Integrated solutions for fully automated grinding of surface flaws on semi-finished products

Integrated solutions for fully automated grinding of surface flaws on semi-finished products High-pressure grinding has proven to be the optimum technology for removing surface defects on semi-finished products prior to further processing. BRAUN s multi-functional grinding machine provides fully automated full surface, corner and flaw grinding of billets, blooms, ingots and other products. Automated grinding of surface flaws, however, also requires accurate detection of the defects in terms of location and depth. The thermo-inductive crack detection system developed by vatron GmbH provides such information, resulting in an integrated, fully automated system for flaw detection and grinding. Authors: Gerhard Richter, Ronald Ofner and Ewald Fauster BRAUN Maschinenfabrik GmbH & Co KG and vatron GmbH r Fig 1 HP grinding machine at BÖHLER Edelstahl, Kapfenberg The surface of cast, forged or rolled products may contain cracks, inclusions and scale and, depending on steel grade and application, these imperfections may need to be removed prior to further processing. High pressure grinding has been proven to be the most reliable and most effective technology to achieve fault-free surfaces. Thanks to its reliability, high capacity and flexible applicability, but also due to its high environmental compatibility, it is superior to other techniques, such as manual grinding, robotic grinding or flame scarfing. Depending on type, quantity and distribution of the surface flaws, either the entire surface or only certain areas of the work piece must be ground. We distinguish between the following three basic applications for high-pressure grinding: ` Bright grinding of the entire surface ` Grinding of the corners (for work pieces with square cross-section) ` Controlled grinding of partial surface flaws DEVELOPMENT OF HP GRINDING MACHINE BRAUN Maschinenfabrik developed its HP (High-Pressure/ High-Performance) grinding technology in 1998-99. The first application was a multifunctional facility for bright, corner and flaw grinding of billets of various dimensions and material grades at BÖHLER Edelstahl, Kapfenberg, Austria, using grinding wheels of 635mm (25 ) starting diameter (see Figure 1). Throughout the design of this first HP grinding facility BRAUN tried its utmost to utilise the latest technical advances available at that time and to develop a substantially improved design. It was of great advantage that this application had some special requirements, such as dealing with occasionally badly distorted billets, some small billet sizes they tend to deflect during grinding and a huge variety of materials including many alloys sensitive to surface cracks and surface decarburisation. Furthermore, it was beneficial that BRAUN took the operational experiences of customers into account during the planning and engineering of the new facility. Since 2004 improved HP grinding machines have been built for a variety of applications: ` Two facilities for bright, corner and flaw grinding of billets for the Russian steel combine OÉMK, Staryi Oskol ` A facility for longitudinal grinding of the outer surface of large-scale seamless steel tubes for the Düsseldorf- Reisholz works of the German-French tube manufacturer Vallourec & Mannesmann ` A facility for longitudinal grinding of the outer surface of large-scale seamless steel tubes for the newly built Chinese production plant of Vallourec & Mannesmann in Changzhou, Jiangsu province 124

Finishing Processes ` A facility for helical grinding of stainless steel ingots for the French steel company Aubert & Duval, Les Ancizes ` A facility for bright, corner and flaw grinding of billets for Saarstahl s Neunkirchen works in Germany FUNDAMENTALS OF GRINDING TECHNOLOGY Hot-pressed grinding wheels (see Figure 2) are the tools used for the high pressure grinding process. In order to meet the requirements of the grinding application with regard to surface roughness, grinding depth, brightness of the surface, etc, it is necessary to select the proper wheel specification (ie, type and size of the abrasive grains). In order to achieve a high-quality grinding result, however, the grinding machine too must comply with certain requirements: r Fig 2 Basic structure of a hot-pressed grinding wheel Grinding pressure The machine must be designed in such a way that the grinding pressure can be selected in accordance with the given application. Furthermore, it is of great advantage if it can be operated with grinding pressures as high as possible in order to achieve high material removal rates. In either case it is essential to keep the set grinding pressure constant, in particular if the surface of the work piece is uneven. Peripheral speed of the grinding wheel To achieve long service life of the grinding wheel, grind quality and, to a somewhat smaller extent, the achievable removal rate the selection of the proper peripheral speed of the grinding wheel is important. The majority of grinding wheels are rated for a maximum peripheral speed of 80m/s (262ft/s), thus the grinding machine must be capable of achieving this permissible speed. In particular, the selected speed must be kept constant as the wheel diameter reduces as it wears. r Fig 3 Ideal grinding finish (tool steel) Relative speed of the work piece in relation to the grinding wheel The relative speed of the work piece in relation to the grinding wheel should be as high as possible. This is not only important for a high removal rate but in particular for the quality of the ground surface. If the relative speed is too low, the work piece can become locally overheated if the wheel dwells too long at a certain position on the surface. Especially for materials with a higher carbon content, this could lead to discolorations and local hardening of the surface. Ideal grinding results are shown in Figures 3 and 4. Furthermore, this high relative speed must be reached as quickly as possible. This means that the grinding machine must be able to accelerate rapidly. Essentially, the design is a table grinding machine. This means that the actual grinding unit is anchored to the foundations and the work piece to be ground is moved back and forth by a grinding carriage (the table). This basic r Fig 4 Decarburisation-free work piece surface (tool steel 1,500 : 1 magnification) a 125

