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316L austenitic stainless steel was plasma nitrided and carburized at low temperatures to produce precipitation-free nitrided and carburized layers, respectively. The reciprocation sliding wear performances of the untreated, nitrided and carburized specimens were compared under both unlubricated (dry) and corrosive (in 0.5 M H2SO4 solution) conditions. The results show that under dry sliding conditions, both the nitrided layer and carburized layer can offer good wear resistance to 316L steel. The total material loss (TML) of the steel is reduced by more than two orders of magnitude by low temperature nitriding, while low temperature carburizing offers a reduction in TML by an order of magnitude. The better dry sliding wear performance of the nitrided layer is attributed to its much higher hardness as compared to the carburized layer. However, under corrosive-wear conditions in 0.5 M H2SO4 solution, the wear performance of the nitrided layer is significantly deteriorated, with TML 100% higher than that of the untreated 316L steel. On the other hand, the carburized layer can still offer good wear resistance in the corrosive environment, with a reduction in TML of 316L steel by 40%. This research has practical implication that low temperature nitriding is the most suitable for applications in dry and non-corrosive environments, while low temperature carburizing is more suitable for applications in H2SO4-containing corrosive environments. [ABSTRACT FROM AUTHOR]

In this study, the surface of industrial pure iron was treated with atmospheric-controlled induction heating fine particle peening (AIH-FPP) using mechanical coating (MC) particles of carbon/steel obtained by mixing carbon powder and fine steel particles using mechanical milling. The surface modification effect and the mechanism of its effect were examined and considered by analyzing of the treated surface. Results showed that a modified layer in which carbon elements were diffused was formed near the treated surface by the AIH-FPP treatment using MC particles. In addition, by examining the influence of the treatment conditions on the formation of the modified layer by the design of experiments, the treatment temperature showed the most significant influence among the treatment temperature, peening time, and gas flow rate. The higher the treatment temperature, the deeper the carbon diffused layer. In addition, when treated at ≥1273 K, the microstructure near the surface became pearlite, and Vickers hardness increased. The time required for carbon diffusion in the AIH-FPP carburizing process using MC particles of carbon/steel was approximately the same as that of a general carburizing treatment using a reactive gas. In the AIH-FPP carburizing process, carbon is considered to be transferred from the particles to the surface, and the grain boundaries and dislocations increased by fine particle peening (FPP) are used as channels to diffuse inside the specimen. This mechanism is entirely different from the conventional carburizing methods. [ABSTRACT FROM AUTHOR]

Conducting the carburizing process on the surface layer of low-carbon steel and subsequent austempering heat treatment can be implemented to obtain nanobainite microstructure on the surface of steels. In this research, steel with 0.23 wt.% carbon was carburized for 3 h at 900 °C in a liquid salt bath containing sodium cyanide, sodium carbonate, and sodium chloride and immediately quenched to room temperature. The samples were then heated to 900 °C for 30 min and isothermally transformed at three different temperatures of 200, 250 and 300 °C for 72, 24 and 12 h, respectively. It was found that nanostructured bainite was formed on the surface layer and the subunits of bainitic ferrite and high-carbon austenite films were almost 60-300 nm thick depending on the heat treatment temperature. It was also found that the samples austempered at these temperatures contained 18, 21 and 28% volume fractions of retained austenite on the surfaces, respectively. Due to the comparable microstructural characteristics, similar friction coefficients were obtained as for ordinary nanostructured bulk bainitic steels with high-carbon content. [ABSTRACT FROM AUTHOR]

To reduce the processing time for in-line carburizing, we performed ultra-rapid carburizing and quenching using induction heating. Conventional gas carburizing and vacuum carburizing (low-pressure carburizing) are treated at 1203-1323 K, which is lower than the eutectic temperature. In contrast, in this study, the carburizing temperature of 1523K was higher than the eutectic temperature for speeding up this process. The SCM420 sample was rapidly heated to the treatment temperature by induction heating. CH4 and N2 were mixed and treated at atmospheric pressure at 5 vol% CH4 and 10 vol% CH4. For ultra-rapid carburizing, the total carburizing depth was proportional to the square root of the carburizing time, following the parabolic law, regardless of the CH4 concentration. We obtained that the amount of carbon penetrating the steel from the atmosphere was proportional to the carburizing time. In other words, the carbon penetration rate was approximately constant during the carburizing process. The processing time for the ultra-rapid carburizing and quenching at 1523K in this study was only 5% of that for the conventional gas carburizing at 1203 K. [ABSTRACT FROM AUTHOR]

