Core Hardness Research Articles

Effects of local overloads on cold rolling mill work rolls

Local overloading is one of the most common reasons for premature failures of cold rolling mill work rolls. Case-hardened rolls demonstrate spatial variation of the material hardness and yield strength with a decreasing profile with depth and belong to the class of so-called plastically graded materials. Contact overloads cause formation of subsurface closed-shape plastic zones, accompanied by a significant perturbation in the stress field. In the present paper the effect of an overload on a case-hardened roll is numerically modelled using finite element, when the parameters of the contact loads are chosen to represent a common situation when an accidentally formed strip fold enters the roll bite. It is shown that plastic deformations in the graded layer originate at depths, which significantly exceed the depth of the maximum effective stresses, determined by the elastic solution. During the contact overload, tensile stresses develop at the upper boundary between the elastic and plastic zones. Tensile stresses at the free surface in the vicinity of the contact zone increase up to ten times due to formation of the subsurface plastic zone. Residual stresses with a very large tensile hydrostatic component are shown to appear in the unloaded material. These tensile stresses, both operational and residual, can cause premature spalling in the contact surface during either the overloading or further operation of the roll. It is shown that an increase in the depth of the hardened case and increase in the roll core hardness allows to reduce plastic deformations during overloads, reduce both operational and residual stresses in the graded layer and shift formation of the plastic zone deeper from the surface and the contact zone, which decreases the chances of premature spalling.

The impact of rock composition on geomechanical properties of a shale formation: Middle and Upper Devonian Horn River Group shale, Northeast British Columbia, Canada

The geomechanical properties of a shale reservoir are essential both to the development of natural fractures and to the formation’s response to hydraulic fracture stimulation. We evaluate the rock mechanical properties in the Middle and Upper Devonian Horn River Group shale, including the Evie and Otter Park Members and the Muskwa Formation, applying core hardness measurements and log-derived Young's modulus, Poisson's ratio and brittleness and relate these to shale composition and texture. Clay content is the most significant factor controlling the brittleness of shale rocks. The effect of quartz content on rock mechanical properties depends on the type of quartz present in the rock: authigenic quartz is positively correlated with brittleness, but detrital quartz has little or no effect. Factor analysis indicates that carbonate increases brittleness, while no obvious correlation between TOC content and brittleness was observed. Depositional facies are related to geomechanical properties because each facies has a distinctive composition. Massive mudstone and pyritic mudstone are relatively brittle because of abundant authigenic quartz cement. Laminated mudstone and bioturbated mudstone are relatively ductile because most detrital carbonate and quartz grains are set in a matrix of clay minerals. Brittleness in the Horn River Group shale shows both geographic and stratigraphic variability. Increasing brittleness in the northwest part of the basin largely results from greater distance from the clastic sediment source. The Otter Park Member represents a period of major relative sea level fall and is relatively ductile because of its high clay content, whereas the underlying Evie Member and the overlying Muskwa Formation are relatively brittle because of high carbonate and biogenic silica content, respectively.

Strength and its prediction models based on fracture mechanics of thin layers copper cladding steel wires under different amount of deformation

The microstructure and mechanical properties of copper clad steel (CCS) wires drawn with different amount of deformation were investigated by electron backscatter diffraction, Vickers micro hardness testing, tensile testing and scanning electron microscope. The results showed that the equiaxed grains of copper and ferrite transformed gradually into elongated grains along the drawing direction with the deformation amount increasing, and the copper and ferrite grains near the interface exhibited finer grain size than those away from the interface after drawing. With the deformation amount rising, the micro hardness of copper and steel core increased, and strength of CCS wires increased and the elongation of CCS wires decreased. There are different fracture mechanics of CCS wires under different amount of deformation during tensile testing. When the deformation amount was 34.4 %, the copper layer and steel core were broken at the same time as the CCS wires were broken during the tensile testing. When the deformation amount was 97.1 %, the steel core was broken firstly, then the copper layer was broken. Therefore, the models of strength prediction of CCS wires based on fracture mechanics were proposed. The models of strength prediction of CCS wires under small and large amount of deformation are σb(ccs) = σb(cu)νcu + σb(Fe)(1 − νcu), η < 90.6 %, and σb(CCS) = σb(Fe), η ≥ 90.6 %, respectively.

