Research on the dissolution behavior of cementite in the GCr15 steel surface layer induced by surface nanocrystallization

Dissolution of cementite was found in the surface layer of 1.0C-1.5Cr steel plates during the process of surface mechanical attrition treatment(SMAT),and its evolution was characterized by transmission electron microscope(TEM),three-dimensional atom probe(3DAP)and Mssbauer spectroscopy.The average grain size contained in the top surface of SMAT specimen was 10nm,and no diffraction ring corresponding to cementite grain was identified in the selected area election diffraction(SAED)pattern,which indicated the disappearance of cementite.3DAP analysis showed the average carbon concentration in ferrite(0.75 at%)after SMAT,which was almost 100 times higher than that in matrix(0.008 at%),which suggested cementite dissolve in the process of SMAT.The results of Mssbauer spectroscopy indicated that partial cementite dissolved in the process of SMAT,the saturation of cementite dissolution is about 47%.Evolution of cementite involved three sub-stages:①inoculation stage,in the first 5 min of treated duration,cementite fraction is reduced only by 0.4%;②dissolution stage,within the following 25 min cementite fraction significantly is reduced from 14.6% to 8.4%;③saturation stage,when treatment exceeds 30 min,the fraction of cementite nearly remains the same.     

表面纳米化诱导GCr15钢渗碳体溶解行为研究

高能机械球磨表面纳米化过程中发现表面纳米晶层内渗碳体发生溶解现象,采用TEM、三维原子探针和穆斯堡尔谱对渗碳体的溶解进行了表征。TEM观察结果表明,在纳米化的最表层平均晶粒为10 nm,在选区电子衍射花样中没有发现渗碳体的衍射环,表明渗碳体可能发生溶解。三维原子探针分析表明,表面纳米化处理后,铁素体中的碳含量为0.75at%,是基体铁素体碳含量的100倍,表明表面纳米化过程中渗碳体发生了溶解,通过穆斯堡尔谱分析结果表明,渗碳体的溶解量约为47%。表面纳米化过程中渗碳体的溶解可分为三个阶段：①诱导阶段,在表面机械研磨的最初5 min,渗碳体的溶解分数仅为0.4%;②溶解阶段,在表面纳米化处理的5-30 min,渗碳体大量溶解,其体积分数由14.6%降低到8.4%;③饱和阶段,当处理时间达到30 min以上,渗碳体的体积分数基本保持不变。     

表面机械研磨纳米化GCr15钢摩擦磨损性能研究

利用表面机械研磨处理（SMAT）技术对GCr15钢制备了纳米结构表层。通过X射线衍射及透射电镜分析了其组织结构,利用UMT—2M型摩擦磨损试验机测试了GCr15钢处理前后的摩擦磨损性能。分析结果表明,试验中处理时间为15 min是提高其摩擦磨损性能的最佳工艺条件。磨痕形貌的扫描电镜观察表明,随着磨损深度的增加,主导磨损机制由黏着磨损转变为磨粒磨损。     

表面纳米化对低碳钢在干摩擦条件下摩擦磨损性能的影响

采用表面机械研磨的方法对低碳钢板材进行表面处理.经X射线衍射及透射电镜分析表明处理后的试样形成了一定厚度的具有纳米晶粒结构的表层.用立式万能摩擦试验机研究处理后试样在干摩擦条件下的摩擦磨损性能,通过分析其磨损表面形貌,探讨表面纳米化对低碳钢磨损行为的影响.结果表明在载荷为15～75N条件下,其摩擦系数较未处理样品明显降...     

