Hot steel yields its secrets

Hot steel emerging from a continuous casting machine can be analysed for defects – both external and internal -- so that process parameters can be allowing real-time adjustment of process parameters. Firing laser pulses at the steel momentarily turns a small area of the surface into plasma. This generates a wave of ultrasound, which is deflected by internal surfaces arising from cracks and inclusions and can be detected and analysed. Pilot scale experiments on moving hot steel demonstrate the efficacy of the method. It is equally applicable to any material emerging from a continuous manufacturing process. The technique is called Laser-EMAT, the letters standing for Electro Magnetic Acoustic Transducers. A related technique has been used to look for defects in railway tracks (see box). EMATs have been suggested as a means of detecting defects in hot steel since the 1960s, but at that time, the available instrumentation and computing were inadequate for the task. More recently, there have been reports of trials using EMAT-EMAT and laser-laser inspection techniques for find surface defects on steel but the work has never been commercialised. Laser-EMAT has been used to find defects in hot, moving steel pipes, but to our knowledge nobody has used it to detect defects inside fast moving steel slab emerging from a caster. Iain Baillie, based in the Corus Teesside Technology Centre is developing this idea as his engineering doctorate project at the University of Warwick. In the new method, a 1J, 1064nm laser pulse lasting only 15ns impacts on the metal surface. “It’s like dropping a brick into water,” says Baillie. “Pulses are sent out 20 times a second. The pulse of metal plasma generated by each laser pulse striking the surface generates surface and internal waves at ultrasonic frequencies.” Speaking at the recent Synergy for Success Conference at the Institute of Physics, he explained that finding defects in steel as it emerges from the caster will minimise scrap and maintain quality before the material proceeds to downstream processing. It also helps to prevent defective or out of tolerance material being supplied to either internal Corus business units or external customers. Possible surface defects include cracks, blowholes and inclusions, while internal defects can include cracks, porosity, inclusions and segregation. The steel emerges from the caster at temperatures of up to 1000 deg C at speeds of 0.5m/s to 5m/s in an environment full of dust, vibration, water spray and steam. Operators currently rely on manual visual inspection, eddy current techniques – which only respond to the top few mm – and CCD cameras, which cannot see through the oxide scale on the surface. Internal inspection requires cutting up cold steel in order to take sulphur prints, or cutting out samples for metallurgical examination. By this time, the steel is likely to have been through the rolling mill and may already be in the hands of customers. EMATs are, in Baillie’s words “essentially magnets with coils wound round them.” The ones used at Corus are water cooled and stood off the metal surface by a few mm. The ultrasonic waves arriving back at the steel surface change the magnetic field in front of the sensor, which induces a current in the coil that can be detected and analysed. In more conventional inspection practice, ultrasound pulses are launched into materials from transducers placed on the surface of the material being monitored, and received by either the same or different transducers on the surface. But this is not an option with hot steel. Using a laser as the sound source has the additional advantage of acting as a point source, improving the resolution of the technique. In the pilot rig, and in the real world deployable system, everything from the firing of the laser, the movement of the steel and the acquisition of data is computer controlled via LabView from National Instruments. First trials were undertaken with a 1.6m long, 0.11m square billet with a known transverse defect, rolled back and forth on a pilot rolling mill. The tests were then repeated with the billet heated to 800ºC. Defects can be detected whether they are to one side of the laser spot and EMAT or between them. If the defect is to one side, reflections can be picked up whose timing relates to how far the defect is from the laser spot and detector. If the defect is between the laser spot and detector, the signal is attenuated. LabView is used for signal capture, visualisation and analysis as well as controlling the system. A finite element model was constructed at Warwick University as part of the investigation in order to understand the various waves passing through the system and to optimise the system design. Baillie reports that results produced using the computer model closely resembled those obtained by experiment. The next stage will be to build EMAT arrays and improve signal to noise ratio, possibly by using a more powerful laser. The prototype system is to be installed on the continuous casting pilot plant at the Teesside Technology Centre. The project was funded in part by the European Research Fund for Coal and Steel and the Engineering and Physical Sciences Research Council. Ultrasonics keep trains on the rails US researchers claim to have identified an improved way of finding defects in steel railway tracks. A team led by Francesco Lanza di Scalea – professor of structural engineering at the University of California, San Diego – has used laser beam pulses to gently ‘tap’ on steel rails. A prototype vehicle rolls down the track delivering ultrasonic laser beam taps at one-foot intervals. This sends waves down the track at 1800m/s. Microphones are positioned a few inches above the rail and 12 inches from the downward-pointed laser beam. They detect any reductions in the strength of the ultrasonic signals, which indicates surface cuts, internal cracks and other defects. A prototype vehicle equipped with the UCSD technology detected at least 77% of internal defects and at least 61% of surface cuts in a recent test run. According to the Federal Railroad Administration, track defects account for about one-third of the 2,200 annual train derailments in the U.S. The UCSD team claims that its method is an improvement on existing methods because it sends the ultrasonic pulse along the rail, rather than firing it from above. This means that signals are not blocked by surface defects such as superficial cracks. Software helps to filter out excess noise. “Our pulsed-laser technique is potentially very effective at finding internal rail defects,” said Lanza di Scalea. “When fully developed, it could allow rapid inspection of tracks at up to 70mph.” University of California San Diego National Instruments Email Iain Baillie Pointers * The technique can detect a wide range of defects in moving, hot, steel billets in a dirty, wet environment * The initiating laser pulses excite a burst of ultrasound from a point source, which is picked up by an EMAT close to the metal surface * A new way of detecting ultrasonic pulses could form a more effective way of finding dangerous internal defects in railway tracks