Digging deep with Wassara
The mining business needs new drilling solutions to tackle the mounting productivity demands placed upon the industry. These solutions need to be relatively environmentally friendly as well as cost competitive compared to current drilling solutions.
Above: Schematic diagram of the down the hole (DTH) hammer technology
There are many complex parts that make up each DTH tool. At the heart of the tool is the sliding case that provides the bi-directional flow of water for the piston assembly. The sliding case requires several internal fluid channels to allow the flow of water, and due to its complexity, it cannot be fabricated from one single part. This complexity in the design makes these parts relatively expensive. Adding to the costs are frequent rejects in production as a result of the joining process, as well as failure due to wear or pitting corrosion increasing the maintenance requirements.
The next hurdle to overcome for the adoption of metal AM was the availability of the correct metal alloy for the specific application. In this case the standard steel alloy used for this part is 527M20, a structural alloy steel that would not normally be considered for metal AM due to the medium carbon content. A more suitable choice of steel alloy for additive manufacturing is 316L stainless, however even though corrosion resistance is high for this alloy, it would not be expected to have sufficient wear resistance and withstand the erosion during usage.
This was the first time that this steel had been tested in this kind of industrial mining application, and therefore the parts were heat treated post-build for maximum hardness.
To test the performance of the part, it was assembled into a full tool and used under standard mining conditions, creating typical long bore channels in a candidate rock face. The drill tool then underwent routine visual inspection and maintenance. The additively manufactured sliding case showed no signs of pitting and only minimal wear, compared to a standard part. The tool was re-assembled and further drilling tests were undertaken before re-inspection.
Following a second test the AM built sliding case did show some signs of wear but despite this there was no evidence of surface pitting which is the second most common failure mode. A further test followed, extending way beyond the expected drilling period to try and establish if the onset of pitting could be found but in actual fact there was no evidence of this. This has led to the initial conclusion that the AM maraging steel part has potentially superior pitting resistance to the conventionally chosen steel for the sliding case.