Effect of Air Pressure on Hardened Layer of U75V 60 kg/m Heavy Rail after Heat Treatment

The samples cut from U75V 60 kg/m heavy rail are heated to 900 ℃ in resistance furnace and then put into air spraying channel to be cooled for 80 s, and change air pressure from 0.16 MPa to 0.33 MPa, and observe the effect of air pressure on hardened layer. The thickness and hardness of hardened layer increases with the increase of air pressure, and the thickness is more than 24 mm at the center and top fillets of rail head, and more than 15 mm at the blow fillets of rail head when air pressure is more than 0.26 MPa. During the tempering after heat treatment, tempering temperature of rail head is more than 570 ℃ when air pressure is separately 0.16 MPa, 0.20 MPa and 0.23 MPa, which is higher than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s according to CCT curve of U75V steel. When air pressure is separately 0.26, 0.30 and 0.33 MPa, the tempering temperature is 529 ℃ lower than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s. Under this condition, pearlite transformation is finished totally, so in order to reduce air consumption and control the cost, proper air pressure for U75V 60 kg/m heavy rail heat treating should be 0.26 MPa; the cooling rate increases with the increase of air pressure, and the average cooling rate on the surface of rail head is more than 3.21 ℃/s when air pressure is more than 0.26 MPa, and the largest cooling rate occurs at the top fillets of rail head.

The Effect of Heat Treatment Atmosphere on Hardening of Surface Region of H13 Tool Steel

The main objective of the die heat treatment is to enhance the surface hardness and wear properties to extend the die service life. In this paper, a series of heat treatment experiments were conducted under different atmospheric conditions and length of treatment. Four austenitization atmospheric conditions were studied and although each heat treatment condition resulted in a different hardness profile, it did not affect the results for gas nitriding. All samples subjected to the nitriding process produced similar thicknesses of hardened case layer with average hardness of 70 - 72 HRC if the initial carbon content is not too low. It was shown that heat treatment without atmospheric control results in a lower hardness on the surface since the material was subjected to decarburization effect. The stainless steel foil wrapping around the sample and heat treatment in a vacuum furnace could restrict the decarburization process, while pack carburization heat treatment resulted in a carburization effect on the material.

INFLUENCES OF MICRO-ALLOY AND HEAT-TREATMENT PROCEDURE ON THE HARDENING OF DJ108TI ALLOY

The influences of micro alloy and heat treatment procedure on the hardening of DJ108Ti alloy, which is based on the electrolytic Al Si(2 05%) Ti(0 25%) Fe(0 30%) multi component alloy, have been investigated. The results indicate that the micro alloy based on the multi component alloy makes α Al dendrites in DJ108Ti alloy refined; that the improved heat treatment procedure makes the eutectic Si phases granulated , the Fe phase refined, and the strengthening phases of Mg2Si and Al2Cu fully precipitated. The micro alloy and the heat treatment procedure harden DJ108Ti alloy and benefit the abrasion resistance of the alloy

20CrMnTiH钢渗碳齿轮弯曲疲劳性能研究

利用扫描电镜、金相显微镜、疲劳试验机和硬度计等测试手段研究20CrMnTiH钢渗碳齿轮弯曲疲劳性能。研究表明:20CrMnTiH钢渗碳齿轮主要沿齿根部位断裂。随着载荷的增加,弯曲疲劳寿命降低,当载荷在50~100kN时,齿轮疲劳寿命增加数倍,建议生产应用在此范围载荷内能发挥该材料最佳性能。

T6态热挤压Al-5Cu-0.8Mg-0.15Zr-0.2Sc-0.5Ag合金的低周疲劳行为

为了揭示经过T6处理的热挤压Al-5Cu-0.8Mg-0.15Zr-0.2Sc-0.5Ag合金在不同温度下的低周疲劳变形与断裂行为,对T6态热挤压合金进行了室温和200℃条件下的低周疲劳实验。结果表明:在室温和200℃下合金塑性应变幅与载荷反向周次之间服从Coffin-Manson公式,而弹性应变幅与载荷反向周次之间服从Basquin公式;合金在室温和200℃下低周疲劳变形时,在较低外加总应变幅下疲劳变形机制主要为平面滑移,而在较高外加总应变幅下疲劳变形机制主要为波状滑移;室温和200℃下合金的疲劳裂纹均萌生于疲劳试样表面并以穿晶方式扩展。

