Einsatzhärten: Untersuchungen zur Änderung der Härteannahme nach isothermischer Umwandlung

Case hardening: Investigations on the change in hardenability after isothermal transformation To increase economic efficiency of case hardening processes great efforts are made in raising carburising temperature. One possibility to remove the coarse grain appearing after high temperature carburising is to integrate an isothermal transformation into the carburising procedure. Apart from a fine martensitic structure two extra effects appear if this technique is used giving reason to more research work. On the one hand there appears a reduction in hardenability of the investigated material. On the other hand bending strength increases towards conventionally case hardened samples.     

Metallurgical influence on distortion of the case‐hardening steel 20MnCr5

In literature, only few works are published that make statements on influences between material properties and distortion. There are some investigations on the influence of carbides and their distribution in the structure. Other authors report on a distinct influence of the texture on distortion. However, the operational sequences during solidification as well as the following deforming processes affect the homogeneity of the chemical composition. Local fluctuations of the chemical composition lead to locally different phase transformations. As a result there will be differences in the structure and therefore inhomogeneous distortion. The hardenability could be suitable as an integrating parameter to describe the effect of the alloying elements as well as their homogeneity in distribution in the structure on distortion. Additionally, the preheating treatment of the material as well as the carburizing process can influence the size alteration and the deformation. Therefore, in this work steel of the quality 20MnCr5 with different hardenability grades were varied concerning their preheating treatment conditions. Three conditions were adjusted: annealing to a specific structure, pre‐aging and the condition after cooling from the hot rolling heat. The extend of distortion was evaluated by measurements on a coordinate measuring apparatus before and after hardening.     

Systematic analysis of the correlation between part geometry and distortion due to heat treatment

Part distortion occurring during hardening combined with narrow tolerances often requires post heat treatment machining. Since corrective machining is expensive, distortion must be kept as low as possible. The current work demonstrates the relevance of the part geometry for distortion. First of all, the physical effects that make the part geometry a significant carrier of distortion potential are outlined. Secondly, the degree to which the part geometry can influence distortion is illustrated on a complex automotive part. Finally, a method is presented that helps to identify the design features of a part that are responsible for crucial deformations following heat treatment.     

Teilautomatisiertes preforming zur Qualitätssicherung und Verkürzung von Fertigungszeiten bei der Herstellung von Rotorblättern von Windenergieanlagen

Quality management and reduction of processing time through automised preforming in the rotorblade manufacturing for wind turbines Rotor blades of wind turbines are mainly manufactured from fibre reinforced plastics. These materials show an excellent performance concering the ratio of stiffness and weight. Basically the composite material of rotor blades is made out of glass fibres and thermosetting material. But growing technological and economical requirements to rotor blades by off‐shore‐application, in particular through increasing rotor blade diameter, have to meet successful. Although reduction of processing time and increased quality of rotor blades play a major role. To achieve the increasing requirements of the rotor blade manufacturing the Bremen Institute for Engineering Design and the rotor blade manufacturer Abeking & Rasmussen Rotec GmbH are working together on the handling and manufacturing of textile preforms. Preforms are made out of stacked and bonded dry textile layers and deposited in the mould as one textile part. Preforming allows the parallel processing of some manufacturing steps. This shows great advantages in the production of fibre reinforced plastics because of the long curing time of the resin component. This parallel processing is required to reach the aim of decreased overall cycle time. This essay is focussing on the handling of textiles and the textile preforming. Summarised aspects of quality management and economics in textile preforming will be discussed with the aim of automising the preforming process.     

Applikation ausscheidungshärtender Bronzeschichten mittels Hochgeschwindigkeitsflammspritzen

Application of age hardenable‐copper‐based‐coatings trough HVOF‐spraying Thermal spraying of age hardenable copper based alloys, enables the producing of wear resistant coatings with high conductivity. With high velocity flame spraying it is possible to create dense coatings with good adhesion to the substrate. The coatings are produced in two steps the thermal spray process itself and the followed heat treatment. The characterisation of the produced specimen shows promising results. The achieved wear resistance of the coatings is similar to bulk material and better than pure copper.     

Simulation‐based optimization of the local material state in the field of cyclically highly stressed case hardened construction details with notch effect*

Steel components very often show construction details, such as cross holes and rounded shaft shoulders which lead to local stress concentrations of the multiaxial stress state in case of mechanical loads (notch effect). Under cyclic loading these stress concentrations (hot spots) can cause crack initiation, crack propagation and finally failure of structural components. The fatigue strength of cyclically loaded components can be considerably increased by the heat treatment case hardening. The shape of the construction detail has a significant influence for the sub‐processes of the case hardening. This can be related to the carbon diffusion process during carburizing and the local heat transfer during quenching. As a result, the local material state following a case hardening process is often not optimal with respect to phase composition and residual stress field. In order to optimize the hardening process a heat treatment simulation based on the Finite Element Method was coupled with procedures for sensitivity analysis and optimization. Taking into account the operational loading conditions for the component, it was possible to adapt technological parameters of the case hardening process for the specific shape of the construction detail, leading to a substantially increased fatigue strength and therewith improvement of the efficiency of the case hardening process itself.     

