磨削用量对球墨铸铁QT400磨削淬硬层及其均匀性的影响

采用逆磨+顺磨的双程平面磨削方式对球墨铸铁QT400进行磨削淬硬试验,研究了磨削深度a_p和试样进给速度v_w对淬硬层及其均匀性的影响。结果表明,磨削后试样表层存在熔化、完全相变淬硬和未完全相变淬硬等3种情况,其中,熔化层组织为二次渗碳体、残留奥氏体和碳化物,完全相变淬硬层组织为针状马氏体、残留奥氏体和球状石墨,未完全淬硬层组织为针状马氏体、铁素体、残留奥氏体和球状石墨。显微硬度分布曲线中高硬度区的平均硬度值在850~950 HV0.2之间,与基体(190~230 HV0.2)相比,显微硬度提高近3倍。随着磨削深度a_p的增大或试样进给速度v_w的减小,试样表层呈现“完全未淬硬→未完全淬硬→完全淬硬→熔化”的变化规律,显微硬度分布曲线中高硬度区的范围也变宽,淬硬层的深度也增大且均匀性良好。     

超声冲击处理S30408不锈钢的微观组织演变与电化学性能

通过超声冲击处理(UIT)方法在S30408奥氏体不锈钢表面构筑梯度纳米结构,通过XRD、硬度测试、光学显微镜、扫描电镜和电化学测试等方法研究了超声冲击时间对试验钢表层纳米微观结构和耐腐蚀性能的影响。结果表明,超声冲击处理后在材料表层产生了一定深度的硬化层,同时引入了残余压应力,且残余压应力和硬度均随着超声冲击时间的增加而增加,在超声冲击时间为300 s时分别达到最大值740.12 MPa和82.22 HRB。超声冲击处理后试样表层观察到呈梯度变化的微观结构,可分为纳米层、剧烈塑性变形层和基体。超声冲击使表层晶粒得到细化,并引发了马氏体转变,超声冲击时间为300 s时的晶粒尺寸最小,马氏体转变量最大,分别为14.82 nm和39.80%。超声冲击处理对S30408不锈钢耐蚀性能的影响是晶粒尺寸、残余应力、马氏体相变含量、表面缺陷等因素共同作用的结果,超声冲击时间为180 s时试样的耐蚀性最好,自腐蚀电流密度为1.22 μA/cm²。     

23Si2MnCrNiMoV钢固体渗碳层的影响因素

研究了渗碳温度、渗碳时间、扩散时间、深冷处理参数对23Si2Mn Cr Ni Mo V钢渗碳层的碳浓度梯度、表层低硬度区深度、有效渗硬层深度(550 HV0.3)、碳扩散距离、微观组织形貌等影响,实验研究的渗碳温度区间为890~970℃,渗碳时间为4~10 h,扩散时间为0~4 h。结果表明,较多的残留奥氏体存在是造成渗碳表层高C低硬度的主要原因,控制C浓度为0.72%~0.86%时,可获得最大硬度,若进一步增加C含量,会形成大量的残留奥氏体,反而降低渗透层的硬度;深冷处理对有效渗硬层深度几乎没有影响,但可使表层低硬度区域从距表面0.7 mm缩至0.3 mm。     

汽车用Q&P钢中的残留奥氏体

研究了在两相区加热与完全奥氏体区加热Q&P工艺下0.2C-1.51Si-1.84Mn钢中的残留奥氏体。结果表明:两相区加热得到的残留奥氏体量最高达13.39%,完全奥氏体化得到的残留奥氏体量最高达5.23%,配分时间为10 s时,两种加热方式的试样都完成碳配分;两相区加热得到的残留奥氏体以两种形态存在:处于马氏体板条间的薄膜状和位于原奥氏体晶界处的块状,而完全奥氏体化后得到的残留奥氏体以薄膜状存在于马氏体板条间;通过电子背散射衍射(EBSD)发现,残留奥氏体的分布与晶界多少有关。     

残留奥氏体量多时测定碳化物级别的简易方法

<正>残留奥氏体量较多(4级以上)时,由于背景亮,分布在其上的白色碳化物块形状模糊,难以区分残留奥氏体和碳化物而造成碳化物评级的困难。常规办法是将该试样在230~250℃的温度下,重新回火30~45 min,以减少或消除残留奥氏体,然后用硝酸酒精侵蚀,进行碳化物评级,该方法比较麻烦。新方法:将渗碳试样抛光后,先用4%(质量分数)的硝酸酒精溶液侵蚀,在放大400倍下观察,如果遇到残留奥氏体级别     

