Elastic-plastic analysis of hardened layers in rims subjected to repeated rolling contacts

This paper examines the effects of shallow, surface, and subsurface hardened layers on the response of rims to repeated, 2-dimensional (plane strain) rolling contacts. The rolling is simulated by translating a Hertzian pressure distribution across a finite element model of an elastic-plastic half-space. Four cases are examined: (1) a homogeneous rim, (2) and (3) rims with 0.2w-deep and 0.4w-deep (2w is the Hertzian contact width) hardened surface layers, and (4) a rim with a 0.4w-deep subsurface layer. The dimension 2w can be viewed as either the macrocontact width or the microasperity contact width. The calculations treat elastic-perfectly plastic, cycle and amplitude independent, Von Mises material behavior with the yield strength of the hardened layers twice the value of the surrounding material. The effects of pure rolling at a peak contact pressure-to-shear yield strength ratiop ₀/k = 5 are examined. The calculations describe the effects of the layers on the displacements of the rim surface, the extent of the plastic zone, the residual stresses, and incremental plastic strains. The results indicate that the response of the material at different depths is weakly coupled. Cyclic plasticity is eliminated in the hardened layers, but is not substantially altered in the adjacent material. The hardened layer must occupy a large part of the respective active plastic zones of the macrocontact and microasperity contact to prevent continuing cyclic plastic deformation in the two regions.     

Influence of plastic deformation upon the half-width of engineering metallic materials in hard state

The half-width values of the X-ray diffraction profiles are frequently used to characterize the static strength of a strengthened surface, or the depth distribution of this mechanical parameter, in a strengthened surface layer, especially in a shot-peening affected layer. However, for the unpeened surface and the base material of the shot-peened specimen of an alloy steel treated in hard state, the experimental results shown in this article indicate that uniaxial tensile or compressive plastic deformation increases the yield strengths while it decreases the half-width values. The half-width values of the shot-peened surface and surface layer greatly decrease, whereas the yield strength of this surface remarkably increases. Accordingly, in the authors’ opinion, the half-width values could not correctly describe the static strengths of hard metallic materials, and, contrary to the viewpoint put forward by a lot of researchers, the shot-peened surfaces of such materials are work hardened instead of work softened. A model demonstrating that plastic deformation reduces the half-width values by decreasing the second kind internal stresses is developed.     

Correlation of microstructure with hardness and wear resistance of VC/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

Correlation of microstructure with hardness and wear resistance of VC/carbon steel surface-alloyed materials fabricated by high-energy electron-beam irradiation was investigated. The mixtures of VC powders and flux (50 pct MgO-50 pct CaO or CaF₂) were deposited on a plain carbon steel substrate, and subsequently irradiated using a high-energy electron beam. The surface-alloyed layers of 1.2 to 3 mm in thickness were homogeneously formed without defects, and contained a large amount (about 10 vol pct) of VC precipitates in the bainitic or martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface-alloyed layers improved hardness and wear resistance. Particularly in the surface-alloyed material fabricated with the lower input energy density, the wear resistance was greatly enhanced over the steel substrate because of the increased size and volume fraction of VC particles, although the thickness of the surface-alloyed layer decreased. Microstructural modifications including melting, solidification, precipitation, and phase transformation of the surface-alloyed layer were also predicted from a thermal transfer modeling and a Fe-V-C ternary phase diagram. The predicted results were found consistent with those data from actual electron-beam irradiation and microstructural analysis.     

Correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

The correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. The mixtures of TiC, SiC, or TiC + SiC powders and CaF₂ flux were placed on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures using an electron-beam accelerator. The surface composite layers of 1.2 to 2.1 mm in thickness were formed without defects and contained a large amount (up to 66 vol pct) of precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates and a hardened matrix in the surface composite layer, improved the hardness and wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate, because 66 vol pct of TiC and Ti₅Si₃ was precipitated homogeneously in the hardened martensitic matrix. These findings suggested that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved hardness and wear properties.     