氦气球阀主密封密封结构研究

阐述了随着我国能源需求的持续增长及能源结构改革需要,核能快堆及高温气冷堆等四代核电技术上也有了突破,对阀门的结构和性能提出了更高的要求。球阀具有大流量、低流阻、操作方便等优点,在高温气冷堆等四代核电势必得到广泛应用。介绍了研究高温气冷堆核级氦气球阀结构选型的意义,通过研究分析确定了上装式低内漏率球阀结构的选型开发,同时为了确保其密封性能能够满足要求,对样机进行了一系列的氦检漏试验,证明了该结构选型的良好密封性能。通过本课题的研究,掌握了核级氦气球阀满足低内漏率要求的主密封结构关键技术,解决了氦气等高渗透性介质工况阀门主密封结构密封性能保证的难题,可以为高温气冷堆核级氦气球阀等具有低内漏率要求球阀的结构设计提供借鉴。

静脉腔内激光消融联合聚桂醇泡沫硬化剂注射治疗下肢静脉曲张的效果

目的探讨静脉腔内激光消融(EVLT)联合聚桂醇泡沫硬化剂注射治疗下肢静脉曲张的效果。方法选取2021年3月至2022年2月我院收治的110例下肢静脉曲张患者,随机将其分为研究组(68例)和对照组(42例)。研究组采用EVLT联合聚桂醇泡沫硬化剂注射治疗,对照组采用传统大隐静脉高位结扎加抽剥术治疗。比较两组的治疗效果。结果研究组的手术时间、住院时间均短于对照组,术中出血量少于对照组(P<0.05)。研究组的术后并发症总发生率低于对照组(P<0.05)。研究组的治疗总有效率高于对照组(P<0.05)。结论EVLT联合聚桂醇泡沫硬化剂注射治疗下肢静脉曲张效果显著,具有创伤小、恢复快、并发症少等优点。

Effect of Heat Treatment on Prior Grain Size and Mechanical Property of a Maraging Stainless Steel

Effect of the heat treatment, including solution treatment （ST） and aging treatment （AT）, on the prior austenite grain （PAG） size, microstructure and mechanical properties of a precipitation hardening maraging stainless steel was investigated. The results indicate that the relations between PAG size and yield strength （σy） under both ST and AT conditions obey the HalI-Petch relationship. Furthermore, after ST at 1050℃ for 1 h＋cryogenic treated （CT） at -70℃ for 8 h＋AT at 535℃ for 4 h, the tested steel showed its ultimate tensile strength （σb） and σy over 1900 MPa and 1750 MPa, respectively.

Effects of pre-strain and baking parameters on the microstructure and bake-hardening behavior of dual-phase steel

In a typical process, C-Mn steel was annealed at 800℃ for 180 s, and then cooled rapidly to obtain the ferrite-martensite microstructure. After pre-straining, the specimens were baked and the corresponding bake-hardening （BH） values were determined as a function of pre-strain, baking temperature, and baking time. The influences ofpre-strain, baking temperature and baking time on the microstructure evolution and bake-hardening behavior of the dual-phase steel were investigated systematically. It was found that the BH value apparently increased with an increase in pre-strain in the range from 0 to 1%; however, increasing pre-strain from 1% to 8% led to a decrease in the BH value. Furthermore, an increase in baking temperature favored a gradual improvement in the BH value because of the formation of Cottrell atmosphere and the precipitation of carbides in both the ferrite and martensite phases. The BH value reached a maximum of 110 MPa at a baking temperature of 300℃. Moreover, the BH value enhanced significantly with increasing baking time from 10 to 100 min.

Effects of Solution Treatment Time and Sr-Modification on Microstructure and Mechanical Property of Al-Si Piston Alloy

The purpose of this paper was to investigate the effects of solution treatment time and Sr-modification on the microstructure and property of the Al-Si piston alloy.It was found that as-cast microstructures of unmodified and Sr-modified Al-Si alloys consisted of a coarse acicular plate of eutectic Si,Cu_3NiAl_6 and Mg_2Si phases in theα-Al matrix but different in size and morphology.Both size and inter-particle spacing of Si particles were significantly changed by increasing the solution treatment time.After a short solution treatment,the coarse acicular plate of the eutectic Si appears to be fragmented.Fully modified microstructure of Sr-modified alloy can reduce the solution treatment time compared to unmodified alloy.The maximum of a peak hardness value is found in the very short solution treatment of both Al-Si piston alloys.Compared to 10 h solution treatment,the solution treatment of 2-4 h is sufficient to achieve appropriate microstructures and hardness. The short solution treatment is very useful to increase the productivity and to reduce the manufacturing cost of the Al-Si piston alloys.

