Effective Case Depth Research Articles

Effect of Different Microstructures on Surface Residual Stress of Induction-Hardened Bearing Steel

JIS SUJ2 steel is most widely used in bearing steel. The advantages are good hardenability, excellent fatigue, wear resistance and comprehensive mechanical properties. The wear resistance and fatigue resistance of this steel are attracting more attention, and the residual stress state and its distribution on the surface of the heat affected zone are critical factors affecting the fatigue life and wear resistance of the parts. In this study, SUJ2 is used as a material by which to study the surface residual stress and retained austenite distribution of induction-hardened steel. Quenching and tempering treatments were used to obtain different microstructures and an induction method was used to re-quench the case region. After the heat treatment, the residual stress and retained austenite volume on the surface were analyzed by X-ray diffraction and analyses of the microstructure and the hardness were also conducted. The results show that the microstructure after heat treatment contains unsolved carbides, tempered martensite and retained austenite. In the induction-hardened area, the residual stress is all compressive, and the values are more than −750 MPa. In conclusion, the microstructures of the specimens before induction hardening have a significant impact on the effective case depth for the same output power condition and the surface residual stress changes from a tensile to a compressive state. In the induction-hardened area, the maximum of the residual compressive stress was increased as the austenitized temperature of quenching increased.

Low‐Pressure Nitrocarburizing in Standard Vacuum Furnaces—Preliminary Studies

For the first time, low‐pressure nitrocarburizing process is performed in a minorly modified standard vacuum furnace on 16MnCr5 alloy steel samples. Ammonia and acetylene are used as a source of nitrogen and carbon, respectively. Lowering the pressure of the process ensures longer and more efficient ammonia dissociation. To prove the safety of the process, exhaust gases are investigated online using residual gas analyzer mass spectrometer, whereas reaction products deposited on the sample's surface are studied by means of time‐of‐flight secondary ion mass spectrometer. Possible reactions taking place in the atmosphere of the chamber are written down. In the results of conducted examinations, it is confirmed that no toxic gases are generated, and no cyanides and polycyclic aromatic hydrocarbons are formed during the process. Additionally, microstructure analysis of the cross sections of nitrocarburized sample is performed using scanning electron microscopy (SEM) and optical microscopy. It confirms the diffusion of nitrogen and carbon into the near surface layer. SEM/energy‐dispersive X‐ray spectroscopy analysis demonstrates precipitation of carbide and carbonitride, what is proved by X‐ray diffractometer and microhardness profile analysis. The presented low‐pressure nitrocarburizing results in an increase in hardness from about 200 to over 500 HV and the formation of an effective case depth of about 20 μm, after only 20 min of the process.

Effects of thermo-chemical treatment and grinding process of external cylindrical surfaces on residual stresses in 13CrMo4-5 steel

This paper presents a study aimed at determining the effect of the carburizing treatment process and the subsequent grinding process on the residual stresses occurring in ring-shaped specimens made of 13CrMo4-5 steel.During the tests, vacuum carburizing was used, achieving an effective case depth ECD = 0.5 mm. Subsequently, the cylindrical outer surfaces of the samples were ground by conventional plunge grinding and with innovative kinematics using a test stand based on a conventional flat-surface grinding machine. As part of the study, microhardness and residual stresses were measured before and after grinding. Measurements were carried out to a depth of 1 mm. The main component of the stand is an original special device that allows the cylindrical specimen to be clamped. Then the angle between its axis of rotation and the axis of rotation of the grinding wheel is set with respect to the plane of the grinding machine’s magnetic table. In the described tests, the axis of rotation of the cylindrical specimen was deviated from its original position by 15 and set at an angle of 75 to the axis of rotation of the grinding wheel. The specimens were ground with a grinding wheel of noble electro-corundum marked 38A60K8V. In both kinematic cases of the grinding process, a machining allowance of 0.01 mm was removed.Grinding using innovative kinematics did not cause any significant changes in the microhardness distribution, either for vacuum or conventional carburizing. In addition, residual stress measurements using the Dawidenkov-Sachs method showed that innovative grinding enables a more favourable distribution than those obtained after conventional plunge grinding.Further research will focus on, among others, selecting the angular settings of the workpiece axes relative to the grinding wheel axes depending on their dimensions. Grinding guidelines should include coverage ratio, infeed value, grinding time, and peripheral speeds. In addition, the plan for future research includes measuring the components of the grinding force and the geometric structure of the surface.Grinding process is a crucial stage of steel treatment in almost every industrial branch. In sustainable manufacturing, it is extremely important to produce high-quality items while reducing the cost of manufacturing and taking care of the environment and workers’ health.The proposed test stand, together with the authors’ device, makes it possible to conduct machining of the external surfaces of cylindrical workpieces on a flat surface grinder. In this case, the innovation of the grinding process consists of the non-parallel alignment of the cylindrical rotational axis of the specimen and the rotational axis of the grinding wheel with respect to the plane of the magnetic grinding table.

