Hardfacing alloy development at TWI has evolved from a broad, comparative evaluation of experimental and commercial alloys to a more detailed investigation of new experimental alloys.
Hardfacing involves the deposition of a thin, wear-resistant layer of material on to a structural metal to create a composite with an optimum combination of bulk physical and mechanical properties and high surface wear resistance. Wear under highly abrasive conditions represents one of industry's most challenging wear-related problems.
Achieving high abrasion resistance generally requires deposition of a microstructure comprising extremely hard particles embedded in a softer, tough matrix. A wide variety of abrasion-resistant hardfacing alloys and deposition methods has been developed and commercialised. But a strong driving force continues for development of high performance hardfacing materials which are both more cost-effective and less dependent on strategic materials.
Previous studies at TWI have demonstrated the effectiveness and versatility of the plasma transferred arc (PTA) process in producing hardfacing deposits. This process allows proportions of matrix and hard particles to be accurately controlled, thereby allowing systematic generation of a range of metallurgical structures for evaluation and optimisation.
Two experimental hardfacing alloy systems which have received appreciable evaluation are produced by co-depositing varying proportions of austenitic stainless steel (SS) particles and hard particles of vanadium-tungsten carbide (V,W)C or chromium boride (CrB). The microstructure of a (V,W)C/SS deposit produced with a 3/1 ratio of (V,W)C to SS particles exhibited coarse, unmelted carbide particles in a background structure of resolidified eutectic. The microstructure of the CrB/SS deposit produced with equal proportions of CrB and SS particles comprised re-solidified acicular plates of CrB in a eutectic matrix, with no evidence of unmelted CrB particles.
The wear resistance of these microstructures was quantitatively evaluated at TWI using a computer-monitored pin-on-disc type wear test machine. Results on both hardfacing alloy systems have indicated that wear resistance is a function of the hard-particle-to-matrix ratio, but that an increase in hard particle content does not necessarily impart a decreased rate of wear. A greater proportion of equiaxed, unmelted (V,W)C particles in the (V,W)C/SS deposit promotes a greater wear resistance.
In contrast, the original CrB particles melt in the deposit, and acicular CrB platelets form during solidification of the CrB/SS deposits. These particles offer less effective resistance to highly abrasive wear. Wear rates of both materials were appreciably less than those determined for conventional Co-based hardfacing alloys.
The precise way in which deposit microstructure influences wear resistance is dependent on more than simply the primary particle size and distribution, and is the subject of continuing investigation. Despite the greater wear rates of the CrB/SS deposits, the lower cost of this material may ultimately show it to be competitive with the carbide/SS deposits.
Further quantitative information on wear resistance of these deposits using both the pin-on-disk and alternative wear testing methods has been developed. In addition, modifications to the PTA deposition process have been made, allowing retention of appreciably greater quantities of unmelted hard particles in the deposit, thereby improving abrasive wear resistance.
For information about TWI's capabilities please contact us.