The effect of locally source carbonaceous materials on the mechanical properties of mild steel

The work focuses on the effects of locally sourced carbonaceous materials such as Animal bone, Snail shell and Periwinkle Shell on the mechanical properties of mild steel carburized at varying treatment temperatures of 700, 800, 900, 1000 and 1100 °C, soaked at varying carburizing time of 5hr, 4hr, 3hr, 2hr and 1hr at the respective treatment temperatures. The samples are quenched in oil and tempered at 350°C for 45 minutes. Prior to the carburization process, standard test samples were prepared from the as received specimen for tensile tests, wear test and micro hardness analysis. After the carburization process, the test samples were subjected to standard test to generate data for ultimate tensile strength, the case and core hardness values, wear rate and wear resistance of the carburized samples. The study shows that the mechanical properties of mild steels were found to be strongly influence by each of the carbonaceous materials.

Effect of Si Content on Wear Performance of Hot Work Tool Steel

With the introduction of modern low-weight high-strength materials, tools and dies in hot metal forming are exposed to increasingly demanding contact conditions. This requires use of surface engineering techniques and proper balance between core hardness and fracture toughness. However, it is not very straight forward which combination to use in terms of wear resistance. The aim of this work was to investigate the effect of Si content on properties of AISI H11-type hot work tool steel in relation to austenitizing and tempering temperature. Work was focused on the core fracture toughness and fracture toughness versus hardness ratio and how they affect galling and wear resistance of plasma nitrided hot work tool steel. In the case of high Si content, increased austenitizing temperature results in high core hardness but considerable drop in fracture toughness and wear resistance. However, for low Si content, increased core hardness is accompanied with improved fracture toughness and greatly improved wear resistance. Galling resistance on the other hand is more or less independent of the substrate properties and mainly depends on surface conditions and plasma nitriding process.

Effect of molding parameters on chemically bonded sand mold properties

Chemically bonded sand molds and cores have better mechanical properties and produce more dimensionally accurate castings, compared to green sand molds, and hence are being increasingly preferred for near net shape metal parts. While it is well known that the properties and quality of a mold depend on its material composition and molding process, this has not been investigated well for chemically bonded molds. In this work, the effect of sand grain size, binder percentage and curing time (each varied in three levels) on the mechanical properties (compression strength, shear strength and core hardness) as well as dimensional changes (shrinkage) of no-bake chemically-bonded molds and cores was studied through lab experiments. Their mechanical properties were found to increase with an increase in binder content and curing time; and decrease with increasing grain fineness number. The shrinkage was found to increase with an increase in all three process parameters. The results are explained in terms of bonding and curing phenomena of binders, and evaporation of solvent. These are also supported by SEM analysis of crosslinked resin bridges between sand grains. The findings were incorporated into a multi-objective optimization model to obtain the desired combination of mold properties, which is solved as a linear programming problem. The model and its results were successfully verified through experiments. This work paves the way for automatic optimization of molding parameters of chemically-bonded sand molds and cores, to achieve the desired quality.

轧机齿轮轴失效原因分析

本文采用光学显微镜、扫描电镜等对某钢厂热连轧机断裂齿轮轴进行了检测及分析，结果表明：非金属夹杂物级别过高，带状组织严重，组织不均匀，轴心部硬度偏低等是导致齿轮轴开裂的主要原因。 Reasons for the fracture of gear shaft for hot continuous rolling mill were comprehensively analyzed by means of optical microscope and scanning electron microscope. The results showed that the fracture could be attributed to the following major reasons: excessive levels of non-metallic inclusion, serious banded structure, uneven metallurgical structure and low hardness of axis core.

Surface mechanical attrition treated AISI 304L steel: role of process parameters

In the present work, AISI 304L steel was used to investigate the effect of surface mechanical attrition treatment (SMAT) parameters on the hardness and microstructure. The surface modified by SMAT was examined using an optical microscope, SEM, electron backscattered diffraction, X-ray diffraction and microhardness measurements. The surface hardness of specimens was more than two times the hardness of non-treated core for all SMAT parameters. High density of shear bands (which formed nanosize rhombic blocks) was observed near the treated specimen surface. Density of shear bands was reliant on the size and number of balls. Deformation induced martensite (α′) was formed in surface mechanical attrition treated region. Volume per cent of α′ was enhanced with an increase in SMAT duration and number of balls. However, it was reduced with an increase in gap between vibrating plate and specimen surface. Microstructure evolution at various SMAT parameters was effectively explained using newly introduced terminology called ‘peening intensity’.