Optimizing the Gas Carburization Process: Strategies for Reducing Total Cycle Time

Abstract: This case study presents the results of an experiment designed to reduce the total cycle time of the gas-Carburizing process for AISI 8620 steel. By optimizing process parameters such as carburizing temperature, soaking time, and carbon potential, the effective case depth was significantly improved. Previous research has indicated that optimizing the duration of the boost/diffusion stages and carbon potential in the carburizing process can lead to a 25% reduction in total process time [1].Our study showed that it is possible to achieve a reduction in total cycle time of up to 21%, leading to cost savings for heat treatment companies while also promoting a safer environmental outcome

Dynamic response characteristics and damage mechanism of impact wear for deep plasma nitrided layer

The impact wear behaviour of deep plasma nitrided layer and its substrate (axle steel) were investigated. The nitrided layer had a 6 µm thick compound layer, and the effective case depth was 0.85 mm. Moreover, it introduced compressive residual stress with a maximum of 715.8 MPa at a depth of 511 µm. The nitrided specimens showed a lower energy absorption ratio and better impact resistance than the untreated specimens. Plastic deformation occurred in the initial test of the untreated specimens, but elastic deformation was predominant for the plasma-nitrided layer. The main wear mechanisms of the untreated samples were pitting, delamination and oxidation wear. In contrast, the plasma-nitrided specimens exhibited better oxidation resistance and slighter peeling-off than the untreated specimens.

Effective Case Depth and Wear Resistance of Pack Carburized SCM 420 Steel Processed Using Different Concentrations of Natural Shell Waste Powders and Carburizing Duration

The efficacy of using coconut shell powder (CSP) and dog conch shell powder (DCSP) as an alternative carburizing media for SCM 420 steel pack carburization was investigated. The effective case depth and wear resistance of quenched specimens prepared using various CSP–DCSP ratios and carburizing times were determined and compared. The effective case depth was measured from the microhardness profile obtained using a Vickers hardness testing machine. A CSP–DCSP ratio of 60%:40% and carburization time of 12 h were found to increase the effective case depth of SCM 420 quenched specimens to 2340 µm. The results clearly showed that the effective case depth increased by increasing carburization time and DCSP concentrations from 0% to 40% as a carburizing media and decreased by further increasing DCSP concentrations to 50%. Moreover, the wear resistance of quenched specimens increased approximately two times as DCSP concentrations were increased from 0% to 40% for a carburization time of 12 h.

