Hans Jörg Stoll and Bernhard Kuhn, DISA Industrie AG, Schaffhausen, Switzerland
(Published in Millenium Steel 2004, the leading review of advanced process technology worldwide)
Founded in 1900, DISA, part of the A.P. Moller-Maersk Group, is a leading supplier of foundry equipment, metal surface finishing systems and air pollution control solutions. The DISA Shot Blast Group, one of the world’s leading producers of shot-blast and shot-peening systems, is able to provide a very wide range of de-scaling, cleaning and surface conservation solutions. The portfolio for shot blasting applications covers every requirement of the manufacturing industry, and in particular, those of the steel sector.
DISA shot blast equipment design brings together the accumulated expertise of four well known names in the industry: Georg Fischer, BMD, Vogel & Schemmann and Goff. Designs include layouts for continuous throughput of product or batch processing. Of special interest to the steel industry are high-speed machines for continuous strip de-scaling on annealing and pickling lines, and wire and bar shot blast systems which can be integrated into automatic drawing layouts. The needs of fabricators and shipyards are addressed with fully automatic plate and rolled section de-scaling and preconditioning systems. Specialised de-scaling designs are available for the application of protective coatings to the surfaces of steel plate and profiles prior to fabrication, or simply to safeguard the material from further corrosion during storage.
The technology
The principle of blast cleaning technology is based on the use of the high kinetic energy of shot and grit resulting from its mass and impact velocity see 
Figure 1 (on the left). A bundled stream of grain-sized hard particles is accurately flung at high speed against a surface or work pieces. Depending on the reason for blast cleaning a wide variety of surface conditions on the work piece treated can be produced. Depending on the application, different materials are used for shot blasting and shot peening, namely:
- Steel shot (spherical cast steel particles of diameter approx. 0.6 – 1 mm) is used for de-scaling of sheet, profiles, billets, wire and forgings. For stainless steel strip in combined lines, a diameter of 0.2-0.4mm is used
- Cut wire (steel wire cut into particles where diameter = length) is used for de-scaling of sheet, profiles billets, wire and bars, forgings but not for steel strip. It is pre-rounded, for shot peening
- Stainless steel (cast or cut wire) is used for stainless steel wire, bars and forgings to prevent corrosion problems caused by embedded carbon steel residues
- Cast steel grit (crushed round particles) is used only for special applications
The hardness of these abrasives is typically 48-56 HRC and normally, for de-scaling, a particle size as small as possible is used in order to keep the roughness low. This helps minimise paint consumption on plates and profiles, residual roughness on wire and bars after cold drawing, and residual roughness on steel strip after cold rolling. The particle size must, however, be large enough to create the required de-scaling quality with a reasonable quantity of abrasive per m2. The different de-scaling degrees are fixed within the ISO 8501-1 standard.
The specific quantity of abrasive required depends on initial surface (scale, rust) and degree of required cleaning, but typically is 120kg/m2 for structural steel, sheet and profiles, 200 kg/m2 for shot peening and for cleaning hot-rolled wire before drawing, and 70-140kg/m2 for stainless steel strip before acid pickling.
Consumption of abrasive depends on quality and size, and ranges between approximately 0.1 and 0.3 kg per kW of blast wheel power. In order to maintain particle quality, worn abrasive, below approximately on third nominal diameter, is continuously extracted from the circuit by an air wash separator. In the shot peening process, particles below half nominal size are removed. The amount of abrasive thrown per wheel and per minute depends on abrasive speed and drive power. At 80m/sec and with a wheel power of 30kW the amount is approximately 400kg/min.
As shot blasting is very wear-intensive, the wearing parts of machines are made of high-alloy cast steel, hardened tool steel or high manganese steel, and are easily exchangeable in terms of maintenance.
Shot blasting and surface protection
A typical surface protection facility (see Figure 2 below), based on one of DISA’s RC horizontal roll-through powered-conveyor machines provides for the progression of plate or profiles through an initial blow-off unit for the removal of loose scale or debris (left-hand side of picture). Next in line is an oven to
remove residual moisture from the product and raise the temperature to a level that will aid drying of a subsequently applied protective coating. The material then enters the shot blast unit where it is de-scaled by shot, directed from wheels mounted above and below the conveyor. Finally, the steel work is progressed through an airless paint spraying station, followed by a paint drying tunnel.
