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Detection of hard spots and other material inhomogeneities on steel plates

: Youssef, Sargon; Amiri, Meisam; Youssef, Amir; Molenda, Daniel; Ballman, Horst

Merck, Peter (Ed.) ; European Federation for Non-Destructive Testing -EFNDT-:
12th European Conference on Non-Destructive Testing, ECNDT 2018. Proceedings : Gothenburg, Sweden, 11-15 June 2018; CD-ROM
Gothenburg: Sweden MEETX AB, 2018
ISBN: 978-91-639-6217-2
Paper ID-ECNDT-0229-2018, 28 pp.
European Conference on Non-Destructive Testing (ECNDT) <12, 2018, Gothenburg>
Conference Paper
Fraunhofer IZFP ()
eddy current testing; magnetic testing

The need for evaluation and determination of mechanical material properties (e.g. hardness) for modern steel plates are increasing in gas and oil industries since the last decade. According to standards like ISO15156, NACE MR0175 and DNV-OS-F101, low-carbon steel should not exceed a Vickers Hardness of 250HV10 for sour service. For economic reasons, operators often prefer to use higher steel grades, e.g. X60 or X65, also for pipelines with sour medium. The basic relation, the higher the grade the higher the hardness, means that a pipe with a grade of X65 typically has a base hardness of 220 HV10 and is close to the maximum allowable hardness threshold. Typically, if the hardness for carbon steels is higher than 300HV10, it will result in the loss of ductility, which leads to increased susceptibility to cracking, such as sulfide stress cracking or hydrogen-induced cracking (SSC, HIC). This might also lead to crack formation during forming a pipe or in service when a mechanical load is applied. Therefore plate mills and their clients (e.g. line pipe producers, pipeline operators) are looking for a reliable method to detect small material inhomogeneities to ensure the required steel quality of the whole plate, directly in the steel mills. To this end, a micromagnetic testing method was presented for characterization of materials inhomogeneities. The background of this micromagnetic testing method is the analogy between the mechanical (e.g. Hardness) and micromagnetic properties (e.g. Coercivity). The 3MA-X8 method (3MA = Micromagnetic Multiparameter Microstructure and Stress Analysis), which is developed at Fraunhofer IZFP, combines three electromagnetic measuring techniques with different sensitivities for the microstructure and stress state as well as different sensitivities regarding disturbing influences or analysis depth. The result of 3MA-X8 is a multitude of measurement parameters, which can be correlated to the mechanical material properties (e.g. Hardness). Therefore 3MA-X8 method is capable to detect hard spots and other inhomogeneities. In practical use, a relative hardness variation of ± 30HV10 and a diameter of 10 - 20mm with a minimum signal-to-noise ratio of 6dB has to be detected during production. For demonstrating the 3MA-X8 performance, different kinds of samples have been generated and investigated in a study including samples directly from the steel mill with varying cooling conditions, test samples by inductive hardening with different hardening depths, case hardening (carbonization) and quenching. In the first step, the samples have been automatically scanned with a mobile hardness device, with a resolution of 2 x 2 mm² in order to obtain a high-quality resolution reference data in which they can be used for training and validation of the 3MA-X8 device. Since the 3MA-X8 device was developed and optimized for rough steel production conditions, the performance of 3MA-X8 on different operating conditions and influencing factors was the main objective of this study including remnant magnetic fields in the steel plates, residual stress, low and high temperatures (-20 °C - +80?C), mill scale (ferritic thin surface layer of different iron oxides) and scanning velocities up to 1m/s. Furthermore the implementation of the 3MA-X8 method including eight sensors was successfully presented on a mobile trolley device as an intermediate inspection solution for the heavy plate measurements. Finally, an outlook is given for an automated inspection system for flat products in rolling mills by showing a prototype in operation. The system could be realized in a short time frame by the development of an intelligent sensor with build-in electronics to be connected to a versatile interface which allows the usage of the 3MA-X8 method with the modular Rosen Inspection Electronics.