The article reviews and analyses the current scientific research in the field of surface treatment of metal surfaces with concentrated energy fluxes (CEF) — the electric-spark (in the literature, known also as electrospark) alloying (ESA), which makes it possible to obtain surface structures with unique physical, mechanical and tribological properties at the nanoscale. The ESA method with a graphite electrode (electrospark carburizing — EC) is based on the process of diffusion (saturation of the surface layer of a part with carbon), and it is not accompanied by an increase in the size of the part. In this article, the influence of the EC parameters on the quality of the carburized layer is investigated. The microstructural analysis shows that the three characteristic zones could be distinguished in the structure: the carburized (‘white’) layer, the finely dispersed transition zone with fine grain, and the base metal zone. The analysis of the results of the durometric studies of the coatings is carried out. To achieve the required parameters of dimensional accuracy and roughness of the working surface of the part after the EC process, it is necessary to use the method of non-abrasive ultrasonic finishing (NAUF). In addition, because of applying the NAUF method, the surface roughness is decreased, the tensile stresses are changed to the compressive ones, and the fatigue strength is increased too. In addition, to reduce the roughness of the treated surface, it is proposed to apply the EC technology in stages, reducing the energy of the spark discharge at each subsequent stage. In order to increase the quality of the carburized layer obtained by the EC process, it is proposed to use a graphite powder, which is applied to the treated surface before alloying. The comparative analysis shows that, after the traditional EC process at Wp = 4.6 J, the surface roughness of steel 20 is Ra = 8.3–9.0 μm, and after the proposed technology, Ra = 3.2–4.8 μm. In this case, the continuity of the alloyed layer increases up to 100%; there increases the depth of the diffusion zone of carbon up to 80 μm as well as the microhardness of the ‘white’ layer and its thickness, which increase up to 9932 MPa and up to 230 μm, respectively. The local micro-x-ray spectral analysis of the obtained coatings shows that, at the EC process carried out in a traditional way, the applying Wp = 0.9, 2.6, 4.6 J provides the formation of the surface layers with high-carbon content depths of 70, 100, 120 μm, respectively, and with the use of a graphite powder, they are of 80, 120, 170 μm. While deepening, the amount of carbon is decreasing from 0.72–0.86% to the carbon content in the base metal — 0.17–0.24%. In the near-surface layer formed with the use of the new technology, the pores are filled with free graphite, which could be used as a solid lubricant to improve the operating characteristics of the friction-pairs parts processed thereby.

In the service process of gears, premature fatigue failure or fracture of gears is often caused by poor surface performance. 20CrNiMo steel is a commonly used material for gears. Laser-shock peening (LSP), carburizing treatment (CT), and hybrid modification of carburizing treatment and laser-shock peening (LSP + CT) were carried out to improve the performance of 20CrNiMo steel. The hardness, residual stress, microstructure, subgrain size, and toughness of the samples were analyzed following various modification methods. It was observed that the properties of the composite-modified gradient structure materials achieved through carburizing and laser-shock peening were superior to those modified using single methods. After the composite treatment of carburizing and laser shocking, the samples exhibited the most significant increase in hardness, up to 916HV0.1, with a surface layer experiencing residual compressive stress as low as −635 MPa. Simultaneously, a gradient microstructure was formed on the surface layer, with 80% of the crystallites being in the nanoscale range. Furthermore, the toughness was notably enhanced. Experimental results confirm the improvement in the properties of 20CrNiMo samples, resulting in the creation of a functionally graded material through the composite treatment of carburizing and laser shocking.

Guide rings and rolling bodies are mainly made of alloy steel with a high chromium content, heat treated, as well as of carburising steel. The efforts to improve the standard of quality of bearing steels are determined by the interest of continuously increasing the adhesion of constructive types. These steels are recommended for large bearings which operate under shock and vibration conditions. Thermochemical treatments are followed by quenching and tempering heat treatments. However, undesirable defects occur sometimes on the surface of the carburised layer and solutions are constantly being sought to identify the causes thereof. [ABSTRACT FROM AUTHOR]

Herein, the carburizing behavior of 20MnCr5 gear steel with different Nb contents after carburizing with the temperature from 900 to 950 °C and the corresponding holding time from 8 to 5 h are investigated. The theoretical results of carbon content and the depth of carburized layer for the two experimental carburizing processes should be the same according to the classical diffusion model. However, the experimental results show that the carbon content of the entire hardened layer increases, and the depth of carburized layer increases by 35% after high‐temperature carburizing. The carbon content of 0.061%Nb steel on the outer surface increases, but the depth of carburized layer remains unchanged. It is found that the theoretical D values at 900 °C and 950 °C are 2.0 × 10−7 and 3.3 × 10−7 cm2 s−1, respectively. However, the experimental D value is much larger than 5.0 × 10−7 cm2 s−1. The carbon diffusion is greatly promoted by high‐temperature carburizing because the number of excess carbides on the outer surface decreases. The diffusion coefficient decreases after the addition of Nb due to forming a large number of small particles of Cr7C3 and NbC, but the effect is only obvious on the outer surface. [ABSTRACT FROM AUTHOR]

This study concerns the improvement of performance of resistance to wear phenomena of the Ti-6Al-4V alloy surface by means of Gaseous carburizing thermochemical treatment. Three-thermochemical treatment durations (2h, 4h, and 6h) were chosen for investigation of the effect of such treatment on this alloy. The hardness test under an indentation load of 0.05 kgf with a Vickers pyramidal indenter revealed that the surface hardness is 335 HV for the untreated samples. The hardness reaches approximately 1500 HV during gas cementation at 930 �C for variable times (2h, 4h, 6h) followed by quenching at 840 �C in an oil medium, which was accompanied by a significant improvement in wear resistance. The characterization of the modified surface layers was studied by means of a microscopic analysis and by X-ray diffraction. The case-hardening made it possible to obtain a wear resistance greater than that of the alloy not treated, minimal loss of mass by dry friction and an improvement in roughness as well as a good coefficient of friction