The specialist nature of DISA de-scaling systems is well illustrated by the HB range of machines. These are units employed within integral production lines used for the annealing, cooling, mechanical primary de-scaling and chemical secondary de-scaling of hot-rolled strip. As a result of the combination of blast cleaning with pickling, a cumulative de-scaling effect can be achieved. The shot blasting procedure results in a massive reduction in the acidic density of the subsequent chemical de-scaling operation and the sludge residue in the pickling baths is substantially reduced.
The HB cleaning machines, which direct the blasting media onto both sides of the strip in a transverse pattern, can be adjusted during the operating process to suit various widths and qualities of materials. The blasting residue left lying on the top surface of the strip is blown off with high-pressure air. As a result, the strip emerges from the machine free from dust. Such HB units, which may use two or four blast wheels, can be employed in series to accommodate exceptionally high throughputs.
Where narrow strip is being processed, DISA offer the HB-4/35-100 and HB-4/45-100 models designed, respectively, to accommodate strip widths of 50 – 350 mm and 100 – 450 mm. Larger machines in the HB range are available to handle strip widths of 600 – 2100 mm (see Figure 3).
De-scaling in fabrication shops
In fabrication shops and shipyards, there is frequently a need to de-scale and de-rust rolled steel products, such as plate, sections or tubing, prior to secondary operations such as cutting, bending and welding. DISA offers roller conveyor-type machines for horizontal cleaning of plate and sections, and units for plate blast cleaning on a slant. In the case of the horizontally-configured models, the blast wheels are so arranged as to bombard the product from all sides with abrasive. The operator can select an appropriate programme, covering plate or section, and then input length, width and thickness, so providing complete control of the entire shot blasting procedure. The registering of the length of the flat work piece ensures that discharge of abrasive only takes place during passage between the blast wheels. The input of the width parameter limits blasting to the area presented to the machine. Where the programme is pre-selected for a section, the direction of the blast stream from the outer wheels is altered automatically, so that the vertical surfaces are cleaned in a single pass.
Depending on the model specified, the machines can accommodate plates up to a maximum width of 3,600 mm or sections with a maximum dimension of 1,000 x 500 mm. Where plates are being cleaned in the slanting position, the units can cope with widths from 1,600 to 3,200 mm.
Where it is necessary to de-scale or clean smaller sized sheet plates and sections, DISA supply the RC series of shot blast units which can accept work pieces with a maximum width of 600 – 2,100 mm and a maximum height of 500 mm, depending on the specification see Figure 4.
Mechanical de-scaling of wire and bar
High throughputs are also achievable with specialist shot blast machines designed for de-scaling wire and bar. The higher-capacity models from the FL series can be integrated into wire drawing lines operating at up to 240 m/min. The blasting envelope extends from 5 to 110 mm diameter, depending on model selected. These machines are equipped with an adjustable plate system to guide and concentrate the blast medium on the workpieces. This specific feature ensures maximum exploitation of energy and abrasive used. Guide and pressure rollers facilitate the smooth passage of wire and bar through the machine. Each transport roller is driven individually and is vertically adjustable, and the layout can also incorporate driving and pre-straightening units. Such models can be sited at the feed-in side of draw benches, or used as the basis of coil-to-coil de-scaling lines (see Figure 5).
The DF series can accommodate throughputs of up to 180 m/min and offer a blasting envelope, depending on model, from 2 to 80 mm diameter. Again, the machines are provided with guide and pressure rollers. A feature of these units is the use of innovative abrasive-stream guide plates which act as directional funnels for the blasting media (see Figure 6 below). These devices are able to increase the impact intensity on the surface of the wire or bar by up to 300%, which results in faster throughput speeds without the need to increase blast wheel power. Depending on the drawing speed, models can be equipped with
two, three, four or six blast wheels. These wheels are staggered along and around the cross-section of the blast cabinet. Each wheel is normally associated with its own set of abrasive stream guide plates, supported outside the blast cabinet. These plates are adjustable, in a matter of seconds, to suit the cross-section of the wire or bar.
