Using advanced gas cooling technology, Solar Atmospheres, Souderton, PA, USA, has developed a unique three-step heat treat process to austenite condition, in situ cool, and bright age harden 17-7 precipitation hardenable (PH) stainless steel in a vacuum furnace without breaking vacuum.
This, states Solar, is highly desirable because it is difficult to keep 17-7 bright during the PH treatment after breaking vacuum and exposing parts to ambient atmosphere required for adequate cooling where it can discolour due to oxidation.
Working with the medical industry, a new in situ process was developed where desirable mechanical properties could be consistently obtained. With this new process, a new medical device design could be based on the established set of mechanical properties and the medical company would reap the benefit of reduced costs owing to reduced pricing associated with the new in situ, three-step process.
17-7 PH is classified as a semi-austenitic stainless steel that has expansive use in aerospace and growing new applications in the medical industry. The reason for this is 17-7 PH’s attractive combination of properties including very high, spring-like strength, good ductility, and excellent corrosion resistance due to its high alloy content. In order to obtain these properties, however, 17-7 must be heat treated in a more controlled manner than many other alloys.
Solar Atmospheres performed a recent study that showed its vacuum heat treatment technology offers a unique processing advantage to achieving these highly desirable properties. As a baseline, the new set of mechanical properties and microstructure characteristics were compared to standard Condition TH 1050, the most common condition specified in industry for 17-7 PH.
A three-step austenite conditioning, in situ cooling and precipitation hardening process resulted in a new condition of 17-7 that Solar Atmospheres has coined SH 950. Compared to 17-7 Condition TH 1050, Condition SH 950 showed comparable yield strength, higher tensile strength, and on average twice the ductility.
These tensile properties were obtained with two different heats of steel with three tensile specimens tested for both heats and conditions resulting in 12 separate tensile test results. The findings are both consistent and inspiring. The microstructure of SH 950 also appeared superior with respect to lower delta ferrite stringers being present.
The results associated with Solar’s Condition SH 950 for 17-7 stainless steel indicates that a new medical device design could be based on the newly established range of mechanical properties. This new vacuum heat treatment technology offers a unique processing advantage for achieving desirable properties while cost effectively producing bright, non-discoloured parts.