Parameter Design for Hardness of Metal Filled Epoxy Matrix

Epoxy is blended with metal fillers to enhance its performance and application for tooling. Material used contains aluminium particles (Rencast CW47), blended with brass separately. Brass is added into the matrix based on 10%, 20% and 30% of the epoxy mix weight ratio. Using Taguchi method, controls factor such as pre-curing temperature (A), filler percentage (B), degassing time (C), curing time (D) and post curing temperature (E) are selected parameters in order to determine optimum conditions. Noise strategy is adopted for material cooling via ambient (N1) and oven temperatures (N2), which gave different hardness values of the epoxy composite. Optimum parameters were decided based on the prediction of S/N ratio, where A = Room temperature, B = 20%, C = 60 minutes, D = 14 hours and E = 1800C. The most significant control factor is de-gassing time for both fillers, and percentage of filler to weight ratio of epoxy mixture is optimum at 20%. It was found that brass gave higher mean hardness values, (Hv = 24.1) compared to copper (Hv = 22.7) and aluminium filled epoxy (Hv = 22.3) based on the prediction run, and suitable as additional filler to improve the surface toughness of epoxy matrix as inserts. Confirmation run was conducted according to the optimum parameters and validated the gain values for brass at 7.98. Fillers enhance the mechanical properties of epoxy matrix by increasing hardness of mould cavity and core. This work helped to design the best control factors to produce optimum hardness conditions for filled epoxy matrix.

Evolution of Drawing Texture for A6 Aluminum Conductor

Evolution of drawing texture for A6 aluminum conductor with the drawing process was investigated by macro and micro-texture analysis. The results show that the fiber-like deformation texture of 111 and 100 formed in the drawing process, and 100 texture reduced while 111 texture enhanced with the increasing strain. The distribution of deformed texture was of homogeneity along the radial direction of wire: the deform texture transformed from strong 100 texture(~52% volume fraction) in the surface to strong 111 texture(~55%) in the center by moderate strains; the radial gradient texture was weakened and a strong 111 texture(70%) formed in the overall wire by high strains. Moreover,the core hardness of the wire was higher than that of the surface, which attributed to the texture gradient distribution along the radial. Adjusting the drawing process to optimize the dislocation density and texture as well as their distribution in the wire is an effective route to improve the strength and conductive properties of the A6 aluminum conductor.

Effect of Binder Composition on the Shrinkage of Chemically Bonded Sand Cores

Modern foundries increasingly use chemically bonded no-bake sand cores and molds because they provide ease of molding, good surface finish, and collapsibility. One of the most popular binder systems is alkyd oil urethane no-bake system comprising three parts: alkyd resin, catalyst, and crosslinking agent. Their amounts and ratios can influence mold shrinkage, and thereby dimensional quality of the resulting casting. With rising emphasis on net-shape casting, there is a need to optimize the binder composition to minimize dimensional errors, while achieving the desired bench life, stripping time and hardness. This work investigates the effect of binder composition on the dimensions of chemically bonded sand cores with respect to time. The maximum shrinkage of 0.15% over length was observed when resin content was 2.4% by weight of sand. The rate of shrinkage increased with the amount of catalyst. Measurement of core hardness and reduction in weight, followed by SEM studies, provided a better understanding of the underlying phenomena, especially formation of resin bridges between compacted sand particles and evaporation of solvent. This work is expected to aid in selection of the most appropriate binder composition for a given set of molding process constraints and cast product requirements.

Temperature and relative humidity influence the ripening descriptors of Camembert-type cheeses throughout ripening

Ripening descriptors are the main factors that determine consumers’ preferences of soft cheeses. Six descriptors were defined to represent the sensory changes in Camembert cheeses: Penicillium camemberti appearance, cheese odor and rind color, creamy underrind thickness and consistency, and core hardness. To evaluate the effects of the main process parameters on these descriptors, Camembert cheeses were ripened under different temperatures (8, 12, and 16°C) and relative humidity (RH; 88, 92, and 98%). The sensory descriptors were highly dependent on the temperature and RH used throughout ripening in a ripening chamber. All sensory descriptor changes could be explained by microorganism growth, pH, carbon substrate metabolism, and cheese moisture, as well as by microbial enzymatic activities. On d 40, at 8°C and 88% RH, all sensory descriptors scored the worst: the cheese was too dry, its odor and its color were similar to those of the unripe cheese, the underrind was driest, and the core was hardest. At 16°C and 98% RH, the odor was strongly ammonia and the color was dark brown, and the creamy underrind represented the entire thickness of the cheese but was completely runny, descriptors indicative of an over ripened cheese. Statistical analysis showed that the best ripening conditions to achieve an optimum balance between cheese sensory qualities and marketability were 13±1°C and 94±1% RH.

