Patent Application
20080025863
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fees.
These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein;
The alloy and associated method of manufacturing may be applied to any powder metal component where high strength is desirable and, more particularly, where surface features and shapes are amenable to near-net shape sintered materials. Applications include automotive drive and valve train components, such as transmission gears. These transmission gears require very high contact and bending fatigue strength.
The powder metal alloy blend may be formed of the following composition: 97.40 wt % (Fe—Mo) pre-alloyed powder, 2.0 wt % nickel powder, 0.6% graphite powder, and 0.6 wt % wax lubricant. In order to form components from the disclosed alloy composition, the mixture is cold compacted in a die at 40-65 psi to form a specimen with an approximate density of 7.2 g/cm3. External lubricants such as zinc stearate or organic lubricants are typically applied to the die to foster proper mold release. The formed compact is then pre-sintered at 1950° F. for 0.5 to 1 hour in an atmosphere of at least 95% nitrogen, balance hydrogen. This pre-sintering step is particularly designed to promote bonding of the powder particles without fostering carbon diffusion beyond the iron grain boundaries of the component materials. This results in a preform which is more prone to withstand the stress of deformation during cold working.
The pre-sintered compact, or preform, is then densified at the working surface of the material by cold forming. This cold forming step may include extrusion, burnishing, rolling or any other method that induces plastic deformation of steel in cold state without the aid of heat other than typical heat generated by friction between two metals in contact under load.
The cold formed preform is then sintered at a temperature around 2100° F. for 25 minutes in a 95% N2/5% H2 atmosphere. The nitrogen-hydrogen atmosphere must be capable of chemically reducing any oxygen and/or impurity content found in the metal powders and the ratio will be primarily dependent upon the particular mixture of alloying elements being utilized. Alternatively, sintering can be carried out under vacuum with similar results. Precise control of the carbon content in the perform is critical and carbon loss by decarburization due to high water vapor content or high levels of oxygen in the furnace should be avoided. In particular situations, carbon can be added in the sintering furnace to increase the overall carbon concentration in the final part beyond the level of admixed graphite in the powder.
Depending on the alloying elements, the sintered components may be fast cooled in the sintering furnace at a cooling rate of at least 1.5° F./sec between 1000° F. and 400° F. to form a microstructure composed of at least 90% martensite in the entire section of the part. Alternatively, the components are hardened by: (1) induction heating the sintered component on critical surfaces for approximately 1-3 seconds and subsequently quenching in oil or water; or (2) austenitization by heating in a furnace under carburizing or neutral carbon atmosphere between 1600° F. and 1950° F. followed by quenching in oil or water. Heat treatment may be performed by sinterhardening, induction or austenitization and quenching, any of which may be followed by tempering in air between 300° F. and 450° F. for 1 to 2 hours.
The prior art materials typically utilized carburization at this point in the manufacturing process to achieve some degree of carbon penetration into the sintered part. Typical carburization parameters for low or carbon free materials include more than 4 hours of exposure to carbon containing gas at high temperatures to achieve high carbon concentration at the surface or near-surface region at a depth determined by the time and temperature that the part is exposed to this carbon atmosphere in furnace. Such carburized parts are characterized by a gradient of carbon concentration from the surface to a predetermined distance where the carbon is 0.4 wt % of the immediately surrounding matrix. The structure of this carbon-rich section corresponds to a martensitic structure which provides high strength only in the relatively shallow surface layer. This process is expensive and cumbersome, especially when utilized for large parts. In some cases, uneven exposure of the part to the carbon gas causes inconsistent penetration of the carbon inside the part resulting in limited or inconsistent structural strength and/or limited performance of the finished component. Beside cost, one significant disadvantage of extended time in the carburization furnace at high temperature is the increase in grain size of the finished part. Increased grain size is undesirable, resulting in lower strength and low fracture resistance.
The sintered components disclosed herein exhibit relatively small grain size, typical of powder metal components, while achieving an approximate carbon concentration of 0.5 wt %, which, moreover, is generally uniformly distributed throughout the component. The high carbon content throughout the part ensures the formation of a martensite structure in the entire section of the part which results in improved performance not only at the part surface, but more uniform strength and fracture resistance throughout the entire component. Additionally, the elimination of the carburization step results in significant cost and time savings in the manufacturing of the components.
Referring now to
The method as disclosed provides a significant amount of flexibility of the final internal microstructure, depending on the post-sintering heat treatment. As seen in
By comparison, the surface densified low carbon material forms a shallow layer of martensite 14 on surface and a soft ferrite structure 16 in most of the part section. The difference in microstructure as shown in this figure has a significant impact on performance especially in the applications requiring high fatigue strength. To characterize the fatigue strength difference between the surface densified materials with different base carbon contents, surface densified components made with the two types of materials were evaluated in a fatigue testing machine. The number of cycles to failure is plotted in
In addition to the improvement in strength, contact and bending fatigue, articles made from the sintered powder metal composition according to this invention display excellent dimensional stability and good machineability, properties that are critical from both a manufacturing and performance point of view.
Although particular embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it is to be further understood that the present invention is not to be limited to just the embodiments disclosed, but that they are capable of numerous rearrangements, modifications and substitutions, as identified in the following claims.
