The problem
The client makes extrusion screws for a variety of applications in the food and chemical industry. For many years it used Stellite alloys overlaid onto the screw flight-heads. This process involved manual welding with bare rods - a slow and costly process, necessitating the use of highly experienced and skilled welders, who are expensive to train. When a number of the most experienced and skilful welders came up for retirement, the company decided to approach Stellite to look at an alternative, more cost-effective process.

Why it was difficult
The screws have varying, complex geometries and any new process would have to produce a coating that would perform as well as the oxyacetylene process that they used. It was particularly important to avoid excessive distortion in the welding process. In order to maintain the wear life of the part, the dilution of the coating material by the base metal would have to be minimised while still insuring a good metallurgical bond. It was also important the process be as automated as possible to minimise the operator skill level required and produce a more consistent quality.

The Stellite solution
Stellite was able to bring its expertise to bear with both equipment and consumables. The solution chosen was a new PTA (plasma transferred arc) welding system, which used powder filler metal. Stellite developed and delivered a complete welding system and trained the customer's operators in the techniques required. To ease the transition, Stellite also developed specific weld procedures using its service capability and provided the initial group of PTA overlaid parts. Stellite also provided PTA powder in the same alloys as the customer had been using but which were cheaper than the existing rod material.

The result
The PTA deposits were an improvement in both quality and consistency. Additionally, as the system was simpler, more operators could be trained, resulting in greater flexibility in handling variations in production demand. The higher deposition rate and productivity of the PTA process resulted in the customer having increased capacity while maintaining the same labour costs. This enabled the company to take advantage of an upturn in its market and significantly increase its productivity.