Influence of carburising on distortion behaviour

Case hardening is a common process to produce steel components, which are characterised by a hard surface combined with a ductile core. As a result of this treatment the parts show a good wear resistance as well as an increase in fatigue strength. High strengthened gear wheels can be mentioned as important examples, being treated in this manner. The case hardening process consists of a carburising and a hardening treatment. The carbon gradient in the surface results in a gradient in the transformation behaviour. The transformation to ferrite, pearlite and bainite is affected as well as the martensite transformation. Distortion due to these chemical inhomogeneities is added to distortion caused by temperature gradients and temperature gradient implied transformation stresses. In this project the effect of the carburising process on distortion behaviour is investigated. The influence of the microstructure before carburising, the carburising depth, the surface carbon content, the course of the process, the carburising temperature, the hardening temperature and the interaction between these parameters are investigated. Design of experiment methods are used to receive the effect of interactions between the varied parameters and to reduce the number of experiments. Additional investigations concern parameters, which can not be varied completely with the other parameters. The influence of low pressure carburising treatments and carburising at high temperatures, for example, are analysed in a spot‐check. The aim of the investigations is to find the decisive parameters affecting distortion in the carburising process, which are later on varied in a design of experiments plan containing the whole production line.     

Hardening distortions of serial produced crown wheels

The hardening distortions of serial produced crown wheels are studied with respect to gear runout, inner diameter and back‐face tilt. The data analysed originates from a production data base from ordinary production as well as from directed experiments carried out in production, resulting in a large set of data. Strong influences are found for steel plants, position of material in ingots and stacking levels on hardening trays. It could be concluded that rectangular strands have a detrimental effect on gear runout, which, however, can be strongly decreased by disabling the magnetic stirring during casting. Furthermore, the inner diameter after quenching is influenced by the stacking level on the hardening tray when free‐hardening or when using a segmented central expander during press quenching. This influence is attributed to variations in hardening temperature. When press quenching using a fixed mandrel, the effect of stacking level disappears. Moreover, it is found that the back‐face tilt strongly depends on the position in the ingot as well as on the stacking level on the hardening tray.     

Löten und Härten im Vakuum zur Herstellung leistungsfähiger Gesteinwerkzeuge

Brazing and hardening in vacuum for manufacturing of high‐performance tools for construction applications High‐performance tools for applications in construction industry are used under severe service conditions. Huge amounts of construction materials have to be removed in short times with long service times. By sophisticated heat treatment processes or combination of materials construction tools are optimized for the dedicated application. Vacuum brazing is a proven technology to join different materials. Case studies will be presented where vacuum hardening of tool steel and vacuum brazing of hard metal to steel improve tool performance significantly.     

Lebensdauerabschätzung an Kunststoffrohren mittels Zeitstand‐Innendruckversuch

Lifetime Prediction of Plastic Pipes by means of Internal Pressure Creep Test The Lifetime Prediction of plastic pipes is based, according to the current set of relevant technical regulations, mainly on the Internal Creep Rupture Test. In this examination, the test for the pipes performed in an accelerated mode with a defined internal pressure and at increased temperatures. In compliance with the applicable Arrhenius rate law, an extrapolation of the measured values then takes place.     

Bandsägen für die Steinbearbeitung – aktuelle Technologieentwicklung

Bandsawing machines for stone cutting – state of the art Due to an EU‐research‐project BaSST a new process was developed to cut slabs and tiles in granite, marble and sandstone out of a huge stoneblock by means of a bandsaw. Such natural stones are very hard, the cutting forces generated to the sawband have to be limited. This was realized with a pivoting sawband, which allows to vary the effective length of engagement depending of the dimension of the material. Simoultaneausly a sawband with diamond cutting segments was developed and tested. Over a long period of optimization a long lifetime of the sawband was reached. The end of the service life was determined by normal wear of the cutting segments and not by breakage of the sawband together with very high cutting rates and excellent cutting surfaces. The outstanding advantage of this new cutting process is a high flexibility for the production, so far unknown till now, a very good cut quality and a remarkable reduced waste of material due to the thin sawband. The number of slabs cut from a block can be doupled due to the thin kerf of the sawband. The new cutting process allows to cut slabs and tiles from a block in a very economic way and under best environmental conditions.     

Konzept zur Verbesserung der Meerwasserbeständigkeit von Wälzlagerstahl

To develop corrosion‐resistant hardenable bearing steels for seawater applications the contents of Mo and Cr were raised in respect to X65Cr14 and the carbon content was lowered by making use of the soluble N content. The steels X51CrN(14)14 and X51CrMoN(14)15‐3, to be molten under normal pressure, and steel X33CrMoN(32)15‐3, to be processed under an N₂ pressure of = 4.7 bar, promise a higher resistance to pitting corrosion and a lower liability of coarse eutectic carbides in segregated areas without discernible disadvantages in respect to hardness and retained austenite. The study has led to a pre‐selection of promising new steels just by thermodynamic calculations and empirical equations without time‐consuming and costly experiments.     

