Conventional Hardening Research Articles

Effect of the yield surface evolution on the earing defect prediction

Although the prediction of earing in the cup drawing process is considerably related to the yield surface shape, the yield surface evolution is also essential for the final ear form. The bending-unbending issue is a fundamental subject occurring on the die and punch shoulders. Since the yield stress is loading path dependent in reversal loadings, the conventional hardening models used in the monotonic loading conditions bring about inaccurate outcomes for predicting the ultimate earing profile, and a kinematic hardening model should be incorporated into the constitutive equations. This study elucidates the yield surface evolution effect involving expansion and translation simultaneously on the ear formation. A sixth-order polynomial yield function was employed to precisely characterize the yield surface shape, while a combined isotropic-kinematic hardening model was implemented to represent the evolution of the yield surface. The translation of the yield surface position was defined by the Armstrong-Frederic hardening model. Punch force-stroke responses and the ear form profiles were predicted by the implemented plasticity model in Marc using the Hypela2 user subroutine and compared with the experimental results. The combined hardening assumption yielded an increase in the mean cup height when compared to the isotropic hardening assumption. Moreover, The HomPol6 coupled with the combined hardening showed a better agreement with the experimental results.

Experiments and/or crystal plasticity finite element modeling of the mechanical properties of pristine and irradiated tungsten single crystal

Tungsten is used for vital applications in nuclear industry, particularly in experimental fusion reactors like ITER. Experimental evaluation of the irradiation-induced hardening is usually carried out by indentation tests. However, accurate prediction of irradiation hardening by indentation simulations faces two challenges: (1) compression data of single-crystal tungsten for indentation model calibration are lacking, since most available data are from tension tests, but tungsten exhibits a tension-compression asymmetry; (2) the irradiation hardening model for tungsten needs to be improved by incorporating more accurate irradiation defect strengthening mechanisms and defect density evolution laws that are quantitatively extracted from the lower scale modeling such as dislocation dynamics (DD) and molecular dynamics (MD). In this work, we present a comprehensive solution for the neutron irradiation hardening prediction of tungsten based on the experiments on pristine single-crystal tungsten and lower scale simulations. First, we conduct compression and Vickers indentation experiments to calibrate and validate the model for indentation simulations, and extend the model with the irradiation defect hardening law and the defect density evolution law informed by DD and MD simulations. Then, we use the irradiation hardening results from the literature to benchmark the model for its effectiveness, and find that the predicted hardness increases match well with the experimental results for different doses. Moreover, we discuss the drawback to the widely-used conventional hardening models, and find that these models tend to overestimate the defect strengthening coefficients.

Response of Partly Recrystallized Microstructure to the Rapid Heat Treatment in TC18 Alloy

The response of TC18 (Ti-5Al-5Mo-5V-1Cr-1Fe) alloy bar to the rapid heat treatment (20 K•s−1) in the 850 to 950°C temperature range followed by ageing at 500°C for 8 hours was studied in this paper. Starting material was only partly recrystallized; share of nonrecrystallized part was not uniform along the bar and consisted 10 to 50% of the cross section. Although both parts, recrystallized and nonrecrystallized, had α+β phase composition, the morphology of a phase in them was different. Due to that and, presumably, to different defect density, the two parts behaved differently upon rapid heating and ageing keeping the properties fluctuated depending on the fraction of the nonrecrystallized part in the particular specimen since nonrecrystallized material remained harder and stronger and therefore more inclined for microcracks to nucleate and grow. Such fluctuation masked a possible temperature dependence of the properties. It resulted in a strength level of around 1700 MPa which is well above the typical strength of this alloy achieved with conventional thermal hardening. Thus, rapid heat treatment can be successfully used to process an extremely high strength condition in titanium alloys to increase their competitiveness in many attractive applications.

Tactical Innovation to Incorporate Post-Consumer Expanded Polystyrene in Artisanal Chains for the Doping of Products

In its original manufacturing purpose, Expanded Polystyrene (EPS) is an industrial product with a short life cycle and waste with high negative environmental impact. Given this externality, this article contributes to the state of the art by proposing reuse EPS as a raw material valuable to the process of manufacturing hats in a Mexican company. The SCAMPER technique is used to formulate a hardener, which is optimized with a Taguchi design. Statistically, there is no difference between the quality of the conventional hardener and the mixture based on post-consumer EPS to make hats based on the standards defined by the company; a subjective analysis supported by the judgment of experts validates the quality of the hats. A contour graph and response surface reflect different combinations of solute and solvent to formulate the glue for the doping of the hat, with the same hardness results. This allows the artisan to assess the formulation from an economical point of view, as well as with respect to the arrangement of materials. These results specifically propose the sustainable alternative of integrating waste from the post–consumer EPS chain into the artisanal hat value chain, and are replicable to other similar products.