DISA also build specialised wire-coil blast machines. This wire-coil blasting principle has been especially developed for alloyed and highly alloyed rolled wires, as well as high-carbon content versions. It avoids repeatedly bending the wire, thus causing undesirable cold work-hardening. The wire is transported and guided by a horizontal hardened comb shaft and, separated, loop by loop, from the coil by a roller, it is forwarded by the rotating comb shaft through the shot blast cabinet. Uniform de-scaling and cleaning of the entire wire surface is ensured by the controlled helical movement, the regular intervals between the free-hanging wire loops, and by the arrangement of the wheels within the cabinet. Two wheels mounted on the cabinet walls project the abrasive parallel to the comb shaft onto the travelling loop. For loops with a winding diameter of more than 750 mm, the coils may be shot blasted directly on the inside by two additional wheels.
Shot peening
DISA has been involved over many years in the design and manufacture of specialised shot-and stress-peening equipment for steel components. Spherical steel shot or round cut wire should be used in this application. The benefits to fatigue life are illustrated in Figure 7 (see below). Typical applications include the processing of a wide variety of gear components; coil, leaf and clutch springs and many other parts which are subject to high alternating stress.
The range of machines covers both batch and continuous throughput models. Whereas batch machines are mainly used for small parts and low to moderate process requirements, machines of the throughput design ensure overall process reliability (defined coverage and Almen values, (a measure of imparted compressive stress) for all parts shot peened. Such units can be either manually or robotically loaded and unloaded and, although machines are of standard design, individual models are frequently adapted to meet client requirements. Figure 8 shows a satellite table DR shot peening machine for gear parts.
For circular or cylindrical work pieces, such as gears, pinions and clutch springs the DR design includes two machine-types with two or four blast wheels and rotating tables incorporating individual satellite turntables. Each turntable bears a fixture which can accommodate one or several vertically stacked work pieces (see Figure 9).
A typical satellite machine, the DR-2/0301 – 100, with up to 17 turntables, can process 600 clutch springs per hour, using two blast wheels. The unit can accommodate work pieces between 150 and 330 mm in diameter, with a maximum height of 50 mm. The work tables, rotated in the shot zone by a toothed belt, driven by a frequency-controlled geared motor, turn at approximately 50 rpm. The blast wheels are each mounted on a movable carriage, the position of which is adjusted to suit a specific clutch spring diameter. The effective shot-peening time for a particular work piece can be varied continuously in accordance with the desired Almen value. Such a unit can be fitted with a fully automatic loading and unloading device. The incorporation of a further robot in the system enables the components to be turned over automatically half-way through the 6 - 15 second cycle.
The RD family also includes shot-peening units for processing suspension springs, leaf springs and valve springs and more conventional batch-type reverse belt units and overhead monorail-type units are available for shot-peening a variety of components.
Recent shot blast projects
The organisation has recently been involved in various projects in the Far East. One of these was concerned with a new hot strip annealing and pickling line for the Baosteel Group Ningbo Baoxin Stainless Steel Company, based in Ningbo, PR of China. The organisation, one of the world’s largest producers of stainless steel strip, placed the order for the line with Sundwig GmbH, a member of the Andritz Group and DISA were sub-contracted to supply two HB 2 x 4 shot blast machines, designed to operate in series for the removal of scale resulting from rolling and annealing, in a reliable and environmentally acceptable manner.
The annealing and pickling line has been designed for a production capacity of 700,000 tonnes of stainless steel strip per year. The hot rolled strip, in widths between 650 and 1350 mm and with thicknesses between 3 and 6 mm, enters the annealing section at speeds of up to 91 m/min. This heat treatment procedure is followed by blast cleaning and pickling. More than 5 km of steel strip can be produced per hr. The line operates continuously for about 7,000 hours/year. The machines use 16 blast wheels, each rated at 110 kW, and throughput of the blasting medium is more than 1100 t/h.
The Andritz Group was also involved in a major contract associated with another Chinese hot-rolled stainless-steel line. This was placed by the Taiwan-based Yieh United Steel Corp. as part of an investment in the organisation’s daughter company, the Lianzhong Stainless Steel Corp. in Guangzhou, PR of China. Again, DISA provided two HB 2 x 4 blast cleaning machines designed to operate in series. With strip widths up to a maximum of 1,600 mm and a thickness of 10 mm, the units are able to de-scale the material at throughputs of up to 127 m/min.
DISA is able to provide these customers with a quick and competent maintenance` support of these plants through its a Chinese subsidiary.