Vacuum heat treatment optimization for improved load carrying capacity and wear properties of surface engineered hot work tool steel

Tools and dies used in hot metal forming are exposed to very demanding contact conditions, including elevated temperatures, high contact pressures and abrasive flow of work material, which lead to different types of tool damage. However, the most critical ones are wear and fatigue fracture. As the focus shifts toward the use of new lightweight high-strength materials, also requirements on tool properties are becoming more demanding. This requires use of surface engineering techniques like nitriding and PACVD coating and proper balance between hardness and fracture toughness. Vacuum heat treatment allows the optimization of the tool steel microstructure, which may result in increased fracture toughness while maintaining or even increasing hardness, which enhances possibilities of using coated forming tools.The aim of this work was to investigate the effect of austenitizing and tempering temperature on properties of AISI H11 hot work tool steel, including fracture toughness and hardness and how these properties effect on wear resistance and load-carrying capacity of plasma nitrided and PACVD-coated tool steel. Results indicate that through optimized vacuum heat treatment and surface engineering resistance of tool steel can be greatly improved. Increasing core hardness improves the resistance of nitrided AISI H11 tool steel to abrasive wear, but sufficient level of fracture toughness needs to be obtained first. Change in fracture toughness/hardness ratio has up to 40% influence on wear resistance. On the other hand, galling resistance mainly depends on the surface condition, with plasma nitrided surface without compound layer providing the very best results. Although TiN/TiB2 coating has no beneficial effect on galling resistance, it gives up to two orders of magnitude better wear resistance than nitride surface. However, for proper coating support high core hardness needs to be provided.

STUDIES ON SELECTED PROPERTIES OF SURFACE LAYER OF C45 STEEL OBJECTS AFTER LASER CUTTING AND MILLING

The article presents the results of studying the effects of technological parameters of milling upon surface roughness and microhardness of C45 steel objects after laser cutting. The metallographic structure formed as an effect of cutting by laser was also analyzed. The milling was performed on a FV-580a vertical machining centre. Depth of cut and feed per tooth were changed within the following range: a p = 0.09–0.18 mm and f z = 0.02–0.17 mm/tooth. To measure the surface roughness a Surtronic 3+ profile graphometer was used, whereas microhardness was measured with the use of a Leco LM 700AT microhardness tester. The surface roughness was significantly improved as a result of milling. The laser beam input and output zones were eliminated. Only a part of the layer hardened by laser cutting was removed while milling, in effect of which after milling the hardness of surface layer is much higher than hardness of the core.

Pack-Boronizing of AISI 4140 Steel: Boronizing Mechanism and the Role of Container Design

In this study, pack-boronizing of AISI 4140 steel was done. Three mild-steel containers with different size, namely, small-size container (SC), medium-size container (MC), and big-size container (BC) were used. Steel specimens were boronized at 950°C for 2 or 3 h. Preconditioning of the containers was observed to be essential before doing final pack-boronizing of steel. MC was found suitable for the formation of monophase Fe2B layer while BC produced unwanted FeB phase along with Fe2B layer. Fe2B phase showed the columnar morphology. Continuous decrease in the hardness from surface to nonboronized core was observed. Maximum surface hardness of about 1367 HV0.1 was obtained (hardness of nonboronized core was about 252 HV0.1) for the boronized steel. As compared to the normalized and hardened–tempered (HT) AISI 4140 steel pins, boronized steel pin showed the lowest “specific wear rate” (0.89 × 10−6 mm3/Nm) and coefficient of friction (COF) (0.55).

Fatigue Fracture of a Locomotive Diesel Engine Cardan-Shaft

A locomotive diesel engine cardan-shaft fractured when trial test for 360 h. Fractographic investigation reveals that the failure mechanism of cardan-shaft is multiple origins fatigue fracture. The crack origins are situated on the external circle surface of the transition fillet root of the splined portion, which acts as a stress concentrator. The macro-structure segregation and corresponding inhomogeneous microstructure are presented in cardan-shaft material. And the presence of more ferrite and abnormal Widmannstatten structure ferrite in core microstructure led to the low core hardness (below the minimum value required). Therefore, the estimated fatigue endurance limit of the cardan-shaft material is lower than the expected value by approximately 20%. Along with the stress concentrator in transition fillet, the fatigue crack initiated and propagated under the torsion load and the axial tensile load, ultimately leading to premature fatigue fracture of cardan-shaft. The defective forging and the heat treatment processes should be responsible for the occurrence of defective microstructure and the consequence of low core hardness.

