17-7PH hardening (RH950) low temperature quenching + aging

17-7PH or Type 631 (UNS S17700) is a chromium-nickel aluminum precipitation hardening stainless steel used for spring applications in numerous industries.

7-7 Precipitation hardening alloys can be formed in the soft austenitic state and hardened to high strength levels by low-temperature heat treatment. The low temperature allows for minimal deformation compared to conventional quench and temper hardening processes. In the soft austenitic annealed condition, 17-7 is highly formable and is ideal for operations such as drawing, bending, and end forming. 17-7 is readily weldable in both the annealed and heat-treated conditions.

Element:
Carbon: 0.09 max

Manganese: 1.00 max

Phosphorus: 0.040 max

Sulfur: 0.030 max

Silicon: 1.00 max

Chrome: 16.00 – 18.00

Nickel: 6.50 – 7.75

Aluminum: 0.75 – 1.50

Iron: Balanced

Physical properties:
Melting point: 2550 – 2640°F (1400 – 1450°C)

Density: 0.282 lb/in3 / 7.8 g/cm3

Tensile Modulus of Elasticity (RH 950 and TH 1050): 29.6 X 10 6 psi / 204 GPA

Mechanical behavior:
Condition: annealed
Tensile Strength Minimum (psi): 130,000

Yield Strength Min. 0.2% Offset (psi): 40,000

2″ Elongation Typical: 35%

Hardness: Rockwell B85

Condition: hardened + aged (TH1050)
Typical Tensile Strength (psi): 200,000

Yield Strength Typical 2% Shift (psi): 185,000

% elongation at 2″: 9%

Hardness: Rockwell C40

Condition: hardened, low temperature quenching + aging (RH950)

Typical Tensile Strength (psi): 235,000

Yield Strength Typical 2% excursion (psi): 220,000

% elongation at 2″: 6%

Hardness: Rockwell C48

Condition: Cold Rolled/Worked + Aged (CH900)
Typical Tensile Strength (psi): 265,000

Yield Strength Typical 2% excursion (psi): 260,000

% elongation at 2″: 2%

Hardness: Rockwell C49

heat treatment:
17-7 requires three basic steps in heat treatment:

Austenitic conditioning.

Cooling transforms austenite to martensite

Precipitation hardening to TH 1050 or RH 950 conditions

Alternatively, to obtain the highest mechanical properties from the alloy, the condition A material is transformed to martensite by cold reduction to condition C in the rolling mill. Hardening to condition CH 900 is accomplished by a single low-temperature aging heat treatment.