The carburizing period for tool steel could be significantly shortened by operating at a higher carburizing temperature. However, grain coarsening happens during the carburizing process, and then results in the deteriorated surface properties in 20CrMnTi gear steel, especially at an elevated carburizing temperature. The relationships between grain coarsening and the precipitates in the developed 20CrMnTi gear steel during pseudo-carburizing were established by microstructure characterization, precipitate analysis and in-situ observation to clarify the coarsening mechanism. The results manifested the Baker–Nutting orientation relationship between the (Ti, Mo)(C, N) particles and the matrix, and then testified to the redissolution and ripening of the (Ti, Mo)(C, N) precipitates pre-formed in the α phase during the carburizing. Coarsening in austenite grain during the carburizing process was mainly caused by the rapid redissolution and ripening of the (Ti, Mo)(C, N) precipitates, although this occurred in a very short pseudo-carburizing time. The area density of the dispersed unripe (Ti, Mo)(C, N) particles markedly decreased from 0.389% in as-hot rolled gear steel to 0.341%, and then from 0.279% in carburized steels at 970 and 980 °C, respectively. Additionally, the redissolution and ripening of the (Ti, Mo)(C, N) precipitates were accelerated by the elevated carburizing temperature of 980 °C, at which time the growing rate in austenite grains was 2.34 μm/min during the prior 1 min (0.79 μm/min during the prior 3 min at 970 °C). The temperature then decreased to 0.003 μm/min in the subsequent carburizing process. The results obtained our current work reflected that the particles with excellent thermal stability should play important roles in the limitation of grain coarsening during the carburizing process.

In this study, the effect of thermochemical processes, such as nitriding and carburizing, on the microstructural evolution of superdúplex stainless steel is presented. The objective is to find a correlation between the microstructure and the mechanical properties of the constitutive phases of this stainless steel (i.e., austenite and ferrite) and analyze the diffusion elements influence on the precipitation of secondary phases, such as sigma (σ), chi (χ), nitrides, and carbides. Hardness (H) and elastic modulus (E) maps were obtained by using a high-speed nanoindentation technique. H and E values for the individual phases were determined through the application of Gaussian Mixture Model statistical analysis. Assessments of the mechanical properties of the different phases were achieved by overlaying field emission-scanning electron microscopy micrographs. An excellent correlation between microstructure and small-scale mechanical properties was attained. Secondary phases have been detected and nanoindentation statistics have proven their influence on the mechanical properties. • Impact of thermochemical processes on the microstructure of superduplex stainless steel. • Excellent correlation between constitutive phases and local mechanical properties is achieved using the Gaussian Mixture Model (GMM) method. • Integration of statistical sampling, high-speed nanoindentation techniques, and histograms enhances understanding of the mechanical properties of individual phases. • High-speed nanoindentation proves advantageous over EBDS. [ABSTRACT FROM AUTHOR]

Herein, it is proposed to obtain a favorable combination of microstructure and mechanical properties of martensitic stainless steel 14Cr14Co13Mo5 by prenitriding method before carburizing and appropriate heat treatment processes. The result indicates that the effective hardened layer depth is increased by 32% to 0.66 mm, and the hardness is significantly improved by prenitriding. The confocal laser scanning microscope and electron backscattered diffraction show that the prenitriding can effectively refine the martensite substructure of the carburized layer after quenching due to the refinement of prior austenite grain. In addition, plenty of second‐phase particles M2N with finer size appear at the same case layer depth instead of coarse M23C6 in the specimen prenitrided, which optimize the microstructure of the case layer. [ABSTRACT FROM AUTHOR]

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear. [ABSTRACT FROM AUTHOR]

Low-temperature thermochemical treatments are particularly suitable for use in the surface hardening of austenitic stainless steels without impairing their corrosion resistance. In fact, when using treatment media rich in nitrogen and/or carbon at relatively low temperatures (

The results of modifying the surface of austenitic stainless steel by anodic plasma electrolytic treatment are presented. Surface treatment was carried out in aqueous electrolytes based on ammonium chloride (10%) with the addition of ammonia (5%) as a source of nitrogen (for nitriding), boric acid (3%) as a source of boron (for boriding) or glycerin (10%) as a carbon source (for carburizing). Morphology, surface roughness, phase composition and microhardness of the diffusion layers in addition to the tribological properties were studied. The influence of physicochemical processes during the anodic treatment of the features of the formation of the modified surface and its operational properties are shown. The study revealed the smoothing of irregularities and the reduction in surface roughness during anodic plasma electrolytic treatment due to electrochemical dissolution. An increase in the hardness of the nitrided layers to 1450 HV with a thickness of up to 20–25 μm was found due to the formation of iron nitrides and iron-chromium carbides with a 3.7-fold decrease in roughness accompanied by an increase in wear resistance by 2 orders. The carburizing of the steel surface leads to a smaller increase in hardness (up to 700 HV) but a greater thickness of the hardened layer (up to 80 μm) due to the formation of chromium carbides and a solid solution of carbon. The roughness and wear resistance of the carburized surface change are approximately the same values as after nitriding. As a result of the boriding of the austenitic stainless steel, there is no hardening of the surface, but, at the same time, there is a decrease in roughness and an increase in wear resistance on the surface. It has been established that frictional bonds in the friction process are destroyed after all types of processing as a result of the plastic displacement of the counter body material. The type of wear can be characterized as fatigue wear with boundary friction and plastic contact. The correlation of the friction coefficient with the Kragelsky–Kombalov criterion, a generalized dimensionless criterion of surface roughness, is shown.

