Microhardness Gradient Research Articles

Porosity and microstructure in pulsed Nd:YAG laser welded Ti6Al4V sheet

The overlapping factor of pulsed laser welding is used to help understand the correlation between welding parameters and the quality of Ti6Al4V welded joints. The number of porosity decreases with the increase in overlapping factor, and the welded joints are almost completely free of porosity when overlapping factor is greater than 75%. This can be attributed to the fact that the remelted volume of the spot region increases with the increase of overlapping factor, which assists porosity formed in the previous pulse wave in escaping from molten pool formed by the subsequent pulse. With the increase of overlapping factor, the weld microstructure becomes much coarser and the width of the fully transformed region of heat affected zone increases, which reduces the microstructure gradient and microhardness gradient from the fusion zone to heat affected zone. A method to evaluate the porosity susceptibility of a specific welding condition prior to actual welding process is presented.

Electric pulse treatment of welded joint of aluminum alloy

Purpose. Explanation of the redistribution effect of residual strengthes after electric pulse treatment of ark welding seam of the aluminum alloy. Methodology. Alloy on the basis of aluminium of АК8М3 type served as the research material. As a result of mechanical treatment of the ingots after alloy crystallization the plates with 10 mm thickness were obtained. After edge preparation the elements, which are being connected were butt welded using the technology of semiautomatic argon arc welding by the electrode with a diameter of 3 mm of AK-5 alloy. Metal structure of the welded joint was examined under the light microscope at a magnification of 200 and under the scanning electronic microscope «JSM-6360 LA». The Rockwell hardness (HRF) was used as a strength characteristic of alloy. Hardness measuring of the phase constituents (microhardness) was carried out using the device PМТ-3, with the indenter loadings 5 and 10 g. The crystalline structure parameters of alloy (dislocation density, second kind of the crystalline grid distortion and the scale of coherent scattering regions) were determined using the methods of X-ray structural analysis. Electric pulse treatment (ET) was carried out on the special equipment in the conditions of the DS enterprise using two modes A and В. Findings. On the basis of researches the previously obtained microhardness redistribution effect in the area of welded connection after ET was confirmed. As a result of use of the indicated treatment it was determined not only the reduction of microhardness gradient but also the simultaneous hardening effect in the certain thermal affected areas near the welding seam. During study of chemical composition of phase constituents it was discovered, that the structural changes of alloy as a result of ET first of all are caused by the redistribution of chemical elements, which form the connections themselves. By the nature of the influence the indicated treatment can be comparable with the thermal softening technologies of metallic materials. Originality. The observed structural changes of alloy and related to them microhardness change in the areas near the welding seam after ET are conditioned by both the change of morphology of structural constituents and the redistribution of chemical elements. In case of invariability of chemical elements correlation in the phase constituents of alloy the reduction effect of gradient microhardness should be far less. Practical value. In practice, the negative effect of the wares embrittlement made using the casting technologies, excluding the pressure casting and quite difficult selection of chemical composition of alloy can be significantly reduced during the treatment of alloy with electric pulses.

Surface nitriding influence on the fatigue life behavior of ASTM A743 steel type CA6NM

Surface treatment such as plasma nitriding has been largely used in several industrial applications because it improves the mechanical, tribological, and chemical properties of materials. This work evaluates the influence of the treatment time on the nitrided layer formation of the martensitic stainless steel ASTM A 743 CA 6NM and its resulting fatigue life behavior. Samples were nitrided at 500°C and a pressure of 4mbar (400Pa) in a gaseous mixture containing hydrogen (H2: 80 vol %) and nitrogen (N2: 20 vol %). Treatment times were 1, 3 and 5h. The properties of the nitrided layers obtained were assessed through microhardness tests, scanning electron microscopy analysis and X-ray diffractometry to identify the phases formed in the layers, construct the S–N curve and determine the fatigue life for each treatment time. For all treatment times, there was a substantial increase in surface hardness, as well as in fatigue life with respect to the non-nitrided material. These results are associated with the generation of the compound layer along with the diffusion layer and the formation of iron (ε-Fe2–3N and γ′-Fe4N) and chromium nitrides. During nitriding at 500°C for 1h, the surface hardness increased by 385% and the fatigue limit increased by 23.76% at 2 million cycles. Higher microhardness gradient between the specimen's surfaces and nuclei was observed for lower nitriding times, resulting in higher compressive residual stress values of the surface for 1h nitriding time and higher fatigue limit.