Werkstoffmechanische Bewertung von Laserstrahl‐Schweißnähten in Feinkornbaustählen

Materials and Mechanics Effects in the Assessment of Laser Beam Weld Joints in Structural Steels For fine‐grained structural steels of different strength levels the consequences of mechanical inhomogeneity (mismatch) in laser‐beam welds on their deformation and failure behaviour without and with defects are quantified; requirements for fitness‐for‐service are derived.     

Influence of processing parameters of selective laser melting on high‐cycle and very‐high‐cycle fatigue behaviour of Ti‐6Al‐4V

Orthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti‐6Al‐4V were fabricated. The results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high‐cycle fatigue (HCF) and very‐high‐cycle fatigue (VHCF) regimes. The S‐N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Then, the tests on the validation group were performed to verify the optimal combination of SLM processing parameters. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack‐of‐fusion defects in the cases of surface and internal crack initiation.     

Grundlagen,Herstellung und Anwendungsaspekte des Supersolidus Flüssigphasensinterns von hochlegierten Werkzeugstählen und Metallmatrix‐Verbundwerkstoffen

Principles, manufacturing and application aspects of super solidus liquid phase sintering of high‐alloyed tool steels and metal matrix composites Iron‐based metal matrix composites (MMC) are applied for abrasive wear resistant applications. A common production route uses hot isostatic pressing (HIP) of metal and carbide powders, a comparatively cost intensive process. Using high‐alloyed tool steels as matrix materials it is possible to obtain dense materials by liquid phase sintering with an internally formed liquid phase. This contribution describes the basic principles of densification of the matrix materials taking thermodynamic calculations into consideration. It points out a production route for processing particulate reinforced, high wear resistant composite materials by sintering. Beside the sintering behaviour concepts for heat treatment as well as the abrasive wear resistance are discussed.     

Laserstrahlschweißen von Mikrodrähten aus Nickel‐Titan‐Formgedächtnislegierungen und austenitischem Stahl

Laserwelding of microwires made of nickel‐titanium shape memory alloys and austenitic steel The special properties of nickel‐titanium shape memory alloys are currently used in micro engineering and medical technology. In order to integrate nickel‐titanium components into existing parts and modules, they often need to be joined to other materials. For this reason, the present contribution deals with the laser welding of thin pseudoelastic nickel‐titanium wires (100 μm) with a neodymium‐doped Yttrium Aluminium Garnet laser. Based on extensive parameter studies, joints without defects were produced. This study deals with the microstructure in the fusion and heat‐affected zones, the performance of the joints in static tensile tests and their functional fatigue. It can be shown that nickel‐titanium/nickel‐titanium joints reach about 75 % of the ultimate tensile strength of pure nickel‐titanium wires. In case of welding nickel‐titanium to steel no interlayer was used. The dissimilar nickel‐titanium/steel joints provide a bonding strength in the fusion and heat‐affected zones higher than the plateau stress level. Nickel‐titanium/steel joints of thin wires, as a new aspect, enable the possibility to benefit from the pseudoelastic properties of the nickel‐titanium component.     

Distortion Behaviour and Mechanical Properties of AlCu4Mg1 Sheet Components after High‐Pressure Gas Quenching in Comparison to Liquid Quenching

The quenching process after solution annealing of age hardenable aluminium alloys is necessary for an improvement of the mechanical properties, but also tends to result in distortion, especially in thin or complex shaped parts, and requires a costly reworking. High‐pressure gas quenching can reduce distortion compared to liquid quenching, because of the better temperature uniformity during quenching. A determination of the distortion behaviour of different serial parts of the aluminium wrought alloy 2024cl (AlCu4Mg1,clad) points out, that high‐pressure gas quenching offers predominantly excellent values regarding the dimensional accuracy after quenching compared to liquid quenchants. In comparison to the conventional heat treatment, similar values in strength, hardness and electrical conductivity have been determined after gas quenching and aging of different aluminium alloys (2024, 6013, and 7075), Furthermore, the residual stresses have been investigated and could be clearly reduced after gas quenching.     

Schweißen von Hohlstrukturen und offenporösen Metallschäumen für den Einsatz in Kombi‐Kraftwerken

Welding of Hollow Structures and Open‐Porous Metal Foams for Application in Combined Cycle Power Plants For applications within the scope of novel cooling concepts joining technologies for sandwich composites and open‐porous metal foams are researched in the context of the Collaborative Research Centre 561 “Thermally highly loaded, porous and cooled multi‐layer systems for combined cycle power plants”. The research motivation and application fields of the different structures are defined. Welding processes and strategies for manufacturing these structures are specified as well as the joining technologies’ characteristics. Planned future works for enhancements of the processes and structures are listed.