Microstructural evaluation of gas nitrided AISI 316 LN austenitic stainless steel

Direct utilization of austenitic stainless steel is limited in numerous industries due to its low surface hardness. To improve the hardness of steels, gas nitriding is a convenient technique to enhance the surface hardness of austenitic stainless steel (316 LN). AISI 316 LN austenitic grade stainless steel were utilized for various applications like nuclear cladding tubes, heaters, turbine parts and blower parts. Nitiriding was carried out for three different hours namely 14, 24 and 72 hrs for the austenitic stainless in the current study. A maximum surface hardness of 1200 HV is obtained for gas nitirding technique when compared to 210 HV for the untreated specimen. Gas nitrided specimen shows a uniform nitride layer on the 316 LN stainless steel. Similarly XRD analysis of the nitrided specimen unveil the distribution of iron nitride phase which occurred during nitriding process. An effective case depth of the compounded layer from gas nitriding was found to be 30.7 µm.

Effect of carburizing process on high cycle fatigue behavior of 18CrNiMo7-6 steel

Carburizing heat treatment is widely used in the fabrication of gears to improve their fatigue performance. The effects of carburizing process on the material properties surface hardness, residual stresses, retained austenite of case-hardened 18CrNiMo7-6 and the consequential fatigue strength have not been studied in detail. In this research, the high-cycle fatigue performance of uncarburized and carburized 18CrNiMo7-6 gear steel were studied. In addition, the evolution processes of hardness, microstructure and residual stress of carburized specimens during the fatigue test were observed quasi-in situ. The initiation of fatigue cracks was also analyzed by SEM and the transformation of retained austenite during the fatigue test was observed by TEM. The results showed that the fatigue strength of carburized specimens has an increase-decrease trend with the increase of effective case depth. Besides, the fatigue failure mode of steel changes from surface failure to internal failure after being carburized, during which retained austenite plays an important part. In addition, if a large amount of retained austenite exists in the specimen with internal failure, the high-density twin martensite region formed by the retained austenite transformation can act as the crack initiation site, which is harmful to fatigue performance.

Influence of carbonitriding conditions on phase composition and residual stresses for 20MnCr5 low alloy steel

This paper reports an investigation of the influence of carbonitriding conditions for 20MnCr5 low alloy steel. Three gaseous carbonitriding conditions were investigated based on different carbon and nitrogen potentials to attain varying levels of carbon between 0.62 and 0.93% mass, whereas for nitrogen between 0.19 and 0.26% mass at the surface. Analysis of retained austenite and residual stress distributions was conducted using X-ray diffraction technique. The effective case depth varied between 900 and 1200 µm. The case microstructures were characterized by varying proportions of retained austenite and martensite, while the core contained essentially bainitic microstructures. The maximum amount of retained austenite which occurred at a depth of 50 µm from the subsurface ranged between 30 and 70% mass and significantly influenced the level of surface micro-hardness whereas the core hardness remaining relatively constant at 450 HV1. High values of residual stresses in martensite phase were observed. The signs, magnitudes, distributions and location of maximum compressive residual stresses were highly influenced by the maximum fraction of retained austenite. Retained austenite of 30%, 50% and 70% mass at the surface lead to peak compressive residue stresses of -280, -227, and -202 MPa at depths of 555, 704, and 890 μm, respectively.
 Keywords: Carbonitriding, retained austenite, martensite, residual stress, XRD.

Investigation of hardness behavior after carburizing and hardening of 15CrNi6 steel

DIN 15CrNi6 is the most representative grade of the case-hardened steels. The present work analyses the influence of carburizing time on hardness of the specific steel. Specimens with similar chemical composition were heated at 900°C in liquid carbonaceous media for one, two, three and four hours, correspondingly. Then samples were oil quenched and tempered at 180°C for two hours. Microhardness was measured across the carburized zone and case profiles were acquired. The effective case depth was determined as function of carburizing holding time. Core macro hardness was carried out and the impact of holding time on the substrate hardness was discussed. The optimum case depth was defined and the carburizing parameters determined. The hardness control is critical in case hardening practice and results provide practical information to heat treaters, useful both to control the treatment parameters and to minimize the risk of failure.

