Ermittlung des Zeitstandverhaltens einer austenitischen Schweißverbindung

Evaluation of the Creep Rupture Behaviour of an Austenitic Welded Joint. Part I: Experimental Determination of the Creep Behaviour The creep rupture behaviour of a welded joint of 18% Cr-11% Ni-steel was investigated on specimens of different geometry and size. Differences in the creep rupture behaviour between welded joint specimens and specimens taken from the different zones of the welded joint are discussed. The plasticity-and creep behaviour of the different zones is described by constitutive equations.     

Untersuchung einer mehrlagigen Schweißnaht eines dickwandigen Rohres aus dem austenitischen Stahl X6 CrNiNb 18 10

Analysis of a multi pass weld of a thick walled tube made of austenitic stainless steel X6 CrNiNb 18 10 In this paper, microstructure and residual stresses of a multi pass welding of a thick‐walled tube made of austenitic stainless steel X6 CrNiNb 18 10 (1.4550) are systematically characterized and assessed. Results of microstructural and phase analyses, residual stress and hardness measurements as well as of a tensile test using micro specimen and SEM analyses are presented. Using these data, plastic deformations occurring during the welding process in the vicinity of the weld seam are evaluated. Finally, consequences of an additional heat treatment at 400 °C/24 h are studied.     

Bruchmechanische Untersuchungen an einer Schweißverbindung von 90 mm Dicke aus Almg 4,5 Mn bei Raumtemperatur und bei −196 °C

Toughness Evaluation for a Thick Weld Joint of AlMg 4,5 Mn at ?196 °C To asses the toughness of a 90 mm thick MIG-weld joint of Al Mg 4.5 Mn fracture mechanics and charpy impact material parameters have been determined at 20 °C and ?196 °C. Additional to the mechanical tests extensive metallographic and fractographic tests on specimens of the weld material and base material were done. The results of the fracture mechanics tests and the fracture analysis show, that now brittle fracture will occur at ?196 °C. The further conclusions are, that charpy energy values are no meaningful basis for a toughness evaluation of such an aluminium weld joint. The joint can be used without restrictions at ?196 °C.     

Einfluß der Abkühlgeschwindigkeit auf die δ-Ferrit-Ausscheidung im austenitischen Schweißgut

Influence of the Cooling Rate on the Separation of δ-Ferrite in Austenite Weld Metal There exists a lot of literature concerning the significance of δ-ferrite in welding joints of austenitic stainless steels. An information about the influence of the cooling rate on the primary cristallisation is even more limited. The present results show, that the δ-ferrite-quantity measured at room temperature is hardly influenced by the cooling rate. Furthermore the cooling rate seems to influence the primary cristallisation. The well-known result was verified, that the orientation of the polished section plane yield different δ-ferrite-quantities. Moreover it is shown, that the toughness and therefore the critical crack length increases with decreasing δ-ferrite-quantity.     

Modellierung des Ermüdungsrisswachstums in Schweißverbindungen unter Berücksichtigung von Schweißeigenspannungen

The present paper contains research results determined within the framework of a project called IBESS (?Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“) by the Materials Mechanics Group of the Technische Universität Darmstadt [1]. Aim is to calculate the fatigue life of welded joints by taking into account the effect of residual stresses and the influence of the weld toe geometry. Here, the fatigue life is regarded as period of short fatigue crack growth. Two and three dimensional finite element models, with cracks as initial defects, are constructed for this purpose. Fatigue crack growth analyses are performed by using the node release technique together with the finite element program ABAQUS. The welding residual stresses as well as the plasticity induced crack closure effects are considered. Structural calculations are performed in order to introduce residual stress fields in finite element models. The calculated compressive residual stress field matches the measured one especially in the weld notch area. The effective cyclic J‐integral (ΔJ_eff) is used as crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a Python code was written for the determination of ΔJ_eff at every crack length phase. The calculated fatigue lives were compared with experimental data and a good accordance between both results was achieved. The impact of welding residual stresses on ΔJ_eff as well as on the fatigue life during short crack growth was investigated. As expected, results revealed that at lower stress amplitude, a compressive residual stress field is favorable to the fatigue life, whilst a tensile residual stress field is unfavorable. The influence of residual stresses can be neglected only for large load amplitudes.     

