogenic treatment decreased with the elevated tempering temperature,and the lowest pitting corrosion potential was found at the tempering temperature of 650℃.The sensitivity of the pitting corrosion potential was attributed to the precipitation of M_(23)C_(6)carbides and Si atoms segregation.Si atoms segregation engendered the formation of Cr-depleted zone near M_(23)C_(6)and impeded the recovery of Cr-depleted zone.展开更多
Additively manufactured(AM)metallic materials commonly possess substantial tensile surface residual stress,which is detrimental to the load-bearing service behavior.Recently,we demonstrated that deep cryogenic treatme...Additively manufactured(AM)metallic materials commonly possess substantial tensile surface residual stress,which is detrimental to the load-bearing service behavior.Recently,we demonstrated that deep cryogenic treatment(DCT)is an effective method for improving the tensile properties of CoCrFeMnNi high-entropy alloy(HEA)samples fabricated by laser melting deposition(LMD),by introducing high compressive residual stress and deformation microstructures without destroying the AM shape.However,carrying out the DCT in a single-step mode does not improve the residual stress gradients inherent from the LMD process,which are undesirable as the mechanical properties will not be homogeneous within the sample.In this work,we show that carrying out the DCT in a cyclic mode with repeated cryogenic cooling and reheating can significantly homogenize the residual stress in LMD-fabricated Co Cr Fe Mn Ni HEA,and improve tensile strength and ductility,compared with single-step DCT of the same cryogenic soaking duration.Under cyclic DCT,the thermal stress is re-elevated to a high value at each cryogenic cooling step,leading to the formation of denser and more intersecting reinforcing crystalline defects and hcp phase transformation,compared to single-step DCT of the same total cryogenic soaking duration in which the thermal stress relaxes towards a low value over time.The enhancement of defect formation in the cyclic mode of DCT also leads to more uniform residual stress distribution in the sample after the DCT.The results here provide important insights on optimizing DCT processes for post-fabrication improvement of mechanical properties of AM metallic net shapes.展开更多
The effect of deep cryogenic treatment on the formation of reversed austenite （RA） in super martensitic stainless steel was investigated. RA was found to form in steels without （A） and with （B） deep cryogenic tr...The effect of deep cryogenic treatment on the formation of reversed austenite （RA） in super martensitic stainless steel was investigated. RA was found to form in steels without （A） and with （B） deep cryogenic treatment. The volume fraction of RA initially increased and then decreased with increasing tempering temperature over 550-- 750 ℃ for the two steels, which were quenched at 1050 ℃. In addition, for both with and without deep cryogenic treatment, the RA content reached a maximum value at 650 ℃ although the RA content in steel B was greater than that in steel A over the entire range of tempering temperatures. Furthermore, the hardness （HRC） of steel B was greater than that of steel A at tempering temperatures of 550--750 ℃. From these results, the basic mechanism for the formation of RA in steels A and B was determined to be Ni diffusion. However, there were more Ni enriched points, a lower degree of enrichment, and a shorter diffusion path in steel B. It needed to be noted that the shapes of the RA consisted of blocks and stripes in both steels. These shapes resulted because the RA redissolved and trans- formed to martensite along the martensitic lath boundaries when the tempering temperature was 650--750 ℃, and a portion of RA in the martensitie lath divided the originally wider martensitic laths into a number of thinner ones. In- terestingly, the RA redissolved more rapidly in steel B and consequently resulted in a stronger refining effect.展开更多
In this study,the effects of deep cryogenic treatment(DCT)on the mechanical and tribological properties of AISI D3 tool steel were investigated together with a systematic correlation between their hardness and wear re...In this study,the effects of deep cryogenic treatment(DCT)on the mechanical and tribological properties of AISI D3 tool steel were investigated together with a systematic correlation between their hardness and wear resistance.It was found that conventionally heat treated AISI D3 tool steel samples were significantly hardened via an additional DCT,which was attributed to the more retained austenite elimination,more homogenized carbide distribution and more reduction in carbide size in the samples.As a result,the hardened AISI D3 samples exhibited reductions in their friction and wear during rubbing against alumina and 100Cr6 steel balls under different normal loads due to the effectively hindered removal of surface materials.The results clearly showed that the DCT was an effective way to improve the mechanical and tribological properties of the AISI D3 tool steel samples as the tribological performance of the tool steel samples was significantly influenced by their hardness.展开更多