Verification of Inverse Quench-Hardening Phenomena in Bearing Steel Specimens by Experiment and Computer Simulation

The appearance of lower hardness at the surface than at the core after through-hardening of steels is known as "inverse quench-hardening." Pioneering work was performed in the 1970s by establishing a simple test procedure where cylindrical specimens were cooled by air and brine sequentially to produce the effect. This phenomenon was described in relation to the polymer quenching and its effect to the improvement in the fatigue strength of the steel in the 1990s. The hardening mechanism in the same specimens as the pioneering work was recently examined using a finite element simulation method. However, this work has not been completed for lack of experimental quenching cooling curves for the comparisons. In this study, the same experiments using 20mm diameter cylindrical specimens as those of the pioneering work were conducted to obtain cooling curves and hardness distributions for comparing simulated results. Experimental cooling curves showed a temperature recovery as predicted by the simulation. Also the inverse quench-hardening phenomena were found in the measured and simulated hardness distributions of the specimens.

Effect of Manganese on As-Cast Microstructure and Hardening Behavior of High Chromium White Cast Iron

The effect of manganese on the as-cast structure and hardening behavior of high chromium white cast iron subjected to sub-critical treatment was studied.The results indicate that the fraction of retained austenite and the manganese distribution in as-cast alloys are controlled by manganese content.The manganese distribution in as-cast alloys is not homogeneous.The manganese content in carbide is higher than that in matrix.Whether the secondary hardening occurs or not and the peak hardness of secondary hardening is controlled by manganese content in retained austenite in as-cast structure.Higher manganese content can cause more retained austenite.The secondary hardening occurs in sub-critical treating process if the fraction of retained austenite is high.

EFFECT OF PROGRESSIVE AGING TREATMENT ON THE PROPERTIES AND MICROSTRUCTUR

A dispersion-hardened high-hard elastic alloy is developed by adding 3%/ of aluminium (Al) to the nickel-chromium alloy. The experimental results show that after a progressive aging of the alloy at 550℃ or 5 h, supersaturated Cr(α)-rich phase (80 wt% Cr)formed. The Cr content of this phase is by 40 wt% higher than that in as-prepared alloy. An analysis of quantitative metallography shows that the seperated Cr(α)-rich phase grains have an average size(D) of 1.8225 mm and the total area of the particles is 16. 7% of the measured area (Pa = 16.7%). The large amount of dispersively distributed supersaturated Cr(α)-rich phase which are seperated out during progressive aging process strengthen the alloy matrix, and raise the hardness and bending strength of the alloy.

Electrokinetic Nanoparticle Treatment Success Factors

Electrokinetic pozzolanic nanoparticle treatments have been reported in the literature to achieve rapid porosity reductions and deeply penetrating strength enhancement of cement and concrete. The high electric fields required to achieve these results have tended to be accompanied by particle suspension instability. Coagulation is an instability that can limit the efficiency and effectiveness of a treatment by removing particles from suspension. The current study examines how electro-coagulation impacts electrokinetic treatment effectiveness. The nano-pozzolan suspension used in this study was Nalco 1056, alumina-coated silica (24-nm). A threshold electric field strength of 0.4 V/cm was identified for avoiding direct electro-coagulation. Treatments conducted at this threshold value exhibited a 50% strength increase. Treatments conducted above this threshold value exhibited significant particle electro-coagulation losses and strength increases of only 25%. This study found that electro-coagulation was influenced by the electric field strength, through a combination of particle crowding at pore entrances and pH shifts as driven by electrolysis.

Effects of pre-aging treatment on subsequent artificial aging characteristics of Al-3.95Cu-(1.32Mg)-0.52Mn-0.11Zr alloys

The effects of pre-aging treatments on subsequent artificial aging response were investigated by means of transmission electron microscopy observations and hardness test in age hardened Al-3.95Cu-(1.32Mg)-0.52Mn-0.11 Zr alloys. In Al-3.95Cu-0.52Mn-0.11 Zr alloy, when the pre-aging temperature is 25 °C, the pre-aging treatment has no evident effect on the peak hardness of subsequent artificial aging, while a positive effect(increase of peak hardness) appears when pre-aging temperature is50 °C. However, in Al-3.95Cu-1.32Mg-0.52Mn-0.11 Zr alloy, it is found that whether the pre-aging temperature is 25 °C or 50 °C,the peak artificial aging hardness is lower than that of T6 treated alloy, that is to say, pre-aging treatment has a negative effect on the peak hardness of subsequent artificial aging in the alloys.

