Stainless Steel - Grade 17-7 (UNS S17700)

Topics Covered

Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
Fabrication and Heat Treatment
Machinability
Forming
Welding
Heat Treatment
Cold Working
Annealing
Aging
Applications

Introduction

Stainless steels are high-alloy steels, which have high corrosion resistance when compared to other steels due to high chromium content of about 4-30% . 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 1.0
Manganese, Mn 1.0
Aluminum, Al 0.75 - 1.50
Carbon, C 0.090
Sulfur, S 0.040
Phosphorous, P 0.040

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-m½ 69.2 ksi-in½
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 µm/m°C 6.11 µin/in°F
Thermal conductivity (@ 100°C/212 °F) 16.4 W/mK 114 BTU in/hr.ft².°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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