Evaluation of Retained Austenite in Powder Metallurgy Pump Ring Components and Alternative Processing Methods to Minimize Loss of Pump Efficiency throughout Extensive Operational Temperature Ranges

Retained austenite is a metastable phase present when a steel microstructure is not fully transformed during the quenching process from a heat treating operation. The martensite finish, M_f, temperature is below room temperature with alloys containing carbon at levels of 0.30% or higher and with normal production quench cycles, appreciable amounts of untransformed austenite may remain intermingled within the martensite phase. Problems can arise when the microstructure is exposed to temperatures lower than that of the initial quench, resulting in further transformation of the metastable austenite. With a 4-6% volumetric change, internal stresses can manifest into cracks or macroscopic dimensional changes can occur that may render a component unusable within its intended application. For a hydraulic oil pump ring in automotive applications where tight dimensional tolerance, high hardness martensite phase for acceptable wear characteristics, high efficiency and long life are required, care must be taken to stabilize the steel phases present for operation in climates with extreme hot or cold temperatures. Due to the pump ring’s complex geometry, alloy requirements and economic needs, powder metallurgy (PM) is typically the best choice in the manufacturing of these components using a common PM alloy with a 0.8-0.9 wt% carbon levels. With these pump components, testing performed at temperatures down to -40C exhibited large macroscopic dimensional changes on the order of 12-15 um resulting in unacceptable loss of pump efficiency. A sequence of tempering processes was found to improve the dimensional change to less than 3-5 um by stabilizing the untransformed austenite, however production challenges still persist and a cryogenic quench processes proved to be expensive. This paper investigates alternative PM alloys that exhibit promise in minimizing the macroscopic dimensional change during cold testing while providing the necessary microstructure to achieve acceptable life performance and temperature stability.