The article reviews and analyses the current scientific research in the field of surface treatment of metal surfaces with concentrated energy fluxes (CEF) -- the electric-spark (in the literature, known also as electrospark) alloying (ESA), which makes it possible to obtain surface structures with unique physical, mechanical and tribological properties at the nanoscale. The ESA method with a graphite electrode (electrospark carburizing -- EC) is based on the process of diffusion (saturation of the surface layer of a part with carbon), and it is not accompanied by an increase in the size of the part. In this article, the influence of the EC parameters on the quality of the carburized layer is investigated. The microstructural analysis shows that the three characteristic zones could be distinguished in the structure: the carburized ("white") layer, the finely dispersed transition zone with fine grain, and the base metal zone. The analysis of the results of the durometric studies of the coatings is carried out. To achieve the required parameters of dimensional accuracy and roughness of the working surface of the part after the EC process, it is necessary to use the method of non-abrasive ultrasonic finishing (NAUF). In addition, because of applying the NAUF method, the surface roughness is decreased, the tensile stresses are changed to the compressive ones, and the fatigue strength is increased too. In addition, to reduce the roughness of the treated surface, it is proposed to apply the EC technology in stages, reducing the energy of the spark discharge at each subsequent stage. In order to increase the quality of the carburized layer obtained by the EC process, it is proposed to use a graphite powder, which is applied to the treated surface before alloying. The comparative analysis shows that, after the traditional EC process at Wp = 4.6 J, the surface roughness of steel 20 is Ra = 8.3-9.0 µm, and after the proposed technology, Ra = 3.2-4.8 µm. In this case, the continuity of the alloyed layer increases up to 100%; there increases the depth of the diffusion zone of carbon up to 80 µm as well as the microhardness of the "white" layer and its thickness, which increase up to 9932 MPa and up to 230 µm, respectively. The local micro-x-ray spectral analysis of the obtained coatings shows that, at the EC process carried out in a traditional way, the applying Wp = 0.9, 2.6, 4.6 J provides the formation of the surface layers with high-carbon content depths of 70, 100, 120 pm, respectively, and with the use of a graphite powder, they are of 80, 120, 170 pm. While deepening, the amount of carbon is decreasing from 0.72-0.86% to the carbon content in the base metal -- 0.17-0.24%. In the near-surface layer formed with the use of the new technology, the pores are filled with free graphite, which could be used as a solid lubricant to improve the operating characteristics of the friction-pairs parts processed thereby. [ABSTRACT FROM AUTHOR]

In the present study, we fabricated tungsten carbide (WC) coatings on tungsten using a recently proposed two-step interstitial carburization method and systematically investigated their mechanical behavior, including their responses to indentation, scratch, and wear. The coatings comprised WC and a very small amount of W 2 C dispersed at the coating–substrate interface. The volume fraction of the carbide phases nearly reached 100 %, and the grain size of WC increased with increasing distance from the surface, thus exhibiting a distinct gradient microstructure with columnar WC grains. Indentation experiments on the coating surface showed that the coating attained a hardness of 25 GPa and cracks were generated when the indentation load exceeded 5 N. The fracture toughness of the coating was estimated to be 2.5 MPa m1/2. The response of the coating to scratching was not sensitive to the scratching speed, and an increase in the scratching speed did not alter the geometry of the grooves, but it resulted in an earlier occurrence and a higher density of cracks; further, it leads to increased acoustic emission. Delamination of the coating did not occur when the normal load was in the range of 0–100 N, indicating that the adhesion strength of the coating was greater than 100 N. We studied the wear behavior using a ball-on-disc tribometer and found that the WC coatings exhibited extraordinarily high wear resistance, and both abrasive wear and adhesive wear mechanisms acted concurrently during wear. Therefore, we verified that the WC coatings prepared by interstitial carburization have a high hardness and excellent scratch and wear resistance. [ABSTRACT FROM AUTHOR]

