Feb_March_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 | F E B R U A R Y / M A R C H 2 0 2 0 4 0 I n light and heavy transport vehicles, power from the en- gine to differential gears is transmitted by axisymmetric shafts. These components are subjected to fatigue under torsional or flexural conditions, and must be sturdy and re- sistant to stresses, so they are forged from alloyed steels and heat treated. The heat treatment in this instance includes surface hardening followed by quenching and tempering. Vacuum carburizing offers excellent uniformity and repeat- ability due to its high process control capability, it improves mechanical properties due to the lack of intergranular oxida- tion, and it potentially reduces cycle times as higher process temperatures are feasible [1] . Gas quenching offers favorable conditions to reduce distortion because convection is the sole heat-transfer mechanism. Quenching gases such as ni- trogen and helium are inert and ecologically benign as they do not leave behind any type of residues on the work pieces or in the heat-treating system. However, in comparison to fluids, a disadvantage of quenching gases is their reduced heat-transfer rate under similar conditions [2,3] . Distortion control is of fundamental importance in the manufacturing of transmission shafts due to the impact that uncontrolled distortion exerts on the transmission system including noise and vibration. Quenching after carburizing is known to affect distortion through microstructural changes, but there are other variables such as racking position, size of the shafts, etc., that influence distortion. EXPERIMENTAL PROCEDURE A series of experiments were designed to study vacuum carburization of transmission shafts. The shafts were manu- factured from steel of the same nominal composition, SAE 8625, but from batches of low and high hardenability as de- termined by the Jominy test, and identified as A and B. The shafts were positioned in the racks either with their flanges facing up or facing down. Carburizing was conducted to ob- tain a minimum case depth of 0.8 mm. Some of the shafts were quenched using nitrogen gas, 20 bar pressure, or by im- mersing in quenching oil at 90°C. The quenched shafts were tempered for one hour. An optical measurement system and a gear analyzer recorded the dimensional characteristics. ASTM E384-17 and ASTM E10-15 standards were followed to obtain the hard- ness of thematerial. X-ray diffraction recorded residual stress according to the EN 15305:2008 standard. RESULTS AND DISCUSSION Figure 1 shows the transmission shaft selected for the study. Figure 2 shows the changes in hardness of the steels as a function of distance from the quenched end of Jomi- ny tests, which confirmed the difference in hardenability of each batch. Figure 3 shows the results of the run out measure- ments at four different positions on the shaft (as indicated) VACUUM CARBURIZATION OF A TRANSMISSION SHAFT Tests conducted on vacuum carburized and quenched shafts identify variables that increase distortion and the magnitude of residual stresses. M.A. Delgado-López, Sistemas Automotrices de México, SISAMEX R. Colás, FASM,* Universidad Autónoma de Nuevo León, Mexico V. Esteve, ECM Technologies Inc. 8 *Member of ASM International Fig. 1 — Schematic of transmission shaft used in the study. Fig. 2 — Hardness changes in Jominy tests.