Core Hardness Research Articles

Response of a DIN 18MnCrSiMo6-4 Continuous Cooling Bainitic Steel to Plasma Nitriding with a Nitrogen Rich Gas Composition

The use of continuous cooling bainitic steels can provide a more energy efficient manufacturing route. However, for their use in mechanical components like gears, it is necessary to improve their surface properties without impacting the core properties to guarantee reliable mechanical performance. The effect of temperature and time on the plasma nitriding response of a DIN 18MnCrSiMo6-4 steel was investigated. The plasma nitriding was performed for 3, 6 and 9 hours, at 400, 450, 500 and 550 °C, using a gas mixture composed of 76 vol.% nitrogen and 24 vol.% hydrogen. Samples were characterized before and after plasma nitriding concerning the microstructure, hardness and microhardness, fracture toughness, phase composition and residual stress states. Based on the results presented, layer growth constants (k) for different temperatures was determined. Moreover, it could be found that 500 °C gave the best results investigated here, as higher temperature took to core and surface hardness decrease. The nitrided samples with thicker compound layers presented a fracture behavior dominated by the formation of Palmqvist cracks. X-ray phase analysis indicated the formation of biphasic compound layer on the surface of all nitrided samples. The diffusion zone presented compressive residual stresses with highest values near the surface.

Effect of slide burnishing of shoulder fillets on the fatigue strength of X19NiCrMo4 steel shafts

The article presents the results of investigations on the influence of the technological parameters of slide burnishing of shoulder fillets on a shaft on the fatigue strength of machine components. Fatigue tests were carried out for shafts made of X19NiCrMo4 steel after the electroslag remelting process. The burnishing process of the shoulder fillets of shafts were carried out with tools characterized by different values of tip radius and at varying angles of tool slope in relation to normal with regard to the generating line of the shaft journal. The burnishing improved fatigue strength by 28.5% compared to the non-burnished specimens. Compared with turning, the surface roughness of specimens subjected to slide burnishing decreased in the range from 64.1% to 85.8%. The measurements of the microhardness of the surface layer showed that through slide burnishing, it is possible to strengthen the surface layer by 32% down to a depth of up to 0.018 mm compared to the core hardness.

Influence of in-service thermal softening on wear and plastic deformation in remanufactured hot forging dies

The need for economic and greener production methods necessitates remanufacturing of used hot forging dies. However, it has been reported from industries that the remanufactured forging dies show higher wear rate, and plastic deformation as compared to new dies. In this work wear and plastic deformation behaviour in new, and remanufactured hot forging dies at different stages of forging cycles are experimentally studied. Die geometry was digitised using three dimensional optical scanner to analyse the wear depth, and plastic deformation. Compared to new dies, significant wear and plastic deformation was found in remanufactured dies. Since in-service microstructural changes are expected to influence wear and plastic deformation behaviour, microhardness tests were conducted on die specimen to measure the variations in hardness from surface to core regions of die. This study reveals that a thermally softened layer grow on the die surface with each forging cycle and in subsequent cycles, this layer gets removed by in-service wear and plastic deformation. In addition, the core hardness in new and remanufactured dies was found to decrease with increasing hot forging test duration. Consequently, remanufactured dies exhibits a lower core hardness, as compared to new dies, which eventually results in higher wear loss and plastic deformation.

The investigation on the fabrication and microstructure of NbTi-(NbTi)C Fe-based composite

NbTi-(NbTi)C Fe-based composites are fabricated by in-situ diffusion reaction. The locally reinforced (NbTi)C cermet layer formed by carrying out the reaction of C + β-NbTi → (NbTi)C, which resulted in the formation of the core-annular shell structure with a ductile core (NbTi) and high hardness annular shell (cermet layer). The (NbTi)C cermet layer is composed of two zones, where ZoneⅠ, near the GCI, is composed of micron and sub-micron-sized particles; and ZoneⅡ, near the remnant β-NbTi, is composed of sub-micron-sized particles. The detailed microstructural, elemental and morphological analysis has been carried out to unveil the phase distribution and microstructural changes in the (NbTi)C cermet layer. The results reveal that the cermet layer consists of (NbTi)C particles with different Ti/(Ti + Nb) ratios, ranging from 0.6308 to 0.8330, which have different morphology: the fusiform particles and near spherical or cubic particles. Moreover, the nanoindentation results of (NbTi)C layer indicate that the maximum hardness and elastic modulus is 31.43 GPa and 473.11 GPa, respectively. It is worth mention that the remnant β-NbTi and α-Fe phases, surrounding the (NbTi)C cermet layer, render high plasticity. Therefore, the combination of β-NbTi, α-Fe and (NbTi)C phases can effectively improve the hardness and plasticity of as-prepared composites.

