Vlaženje za vrijeme kaljenja u tekućem sredstvu

Immersion quenching is a widely used technique in heat treatment, because this technique provides higher heat transfer coefficients than the most alternative gas quenching techniques. The disadvantage of immersion quenching in evaporating fluids is the complex heat transfer mechanism which consists of three phases, namely film boiling, nucleate boiling and convective cooling. Especially the transition from film to nucleate boiling – the rewetting of the sample surface – is a complicated process which leads to an extreme local dependence of the heat transfer coefficient. Therefore immersion quenching is very difficult to control and often leads to distortion of the quenched workpieces. In the present paper the rewetting behaviour was investigated by means of hollow and solid cylinders. These workpieces offer the possibility to investigate different distributions of surface temperatures of samples with the same outer geometry/curvature. The observation of the rewetting process was done by means of video films. Additionally, the cooling curves at two positions near the surface were measured. From these data, heat transfer for the quenching simulation by the Finite Element Method was estimated and rewetting curves were calculated.Kaljenje u tekućem sredstvu je tehnika toplinske obrade koja se široko primjenjuje. Ta tehnika omogućava više koeficijente provođenja topline u odnosu na većinu alternativnih tehnika kaljenja na bazi plina. Nedostatak kaljenja u hlapljujućim fluidima je kompleksan mehanizam provođenja topline koji se sastoji od tri faze, redom filmsko ključanje, mjehurićasto ključanje i konveksno hlađenje. Posebno prijelaz s filmskog ključanja na mjehuričasto ključanje – vlaženje površine uzorka - je kompliciran proces koji dovodi do ekstremne lokalne ovisnosti koeficijenta provođenja topline. Zbog toga je kaljenje u tekućem sredstvu vrlo teško kontrolirati i često dovodi do distorzije zakaljenih uzoraka. U radu je istraživano vlaženje sredstvima šupljeg i punog cilindra. Ovakvi uzorci nude mogućnost istraživanja distribucije temperature po površini uzoraka sa jednakom vanjskom geometrijom/zakrivljenosti. Procesa vlaženja promatrana je sredstvima video filma. Dodatno, mjerene su krivulje hlađenja na dvije pozicije blizu površine. Od dobivenih podataka, procijenjeno je provođenje topline za kaljenje simuliranjem metodom konačnih elemenata (FEM) i izračunate su krivulje vlaženja

Austenitic grain size prediction in hot forging of a 20MnCr5 steel by numerical simulation using the JMAK model for industrial applications

Yield strength and toughness in steels are directly associated with hot forging processes, especially by controlling austenitic grain size and cooling conditions. The phenomenological JMAK model in macroscale has been applied in different material classes to predict grain size after hot forming. However, on an industrial application, there is still a lack of understanding concerning the synergic effects of strain rate and temperature on recrystallization. This preliminary study aimed at investigating the applicability of coupled semi-empirical JMAK and visco-elastoplastic models in numerical simulation to predict austenitic grain size (PAGS). Hot forging of cylindrical samples of a ferritic-perlitic DIN 20MnCr5 steel was performed followed by water quenching. The main influences, such as temperature, strain and strain rate fields following the recrystallization model were investigated using the subroutine of FORGE NxT 2.1 software. The results were evaluated by comparing experimentally measured and simulated PAGS at process end. The forging process generates different strain and strain rate fields in the workpiece, which in turn lead to a variation in the PAGS and recrystallization fractions. The simulation was able to detect the PAGS variation showing a good agreement between the experimental forging results and the applied model

Investigation of the influence factors on distortion in induction-hardened steel shafts manufactured from cold-drawn rod

In this study, the distortion of steel shafts was investigated before and after induction hardening. Several essential influencing factors in the manufacturing process chain regarding cold drawing, cutting method, notches on the shafts, and induction hardening were analyzed by design of experiment (DoE). Further necessary examinations of microstructures, hardness profile, segregation of chemical composition, and residual stress state were conducted for understanding the distortion behavior. The results of the statistical analysis of the DoE showed that the drawing process is the most important factor influencing distortion. The surface hardening depth of induction hardening is the second main factor. The relationship between inhomogeneities in the work pieces and the distortion was finally discussed

