Division of Information Technology, Engineering and the Environment
Agricultural Machinery Research Design Centre
Agricultural Machinery Research Design Centre
The Abrasive Wear of Agricultural Ground Tools
The wear behaviour of agricultural ground tools in current use in Australia has been evaluated and compared with that of components fabricated from some potential alternative steels and heat treatments. A literature search revealed that, broadly, there were two different philosophies in ground tool production. The first, and most common, was that used in Australia, based on a high carbon steel (generally 0.7 to 0.8 %C) and heat treated to 40-45 HRc, in contrast to a second practice based on medium carbon content (generally about 0.4%C) and heat treated to yield a hardness of some 50 HRc or higher. The high carbon steel variants in some cases contained alloy additions, most commonly 0.5%Cr, while the medium carbon steels were generally alloyed with Cr, Ti, V, B.
The present program of work was designed to appraise both these philosophies, involving materials of a range of experimental hardness's, and to compare the performance with production ground tools. The test program was designed to compare wear rates measured under relatively controlled conditions on a test track and in selected abrasive field sites. The use of the standardised test track facility allowed evaluation of a number of steel type/heat treatment combinations that it was not feasible to test in field conditions.
The results from both field and track tests also established that there was strong correlation between wear rate and carbon content at 40 and 45 HRc. The results substantiate published data and commonly-accepted theories of wear resistance as being related to carbon content. At 50 HRc however, a different picture emerged with the test track results, where there appeared to be no apparent difference between steels of differing carbon content and wear rate.
Results from both track and field testing showed that there was significant improvement in wear resistance with increasing hardness for the same steel. This was evident with all the steels tested in all soil types. The results indicated that a medium carbon steel of 50 HRc would have wear resistance equal to, or better than, a normal production component, in agreement anecdotal evidence from farmers.
A matter of particular interest was that austempered on bainitic structures were found to exhibit much greater ductility at high hardness than the equivalent hardness tempered martensite structure. The most wear resistant ground tools proved to be the high carbon steels with an upper bainite structure. The upper bainite of high carbon steels exhibited a very fine structure, much finer than the equivalent tempered martensi tic structure, suggesting that wear is critically dependent on the nature of the ferrite/carbide aggregate.
Metallographic examination of abraded surfaces showed that the
principal form of wear was through gouging and ploughing. There is also
evidence of smearing of displaced material over scratches and implanted
soil particles, as there was of the retention of impacted or implanted
particles on the abraded surfaces.
The results and findings are discussed in the context of the competitive
development and manufacture of ground engaging tools.
