Surface hardening of a ductile-cast iron roll using high-energy electron beams

The effects of high-energy electron beam irradiation on surface hardening and microstructural modification in a ductile cast iron (DCI) roll are investigated in this study. The DCI roll samples were irradiated by using an electron accelerator (1.4 MeV), and then their microstructures and hardnesses were examined. Upon irradiation, the unirradiated microstructure containing graphites and the tempered bainite matrix was changed to martensite, ledeburite, and retained austenite, together with the complete or partial dissolution of graphites. This microstructural modification improved greatly the surface hardness due to transformation of martensite whose amount and type were determined by heat input during irradiation. In order to investigate these complex microstructures, a simulation test including thermal cycles of abrupt heating and quenching was carried out. The simulation results indicated that the irradiated surface was heated up to about 1100 °C to 1200 °C and then quenched to room temperature, which was enough to obtain surface hardening through martensitic transformation. Thermal analysis of the irradiated surface layer was also carried out using a finite difference method to understand the surface hardening of the DCI roll and to compare with the simulation test results.     

Microstructural modification of plain carbon steels irradiated by high-energy electron beam

The main objective of the present study is to analyze the microstructural modification of the surface hardened by the irradiation of high-energy electron beam in 0.18 pct C and 0.38 pct C plain carbon steels. Steel samples were irradiated using an electron accelerator (1.4 MeV), and the detailed microstructures of the irradiated surface were examined. Upon irradiation, the ferrite-pearlite structure near the sample surface was changed to the dual-phase structure, i.e., ferrite and martensite, and fine particles or needlelike lamellae were observed in the ferrite/martensite interface. In order to investigate these complex microstructures as well as the martensitic transformation mechanism, the simulation test, including thermal cycles of abrupt heating and quenching, was carried out. The test results indicated that the irradiated surface was heated up to about 1100°C and then quenched to room temperature, which was enough to obtain the surface hardening through martensitic transformation. Thermal analysis of the irradiated surface was also carried out for systematic understanding of the microstructural modification in terms of the irradiation parameters such as beam travel speed.     

Influence of Subsurface Structure on the Linear Reciprocating Sliding Wear Behavior of Steels with Different Microstructures

The present work investigates the influence of subsurface microstructure on the linear reciprocating sliding wear behavior of a number of steels with ferrite-pearlitic, pearlitic, bainitic, and martensitic microstructures under dry unlubricated condition. The change in the underlying microstructure with depth from worn-out surface of steel sample intimately relates to the associated hardness variation and wear volume. The present paper is not about comparison of wear resistance of steels with different structures; rather it is on mutual influence of wear and substructure for individual microstructure. Inherent toughness of the matrix and ability of microstructural components to get deformed under the reciprocating action of the ball decide the wear resistance of the steels. Bainite has good amount of stability to plastic deformation. Ferrite shows severe banding due to wear action. Work hardening renders pearlite to be wear resistant. Temperature rise and associated tempering of martensite are observed during wear.     

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.     

Microstructure of TiB2/carbon steel surface-alloyed materials fabricated by high-energy electron beam irradiation

The processing and the microstructural analysis of TiB₂/carbon steel surface-alloyed materials using the irradiation of a high-energy electron beam were investigated in this study. The mixtures of TiB₂ powders and flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The microstructure of the irradiated surface layer was composed of a melted region, an interfacial region, a coarse-grained heat-affected zone (HAZ), and a fine-grained HAZ. A few residual micropores were found in the melted region of the specimen processed without flux because of irregular thermal transfer, but their number was decreased in the specimens processed with a considerable amount of flux. As a result of irradiation, the Ti content was homogeneously maintained throughout the melted region, whose hardness was greatly improved. This was associated with the microstructural modification including the segregation of Ti and B along solidification cell boundaries and the formation of fine Ti(C, N) particles. The proper flux mix ratio was 15 to 30 pct to obtain excellent surface alloying and a homogeneous microstructure.     

