Steel sprayed to hard formation

Tom Shelley reports on recent advances in the direct spraying of steel and other materials for the manufacture of hard tools and coatings

New, improved predictive controls have been developed for the manufacture of sprayed steel moulding and press tools. Improvements in sprayed steel toolmaking continue to advance, with one recent breakthrough allowing the inclusion of cooling pipes. And the developers are also looking at new coating applications for the technology. Novarc, based near Oxford, has an exclusive license to sell the Sprayform process originally developed by Sprayform Developments and acquired by Ford in 1999. The success of the process hinges on the discovery that 0.8% carbon steel, when arc sprayed and cooled rapidly, shrinks by 4.2% during cooling, but then expands by exactly the same amount when it shear transforms to martensite and lower bainite. But, in order to achieve the desired effect, you need to have a control system capable of anticipating what’s going to happen next, in order that temperatures are maintained within +/-5 degrees C. The coating, undertaken by four Sulzer Smart Arc guns, and the workpiece are monitored using a military standard infra-red camera making five observations per second, with a thermal model running in the background so the software can anticipate changes. The monitoring software includes an algorithm to cut out parts of the image obscured by cold robot parts; once a minute the robot is moved out of the way to allow a full view. If one gun stops, the feed rates of the others are raised to compensate. Managing director, David Field, describes it as, “standard equipment put together in an intelligent way”. The firm has entered into a joint marketing relationship with Sulzer Metco. New developments include laying down an initial 4mm of steel, then tacking down copper pipework and re-starting the spray process to bury cooling pipes within moulds. Other possibilities being investigated for the control system include the ceramic coating of gas turbine engine blades and improved metal arc spraying of other metals and alloys for EMC shielding. The most promising markets for sprayformed tools are rotational, polyurethane and injection moulding, and stamping tools backed up with metal filled resin; with manufacturing tolerance being 0.1mm. The alumino silicate ceramic on which spraying commences may also be worth a further look. Developed at Bristol University, it starts out as a slurry which is then frozen before being oven cured. The material has the right porosity for molten steel to stick to and good thermal shock resistance.