Microstructure and properties of laser-borided 41Cr4 steel

Laser-boriding, instead of diffusion-boriding, was applied to formation of boride layers on 41Cr4 steel. The microstructure and properties of these layers were compared to those obtained after typical diffusion-boriding. Three zones characterized the microstructure of laser-borided layer: laser-borided zone, hardened medium-carbon zone (heat affected zone) and medium-carbon substrate without heat treatment. The through-hardened laser-borided steel was also analyzed. In this case two zones characterized the microstructure: laser-borided zone and hardened medium-carbon substrate. The microstructure of laser-borided zone consisted of eutectic mixture of borides and martensite. This phase composition (especially martensite presence) was the reason for microhardness decrease at the surface in comparison with diffusion-borided steel. However, the use of laser-boriding causes the decrease in microhardness gradient between the surface and the substrate in comparison with typical diffusion-boriding process. The value of mass wear intensity factor of the hardened laser-borided layer was comparable to that obtained in case of diffusion-boriding and through-hardening. The use of laser-borided layers instead of typical diffusion-borided layers may be advantageous under conditions of high abrasive wear of mating parts. For the experimental condition used, the laser-boriding process presented worst results concerning the fatigue strength. The cracks formed on the surface during laser re-melting were the reason for relatively quick first fatigue crack. In case of elements, which require high fatigue strength, the use of modified laser processing parameters would be necessary. The better results should be obtained by increasing of tracks overlapping. Although the cohesion of laser-borided layer was sufficient, the diffusion-borided layer showed a better cohesion.

Estimation of Hydrogen Diffusivity in Austenitic Stainless Steels by Microhardness Gradient Technique

The present study developed a subsurface microhardness gradient technique to estimate hydrogen diffusivity of stainless steels, as per the similarity between concentration distribution and hardness gradient. Cathodic charging were performed on 304 stainless steels for 24 h in a 0.5 mol/L H2SO4 solution using a current density of 100 mA/cm2, with 0.25g/L Na2S as the hydrogen recombination poison. Microhardness in the cross-sectional region had an increase than the uncharged materials due to the hardening mechanism as found by martensite transformation. Hydrogen diffusivity was estimated using the microhardness data and the diffusion equation. The estimated diffusivity of hydrogen at 306 K in 304 stainless steels is 3.28×10-13 m2/s, which has good agreements with the one measured by time-lag electrochemical method in a previous research.

Improvement in Wear and Corrosive Wear Resistance of Ti6Al4V Alloy by Double Glow Plasma Surface Alloying with W-Mo and W-Mo-N

W-Mo and W-Mo-N surface-modified layers on Ti6Al4V alloy were obtained using a double glow plasma surface alloying technique. The morphology, microstructure, and chemical composition distribution of the modified layers were analyzed by scanning electron microscope, Xray diffraction, and glow discharge optical emission spectrometry. The hardness and toughness of the modified layers were measured using a micro-hardness tester, and dynamic repeating press equipment. The wear resistance in ambient air and the corrosive wear resistance in NaCl solution were evaluated using a ball-on-disk wear tester. The results show that W-Mo and W-Mo-N surface modified layers are composed of the alloying layers which vary in composition and phase form along the depth. A microhardness gradient was observed in the modified-surface layers. The surface hardness of the W-Mo-N and W-Mo modified layers was 25.3 and 14.2 GPa, which is seven-fold and 3.9-fold harder than the Ti6Al4V substrate, respectively. W-Mo and W-Mo-N surface-modified layers significantly improved the wear and corrosion resistance of Ti6Al4V. It seems that the wear resistance of W-Mo and W-Mo-N surface-modified layers in NaCl solution is better than that in ambient air owing to the strong lubricating effect of NaCl solution and the excellent corrosion resistance of the modified layers.

Ageing Embrittlement of 15Cr1Mo1V Steel Welded Joints Evaluated by Small Punch Test

The microstructure and property of 15Cr1Mo1V steel welded joints after 70,000h service have been researched by OM, SEM, Vickers hardness test and small punch test (SPT). The microstructure of 15Cr1Mo1V steel welded joints after service became coarse and the grain boundary became vague. More carbide particles were observed in welded joints after service. The microhardness gradient curve of 15Cr1Mo1V steel welded joints before service was higher than that after service. The SPT properties gradient curves, including yield strength gradient curve and tensile strength gradient curve by SPT, had the same shape and trend as the microhardness gradient curve before and after service. The SEM fracture appearance of SPT specimens was the characteristic of ductile fracture, irrespectively of before or after service, though the dimple size was a little bit big and deep before service and some cluster particles were observed after service for the ripening of carbides.

