Home » Solar Develops Heat Treat Process for Bright 17-7 PH Stainless Steel
Solar Develops Heat Treat Process for Bright 17-7 PH Stainless Steel
August 20, 2014
Using advanced gas cooling technology and out-of-the-box thinking, Solar Atmospheres has developed a unique three-step process to austenite condition, transform, and bright age harden 17-7 precipitation hardenable (PH) stainless steel without removing the material from the furnace for transformation.
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. Interestingly, even within these controlled parameters, flexibility exists to obtain attractive properties other than those classified as the standard RH and TH conditions. By adjusting the austenite conditioning temperature to raise the martensite finish (Mf) temperature to its highest theoretical point, in situ cooling to fully transform the material is now possible using Solar’s advanced technology. As a baseline, the new set of mechanical properties and microstructure characteristics were compared to standard Condition TH1050, likely the most common condition specified in industry for 17-7 PH. Solar Atmospheres has coined the new condition “SH950.” Compared to 17-7 PH Condition TH1050, Condition SH950 shows comparable yield strength, higher tensile strength, and on average twice the ductility. The 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 SH950 also appeared superior with respect to lower delta ferrite stringers being present.
There are two reasons why the continuous process is desirable. First, in order for 17-7 PH heat treated to the TH condition to fully transform from the austenite conditioning temperature one must cool to below 60°F (Mf) which is not practical inside a vacuum furnace. Therefore the product must be removed from the furnace and cooled (sometimes artificially in a freezer or cooler) to obtain transformation. This takes time and requires additional equipment. Solar’s SH condition eliminates the removal requirement due to the Mf being higher than 60°F. The second reason Solar’s continuous process is desirable is that by removing the parts from the furnace the risk of discoloration is increased as water vapor is reintroduced into the vacuum furnace upon unloading and reloading. Solar’s continuous process reduces this risk.
The positive results associated with Solar’s Condition SH950 for 17-7 PH indicate that a new medical device design could be based on the newly established range of mechanical properties while reducing both the risk of discoloration and the cost of heat treating.