Introduction
Stainless steels are high alloy steels, which have high corrosion resistance when compared to other steels due to high chromium content of about 4% to 30% . Based on their crystalline structure, they are further divided into ferritic, austenitic, and martensitic steels.
Grade 17-7 stainless steel is a precipitation hardenable chromium nickel-aluminum stainless steel. The following datasheet gives an overview of grade 17-7 stainless steel.
Chemical Composition
The following table shows the chemical composition of grade 17-7 stainless steel.
| Element | Content (%) |
| Iron, Fe | 70 - 75% |
| Chromium, Cr | 16.0 - 18.0 |
| Nickel, Ni | 6.50 - 7.75 |
| Silicon, Si | 1 |
| Manganese, Mn | 1 |
| Aluminum, Al | 0.75 - 1.50 |
| Carbon, C | 0.09 |
| Sulfur, S | 0.04 |
| Phosphorous, P | 0.04 |
Physical Properties
The physical properties of grade 17-7 stainless steel are given in the following table.
| Properties | Metric | Imperial |
| Density | 7.80 g/cm3 | 0.282 lb/in3 |
Mechanical Properties
The mechanical properties of grade 17-7 stainless steel are outlined in the following table.
| Properties | Metric | Imperial |
| Tensile strength | 1275 MPa | 184900 psi |
| Yield strength | 1030 MPa | 149000 psi |
| Fracture toughness (longitudinal, 44 HRC) | 76 MPa-m� | 69.2 ksi-in� |
| Elastic modulus | 204 GPa | 29588 ksi |
| Poisson's ratio | 0.27-0.30 | 0.27-0.30 |
| Elongation at break | 6% | 6% |
| Reduction of area | 10% | 10% |
| Hardness, Rockwell C | 41 | 41 |
Thermal Properties
The thermal properties of grade 17-7 stainless steel are displayed in the following table.
| Properties | Metric | Imperial |
| Thermal expansion co-efficient (@ 0-100 �C/ 32-212�F) | 11 �m/m�C | 6.11 �in/in �F |
| Thermal conductivity (@ 100�C/212 �F) | 16.4 W/mK | 114 BTU in/hr.ft�.�F |
Other Designations
Other designations that are equivalent to grade 17-7 stainless steel are listed in the following table.
| AISI 631 | AMS 5678 | ASTM A705 (631) | ASME SA705 |
| AMS 5528 | ASTM A313 (631) | DIN 1.4568 | ASTM A313 |
| AMS 5529 | ASTM A564 (631) | MIL S-25043 | MIL-S-25043 |
| AMS 5568 | ASTM A579 (631) | AMS 5673 | MIL-W-46078 |
| AMS 5644 | ASTM A693 (631) | AMS 5824 | SAE J217 |
Fabrication and Heat Treatment
Machinability
The machinability of grade 17-7 stainless steel is rated as 75 compared to that of B1112 alloy. Slow speed and constant feeds are preferred to obtain best results.
Forming
Grade 17-7 stainless steel has forming characteristics similar to that of 301 stainless steel. Intermediate annealing can be done for radical forming operations.
Welding
Grade 17-7 stainless steel can be welded using arc and resistance welding techniques. Inert gas shielding is preferred during fusion welding in order to reduce oxidation of aluminum. It is not necessary to perform pre heating and post weld annealing processes.
Heat Treatment
Solution treated material is heavily cold worked which results in condition C material. Condition C material is heated at 482°C (900°F) for 1 h in condition CH900 followed by cooling. Solution treated material is heated at 955°C (1750°F) for 10 min after fabrication in condition A1750 followed by rapidly cooling at room temperature.
Within 1 h of treating to condition A1750, it is cooled at -73°C (-100°F) for 8 h in condition R100. It is then heated at 510°C (950°F) for 90 min in condition RH 950 followed by cooling. After fabrication, it is heated at 760°C (1400°F) for 90 min in condition T.
Within 60 min of cooling, it is then cooled at 13°C (55°F) for 30 min. From condition T, it is heated at 565°C (1050°F) for 90 min in condition TH1050 followed by cooling.
Cold Working
Grade 17-7 stainless steel can be easily cold worked using common methods in the condition A.
Annealing
Grade 17-7 stainless steel is annealed at 1066°C (1950°F) for 3 min for 0.1 in. (2.5 mm) of thickness followed by air cooling.
Aging
Aging of grade 17-7 stainless steel can be done at 1177°C (2150°F) for 4 h followed by cooling. This process is followed by heating at 1078°C (1975°F) for 4 h and cooling. It is again heated at 844°C (1550°F) for 24 h and cooled, and heated at 760°C (1400°F) for 16 h followed by cooling to complete precipitation hardening process.
Applications
Grade 17-7 stainless steel is mostly found in washers and springs. It is used in applications which require high strength and corrosion resistance. It is also used in intricate parts due its low distortion on heat treatment.
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