Influences of copper on solidification structure and hardening behavior of high chromium cast irons

The influences of copper on microstructure and the hardening behavior of high chromium cast irons subjected to sub-critical treatment were investigated. The results show that the microstructure of the as-cast high chromium cast irons consists of retained austenite, martensite and M7C3 type eutectic carbide. When copper is added into high chromium cast irons, austenite and carbide contents are increased. The increased addition of copper content from 0% to 1.84% leads to the increase of austenite and carbide from 15.9% and 20.0% to 61.0% and 35.5%, respectively. In the process of sub-critical treatment, the retained austenite in the matrix can be precipitated into secondary carbides and then transforms into martensite in cooling process, which causes the secondary hardening of the alloy under sub-critical treatment. High chromium cast irons containing copper in sub-critical treatment appear the second hardening curve peak due to the precipitation of copper from supersaturated matrix.

Effect of MWCNT reinforcement on the precipitation-hardening behavior of AA2219

Aluminum alloy matrix composites have found a predominant place in research, and their applications are explored in almost all industries. The aerospace industry has been using precipitation-hardenable alloys in structural applications. However, insufficient literature is available on the influence of multiwalled carbon nanotubes （MWCNTs） on precipitation-hardenable alloy composite materials; thus, this work was designed to elucidate the effect on MWCNT reinforcement on AA2219 with and without precipitation hardening. Reinforcement with MWCNTs has been reported to accelerate precipitation and to achieve greater hardness within a much shorter time. The addition of 0.75wt% MWCNTs resulted in maximal hardness at 90 min, which is approximately 27% of improvement over the maximum hardness achieved by the corresponding monolithic alloy after 10 h of aging. The sample reinforced with 0.75wt% MWCNTs showed an improve- ment of 82% in hardness by solutionizing and aging compared to that achieved by sintering.

Single Track Laser Surface Hardening Model for AISI 4340 Steel Using the Finite Element Method

Laser surface hardening becomes one of the most effective techniques used to enhance wear and fatigue resistance of mechanical parts. The characteristics of the hardened surface depend on the physicochemical properties of the material as well as the heating system parameters. To adequately exploit the benefits presented by the laser heating method, it is necessary to develop a comprehensive strategy to control the process parameters in order to produce desired hardened surface attributes without being forced to use the traditional and fastidious trial and error procedures. This study presents a comprehensive approach used to build a simplified model for predicting the hardness profile. A finite element method based prediction model for AISI 4340 steel is investigated. A circular shape with a Gaussian distribution is used for modeling the laser heat source. COMSOL MULTIPHYSICS software is used to solve the heat transfer equations, estimate the temperature distribution in the part and consequently predict the hardness profile. A commercial 3 kW Nd:Yag laser system is combined to a structured experimental design and confirmed statistical analysis tools for conducting the experimental calibration and validation of the model. The results reveal that the model can effectively lead to a consistent and accurate prediction of the hardness profile characteristics under variable hardening parameters and conditions. The results show great concordance between predicted and measured values for the dimensions of hardened and melted zones.

A Predictive Modeling Based on Regression and Artificial Neural Network Analysis of Laser Transformation Hardening for Cylindrical Steel Workpieces

Laser surface hardening is a very promising hardening process for ferrous alloys where transformations occur during cooling after laser heating in the solid state. The characteristics of the hardened surface depend on the physicochemical properties of the material as well as the heating system parameters. To exploit the benefits presented by the laser hardening process, it is necessary to develop an integrated strategy to control the process parameters in order to produce desired hardened surface attributes without being forced to use the traditional and fastidious trial and error procedures. This study presents a comprehensive modelling approach for predicting the hardened surface physical and geometrical attributes. The laser surface transformation hardening of cylindrical AISI 4340 steel workpieces is modeled using the conventional regression equation method as well as artificial neural network method. The process parameters included in the study are laser power, beam scanning speed, and the workpiece rotational speed. The upper and the lower limits for each parameter are chosen considering the start of the transformation hardening and the maximum hardened zone without surface melting. The resulting models are able to predict the depths representing the maximum hardness zone, the hardness drop zone, and the overheated zone without martensite transformation. Because of its ability to model highly nonlinear problems, the ANN based model presents the best modelling results and can predict the hardness profile with good accuracy.