To investigate the microstructure and damage of friction-fatigued carburized martensitic steels for the reliability of remanufacturing parts, the retained austenite (γ) phase and residual stress were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). We evaluated their changes before and after roller pitching tests, and before and after the operation of the gear parts. In the roller pitching tests, the retained γ phase decreased with increasing load and number of cycles, presumably due to martensitic transformation caused by the cyclic load. The residual stress ratio (after/before the test) was significantly lower at high loads than that before testing, which was ascribed to the appearance of surface microcracks and the resultant release of internal stress. From SEM observations of the cross-section of the friction surface, we confirmed that the changes in the retained γ phase and residual stress ratio reflect the process of formation of multiple microcracks in the 10µm surface layer. The decreases in both the retained γ phase ratio and the residual stress ratio would therefore appear to rule out reuse. A decision on the potential for gear reuse can be made by means of non-destructive testing, i.e., investigating the relationship between the retained γ phase ratio and the residual stress ratio. [ABSTRACT FROM AUTHOR]

Fin-pass rolls are the latest series of rolls in electric resistance welding (ERW) pipe production lines that form the sheets to tubular shape and adjust the edges of the sheet for welding. The rolls (made of AISI 8622 steel) lose their proper function after about 10 years of operation due to severe wear and change of their original surface profile. The worn portions were removed by grinding and replaced by an AISI D2 high carbon steel ring to repair these rolls. After a short time of service (about one year), the edge of the repair ring was exposed to severe spalling and fracture. The present study investigated the causes of the rapid failure of the AISI D2 repair ring and proposed a solution to the problem. The surface morphology, hardness, and wear resistance were studied. Moreover, the stress analysis of fin-pass rolls was studied using ABAQUS 6.14 finite element software for the closer investigation of the failure mechanism. The leading cause of spalling was the inherent brittleness of the AISI D2 steel and the presence of a high-stress concentration at the edges of the repair ring. To overcome this problem, carburized AISI P20 steel, case hardened AISI 4140 steel, and hard chromium electroplated AISI 4140 steel were replaced, and the resulting properties were studied. The highest resistance to spalling and wear occurred with carburized AISI P20 steel because of the high surface hardness and the gradual increase of toughness from the surface to the depth in the carburizing process, increasing the wear resistance and retarding the growth of fatigue cracks. [ABSTRACT FROM AUTHOR]

This study concerns the improvement of performance of resistance to wear phenomena of the Ti-6Al-4V alloy surface by means of Gaseous carburizing thermochemical treatment. Three-thermochemical treatment durations (2h, 4h, and 6h) were chosen for investigation of the effect of such treatment on this alloy. The hardness test under an indentation load of 0.05 kgf with a Vickers pyramidal indenter revealed that the surface hardness is 335 HV for the untreated samples. The hardness reaches approximately 1500 HV during gas cementation at 930 °C for variable times (2h, 4h, 6h) followed by quenching at 840 °C in an oil medium, which was accompanied by a significant improvement in wear resistance. The characterization of the modified surface layers was studied by means of a microscopic analysis and by X-ray diffraction. The case-hardening made it possible to obtain a wear resistance greater than that of the alloy not treated, minimal loss of mass by dry friction and an improvement in roughness as well as a good coefficient of friction. [ABSTRACT FROM AUTHOR]

The work aims to study the influence of carburizing and nitriding on fatigue properties of 18Cr2Ni4WA high strength steel in very high cycle fatigue regime. Very high cycle fatigue tests were carried out on 18Cr2Ni4WA Steel after carburizing and nitriding respectively. The micro morphology of fatigue fracture was observed by scanning electron microscope, the failure mode and failure mechanism were discussed. The relationship between fatigue life and defect size, FGA size, fish eye size of fracture was analyzed. The characteristic size of defects is evaluated by Gumbel, Weibull and GEV distribution functions, and a modified Akiniwa fatigue life prediction model considering the relationship between FGA size and inclusion size was established. The results showed that, nitriding and carburizing treatment improve the surface fatigue limit of the steel. The fatigue life decreases with the increase of internal defect size and FGA size. After carburizing and nitriding treatment, the internal fatigue strength of the specimen decreases slightly. When the failure probability is 99%, the internal defect sizes of nitrided specimens calculated by Weibull, Gumbel and GEV distributions are 141.5 µm, 148.4 µm and 211.7 µm respectively. The calculated internal defect sizes of carburized specimens are 47 µm, 67.8 µm and 40 µm respectively. Compared with the experimental data, the fatigue strength predicted by GEV is the most appropriate. carburizing and nitriding treatment can improve the surface fatigue strength of 18Cr2Ni4WA steel, but slightly reduce the internal fatigue strength. The prediction result of the new model is conservative when the failure probability is 99%, which is suitable for engineering application. [ABSTRACT FROM AUTHOR]

The boundary conditions for describing the mass transfer of carbon from a low-pressure oxygen-free atmosphere into steel under vacuum carburizing are analyzed. Experimental results and thermodynamic calculations are used to determine the numerical value of the activity of carbon in an environment of low-pressure acetylene. The calculated value of the carbon potential is used in the boundary condition of kind III in the form of a Langmuir expression applied to a mathematical model of vacuum carburizing based on numerical solution of the diffusion equation. The calculated concentration profiles are shown to describe adequately the experimental results. Different nonequilibrium processes of vacuum carburizing of steels are shown to have similarity. [ABSTRACT FROM AUTHOR]