The effect of high-energy electron-beam irradiation on microstructural modification of a high-speed steel roll

The purpose of this study is to investigate the microstructural modification in a high-speed steel (HSS) roll irradiated with an accelerated high-energy electron beam. The HSS roll samples were irradiated at the beam travel speeds of 2.5 to 25 mm/s using an electron accelerator (1.4 MeV). The microstructure was examined with a scanning electron microscope (SEM) capable ofin situ fracture testing and simultaneous measurement of the apparent fracture toughness. Irradiation changed the matrix phase from tempered martensite to a mixture of retained austenite and martensite. Coarse primary carbides were partially or completely dissolved, depending on the heat input. Irradiation greatly improved the fracture properties because of the presence of retained austenite, which could retard crack propagation, although hardness was decreased. Occasional interior quench cracks were found in the heat-affected region. Appropriate processing methods, such as pre- or postirradiation, were suggested. A heat transfer analysis of the irradiated surface layer was also carried out to elucidate the influence of the irradiation parameters on the microstructure.     

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.     

Correlation of microstructure with wear and fracture properties of two-layered VC/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with the hardness, wear resistance, and fracture toughness of two-layered VC/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. A mixture of VC powders and CaF₂ flux was deposited on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these powder mixtures to fabricate an one-layered surface composite. A two-layered surface composite was fabricated by irradiating an electron-beam again onto the powder mixture deposited on the one-layered surface composite. The composite layers of 1.2 to 1.5 mm in thickness were homogeneously formed without defects and contained a large amount (25 to 40 vol pct) of carbides in the martensitic or β-Ti matrix. This microstructural modification, including the formation of hard carbides and hardened matrix, improved the hardness and wear resistance. Particularly in the two-layered surface composite containing more carbides, the wear resistance was greatly enhanced to a level 7 times higher than that of the Ti-6Al-4V substrate. In-situ observation of the fracture process showed that microcracks were initiated at carbides and propagated along these microcracked carbides and that shear bands were formed in the matrix between these microcracks. In the two-layered surface composite, numerous microcracks were initiated at many carbides and then rapidly propagated along them, thereby lowering the fracture toughness.     

Effect of partitioning on microstructure and properties of a medium carbon Ti Mo bearing high strength Q&P steel

In order to investigate the effect of partitioning procedures on microstructure and properties of a medium carbon Ti Mo bearing steel, the salt bath experiment, field emission SEM, TEM and tensile tests as well as wear tests were utilized to clarify the microstructural evolution and property changes treated by different partitioning parameters. The results show that the microstructures consist of tempered lath martensite, cementite and (Ti, Mo)C particles. With prolonging partitioning time or increasing partitioning temperature, the amount of laths martensite decreases and thicker martensite plates with blunt boundaries appear. In addition, with increasing partitioning temperature from 310℃ to 400℃, the tensile strength, hardness and low temperature toughness are simultaneously decreased, and the reduction reaches up to about 300MPa, 100HV and 20J, respectively. Finally, the wear performance of the samples partitioned below Ms is obviously better than that of samples partitioned above Ms. The morphology of worn surface of samples partitioned below Ms is characterized by plastic fatigue morphology, while the worn surfaces of samples partitioned above Ms are mainly dominated by grooves.     

配分工艺对中碳Ti-Mo高强Q&P钢组织和性能的影响

摘要：采用盐浴实验、扫描电镜、透射电镜、拉伸实验和磨损实验等手段，研究了配分工艺对中碳Ti Mo钢组织和性能的影响，分析了不同配分工艺处理下的组织演变和性能变化。结果表明，显微组织主要由回火马氏体、渗碳体、(Ti，Mo)C粒子组成。随着配分时间的延长和配分温度的升高，板条马氏体数量减少，马氏体板条厚度增加，边界钝化。此外，随着配分温度从310℃提高至400℃，抗拉强度、硬度和低温冲击韧性同时下降，分别降低约250MPa、56HV和15J。最后，Ms以下温度配分（310℃）试样的耐磨损性能明显优于Ms以上温度配分（400℃）试样。Ms以下温度配分试样磨损表面形貌以塑性变形为主，Ms以上温度配分试样磨损表面以犁沟为主。     

Effect of Sinter Hardening on Microstructure and Mechanical Properties of Astaloy 85Mo

 Effect of sinter hardening on the microstructure, density, hardness and tensile properties of Astaloy 85Mo+0.7% graphite was investigated. For this purpose, Astaloy 85Mo, a pre-alloyed powder, was mixed with 0.7% UF4 graphite and then pressed in single action die and sintered at 1120 ℃ for 30 min in N2-10%H2 atmosphere. Then samples were cooled from 0.5 to 3 ℃/s sintering temperature in accordance with different cooling rates. The difference in microstructure, hardness, density and tensile properties of the samples associated with different cooling rates from sintering temperature has been investigated. The results show that the microstructure remains bainitic by changing cooling rate, but it becomes finer and then the hardness and tensile strength of the samples will increase by increasing the cooling rate from sintering temperature.     

