MECHANICAL PROPERTY DETERMINATION OF HEAVY STEEL PLATES AND COLD ROLLED STEEL SHEETS BY MICRO-MAGNETIC NDT
The presentation is to results recently obtained in collaborative research in the European steel programme performed by Fraunhofer-IZFP and co-operative steel industry producing rolling products. The objective was and is to characterize i.e. to predict mechanical properties like hardness, yield strength, tensile strength, deep drawing parameters rm and ∆r, etc., mainly online by use of contact-less NDT. The approach performed developed a decade before is by micro-magnetic NDT was optimized and especially adopted to sensitively classify a wider range of steel grades. The contribution reports to the technology development, the transducer optimization and the technology qualification. Introduction: Mechanical properties are quality parameters characterizing the fitness for use of rolled products. State of the art to determine yield strength and tensile strength is the selection of standardized specimens at the end of the process and destructive testing according to the definition in the inspection laboratory using standard tensile testing machines. Hardness measurements are performed by using standardized indentation techniques according to Brinell or Vickers. Not satisfying for the producer is the fact that the material – per definition – is destroyed or locally damaged and that this kind of material examination can be performed only statistically and not continuously online. There is therefore the tendency to develop non-destructive (ndt) techniques which – after a calibration by using the destructive techniques can replace the destructive test. In the institute of the authors the topic is an objective and for years ndt-technology was developed and introduced into online process monitoring and control of production lines in the steel industry [1]. Besides ultrasonic techniques based on phase velocity measurements to characterize texture in cold rolled steel sheets and the vertical and planar anisotropy parameter (rm and ∆r) by using EMAT [2] mainly electromagnetic, respectively micro-magnetic techniques were applied [3] and introduced into practice under the acronym 3MA (Micro-magnetic, Multiparameter, Microstructure and Stress Analysis). Micro-magnetic techniques are especially suitable to determine mechanical properties because there are many similarities between the interaction with microstructure parameters and lattice defects and movement of dislocations under mechanical loads and the movement of Bloch-walls under magnetic loads. Figure 1 documents this fact. microstructure parameter: vacancies dissolved atoms dislocations precipitates grain boundaries phase boundaries inclusions pores mechanical properties hardness, yield strength tensile strength, upper shelf FATT Micro-magnetic parameters 3MA Strengthening by lattice defects is impeding of dislocation movement lattice defects impede Bloch wall movements Figure 1: Micro-magnetic microstructure characterization, 3MA Figure 2: 3MA inspection System 3MA is the combination of different micro-magnetic testing procedures: The measurement of the magnetic Barkhausen noise, the incremental permeability, the higher harmonic analysis of the tangential magnetic field and an eddy current impedance measurement. Over all, 22 different parameters are derived by the measurement and can be combined in a multiple regression model in order to determine the mechanical properties [3]. The special intelligence is designed into the transducer head, based on a u-shaped yoke magnet, which by time-multiplexing allows the allocation of all parameters. The 3MA-systen is shown in Figure 2. The magnetic field strength applied was up to 40A/cm. Application on heavy steel plates: In co-operation with the companies Dillinger Hütte AG, IRSID, Fabrique de Fer, Salzgitter AG and ThyssenKrupp-Stahl AG a research project was performed under the umbrella of the European Community of Coal and Steel. Table 1 documents the material selected by the companies of the different partners. Coupons were taken from the running production and destructively tested in the inspection laboratory of one partner. Each of the 88 coupons was measured at 8 different positions in order to have a better statistical approach. The values from position 1-4 were used for calibration and the values from position 5-8 for verification. The regression model applied here was based on 17 unknown coefficients and polynomials in the micro-magnetic parameters up to the 2 order but also non-linear terms like mixed products and quotients. As target functions hardness, yield strength and tensile strength were selected. As function of the hardness a regression coefficient R with R= 0.97 can be found and a residual standard deviation ∆Hardness=11.9 HB-units. Table 1. Inspected material Steel grade Heat treatment Thickness range in Specimens
Robust solutions of inverse problems in electromagnetic non-destructive evaluation
Non-destructive testing (NDT) is the application of physical measurement technology based on energy interaction with the material and its nonconformities. The material's response is sensed by transducers and sensors which?in most cases?scan the component and document the results in inspection images. However, NDT measures a physically defined quantity or even an intrinsic property. The difference between non-destructive evaluation (NDE) and NDT is in the interpretation of the inspection data. NDE has to discuss the inspection results in terms of quality elements and characteristics which are relevant to describe the fitness of the material for use. In the case of macroscopic defects these are the kind of defect (cracklike, globular) and its size and orientation to the main stress directions; in the case of material property determination the parameters are mainly mechanical properties. Therefore, in NDE one has to solve inverse problems. The solution of inverse problems based on mathematical procedures such as integral equations is a strong developing discipline and most of the articles prepared for this special issue of the journal have the objective of discussing the latest state of the art in that field. However, practical NDE needs robust and quick solutions which are to be applied mainly online. Therefore, we present here inversion procedures based on multiple linear regression algorithms applied to inspection data. We describe the calibration procedure to fit the free parameters of the model functions and give examples of practical applications in industry.
Korrosionsbeständigkeit metallischer Werkstoffe in mittelkonzentrierter heißer Schwefelsäure
Fur den neu entwickelten Chrom-Basiswerkstoff Nicrofer 3033 (1.4591) wurde die Abtragungsgeschwindigkeit in unbelufteter p.a. Schwefelsaure im Konzentrationsbereich von 15—80 Massenanteile in % bei Temperaturen von 50—90°C gravimetrisch bestimmt. Daraus resultieren kritische Bestandigkeitsgrenzen unter ungunstigen Beanspruchungsbedingungen. Mit steigender Temperatur erfolgt ein sprunghafter Anstieg der Korrosionsgeschwindigkeit um Grosenordnungen, Ausdruck des Wechsels vom passiven in den aktiven Zustand. Dieser Ubergang in den aktiven Zustand verschiebt sich mit zunehmender Schwefelsaurekonzentration zu niedrigeren Temperaturen. Gegenuber den Referenzwerkstoffen fur Schwefelsaurebeanspruchung Nicrofer 4221 (2.4858) und Nicrofer 3620 Nb (2.4660) besitzt Nicrofer 3033 (1.4591) deutliche Vorteile im passiven Zustand. Polarisationsuntersuchungen in bewegten sauerstoffhaltigen und entlufteten schwefelsauren Losungen ergeben eine leichtere Passivierbarkeit und einen bestandigeren Passivzustand verglichen mit den Referenzwerkstoffen. Das Vorhandensein geringer Mengen an Oxidationsmittel wie Eisen-III-sulfat, Salpetersaure und Kaliumdichromat fuhrt zu einer Ausweitung des Bestandigkeitsbereiches von Nicrofer 3033 im untersuchten Temperatur-Konzentrationsfeld. Die dafur erforderlichen Mindestkonzentrationen an Oxidationsmittel wurden ebenfalls durch Bestimmung der Abtragungsgeschwindigkeit von Nicrofer 3033 ermittelt. Polarisationsuntersuchungen sowie Ruhe- und Redoxpotentialmessungen dienen zur Aufklarung der Wirkungsweise dieser Oxidationsmittel. 
 