Failure Analysis of a Vehicle Engine Crankshaft

An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.

The influence of carbon content in the borided Fe-alloys on the microstructure of iron borides

This paper presents the results of Electron Back-Scatter Diffraction (EBSD) analyses of the borided layers produced on substrate of varying carbon content. Two types of materials were investigated: borided Armco iron of very low carbon content and borocarburized chromium- and nickel-based steels of high carbon content beneath iron borides. The tetragonal phase Fe 2B was identified in all materials studied. It was difficult to obtain an EBSD pattern from iron boride (FeB) because of its presence at low depths below the surface, and because of the rounded corners of the specimens. EBSD provided information on the orientation of Fe 2B crystals. In case of the low-carbon Armco iron the crystallographic orientation was constant along the full length of the Fe 2B needle. The EBSPs obtained from borocarburized steel indicate that the crystallographic orientation of the Fe 2B phase changes along the length of the needle. This is the result of hindered boron diffusion due to boriding of the carburized substrate. The increased resistance to friction wear of borocarburized layers arises from two reasons. One is the decreased microhardness gradient between the iron borides and the substrate, which causes a decrease in the brittleness of the iron borides and an improved distribution of internal stresses in the diffusion layer. The second is the changeable crystallographic orientation of iron borides, which leads to the lower texture and porosity of borided layers. These advantageous properties of the borocarburized layer can be obtained if the carbon content beneath the iron borides is no more than about 1.0–1.2 wt.% C.

Two-step treatment carburizing followed by boriding on medium-carbon steel

Combined surface hardening with boron and carbon was used for medium-carbon 41Cr4 steel. The microstructure, carbon profiles and chosen properties of borided layers produced on the carburized 41Cr4 steel (similar in chemical composition as AISI 5140 steel) have been examined. This two-step treatment carburizing followed by boriding is termed borocarburizing. The microhardness profiles and wear resistance of these layers have been studied. In the microstructure of the borocarburized layer two zones have been observed: iron borides (FeB+Fe 2B) and a carburized layer. The depth (100–125 μm) and microhardness (1450–1900 HV) of iron borides zone have been found. The carbon content (0.94–1.41 wt.%) and microhardness (950 HV) beneath iron borides zone have been determined. The microhardness gradient in borocarburized layer has been reduced in comparison with the only borided layer. The increase of distance from the surface is accompanied by the decrease of carbon content and microhardness in the carburized zone. The carbon and microhardness profiles of borided, carburized and borocarburized layers have been presented. A positive influence of borocarburizing on the wear resistance was determined. The wear resistance of the borocarburized layer was determined to be higher in comparison with that for borided or carburized layers.

Characterization of complex (B + C + N) diffusion layers formed on chromium and nickel-based low-carbon steel

Combined surface hardening with boron, carbon and nitrogen was used for low-carbon chromium and nickel-based steels. The microstructure, boron and nitrogen contents, carbon profiles and chosen properties of borided layers produced on the carbonitrided steels have been examined. These complex (B+C+N) layers are termed borocarbonitrided layers. Gas boriding applied to these steels that have been previously carbonitrided enables the production of wear-resistant diffusion layers. After combined surface hardening with boron, carbon and nitrogen in the microstructure two zones have been observed: iron borides (FeB+Fe 2B) and carbonitrided zone. The iron borides show the tendencies towards lose of the needle-like nature. Although borocarbonitriding causes reducing of the depth of iron borides zone, it also reduces the microhardness gradient across the case. Probably, the brittleness of borocarbonitrided layer is lower. In the consequence, the frictional wear resistance of borocarbonitrided layers is essentially higher than that obtained in case of only borided layers. The similar values of wear intensity factors are characteristic for borocarburized layers. The carbonitriding used before boriding make possible the application of lower temperature and shorter time in comparison with carburizing process.

The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low-carbon steel

The complex (B+C) diffusion layers have been formed on chromium- and nickel-based low-carbon steels. Gas boriding applied to these steels that have been previously carburized enables the production of wear-resistant borocarburized layers. After combined surface hardening with boron and carbon in the microstructure two zones have been observed: iron borides (FeB+Fe2B) and carburized zones. The iron borides in borocarburized layer show the tendencies towards lose of the needle-like nature. The depth of iron borides zone depend on pre-carburizing parameters. Although borocarburizing causes reducing of the depth of iron borides zone, it also reduces the microhardness gradient across the case. An increase of distance from the surface is accompanied by a gradual decrease of carbon content and microhardness in carburized zone. Probably, the brittleness of borocarburized layer is lower. In the consequence, the frictional wear resistance of borocarburized layers is essentially higher than that obtained in case of only borided layers. There is every indication that the carbon content beneath iron borides has an important effect on wear resistance, too. The highest wear resistance of borocarburized layer has been observed in case of about 1.2% C below iron borides.