氧化膜对14Cr12Ni2WMoVNb钢QPQ渗层的室温摩擦磨损和腐蚀性能的影响

为了研究14Cr12Ni2WMoVNb钢QPQ(淬火-抛光-淬火)处理后的氧化膜对渗层室温摩擦磨损和腐蚀性能的影响,利用金相、X射线衍射分析、扫描电镜、能谱分析、划痕仪、摩擦磨损试验机和电化学工作站对试样进行了表征.结果表明:氧化膜对渗层室温摩擦学性能的影响与载荷大小有关.在摩擦时间均为4 min情况下,载荷较小(50 N)时,氧化膜可以降低摩擦系数和体积磨损率;载荷较大(100 N)时,氧化膜被破坏无法降低体积磨损率.氧化膜可明显提高渗层的耐腐蚀性能.含氧化膜试样的极化曲线有明显的钝化区,点蚀电位为-13 mV,去除氧化膜试样在盐雾腐蚀12 h后表面有大范围的腐蚀区域,而含氧化膜试样盐雾腐蚀48 h后才有大区域腐蚀发生.      

冷轧IF钢表面纳米化后的组织与性能

利用机械研磨处理（SMAT）在冷轧IF钢表面制备出具有纳米晶体结构特征的表面层．利用金相观察和透射电镜分析研究了表面纳米层的结构特征，测定了硬度沿厚度的变化，并分析了表面纳米化（SNC）对力学性能的影响．结果表明：（1）表面机械研磨处理后，冷轧（CR）IF钢表面形成了平均晶粒尺寸为7～9nm、晶粒取向呈随机分布的纳米晶组织；（2）表面层附近的硬度显著提高；（3）纳米表层的存在使得材料强化的同时成形极限也得到提高．     

Microstructural Evolution of Surface Layer of TWIP Steel Deformed by Mechanical Attrition Treatment

 A nanocrystalline layer was synthesized on the surface of TWIP steel samples by surface mechanical attrition treatment (SMAT) under varying durations. Microhardness variation was examined along the depth of the deformation layer. Microstructural characteristics of the surface at the TWIP steel SMATed for 90 min were observed and analyzed by optical microscope, X-ray diffraction, transmission and high-resolution electron microscope. The results show that the orientation of austenite grains weakens, and α-martensite transformation occurs during SMAT. During the process of SMAT, the deformation twins generate and divide the austenite grains firstly; then α-martensite transformation occurs beside and between the twin bundles; after that the martensite and austenite grains rotate to accommodate deformation, and the orientations of martensite and between martensite and residual austenite increase; lastly the randomly oriented and uniform-sized nanocrystalline layers are formed under continuous deformation.     

0.9C-9Mn-2Cr-Mo中锰耐磨钢的冲击滚动复合磨损性能

通过M2000多功能摩擦磨损试验机研究0. 9C-9Mn-2Cr-Mo中锰钢和Hardax400(0. 22C-1.6Mn-1.4Cr-Mo)以及Hardex500(0.27C-1.0Mn-0.94Cr-Mo)耐磨钢的冲击和滚动复合摩擦磨损性能,并利用XRD、SEM和TEM等分析了组织转变及磨损机理。实验结果表明,热轧中锰钢比Hardox马氏体耐磨钢表现出更好的抗冲滚磨料磨损性能。中锰钢冲滚磨损表面存在厚度达1000μm的硬化层,最高显微硬度达HV490,洛氏硬度达HRC53。中锰钢磨损机制以凿削破坏为主,伴随局部的疲劳剥落破坏;位错强化、形变孪晶和马氏体相变是中锰钢硬化和抗磨损性能改善的主要原因。     

铝合金表面纳米化——微弧氧化复合涂层摩擦行为

通过表面机械研磨处理在LY12CZ铝合金表面制备表面纳米化(SNC)过渡层,再采用微弧氧化(MAO)技术对纳米晶过渡层进行微结构重构,设计制备出纳米化-微弧氧化(SNC-MAO)复合涂层,并对比研究了铝合金表面微弧氧化涂层及纳米化-微弧氧化复合涂层的摩擦学行为.与微弧氧化涂层相比,纳米化-微弧氧化复合涂层因硬度较高而具有较好的耐磨性.微弧氧化涂层及纳米化-微弧氧化复合涂层与GCr15钢球对磨时具有相同的磨损机理,为对磨钢球向涂层的材料转移和氧化磨损.      