The microstructure and elements distribution of the deep cryogenic treatmentelectrodes and non-cryogenic treatment electrodes for spot welding hot dip galvanized steel areobserved by a scanning electrical microscope. ...The microstructure and elements distribution of the deep cryogenic treatmentelectrodes and non-cryogenic treatment electrodes for spot welding hot dip galvanized steel areobserved by a scanning electrical microscope. The grain sizes, the resistivity and the hardness ofthe electrodes before and after deep cryogenic treatment are measured by X-ray diffraction, the DCdouble arms bridge and the Brinell hardness testing unit respectively. The spot welding processperformance of hot dip galvanized steel plate is tested and the relationship between microstructureand physical properties of deep cryogenic treatment electrodes is analyzed. The experimental resultsshow that deep cryogenic treatment makes Cr, Zr in deep cryogenic treatment electrodes emanatedispersedly and makes the grain of deep cryogenic treatment electrodes smaller than non-cryogenictreatment ones so that the electrical conductivity and the thermal conductivity of deep cryogenictreatment electrodes are improved very much, which make spot welding process performance of the hotdip galvanized steel be improved.展开更多
To further improve the mechanical performance of a new alloyed austempered ductile iron（ADI）, deep cryogenic treatment（DCT） has been adopted to investigate the effect of DCT time on the microstructure and mechanic...To further improve the mechanical performance of a new alloyed austempered ductile iron（ADI）, deep cryogenic treatment（DCT） has been adopted to investigate the effect of DCT time on the microstructure and mechanical behaviors of the alloyed ADI Fe-3.55 C-1.97 Si-3.79 Ni-0.71 Cu-0.92 Mo-0.64 Cr-0.36 Mn-0.30 V（in wt.%）. With increasing the DCT time, more austenite transformed to martensite and very fine carbides precipitated in martensite in the extended period of DCT. The amount of austenite decreased in alloyed ductile irons, while that of martensite and carbide precipitation increased. The alloyed ADI after DCT for 6 h had the highest hardness and compressive strength, which can be attributed to the formation of more plate-like martensite and the finely precipitated carbides. There was a gradual decrease in hardness and compressive strength with increasing the DCT time to 12 h because of the dissolution of M3 C carbide. After tempering, there was a decrease in mechanical properties compared to the direct DCT sample, which was caused by the occurrence of Ostwald ripening of precipitated carbides. The optimum wear resistance was achieved for the alloyed ADI after DCT for 6 h. The wear mechanism of the alloyed ADI in associating with DCT is mainly consisted of micro-cutting wear and some plastic deformation wear.展开更多
The effects of the deep cryogenic heat treatment on the microstructural changes,wear resistance,and hardness of carburized DIN 1.7131 grade steel were investigated.Results show that cryogenic heat treatment reduced th...The effects of the deep cryogenic heat treatment on the microstructural changes,wear resistance,and hardness of carburized DIN 1.7131 grade steel were investigated.Results show that cryogenic heat treatment reduced the retained austenite and increased the carbide amount.In addition,after the cryogenic heat treatment,carbide shows a more uniform distribution,as compared to the conventionally treated ones.It was also clarified that the hardness of the cryogenically treated samples was improved,but the relative improvement decreases with the distance as the surface increases.It has been shown that the wear resistance improves due to the cryogenic heat treatment,and the predominant wear mechanism is a combination of the adhesive and tribo-chemical wear.展开更多
