Case study 1: 5 ton R55-7 imitation finaldrive metal testing results.
Please find attached report as discussed. (CLICK HERE TO DOWNLOAD REPORT).
After 30 years of examining and testing metal components you assume you have seen it all before but this confirms we never stop learning.
The annulus we would describe as an “inedible sponge cake”. Perhaps the shrinkage present is a new form of lubrication channels ???
The gear has been manufactured from steel conforming to AISI 5120 grade steel or equivalent. This is a common medium duty carburizing steel.
Effective case depth was determined to be 0.95mm at 550HV.
Surface hardness of the gear teeth was determined to be 710HV in a tempered martensite matrix with a bainite core of hardness 430HV.
We are advised that this is a heavy duty gearbox and hence we would have expected the gear to have been manufactured from nickel-chromium-molybdenum steel such as EN39B or E110 for improved toughness.
The annulus has been manufactured from ductile (SG) cast iron.
The annulus is in the as-cast condition and has not been annealed or normalized as evident by the presence of “bullseye” ferrite surrounding the graphite nodules
Metallurgical examination finds graphite morphology to have 95% nodularity with a 50/50 ferrite pearlite matrix with graphite size 5.
Hardness was determined to be 215HV.
Hence we have a relatively soft annulus gear – 215HV - running against a gear with a surface hardness of 710HV which will likely result in premature wear of the annulus.
However what was incredulous to find was casting shrinkage porosity at the root of the annulus gear teeth which results in little or no impact resistance for the annulus.
Additionally the annulus is in the as-cast condition and has not been annealed or normalized. SG iron castings such as this annulus given the service conditions in which it operates should never be used in the as-cast condition.
The combination of an as-cast ductile iron annulus with shrinkage porosity in contact with a carburized gear is an unbelievably poor combination of materials for a heavy duty gearbox.