Surface processing to improve the fatigue strength of bainitic steels : an overview

Currently, one of the major challenges for automotive industries is to reduce the weight and energy consumption of vehicles by using stronger and advanced low-cost materials. Conventional solutions using quenched and tempered steels not always fulfill the desired technical, economic and environmental requirements. Modern continuous cooling bainitic steels can provide a good combination of mechanical strength and toughness, being considered an excellent alternative to replace quenched and tempered martensitic steels in the manufacture of forged components. To meet the desired industry standards in highly loaded components, properties like surface hardness, fatigue strength, wear and friction resistance of these steels can be further improved by subsequent mechanical and thermochemical treatments. Therefore, this paper presents the state of the art in the use of continuous cooling bainitic steels for forging and low energy consumption surface improvement techniques such as: deep rolling and plasma nitriding. Finally, case studies are presented, and conclusions drawn on the current trends and reported practices. Surface modification techniques must be carefully controlled and combined with the material of interest to ensure that undesirable characteristics are not introduced during the manufacturing of the components. The development of processes based on the use of forged continuous cooling bainitic steels can be an excellent alternative to replace the conventional quenching and tempering treatment with considerable reduction of the energy consumption

Microestrutura e Propriedades de Desgaste do Aço Bainítico de Resfriamento Contínuo DIN 18MnCrSiMo6-4 Nitretado a Plasma em Diferentes Temperaturas

Os novos aços bainíticos de resfriamento contínuo (CCBS) são uma excelente alternativa quando se requer elevada dureza, tenacidade e resistência à fadiga. Contudo, estima-se que a taxa de desgaste dos CCBS seja insuficiente para aplicação em componentes com alta solicitação mecânica, sendo necessário aprimorar suas propriedades de superfície. O objetivo deste estudo é o de analisar os efeitos da nitretação a plasma sobre à microestrutura e propriedades de desgaste do aço bainítico de resfriamento contínuo DIN18MnCrSiMo6-4. Portanto, tratamentos com duração de 6 horas foram conduzidos com pressão de 3 mbar, mistura gasosa composta por 76% N2 + 24% H2, e temperaturas de 400, 450, 500 e 550 °C, respectivamente. As amostras foram analisadas através de microscopia eletrônica de varredura, difração de raios-X, microdureza e ensaios de desgaste por deslizamento recíproco. Nas condições investigadas, foi possível obter uma camada de compostos e atingir um aumento de até 393% na dureza superficial, entretanto, o uso de temperaturas mais elevadas durante os tratamentos favoreceu a formação de camadas mais profundas. Em todas as amostras nitretadas, constatou-se a formação predominante da fase de nitretos ε-Fe2-3N e, em menor quantidade, da fase de nitretos γ’-Fe4N. Nos ensaios de deslizamento recíproco houve diminuição do desgaste das amostras nitretadas na temperatura de 550°C. Palavras-chave: Aços Bainíticos de Resfriamento Contínuo, Nitretação a Plasma, Microestrutura e Propriedades de Desgaste

Influence of inclusion type and size on the fatigue strength of high strength steels

In this investigation the influence of the inclusion type and size on the fatigue strength of different steels is analysed. As database case hardened, quenched and tempered as well as bearing steels in different heat treatment conditions, which have been investigated in several research projects over the last two decades, are used. Based on the approach of Murakami the local stress intensity at inclusions was determined to evaluate the influence of fracture causing inclusions on the fatigue strength. Different fatigue criteria have been used to calculate the local equivalent stress amplitudes considering residual stresses in the specimens, mean stresses during loading and multiaxial stresses in notched specimens. Since many run out specimens were subsequently fatigued at a higher stress amplitude, the critical inclusion type and size which have survived the initial stress amplitude could be determined. As a result the local stress intensity at inclusions which leads to no fatigue failure could be calculated as well. With the knowledge of the stress intensity factor range of the fractured and run out specimens a threshold stress intensity factor range could be derived for the different steels and inclusion sizes

