Thermal spray service provider Thermaspray is expected to complete a Stellite alloy 6 ball valve, which will be laser-clad to the SAF 225 alloy, allowing for the ball valves to be used in highly corrosive environments at wood and paper pulp and paper-based solutions provider Sappi’s specialised cellulose Saiccor mill, this year.
Thermaspray MD Dr Jan Lourens notes that to successfully clad the Stellite 6 alloy to the SAF 225 alloy is a difficult process and there have been many attempts globally to dimensionally restore ball valves made of SAF 225, but with little success, owing to the complexity of the alloy.
“Sappi posed the challenge to Thermaspray, which, in turn, required the involvement of our technology partner, coatings and surface technology arm of Sulzer Corporation, Sulzer Metco. It was decided to restore the ball valve with a laser-cladding process in two steps using the Haynes 25 as a buffer layer and the Stellite 6 as a final layer.
“Stellite 6 is notoriously difficult to apply as a final layer, particularly on SAF 225, owing to probable cracking. The Haynes 25 and the Stellite 6 layers, however, were applied with zero porosity and no cracking. The success of this project enables Sappi to refurbish ball valves, which have been previously replaced, resulting in a significant cost savings,” Lourens explains.
The project started in April 2013 and is valued at R500 000.
Meanwhile, Thermaspray successfully completed the thermal coating of an 8-m-long hydraulic cylinder with chrome carbide in December last year for hydraulic engineering company Hydron Hydraulics using the high- velocity oxy-fuel (HVOF) process.
Lourens notes that the cylinder is applied in an aggressive sea-going environment where conventional chrome plating would not provide the required long-term life.
“The alternative to hard-chrome protection, in this type of application, has routinely been the application of a chrome-carbide thermally sprayed coating using the HVOF process. The biggest challenge with this R450 000 project was the length of the hydraulic cylinder, as it was the first time a component of this length had been coated and ground in South Africa,” he adds.
He highlights that the cylinder was significantly more damaged than initially expected and significant material had to be removed to ensure dimensional integrity of the workpiece, prior to the rebuild and repair process.
“The rod had been repaired before and the previous work was of a particularly poor standard. Although no pinholes or cracks were observed in the previously applied coating, it was clear that the coating was not applied to virgin material, with a real chance, therefore, of coating integrity being poor.
“The seal areas were preground and then coated with a chrome carbide/nickel chrome coating, HVOF thermally sprayed and, finally, ground to the required size and surface finish,” he enthuses, adding that this is a big advantage for South Africa, as work of this magnitude had to be sent to Europe, owing to a lack of local capability.
Thermaspray also completed application on an acid conversion project for platinum mining major Anglo American Platinum in December 2013 to the value of R380 000.
During the project, the reactor u-tubes for the smelter needed a mechanism to extend the service life of the reactor u-tubes. The project is now in the evaluation phase, with various coatings being evaluated in order to select the best coating for the application. Three very high-technology coatings were applied in this application to determine what would benefit the extension of the service life of the u-tubes.
The materials coated were a chromium (Cr)/molybdenum (Mo)/tungsten/carbon/manganese/niobium/iron alloy, a nickel (Ni)/Cr/titanium alloy and a Ni/Cr/Mo alloy.
“The three coatings are currently on trial, but they have been scientifically assessed and have shown to enhance the service life of the u-tubes – the issue now is to determine the maximum service life extension,” Lourens points out.
He notes that gaining access to the coating area on site and transporting the neces- sary equipment were particularly challenging during the project, which started in June last year.