Guillermo Reyes Castellanos

Ductile iron camshafts low alloyed with 0.2 and 0.3 wt % vanadium were produced by one of the largest manufacturers of the ductile iron camshafts in México “ARBOMEX S.A de C.V” by a phenolic urethane no-bake sand mold casting method. During functioning, camshafts are subject to bending and torsional stresses, and the lobe surfaces are highly loaded. Thus, high toughness and wear resistance are essential for this component. In this work, two austempering ductile iron heat treatments were evaluated to increase the mechanical properties of tensile strength, hardness, and toughness of the ductile iron camshaft low alloyed with vanadium. The austempering process was held at 265 and 305 °C and austempering times of 30, 60, 90, and 120 min. The volume fraction of high-carbon austenite was determined for the heat treatment conditions by XRD measurements. The ausferritic matrix was determined in 90 min for both austempering temperatures, having a good agreement with the microstructural and hardness evolution as the austempering time increased. The mechanical properties of tensile strength, hardness, and toughness were evaluated from samples obtained from the camshaft and the standard Keel block. The highest mechanical properties were obtained for the austempering heat treatment of 265 °C for 90 min for the ADI containing 0.3 wt % V. The tensile and yield strength were 1200 and 1051 MPa, respectively, while the hardness and the energy impact values were of 47 HRC and 26 J; these values are in the range expected for an ADI grade 3.

In the present study, ductile iron camshafts low alloyed with 0.2 and 0.3 wt % vanadium were produced to study the microstructural and mechanical evaluation of lobes and camshaft. For this purpose, camshafts were produced in one of the largest manufacturers of the ductile iron camshaft in México by the phenolic urethane no-bake sand mold casting method. The microstructure of the lobes was studied in three zones located at the top, middle, and bottom of the lobes by optical microscopy, and mechanical tests were performed on lobes and camshafts. A homogeneous distribution of spheroidal graphite with high nodularity for both castings was obtained from the regions of the lobes analyzed. The high cooling rate on the lobe surfaces enabled us to obtain a high nodule count of a smaller size instead of the middle region where big nodules with a low nodule count are presented. An inverse chill behavior was found in the middle region of the lobes where there is an increase in the concentration of carbide-forming elements, leading to the highest micro-hardness values in this region. The tensile properties were increased when the vanadium contents were increased; however, the toughness and ductility of the as-cast alloys were decreased as a result of the increase of the volume fraction of carbide particles.

Samples of ductile iron alloyed with 0.88 % Ni with a nodule count of 606, 523, and 290 nod/mm2 were obtained from sand cast plates of different thickness in the range from 8.46 to 25.4 mm. The effect of the nodule count was evaluated during the austempering process held at 285?C and austempering times of 15, 30, 45, 60, 70, and 90 min. The volume fraction of high carbon austenite increased when the nodule count increased, however, the carbon content of the high carbon austenite kept almost constant. The process window was narrow, requiring a lower austempering time when the nodule count increased. The combination of a higher nodule count and low austempering temperature allowed obtaining a fine ausferritic microstructure which led to higher Brinell hardness and tensile strength. The process window was determined by XRD measurements and it was in good agreement with the microstructural and hardness evolution as the austempering time increased.

An initial assessment of the gray cast irons production by injecting an inoculant with a conveying gas into a molten iron bath was evaluated at a laboratory scale. A numerical simulation was carried out to determine the hydrodynamic behavior between the inoculant particles injected into the molten iron. It was determined that an optimal interaction between the particles with the molten iron occurred at a lance depth of 7 cm and for the particle sizes fine (211 to 297 ?m) and medium (421 to 590 ?m), and the residence time was of 0.38 and 0.4 s, respectively. The melting time was calculated at 0.0008 and 0.003 s for the particle sizes fine and medium, respectively. After the injection, the FeSi of the inoculant melted quickly, releasing the elements of the inoculant which interacted with the molten iron and formed oxides and sulfides creating nucleating sites during solidification. The injection technique allowed obtaining a type-A graphite distribution for the fine and medium particle sizes. The number of eutectic cells increased when the inoculant particle size decreased despite of the low graphitisers elements and manganese contents used in the gray cast iron manufacturing.