Ermittlung bruchmechanischer Kennwerte an austenitischen Werkstoffen nach der Einprobentechnik mittels Gleichstrompotentialsondenverfahren

J-R-Curve Determination on Austenitic Steel Using Single Specimen Technique with DC-Potential-Drop For the evaluation of fracture mechanical properties several methods will be normally used for ductile materials. For the fracture mechanical investigation of very ductile materials there is no standard method achievable at present. Performing continuous measurements of crack length in materials of high ductility appears most difficult. This paper deals with the continuous determination of crack length in two different samples of high ductile austenitic steel by means of DC-potential drop. Due to the indirect way of crack length determination using this method a correction factor depending on deformation has to be introduced. By this it is possible to construct the J-R-curve using single-specimen technique and to evaluate a J₀ value using fractographical methods. Because of rather large displacements arising with fracture mechanical tests of austenitic steels a formula for the correction of the load line displacement is introduced and discussed. The method presented leads to reproducible results which correspond with results obtained using multiple specimen technique.     

Verfahren zur vollständigen Ermittlung der R‐Abhängigkeit des Rissausbreitungsverhaltens mit nur einer Probe

Procedure for the determination of the complete R‐dependency of the crack growth behaviour with only one specimen A new concept for fatigue crack propagation tests has been developed. Using a single specimen, it is possible to determine fatigue crack growth curves (da/dN ‐ ΔK) for every stress ratio between R = 0.9 and R = ‐1. Additionally, the new concept also provides threshold values for fatigue crack growth for different values of R and K_max. In combination with a continuous crack length measurement tool (such as the DC potential drop method) this testing procedure can be performed with minimal effort of personnel and time. The test procedure consists of a sequence of K_max‐constant tests with decreasing crack growth rates. As the applied K_max is increasing stepwise there should be no load history effects. According to the procedures described in the ASTM Standard E 647, the results using this new testing procedure fit very well to the da/dN ‐ ΔK curves generated with different specimens. The tests also fulfil all the requirements of ASTM Standard E 647.     

Messen des Korrosionspotentials an Schweißverbindungen im Kleinbereich

The small scale measurement of corrosion potential in welded joints . The corrodibility of welded joints is reported on. Potential curves give the possibility to backing the incomplete results in this field and permit a well defined statement concerning the corrosion behaviour. Failures of interpretation due to polarisation are indicated. The pulsating method helps to prevent changes of the concentration by reactions and alteration of the pH-value. This method delivers useful informations and detects the corrosion sensibility of welded joints.     

Ermittlung von Spannungsintensitätsfaktoren aus Isochromaten mittels einer Vielparameter-Vielpunkt-Methode

Determination of Stress Intensity Factors from Isochromatics by means of a Multiparameter-Multipoint-Method A multi-parameter-multiple-point method (VPVP-method) for determination of mode-1 stress intensity factors from isochromatic fringe patterns has been developed and tested in connection with practical examples. In particular the stress field behaviour in compact-tension specimens is thoroughly investigated where variable numbers of series coefficients in the stress field expansion and a variety of data-point combinations featuring remoted and close field influences has been utilized. Cracks approaching free specimen boundaries have been given special attention.     

Anwendung des Kerbspannungskonzeptes auf die reale Geometrie von Schweißnahtenden

The process used to manufacture modern weld structures often lead to weld start and end points. Under cyclic loading, crack initiation occurs predominantly at the weld start. In particular, for special investigations into thin sheet structures, no approach for the determination of the fatigue life has been established thus far. Therefore, in this research, we have attempted to find a general approach for the assessment of weld ends using the notch stress concept. The primary aim was to obtain the real geometry of weld ends with high precision using a three dimensional scanner. We were able to find an idealised weld end model with the mean toe radius of r = 0,2 mm that provides almost the same notch stresses like the real geometry. The applied load cases were longitudinal and transverse tension as well as bending. The crack always occurs at the weld toe. The notch stress S‐N‐curve was derived from the fatigue life of the cyclic testing so that the assessment of weld ends is possible for the first time. Moreover, some conversion rules have been developed so that the idealised weld end geometry can be modelled with an arbitrary toe radius. Going one step further, some analyses have been performed regarding to unify the notch stress concept. The existing results of Olivier – who examined long welds with no start and end points – were re‐evaluated to unify the results of long regular welds with the local weld end under one scatter band. Using the scanned geometry a good agreement could be achieved. The use of an idealised model will be validated soon.     

Wirkung von Oberflächenschutzmaßnahmen auf hochwarmfeste Gasturbinenwerkstoffe. Teil I. Beeinflussung des Zeitstandverhaltens

Influence of coating on the Properties of Heat Resistant Gas Turbine Alloys. Part 1. Creep-rupture Behaviour The influence of coatings and coating thermal cycles on the creep and creep-rupture behaviour of several nickel based gas turbine blade and vane alloys was evaluated. As a result of creep-rupture tests, the scatter bands of the rupture stress and of the stress to reach 0.2% plastic strain show in comparison to the uncoated materials decreased mean values and even more decreased lower bound values. At an austenitic steel for exhaust valves, no influence of coating was observed.     