Laser Hardening of Unimax Stainless Steel

Nowdays, laser hardening of materials brings a comparative advantage over the conventional hardening technique. Fast cooling rate due to the heat distribution through its own bulk material, self-quenching property (rapid cooling without external water or oil), environmentally friendly characteristics since the procedure does not exhaust smoke, the localized heat input due to adjustable laser spot size to avoid distortion and minimum time to finish the operation are some of the advantages to mention. NIKO is a company specialized in making electrical products like socket outlets and switches by using injection molding techniques. Unimax is a kind of stainless steel used by the company to prepare some parts of the injection molding components like a Nozzle. This time, the company is using more and more fiber-reinforced polymers throughout their product line. These composites are far stronger than the polymer, but on the downside, the fibers are quite abrasive. The objective of this research was to harden the Unimax stainless steel using Nd:YAG (neodymium-doped yttrium aluminum garnet) laser technique. First, the laser transverse speed and spot size were identified as the primary process parameters. Then, the traverse speed of 100, 150 and 400 mm/min and spot size of 2164, 2169, 2288 and 2412 um were assigned with 3 replications. Afterwards, thermal simulation was done using COMSOL Multiphysics© followed by the real test on the metal bar. Therefore, the highest hardness of 650 HV was obtained at a speed of 150 mm/min and a spot size of 2169 um diameters. Finally, the corresponding depth of hardness and roughness values of 200 um below the surface and unmelt samples respectively were obtained. HIGHLIGHTS Laser hardening of materials brings a comparative advantage over the conventional hardening technique The laser transverse speed and spot size were identified as the primary process parameters. Afterwards, thermal simulation was done using COMSOL Multiphysics© followed by the real test on the stainless steel bar The depth of hardening and Vickers hardness (HV) increased with the smaller spot size and slow traverse speed of the ND:YAG laser, but this resulted in a melt on the surface of the hardened metal One of the problems of making products using injection molding techniques using fiber-reinforced polymers is the abrasive nature of the fibers which widen the injection nozzle GRAPHICAL ABSTRACT

In Situ X‐Ray Diffraction Analysis of Microstructure Evolution during Deep Cryogenic Treatment and Tempering of Tool Steels

Deep cryogenic treatment of tool steels, incorporated in conventional hardening and tempering, has been a topic of intensive research in recent years. Yet, the governing microstructural mechanisms involved in the deep cryogenic treatment of high‐alloyed tool steels are still controversial. Thus, an in situ X‐ray diffraction study is conducted on three tool steels X38CrMoV5‐3, X153CrMoV12, and ~X190CrVMo20‐4 to shed light on microstructural evolution during cryogenic treatment and subsequent tempering. For these high‐alloyed tool steels, the transformation of retained austenite into martensite is detected during the cooling phase of the cryogenic treatment. A change in tetragonality of martensite occurs mainly in the heating phase of the subsequent tempering process, which indicates the diffusion of carbon and carbide precipitation from the martensite. The microstructure evolution of the tool steels after hardening, cryogenic treatment, and tempering is further examined by scanning electron microscopy.

Research of weight and linear wear from resource indicators of cultivator paws hardened by combined method

The article presents the results of theoretical and experimental studies of weight and linear wear from resource indicators of cultivator paws hardened by the combined method. It is established that in order to achieve an increased resource of cultivator legs, world manufacturers use special alloying materials, design features and technological methods of hardening, namely heat treatment, application of wear-resistant materials. The most commonly used methods for strengthening the working surfaces of parts are conventional hardening of medium-carbon, high-carbon and alloy steels. The hardness of the metal can be obtained in the range of 45 HRC for steel 45 and up to 65 HRC for steel 65G and alloy steels. To determine the change in the geometric parameters of the blades of cultivator legs during production tests recorded linear wear, weight wear and the radius of rounding of the cutting edge of the working bodies with an operating time of 8, 23, 42 and 54 ha. According to the results of linear wear of the wings of experimental cultivator paws during production tests, the materials that provide the best performance against abrasive wear were determined. As a result of the analysis of cultivator paws with a yield of 54 ha, it was found that the working bodies do not have visible damage and extreme wear and are suitable for further use. At the same time, measurements showed that the amount of wear on the width of the blade is 5.3–11.9 mm.