边界润滑条件下18Cr2Ni4WA钢的摩擦诱发马氏体相变及其耐磨特性的研究

用18Cr2Ni4WA钢,经渗碳、淬火和冷处理,以获得不同残留奥氏体含量的试样,进行了与T10钢金属盘对磨的边界润滑磨损试验研究.结果表明,磨损过程中确实存在摩擦诱发马氏体相变,且碳含量低的残留奥氏体较碳含量高的残留奥氏体更易发生诱发相变;摩擦诱发马氏体对提高材料的耐磨性是有利的,而残留奥氏体对耐磨性的影响却存在一个临界应力σc;当外加应力＞σc时,残留奥氏体对提高材料的耐磨性有利;当外加应力＜σc时,残留奥氏体对提高材料的耐磨性不利,并且当残留奥氏体和外加应力搭配适当时,耐磨性可达最高值.     

82B盘条奥氏体晶粒度检测方法比较

采用氧化法和直接淬硬法对82B系列高碳钢盘条的奥氏体晶粒度进行了研究。结果表明:氧化法和直接淬硬法均能清晰显示82B盘条奥氏体晶界,氧化法的关键在于热处理后的磨抛过程;直接淬硬法的关键在于奥氏体晶界的侵蚀过程。使用氧化法比直接淬硬法测得的82B奥氏体晶粒度大0.5级,其原因为试样表层的脱碳。     

短时奥氏体化条件下25Cr2Ni4MoV钢的连续冷却相变动力学

采用DIL 805A/D/T多功能淬火膨胀仪,结合显微组织表征和硬度测试,研究了25Cr2Ni4MoV钢在短时奥氏体化条件下的连续冷却转变(CCT)动力学和组织演变规律。结果表明:在850℃短时奥氏体化条件下,连续冷却相变发生在450～150℃区间;当冷速大于2℃/s时得到的室温组织为马氏体,随着冷速降低,试样中出现贝氏体;当冷速小于0.5℃/s时其显微组织主要为贝氏体组织;随着冷速的进一步降低,当冷速为0.02℃/s时,除了贝氏体以外还有少量的马氏体/奥氏体岛和残留奥氏体。冷速从2℃/s降低至0.5℃/s时硬度变化较明显,这与组织中形成的马氏体与贝氏体的比例有关。由于短时奥氏体化条件下存在未溶解的碳化物,基体碳浓度较低,其Ms温度较高;贝氏体转变速率也较快,这可能与奥氏体的晶粒尺寸小和存在未溶碳化物有关。     

热处理对表面纳米化工业纯锆疲劳性能影响研究

采用表面机械研磨处理(SMAT)对工业纯锆进行表面强化,使材料表面组织细化并引入残余压应力,通过热处理(HT)使表层残余压应力释放而纳米晶尺寸保持不变。利用光学显微镜(OM)和透射电子显微镜(TEM)对试样表层显微组织进行表征,利用X射线应力仪测试距试样表面不同深度处残余应力,通过四点弯曲疲劳实验对热处理前后试样疲劳性能进行测试,利用扫描电子显微镜(SEM)对疲劳断口形貌进行观察,探讨晶粒细化及残余压应力对疲劳性能的影响。结果表明：SMAT使工业纯锆表层形成150μm左右变形层且最表面晶粒细化至35 nm左右,并得到深度为334 μm最大应力为-695.5MPa的残余压应力层;热处理后SMAT处理工业纯锆表层残余压应力场深度减至115μm、最大压应力降为-148.8MPa,残余压应力场的变化对裂纹源位置及材料的疲劳极限影响明显。SMAT处理使工业纯锆疲劳极限较未处理试样提升23%;通过热处理使其表层残余压应力释放后,其疲劳极限较未SMAT处理试样疲劳极限提高13%。     

一种新型高硅热成形钢的淬火-配分处理及其性能

研究了不同温度和时间的淬火-配分(Q-P)处理工艺对新型高硅(Si含量为0.82 mass%)热成形钢组织与力学性能的影响,采用光学显微镜(OM)和扫描电镜(SEM)对试样的微观组织进行了观察。结果表明,材料为板条马氏体+残留奥氏体组织,存在较多的M/A岛。当淬火温度为260℃,配分300 s时,材料的强塑积最高,达到22.3 GPa·%,且伸长率为13.3%,远高于商用热成形钢的6.6%。XRD和磁性法测得该处理条件下的残留奥氏体含量分别为5.2%和7.98%,透射电子显微镜(TEM)观察到薄膜状残留奥氏体。分析表明,淬火-配分处理后,保留至室温的富碳残留奥氏体对于提高材料的塑性起到了主要作用。     

