J. Hoffmeister

Residual Stress Depth Distribution after Piezo Peening of Quenched and Tempered AISI 4140

Piezo peening is a new alternative mechanical surface treatment process. Thereby a piezo actuator with an indenter is causing mechanical deformation of the surface area by multiple impacts in a defined way. First results for quenched and tempered AISI 4140 show a great potential: large surface compressive residual stresses of up to -1200 MPa could be generated. In order to obtain a process understanding different process parameters are systematically changed. After the mechanical surface treatments the residual stress and full width at half maximum (FWHM) depth distribution were measured using X-ray diffraction technique. By varying the process parameters path distance, feed rate and amplitude several combinations of residual stress and FWHM depth distributions were generated. The dependency of surface residual stresses, penetration depth, FWHM on the process parameters is analyzed.

Residual stresses under quasi-static and cyclic loading in shot peened Inconel 718

Abstract The residual stress state induced by shot peening should be taken into account in the dimensioning of turbine components. Understanding the changes in the residual stress state caused by the application of quasi-static and cyclic loads is a prerequisite. In order to describe the residual stress state after quasi-static loading, several different shot peened Inconel 718 specimens were loaded isothermally up to specific tensile loadings. To analyze the residual stress state after cyclic loading, isothermal low cycle fatigue tests were performed. These tests were stopped after a defined number of cycles. Finally, after the specimens had been subjected to different loads, the surface residual stresses and — for special loadings — the residual stress depth distributions were determined experimentally by using X-ray diffraction. The surface — core model was adapted so that the complete residual stress depth distribution after quasi-static and cyclic loading can now be described.

Finite element modelling of coverage effects during shot peening of IN718

Abstract Current 3D shot peening simulation models proposed in literature do not take into account coverage as a process parameter influencing the residual state after shot peening. In this study a classic approach, using an ordered dimple pattern, and a new approach, using a stochastic dimple pattern were tested to describe the correlation between coverage, the surface topography and the residual stress state. Model verification was conducted based on X-ray and confocal white light microscopy measurements on shot peened test specimens. The test material was age hardened IN718. Simulations showed that the dimple pattern and the impact order of the shots can have a strong influence on the calculated macroscopic residual stress state. The stochastic approach enabled a realistic prediction of the surface topography and the residual stress state for arbitrary values of coverage while the classic approach strongly underestimated the number of shot impacts needed to achieve a certain value of coverage.

Experimental Determination of Process Parameters and Material Data for Numerical Modeling of Induction Hardening

Induction surface hardening is a widely used manufacturing process to improve the mechanical properties of components. However, better process understanding as well as process development requires numerical modeling. The modeling itself depends on the input data in terms of process parameters and the material behavior. Data acquisition is a rather difficult task due to very short processing times, as seen in contour hardening of gears. The article will give an overview over critical aspects regarding the acquisition of input data. A short presentation of the numerical model used to compare experimental and numerical results shall promote better understanding for improving the modeling or reducing the model complexity necessary for good predictability.

Process Simulation of Single and Dual Frequency Induction Surface Hardening Considering Magnetic Nonlinearity

A 2D computational model of single and simultaneous dual frequency induction hardening has been developed. Process specific aspects such as the nonlinear magnetic material behavior and phase transformation kinetics of quenched and tempered AISI 4140 are considered. Induction surface hardening experiments have been conducted for validation purposes. The measuring methodologies used to extract input data such as the magnetic material behavior, the current, and the effective heating time are presented. Metallurgical characterization and hardness profiles are compared with the results obtained from simulations. The temperature history, hardness depth profile, and hardness distribution are in good agreement.

Generation and Determination of Compressive Residual Stresses of Short Penetration Depths

Generating compressive residual stress states with high gradients and low penetration depths offers high capability regarding increase of fatigue limit of parts. In this work the determination of such specific residual stress distributions by using X-ray diffraction and a little material removal is introduced. Measurements are compared using two interference peaks of different penetration depths, at which confocal microscopy enables high accuracy in determination of the step sizes in electrochemical machining. Furthermore the realisation of these states by two different peening processes using micro blasting media is described. The suitability of the processes micro peening and ultrasonic wet peening as surface treatment methods to improve fatigue limit are shown. Micro peening is based on the shot peening principle with small shots and ultrasonic wet peening on the acceleration of small blasting particles by cavitation. The investigations were conducted at AISI 4140 in a quenched and tempered state. Besides the residual stresses and the integral width of interference peaks as well as the depth distributions, the surface topography was examined. The beneficial effects of these conditions on the fatigue limit in bending tests are described.

