Numerous mechanical components such as bearings and gears are manufactured from seamless steel tubing, which offers superior mechanical properties. The tube making process oftentimes causes wall thickness variations in a helical pattern along the tube length. Therefore, additional clean-up stock is added to the tube. Process control to reduce this variation would achieve considerable savings through improved material utilization and reduced tubing scrap and re-work. For components machined from tubing, additional savings are realized from reduced machining time and tool wear. A laser-ultrasonic system was installed immediately after the final operation in seamless tube making. The system consists of generation and detection lasers and an interferometer located in a cabin outside the tube mill, and a fiber coupled inspection head located above the process line. Tube wall thickness and temperature measurements are used to guide mill adjustments to achieve the desired tolerances. Nearly 1,000,000 tubes were inspected in the first two years of operation. In a separate effort, additional functionality was added using the same signals that measure the tube wall thickness to sense the size of the austenitic grains. The austenitic grain size of carbon and alloy steel is of high importance due to the impact on mechanical properties of the final product. The time delay between the back wall reflections is a function of the wall thickness, while the attenuation between these same signals is related to the grain size. The target austenitic grain size is achieved by controlling the deformation and thermal process parameters operation in seamless tube making. Introduction: Seamless tubes are used for numerous applications, such as hydraulic cylinders and power transmission components (gears and bearing races) where microstructural variation due to a weld seam is not acceptable. The tube making process begins with heated cylindrical billets that are formed into hollows by cross rolling over a piercing plug (see Fig. 1 for a sketch of the process used at The Timken Company). The hollows are elongated into shells by cross rolling over a mandrel bar and formed into the final tube size in a reducing mill. The piercing process can cause wall thickness variations, which often follow a helical pattern. The need of a thickness measurement sensor follows not only from the requirement of controlling the wall thickness to specification, but also from the desire for improved process control to increase yield and reduce scrap and rework. Although penetrating radiation (γ-rays) techniques have been developed and used for thickness gauging tubes, they have various limitations and, in particular, they cannot measure tubes with a mandrel inside. For cold tubes, ultrasonic techniques are widely used to measure thickness along tubes based on the time delay between successive echoes combined with the material velocity. The ultrasonic waves are usually generated and detected by piezoelectric or EMAT transducers and coupled to the inspected part either by direct contact (or a few millimeters distance in the case of EMATs) or through a water bath or a water jet. For hot tubes during the production, these ultrasonic techniques are not applicable, firstly because of the relatively high tube temperature (in the range of 1000 °C) and secondly, because the tube is not very precisely guided. Laser ultrasonics [1], which uses lasers for the generation and detection of ultrasound at a distance (typically several tens of centimeters to one meter and even more), does not have such limitations and was the elected technique to develop the mill-worthy system. Although there has been previous in-plant demonstration of this technique for on-line tube gauging [2-4], this is the first time a system is continuously used in production. The austenite grain size is well known to be a microstructural parameter that determines to a great extent the final microstructure and consequently the mechanical properties of steels. While the time propagation of an ultrasonic echo can provide information about the thickness of the tube, ultrasonic attenuation can provide information about the grain size in the austenitic phase [5], as the normal practice for steel products is to do the hot-rolling in that phase. Continuous inline monitoring of grain size facilitates a controlled process that leads to an optimized microstructure, avoiding post-production modifications usually performed through costly heat treatments. This paper describes the laser ultrasonic system that has been running at The Timken Company in Ohio, USA, for more than two years and reports its performance in the continuous monitoring of both wall thickness and austenitic grain size. Heat Pierce Elongate Reduce Rotary Size Induction Heat Laser-Ultrasonic Gauge Figure 1. Stages of a seamless tube producing process. Description of the System: Generation of ultrasound is performed in the ablation regime by a sufficiently strong laser pulse. The recoil effect following material ejection off the surface (essentially surface oxide) and plasma pressure produce strong longitudinal wave emission perpendicular to the surface. The ultrasonic waves after reflection by the inner wall of the tube cause a small surface motion on the outer surface (typically in the nanometer range) (see Fig. 3a). Detection uses a second laser with a pulse duration sufficiently long to capture all the ultrasonic echoes of interest (typically 50 μs) and very stable in frequency and intensity. The ultrasonic surface motion produces a Doppler frequency shift on the scattered light that is demodulated by an interferometer. Figure 3b shows an ultrasonic signal obtained on-line. Specially designed Doppler velocimeters (translation and rotation) and two-color pyrometer allow the measurement of the position and temperature of the measurement location (see Ref. 6 for details). -1500 -1000 -500 0 50