Analysis of nitrided H13 steel in rotating bending fatigue

Abstract Fatigue behavior of tool steels like AISI H13 needs to be investigated since these materials undergo heavy surface loading conditions during mechanical working operations such as forging, extrusion, etc. Nitriding is one of the methods employed to improve the surface hardness if the materials have to face surface contact or surface loading as the cracking might begin from the surface especially if there is fatigue loading pattern. This work involves heating the H13 steel specimens to 1020 °C, quenching in oil thereafter double tempering at 520 °C for two hours. The nitriding was performed at 550 °C in the atmosphere of atomic nitrogen for two hours to get the effective case depth of 200 µm. These specimens were subjected to rotating bending fatigue at room temperature conditions at a speed of 3000 rpm and constant amplitude loading. The H13 specimens were characterized for hardness, surface roughness and fractured specimens were investigated with scanning electron microscope. It was established nitrided H13 specimens indicated reduced fatigue life which could be attributed to initiation of fatigue cracks from the specimen surface or interface between the nitrided layer and the core matrix.

A Sensitivity Mapping Technique for Tensile Force and Case Depth Characterization Based on Magnetic Minor Hysteresis Loops

In the nondestructive testing and evaluation area, magnetic major hysteresis loop measurement technology are widely applied for ferromagnetic material evaluation. However the characterization ability of major hysteresis loop measurement technology greatly varies as the evaluated target properties. To solve this limitation, magnetic minor hysteresis loops, which reflect the responses of ferromagnetic material magnetization in a systematic way, is recommend. Inspired by plenty of information carried by the minor loops, the sensitivity mapping technique was developed to achieve the highest sensitivity of minor-loop parameters to the nondestructively evaluated targets. In this study, for the first time, the sensitivity mapping technique is used to measure the tensile force in a steel strand and evaluate the effective case depth in induction-hardened steel rods. The method and procedures for the sensitivity mapping technique are given before experimental detection. The obtained experimental results indicate that the linear correlation between the induced voltage (or the magnetic induction intensity) and the tensile force (or effective case depth) exists at most of the locations in the cluster of minor loops. The obtained sensitivity maps can be used to optimize the applied magnetic field (or excitation current) and the analyzed locations at the minor loops for achieving the highest sensitivity. For the purpose of tensile force measurement, it is suggested that the strand should be firstly magnetized to the near-saturation state and then restored to the remanent state. In this way, the highest sensitivity is obtained as about 15.26 mV/kN. As for the induction-hardened steel rods, the highest sensitivity of magnetic induction intensity to the effective case depth occurs under low magnetic field conditions and the absolute value of the highest sensitivity is about 0.1110 T/mm. This indicates that if the highest sensitivity is required in the case depth evaluation, the induction-hardened steel rods are only required to be weakly magnetized. The proposed sensitivity mapping technique shows the good performance in the high-sensitivity evaluation of tensile force and case depth in ferromagnetic materials and its application scope can be extended to other nondestructive detection fields.

Study on Tooth Interior Fatigue Fracture Failure of Wind Turbine Gears

Gear contact fatigue has becoming a bottleneck restricting the safety and reliability of wind turbine transmission systems. Tooth interior fatigue fracture (TIFF) failure is commonly observed in case-hardened wind turbine gears. In this work, a contact fatigue model is developed to investigate the effect of design parameters and material properties on TIFF of wind turbine gears. The sub-surface stress field is obtained by superposing the load-introduced stress and initial residual stress. The hardness gradient of the hardened layer is measured by Vickers hardness tests. Based upon the stress field and material properties, a fatigue parameter analysis is presented to characterize the risk of TIFF by using the Dang Van multiaxial fatigue criterion. Results show that the risk of TIFF increases with increasing external load. Increasing pressure angle and residual compressive stress could reduce TIFF risk. The combination of high surface hardness, low core hardness, and large effective case depth could inhibit the crack initiation.

