LOW TORR-RANGE VACUUM NITRIDING OF 4140 STEEL

摘要

The experimental work completed thus far indicates that the calculated alpha - gamma' phase boundary at 125 torr is within 20 percent of the experimental values. However/additional experimental work is required to determine whether the actual boundary at 125 torr is closer to the conditions that produced white layer, or those that did not. Nevertheless, the preliminary results indicate that the method used to calculate the boundary at 125 torr is reasonable. Low-torr nitriding requires higher critical input ammonia content (higher input nitriding potential) as the nitriding pressure is reduced. The modification of the input ammonia content is the result of reduced partial pressures of ammonia and hydrogen in addition to the differential pump-out of ammonia compared with hydrogen. Although the nitrided surfaces from atmospheric pressure nitriding and low-torr nitriding are similar, there are some advantages of the low-pressure nitriding. The turn around time for a nitriding cycle is quicker owing to the fact that in a vacuum furnace there is no retort to heat up and cool down. Further, accelerated cool-down is accomplished by circulating inert gas across a water-cooled heat exchanger, similar to typical vacuum furnace quenching, Vacuum processing produces a very pure starting atmosphere, eliminating the need for a nitrogen purge, and provides the ability to clean the surfaces of the parts using hydrogen prior to the nitriding step of the cycle. The hydrogen treatment can be accomplished at less than 150 torr, and this greatly minimizes any explosion Risk Low pressure processing also maintains a dynamic/uniform atmosphere in the chamber, eliminating the need for a convection fan. And of particular significance, the vacuum chamber provides an extremely tight thermally and atmospherically uniform furnace, which is ideally suited for precise functionality of modern-day instrumentation control of the nitriding potential.