Laser surface hardening of SS316L

Laser surface hardening is one of the effective method to enhance the mechanical properties of localised surface area of engineering parts made of different types of steels and other metals like titanium etc. Laser surface hardening has many advantages over conventional hardening process like, self-quenching, very fast, control over energy input and localised hardening etc. This work presents the effect of different laser parameters on the micro hardness of SS316L after laser hardening. The experimental work was carried out using 30W Diode-pumped nanosecond (ns) pulsed laser. Design of Experiments One Variable at a Time (OVAT) approach was used for experimentation. Laser power, laser scanning speed, frequency and number of passes were the process parameters used for the experiments. Effects of these parameters over the micro hardness of the surface are described. Along with this, discussion of features of microstructures using optical microscope are done. It has been found that, there is around 25% increase in hardness after laser hardening. Hardened depth up to 400 microns was obtained.

Simulation of Cyclic Loading on Pipe Elbows Using Advanced Plane-Stress Elastoplasticity Models1

Abstract This work investigates the response of industrial steel pipe elbows subjected to severe cyclic loading (e.g., seismic or shutdown/startup conditions), associated with the development of significant inelastic strain amplitudes of alternate sign, which may lead to low-cycle fatigue. To model this response, three cyclic-plasticity hardening models are employed for the numerical analysis of large-scale experiments on elbows reported elsewhere. The constitutive relations of the material model follow the context of von Mises cyclic elasto-plasticity, and the hardening models are implemented in a user subroutine, developed by the authors, which employs a robust numerical integration scheme, and is inserted in a general-purpose finite element software. The three hardening models are evaluated in terms of their ability to predict the strain range at critical locations, and in particular, strain accumulation over the load cycles, a phenomenon called “ratcheting.” The overall good comparison between numerical and experimental results demonstrates that the proposed numerical methodology can be used for simulating accurately the mechanical response of pipe elbows under severe inelastic repeated loading. Finally, this paper highlights some limitations of conventional hardening rules in simulating multi-axial material ratcheting.

Deep Cryogenic Treatment of Plain-Carbon and Low-Alloy Steels

Abstract To improve the mechanical properties of steels, cryogenic treatment is often carried out as an intermediate step between quenching and tempering. The transformation of retained austenite into martensite is the main objective and principle behind the cryogenic treatment. Generally, high-alloy and high-carbon steels are subjected to cryogenic treatment, as they produce a high fraction of retained austenite during quenching as compared with low- and medium-alloy steels. This work analyses the effect of cryogenic treatment on EN8, a plain-carbon steel, and EN47, a low-alloy steel, through microstructural characterization, hardness measurements, and tensile tests. The work shows that influence of cryogenic treatment carried out after tempering is only marginal, whereas cryogenic treatment carried out before tempering yields much better mechanical properties. Experimental results obtained in this work confirm that EN47 responds slightly better than EN8 suggesting that low-alloying additions influence the cryogenic treatment of steels. This work established that by properly applying cryogenic treatment, the strength and hardness of both EN8 and EN47 steels can be increased by about 50 MPa and 3 HRC, respectively, as compared with conventional hardening and tempering treatment.

Self-heating of rolled ZnCuTi sheets

Self-heating effect for thermomechanical forming processes turns out to be dramatically significant for material with a very low melting point, such as zinc alloys. Zinc low melting point temperature (419.54°C) accentuates metallurgical effects when it is formed in cold or warm manufacturing processes. During deformation, self-heating can therefore induce a relative softening in the behavior law (that could improve formability) which competes with the conventional plastic hardening of the material. Hence, the correct identification of zinc material behavior has to account for both softening and hardening phenomena. This paper studies the effect of plastic work energy in the material identified by means of tensile tests by combining digital image correlation and infrared thermography technique.