Carburizing Behavior of AISI 4115 Steel According to the Flow Rates of Acetylene in Vacuum Carburizing

The vacuum carburizing behavior of AISI 4115 steel according to acetylene flow rates (1, 5, 10, 20, 30, 40, and 50 sccm) and temperatures (900, 950, and 1000 <sup>o</sup>C) was quantitatively investigated through microstructure, carbon concentration and hardness analyses. As the acetylene flow rate increased, the penetration rate of carbon into the surface of the samples (surface-carbon flux) increased, resulting in an increase in the solute carbon concentration at the surface up to the solubility of carbon in austenite. At the same time, the amount of carbon diffusion into the interior of the specimen and overall carburizing depth increased. When the carbon concentration in the surface of the carburized specimen exceeded the solubility of carbon in the austenite above a certain flow rate, cementite precipitated in the austenite grain boundaries. Accordingly, the surface hardness decreased and the amount of carbon diffusion into the specimen became constant as the solute carbon concentration in the austenite matrix at the surface of the specimen became constant. As a result, the overall solute carbon concentration and the effective case depth no longer increased. Meanwhile, the tendency of vacuum carburizing behavior according to temperature was the same, but as temperature increased, the surface-carbon flux and rate of carbon diffusion into the interior of the sample increased, resulting in increased carburizing depth. The appropriate flow rates (10, 20, and 30 sccm) and the corresponding surface-carbon fluxes (62.3, 135.7, and 216.7 g/m<sup>2</sup>h) were derived according to carburizing temperatures (900, 950, and 1000 <sup>o</sup>C), respectively.

Quantitative non-destructive single-frequency thermal-wave-radar imaging of case depths in hardened steels

Single-frequency thermal-wave radar (SF-TWR) imaging was used to produce dynamic images of effective case depths from phase image frequency scans in AISI 9310 and Pyrowear 53 steels. SF-TWR, as a fast non-destructive testing technique, was also compared with conventional photothermal radiometry measurements in these two types of steel samples using a three-layer theoretical thermal-wave model. In this paper, a novel approach of SF-TWR imaging, combining a three-distinct-layer thermal-wave model and radial phase profiles to image mean value case depths and their lateral non-uniform distributions, yielded quantitative images of case depths in the two hardened steels and exhibited very good correlation with standard Vickers measurements. The SF-TWR images further revealed strong inhomogeneities in the case depth thickness profiles to be used as important feedback to the heat treating manufacturing industry toward the optimization of their case depth hardening process.

The influence of grinding conditions on surface roughness parameters of 20MnCr5 steel vacuum-carburised by single-piece flow method

The purpose of the experimental tests described in the article was to determine the effect of selected abrasive machining conditions on the value of surface roughness of 20MnCr5 steel samples. The samples were subjected to vacuum carburising (LPC) using the ‘single-piece flow’ method and high-pressure gas quenching (HPGQ) in a 4D Quenching chamber to obtain an effective case depth (ECD) of 0.4 mm. Next, the samples were ground with a Norton Vortex corundum grinding wheel. Cooling and lubricating fluid was delivered to the grinding zone using the flooding method (WET) and with the minimum quantity lubrication method (MQL). The samples were ground in one pass of the grinding wheel using two grinding depths – ae = 0.01 mm and ae = 0.02 mm. For each grinding test, surface roughness measurements were made using a stationary laboratory profilometer T8000 from Hommel Werke. These measurements enabled the determination of the 2D and 3D roughness parameters. Analysis of the measurement results showed that for the adopted test conditions, the use of the MQL method resulted in a decrease in surface roughness compared to the surface ground using the WET method. This property applies to both tested grinding depths.

