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17-7 stainless steel (also called 17-7 PH, UNS S17700, or AISI 631), has a chemical composition of 17% chromium and 7% nickel—thus 17-7. However, there is also a small addition of aluminum that gives this material interesting properties in both cold-worked and precipitation-hardened condition.

Compared with 304 stainless steel, 17-7 has similar corrosion resistance, but in a similar cold-worked condition it has increased strength and better machinability. When heat-treated, 17-7 exhibits increased hardness and tensile strength, which is not achieved by cold-working alone.

Stainless steel has great wearability, good formability, good corrosion resistance, and has minimal distortion upon heat treatment. It is the material of choice for medical, aerospace, and commercial applications.

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  • In cold-worked condition, 17-7 has excellent machinability. A tensile strength above 160ksi provides good strength and dimensional stability when machining. Care must be exercised not to generate high heat when cutting to avoid hardening the material. This is mostly taken care of by machining with a cutting fluid flood of the area.
  • In annealed condition (Condition A), 17-7 has better machinability than annealed 304.

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17-7 has excellent weldability using tungsten inert gas welding (TIG). TIG is also called gas tungsten arc welding (GTAW).

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In annealed condition (Condition A), in cold-worked, or heat-treated condition (RH950 or TH1050), 17-7 is slightly ferromagnetic.

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  • Condition A (Annealed), 145ksi MAX tensile strength — components fabricated from this material can be heat treated to RH950 or TH1050.
  • As-Drawn — Condition C — not all tubing sizes and wall conditions can be manufactured to meet the condition C requirement of 200ksi MIN tensile strength. Tubing may fracture during the straightening process.  ACCU-TUBE recommends targeting a tensile of 160ksi MIN.
  • Heat treated to CH900 Process, 200ksi MIN tensile – not all tubing can meet the requirement of 240ksi MIN due to the possibility of fracturing during the straightening process.  ACCU-TUBE recommends targeting 200ksi for tubing that is heat treated using the CH900 Process.

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  • Heat treated RH950 and TH1050 — complete heat treating after the component is fabricated.
  • Use a specification that defines a tensile requirement, not a hardness specification. Measuring hardness is challenging on small diameter tubing due to the size and geometry of the tubing.
  • Measure the tubing that is being tensile tested. A shift in the tolerance range can impact the tensile calculation significantly. If you use the nominal ID and OD specifications versus actual measurements, your calculations may be off significantly.
  • Use a vacuum heat treating vendor that has significant experience with heat treating 17-7 materials.
  • If bright parts are desired after heat treat, mechanical or electropolishing can be performed.