Investigation of the influence of hardness characteristics of the collet-part lathe system

The results of the study of the hardness characteristics of the collet-part lathe system, which are essentially nonlinear and influence the damping parameters of the spindle assembly unit, are presented. The aim of the work is to study the characteristics of the nonlinear hardness of the elements of the collet-part technological system. In order to realize this aim, in the course of experimental research, static and dynamic characteristics of the spindle core hardness are defined, which are looped hysteresis curves that are characteristic of mechanical systems with a large number of bonds. Measurements of nonlinear parameters of the hardness of the collet-part system are carried out. Measurements are made according to the original method. The basis of the technique is the multi-cycle static load of the spindle in the processing zone in the direction of the vector of the dynamic component of the cutting force by the developed equipment. The equipment includes a screw loader, a two-way ring dynamometer and a mandrel fitted in the spindle of the machine. As a result of the research, the features of the nonlinear characteristics of the hardness of the collet-part lathe system are determined. For the collet-part system, the width of the hysteresis loop in the cold state can reach 20...70 microns. The hysteresis characteristics of the collet-part system in the heated state have the width of the hysteresis loop of 50...200 microns with clamping the part with a diameter of 80...115 mm and a radius of 100...120 mm. Recommendations on the assessment of the hardness change limit depending on the design of the collet are developed. The proposed technique allows one cartridge position to be measured by pressing according to the scheme of loading «on a cam» and «between cams», which reduces twofold the amount of experimental research.

Low Temperature Plasma Nitriding of Low Alloy Steel for the Enhancement of Hardness

Abstract The present concerns surface modification of low alloy CrMoV steel by following the route of plasma nitriding for the improvement of hardness. Plasma nitriding was performed at a low temperature of 450 oC. The maximum hardness at this lower temperature was found to be ∼1270 Hv after nitriding. At 550 oC though the hardness improvement near the surface region was similar but the core hardness was reduced. XRD studies revealed various nitrides of iron (γ′ (Fe4N) and e (Fe2–3N)) with the e nitride as the dominant phase. SEM analyses revealed the surface microstructure with almost no white layer. EDS point analyses shown the signature of N on the surface layer. It has been concluded in this study that the hardness of 90CrMoV8 steel can be improved significantly even at a low temperature of 450 oC without any of risk of the loss of core hardness.

Characterization of ferroniobium topologically close-packed phase inclusion in nickel-based 152 cladding of dissimilar metal welds

Ferroniobium topologically close-packed (TCP) phase inclusions were found in the nickel-based152 nickel-based cladding of dissimilar metal weld joint in pressurized water nuclear reactor. A systematical investigation on the morphologies, microstructures and mechanical properties was conducted by using multi-scale techniques. The formation causes of the TCP-phase inclusion and its possible influences on crack initiation and propagation during long-term operation are also discussed. The ferroniobium TCP-phase inclusions originate from large-size ferroniobium supplement particles in 152 welding electrode flux during welding. The inclusion with core has a homogeneous kernel and a continuous transition zone between the core and eutectic zone, and the transition zone consists of columnar grain zone and isometric grain zone. The inclusion core is confirmed as single-phase rhombohedral Fe7Nb6 (μ phase), and columnar grain zone in the transition zone is also mainly Fe7Nb6, while the isometric grain zone and the Nb-rich phase in the eutectic zone are both mainly hexagonal Fe2Nb (Laves phase), the other phase in the eutectic zone is austenite. The Fe2Nb is formed by eutectic metallurgical reaction between inclusion core and 152 cladding. The average hardness of inclusion core (17.89 GPa) is over 4.5 times as high as cladding matrix (3.91 GPa). The TCP-phase inclusion is of higher cracking susceptibility than 152 cladding, and acts as the initiation of brittle cracks during in-situ tensile test. The ferroniobium TCP-phase inclusions are of higher cracking susceptibility than 152 cladding.