r Fig 5 Grinding head schematic r Fig 6 Grinding head in operation structure features the following advantages compared to a pendulum grinding machine where the work piece to be ground is in a fixed position and the actual grinding unit the pendulum is moved back and forth: ` Significantly higher stability of the grinding unit ` Excellent and consistent visibility for the operator who sits in a control booth ` Substantially better encapsulation of the grinding area as well as a controlled removal of the swarf and dust r Fig 7 Control booth KEY DESIGN FEATURES OF HP GRINDING MACHINE ` Grinding drive with powerful, frequency-controlled motor allows maintenance of a constant peripheral speed of the grinding wheel independent of the wheel diameter ` Wheel wear compensation system by measuring the actual grinding wheel diameter and automatic adjustment of peripheral speed ` Highly efficient and flexible design of the grinding head with weight-saving yet robust construction allows grinding pressures up to 1,400kg (3,100lbs), exact adherence to the pre-selected grinding pressure and uniform material removal even if the surface of the work piece is rough or curved (see Figures 5 and 6) ` If the need arises, the possibility exists to retrofit elements enabling stepless adjustment of the grinding head between 90 and 45 (grinding axis = pivot axis of grinding head, thus allowing re-adjustment of the grinding head even during the grinding process) ` Sensitive and fast-reacting hydraulic-electronic grinding pressure control system ensures a uniform grinding pressure even for uneven or curved work pieces ` Automatic and exact detection of both ends of the work piece ensures a smooth and jolt-free touch-down 128

Finishing Processes and lifting of the grinding wheel which is especially important for longitudinal grinding ` A defined position on the work piece surface can be approached accurately (a precondition for a fully automatic flaw grinding) ` Comfortable, quiet control booth with special operator s seat and panoramic window for the highest degree of operational convenience and unrestricted visibility of the grinding process (see Figure 7) ` Clear, user-friendly process visualisation and data collection can also be connected to a higher-level process control system if the need arises (see Figure 8) Whilst the machine is designed for a fully automated operation and, alone, is able to accomplish this for bright grinding of the complete surface of the work piece or for corner grinding, or for fully automated grinding of partial surface defects, the machine needs to know where the flaws are and how deep they are. Therefore, in addition to the grinding machine, a facility to reliably detect surface defects (cracks) is a precondition for a fully automated flaw grinding complex. This will now be described. r Fig 8 Process visualisation screen example CRACK DETECTION In addition to manual visual testing methods such as application of penetrating agents, automated testing using magnetic powder is well known in the steel industry. However, magnetic powder testing provides a number of serious disadvantages: ` It is only applicable to alloys which can be magnetised ` Crack depth cannot be determined so steel with deep cracks may require grinding, checking and re-grinding ` Older facilities use fluids containing the toxic cyanides ` Magnetic powder, either as a liquid or in dry form, is a consumable and hence incurs ongoing costs Thermo-inductive crack detection system vatron s thermo-inductive testing approach is significantly more flexible and modern, but not yet well known in the steel industry. It provides the following advantages: ` Work pieces can be tested fully automatically and with high reproducibility directly in the production line ` There is no need for preparation or pre-processing of the materials prior to the tests ` Pollutants and toxic substances are avoided ` Energy-intensive magnetisation of the work pieces is omitted r Fig 9 Magnetic field generation and temperature measurement r Fig 10 Example of measuring the defective surface of a billet Source: Institute for Automation, University of Leoben In this test method the work piece is penetrated with a high-frequency magnetic field which is produced by a highfrequency generator and an induction coil (see Figure 9). a 129