Short thermal cycle treatment with laser of vanadium microalloyed steels

Abstract Improvement of crankshaft fatigue properties can be approached by altering its mechanical properties in the surface, such as laser surface treatment. Laser beam treatment offers efficient and precise surface hardening processing with possibility of reducing the production cost compared to the conventional hardening techniques. However, its characteristic of having short thermal cycle can be a challenge for the development of laser surface hardening techniques, such as inadequacy of literatures in phase transformation and resulting mechanical properties under rapid heating and cooling rate. Therefore, this work investigated the impact of short thermal cycles induced by the laser beam on the resulting microstructure and hardness properties in the surface of 38MnSiVS5 and 44MnSiVS6 microalloyed steels. Temperature cycles during the process were recorded and examined with the resulting microstructure along with microhardness values. 44MnSiVS6 microalloyed steel, which contains ca. double the amount of vanadium compared to 38MnSiVS5 steel, produces finer ferrite grains in the treated area for all investigated short thermal cycles. This fine-grained microstructure leads to steady hardness distributions in the treated area. The short thermal cycle was assumed to be unable to dissolve the vanadium precipitates that reside in the ferrite grains, which then initiate precipitation hardening.

Tailored bainitic-martensitic microstructures by means of inductive surface hardening for AISI4140

Inductive surface hardening processes are widely used in the manufacturing of automotive parts. They combine short process times with high economic and energy efficiency. Because of their high surface hardness, induction hardened steel parts feature beneficial wear resistance and also good fatigue properties due to the induced compressive residual stresses in the surface. Considering the positive effects on the mechanical properties by creating a bainitic-martensitic microstructure, outlined by severel authors, this study includes the implementation of a temperature controlled inductive surface hardening process on a conventional hardening machine. Therefore an innovative inductor design, which allows a controlled sample cooling is presented. Supported by dilatometric studies as well as different heat treatment strategies, limitations are identified to gain a profund process understanding. Hereby an accelerated bainite formation after short time austenitization is detected. By using different heat treatment strategies, the fraction of the bainitic phase can be adjusted in the surface of the part made of AISI 4140. The results of this innovative heat treatment method are compared to short time inductive hardening and tempering processes regarding microstructure, hardness and residual stresses. The generated surface layer states, including mixed microstructures, show promising properties resulting in a possible enhancement of the fatigue strength of induction hardened parts.

Laser Surface Hardening of Ni-hard White Cast Iron

Laser surface treatment on two different types of nickel–chromium white cast iron (Ni-hard) alloys (Ni-hard 1 and Ni-hard 4) was investigated. Nd:YAG laser of 2.2-kw with continuous wave was used. Ni-hard alloys are promising engineering materials, which are extensively used in applications where good resistance to abrasion wear is essential. The conventional hardening of such alloys leads to high wear resistance nevertheless, the core of the alloy suffers from low toughness. Therefore, it would be beneficial to harden the surface via laser surface technology which keeps the core tough enough to resist high impact shocks. A laser power of different levels (600, 800 and 1000 Watts) corresponding to three different laser scanning speeds (3, 4 and 5 m·min−1) was adopted hoping to reach optimum conditions for wear resistance and impact toughness. The optimum condition for both properties was recorded at heat input of 16.78 J·mm−2. The present findings reflect that the microhardness values and wear resistance clearly increased after laser hardening by almost three times due to laser surface hardening, whereas, the impact toughness was increased from five joules obtained from conventionally heat-treated samples to 6.4 J as gained from laser-treated samples.

Residual Stress Control on Bearing Steel by Surface Cooled Induction Heating Fast Tempering

In order to reduce the amount of CO2 emission during carburizing while keeping the advantages of case hardening, a surface cooled induction heating fast tempering process has been developed for the production of bearing elements. This process is applied to SUJ2 bearing steel, followed by conventional hardening. By surface cooled induction heating fast tempering, high temperature at the center of the test piece and relatively low temperature at the surface cause a difference in tempering temperature, leading to compressive residual stresses at the surface because of the difference in volume change during tempering. As a result, the bearing made by this process has the same rolling contact fatigue life as a carburized bearing due to high compressive residual stress. Process simulation of surface cooled induction heating fast tempering shows that residual stress is mainly caused by the difference in volume change of martensite during this process between the surface and the center. However, the decomposition of retained austenite appears to shift residual stress toward tensile stress.

Evolution of ultrafine precipitates and its influence on wear mechanism in cryoprocessed high nitrogen martensitic steel

ABSTRACTDevelopment of the space engines with liquid propellants necessitates the use of bearings of high quality. During recent years, a new family of high strength corrosion resistant steels, with partial replacement of carbon by nitrogen, known as high nitrogen steels were developed having higher impact strength and fracture toughness coupled with high hardness for antifriction bearing of cryopumps in rocket. In this research work, wear characteristics of cryogenically treated high nitrogen martensitic stainless steels were investigated by Pin-on-disc wear testing machine. After conventional hardening treatment, specimens were subjected to cryogenic treatment at −196°C with cryosoaking period varying from 8 to 32 h in the interval of 8 h followed by soft tempering. These specimens were analysed for hardness, wear resistance, subsurface and worn surface features by SEM. It is established that there is a profound influence of nucleation and growth of precipitates on the wear behaviour and the underlying wear mechanisms.