高温渗氮工艺对高氮无镍不锈钢组织及力学性能的影响

采用高温渗氮在奥氏体/铁素体双相不锈钢表面形成了奥氏体高氮层。试验结果表明,渗氮层氮含量可达1.0%,与原材料相比氮含量增加了2倍。原始双相组织已经转变为奥氏体,渗氮层深度达到2 mm以上。采用合理优化的高温渗氮工艺,可在提高不锈钢强度、硬度的同时,其伸长率、断面收缩率仍然保持较高的水平。高温渗氮工艺制备高氮无镍不锈钢的最佳工艺参数为:加热温度1200℃、氮气压力0.3 MPa、保温时间24 h。     

离子研磨对TRIP780钢中残留奥氏体含量和稳定性的影响

研究了经离子研磨对TRIP780钢中残留奥氏体含量与稳定性的影响。首先利用电解抛光技术对TRIP780钢进行样品制备以去除样品表面的应力层,然后利用离子研磨仪对所得样品进行离子研磨,再借助场发射扫描电镜对TRIP780钢中的残留奥氏体进行形貌观察与分析,利用电子背散射衍射技术(EBSD)对离子研磨前后的TRIP钢中残留奥氏体含量进行定性和定量分析。结果表明,与电解抛光相比,离子研磨技术同样可以很好的去除样品表面应力层,但是对于TRIP780钢,经过离子研磨后样品中残留奥氏体的含量大幅度减少,从原来的10.1%骤降至0.02%。从残留奥氏体的菊池花样可以看出,经离子研磨后的残留奥氏体菊池花样明显变差,甚至模糊不清。经离子研磨后的TRIP780钢中残留奥氏体含量明显下降,同时其相结构确实发生了转变,由原来的面心立方结构转变为体心立方结构(即fcc→bcc),由此表明残留奥氏体在受到离子轰击后极其不稳定,易发生相变,这一点在残留奥氏体的准原位试验中得到了进一步的验证。同时离子研磨诱发了晶格的畸变,导致菊池花样清晰度明显下降,花样分辨率降低。     

Structure and Mechanical Properties of Nitrogen Austenitic Steel after Ultrasonic Forging

Electron microscopy and X-ray diffraction have been used to investigate a nitrogen 07Kh17AG18 steel with an austenitic structure after the surface deformation treatment—ultrasonic forging. During ultrasonic forging, an austenitic structure transforms into a new structure with an elevated concentration of deformation-induced stacking faults, a lot of deformation microtwins, ε-martensite crystals. The austenite lattice parameter is found to be decreased in the surface layer. After ultrasonic forging, nitrided steel exhibits enhanced strength properties with retained high plasticity.     

On The Enhancement of Wear Resistance of Hardened Carbon Tool Steel （AISI 1095） With Cryogenic Quenching

Many experimental investigations reveal that it is very difficult to have a completely martensitic structure by any hardening process. Some amount of austenite is generally present in the hardened steel. This austenite existing along with martensite is normally referred as the retained austenite. The presence of retained austenite greatly reduces the mechanical properties and such steels do not develop maximum hardness even after cooling at rates higher than the critical cooling rates.Strength can be improved in hardened steels containing retained austenite by a process known as cryogenic quenching.Untransformed austenite is converted into martensite by this treatment. This conversion of retained austenite into martensite results in increased hardness, wear resistance and dimensional stability of steel. Wear can be defined as the progressive loss of materials from the operating surface of a body occurring as a result of relative motion at the surface. Hardness, load,speed, surface roughness, temperature are the major factors which influences wear. Many studies on wear indicate that increasing hardness decreases the wear of a material. With this in mind, to study the surface wear on a surface modified (Cryogenic treated) steel material an attempt has been made in this paper. In this study as a Part -I Hardening was carried out on carbon tool steel (AISI 1095) of different L/D ratio with conventional quenchants like purified water, aqueous solution and Hot mineral oil. As a Part -II hardening was followed by quenching was carried out as said in Part- I and the hardened specimen were quenched in liquid Nitrogen which is at sub zero condition. The specimens were tested for its microstructure, hardness and wear loss. The results were compared and analyzed. The alloying elements increases the content of retained austenite hence the material used was AISI 1095 (Carbon 0.9%, Si 0.2%, Mn0.4% and the rest Iron)     

Effect of simultaneous surface modification process on wear resistance of martensitic stainless steel