r Fig 11 Principal components for thermo-inductive testing Thermo-inductive testing is suitable for all electro-conductive materials, especially steel. As a result of the high electrical permeability of steel, the eddy current is induced in a thin layer on the surface of the material. When the crack depth is comparable with the thickness of this layer, the induced eddy current is deflected by the crack. This leads to a higher density of the current at the edges of the crack, which results in higher temperatures along the crack (see Figure 10). Areas of increased temperature can be detected and automatically evaluated by means of infrared cameras with digital image processing. The temperature increase depends on the crack depth: the deeper the crack, the higher the temperature increase. This enables the determination of the crack depth as a result of the observed temperature distribution. For the testing of long products (bars and billets), the material runs through the induction coil. Four infrared cameras measure the temperature distribution on the entire surface of the work piece (see Figure 11). r Fig 12 Schematic of simple integrated facility for automatic flaw detection and removal Technical item Parameter Optical resolution 1,400 x 1,040 pixel (monochrome) Acquisition rate 30Hz Laser power 40mW Wavelength 660nm Power supply 24V DC Data transmission Gigabit Ethernet Protection class IP 65 r Table 1 Technical data of the HotProfile light section-measurement head INTEGRATED INSPECTION AND CONDITIONING The multi-functional HP grinding machine and the thermoinductive crack detection system described above are the core elements of a fully automated, integrated facility for surface inspection and conditioning of semi-finished steel products. Depending on the specific application, the requirements of the plant, the type of material and the quantity of products which need to be inspected and processed, the set-up of such integrated facility will be different from case to case. CONCEPT EXAMPLE 1 In the most simple case, the grinding machine is situated directly after the crack detection system (see Figure 12). The inspected work pieces are moved towards the grinding machine and are transferred onto the grinding carriage or, if rejected, to a take-out position. This solution allows transfer of the crack coordinates and necessary grinding depth from the crack detection system directly to the grinding machine control system. After automatically detecting both ends of the work piece the flaw positions are automatically approached and the cracks are ground. As certain alloys are more crack sensitive, it is more efficient to do a complete bright grinding of the entire surface and such work pieces can be transferred to the grinding machine without prior surface inspection, if desired. Automatic re-inspection of the ground areas can be achieved either by re-routing the material through the thermo-inductive inspection facility or by inspection of the material surface by means of a light section-measurement head HotProfile a product of vatron GmbH). The former method is preferable for small production volumes as in 130

Finishing Processes most cases the time needed for re-running the material through the inspection facility is shorter than the time required for grinding the defective areas. Product profile measurement For higher production volumes, the HotProfile measurement system is preferred and the heads are mounted directly after the HP grinding machine. This enables both the geometry of the ground area as well as the grinding depth to be verified. Each of the light section sensors is equipped with a monochrome industrial camera together with a data conversion module as well as a laser module with a cylindrical lens in order to project linear laser light onto the material to be inspected. As a result of its set-up (housing, protection class, data interface, power supply), the sensors are well suited for industrial applications. See Table 1 for details. The cross-section of the ground material is measured by means of the light section-measurement sensors. Here, round bars and square billets are distinguished. The laser module projects a light plane onto the material and the resulting intersection line is acquired with a monochrome industrial camera. The geometry of the material can then be computed from the acquired image by means of digital image processing algorithms. First, sets of data points on the intersection line are extracted, then the model of an ideal circle is fitted to the data points. As a result of the orthogonal distances of the individual data points to the circular model, the type of product (eg, bar or billet) can be determined. According the type of material, the following geometric parameters are computed: ` Diameter and centre point coordinates for bars ` Side length, corner radius and centre point coordinates for billets By measurement of the cross-section along the entire length of the work piece, the grinding depth can also be determined. Furthermore, it can be verified whether the actual grinding depth is sufficient for the material defects to be completely removed. Figure 13 shows the light section-measurement head as well as the image of an intersection line acquired with the monochrome camera of the sensor. In the case of cracks remaining after the first run-through, the defective areas can immediately be re-ground. For billets, the re-inspection can be performed after grinding all defective areas of a side surface, thus turning of the billet prior to the re-inspection can be avoided. Product identification In each case a permanent marking of the work piece that allows automatic identification is ensured in order to clearly allocate the data that are generated during the travel of the work piece through the r Fig 13 HotProfile head (left); intersecting line resulting from the inspection of a billet (right) r Fig 14 Front face of a billet stamped with alphanumeric and bar codes individual stations of the entire facility to that work piece. Ideally, the work pieces are stamped when they are still hot (eg, directly after casting, forging or rolling). If they are stamped with an alpha-numeric code and a bar code they can be identified both visually and with an automatic bar code reader ahead of each individual step (see Figure 14). The advantage of such a marking by means of stamping is its permanence and the fact that it is still readable if the stamped surface is scaled or uneven. CONCEPT EXAMPLE 2 For large production volumes (eg, 1Mt/yr or more) and a broad product mix, the set-up will certainly be more complex and sophisticated than example 1 and several grinding machines will probably be required to reach the desired output. Furthermore, for logistics reasons it may be necessary to put inspected work pieces into intermediate storage before they are ground. Defect marking and detection Instead of having to store huge quantities of data or to loop them through from station to station, it is useful to mark the cracks that were detected by the crack inspection system. This also allows manual interference if the need arises (eg, in the case of malfunctions of the material data tracking system) or in extreme cases to manually grind the flaws with the HP grinding machine. Figure 15 shows a possible set-up of such a complex integrated facility. The cracks detected with the thermo-inductive testing a 131