The effect of deep cryogenic cycle treatment(DCT)on Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10)Be_(22.5)(Vit-1)bulk metallic glass(BMG)prepared from high-purity raw materials was investigated.After DCT,no obvious rejuvenation ...The effect of deep cryogenic cycle treatment(DCT)on Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10)Be_(22.5)(Vit-1)bulk metallic glass(BMG)prepared from high-purity raw materials was investigated.After DCT,no obvious rejuvenation of the samples was detected.With an increasing number of cryogenic cycles,the hardness of the samples first decreased and then increased,the room-temperature compression plasticity first increased and then generally remained unchanged,and the impact toughness underwent almost no obvious change.The absence of rejuvenation was attributed to the high fragility index(47-50)and high glass forming ability(GFA)of the material.As lower purity of the raw materials is expected in practical applications,DCT of Vit-1 BMG prepared from low-purity raw materials was also performed.After DCT,the samples prepared with the lower-purity raw materials were clearly rejuvenated,and the room-temperature mechanical properties improved significantly.Both the compression plasticity and impact toughness reached peak values after 5 cryogenic cycles.The initial impurities(including Y and O)had a complex and comprehensive effect on the deformation mechanism of the BMG during DCT.Our findings indicate that the structural heterogeneity,fragility index,and GFA of the BMG alter the effect of DCT.展开更多
文摘The study focused on investigating the effect of Deep Cryogenic Treatment(DCT)on the mechanical and ballistic properties of AA7075-T6 aluminum alloy.The microstructure,microhardness,tensile strength,and impact strength of the Base Material(BM)and DCT-treated 7075 samples were analyzed through metallographic analysis and mechanical tests.The microstructure of the DCT-treated 7075 samples revealed fine grains and a distribution of secondary phase particles.The tensile strength,impact strength,and microhardness of DCT-treated samples increased by 7.41%,4%,and 9.68%,respectively,compared to the BM samples.The fractography analysis of the tensile samples showed cleavage facets,microvoids,and dimples in both the samples.The ballistic behavior of the BM and DCT target plates were studied by impacting hard steel core projectiles at a velocity of 750±10 m/s.The target plates failed due to petaling and ductile hole enlargement,and the depth of penetration(DOP)of the DCT target was less than that of the BM target,indicating a higher ballistic resistance.The post-ballistic microstructure examination of the target plates showed the formation of an Adiabatic Shear Band(ASB)without any cracks.It was concluded that the DCT treatment improved the mechanical and ballistic properties of the aluminum alloy due to grain refinement and high dislocation density.
基金Project(12JJ8018)supported by the Natural Science Foundation of Hunan Province,China
文摘The effects of deep cryogenic treatment on the microstructure and properties of WC-11 Co cemented carbides with various carbon contents were investigated.The results show that after deep cryogenic treatment,WC grains are refined into triangular prism with sound edges via the process of spheroidization,but WC grain size has no evident change compared with that of untreated alloys.The phase transformation of Co phase from α-Co（FCC） to ε-Co（HCP） is observed in the cryogenically treated alloys,which is attributed to the decrease of W solubility in the binder（Co）.Deep cryogenic treatment enhances the hardness and bending strength of the alloys,while it has no significant effects on the density and cobalt magnetic performance.
基金Project (51171104) supported by the National Natural Science Foundation of China
文摘Combining with the low temperature material properties and the boiling heat transfer coefficient of specimen immersed in the liquid nitrogen, a numerical model based on metallo-thermo-mechanical couple theory was established to reproduce the deep cryogenic treatment （DCT） process of a newly developed cold work die steel Cr8Mo2SiV （SDC99）. Moreover, an experimental setup for rapid temperature measurement was designed to validate the simulation results. The investigation suggests that the differences in temperature and cooling rate between the surface and core of specimen are very significant. However, it should be emphasized that the acute temperature and cooling rate changes during DCT are mainly concentrated on the specimen surface region about 1/3 of the sample thickness. Subjected to DCT, the retained austenite of quenched specimen continues to transform to martensite and finally its phase volume fraction reduces to 2.3%. The predicted results are coincident well with the experimental data, which demonstrates that the numerical model employed in this study can accurately capture the variation characteristics of temperature and microstructure fields during DCT and provide a theoretical guidance for making the reasonable DCT procedure.
基金This project is supported by Natural Science Foundation of Shanxi Province,China(20051063)the Education Department Science and Technology Development Foundation of Shanxi Province,China(200262)Doctor Research Foundation of Taiyuan University of Science and Technology,Taiyuan,Shanxi Province,China(200271).