To investigate the microstructure and damage of friction-fatigued carburized martensitic steels for the reliability of remanufacturing parts, the retained austenite (γ) phase and residual stress were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). We evaluated their changes before and after roller pitching tests, and before and after the operation of the gear parts. In the roller pitching tests, the retained γ phase decreased with increasing load and number of cycles, presumably due to martensitic transformation caused by the cyclic load. The residual stress ratio (after/before the test) was significantly lower at high loads than that before testing, which was ascribed to the appearance of surface microcracks and the resultant release of internal stress. From SEM observations of the cross-section of the friction surface, we confirmed that the changes in the retained γ phase and residual stress ratio reflect the process of formation of multiple microcracks in the 10μm surface layer. The decreases in both the retained γ phase ratio and the residual stress ratio would therefore appear to rule out reuse. A decision on the potential for gear reuse can be made by means of non-destructive testing, i.e., investigating the relationship between the retained γ phase ratio and the residual stress ratio. [ABSTRACT FROM AUTHOR]

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear.

Ametallographic study of advanced low-carbon martensitic steel with high resistance of the supercooled austenite to the transformations by pearlitic and bainitic mechanisms under continuous cooling is performed. The critical temperatures of the transformations occurring under heating and cooling are determined. The kinetics of the isothermal transformation of the austenite in the temperature range of 300 – 400°C is analyzed. The possibility of successful implementation of isothermal quenching of the steel after carburizing is demonstrated. [ABSTRACT FROM AUTHOR]

The very high cycle fatigue (VHCF) properties of case hardening steel 18CrNiMo7-6 after carburizing and pseudo-carburizing have been investigated by means of ultrasonic fatigue tests. Results showed that the pseudo-carburized specimens with the hardness of 442 HV did not show VHCF fracture, and fatigue fracture did not occur even after 109 cycles when the applied stress amplitude was lower than the high cycle fatigue limit of 650 MPa obtained at 107 cycles. When carburized thoroughly with the hardness of 700 HV, the experimental steel showed VHCF phenomenon, and it might fracture even at nearly 109 cycles when the stress amplitude was about 600 MPa. Observations of the fracture surfaces of the fatigue specimens showed that after carburizing, the fatigue crack initiation sites changed from martensite matrix to nonmetallic inclusions, forming granular bright facet (GBF) and fish-eye before final abrupt fracture. The stress intensity factor at GBF area, which was calculated to be 3.5 MPa m1/2, could be used as a critical value for VHCF fracture for the carburized specimens of the experimental steel. The VHCF fracture of the carburized specimens of the experimental steel could be related to the high hardness and large amount of retained austenite as a result of carburizing. [ABSTRACT FROM AUTHOR]

Microstructure refinement strategies in simulated carburized microstructures were evaluated because of their potential for improving the fatigue performance of case-carburized components. Commercial 52100 steel was used to simulate the high-carbon content in the case. Specimens were subjected to various thermal treatments in a quenching dilatometer. Reheating cycles to austenitizing temperatures were evaluated with respect to both prior austenite grain size (PAGS) and associated martensite and retained austenite (RA) refinement. Quantitative stereological measurements were performed to evaluate the microgeometry of plate martensite and the size distribution of RA regions. Decreasing the reheating temperature resulted in finer PAGS, and multiple reheating cycles resulted in a narrower PAGS distribution. Refinement in PAGS led to a reduction in martensite plate size and finer distribution of RA. Additionally, interrupted quenching below martensite start (MS) temperature was evaluated. This processing route results in a refinement of martensite plates and more stable RA. The stabilization of austenite may be mechanical or chemical in nature, owing to the deformation of austenite during primary transformation, or due to partitioning of carbon into austenite similar to quenching and partitioning steels. [ABSTRACT FROM AUTHOR]

As a novel manufacturing technology, additive manufacturing (AM) has many advantages such as energy saving, reduced material waste, faster design-to-build time, design optimization, reduction in manufacturing steps, and product customization compared to conventional manufacturing processes. Heat treatment is widely used to improve the properties of conventional manufactured steel parts. The response of additively manufactured steel parts to heat treatment may be different from conventionally manufactured steel parts due to variations in the as-deposited alloy microstructure. An understanding of heat treatment processes for additively manufactured steel parts is necessary to develop their heat treatment process parameters. In the present work, 20MnCr5 steel was selected to investigate the carburizing heat treatment of additively manufactured parts. These parts were fabricated by selective laser melting (SLM) for the carburizing study. It was found that the AM parts fabricated by the SLM process show the microstructure of tempered martensite, while the microstructure of as-received wrought part is ferrite and pearlite. It was also experimentally found that the SLM process decarburizes the entire SLM part. Before carburizing, a normalization process was conducted on both SLM and wrought 20MnCr5 parts to reduce the effect of the pre-carburizing microstructure. The objective of this project is to determine the carburizing performance of additively manufactured steel parts. The results for the SLM parts in terms of carbon concentration and microhardness profiles are compared with the results for the wrought steel. It was found that the carburized SLM part in the present work has higher carbon concentration near the surface, deeper case depth, and higher total carbon flux than the carburized wrought part. [ABSTRACT FROM AUTHOR]

