Solar Atmospheres, Souderton, Pa., has developed a unique three-step process to austenite-condition, transform, and bright-age-harden 17-7 precipitation hardenable stainless steel without removing the material from the furnace for transformation. The company has named 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.
17-7 PH is classified as a semi-austenitic stainless steel that has many uses in aerospace and growing applications in the medical industry. The reason for this is its combination of properties, including very high, spring-like strength, good ductility, and excellent corrosion resistance.
There are two reasons that the continuous process is beneficial. First, in order for 17-7 PH heat treated to the TH condition to fully transform from the austenite conditioning temperature, the material 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) for 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 beneficial is that while 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.
Tensile properties were achieved with two different heats of steel, with three tensile specimens tested for both heats and conditions, resulting in 12 separate tensile test results. The microstructure of SH950 also appeared superior with respect to the reduced presence of delta ferrite stringers.
However, to obtain achieve these properties, 17-7 must be heat treated in a more controlled manner than many other alloys. 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 via 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.