Microstructure and properties of composite (B + C) diffusion layers on low-carbon steel

Combined surface hardening with boron and carbon was used for low-carbon 5120 steel. The microstructure, carbon profiles and chosen properties of borided layers produced on the carburized 5120 steel have been examined. These composite (B + C) layers are termed borocarburized layers. The microhardness profiles and wear resistance of these layers have been studied. In the microstructure of the borocarburized layer two zones have been observed: iron borides (FeB + Fe2B) and a carburized layer. It has been found the depth (100–125 μm) and microhardness (1500–1900 HV) of iron borides zone. The carbon content (0.83–1.46 wt pct) and microhardness (950 HV) beneath iron borides zone have been determined. The microhardness gradient in borocarburized layer has been reduced in comparison with the only borided layer. An increase of distance from the surface is accompanied by a decrease of carbon content and microhardness in the carburized zone. The carbon and microhardness profiles of borided, carburized and borocarburized layers have been presented. A positive influence of composite layers (B + C) on the wear resistance was determined. The wear resistance of the borocarburized layer was determined to be greater in comparison with that for only borided or only carburized layers.

Steel surface treatment by a dual process of ion nitriding and thermal shock

Samples of AISI 4140 steel were surface treated under two different processes: ion nitriding and high energy pulsed plasma irradiation. Ion nitriding was performed with a 100 Hz square wave glow discharge, in an atmosphere of an 80% N 2 and 20% H 2 mixture, under a total pressure of 5.6 mbar. Pulsed plasma irradiation consisted in the surface irradiation with a predetermined number of pulses of high energy and short duration argon plasmas, accelerated in a Z-Pinch experiment. Each pulse can induce high temperatures in a short time (<200 ns), followed by an also fast (∼10 μs) cooling down. The samples, ion nitrided and post-irradiated with pulsed plasmas showed important surface property improvements with respect to samples subjected only to ion nitriding. Those improvements consisted of an increase in the thickness of the hardened layer, and in a reduction of the micro-hardness gradient. These results show a complex surface layer structure that improves the support base for loads, reducing the probability of surface layer loosening.

Characterization of complex (B + C) diffusion layers formed on chromium and nickel-based low-carbon steel

Combined surface hardening with boron and carbon was used for low-carbon chromium and nickel-based steels. The microstructure, boron contents, carbon profiles and chosen properties of borided layers produced on the carburized steels have been examined. These complex (B+C) layers are termed borocarburized layers. The microhardness profiles and wear resistance of these layers have been studied. In the microstructure of the borocarburized layer two zones have been observed: iron borides (FeB+Fe 2B) and a carburized layer. The depth (70–125 μm) and microhardness (1500–1800 HV) of iron borides zone have been found. The carbon content (1.2–1.94 wt.%) and microhardness (700–950 HV) beneath iron borides zone have been determined. The microhardness gradient in borocarburized layer has been reduced in comparison with the only borided layer. An increase of distance from the surface is accompanied by a decrease of carbon content and microhardness in the carburized zone. The carbon and microhardness profiles of borided, carburized and borocarburized layers have been presented. A positive influence of complex layers (B+C) on the wear resistance was determined. The wear resistance of the borocarburized layer was determined to be greater in comparison with that for only borided or only carburized layers.

Laser assisted deposition of graded overlay of Stellite 6 on austenitic stainless steel

Hardfacing by Stellite is extensively employed in many engineering applications for enhancing high temperature wear, oxidation and corrosion resistance of austenitic stainless steel components. These overlaid components, because of large difference in thermo-physical and mechanical properties between clad and substrate, are largely prone to cracking. Cracking resistance of Stellite 6 clad austenitic stainless steel components can be greatly enhanced by providing smooth composition gradient across substrate-clad interface. This article presents development of graded overlay of Stellite 6 on AISI 304 stainless steel substrate using 2.5 kW continuous wave CO 2 laser and their comparison with directly Stellite 6 clad AISI 304 stainless steel specimens. Graded overlaying of Stellite 6 effected significant reduction in microhardness gradient across substrate-clad interface with respect to that of directly Stellite 6 clad specimens. Thermal cycling tests, performed on laser-cladded specimens, demonstrated superi...