In the present study, we have examined the wear properties of the sintered pure iron subjected to two distinct heat treatments such as nitriding-quenching (NQ) and carburizing-quenching (CQ). Based on our current observations, the martensite layer was formed on the surface layer following each treatment, whereas the hardness of the NQ martensite was much higher than that of the CQ one. The wear of the CQ specimen was slightly smaller than that of the NQ martensite, despite the lower value of the initial hardness. Meanwhile, the hardness of the CQ surface after the sliding tests significantly elevated relative to the NQ surface resulting in the better wear resistance. EBSD analysis demonstrated that the plastic deformation on the CQ surface along the sliding direction. Furthermore, the micro area X-ray diffraction along the surface layer of the CQ surface showed that a small amount of the retained austeite which reduced locally during the test. Therefore, the CQ-treated surface showed the excellent wear resistivity due to the surface hardening by the stress-induced transformation of the retained austenite dispersed in the martensite, in addition to the strain hardening of the martensite itself. In contrast, the worn surface of the NQ specimen showed slight plastic deformations of the ferrite grains beneath the martensite layer, but not in the surface martensite layer. This deformation under the martensite layer was due to the hardness gap between inward and the heat-treated surface, and might contribute to form the concave profile on the sliding surface. Consequently, this study could demonstrate such the difference in the wear mechanisms between the CQ and the NQ specimens. [ABSTRACT FROM AUTHOR]

Typically, a cost-effective and versatile carburizing process is used to improve the surface integrity of high-value-added gear components. In this study, carburized treatment and wet shot peening (WSP) were applied to increase the corrosion resistance of 18CrNiMo7-6 gear steel. The surface integrity and corrosion resistance after carburizing and combined treatment (carburized treatment + WSP) were analyzed. The results indicated that the combined treatment converted the retained austenite surface layer within approximately 45 μm from the surface into the martensitic phase, with a 13.9 % reduction in equivalent grain size and a 23.6 % increase in dislocation density compared with carburizing. Furthermore, electrochemical experiments in a 3.5 wt% NaCl solution indicated that the combined treatment process increased the corrosion resistance compared with carburizing (55.4 % lower corrosion current density and a higher Fe2+/Fe3+ ratio). Despite the high surface roughness obtained with combined treatment, the improved corrosion resistance is primarily ascribed to the high compressive residual stress and dislocation density. This study provides insights into the surface strengthening design of 18CrNiMo7-6 gear steel for corrosive environments—particularly the synergistic strengthening effect of carburizing and WSP. • WSP was introduced to improve corrosion resistance and surface integrity. • Corrosion resistance of 18CrNiMo7-6 steel cannot be enhanced by carburized treatment. • Competing effects of surface integrity on corrosion resistance were investigated. [ABSTRACT FROM AUTHOR]

This work compares the tribological behavior of surface layers obtained by three different hardening processes. The layers were formed on the surface of AISI 4140 steel by applying three different thermochemical treatments. Wear resistance was evaluated using a standardized tribological machine for abrasive wear, according to the limits established by the ASTM G65 “Standard Test Method for Measuring Abrasion Using Dry Sand/Rubber Wheel Apparatus”. According to the results, the boride layers exhibited the highest wear resistance, as compared to nitrided and carburized layers. In contrast, the carburized layers presented the highest loss of volume. Scanning electron microscopy (SEM) was used to analyze the worn surfaces to examine the wear mechanisms. Abrasive wear was identified in all the samples, as the main abrasive wear mechanism. The mean values of the coefficient of friction (CoF) of the hardened surfaces were 0.39, 0.55, and 0.65 for carburizing, nitriding, and boriding samples, respectively, indicating that the wear process may not always be related to a low CoF. The results suggest that the highest hardness is normally associated with high wear resistance, but the coefficient of friction could be not directly related to the hardness of the materials. Finally, a statistical study demonstrates the random nature of the layers obtained by three different hardening processes.

Low-temperature treatments have become a valuable method for improving the surface hardness of stainless steels, and thus their tribological properties, without impairing their corrosion resistance. By using treatment temperatures lower than those usually employed for nitriding or carburizing of low alloy steels or tool steels, it is possible to obtain a fairly fast (interstitial) diffusion of nitrogen and/or carbon atoms; on the contrary, the diffusion of substitutional atoms, as chromium atoms, has significantly slowed down, therefore the formation of chromium compounds is hindered, and corrosion resistance can be maintained. As a consequence, nitrogen and carbon atoms can be retained in solid solutions in an iron lattice well beyond their maximum solubility, and supersaturated solid solutions are produced. Depending on the iron lattice structure present in the stainless steel, the so-called “expanded austenite” or “S-phase”, “expanded ferrite”, and “expanded martensite” have been reported to be formed. This review summarizes the main studies on the characteristics and properties of these “expanded” phases and of the modified surface layers in which these phases form by using low-temperature treatments. A particular focus is on expanded martensite and expanded ferrite. Expanded austenite–S-phase is also discussed, with particular reference to the most recent studies.