Long-term operation surface changes in differentially quenched 100-m rails

By optical microscopy and transmission electron diffraction microscopy, the evolution of the structural and phase states in the surface layers over a depth of 10 mm in the head of differentially quenched rail (category DT350) is studied, as the rail is subjected to passed tonnage of 691.8 million t at the experimental loop of AO VNIIZhT. In the initial state, the following structural components are present in the rail head: plate-pearlite grains (relative content 0.7); mixed ferrite–carbide grains (0.25); and grains of structure-free ferrite. After experiencing a passed tonnage of 691.8 million t, this state only remains beyond a depth of 10 mm. At that depth, a large quantity of bend extinction contours is observed. That indicates elastoplastic distortion of the material’s crystal lattice. The stress concentrators in the steel are intraphase and interphase boundaries of the ferrite and pearlite grains, cementite and ferrite plates in pearlite colonies, and globular cementite and ferrite particles. Structural transformations are observed at the macro level: microcracks appear, running at acute angles from the surface to a depth of 140 μm; and a decarburized layer is formed. At the micro level, elastoplastic stress fields are formed, and the cementite plates in the pearlite colonies break down. The stress concentrators in that case are intraphase and interphase boundaries of the ferrite and pearlite grains, cementite and ferrite plates in pearlite colonies, and globular cementite and ferrite particles. In structure-free ferrite grains, cementite nanoparticles are formed. The results are compared with the evolution of the structural and phase states at the surface of a recess in bulk-quenched rail as the rail is subjected to gross loads of 500 million t: the transformation of the structural and phase states in the surface layers is more pronounced. Plate pearlite is characterized by solution of the cementite plates. That leads to the formation of chains of globular carbide particles at the sites of the cementite plates. This may be associated with transfer of the carbon atoms from the cementite lattice to dislocations.     

温轧表面纳米晶化304不锈钢钢板的多尺度晶粒组织

采用表面机械研磨处理后进行温轧的工艺,制备超细晶粒结构的304不锈钢钢板;采用XRD、TEM分析表征材料的微观组织结构;测试材料的维氏硬度。试验结果表明,所获得的材料晶粒尺寸呈梯度分布,从表层的纳米晶层逐渐过渡为心部的微米晶层,其中微米晶层的体积分数约为40%;同时,此多尺度晶粒分布的304SS板由奥氏体和马氏体双相组成,并同样呈梯度分布,具体为马氏体的体积分数从表层到心部逐渐减少,而奥氏体则反之;所制备的钢板表层维氏硬度显著提高,与原始未处理的304SS相比提高了150%;硬度的大幅度提高源于晶粒细化和应变诱导马氏体相变;维氏硬度分布与微观组织变化一致,表明此样品实现了高强高延性的良好结合。     

细化渗碳体对高碳纳米贝氏体轴承钢的影响

研究了GCr15Si1Mo轴承钢中渗碳体细化后,对材料微观组织、常规力学性能、耐磨性及滚动接触疲劳性能的影响。通过工艺的调控,使材料组织中的渗碳体尺寸从0.49细化到了0.20 μm,贝氏体铁素体板条尺寸从66细化至41 nm,并且渗碳体的分布密度也随之提高。随后通过SEM、TEM、XRD、硬度、冲击、摩擦磨损及滚动接触疲劳等试验,得到了材料的宏观性能与微观组织。研究结果表明,细化渗碳体后,材料的耐磨性能和滚动接触疲劳性能比常规工艺的优异,但是韧性相对于常规工艺有显著降低。研究证明了较细小渗碳体可以对轴承钢的组织结构、常规力学性能、耐磨性能及疲劳性能产生一定的影响,为后期轴承钢中渗碳体的调控研究提供支持。     

Wear 110Γ13 of steel with ultrasound-induced nanostructuring of the surface layer part 2. Structural research

The structural modification on friction with boundary lubrication is considered for 110Γ13 steel with and without impact ultrasound treatment. The change in fine structure of the surface and near-surface layers is analyzed on the basis of transmission electron microscopy. The differences in wear of the treated and untreated samples are interpreted.     