Investigation of the Tempering Effect during Nitriding*

Abstract Nitriding is a widely used treatment to increase the performance of unalloyed and alloyed steels. Because nitriding majorly affects the hardness of the surface region of the material it is common to perform a quenching and tempering treatment before nitriding in order to get a suitable core hardness and microstructure for application. The hardness from pre-heat treatment is nearly not affected by the following nitriding process if the temperature of tempering exceeds the nitriding temperature more than ca. 30 °C. Nevertheless in some cases where a high nitriding depth, i. e. 0.8 mm and more, is necessary, the nitriding parameters temperature and time can affect the material's state, which was achieved by tempering, even though the nitriding temperature is lower than the temperature of the tempering treatment. The current investigations concern the tempering behaviour of typical nitriding and heat treatable steels. In nitriding experiments with different treatment temperature and time the stability of the pre-heat treatment state was investigated and the Hollomon-Jaffe parameter was used to describe the effect of temperature and time on the material's properties. Furthermore it was shown, that the Hollomon-Jaffe parameter is also suitable to predict the tempering effect of a long time nitriding process.

Microstructural evolution and sintering kinetics during spark plasma sintering of pure tantalum powder

Spark plasma sintering (SPS) technology can significantly inhibit grain coarsening due to its characteristics of rapid sintering and densification, and obtain a material with high density and uniform microstructure. We completed the SPS sintering of tantalum powder, and characterized the microstructure and mechanical properties of the samples at different sintering temperatures. The results show that during the pure tantalum sintering process, the rapid densification temperature range is 800 °C–1300 °C, and the maximum shrinkage rate of the sample is 1100 °C. A creep model was applied to determine the densification mechanisms involved in the densification stage, which can be elucidated by evaluating the stress exponent (n) and the apparent activation energy (Qd) from the densification rate law. It shows that a series of interfacial reactions and grain boundary diffusions govern the densification process at low effective compaction stresses (n = 1.6, Qd = 107.01 kJ/mol), while dislocation climbing operate at higher effective compaction stresses (n = 3.1 and n = 4.1), which is confirmed by TEM. When the sintering temperature of tantalum powder increased from 1500 °C to 1700 °C, the density and grain size of the sample increase gradually, the tensile strength and flexural strength also increase. The surface hardness of the sample increases gradually. Moreover, surface hardness is much higher than core hardness. The fracture mode of the specimen changes from brittle fracture to ductile fracture. This study has enabled the selection of high quality tantalum alloy sintering routes and optimization of the sintering process.

Influence of plasma nitriding pretreatments on the tribo-mechanical properties of DLC coatings sputtered on AISI H11

The duplex treatment, consisting of plasma nitriding and the deposition of a DLC coating, was carried out on the hot-work tool steel AISI H11. The coating structure, composed of Cr-based interlayers and a hydrogenated carbon layer, was sputtered on non-nitrided, nitrided, as well as nitrided-repolished AISI H11 steel with an either annealed or quenched and tempered base condition to examine the influence of the pretreatment condition on the tribo-mechanical properties of the DLC coating.Besides the graded hardness profile, plasma nitriding leads to a roughness increase, which affects the microstructure as well as the mechanical properties of the DLC coating. The rougher surface favors a film growth of a carbon layer with larger cluster-like structures. As a result, these DLC coatings exhibit hardness values below 22 GPa, while the coating systems sputtered on substrates with smoother surfaces reach values of approximately 26 GPa and showed a good adherence. The heat treatment condition influences the load-bearing capacity of the nitrided substrate as the higher core hardness enhances the mechanical support of the coating and reaches the highest adhesion class HF1 in the Rockwell C tests. Due to the lower film adhesion and the low hardness of the DLC coatings sputtered on nitrided non-repolished AISI H11, high coefficients of frictions and wear coefficients of up to 0.59 and 3.19 ∗ 10−5 mm3/N∗m were determined in tribometer tests against WC/Co counterparts. In contrast, the nitrided repolished steel exhibits a low coefficient of friction of 0.12 as well as a low wear coefficient of 0.06 ∗ 10−5 mm3/N∗m. Therefore, a repolishing of the nitrided AISI H11 with quenched and tempered base condition ensures the highest load-bearing capability of the substrate as well as an improved friction and wear behavior of the DLC coating.