Cracking resistance and strength of carbide steels

Conclusions The carbide steels with binders of austenitic or austenitic-martensitic steels have high cracking resistance which is not inferior to that of the tungsten carbide hard alloys (at the same volume content of the metallic phase). This can be explained by high ductility of the binder of these materials.There is a relationship between cracking resistance and certain properties of the carbide steels, such as hardness, proof stress, and the limiting plastic strain in compression: a reduction of HV, _0.1, and an increase of ₁ are accompanied by an increase of K_1c. Consequently, it is possible to evaluate the cracking resistance on the basis of these available and relatively easy to determine mechanical properties.Translated from Poroshhkovaya Metallurgiya, No. 1(325), pp. 90–94, January, 1990.     

Effect of intense thermomechanical treatment on the structure and properties of titanium pseudo-α alloys during electromechanical treatment

The possible control of the surface structure of a titanium pseudo-α alloy using surface plastic deformation with simultaneous heating of the contact zone by an electric current passing in the tool-part pair is investigated. The electric current is the main parameter to control the process. The optimum electric current responsible for the maximum microhardness and structural features of the hardened layer is established.     

Fracture toughness and fatigue crack growth in rapidly quenched Nb-Cr-Ti In situ composites

In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcing intermetallic phase. Two-phase microstructures with small Cr₂Nb particles in a Nb(Cr, Ti) solid solution alloy matrix were produced for several compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of these composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing volume of Cr₂Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conventionally processed composites with larger particles of the intermetallic compound. The fatigue crack growth rate curves exhibit steep slopes and a low critical stress intensity factor at fracture. The lack of fracture and fatigue resistance is attributed to the contiguity of the intermetallic particles and the absence of plastic flow in the Nb solid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constraint caused by the hard Cr₂Nb particles.     

Formation of electrospark coatings of the VK8 hard alloy with the Al2O3 additive

The results of studying the formation of coatings by electrospark alloying with the use of an electrode material based on the VK8 tungsten-containing alloy with the addition of an aluminum oxide nanopowder are presented. It is shown that introducing Al₂O₃ into the hard alloy promotes an increase in the mass transfer coefficient, microhardness, and wear resistance of the formed coatings. The duration of electric discharges exerts a significant effect on the phase composition of the alloyed layer.     

Effect of Specific Energy Input on Microstructure and Mechanical Properties of Nickel-Base Intermetallic Alloy Deposited by Laser Cladding

This article describes the microstructural features and mechanical properties of nickel-base intermetallic alloy laser-clad layers on stainless steel-316?L substrate, with specific attention on the effect of laser-specific energy input (defined as the energy required per unit of the clad mass, kJ/g) on the microstructure and properties of the clad layer, keeping the other laser-cladding parameters same. Defect-free clad layers were observed, in which various solidified zones could be distinguished: planar crystallization near the substrate/clad interface, followed by cellular and dendritic morphology towards the surface of the clad layer. The clad layers were characterized by the presence of a hard molybdenum-rich hexagonal close-packed (hcp) intermetallic Laves phase dispersed in a relatively softer face-centered cubic (fcc) gamma solid solution or a fine lamellar eutectic phase mixture of an intermetallic Laves phase and gamma solid solution. The microstructure and properties of the clad layers showed a strong correlation with the laser-specific energy input. As the specific energy input increased, the dilution of the clad layer increased and the microstructure changed from a hypereutectic structure (with a compact dispersion of characteristic primary hard intermetallic Laves phase in eutectic phase mixture) to near eutectic or hypoeutectic structure (with reduced fraction of primary hard intermetallic Laves phase) with a corresponding decrease in the clad layer hardness.     

Effect of environment and grain size on cyclic deformation and surface hardening of iron

Specimens of commercially pure vacuum-melted iron with different grain sizes (0.02 to 0.9 mm diameters) were subjected to push-pull cyclic loading under plastic strain control at an amplitude of 5 × 10^-4 in both ultrahigh vacuum (UHV) and oxygen to investigate the effects of grain size and environment on cyclic surface deformation and microcrack initiation. The cyclic stress response curve is not affected by the environment. Fatigue in UHV produces diffuse, wavy but discrete slip lines in which tiny microcracks are initiated. In the presence of oxygen, fatigue produces prominent slip bands along which intense slip band cracks develop. Slip lines developed in UHV, although weaker in intensity, cover more surface area than those developed in oxygen. With increasing grain size, initiation of fatigue cracks along grain boundaries becomes more important. Surface hardening of the specimens and the depth of the surface hardened layer were examined by microhardness measurements after cyclic deformation. In both environments, the surface layer shows an enhanced microhardness over that in the bulk, and the depth of the hardened surface layer increases with grain size. The depth profile of microhardness is not significantly affected by the environment. formerly Graduate Research Assistant at Northwestern Uni-versity,     