The aim of this paper is to consider the possibility of using Glow Discharge Optical Emission Spectroscopy (GDOES) for determination of carburized layer thickness. Carburized layer increases the hardness of basic material (low carbon steels with carbon content of 0.2%), thereby it increases its wear and abrasion resistance and it decreases forming of oxidation layer too. The thickness of this layer is usually determined by hardness measuring according to the ČSN EN ISO 2639 standard. The results of this standardized method are compared with the results obtained by two methods of GDOES and with pictures of optical microscopy. It has been found that the method of “Bulk” GDOES analysis with gradual grinding is suitable for determining the thickness of carbon enriched layers, while carburized layers which would correspond to standardized hardness 550 HV 1 may be only guessed from the graph of dependence of carbon content on the depth, in which analysis was performed. The pictures from the optical microscope in cross section do not have sufficient predictive value.

The effects of cryogenic treatment and low temperature tempering on the microstructure, mechanical properties and distortion of the 20Cr2Ni4A and 17Cr2Ni2MoVNb carburized gear steels were investigated. The results showed that the case hardness of the experimental steels was increased after the cryogenic treatment, due to the decrease of the retained austenite content and the precipitation of the tiny carbides. The wear resistance of the two steels after cryogenic treatment was improved, although the wear mechanisms were different for 17Cr2Ni2MoVNb and 20Cr2Ni4A steels. The distortion of the Navy C-ring specimens underwent shrinkage before expansion during the cryogenic process, and the distortion of 17Cr2Ni2MoVNb steel was smaller than that of 20Cr2Ni4A steel.

The cryogenic treatment (CT) process of carbide-free bainite steel after carburizing was optimized by the method combining thermal magnetic analysis, microhardness analysis and direct reading spectrometric analysis. The results show that cryogenic treatment temperature of the hardened layer should be lower than 134K by measuring thermal magnetic curve of the sample after carburizing at 1193K and air cooling (AC). After cryogenic treatment at 123K and tempering (T) at 463K, retained austenite content of the hardened layer is about 12.2% (mass fraction). The near surface layer of carburized steel is hardened dramatically through the cryogenic treatment, and the hardness of near surface layer reaches about 810HV1.0 after low temperature tempering. The distribution of hardness gradient of carburized steel tends to be reasonable.

In the production, transportation and processing of biological resources different electrical equipment is used, the integral part of which are DC machines. Energy efficiency and reliability of the production depends on the drives. The breakdown even of one engine may stop the production process and consequently entail losses. One reason for the failure of the DC motors is insulation damage due to exposure to partial discharge, however, a partial discharge detection technique has not been developed yet. For early warning of failure of DC machines and reducing the cost of unplanned stoppage on the replacement of a failed motor it is proposed to detect partial discharges at an early stage. There are many developments related to the detection of partial discharges. The majority of methods refer to equipment operating with AC voltage. For equipment operating at a constant voltage, reliable methods for partial discharge detection are not developed yet. Therefore, now the question of the diagnosis of partial discharge in the DC machine is quite relevant. Objective: to provide a method for detecting the partial discharge pulse applied using the DC voltage to the test object. Methods. A physical model for the detection of partial discharges was created. With the resulting model and electronic Tektronix oscilloscope type TDC-2012 the registration of partial discharge was carried out, for further processing and analysis on a computer. Results. The authors developed a method for the detection of partial discharge constant voltage by applying a short pulse. A series of tests, which revealed a number of certain laws by which we can judge the presence or absence of partial discharge in the test object, were performed by means of the created physical model. The authors obtained a patent on the basis of this research.

Carburizable martensitic stainless steels (MSSs) are attractive candidates for bearings due to their high corrosion resistance, high hardness, and high temperature performance. Wear performance in tribocorrosion applications is strongly influenced by the surrounding environment. Electrochemical testing was used to evaluate three different surface treatments on AMS 5930 steel developed for advanced gas turbine engine bearing applications: low temperature (LTT), high temperature (HTT), and carbonitrided (CN). HTT had a higher corrosion rate that increased with time, whereas LTT and CN had lower corrosion rates that were stable over time. Accelerated testing revealed that surface treatment significantly influenced how corrosion propagated: HTT was more uniform; conversely, LTT and CN showed localized attack. Degradation mechanisms developed from electrochemical methods provide rapid insight into long-term wear behavior. [ABSTRACT FROM AUTHOR]

As a polycrystalline fiber, continuous zirconia (ZrO 2) fiber used at high temperatures will cause grain coarsening and strength degradation due to grain growth. In order to suppress the grain coarsening, zirconium carbide (ZrC) and carbon were introduced into ZrO 2 continuous fibers by pack carburizing, and ZrC/ZrO 2 composite fibers with a radial gradient were subsequently prepared. Due to the pinning effect of ZrC or carbon on the grain boundary, the movement of the grain boundary is limited. The composite fibers obtained in an argon atmosphere at 1400 °C exhibited fine grain, compact micromorphology, and a tensile strength of 784 MPa. The formation process of the composite fiber was accomplished by the interaction of external graphite carbon and internal residual carbon with ZrO 2. This work provides a simple and economical method for preparing carbide composite fibers. [ABSTRACT FROM AUTHOR]