Bernhard Redmer

Assessment of Hot Cracking Behaviour in Welds

Abstract Hot crack assessment during production and processing of metallic materials is an essential prerequisite for the safety of welded structures. The hot cracking investigations presented here were carried out as part of a study aimed at the development of Cr/Ni low transformation temperature filler materials. Low transformation temperature alloys open up the possibility for welding high strength steels. The externally loaded Modified Varestraint Transvarestraint hot cracking test was employed in the experiments described. The hot cracking resistance was evaluated with the help of light microscopy applied at the specimen surface. The proportionality between hot cracking susceptibility and Cr/Ni alloy content was explained by the altered solidification kinetics and by the enlarged solidification interval. The internal crack paths and the three-dimensional structure of the crack net in the material volume were examined using X-ray computer tomography. The total crack lengths for different material depths and circumjacent rectangular volumes, respectively, were be determined. An increasing hot cracking susceptibility with increasing Cr/Ni alloy content was also be established for the specimen volume.

Hochenergieradiografie – Erkennung von Einzelheiten in großen, komplexen Schichttiefen

Kurzfassung Zur Durchstrahlung großer, dichter Objekte reichen die Energien, die mit normalen Röntgenröhren erzielt werden können, ab gewissen Schichtdicken nicht mehr aus. Als hochenergetische Strahlenquellen stehen Kobalt-60 und Elektronenbeschleuniger zur Verfügung. Als Einschränkung der Hochenergieradiografie ist der geringere Kontrast besonders beim Vorhandensein leichterer Objekte umgeben von Körpern bestehend aus schweren Elementen anzusehen. Es ist daher zu erwarten, dass Objekte aus organischen Substanzen hinter dicken Schwermetallwänden oder in Bohrungen von Metallblöcken schwer zu erkennen sind. Im Unterschied zu einem Gammastrahler wie Kobalt-60 mit den beiden Spektrallinien um 1,3 MeV besitzt die Bremsstrahlung aus einem Beschleuniger einen wesentlichen Anteil an niederenergetischer Strahlung, der fließend zu höheren Energien übergeht. Es wird hier untersucht, welche Signaturen von leichten Materialien in einer Umgebung aus Schwermetall bis zu welcher Dicke und bis zu welchem Komplexitätsgrad erkennbar sind. Mit einem Betatron (JME X-ray Betatron 7,5 MeV) und einem Matrixdetektor (Perkin Elmer XRD 1621) wurden Aufnahmen von unterschiedlichen leichten Objekten angefertigt, die zunehmend in eine Umgebung aus Schwermetallen gestellt wurden. Mit unterschiedlichen Energieeinstellungen wurde untersucht, inwieweit eine Materialerkennung hinter welcher Abschirmung möglich ist. Die experimentellen Ergebnisse werden mit Simulationen verglichen, die mit einer Software zur Modellierung von Durchstrahlungsverfahren (aRTist) erzeugt wurden. Dabei wird dem Problem der Aufhärtung bei Anwendung von Bremsstrahlung Rechnung getragen. Die Ergebnisse können sowohl zur Erkennung von Fremdkörpern in Maschinen oder Pumpen als auch zur Überprüfung von Frachtladungen im Bereich der öffentlichen Sicherheit und bei Zollkontrollen dienen.

Reliability Evaluation of NDT Techniques for Cu-Welds for Risk Assessment of Nuclear Waste Encapsulation

Abstract In order to handle the long living radioactive waste (spent nuclear fuel) SKB is planning to build a deep repository that requires no monitoring by future generations. The spent nuclear fuel will be encapsulated in copper canisters consisting of a graphite cast iron insert shielded by an outer 30–50 mm thick copper cylinder for corrosion protection. The most critical part of the encapsulation process is the sealing of the canister, which is done by welding the copper lid to the cylindrical part of the copper shell using friction stir welding and electron beam welding. The quality of the welding process and the reliability of the NDT system – radiographic and ultrasonic testing – must be satisfactorily determined and combined to derive assumptions regarding the frequency of undetected welding defects for a set of canisters as input for the risk assessment. This is done using the POD method according to the “Reliability Handbook MIL 1823” and its generalization.

Mechanized weld inspection for detection of planar defects and depth measurements by tomosynthesis and planartomography

A digital radiography (DR) system was developed featuring a high sensitivity line camera, a custom manipulator, and a 225 kV x-ray tube. This system is designed to scan circumferential pipe welds. The pixel resolution is 50 μm with 2048 pixels per line. The camera and tube are supported 180° apart and rotate together around the pipe to acquire DR images line by line. The technique of line scanning yields several advantages in comparison to film radiography. The radiation direction corresponds to the central projection technique which always employs radial penetration and avoids long tangential paths. Thus radial flaws are detected with maximum available contrast. The slit collimator of the camera significantly reduces the proportion of scattered radiation in the image. This DR system thus provides image quality superior to film when properly aligned. The mechanism of this system allows the adjustment of the radiation direction for inspection of side wall flaws by a second translational manipulator. Scanni...

Mobile X-Ray Computed Tomography for Nuclear and Aviation Industry

Abstract Digital detectors such as phosphor imaging plates (IP) and digital detector arrays (DDA) enable radiographic inspection with higher efficiency and improved image quality in comparison to the classic film technique. The mobile mechanised tomographic system TomoCAR (tomographic computer aided radiology) was developed first for inspection of circumferential welded seams of pipes. It consists of the manipulator based position control of an X-ray tube in front of the region to inspect and a digital detector array behind it. The high contrast sensitivity technique was combined with planar tomography. The tomographic reconstruction allows the three-dimensional (3D) representation of the material structure and included defects, equivalent to a metallographic cross sectioning. The hardware design is based on a specially developed flat X-ray tube and a direct converting CMOS detector array on CdTe basis. TomoCAR is applied for sizing of volumetric and planar defects. It allows the reliable detection of planar defects with openings larger than 25μm by subpixel resolution due to the achieved high signal-to-noise ratio. TomoCAR was successfully qualified as the result of a German pilot study on the basis of the European Network of Inspection and Qualification (ENIQ) guidelines for application in the nuclear power industry. Application examples are presented. The TomoCAR design was modified for in situ inspection of large aircraft components under production conditions. A gantry gate based planar tomograph was constructed and tested for inspection of the integrity of large flat CFRP panel components of up to 3 m × 9 m size. The integrity of imbedded stringers has been successfully observed. Different trials were performed to prove the finding rate of cracks in embedded stringers, which will be presented and discussed.

Bayesian restoration of crack images in welds from incomplete and noisy X-ray data

The restoration of crack images in welds calculated from few x-ray projections only is connected with uncertainties which may corrupt the final images, namely: lack of admissible x-ray projections, lack on the projections of data, which definitely indicate the presence of crack, the crack appearance in the projection is frequently hidden by the images of other defects like undercuts, lack of fusion, etc. A complete Bayesian based technique was developed for a multi-step reconstruction of 3D crack images from limited number of 2D projections and checked using simulated and experimental data. A specific equipment for simultaneous circular rotation and out-of-plane tube positioning in connection with a photo diode line camera was developed and tested for in-field inspection and used for measuring the x-ray projections. The Bayesian quasi-3D restoration with Gibbs prior is applied at the last step being preprocessed by the following procedure: estimate the noise level in the 2D images of the object and select...