U. B. PATHAK, Tata Motors Limited, PUNE, India
Pinion shaft holds differential gears in place and facilitates differential action. This is the major differential component, which often looses lubrication film under heavy loads, thus causing friction leading to heat generation and pressure welding of diff gears on pinion shafts. Conventional differential designs use case carburised pinion shaft and are capable of taking normal loads. These carburised diff shafts are suitable for normal automobile and European off-highway applications. In Asian countries off-highway application undergoes heavy spike loads. In the severe applications there are several cases of gear seizure leading to the failure of differential case and lot of secondary failures. Hence the best option is to go for the pinion shafts with better tribological properties and core properties. Gas nitrided pinion shaft with heavy white layer and case depth, with medium core hardness was considered as the best option. Conventional gas nitriding practices around the world typically uses pit type furnace with ammonia purging and process control through control on ammonia dissociation. Generally, two stage gas nitriding cycles are used. Most of the conventional cycles take around 72 hrs to develop white layers as low as 5 to 15 microns. Under such operating conditions getting 20 to 30 microns white layer, would necessitate for gas nitriding cycles of more than 100 Hrs. Also, higher temperatures of conventional pit type furnace gas nitriding cycles results into drop of core hardness below acceptable limits. In addition, the dimensional inconsistency due to warpage becomes another major issue. Under such conditions the innovative process idea was to go for gas nitriding using fluidized bed furnace. This case study discusses the smaller duration (24-26 Hrs) efficient gas nitriding cycles using fluidized bed furnaces. Consistency in the end results was established over more than 15 trials.
Summary: It is possible, to achieve heavy white layer to the tune of 0.025 – 0.050 mm with diffusion zone of 0.6 – 0.8 mm using a smaller duration gas Nitriding cycles at 525 ± 5º C, in fluidized bed furnace compared to longer gas Nitriding cycles in conventional pit type furnaces. Nitriding cycle time can be reduced to 60% of the cycle time, using fluidized bed Nitriding furnace. Prior hardening and tempering cycles governs post gas nitriding core properties. Diffusion zone microstructure consisting of fine nitride network can be achieved in fluidized bed furnace by adjusting proper process parameters. Consistency in dimensional distortion (growth) can be established and is dependent on prior tempering temperature, for a given compound layer thickness.