The influence of grinding wheel type on microhardness and residual stresses in vacuum-carburised 20MnCr5 steel using the single-piece flow method

The aim of the experiment described in the paper was to determine the effect of selected conditions of abrasive machining on the size and distribution of microhardness and residual stresses developed in the technological surface layer of flat specimens made of 20MnCr5 steel. The specimens were subjected to single-piece flow low-pressure carburising (LPC) and high-pressure gas quenching (HPGQ) in a 4D Quenching chamber in order to achieve an effective case depth of ECD=0.4 mm. This was followed by grinding the specimens with Quantum and Vortex alumina grinding wheels. Cooling and lubricating fluid were supplied to the grinding zone using the flood (WET) method. The samples were ground in one pass of the grinding wheel using one grinding depth (ae = 0.01 mm). The measurements for each specimen were made twice - after the thermo-chemical treatment and after the grinding. Microhardness and residual stress were measured using the X-ray method sin2Ψ. The final part of the article provides an analysis of the measurement results and presents conclusions and recommendations for further studies.

The study on mechanical properties of pack carburized low carbon steel using BaCO3 as energizer

Machine components such as crankshaft and gear require hard surface and ductile core characteristics. One method to obtained such characteristics is pack carburizing. This research concern to compare the mechanical properties of carburized low carbon steel under different carburizers; GBC (80% goat bone charcoal + 20% BaCO3) and BC (80% bamboo charcoal + 20% BaCO3) and derived from reference. Pack carburizing was carried out by heating up to a temperature of 950°C, holding for 4 hours and quenching in the oil. The results showed that hardness, yield strength, tensile strength and modulus elasticity of carburized steels increased significantly, however, there was a decrease in elongation. The closer to the core the lower of hardness obtained until reach a total case depth of 1.5 mm and 2 mm for carburized using BC and GBC, respectively. The use of carburizer GBC produces carburized steel with a higher yield strength (51.75 kg.mm−2), tensile strength (74.58 kg.mm−2), and elongation (5.78 %) than carburizer BC but lower in hardness (558.13 HV1), modulus elasticity (256.79 GPa), and effective case depth (0.33 mm). The mechanical properties of carburized low carbon steels have fulfilled requirements as the material of gear or shaft.

Surface Characteristics and Distortion Analysis of the Case-Hardened Helical Gears: A Comparison of Different Case-Hardening Treatments

In this study, the effect of different case-hardening treatments on microstructure, hardness distribution, surface roughness parameters, and dimensional changes was investigated for helical-toothed gears made of 16MnCr5 and 21NiCrMo2 steels. Gas carburizing, gas carburizing–double quenching, low-pressure carburizing, and plasma-nitriding treatments were performed as case-hardening processes. The surface characteristics and distortion analyses of the case-hardened helical gears were examined using an optical microscope, hardness tester, profilometer, and micrometer. In order to ensure adequate pitting and bending strength, the required minimum effective case depth and minimum total case depth values were provided by the performed case-hardening treatments. In the gears, the largest dimensional increase after the heat treatments occurred at the end of the gas carburizing due to the long treatment times at high processing temperatures. The surface roughness values obtained through all performed case-hardening treatments necessitated the finishing processes.

Verification on Possibility of Nitrogen Diffusion into Steel by High Temperature N-Quench

N-Quench, which is a new surface heat treatment to infiltrate nitrogen into steel parts followed by quenching to achieve hardening, is gathering attention in the nitriding field as it affords low distortion while maintaining a higher effective case depth (ECD) compared with conventional nitriding. N-Quench is conducted mainly between 680°C and 800°C, where the two-phase region of ferrite and austenite exists in the Fe-N phase diagram. However, a few studies have reported on nitriding at temperatures higher than 800°C due to decomposition of NH3, which is a key source of nitrogen infiltration. Our results revealed that in a conventional furnace such as resistance heating furnace, no nitrogen infiltrated the specimen at 930°C, which is the general carburizing temperature. On the other hand, in the infrared heating furnace, nitrogen infiltrated the specimen at 930°C successfully with lesser NH3introduction than that required by the conventional furnace. Therefore, in this study, the limit of NH3decomposition is assessed and possibility of extending the applicability of N-Quench, especially increasing the ECD while maintaining a low distortion, is examined.