November_December_2021_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 1 4 6 T he primary function of a crown wheel pinion is to transfer power from the transmission to the wheel ends. Dimensional accuracy is extremely important to meet the part life requirement. A study was carried out to test the effects of an additional annealing step during heat treatment on a hypoid gear set, particularly on distortion. Heavy truck, full float axle gear sets made of DIN 20MnCr5 were used in this case study. Distortion in hypoid gear sets is unavoidable and happens during case hardening [1] . Complete elimination of distortion is not practical because it involves a microstruc- tural phase change from FCC austenite to BCT martensite. These crystallographic changes impart dimensional im- perfection in the part, or distortion. But distortion can be minimized using heat treatment. The gear sets are manufactured in a dry gear cutting machine. Batch A (three gear sets) serves as the control group and is heat treated via the typical process. Batch B (three gear sets) is annealed after the soft gear cutting step. Residual stress analysis test results show a reduction in stress levels in batch B gear sets. This residual stress re- duction is also evidenced by lower distortions in batch B gear sets. MECHANISM OF GEAR DISTORTION Distortion as defined metallurgically, occurs when crystallographic changes take place. Whenever gears are heat treated microstructural changes occur. At the soft gear stage, the microstructure is ferrite and pearlite, and heat treating to a case carburizing temperature of 925°C changes its phase to austenite. Upon quenching the micro- structure changes to martensite [2] . During this phenomenon, the crystal lattices of the soft gears is body centered cubic (BCC), changes to face centered cubic (FCC) at carburizing temperature, and upon quenching the crystal lattice changes over to body cen- tered tetragonal (BCT). The carburizing process diffuses the carbon atom to the gear surface. These carbon atoms occupy the void space available inside the crystal lattice. While quench- ing, the phase changes from austenite FCC to martensite BCT and the rearrangement of crystal lattice takes place. Fe atoms present in the FCC lattice move to the position and new orientation to form a BCT lattice. This complexity in the crystal lattice system generates the dimensional im- perfection called distortion. HEAT TREATMENT OF HYPOID GEARS Case carburizing, hardening, and tempering are per- formed on both batches of test gears. Three samples SN1, SN2, and SN3 are taken from each batch. The annealing heat treatment for batch B is carried out in an inert nitro- gen N 2 atmosphere to avoid scaling and decarburization. Furnace loading fixture and loading quantity are kept CASE STUDY: CONTROLLING THE DISTORTION OF A CROWN WHEEL (HYPOID GEAR) This study looks at whether adding an annealing step to the heat treating process has an effect on residual stress and distortion on hypoid gear sets. Yathish Rao* Industry Professional, Pune, India *Member of ASM International Fig. 1 — Residual stress measurement locations ABCD. A B D C D C B A 12