The effect of thermal maturity on geomechanical properties in shale reservoirs: An example from the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin

Abstract Shale reservoirs are characterized by low matrix permeability and therefore require effective models of geomechanical properties to optimize drilling and hydraulic fracturing strategies. Both initial rock composition and thermal maturity are potentially critical controls on geomechanical properties. We investigate the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, a major shale gas target that spans a wide range in rock compositions and thermal maturity to identify relationships between these parameters and geomechanical properties. Core hardness measurements and dipole sonic and density log data were used to characterize the geomechanical properties. Major element chemical analysis, X-ray diffraction analysis and LECO combustion were used to determine mineralogy, bulk rock chemistry and total organic carbon (TOC) content and to distinguish biogenic from detrital silica. Scanning electron microscopy (SEM) images with complementary energy-dispersive spectroscopy (EDS) maps were obtained for representative samples to document the rock fabric and distribution of organic matter and minerals. Hardness and Al2O3 concentrations are strongly negatively correlated in all cores, regardless of thermal maturity, suggesting that clay minerals are the most significant factor controlling geomechanical properties. Biogenic silica is positively correlated to hardness. Detrital silica is negatively correlated to hardness, an artifact of the positive correlation between detrital clay minerals and detrital quartz. The positive correlations between CaO content and hardness in all cores suggest the brittle behavior of carbonate minerals. Increased thermal maturity from immature to oil window results in greater hardness for rocks of similar geochemical compositions. We propose that this results from: (1) enhanced mechanical compaction; (2) carbonate cementation; (3) increased stiffness of kerogen; and (4) partial conversion of kerogen into expelled hydrocarbons that reduces the load-bearing function of the organic matter. At maturities greater than oil window, thermal maturity does not exert a major control on the rock strength; thus, there are no significant changes in hardness between samples from oil window and dry gas window. Geomechanical properties can be related to mineralogy, which exerts a major impact on the geomechanical properties. Quartz cements sourced from biogenic silica rather than smectite-to-illite transition are the primary contributor to the rock strength in the Duvernay Formation. However, it is noteworthy that thermal maturity should also be considered as an important factor when predicting geomechanical properties from mineralogy.

Identifying process parameters influencing gear runout

AbstractThe hardening distortions with respect to base body, clutch teeth and helical gears are investigated for a serial‐produced main shaft gear of a 20NiCrMoS6‐4 steel. The influences of casting geometry, annealing heat treatment and stress relief annealing of blanks, as well as vertical and horizontal loading arrangements during case hardening, are studied. The concentricity, roundness and runout of clutch teeth and helical gears are measured in the soft machined, hardened and hard‐machined conditions. The Brinell hardness is measured on blanks obtained from different manufacturing routes showing differences in hardness and scatter. Stress relief annealing lowers the hardness and the scatter for all groups, but has no significant effect on distortions. The case depth, core hardness and surface hardness are measured after hardening. The study shows that the surface hardness correlates with the oil flow measured in the quench tank. The effect of casting geometry is stronger for the clutch teeth compared to the helical gears. For the clutch‐teeth roundness and runout, significantly lower values are found for square geometry compared to rectangular. It is also seen that the major part of the runout comes from roundness errors which are mainly induced by the hardening. Horizontal loading reduces roundness errors and runout but produces conical base‐bodies with worse backplane flatness.

Mechanical Properties of Cubic (U,Zr)O2

Evaluation of fuel debris properties in the Fukushima Daiichi nuclear power plant (1F) is required to develop fuel debris removal tools. In the removal of debris resulting from the Three Mile Island unit 2 (TMI-2) accident, a core-boring system played an important role. Considering the working principle of core boring, hardness, elastic modulus, and fracture toughness were found to be important fuel debris properties that profoundly influenced the performance of the boring machine. It is speculated that uranium and zirconium oxide solid solution (U,Zr)O2 is one of the major materials in the fuel debris from 1F. In addition, the Zr content of the fuel debris from 1F is expected to be higher than that of the debris from TMI-2 because the 1F reactors were boiling-water reactors. In this research, the mechanical properties of cubic (U,Zr)O2 samples containing 10%–65% ZrO2 are evaluated. The hardness, elastic modulus, and fracture toughness are measured by the Vickers test, ultrasonic pulse echo method, and indentation fracture method, respectively. In the case of (U,Zr)O2 samples containing less than 50% ZrO2, Vickers hardness and fracture toughness increased, and the elastic modulus decreased slightly with increasing ZrO2 content. Moreover, all of those values of the (U,Zr)O2 samples containing 65% ZrO2 increased slightly compared to (U,Zr)O2 samples containing 55% ZrO2. ZrO2 content affects fracture toughness significantly in the case of samples containing less than 10% ZrO2. Higher Zr content (exceeding 50%) has little effect on the mechanical properties.