Influence of magnetic field on surface modification and the friction behavior of sliding couple aluminium/XC 48 steel

We have studied the surface modification and tribological behavior of the paramagnetic aluminum alloy/ferromagnetic steel XC 48 sliding couple in the presence and absence of a d.c. magnetic field. Experiments were conducted on pin/disc tribometer in ambient atmosphere under an applied normal load P = 3.4 N and a sliding speed v = 0.67 m s -1 . The magnetic field was applied to the aluminum pin and remained constant during the test. Our experimental results show that the presence of a magnetic field around the tribocontact modifies the mechanical properties of the contact surfaces and their friction behavior. The modifications induced in contact surfaces and subsurfaces of aluminum and steel by a magnetic field of H=4500 A m - after sliding for I h are analyzed by Vickers microhardness, scanning electron microscopy and energy dispersive spectroscopy (EDS) analysis. The microhardness gradient in the pin is about 12 H v μm - 1 in a thin layer of 30 μm of material, and the mirohardness gradient in the disc is about 9.5 H v μm -1 in 20 μm of the material depth. The oxygen consumed by the magnetized sliding surfaces increases by a factor of 4. The friction coefficient becomes more stable and decreases from μ=0.55 to 0.45. The wear rates of both the pin and the disc are increased.

Behaviour of sintered 410 low carbon steels towards ion nitriding

Sintered stainless steel samples (AISI 410, low carbon, 6.45 and 6.8 g cm density) have been subjected to ion nitriding at 590°C under different N2H2 mixes and for different times. Investigations have been carried out by means of metallographic techniques, electron probe microanalysis. XRD, hardness and microhardness measurements. The hardness values show that current sintering in industrial conditions originate some scattering of physical properties. The hardness and microhardness gradients show a very limited influence of material density, confirming the suitability of ion nitriding to porous steels. Features and behaviour of the nitrided layers were explained on the basis of the different processing conditions. A more complete survey will analyse the wear and corrosion resistance of AISI 410 sintered and ion nitrided specimens.

Plastic deformation and damage accumulation below the worn surfaces

Delamination of material layers adjacent to the worn surfaces is a commonly observed form of wear in unlubricated or poorly lubricated surfaces. In ductile materials, the delamination process usually involves large plastic deformation and subsurface damage. In this study, metallographic techniques have been used to determine the extent of plastic deformation and strain localization events during the sliding wear of annealed OFHC copper samples. Tests were performed using a block-on-ring type wear machine under constant load and constant velocity conditions. Subsurface displacement and microhardness gradients were measured as a function of sliding distance. It was observed that both the magnitude of plastic strain (and stress) gradients and the depth of highly deformed layers vincreased with the sliding distance. The flow stress and strains at the subsurface regions are shown to obey a Voce type constitutive equation. Wear proceeded mainly by a mechanism of delamination via subsurface crack growth. It is proposed that the competition between the plastic strain which enhances void growth and the hydrostatic pressure which suppresses it is responsible for the generation of a damage gradient so that the delamination takes place at a certain depth where the damage accumulation rate is maximum. A model based on the Rice and Tracey analysis of ductile void growth is developed and used to determine the location of subsurface crack propagation.

Delamination wear in ductile materials containing second phase particles

Plastic deformation and damage accumulation below the contact surfaces play an important role in sliding wear of ductile materials. In this study, metallographic techniques have been developed and used to determine the magnitude of the shear strains and the microhardness gradients in near surface regions in an aluminum-7% silicon alloy. Under dry sliding wear conditions, both the magnitude of plastic strains and the depth of heavily deformed zones increased with sliding distance and applied load. The flow stress and the plastic strains in the deformed zones are shown to obey a work hardening law which can be expressed in the form of a Voce type constitutive equation. Scanning electron microscopy (SEM) studies on the longitudinal sections below the worn surfaces indicated that thin flake-shaped debris particles were generated by a process of subsurface delamination occurred via cracks which originated from silicon particles within the deformed zones (but not at the contact surface) and propagated parallel to the surface. A model based on the hypothesis that delamination cracks are formed by the coalescence of voids at a critical depth below the worn surfaces has been proposed. It is shown that the critical depth for maximum rate of damage accumulation is determined by a competition beetween the plastic strain which enhances void growth and the hydrostatic pressure which suppresses it.