Stainless Steel Grade 17-7 PH Precipitation Hardened (UNS S17700)

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

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