Microstructure and mechanical properties of Fe-Cr-2Ni-Mo-V steel in carburizing process

The microstructural transition and mechanical properties of Fe-Cr-2Ni-Mo-V steel were investigated in a carburizing process with carburizing at 935 °C, oil quenched, re-quenched at 880 °C,and low-temperature tempered at 180 °C. X-ray diffraction was employed for phase analysis and Ir Carbon-sulfur Spectrometer was used for carbon content detection. The morphologies of martensite were observed by scanning electron microscopy. The Vickers hardness was measured and the impact toughness was measured by means of Charpy’s V-notch tests. The results show that the carbon concentration and hardness gradients are established after carburizing process as a function of distance from the case to core. The carbon content gradient results in a gradient of martensitic microstructures, ranging from plate martensitic morphologies with large amounts of retained austenite in case to lath martensitic microstructures with essentially no retained austenite in core. The mechanical properties are significantly improved with adding re-quenching and low-temperature tempering. The case hardness, core hardness, tensile strength and impact energy are 851 HV, 402 HV, 1310 MPa and 92 J, respectively.

Abrasion wear of cutting tool developed from recycled steel using palm kernel shell as carbon additive.

Most of the cutting tools in use are imported and the cost of replacement is high, hence there is need to develop them locally. This research work studied the abrasion wear of cutting tools developed from recycled steel using palm kernel shell (PKS) as carbon additive. The chemical composition of the materials was analysed before and after the melting. Alloy additives were added and then carburized in a furnace at different temperatures and time ranges. The toughness, surface and core hardness were carried out given 24.0 J/cm2, 76.8 and 47.9 HR with 17 J/cm2, 76.3 and 46.1 HR for the control. It was observed that the tool cut the work piece easily on a lathe machine with the best minimum length of cut 50 mm for the time of cut 1.19 and 2.02 minutes mm. It can be concluded that the developed cutting tool showed significant performances during machining.

Parametric Effects of Carburization Time and Temperature on the Mechanical Properties of Carburized Mild Steel

The study of process parameters in metals during heat treatment has been of considerable interest for some years but there has been relatively little work on process variables during the surface hardening process since controlling parameters in carburization is a complex problem. This work focuses on the parametric effects of the carburizing temperature and time on the mechanical properties of mild steel carburized with activated carbon, at 850, 900 and 950 °C, soaked at the carburizing temperature for 15 and 30 minutes, quenched in oil, tempered at 550 °C and held for 60 minutes. Sequel to carburization process, standard test samples were prepared from the as received specimen for tensile and impact tests. After carburization process, the test samples were subjected to the standard test and from the data obtained, ultimate tensile strength, engineering strain, impact strength, Youngs’ moduli were calculated. The case and core hardness of the carburized tempered samples were measured. Observation shows that the mechanical properties of mild steels were found to be strongly influenced by the process of carburization, carburizing temperature and soaking time at carburizing temperature. It was concluded that the optimum combination of mechanical properties is obtained at the carburizing temperature of 900°C followed by oil quenching and tempering at 550 °C.

Quality-cost analysis of modern carburising technology implementation in the production cycle of cog elements for the aviation industry

