Einfluss höchster Reinheitsgrade auf die Lochkorrosionseigenschaften austenitischer Stähle

Influence of highest degree of cleanliness on the pitting corrosion behaviour of austenitic steels Stainless steel grades of the type material. no. 1.4401, produced in an industrial scale, have been vacuum arc re‐melted up to three times to achieve lowest inclusion contents. These materials have been corrosion tested to get information about the influence of non‐metallic inclusion content on pitting corrosion behaviour. By the use of potentiodynamic polarisation curves on basis of ASTM G61‐86 a clear effect of steel cleanliness was detected. The highest degree of cleanliness and best corrosion behaviour has been achieved with vacuum arc re‐melting material where non‐metallic inclusions are moved to the ingot surface area and minimum gas contents and micro‐segregations are gained.     

Korrosionsverhalten wetterfester Baustähle unter Berücksichtigung konstruktionsbedingter Spalte

Corrosion behaviour of weathering steels in consideration of constructional crevices Lattice constructions made from the steels WT St 37‐2, COR‐TEN A, KT 62/ ST 37‐2, and WT Neu had undergone a weathering in Zinnwald/Erzgebirge over a period of 8 years. Stationary protection layers, consisting of an inner amorphous barrier layer and a semicrystalline outer layer, had formed on the free surfaces, whereas this layer formation was not or only partially found in crevices of different geometry. In the present work the thickness‐loss of the steel sheets and profiles is evaluated in dependence of the design of the crevice. Furthermore, the structure and the chemical composition of the rust layers are described with regard to a crevice design which is favourable or unfavourable for the corrosion behaviour.     

Advanced studies on the influence of manganese on the potential increase of stainless steels

The influence of manganese on the time course and on the final value of the open‐circuit potential of microbially influenced stainless steel (SS) has been examined by variation of the concentration of manganese in the aqueous phase and on the surface of the metal sample. Under the influence of bacteria stable final values of the open‐circuit potential are attained at 540 mV or 620 mV depending on the manganese concentration in the aqueous phase, the duration of the experiment, and the supply of oxygen. These potential values can also be fixed electrochemically by anodic deposition of MnO₂ on the metal surface. According to the results obtained with different techniques, the electron flow from the metal towards the oxygen is a combination of biotic and abiotic processes. Besides the microbial oxidation of manganese the electron flow via a Mn‐based redox enzyme system has to be discussed.     

Prediction of Distortion of Cylinders without Phase Transformations

The rule of Ameen whereby steel made cylinders with no phase transformations during quenching approach the spherical shape is contrary to the finding of Berger, who observes an elongation of thin and long work pieces. If the prediction of Ameen is correct, cylinders should decrease in length during heat treatment. This paper describes a first step of broader investigation of strain hardening and distortion of cylinders during gas quenching in a gas nozzle field. To make more general predictions about the distortion of cylinders which show no phase transformations during heat treatment, different dimensions of cylinders (lengths 50 mm, 100 mm and 200 mm, diameters 10 mm up to 100 mm) were investigated by means of numerical and experimental methods. The prediction of dimension and shape changes during gas quenching of steel cylinders has been performed by numerical simulation using the commercial Finite Element Program SYSWELD. The austenitic steel SAE30300 (German grade X8CrNiS18.9) was selected as investigated material; it shows no phase transformations during performed heating and cooling. The investigations show a good agreement between the kinematic strain hardening model and the experimental data, whereas the isotropic model is not in line with the experimental data. If the ratio of length and diameter is greater than 3, the relative changes in length displayed against the Biot number give uniform curves for all investigated geometries and heat transfer coefficients.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle (Teil II)