Mechanische Oberflächenbearbeitung durch Mikrostrahlen

Kurzfassung Vielfältige Verfahren der mechanischen Oberflächenbehandlung erlauben Änderungen der Randschichteigenschaften, die zur Steigerung der Wechselfestigkeit genutzt werden können. Diese Verfahren beruhen auf einer plastischen Verformung der Oberfläche und führen optimalerweise zu Druckeigenspannungen nahe der Oberfläche und einer Verfestigung durch Änderungen des mikrostrukturellen Zustandes, wie der Korngröße oder der Versetzungsdichte. Damit ist auch stets eine Änderung der Topographie verbunden. Häufig wird das Kugelstrahlen verwendet, welches auch die Bearbeitung komplexerer Geometrien erlaubt. Eine Weiterentwicklung davon stellt das Mikrostrahlen dar, bei dem Strahlmittel mit Durchmessern unter 100 μm eingesetzt wird. Neue Anwendungsmöglichkeiten bietet das Verfahren bei der Bearbeitung dünnwandiger Bauteile, die durch die geringeren Strahlintensitäten ermöglicht wird, oder auch bei schwer zugänglichen Bauteilbereichen infolge kleinerer Geometrien des Strahlsystems. Die Charakterisierung der Randschichteigenschaften erfolgt durch Röntgendiffraktometrie, Konfokal- und Rasterelektronenmikroskopie und Aufnahmen an Querschnitten, die mittels fokussierten Ionenstrahls hergestellt wurden. Die Untersuchungen zeigen, dass sich die resultierenden Oberflächeneigenschaften durch Druckeigenspannungen, eine Verfestigung durch Erzeugung nanokristalliner Randschichten sowie eine gegenüber dem Kugelstrahlen vergleichsweise gute Topographie auszeichnen. Die Eindringtiefe des Verfahrens ist hierbei deutlich stärker auf oberflächennahe Bereiche begrenzt als etwa beim Kugelstrahlen. Wechselbiegeversuche zeigen, dass damit eine deutliche Steigerung der Wechselfestigkeit erzielt werden kann. Die Steigerungen der Wechselfestigkeit beim Kugelstrahlen werden dabei für die untersuchten Probengeometrien nach Mikrostrahlen deutlich übertroffen, sodass sich das hohe Potenzial des Verfahrens zur mechanischen Oberflächenbehandlung zeigt.

Modelling of Strain Rate and Temperature Dependent Flow Stresses of Supercooled Austenite for AISI 4140

The numerical modelling of heat treatment has become an essential tool in understanding distortion potentials for case hardening. When looking at other surface hardening processes such as induction or laser hardening, high heating and cooling rates automatically lead to higher strain rates during the heat treatment cycle. So far, there have been almost no investigations on the strain rate as well as temperature dependency of the mechanical properties of supercooled austenite. In this paper, the typical induction and laser hardening steel AISI 4140 has been used in order to determine the influence of strain rate and temperature on the mechanical behaviour. The experiments are based on tensile tests, using a specifically designed thermo-mechanical simulator. The experimental results show that a positive strain rate sensitivity for strain rates up to 1 s-1 results. Especially in the temperature interval where austenite formation occurs during heating, the strain rate sensitive flow stress might lead to an alteration of the plastic strains in comparison to conventional heat treatments at low heating rates. The material model presented in this paper allows a good reproduction of the experimental data over a wide range of strain rates and temperatures.

Simulation des Einsatzhärtens gradiert poröser Bauteile: Materialmodellierung*

Kurzfassung Um eine porositätsbedingte Durchkohlung von pulvermetallurgisch erzeugten Bauteilen beim Einsatzhärten zu vermeiden, kann der Wärmebehandlung eine mechanische Oberflächenbehandlung vorangestellt werden. Dabei wird die durchgängig offene Porosität in eine verdichtete, gradiert poröse Randschicht überführt. Als Folge der Prozessverkettung werden sowohl das Kohlenstoffprofil als auch die Umwandlungskinetik durch die in der Randschicht vorliegende Porosität bestimmt. Eine Modellierung der Diffusionskinetik ist mittels effektiver Diffusionskoeffizienten möglich. Die Umwandlungskinetik kann auf Basis eines dichte-, kohlenstoff- und temperaturabhängigen Geschwindigkeitskoeffizienten, der das Ergebnis einer mechanismengetriebenen Modellierung ist, beschrieben werden. Zusammen ermöglichen beide Kinetikmodelle eine Simulation der Gefügeentwicklung beim Einsatzhärten randschichtverdichteter, gradiert poröser Bauteile.

Austenite-Bainite Transformation Kinetic Model for the Powder-Metallurgical Steel Astaloy 85 Mo

A new approach for case hardening of powder metallurgical steels is surface densification prior to heat treatment, hence avoiding hardening to the core caused by open porosity. With regard to this process chain a porosity and carbon dependent model of the transformation kinetics is essential. In powder metallurgical materials the transformation behavior is mainly influenced by the chemical composition, homogeneity and porosity. Using a prealloyed powder, e.g. Astaloy 85 Mo, a homogeneous distribution of alloying elements after sintering can be assumed and the transformation behaviour is mainly determined by pores and the carbon profile caused by case hardening. The effect of carbon is widely known but up to now, only a few details about the effect of porosity on the transformation can be found in literature. It is reported that a decreasing relative density causes a reduction of incubation and overall isothermal transformation time. In the present study, the transformation kinetics of a powder metallurgical steel based on Astaloy 85 Mo were investigated for the carbon levels 0.5 and 0.8 wt% as well as the relative densities 6.8, 7.2 and 7.8 g/cm³. The investigations were carried out using a high-speed quenching dilatometer. The isothermal time temperature transformation diagrams for this powder-metallurgical alloy are presented and Avrami-type equations are fitted to the measured data. A good correlation can be found for the transformation model and the experimental results verifying the used modeling approach showing the potential to be applied within case hardening simulations.