Purpose: The aim of the study was to analyse the possibility of introducing vacuum carburising technology with pre-nitriding in the aviation industry. Design/methodology/approach: The samples have been tested with new technology and the planet gear requirements for aviation applications were determined. The effect of pre-nitriding technology used before vacuum carburising was also investigated for antidiffusional surface protection. A cost analysis of replacing the conventional technology with a new low-pressure carburising technology has also been introduced. Findings: Structural characteristics of the obtained layers were determined in terms of carbon concentration distribution, effective and total thicknesses, hardness of the surface and core, structure, grain size, residual austenite content and lack of decarburization and internal oxidation. The degree of degradation of various forms of anti-diffusion protections in the form of pastes and coppering and their effect on the surface layer have also been investigated. The comparative economic analysis of the use of conventional and vacuum technologies, namely the carburization in a function of the required thickness of the hardened layer and temperatures of processes, has also been made. Research limitations/implications: Basing on the studies and analyses carried out in this work, it can be concluded that the introduction of pre-nitriding carburising technology instead of conventional carburising technology in endothermic atmosphere is justified qualitatively and economically alike. Practical implications: The analysis of the test results confirms that it is possible to meet the requirements of the aviation industry through high temperature pre-nitriding vacuum carburising. The results of anti-diffusion protection tests confirm that it is possible to use anti-fouling pastes instead of electrolytic copper. Cost analysis confirms the economic benefits of implementing a new technology. The implication of the above is the willingness of the proposed technology to allow testing in the aviation industry. Originality/value: The research confirming the fulfilment of quality requirements set for hardened layers in aviation applications which also have an economic advantage.

Characterization of simultaneous aged and plasma nitrided 17-4 PH stainless steel

Microstructural characteristics (using TEM, SIMS, SEM, optical profilometry and XRD) and residual stress (using nanoindentation and XRD) of simultaneously aged and plasma nitrided martensitic 17-4 PH stainless steel were investigated. It was observed that morphology and microstructure produced during plasma nitriding of 17-4 PH stainless steel were very dependent on the treatment temperature and time. An unetchable nitrided layer formed on specimen surface nitrided at 400°C which contained expanded martensite, whereas nitrided layers at 450 and 500°C contained dark phases of nitrides in addition to expanded martensite. Nitride precipitates and expanded martensite laths were observed in TEM images and their corresponding diffraction patterns. Lack of incoherent copper rich precipitates in TEM confirmed that aging and plasma nitriding were performed simultaneously. SIMS profiles revealed that the total diffusion depth of nitrogen is 3–4 times thicker than the nitrided case. The lowest and highest surface residual stresses were contributed to PN400C5h and PN400C10h, respectively. Although hardness values increased by increasing temperature/time of plasma nitriding process, i.e. increasing nitrided layer thickness, residual stress was a function of residual stress relaxation at high temperature in addition to thickness of nitrided layer. Residual stress depth profile showed that maximum compressive residual stress at the surface, dropped to zero just below the nitrided case, then increased to a maximum value of tensile residual stress; then decreased gradually to zero. Hardness depth profile measurements revealed that hardness was very dependent on residual stress induced by nitrogen atoms diffused interstitially during plasma nitriding. Core hardness of specimens made it evident that aging took place during plasma nitriding.

Influence of simultaneous aging and plasma nitriding on fatigue performance of 17-4 PH stainless steel

This paper investigates fatigue behavior of precipitation hardenable 17-4 PH stainless steel after simultaneous aging and plasma nitriding. For this purpose, “solution treated”, “aged” and “simultaneous aged and nitrided” specimens were compared in terms of hardness, phase analysis, residual stress and fatigue strength. Hardness values of aged specimens were recorded during aging process to find the optimum condition in terms of time and temperature for plasma nitriding at which specimens can be simultaneously nitrided and aged. Plasma nitrided specimens were analyzed for residual stresses, and its effect on mechanical properties including hardness and fatigue strength. The specimens that were plasma nitrided at lower temperatures had the highest core hardness. Increasing the nitriding temperature/time caused an increase in residual stress and, consequently, a higher surface hardness. Both nitriding and aging processes improved fatigue life by more than 40%. Plasma nitriding imparts beneficial effect mainly during high stress fatigue while aging treatment is more effective on low stress fatigue properties. Specimens nitrided at 500°C for 5h experienced longer fatigue life for high stress conditions while specimen aged/nitrided at 400°C for 10h exhibited the highest fatigue strength.

The Nitrocarburising Response of Low Temperature Bainite Steel

The nitrocarburising response of low transformation temperature ultrafine and nanoscale bainitic steel was investigated and compared with martensite and pearlite from the same steel composition. It was found that the retained austenite content of the bainitic steel dictated the core hardness after nitrocarburising. The refined bainitic structure showed improvements in the nitriding depth and hardness of the nitrocarburised layer, compared to coarser grained martensitic and pearlitic structures, possibly due to the fine structure and the distribution of nitride forming elements.