文摘Deep cryogenic treatment technology of electrodes is put forward to improve electrode life of resistance spot welding of aluminum alloy LF2. Deep cryogenic treatment makes electrode life for spot welding aluminum alloy improve. The specific resistivity of the deep cryogenic treatment electrodes is tested and experimental results show that specific resistivity is decreased sharply. The temperature field and the influence of deep cryogenic treatment on the electrode tip temperature during spot welding aluminium alloy is studied by numerical simulation method with the software ANSYS. The axisymmetric finite element model of mechanical, thermal and electrical coupled analysis of spot welding process is developed. The numerical simulation results show that the influence of deep cryogenic treatment on electrode tip temperature is very large.
基金[This work was financially supported by the National Natural Science Foundation of China (No. 50175080) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (SRFDP: No. 20030056003).]
文摘The effects of deep cryogenic treatment on mechanical behavior of a Cu-Cr-Zr alloy for electrodes of spot welding were investigated employing Brinell-hardness testing unit, abrasion examination machine, electronic almighty testing machine and X-ray stress analyzer. Tensile fracture surfaces of the alloy were characterized by scanning electronic microscope （SEM） with energy dispersive X-ray spectroscopy （EDS）. The results showed that, after deep cryogenic treatment, σb and σ0.2 increased 23 MPa and 21 MPa respectively, the wear rate of the alloy exhibited the trend of decrease with the decreasing temperature and increasing time of deep cryogenic treatment, and the surface residual stress of the alloy was partially eliminated by deep cryogenic treatment.
基金This project is supported by Nature Science Foundation of Shanxi Province （ No. 2009011028-2） , Talent Star Special Foundation of Taiyuan （No. 09121013 ）, College Students Innovation Foundation of Shanxi Province（ No. UIT20090065 ）.
文摘Cr-Zr-Cu alloy electrodes for resistance spot welding of aluminium alloy are treated by deep cryogenic treatment processes. The Cr-Zr-Cu alloy electrodes are analyzed by transmission electron microscope（ TEM ） , and results show that the common dislocation in Cr-Zr-Cu alloy electrodes is changed into the dislocation loop, and twin crystal is found after deep cryogenic treatment. The parallel twin crystal band is observed by selected electron diffraction（SED） and the twin crystal plane is marked as （ 111 ）. The Cr-Zr-Cu alloy electrode is studied by X-ray diffraction（ XRD ） and results show that the intensity of diffraction peak is obviously changed after deep cryogenic treatment, and the grain rotates to preferred orientation. The Cr-Zr- Cu alloy electrode is studied by positron annihilation technique （PAT） and results indicate that the amount of vacancy defects is less than that of Cr-Zr-Cu alloy before deep cryogenic treatment. The main elements in Cr-Zr-Cu alloy are studied with X- ray photoelctron spectroscopy（ XPS ） and the intensity of spectrum peak is increased after deep cryogenic treatment.
基金financially supported by the Beijing Natural Science Foundation (No.2164072)the Youth Innovation Promotion Association of CAS (No.2016021)the Key Laboratory of Cryogenics,TIPC,CAS (No.CRYOQN201504)。
文摘The effect of deep cryogenic treatment(DCT)on the electrochemical behavior of Ti-6Al-4V alloy in Hank's solution was experimentally investigated in the present work.Deep cryogenic treatments at-196℃soaking for 24(DCT-24) and 48(DCT-48) hours were conducted on the commercial annealed Ti-6Al-4V alloy.The methods of polarization and electrochemical impedance spectroscopy(EIS) were employed to evaluate the electrochemical behavior and corrosion mechanism.Furthermore,atomic force microscopy(AFM) was used to detect the corrosion surface.The results showed that deep cryogenic treatment shifted the corrosion potentials(E_(corr))to the positive direction.The corrosion current densities(i_(corr))of samples treated by DCT-24 and DCT-48decreased from 153.1 nA·cm^(-2) in the untreated(UT)sample to 86.3 nA·cm^(-2) and 43.3 nA·cm^(-2),respectively.Furthermore,the roughness of corrosion surfaces of samples subjected to DCT was smaller than that of the UT sample,which indeed demonstrated the improvement in corrosion resistance of Ti-6Al-4V alloy in Hank's solution.The results of EIS test indicated that deep cryogenic treatment had no influence on the corrosion mechanism of Ti-6Al-4V alloy in Hank's,while enhanced resistance value of outer porous layer R_p,which makes the surface more efficient to prevent the corrosion.The reduction of β-phase particles,improvement in dislocation density and release of residual stress caused by deep cryogenic treatment had great contribution to the improvement in corrosion resistance of Ti-6Al-4V alloy in Hank's solution.