Comparison of the microstructures and abrasive wear properties of stellite hardfacing alloys deposited by arc welding and laser cladding

The microstructure of cobalt-based hardfacing alloys deposited by manual metal arc (MMA) welding, tungsten inert gas (TIG) welding, and laser cladding has been investigated as part of a study attempting to establish the relationship between microstructure and abrasive wear properties. For typical deposition conditions, the differences in freezing rates associated with the three processes are found to give rise to large differences in microstructure. The MMA process is found to lead to the largest degree of dilution of the hardfacing deposit; the TIG and laser deposits exhibited much lower levels of mixing with the base plate. For the deposition conditions used in this study and for the alloys examined, the scale of the microstructure decreases in the order MMA, TIG, and laser cladding, leading to an increase in the deposit hardness in the same order. It is found that with alumina as an abrasive, the wear rate persistently is higher with the MMA deposits (which have the coarsest microstructure with the lowest starting hardness), the weight loss being approximately linear with time. The laser and TIG deposits, which have more refined microstructures and slightly higher carbon concentrations, both are found to exhibit significantly lower wear rates. Initially, the TIG samples are the most resistant to abrasion, even though their microstructure compares with that of the laser samples; this is a consequence of their higher ductility associated with a lower rate of strain hardening. The laser samples, which contain a lower matrix iron concentration, strain harden more rapidly; consequently, they exhibit an initial decrease in wear rate. With the much harder silicon carbide abrasive, all samples show similar wear rates which do not decrease with time. The wear data are found to correlate with scanning and transmission electron microscopy observations, and it is possible to rationalize the interaction among microstructure, abrasive, and alloy deposition processes.     

Effect of Microstructures on Resistance of a Hot Elevated-Temperature Wear Working Die Steel

 Elevated-temperature wear tests under atmospheric conditions at 400 ℃ were performed for a hot working die steel H21 on a pin-on-disk wear tester. The phase and morphology of worn surfaces were examined using XRD and SEM, and the relation of wear resistance to tempered microstructures was studied for H21 steel. XRD patterns exhibit that oxidative wear is a predominated wear mechanism with Fe3O4 and Fe2O3 on worn surfaces. It is found that with increasing normal load, obvious plastic deformation of substrate appears on worn surfaces. Microstructures start to affect apparently wear resistance of the steel with an increase of load. Under loads of 50-100 N, wear losses of steel retain low values and relatively approach for steels with various microstructures. As loads are increased to 150-200 N, wear losses of steel start to increase obviously and present apparent difference for steel with various microstructures. Wear resistance is found to increase in the sequence as follows: tempered sorbite, tempered martensite, tempered troostite without secondary hardening and tempered troostite with secondary hardening or upcoming one. Higher strength and microstructural stability are required for steels with excellent wear resistance.     

Laser Processing of Cast Iron for Enhanced Erosion Resistance

The surfaces of nodular and gray cast iron specimens have been modified by CO₂ laser processing for enhanced hardness and erosion resistance. Control of the near-surface microstructure was achieved primarily by controlling resolidification of the laser melted layer through variations in laser beam/target interaction time and beam power density. Typical interaction times and power densities used in this study were 5 msec and 500 kW/cm². Analysis of the laser melted surface showed a dramatic increase in hardness and a greatly refined microstructure. Depending on the processing parameters, two basic kinds of microstructure can be produced in the laser hardened layer—a feathery microstructure with a very high hardness (up to 1245 HV) and a dendritic microstructure with a metastable, fully austenitic matrix and a lower hardness (600 to 800 HV). Erosion testing was done in a rotating paddle device using slurries of SiO₂ or SiC in water. Weight loss and crater profile measurements were used to evaluate the erosion characteristics of the various microstructures. Both ductile and gray cast iron showed marked improvement in erosion resistance after laser processing.     

Effect of low-pressure carburizing and plasma nitriding on mechanical properties and fatigue endurance limits of low alloy sintered steels

ABSTRACT

Thermochemical treatments like plasma nitriding or surface carburizing are commonly used to enhance surface hardness of steel components. An important difference between these treatments is the temperature at which they are carried out. In the present paper, the surface carburizing was carried out following a recently reported non-isothermal low pressure carburizing (LPC) treatment. In order to gain a comparative view of the effect of different treatments on the microstructure, microhardness, fatigue and impact properties, materials with distinct hardenability and widely used in the industrial production were evaluated. Tests were also carried out using industrially processed components aimed to an application demanding high wear resistance. The microstructural evolution during case hardening was studied by optical and electron microscopy.     