 
 
Corrosion resistance of metallic materials in medium concentrated hot sulfuric acid 
 
 
 
For the newly developed chromium-base material Alloy 33 (German material No. 1.4591) the corrosion rate has been gravimetrically determined in non aerated high purity grade sulfuric acid in the concentration range from 15 to 80 mass-% at temperatures from 50 to 90°C. This way critical limits of corrosion resistance behaviour under very unfavourable conditions of exposure result. An erratic increase of the corrosion rate by a few orders of magnitude takes place with increasing temperature according to the transition from the passive into the active state. This transition into the active state is shifted towards lower temperatures with increasing concentrations of sulfuric acid. Alloy 33 (1.4591) in the passive state possesses clear advantages against the reference materials for use in sulfuric acid: Alloy 825 (2.4858) and Alloy 20 (2.4660). Polarization measurements in agitated oxygen containing and deaerated sulfuric acid solutions show a higher tendency for passivation and a more stable passive state in comparision with the reference materials. The presence of small amounts of oxidants such as ferric sulfate, nitric acid or potassium dichromate results in a widening of the range of corrosion resistance of Alloy 33 in the studied temperature-concentration field. In addition the minimum concentrations of oxidants necessary for this behaviour have been determined by investigation of the measured corrosion rates of Alloy 33 (1.4591). Polarization tests as well as potential measurements of rest potential and redox potential serve to further explain the mode of operation of these oxidants.