Messung der Schweißkraft,des Abbrandes und des Kontaktwiderstandes von Kontaktwerkstoffen für die Energietechnik

Measuring of welding force and contact resistance with a testing equipment to classify contact materials for power engineering . Knowledge regarding welding force, erosion and contact resistance is very essential to select and evaluate contact materials which are to be used in electric switchgear. Measuring systems and results which are found in the literature are mostly carried out under test conditions not close enaugh to practice. An automatic test switchgear with which the mentioned technological characteristics can be evaluated is described. Test conditions can be varied in a very wide range so that every condition can be selected suiting the switchgear. Data on welding force, erosion rate and contact resistance of the contact materials like Ag 1000, AgNi 10, AgCdO 12 and AgC3 are given. A statement on the quality of braze between contacts and carrying material can be given.     

Verbesserung des Einzeldraht-Anemometer-Verfahrens zur Ermittlung des Grundgeschwindigkeitsvektors und des Reynoldsschen Spannungstensors

Ohne Zusammenfassung     

Ermittlung des Spannungsintensitätsfaktors KI mit dem spannungsoptischen Verfahren

Determination of Stress Intensity Factor K_I by means of Photoelastic Technique A survey is given of the methods applied for determining stress intensity factor K_I by means of photo-elasticity. In this connection the scope of validity and the accuracy of these methods are discussed.     

Ein Verfahren zur Ermittlung der Rissentwicklung während des Feuerverzinkens

Hot‐dip galvanizing represents a very effective method to protect components against corrosion. During the hot‐dip galvanizing process, however, there is a risk that cracks form in the component. With an increase of the temperature of the zinc bath, the danger of crack formation also increases. This article describes a method to determine the start of the crack formation during hot‐dip galvanizing. With a few tests, this method allows to determine a stress‐strain limit (energy limit) below which no liquid metal assisted cracking (flüssigmetallinduzierte Rissbildung) can occur during the transient phase of hot‐dip galvanizing.     

Untersuchungen zum Ermüdungs-Rißwachstumsverhalten des austenitischen Stahles X 6CrNi 18 11 (1.4948)

Investigations of the Fatigue Crack Growth Behaviour of the Austenitic Stainless Steel X 6CrNi 18 11 (1.4948) The fatigue crack growth behaviour of the heat resistant austenitic stainless steel X 6CrNi 18 11 (material number 1.4948), which is used as the structural material for the primary and secondary heat transfer systems of the liquid metal fast breeder reactor SNR 300, is described. The parameters under consideration are the temperature in the range from 308 K up to 973 K, the frequency from 0.5 Hz up to 20 Hz, the stress ratio R as well as the heat-to-heat variation. The minimum ligament size criterion as well as the influence of the test technique on crack propagation are discussed. For the various test parameters the constants C and n of the Paris equation are calculated. The test results demonstrate a small influence of the heat-to-heat variation on the crack growth. At low stress ratios and at room temperature the influence of frequency is very small. A strong influence of the stress ratio R on the crack growth rate is observed. For higher R-values and high temperatures a distinct influence of the frequency on the fatigue crack propagation is shown.     

Eine effiziente Methode zur Ermittlung des Schädigungsbereiches bei Wöhleruntersuchungen

An Efficient Method for the Estimation of the “French Damage Area” By a statistically planned and evaluated variant of French's method, the French damage line may be transformed into a quantitatively describable “damage area”. For this purpose a considerable amount of tests has to be applied (app. 400 specimens). A new evaluation of statistically planned investigations of Panagopoulos [23] showed two remarkable results:

• 100% probability of fracture is nearly related to 100% probability of damage
• the arcsin √-transformation is a well fitting model describing the distribution of damage.

A new efficient method for the estimation of the damage area based on this observations was derived and described. The damage area is discussed regarding two level and multi-level tests.     

Vorhersage der Schwingfestigkeit von Schweißverbindungen auf der Basis des statistischen Größeneinflusses

Prediction of the fatigue strength of welded joints based on the statistical size effect With increasing length of welding seams the fatigue strength decreases. This effect is confirmed in fatigue tests on butt joints and cruciform joints. The influence of the length of the welding seam on the fatigue strength depends on the scatter of the parameters describing the geometry of the welding seam. The theory of the statistical size effect worked well in fitting and predicting the test results. The fatigue strength of welded joints is well predicted, if no other influences like e. g. residual stresses change the fatigue strength. Additionally a method for the calculation of crack propagation is proposed, which takes in account angular and linear misalignement as well as local changes of the geometry of the welding seam.