Phase structure dependence of acceptor doping effects in (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ceramics

The introduction of acceptor dopants into perovskites is a common method for designing hard piezoelectric ceramics by forming defect dipoles. It is interesting to find that the doping effect of a few amount of MnO2 in (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ceramics significantly depends on the local symmetry. A conventional hardening effect corresponding to an obviously enhanced mechanical quality factor and a decreased piezoelectric coefficient can be generated in the tetragonal phase rich zone, rather than in the rhombohedral-rich zone, where a typically amphoteric characteristic can be observed unexpectedly. The result was well interpreted by means of the formation of a local random field as a result of the local symmetry-breaking effect of point defects after doping MnO2 in addition to a traditional internal bias field as a result of the pinning effect of defect dipoles. Compared with tetragonal-rich ones, the obviously enhanced random field in rhombohedral-rich compositions tends to induce a weakening ferroelectricity, as evidenced by composition dependent normal ferroelectric-relaxor ferroelectric phase transition. This work provides a new understanding of how charged defects affect the domain stability in the matrix with different symmetries through the competition between random electric fields and internal bias fields.

Void Growth and Coalescence in Porous Plastic Solids With Sigmoidal Hardening

Abstract This paper presents an analysis of void growth and coalescence in isotropic, elastoplastic materials exhibiting sigmoidal hardening using unit cell calculations and micromechanics-based damage modeling. Axisymmetric finite element unit cell calculations are carried out under tensile loading with constant nominal stress triaxiality conditions. These calculations reveal the characteristic role of material hardening in the evolution of the effective response of the porous solid. The local heterogeneous flow hardening around the void plays an important role, which manifests in the stress–strain response, porosity evolution, void aspect ratio evolution, and the coalescence characteristics that are qualitatively different from those of a conventional power-law hardening porous solid. A homogenization-based damage model based on the micromechanics of void growth and coalescence is presented with two simple, heuristic modifications that account for this effect. The model is calibrated to a small number of unit cell results with initially spherical voids, and its efficacy is demonstrated for a range of porosity fractions, hardening characteristics, and void aspect ratios.

Status of laser transformation hardening of steel and its alloys: a review

The state of laser transformation hardening in surface modification of steel and its alloys is reported in this review paper. The laser hardening process involves the use of high-intensity laser radiation to heat the surface of steel into the austenitic region rapidly. Due to high rates of heat transfer, steep temperature gradients result in rapid cooling by conduction. This causes the transformation from an austenite to a martensite microstructure without the need for external quenching and generates a hard wear-resistant surface. The introduction of high-power lasers, such as neodymium-doped yttrium aluminum garnet (Nd:YAG), diode and fiber lasers, has made laser-assisted hardening an attractive candidate for surface hardening processes. Recently, fiber lasers appeared as a competitor to Nd:YAG and diode lasers for material processing. This review paper is a synopsis of the fundamentals of laser hardening, outlining some of its benefits compared with those of conventional hardening techniques and its industrial applications. A selective review of the experimental and numerical research carried out on steel in this area is also presented. Finally, it is concluded that the overall state of research on laser hardening in surface engineering of steel is well developed, and a still-growing industrial application is observed.

Manufacturing of hybrid gears by incremental sheet-bulk metal forming

Abstract The weight of automotive components has to be reduced and additionally a higher integration of functions is aspired. In the case of geared components, the incremental sheet-bulk metal forming technology enables the manufacturing of load- and weight-adapted functional parts. Due to locally adapted mechanical properties by strain hardening, the subsequent conventional hardening and heat treatment can be avoided for certain cases. However, with the application of high strength steels, the strain hardening reduces the tool-lifetime of the filigree gear forming tools. One possibility to increase the tool-lifetime and also the possible gradient of the mechanical properties can be provided by the manufacturing of hybrid gears. In this case, a stack of aluminum and steel sheets with different initial diameters is joined by the combination of a consecutive edge-thickening and gear forming process. This paper presents the experimental results. A reduction of the tool-load by 20-30% compared to the monolithic process was achieved. Furthermore the metallographic investigation proves the coexistence of a form-fit and force-fit as the resulting joining-principle.