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.
Hygroscopic Expansion of some Casting Investments
CREDIT is generally given to Taggart1 for introducing the cast gold inlay to the dental profession. Although he realized that gold shrinks when cooled from molten state to solid state of room temperature, the investment available to him did not supply sufficient compensation. In 1910, Van Horn2 published a paper dealing with a method by which he obtained the compensation by means of wax expansion. The next major research in connection with this problem was the work of Coleman3 at the National Bureau of Standards. He determined the gold shrinkage to be 1.25 per cent and presented a casting technic with compensation values of quantitative nature. The total compensation for the gold shrinkage was obtained by the combination of the setting and thermal expansion of the mold. This procedure is the general practice followed at the present time for compensation. There are some disadvantages to this method. Skinner has indicated the difficulty of obtaining smooth castings and the improper compensation for the casting shrinkage.4 It has been found by Hollenback5 and Scheu6a that castings made at a high temperature are rougher than those made at a low temperature (450° C.), depending upon the great difference in thermal expansion between plaster of Paris and silica, which causes a poor union of the fine particles. This may also result in friability and there may be some warpage due to the differential expansion. A method which provides sufficient compensation at a low mold temperature is the hygroscopic casting technic.b Wilson7 showed that certain plasters expand as much as 0.0044 inch to the inch by taking up moisture from the air. With this information at hand, Scheula, b tested some commercial investment to determine what results the addition of moisture would have. He found6b: (1) All of the casting investments that were tested expanded. (2) The thicker the mix, the greater the expansion. (3) A mix with hot water expanded more than one with cold water. (4) The time at which the mix is placed in the water is of great importance. It was found that if the mix is placed in the water early, it would not expand so much as when placed in a little later. It was also found that if the mix was placed in the water after a certain time, it could not expand to the maximal amount. (5) The temperature of the water in which the mix is placed is important, for two reasons; (a) the temperature of the water controls slightly the expansion of the investment, and (b) water at a temperature of 1000 F. slightly softens the wax pattern so that the fine margins are not marred while expansion is taking place. The mix should remain in the water at least 20 minutes to obtain the full expansion, after which time no further expansion is obtained.