Nanocrystallization and magnetic properties of Fe-30 weight percent Ni alloy by surface mechanical attrition treatment

A nanostructured surface layer was formed in Fe-30 wt pct Ni alloy by surface mechanical attrition treatment (SMAT). The microstructure of the surface layer after SMAT was investigated using optical microscopy, X-ray diffraction, and transmission electron microscopy. The analysis shows that the nanocrystallization process at the surface layer starts from dislocation tangles, dislocation cells, and subgrains to highly misoriented grains in both original austenite and martensite phases induced by strain from SMAT. The magnetic properties were measured for SMAT Fe-30 wt pct Ni alloy. The saturation magnetization (M _s ) and coercivity (H _c ) of the nanostructured surface layers increase significantly compared to the coarse grains sample prior to SMAT. The increase of M _s for SMAT Fe-30 wt pct Ni alloy was attributed to the change of lattice structure resulting from strain-induced martensitic transformation. Meanwhile, H _c was further increased from residual microstress and superfined grains. These were verified by experiments on SMAT pure Ni and Co metal as well as liquid nitrogen-quenched Fe-30 wt pct Ni alloy.     

Influence of Surface Mechanical Attrition Treatment Duration on Fatigue Lives of Ti–6Al–4V

This work deals with the influence of surface mechanical attrition treatment (SMAT) duration on fatigue lives of Ti–6Al–4V. The SMAT process was carried out in vacuum with SAE 52100 steel balls of 5 mm diameter for 30 and 60 min at a vibrating frequency of 50 Hz. SMAT treated surface was characterized by electron microscopy. Surface roughness, nano-indentation hardness, residual stress, and tensile properties of the material in both SMAT treated and untreated conditions were determined. SMAT enabled surface nanocrystallization, increased surface roughness, surface hardness, compressive residual stress and tensile strength but reduced ductility. Samples treated for 30 min exhibited superior fatigue lives owing to positive influence of nanostructured surface layer, compressive residual stress and work hardened layer. However, fatigue lives of the samples treated for 60 min were inferior to those of untreated samples due to presence of microdamages or cracks induced by the impacting balls during the treatment.     

Surface nanocrystallization in AISI 52100 steel induced by surface mechanical grinding treatment(SMGT)

Surface nanocrystallization(SNC) has proved to be an effective approach to improve the overall properties of bulk metallic materials.Recently,a new surface nanocrystallization technique,i.e.,surface mechanical grinding treatment(SMGT),was developed.In this work,a gradient nano-micro structure was achieved in the surface layer of the AISI 52100 steel by using SMGT.We obtained a minimum grain size of about 7nm in the top surface layer.The total thickness of the deformed layer is over 200 micrometer.Meanwhile the surface roughness is rather low. Ferrite grains were deformed to different extents varying with depth from the top surface.Gradient grain sizes were formed from top surface to deep matrix which offered a great opportunity to study the refinement process of the ferrite grains.It is found that dislocation activities play a dominant role in the process.At the initiate stage, dislocations accumulated and interacted to form dense dislocation walls and cells.Increasing strain and strain rate induced more dislocation walls in cells,forming finer cells.This procedure continued until nanograins formed at the top most surface. The existence of cementite particles in ferrite matrix greatly facilitates the ferrite refinement process.Boundaries between ferrites and cementites offered many dislocation sources which accelerate the propagation of dislocations. Dislocation walls were blocked by cementites which certainly lead to finer dislocation cells.The existence of cementites makes it easier to generate fresh dislocation walls in sub-micron grains.A strain gradient was formed from a cementite particle to surrounding ferrite grains.This strain gradient gives rise to more geometric necessary dislocations. As ferrite grain size decreased less than that of cementite particles,fragmentation occurred in cementites.Hard second phase was usually considered as brittle.In this work,evidences of deformation(traces of dislocation activities) in cementites were distinct.Since the stress concentration in the phase boundary(especially triple junction) excesses the shear modulus of cementite,dislocation emission was triggered.It is found in this work that dislocations tend to slip along parallel planes,possibly on(001),(01 0),(100),(110),(10 1 ) and(011) planes,depending upon as the load directions.     