基金financially supported by the Major Special Project,Ministry of Industry and Information Technology,China (No.2012ZX04003061)
文摘Effect of deep cryogenic treatment （DCT） on the microstructures and mechanical behavior of ultrafine-grained WC-12Co cemented carbide was investigated by using XRD, SEM, and DSC. The phase transformations of pure Co and binder phase Co in cemented carbide were analyzed in detail to correlate the strengthening mechanism with its x -ε, phase transition. The results show that DCT resulted in a slight increase in hardness and bending strength of ultrafine- grained WC-12Co cemented carbide. For the ultrafine-grained cemented carbide after DCT, there is no significant change in the microstructure and the elemental distribution of the cemented carbides, but the fractured morphology shows a feature of plastic deformation. In the cases of pure Co and the binder phase Co in WC-12Co cemented carbide, they exhibit different features of phase transformation. The improvement of mechanical property of cemented carbide can be attributed to the increased amount of ε-Co in WC-12Co composites after DCT.
基金The work was financially supported by the National Natural Science Foundation of China(Grant No.51761022).
文摘The effects of traditional heat treatment(quenching and then tempering)and deep cryogenic treatment on the microstructure and mechanical properties of a low-carbon high-alloy martensitic bearing steel were studied by Rockwell hardness test,X-ray diffractometry,scanning electron microscopy and transmission electron microscopy.The results show that the deep cryogenic treatment promotes the transformation of the retained austenite to martensite during cooling,which leads to the hardness of the sample after deep cryogenic treatment higher than that at the quenched state.Also,the carbon content in the martensite matrix after different treatments was calculated and the results indicated that deep cryogenic treatment can promote the segregation of carbon atoms in martensite to dislocations.The segregated carbon atoms act as and grow into nuclei for the formation of fine carbide particles during subsequent tempering.And this resulted in the fact that the hardness of the tempered experimental steel after deep cryogenic treatment is higher than that without deep cryogenic treatment.
文摘The effects of the prior austenite grain size in deep cryogenic treatment on the hardness, the structural change and the wear resistance of D6 tool steel were investigated. The wear resistance of the cryogenically treated samples was determined using the pin-on-disk wear machine. The microstructural characteristics and phases present in heat treated samples were determined using SEM and XRD techniques. The results showed that the retained austenite is completely transformed to martensite during the cryogenic treatment. Besides, there is an optimum grain size of which the maximum wear resistance and hardness are obtained.
基金The authors gratefully acknowledge the financial support received from the Central Iron and Steel Research Institute and the Research Center for Analysis and Measurement of Kunming University of Science and Technology(No.2017M20152230069).
文摘The microstructure evolution and the pitting corrosion resistance of a supermartensitic stainless steel after deep cryogenic treatment process were clarified through X-ray diffraction,field emission scanning electron microscopy,transmission electron microscopy(TEM)and electrochemical methods.The results showed that the microstructure of supermartensitic stainless steel mainly consisted of reversed austenite,tempered martensite,and M_(23)C_(6)carbides after tempering.The deep cryogenic treatment promoted the refinement of the martensite laths and the precipitation of the carbides in comparison with the traditional process.TEM analysis indicated that the segregation of Si atoms at the boundary was found at the interface between carbide and martensite.The pitting corrosion potential of the specimens subjected to deep cryogenic treatment decreased with the elevated tempering temperature,and the lowest pitting corrosion potential was found at the tempering temperature of 650℃.The sensitivity of the pitting corrosion potential was attributed to the precipitation of M_(23)C_(6)carbides and Si atoms segregation.Si atoms segregation engendered the formation of Cr-depleted zone near M_(23)C_(6)and impeded the recovery of Cr-depleted zone.