Microstructure, Surface Characterization and Fatigue Assessment of 56SiCr7 Spring Steel

A high number of cycles sustained before failure, under cyclic loading application, is one of the key performance requirements of springs. Heat and surface treating of the spring steel can have a significant influence on its fatigue life. The main purpose of heat treating is to achieve a tempered-martensitic microstructure with appropriate surface hardness, in order to increase the fatigue limit of the spring. The heat treatment aims to an end martensitic formation from an initial ferritic/perlitic microstructure through the steps of heating, quenching and tempering. This study shows that the correct parameters chosen for each of these steps for 56SiCr7 steel can result to an appropriate microstructure, and therefore increase the steel surface hardness up to approximately 550-600 HV. With the aid of optical microscopy, the thickness of the decarburized surface layer is determined, in order to distinguish between the core and surface microstructure hardness of heat treated steel. Surface treatment, through shot-peening, induces compressive residual stresses on the surface of the steel, thus increasing the hardness by at least 204 HV, compared to the raw material, and doubling the number of cycles to failure. Vickers micro-hardness measurements conducted on a cross-section, at different depths from the surface of the steel, show the trend of hardness increasing towards its core, and verify the dependence of the surface hardness of the steel on heat treatment.

The Surface Stereometry and Microstructure of Steel 100CrMnSi6-4 after Ablation Laser Texturing

Publication presents an analysis of possibility of applying ablation laser texturing of 100CrMnSi6-4 bearing steel in the tribological aspect. Selection of different variants of laser micromachining, including pulsed radiation (λ=1.064 μm), and different variants of geometric surface texture, characteristic surface stereometry was obtained with regular arranged lubrication microchannels on the surface roller bearing steel. Maximal hardness in the melted and laser hardened zone, inside the micro-container and in its nearby zone, increased up to 800÷1000 HV0.05 (about 60÷75% increase) in comparison to the core hardness (610÷660 HV0.05). Local hardness increase even up to 1100 HV0.05 was noticed. No micro-cracks in the surface layer neither in the lubricant micro-containers were found. Moreover, no material degradation in the cross-sections in the micro-containers was noticed.

Hardened Case Properties and Tensile Behaviours of TMT Steel Bars

Nowadays, high strength steels are getting very popular all over the world for various applications such as in auto body making, machine parts manufacturing, steel structure buildings, concrete reinforcement, etc. In Bangladesh, use of high strength TMT steels bars has got a momentum in the construction of flyovers, bridges and high rise buildings because of its good combination of the required mechanical properties. In this research work, locally produced high strength TMT reinforcing steel bars (20 mm dia.) of two different companies have been investigated. As a part of this research work, at first, details microstructural morphologies of hardened case, transition zone and core of both steels were investigated by optical microscope and they were then photographed. After that, Rockwell hardness was measured on different zones. Tensile tests were also carried by a Universal Testing Machine at room temperature. After tensile tests, details of the fracture modes of the steel bars were studied carefully. The fracture modes were then discussed in terms of chemical compositions and carbon equivalents, resulted microstructures, case and core hardness, depths of hardened case and transition zone of the steel bars.

The Effect of Heat and Surface Treatment on the Fatigue Behaviour of 56SiCr7 Spring Steel

Heat and surface treatments of spring steels can greatly affect fatigue life by increasing the number of cycles to failure. Heating, quenching and tempering of spring steel will increase its fatigue limit, as tempered martensite formation with an appropriate surface hardness is achieved from an initial ferritic/perlitic microstructure. This study deals with the parameters of the manufacturing process to transforming the initial microstructure of 56SiCr7 steel to tempered martensite, and increasing the surface hardness and fatigue resistance. Microscopy helps determine the decarburized surface layer thickness. Macro- and micro hardness measurements distinguish between core and surface microstructure hardness of the heat-treated steel. Shot-peening induces compressive residual stresses to the surface of the steel, therefore increasing its surface hardness and consequently the number of cycles to failure. This study shows significant hardness increase on the surface of the steel, and verifies how heat- treatment and shot-peening affect surface hardness. Experimental and analytically calculated (FKM guideline) S-N curves at 4- point cyclic bending quantify the individual effects resulting from the applied heat treatment and shot peening on fatigue life.