Sep 26 2012
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.