Finishing Processes r Fig 15 Schematic of complex integrated facility for automatic flaw detection and removal r Fig 16 Colour detection unit (left) Colour images acquired (right) facility are marked with colour sprayed onto the surface of the material. There are two different colours: one is the grinding colour used to mark defects for grinding, the other is the rejection colour which is used to mark defects exceeding a specified crack depth. Internal defects can be detected by means of ultrasonic facilities which can also be marked with the rejection colour. Rejected material can subsequently be sorted or optionally be conveyed to a BRAUN automated cutting facility where only the defective parts of the work piece will be cut out and rejected. A vatron GmbH colour camera is used to detect the colour markings (see Figure 16 and Table 2). The camera is protected by means of a robust housing and is mounted directly before the cutting facility. Together with the data converting module which is also mounted in the housing, the camera represents a colour detection unit for industrial applications. As the entire surface is inspected with the colour camera, colour images are acquired at a rate of up to 30Hz and subsequently processed online by means of digital image processing algorithms, as follows: First, the Bayer-Array transmitted by the colour camera is converted using a simple mosaic dissolving algorithm to an RGB image. This is subsequently transformed to the Hue-Saturation-Luminenscence (HSL) colour model, which is better suited for colour detection than the RGB colour model. For efficiency reasons the data conversion of RGB to HSL is implemented using a look-up table. The colour detection is finally realised in the HSL image. After segmentation by means of three pairs of threshold values (one pair for each of the three dimensions of the HSL colour model), a binary image is obtained (see Figure 16). The foreground of this binary image consists of pixels corresponding to the grinding colour. Adjacent foreground pixels are collected to colour areas, which are subsequently enclosed by rectangles, then the resulting rectangles are used to describe both the position and the size of the images acquired. 132

Technical item Parameter Optical resolution 700 x 520 pixel (RGB) Acquisition rate 30Hz Power supply 24V DC Data transmission Gigabit Ethernet Protection class IP 65 r Table 2 Technical data of the colour detection unit CONCLUSIONS AND OUTLOOK Integrated solutions for fully automated detection and grinding of surface flaws on metallic products are not only feasible but can also be adapted to specific customer requirements. With BRAUN s HP grinding machine and vatron s system for thermo-inductive crack detection, the two core components for such an integrated facility are available; realising the following user advantages: ` Reliable detection of position, size and depth of surface cracks ` Reliable transmission of the flaw coordinates from the crack detection system to the grinding machine ` High quality of ground surfaces ` Automatic verification of the actual grinding depth ` Reliable identification of the work pieces to be inspected or ground ` Reliable material data management ` Improved quality assurance ` Minimal staffing ` Integration of additional inspection units (eg, ultrasonic testing) and conditioning stations (eg, friction saws or abrasive cut-off machines) into the automated system if required ` Flexible solutions for different production requirements (including intermediate storage of work pieces) Thanks to their specific know-how, their long-time experience and their targeted R&D cooperation, BRAUN and vatron GmbH are ideal partners for the international steel industry. MS Gerhard Richter is Vice President, Steel Cutting & Grinding Machines Division, BRAUN Maschinenfabrik GmbH & Co KG, Vöcklabruck, Austria. Ronald Ofner is Business Unit Manager, Optical Solutions for Industrial Plants and Ewald Fauster is Project Manager, Optical Solutions for Industrial Plants, both at vatron GmbH, Leoben, Austria CONTACT: g.richter@braun.at ronald.ofner@vatron.com 133