Features of structure formation of the surface layer in the course of electroexplosive alloying tungsten carbide hard alloy

The surface of the VK10KS hard alloy is hardened to 25 GPa; it is affected by pulsed plasma jets formed by the electric explosion of coal-graphite fibers and aluminum or titanium foil. It is established that intense hardening is inherent in alloys after electroexplosive alloying with titanium due to the formation of TiC and (Ti, W)C carbides in the surface layer.     

Correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

This study is concerned with the correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation. The mixtures of TiC, SiC, Ti + SiC, or TiC+SiC powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures. The surface composite layers of 1.2 to 2.1 mm in thickness were homogeneously formed without defects and contained a large amount (30 to 66 vol pct) of hard precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates in the surface composite layer, improved the hardness and abrasive wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the abrasive wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate because of the precipitation of 66 vol pct of TiC and Ti₅Si₃ in the hardened martensitic matrix. During the sliding wear process, hard and coarse TiC and Ti₅Si₃ precipitates fell off from the matrix, and their wear debris worked as abrasive particles, thereby reducing the sliding wear resistance. On the other hand, needle-shaped Ti₅Si₃ particles, which did not play a significant role in enhancing abrasive wear resistance, lowered the friction coefficient and, accordingly, decelerated the sliding wear, because they played more of the role of solid lubricants than as abrasive particles after they fell off from the matrix. These findings indicated that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved abrasive and sliding wear resistance, although the abrasive and sliding-wear data should be interpreted by different wear mechanisms.     

Structure of the surface layer of a hard-metal cold-upsetting tool

Conclusions In the structure of the WC and Co phases making up VK20K hard metal the presence was discovered, in the as-sintered condition, of dislocations and twins, which may be attributed to the generation, during the postsintering cooling of the alloy, of large thermal stresses as a result of the difference in the coefficients of thermal expansion of its tungsten carbide grains and metallic cobalt interlayers. Operation increased the number of dislocations and twins and brought about fragmentation of twins in the Co and WC phases, which is evidence that their wear was preceded by appreciable plastic deformation. In the structure of VK20K alloy in the as-sintered condition and in its surface layer after wear the carbide W₂C was found on the surface of the carbide WC between the WC grains and Co-phase interlayers, and it is therefore reasonable to assume the existence under such conditions of an extremely thin transition layer of virtually pure tungsten.Translated from Poroshkovaya Metallurgiya, No. 12(216), pp. 83–88, December, 1980.     

Role of the phase composition of tungsten containing hard alloys in the formation of a layer of titanium carbide in gas titanizing

Conclusions In gas titanizing of tungsten-containing hard alloys a continuous layer of titanium carbide forms on their surface; its composition, thickness, and microhardness depend on the phase composition of the alloy, in particular on the amount of cobalt phase.When hard alloys contain the phase (Ti, W)C, the thickness of the layer of titanium carbide increases.We discovered selective solubility of the solid solution (Ti, W)C in the cobalt phase in proportion to its decarburization during titanizing; this leads to the formation of ring structure of the grains of (Ti, W)C with higher titanium content on the periphery which, as is known [9], increases the wear resistance of hard alloys.Translated from Poroshkovaya Metallurgiya No. 7(319), pp. 74–79, July, 1989.     