Compound Layer Design for Deep Nitrided Gearings

Deep nitriding is used to obtain a nitriding hardness depth beyond 0.6 mm. The long nitriding processes, which are necessary to reach the high nitriding hardness depths, mostly have a negative influence on the hardness and strength of the nitrided layer as well as on the bulk material. The compound layer often is considered less, because in most practical cases, it is removed mechanically after nitriding, to avoid spalling in service. However, in former investigations, it was shown, that thick and compact compound layers have the potential for high flank load capacity of gears. The investigations focus on the simultaneous formation of a high nitriding depth and a thick and compact compound layer. Beside the preservation of the strength, a challenge is to control the porosity of the compound layer, which should be as low as possible. The investigations were carried out using the common nitriding and heat treatable mild steel 31CrMoV9, which is often used for gear applications. The article gives an insight on the development of multistage nitriding processes studied by short- and long-term experiments aiming for a specific compound layer build-up with low porosity and high strength of the nitride layer and core material

Development and application of methods to characterize micro semi-finished products and micro components

In the production of semi-finished products for the production of microcomponents and the components themselves, the characterization of their physical properties is of particular importance. Due to the often oligocrystalline character of these semi-finished products and components, it is necessary to use a suitable testing technique for static and dynamic investigations, as the mechanical properties are not transferable from the macroscopic point of view. In addition, the micro semi-finished products and components often show inhomogeneities induced by the manufacturing process. On the one hand, these are directly reflected in the microstructure and on the other hand they have an effect on quantities such as hardness or residual stresses, which play a decisive role in the application. Mechanical testing, conventional metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), ultra-microhardness testing and X-ray residual stress analysis were used as measuring and analysis techniques suitable for the sub-millimeter range. In the following, the possibilities and limitations of two these methods are illustrated using the example of mechanical testing and EBSD. In this paper several examples for possible characterization techniques are given

Influence of inclusion type and size on the fatigue strength of high strength steels

In this investigation the influence of the inclusion type and size on the fatigue strength of different steels is analysed. As database case hardened, quenched and tempered as well as bearing steels in different heat treatment conditions, which have been investigated in several research projects over the last two decades, are used. Based on the approach of Murakami the local stress intensity at inclusions was determined to evaluate the influence of fracture causing inclusions on the fatigue strength. Different fatigue criteria have been used to calculate the local equivalent stress amplitudes considering residual stresses in the specimens, mean stresses during loading and multiaxial stresses in notched specimens. Since many run out specimens were subsequently fatigued at a higher stress amplitude, the critical inclusion type and size which have survived the initial stress amplitude could be determined. As a result the local stress intensity at inclusions which leads to no fatigue failure could be calculated as well. With the knowledge of the stress intensity factor range of the fractured and run out specimens a threshold stress intensity factor range could be derived for the different steels and inclusion sizes

Sensor system for controlled carbonitriding

Carbonitriding is mainly used to enhance the hardenability and wear resistance of unalloyed steels. Last year investigations have shown that improved component properties can efficiently be obtained by carbonitriding also low alloy and even higher alloy steels. The importance of carbonitriding rises. In the past, carbonitriding was done with constant ammonia additions, e.g. 2–5 vol% are added to the carburizing atmosphere without controlling or measuring the nitriding potential of the atmosphere. The results scatter in a wide rage depending on furnace, batch size and load and many other influencing factors. The need for a control system is obvious. The presented sensor system allows carbonitriding processes, where carbon and nitrogen potentials of the atmosphere are controlled. This allows adjusting surface carbon and nitrogen contents to desired values independently and guarantees constant product quality. The sensor system consists of a combination of an ammonia and an oxygen sensor and simulation software. By this way online diffusion profile calculation for carbon and nitrogen are possible even for alloyed steels taking alloy factors for both into account. These alloy factors, measured for many steel grades and phase boundaries, were calculated with Thermocalc