ABSTRACT

In heavy plates and thick walled tubes, defects can occur near the surface. These defects mostly are located only a few micrometers below the surface, so that non-destructive detection methods such as ultrasonic testing or optical monitoring cannot be used for detection. Laser thermography can solve this problem and is particularly suitable for fast-running, process-integrated applications. Practical tests for the detection of artificial and real near-surface defects were carried out with a high-performance solid-state laser. Laser-activated thermography shows a high potential in terms of increasing quality of thick walled steel products and therefore enhancing safety and reliability of offshore constructions.

INTRODUCTION

Internal defects within the material pose a special challenge for quality assurance. In particular, defects close to the surface, such as scales, shells or sliver, are currently difficult to detect reliably using optical and ultrasonic test methods, which are widely used in the production process. Regarding the optical methods this is due to the lack of macroscopic visibility on the surface. Also with ultrasonic testing methods near surface defects cannot be detected securely, because the defect lies in the dead zone directly beneath the ultra sonic testing probe. This dead zone is generally caused by the influence of the sending impulse, which cannot be avoided with the widely used combined send/receive testing probes (Schiebold 2015).

Only during the subsequent forming process do some of these defects break through the surface and become visible to the naked eye. Due to the interaction of the defect position and the degree of deformation of the component in this area, only a fraction of all existing near-surface defects are visible in this process step, while the rest remains undetected. The method presented here is intended to help to localize these hidden and difficult-to-detect defects directly after the rolling mill before they enter further processing, thus helping to avoid unnecessary costs due to rejects and warranty claims.

In 2019, an estimated 1.755 million tons of hot-rolled steel products were produced worldwide, of which a significant part is produced as heavy plates–among other uses also for the offshore industry. These production dimensions require a process-integrated high-speed inspection method that enables cost-effective overall area monitoring during the production process.

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