Summary of Test Results on Bucket Teeth Test Program
Three different sizes of teeth were used, namely Hensley X550, Caterpillar 9W8452 and 9W8552. Height and weight measurements were recorded on each tooth in the as-received condition and after different intervals of service time. This enabled us to measure the wear rate in inches per hour. Additionally, various potential means of extending the service life of teeth were investigated including cryogenic processing and surface hardening. Comparative metallurgical and chemical properties of the bucket teeth were also evaluated. The test data contained herein will be useful background information to any follow-up testing program that may be undertaken.
Chemical / Metallurgical Tests
Specimens were excised from sample teeth from each supplier and analyzed using Glow Discharge Optical Emission Spectroscopy and combustion analysis was used for carbon determinations. The results are as follows:
While some differences in chemistry are evident all the teeth tested are considered to have been manufactured from alloy steel. The carbon content in some cases was surprisingly low and ranged from 0.236% to 0.37%. Carbon is a significant element which influences the hardness and hence wear resistance of a heat-treated material.
Samples were excised from different size teeth from the four suppliers. The specimens were polished and etched in 2% nital for microscopic examination. All the samples exhibited a normal tempered martensitic microstructure which is consistent with a properly heat treated material. Some samples showed a minor degree of partial decarburization which is considered to be of an inconsequential nature given the commercial application involved. The presence of decarburization (i.e. loss of carbon at and below the surface) results in a reduction of surface hardness and consequential loss in wear resistance. The severity to which it exists is a function of the heat treatment parameters used (i.e. Temperature, time at heat, furnace atmosphere). It can vary from heat treatment batch to batch and various measures are normally used to minimize its presence.
Hardness tests were conducted on samples from each supplier the results of which shown below.
|SUPPLIER||Average Core Hardness (HV500)||Approx. EQ. HRC|
As noted above, the spread in hardness between the teeth from the various suppliers is approximately 3 points on the Rockwell scale. The teeth from Supplier H gave the highest value (HRC55) followed in descending order by suppliers C, F, R.
Wear Testing of Teeth
Field wear tests were carried out on several hundred teeth in the referenced program both in the as-received and modified condition.
As noted earlier, three different sizes of teeth were used, namely, Hensley X550, and Caterpillar 9W8452 and 9W8552. Height and weight measurements were recorded on each tooth before (i.e. as received condition) and after different intervals of service time. The results were recorded and used to measure the wear rate in inches per hour. It was recognized that the results obtained were subject to a number of uncontrollable variables such as, tooth location on the bucket (end teeth always exhibited the highest wear rate), variations in the composition of the excavated material from area to another, machine operator, etc., and this was taken into account in the overall assessment of the results.
Based on the overall wear tests conducted, it was concluded that the representative products from the four suppliers gave comparable wear test results.
It is probable that further comparative wear tests conducted under laboratory conditions will be less subject to variables and consequently, yield more meaningful results for marketing purposes. As discussed at our recent meeting, an appropriate test facility will be explored for this purpose.
Methods Investigated to Extend Service Life of Teeth
A further objective of the program was to investigate various ways of extending the service life of bucket teeth operating in harsh environments e,g, excavation of hard rock, as a cost-saving measure. The two principal methods investigated included cryogenic treatment and hardfacing.
The cryogenic treatment entails exposing the parts to sub-zero (approx.-300 degrees F) for an extended period of time. The process transforms any residual retained austenite in the heat treated microstructure to martensite and thereby makes the material harder and more wear resistant. Not all steels respond to this treatment. The carbon content of the alloy steels used in the bucket teeth is insufficient to make the material respond to the cold treatment. This was challenged by the cryogenic processor but confirmed by field wear testing a large quantity of cold treated vs as-received teeth.
The hardfacing process involves the choice of a welding consumable and a weld procedure. A large variety of hardfacing alloys are available. To maximize the benefits of hardfacing, the hardfacing material should have a greater hardness than the material causing the wear, and exceed the hardness of the substrate material. Several different alloys and weld patterns (waffle, herringbone, stringer, dot) were tried. Numerous comparative field wear tests were carried out on hardfaced vs as received teeth. The former in some cases showed improvement in the service life. However, it was concluded on the basis of these tests that while the wear resistance of bucket teeth can be improved if the proper hardfacing materials and associated process are carefully selected and controlled, the degree of improvement, in our case, was not sufficient to offset the cost of hardfacing. Moreover, a number of the hardfaced teeth cracked prematurely in service.
As a general rule, it is advisable to undertake a cost/benefit analysis in advance to ensure the process will be cost-effective.
Romac was ultimately selected as the primary supplier of bucket teeth for a major excavation (rock/earth) project due to their comparable performance and significant cost advantage.
Report By: Thomas J., Large Mine Located in Canada
Date: March 25th, 2017