基金the National Natural Science Foundation of China(Nos.52171154,and 51871076)the National Key Research and Development Programs of China(Nos.2018YFB1105200 and 2019YFA0209904)+1 种基金the Guangdong Province Basic and Applied Research Key Projects(Nos.2020190718102)the National Key R&D Programme,Ministry of Science and Technology of China(No.2019YFA0209)。
文摘Additively manufactured(AM)metallic materials commonly possess substantial tensile surface residual stress,which is detrimental to the load-bearing service behavior.Recently,we demonstrated that deep cryogenic treatment(DCT)is an effective method for improving the tensile properties of CoCrFeMnNi high-entropy alloy(HEA)samples fabricated by laser melting deposition(LMD),by introducing high compressive residual stress and deformation microstructures without destroying the AM shape.However,carrying out the DCT in a single-step mode does not improve the residual stress gradients inherent from the LMD process,which are undesirable as the mechanical properties will not be homogeneous within the sample.In this work,we show that carrying out the DCT in a cyclic mode with repeated cryogenic cooling and reheating can significantly homogenize the residual stress in LMD-fabricated Co Cr Fe Mn Ni HEA,and improve tensile strength and ductility,compared with single-step DCT of the same cryogenic soaking duration.Under cyclic DCT,the thermal stress is re-elevated to a high value at each cryogenic cooling step,leading to the formation of denser and more intersecting reinforcing crystalline defects and hcp phase transformation,compared to single-step DCT of the same total cryogenic soaking duration in which the thermal stress relaxes towards a low value over time.The enhancement of defect formation in the cyclic mode of DCT also leads to more uniform residual stress distribution in the sample after the DCT.The results here provide important insights on optimizing DCT processes for post-fabrication improvement of mechanical properties of AM metallic net shapes.
文摘The effect of deep cryogenic treatment on the formation of reversed austenite （RA） in super martensitic stainless steel was investigated. RA was found to form in steels without （A） and with （B） deep cryogenic treatment. The volume fraction of RA initially increased and then decreased with increasing tempering temperature over 550-- 750 ℃ for the two steels, which were quenched at 1050 ℃. In addition, for both with and without deep cryogenic treatment, the RA content reached a maximum value at 650 ℃ although the RA content in steel B was greater than that in steel A over the entire range of tempering temperatures. Furthermore, the hardness （HRC） of steel B was greater than that of steel A at tempering temperatures of 550--750 ℃. From these results, the basic mechanism for the formation of RA in steels A and B was determined to be Ni diffusion. However, there were more Ni enriched points, a lower degree of enrichment, and a shorter diffusion path in steel B. It needed to be noted that the shapes of the RA consisted of blocks and stripes in both steels. These shapes resulted because the RA redissolved and trans- formed to martensite along the martensitic lath boundaries when the tempering temperature was 650--750 ℃, and a portion of RA in the martensitie lath divided the originally wider martensitic laths into a number of thinner ones. In- terestingly, the RA redissolved more rapidly in steel B and consequently resulted in a stronger refining effect.
文摘In this study,the effects of deep cryogenic treatment(DCT)on the mechanical and tribological properties of AISI D3 tool steel were investigated together with a systematic correlation between their hardness and wear resistance.It was found that conventionally heat treated AISI D3 tool steel samples were significantly hardened via an additional DCT,which was attributed to the more retained austenite elimination,more homogenized carbide distribution and more reduction in carbide size in the samples.As a result,the hardened AISI D3 samples exhibited reductions in their friction and wear during rubbing against alumina and 100Cr6 steel balls under different normal loads due to the effectively hindered removal of surface materials.The results clearly showed that the DCT was an effective way to improve the mechanical and tribological properties of the AISI D3 tool steel samples as the tribological performance of the tool steel samples was significantly influenced by their hardness.
基金This projecti is supported by National Natural Science Foundation of China (No.50175080).
