Jul 31 2013
Topics Covered
Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
Fabrication and Heat Treatment
Machinability
Heat Treatment
Forming
Welding
Annealing
Aging
Applications
Introduction
The two main properties of precipitation-hardened stainless steels are high corrosion resistance and high strength. Stainless steel grade 17-7 PH is a precipitation hardened stainless steel that possesses high strength and hardness, excellent fatigue properties, good formability, good corrosion resistance, and minimal distortion upon heat treatment. The properties of this alloy are well suited for aerospace applications.
Stainless steel grade 17-7 PH is suitable for use in fresh water and industrial atmospheres, and mild chemical and oxidizing environments. It is recommended that 17-7 PH should not be used in salt water or reducing environments.
The following sections will discuss in stainless steel grade 17-7 PH in further detail.
Chemical Composition
The chemical composition of stainless steel grade 17-7 PH is outlined in the following table.
|
Element |
Content (%) |
|
Chromium, Cr |
16-18 |
|
Nickel, Ni |
6.50 - 7.75 |
|
Manganese, Mn |
1 max |
|
Silicon, Si |
1 max |
|
Aluminum, Al |
0.75 - 1.50 |
|
Carbon, C |
0.09 max |
|
Phosphorous, P |
0.040 max |
|
Sulfur, S |
0.030 max |
|
Iron, Fe |
Balance |
Physical Properties
The physical properties of stainless steel grade 17-7 PH are tabulated below.
|
Properties |
Metric |
Imperial |
|
Density |
7.80 g/cm3 |
0.282 lb/in3 |
Mechanical Properties
The following table shows mechanical properties of stainless steel grade 17-7 PH (TH1050, bar and forgings).
|
Properties |
Metric |
Imperial |
|
Tensile strength |
1170 MPa |
170000 psi |
|
Yield strength |
965 MPa |
140000 psi |
|
Modulus of elasticity |
204 GPa |
29600 ksi |
|
Poisson’s ratio |
0.27-0.30 |
0.27-0.30 |
|
Elongation at break |
6% |
6% |
|
Hardness, Rockwell C |
38 |
38 |
Thermal Properties
The thermal properties of stainless steel grade 17-7 PH are given in the following table.
|
Properties |
Metric |
Imperial |
|
Thermal conductivity |
16.4 W/mK |
114 BTU in/hr.ft2.°F |
|
Thermal expansion co-efficient (@0.000-100°C/32-212°F) |
11 µm/m°C |
6.11 µm/m°C |
Other Designations
Other designations that are equivalent to stainless steel grade 17-7 PH include the following.
|
AISI 631 |
AMS 5528 |
AMS 5529 |
AMS 5568 |
AMS 5644 |
|
AMS 5678 |
ASTM A313 (631) |
ASTM A564 (631) |
ASTM A579 (631) |
ASTM A693 (631) |
|
ASTM A705 (631) |
DIN 1.4568 |
MIL S-25043 |
DIN 1.4568 |
AMS 5673 |
|
AMS 5824 |
ASME SA705 |
ASTM A579 (grade 62) |
MIL SPEC MIL-W-46078 |
SAE J217 |
|
SAE J467 |
|
|
|
|
Fabrication and Heat Treatment
Machinability
Machinability of stainless steel grade 17-7 PH is rated at 75 % of B1112. On machining, this alloy displays long gummy chips and requires chip breakers. It is recommended that slow speeds and constant feeds while machining will provide excellent results.
Heat Treatment
For heat treating stainless steel grade 17-7 PH, three important steps need to be carried out - austenite conditioning, cooling to transform the austenite to martensite, and precipitation hardening to condition TH 1050 or RH 950.
Forming
Forming characteristics of stainless steel grade 17-7 PH are similar to stainless steel grade 301. For radical forming operations, intermediate annealing may be required.
Welding
Common arc and resistance methods can be used with stainless steel grade 17-7 PH. It is recommended that during fusion welding inert gas shielding should be performed to reduce the oxidation of aluminum during the process.
Annealing
Annealing of stainless steel grade 17-7 PH is performed at 1066°C (1950°F) for 3 min for every 0.1 inch (2.5 mm) of thickness, and then air cooled.
Aging
Stainless steel grade 17-7 PH is solution annealed at 1177°C (2150°F) for 4 h and air cooled. Same step is performed at 1079°C (1975°F) for 4h and air cooled, then at 843°C (1550°F) for 24h and air cooled, and at 760°C (1400°F) for 16h and air cooled. This completed the precipitation hardening process. For excellent results and performance, the material should be clean and free from scale and impurities.
Applications
Stainless steel grade 17-7 PH is primarily used for applications requiring corrosion resistance, good strength, and mechanical properties up to 427°C (800°F) in operation. It is also commonly used for intricate parts due to its low distortion in heat treatment.
Stainless steel grade 17-7 PH finds uses in the following:
-
Variety of springs and washers
-
Chemical processing equipment
-
Heat exchangers
-
Power boilers
-
Superheater tubes
-
Components used in high-strength/high-temperature conditions