In order to improve the wear resistance of martensitic stainless steel, a surface treatment system was developed that combines high-frequency induction heating (IH) with fine particle peening (FPP). In this system, a compressed air spray from the FPP nozzle rapidly cools the specimen surface, which is heated by the IH system. The specimen surface can be simultaneously modified by work hardening and quenching. Vickers hardness and retained austenite measurements were conducted to characterize the surface-modified layer generated by the developed process. Surface microstructures were also observed by scanning electron microscopy (SEM) and optical microscopy. The process created a surface with a high hardness and an extremely fine-grained microstructure. The fine-grained microstructure was generated by dynamic recrystallization. The process reduced the amount of retained austenite in the surface layer because it increased the precipitated chromium carbide content. Reciprocating sliding wear tests were conducted to evaluate the wear resistance of the surface. The specimen modified by the developed process exhibited higher wear resistance than specimens that had only been quenched. This implies that the developed simultaneous process can significantly improve the wear resistance of steel surfaces.     

TIG表面重熔对堆焊层耐空泡腐蚀的影响

用氩弧熔化焊(MIG)在4020碳钢表面堆焊耐空泡腐蚀材料,然后采用TIG（钨极氩弧焊），将堆焊层改性;通过与传统磨削表面加工对比，研究了TIG表面重熔对空泡腐蚀的影响.结果表明 : 在45 h空泡腐蚀试验后，磨削试样的累积失重量是TIG表面重熔试样的157倍；奥氏体到马氏体的相变是Stellite 21材料吸收空泡冲击能的主要途径，而TIG表面重熔加工可以延迟相变，延长吸收空泡冲击的时间，减缓空蚀；TIG重熔表面抑制了片层状马氏体的裂纹发展，避免了大的物质剥落.     

Residual stress state after the laser surface remelting process

Residual stresses are a result of elasto-plastic deformations induced in the workpiece material during the heat treatment process. The extent and magnitude of internal stresses depend on temperature conditions in heating and cooling and physical properties of the workpiece material. This contribution discusses the extent and distribution of residual stresses after laser remelting a thin surface layer on ductile iron 80-55-06 (ASTM specification) or Gr 500-7 according to ISO. Residual stresses are not only induced by temperature differences but also result from stresses due to microstructural changes between the surface and the core of the specimen subsequent to cooling to the ambient temperature. The distribution and extent of residual stresses in the remelted thin surface layer depend mostly on melt composition and cooling conditions. Different rates of solidification and subsequent cooling of the remelted layer are reflected in the volume proportions of the created cementite, residual austenite, and martensite in the microstructure. The rate of heating and cooling of the thin surface layer is a function of laser power, beam diameter on the workpiece surface, and interaction time. In addition, the number of passes of the laser beam over the workpiece surface and different degrees of laser trace overlapping were increased to see how these can affect the thermal conditions in the workpiece. To determine the residual stresses, the relaxation method was used. This is based on measuring the specimen strain during electrochemical material removal.     

Characterization of ultrasonically peened and laser-shock peened surface layers of AISI 321 stainless steel

The effects of ultrasonic impact peening (UIP) and laser-shock peening (LSP) without protective and confining media on microstructure, phase composition, microhardness and residual stresses in near-surface layers of an austenitic stainless steel AISI 321 are studied. An X-ray diffraction analysis shows both significant lines broadening and formation of strain-induced ε- and α-martensite after UIP with additional peaks found near austenite ones in the low-angle part after LSP supposedly due to formation of a dislocation-cell structure in the surface layer. TEM studies demonstrate that a nano-grain structure containing either only austenitic grains with ε-martensite (at strains up to 0.42) or both austenite and α-martensite grains (at higher strains) can form in the surface layer after UIP. Highly tangled and dense dislocation arrangements and even cell structures in fully austenitic grains are revealed both at the surface after LSP and in the layer at a depth of 80 μm after UIP. UIP is found to produce a sub-surface layer 10 times thicker and about 1.4 times harder than that formed by LSP. A mechanism of formation of the dislocation-cell structure in such steels (with a low stacking fault energy) is discussed. A nucleation process of α-martensite is discussed with respect to strain, strain rate, local heating and mechanical energy accumulated/applied to the surface layer under conditions of UIP and the LSP and compared to literature data for different loading schemes.     

IMPROVING CONTACT FATIGUE STRENGTH OF CARBONITRIDING GEAR STEEL 25MnTiBR WITH RETAINED AUSTENITE

本文通过对碳氮共渗齿轮材料接触疲劳性能试验和接触表面剥落过程的断裂力学分析及裂纹顶端塑性区的能量计算,研究了表层组织中的残留奥氏体在提高此类材料接触疲劳强度中所起的作用。