Roles of Friction-Induced Strain Hardening and Recrystallization in Dry Sliding Wear of AZ31 Magnesium Alloy

Dry sliding wear tests were performed on AZ31 alloy using a pin-on-disc configuration under the loads of 5–360 N and sliding speeds of 0.1–1.5 m/s. Friction and wear characteristics of AZ31 alloy were investigated as a function of the load and sliding speed. Wear mechanisms for AZ31 alloy were characterized by scanning electron microscopy. The wear behavior in mild and severe wear regimes was described in terms of plastic deformation and microstructure evolution in subsurface, and surface hardness change and temperature rise of worn surfaces. The results revealed that surface strain hardening caused by large plastic deformation played an important role in maintaining a low slope of wear rate in mild wear regime, while surface thermal softening originating from dynamic recrystallization and surface melting were responsible for a rapid wear in severe wear regime.     

Microstructural Aspects and Wear Behavior of Sinter Hardened Distaloy HP

Effect of cooling rate during sinter hardening on the microstructure and wear behavior of sintered steel grade Distaloy HP has been studied. Wear performances are closely related to macro and micro hardness of the materials. Dry sliding wear tests have been conducted using a reciprocating pin on flat wear testing machine under normal loads of 25, 35 and 45N and at a constant speed of 0.3 m/s. The samples were sinter hardened at different cooling rates 0.5–3 °C/s in order to investigate the influence of microstructure and hardness on wear behavior. It has been shown that, sintering process and cooling rate change the microstructure and hence the hardness and wear behavior of the material. The best wear resistance was detected at a cooling rate of 3 °C/s. At this cooling rate the material had an almost martensitic microstructure and the wear rate was some how independent of the applied load.     

Improvement of hardness and wear resistance of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

The objective of this study is to investigate microstructure, hardness, and wear properties of three kinds of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of Ti+C, TiC+TiB₂, and Ti+B₄C powders and CaF₂ flux were deposited on a Ti-6A1-4V substrate, and then high-energy electron beam was irradiated on these mixtures. The surface-alloyed layers of 0.9 to 1.6 mm in thickness were homogeneously formed, and contained a large amount (30 to 44 vol. pct) of hard precipitates such as TiC and TiB in the martensitic matrix. This microstructural modification improved the hardness and wear resistance of the surface-alloyed layer 2 times and 6 to 9 times, respectively, greater than that of the substrate. Particularly, the surface-alloyed material fabricated with Ti+B₄C powders had a larger volume fraction of TiB and TiC homogeneously distributed in the martensitic matrix, and thus showed the best hardness and wear resistance. These findings suggested that the surface-alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-base surface-alloyed materials with improved hardness and wear properties.     

Improvement of the hardness and wear resistance of (TiC, TiN)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis and improvement of the hardness and wear resistance of Ti-6Al-4V surface-alloyed materials fabricated by a high-energy electron beam. The mixtures of TiC, TiN, or TiC + TiN powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then the electron beam was irradiated on these mixtures. In the specimens processed with a flux addition, the surface-alloyed layers of 1 mm in thickness were homogeneously formed without defects and contained a large amount (over 30 vol pct) of precipitates such as TiC, TiN, (Ti _x Al_1−x )N, and Ti(C _x N_1−x ) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification, including the formation of hard precipitates and hardened matrices in the surface-alloyed layers, improved the hardness and wear resistance. Particularly in the surface-alloyed material fabricated by the deposition of TiN powders, the wear resistance was greatly enhanced to a level 10 times higher than that of the Ti alloy substrate. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-based surface-alloyed materials with improved hardness and wear resistance.     

Microstructural analysis of vanadium carbide/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis of vanadium carbide (VC)/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of VC powders and MgO-CaO flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron-beam accelerator. Microstructures of the irradiated surface regions were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Residual pores were found in the specimen processed without flux, but hardly found in the specimens processed with a considerable addition of flux. As a result of irradiation, vanadium content was homogeneously maintained throughout the melted region, and fine vanadium carbides were formed in the melted region. These microstructural modification including the formation of vanadium carbides greatly improved hardness, especially high-temperature hardness up to 500 °C.