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels (Part II) Austenitic stainless steel (18% Cr, 12% Ni, Mo gradation between 0,06 to 3,6%) had been solution nitrided. By step‐by‐step removing, the samples could be prepared with various surface contents of nitrogen from 0.04 to 0.42%. In two test series the influence of nitrogen had been determined. The susceptibility against pitting corrosion of these samples had been tested by the chronopotentiostatical method. For the investigated steel composition and the used corrosion system there is no infuence of molybdenum on the effectiveness of nitrogen. The effectiveness of nitrogen can be described by the factor 25 in the PRE. By the investigation of the surfaces with the XPS analysis, it could be shown that the passivation and the pit nucleation is influenced by nitrogen. In these ranges NO_x, NH_x, and NH_z‐spectra have been detected. Bound Mo was found in steels containing molybdenum. It is assumed that the repassivation mechanisms of N and Mo work independently of each other. With the results efforts are supported to improve the pitting corrosion resistance also at molybdenum poor steels by surface nitriding or nitrogen alloying. The achieved results justify the assumption that the observed positive effect of the nitrogen may be extented to even higher nitrogen contents. A prerequisite for this is avoiding secondary phases in the matrix. The adverse influence of small particles is known well.     

Fatigue assessment of laserbeam welded PM steel components by the notch stress approach

The local fatigue strength of a laserbeam weld of a complex engine component, which joins a PM with a formed sheet component, was assessed by the notch stress concept with the fictitious reference radius of r_ref = 0.05 mm. First, simplified specimens, following the main geometric dimensions of the parts, were manufactured. On these specimens the fatigue strength was identified by tests and the notch stresses calculated by finite element analysis. Based on these results a design SN‐curve was derived to assess the fatigue strength of the engine component. The numerical assessment of the welded joint was verified by proof tests with the component. The assessment could be improved by considering statistical and stress gradient dependent size effects according to the concept of the highly stressed volume.     

A study on diffusion bonding of steel and copper alloy

In combustion chamber of liquid propellant launch vehicle, the inner shell of the chamber is copper alloy with cooling channels for regenerative cooling and outer shell is steel to maintain high pressure inside the chamber. The purpose of this study is to find the optimum condition for diffusion bonding of copper and steel and the experimental conditions were 3 different pressures at temperatures from 800°C to 950°C. In order to characterize the flow strength of materials at high temperatures, several tensile tests were performed at several temperatures from 800°C to 950°C. This information is used to estimate the test condition for diffusion bonding and superplastic forming. Mechanical properties of bonded specimen were evaluated with single lap joint tests and shear tests. Microstructure of bonded layer has been also observed with SEM with EDX. It is shown that the optimum condition of diffusion bonding is 7 MPa at 890°C, for one hour. Pressurization test of bonded specimen with cooling channels was performed with hydraulic pressure of 87 MPa without failure.     

Microstructure – A Dominating Parameter for Chip Forming During High Speed Milling

High speed milling experiments with cutting speeds of up to 7000 m/min have been performed on an aluminium alloy (AlZnMgCu1.5, AA 7075) and a steel (40CrMnMo7). Both materials were heat treated to produce a variety of microstructures in order to investigate the influence of microstructure on segmentation during chip formation. In case of the aluminium alloy it has been shown that chip formation is governed by the precipitation state, but not by the hardness of the material. In contrast, hardness can be used for a qualitative prediction of chip shape of the steel. In all cases, the chip formation mechanisms remain essentially unaffected by cutting speed, i.e. no transition from continuous to segmented chips occurred.     

Stainless steels suited for solution nitriding

Solution nitriding is a new heat treatment to yield a high nitrogen case on stainless steels at 1100 ± 50°C. Combining experimental results and thermodynamic calculation steels are selected to give a hard martensitic or high strength austenitic case. Especially developed steels are discussed as well as the suitability of standard grades. A martensitic case is combined with a martensitic core in steel Cr13C0.2 and with a softer ferritic‐martensitic core in steel Cr13C0.1. The nitrogen content of an austenitic case increases with the Cr/Ni ratio, e.g. in the order of Cr17Ni12Mo2, Cr18Ni10, Cr22Ni5Mo3N0.2. The duplex microstructure of the latter provides the highest yield strength in the core. It is essential to stay clear of the austenite/austenite + M₂N boundary and avoid precipitates which deteriorate the fatigue and corrosion resistance. Seventeen steels are assessed in this report.