文摘The microstructure and elements distribution of the deep cryogenic treatmentelectrodes and non-cryogenic treatment electrodes for spot welding hot dip galvanized steel areobserved by a scanning electrical microscope. The grain sizes, the resistivity and the hardness ofthe electrodes before and after deep cryogenic treatment are measured by X-ray diffraction, the DCdouble arms bridge and the Brinell hardness testing unit respectively. The spot welding processperformance of hot dip galvanized steel plate is tested and the relationship between microstructureand physical properties of deep cryogenic treatment electrodes is analyzed. The experimental resultsshow that deep cryogenic treatment makes Cr, Zr in deep cryogenic treatment electrodes emanatedispersedly and makes the grain of deep cryogenic treatment electrodes smaller than non-cryogenictreatment ones so that the electrical conductivity and the thermal conductivity of deep cryogenictreatment electrodes are improved very much, which make spot welding process performance of the hotdip galvanized steel be improved.
基金Financial support from the National High Technology Research and Development Program of China(No.2012AA03A508)is greatly appreciated
文摘To further improve the mechanical performance of a new alloyed austempered ductile iron（ADI）, deep cryogenic treatment（DCT） has been adopted to investigate the effect of DCT time on the microstructure and mechanical behaviors of the alloyed ADI Fe-3.55 C-1.97 Si-3.79 Ni-0.71 Cu-0.92 Mo-0.64 Cr-0.36 Mn-0.30 V（in wt.%）. With increasing the DCT time, more austenite transformed to martensite and very fine carbides precipitated in martensite in the extended period of DCT. The amount of austenite decreased in alloyed ductile irons, while that of martensite and carbide precipitation increased. The alloyed ADI after DCT for 6 h had the highest hardness and compressive strength, which can be attributed to the formation of more plate-like martensite and the finely precipitated carbides. There was a gradual decrease in hardness and compressive strength with increasing the DCT time to 12 h because of the dissolution of M3 C carbide. After tempering, there was a decrease in mechanical properties compared to the direct DCT sample, which was caused by the occurrence of Ostwald ripening of precipitated carbides. The optimum wear resistance was achieved for the alloyed ADI after DCT for 6 h. The wear mechanism of the alloyed ADI in associating with DCT is mainly consisted of micro-cutting wear and some plastic deformation wear.
基金the Majlesi Branch,Islamic Azad University for the support of this work
文摘The effects of the deep cryogenic heat treatment on the microstructural changes,wear resistance,and hardness of carburized DIN 1.7131 grade steel were investigated.Results show that cryogenic heat treatment reduced the retained austenite and increased the carbide amount.In addition,after the cryogenic heat treatment,carbide shows a more uniform distribution,as compared to the conventionally treated ones.It was also clarified that the hardness of the cryogenically treated samples was improved,but the relative improvement decreases with the distance as the surface increases.It has been shown that the wear resistance improves due to the cryogenic heat treatment,and the predominant wear mechanism is a combination of the adhesive and tribo-chemical wear.
基金financially supported by the National Science Foundation for Distinguished Young Scholars of China(Grant No.51725504)the open funding via State Key Laboratory of Materials Processing and Die&Mould Technology(Grant No.P2019-011)+1 种基金the Guangdong Provincial Natural Science Foundation of China(Grant No.2020A1515011524)the Fundamental Research Funds for the Central Universities,HUST(Grant No.2018KFYRCPT001).
文摘The effect of deep cryogenic cycle treatment(DCT)on Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10)Be_(22.5)(Vit-1)bulk metallic glass(BMG)prepared from high-purity raw materials was investigated.After DCT,no obvious rejuvenation of the samples was detected.With an increasing number of cryogenic cycles,the hardness of the samples first decreased and then increased,the room-temperature compression plasticity first increased and then generally remained unchanged,and the impact toughness underwent almost no obvious change.The absence of rejuvenation was attributed to the high fragility index(47-50)and high glass forming ability(GFA)of the material.As lower purity of the raw materials is expected in practical applications,DCT of Vit-1 BMG prepared from low-purity raw materials was also performed.After DCT,the samples prepared with the lower-purity raw materials were clearly rejuvenated,and the room-temperature mechanical properties improved significantly.Both the compression plasticity and impact toughness reached peak values after 5 cryogenic cycles.The initial impurities(including Y and O)had a complex and comprehensive effect on the deformation mechanism of the BMG during DCT.Our findings indicate that the structural heterogeneity,fragility index,and GFA of the BMG alter the effect of DCT.