Importance of slip mode for dispersion-hardened β-titanium alloys

The mechanical properties of Ti-7 Mo-7 Al and Ti-7 Mo-16 Al (in at. pct) were correlated to the microstructure. The mechanical properties of the alloy with low aluminum content, consisting of α+ β phases, were dependent on the size of the α particles. Although the α phase is softer than the β phase, the small α particles, upon plastic deformation of the alloy, functioned as typical hard agents in a dispersion-hardened system and the volume fraction of the particles controlled the macroscopic ductility. A rapid strain-hardening behavior of the small α particles seemed to be responsible for this effect. Large α particles behaved like soft, incoherent particles, the volume fraction having little effect on the inherent ductility of the alloy. The two phase (β+ Ti₃Al) microstructure of the alloy with high aluminum content resulting from high temperature aging to 900°C exhibited a yield stress of 130 ksi and an elongation to fracture of 5 pct. The ductility of this microstructure was controlled by the volume fraction of the Ti₃Al particles inducing homogeneous slip. The favorable ductility properties of the microstructures with low Ti₃Al volume fraction were lost if the slip mode was changed from homogeneous slip to planar slip. Formerly Staff Member, Materials Research Center, Allied Chemical Corp., Morristown, N. J.     

Surface-alloy formation in film–substrate melting by intense pulsed electron beam. Part 2

The elemental and phase composition and defect structure of the surface layer on 5140 steel is considered after alloying by melting of a film–substrate (5140 steel) system under irradiation by intense pulsed electron beam; the film consists of aluminum or titanium. Treatment of the titanium–5140 steel system results in alloying to the depth of the molten layer (about 15 μm); polycrystalline structure (with submicron grains) based on α-phase hardened by titanium-carbide nanoparticles is formed. For the aluminum–5140 steel system, only a thin surface layer (about 2 μm) is alloyed, on account of the evaporation of aluminum from the steel surface. Martensit structure hardened by iron-aluminide nanoparticles is formed.     

磁悬浮列车闸片用硬质涂层的相结构及其性能研究

为改善磁悬浮列车闸片的制动性能,延长使用寿命同时降低生产成本。采用超音速喷涂工艺,在磁浮列车制动用闸片基体表面分别制备了WC-12Co和Cr_3C_2-25NiCr两种硬质涂层。硬质涂层的性能不仅与硬质相晶粒的大小有关,而且还与相组成及相转变密切相关。本研究利用扫描电子显微镜(SEM)、氧氮分析仪、万能试验机和显微硬度计等测试表征方法,分别对涂层的显微组织、氧化物含量、结合强度和显微硬度等结构和力学性能进行分析探究。从热力学的角度出发,对涂层中碳化物的生长条件及相变过程进行了讨论。结果显示,在本工艺参数条件下,超音速喷涂制备的WC-12Co和Cr_3C_2--25NiCr硬质涂层显微组织致密,孔隙率小于1%,涂层中的氧含量分别为0.78%和1.25%,平均结合强度分别为75.2 MPa和73.8 MPa,平均显微硬度分别为1355.8HV_1和1160.5HV_1。因此,通过材料设计的热力学分析,优化喷涂工艺,有利于高性能硬质陶瓷涂层的研制。     

Penetration of molten nickel into sintered titanium carbide hard metals

Conclusions A study was made of some aspects of molten nickel penetration into titanium carbide hard metals. It is shown that, when such a liquid phase migrates through a semiinfinite rod, the variation of the concentration of the binder metal along the length of the rod may be described by a hyperbolic function. The dependence of the thickness of the layer of increased nickel content on time of contact between the specimen and the melt was determined. It is demonstrated that, by employing the method of infiltration of a hard metal with a liquid metal, it is possible to produce parts possessing varying physicomechanical properties.Translated from Poroshkovaya Metallurgiya, No. 11(215), pp. 25–38, November, 1980.     

Fine structure of interphase boundaries in hard alloys of the chromium carbide–titanium system

The structure of interphase boundaries in hard alloys fabricated by the explosive compacting of the powder mixtures of chromium carbide (Cr₃C₂) and titanium is investigated. It is established by electron microscopy that similar boundaries have thicknesses of about 100 nm, over the extension of which a smooth variation in chromium and titanium contents is observed at the almost identical carbon concentration. The boundary structure is nonuniform over the thickness, notably, a layer with a thickness of 5–7 nm and an amorphous structure is revealed between two crystalline interlayers. It is shown that the revealed layers are the layers of specific “boundary phases” not corresponding to any phase of the equilibrium phase diagram of the Cr–C–Ti system.