Formation of internal stress fields in rails during long-term operation

The structure and the internal stress fields in R65 rails withdrawn from operation because of side wear after long-term operation are studied and estimated. A high scalar dislocation density (higher by a factor of 1.5–2), the fragmentation of cementite lamellae, and the precipitation of carbide particles are detected in the layers adjacent to the roll surface. The stresses at the boundaries of the particles with the ferrite matrix can exceed the ultimate strength of the steel.     

Effect of Surface Mechanical Attrition Treatment on Fatigue Lives of Alloy 718

The effect of surface mechanical attrition treatment (SMAT) and its duration on fatigue lives of alloy 718 has been studied. The SMAT process was carried out in vacuum (?0.1?MPa) with SAE 52100 steel balls of 5?mm diameter for 30 and 60?min at a vibrating frequency of 50?Hz. SMAT resulted in surface nanocrystallization, higher surface roughness, higher surface hardness, higher compressive residual stress, higher tensile strength, reduced ductility and superior fatigue lives. The enhancement in the fatigue lives of treated samples can be attributed to the positive influence of nanostructured surface layer, compressive residual stress and work hardened layer, which surpassed the negative effect of increased surface roughness. There was no significant difference between the fatigue lives of samples treated for 30 and 60?min.     

Laser transformation hardening of iron-carbon and iron- carbon- chromium steels

The laser transformation hardening response of Fe-0.5C-0.8Mn and Fe-0.5C-0.8Mn-0.8Cr steels was examined. A 2 kW CO₂ laser was used to scan the steel surfaces at various rates. Complete transformation of pearlite to austenite, and hence to martensite, occurred in the laser heated surface layer of the Fe-C-Mn steel. During equivalent heat treatment of the Fe-C-Mn-Cr steel, incomplete austenitization of the pearlite colonies left the cementite plates largely undissolved. However, the maximum surface hardness was approximately the same for both alloys. Comparison of calculated and measured hardened depths yielded values of the effective coupling coefficient of the laser beam to the steel which varied as a function of beam interaction time. Modeling the process allowed a dis-tinction to be made between the effects of alloying elements and of pearlite spacing upon the depth of complete austenitization. In this case, the effect of the difference in pearlite spacing between the two steels was negligible. In the alloy steel, Cr and Mn were strongly partitioned to the cementite before heat treatment, and remained so after laser processing. Incomplete austenitization of that steel is attributed to partitioning of alloying elements to the cementite and their retarding influence on the diffusion controlled dissolution kinetics of the alloyed carbide.     

17-4PH马氏体沉淀硬化不锈钢表面防护技术研究进展

17-4PH马氏体沉淀硬化不锈钢具有良好抗衰减性能、抗腐蚀和抗空蚀性能以及优异的力学性能,在核能、海洋和航天等工业领域有广泛应用。由于长期服役于高温、腐蚀、冲击和摩擦的复杂环境中,17-4PH零部件表面受到水蚀、空蚀、腐蚀和摩擦磨损等严重破坏,亟需开展针对性防护研究。为此综述了目前17-4PH不锈钢表面防护的研究进展,主要从表面强化、表面改性和表面涂敷3个角度分析其优缺点、应用现状,并对其未来发展趋势进行展望。