Among the family of advanced stainless steels used in aerospace, chemical processing, and power generation, 15-7PH stainless steel stands out for its combination of high strength, good corrosion resistance, and heat treatment versatility. One of the most frequently asked technical questions about this alloy concerns its density and melting range, two fundamental parameters for engineers calculating weight, thermal performance, and casting or forging operations.
In this article, SAKYSTEEL provides a detailed explanation of the density and melting range of 15-7PH stainless steel, along with insights into how these physical properties influence design, fabrication, and performance in demanding environments.
1. Introduction to 15-7PH Stainless Steel
15-7PH (UNS S17700, also known as Type 631) is a precipitation-hardening martensitic stainless steel developed to provide high strength, hardness, and corrosion resistance combined with good fabrication characteristics. The name “15-7PH” comes from its approximate composition of 15% chromium and 7% nickel, with aluminum added for precipitation hardening.
The alloy undergoes a special heat treatment process where fine intermetallic compounds (mainly Ni₃Al) form within the matrix, greatly increasing strength without sacrificing toughness. Because of this, 15-7PH is widely used in:
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Aerospace springs and diaphragms
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Chemical process equipment
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High-temperature fasteners
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Power generation components
Understanding its density and melting characteristics is essential for accurate weight calculations and thermal process design.
2. Chemical Composition of 15-7PH
| Element | Typical Content (%) | Purpose |
|---|---|---|
| Chromium (Cr) | 14.0 – 16.0 | Corrosion resistance and strength |
| Nickel (Ni) | 6.5 – 7.8 | Toughness and martensitic stability |
| Aluminum (Al) | 0.75 – 1.5 | Precipitation hardening |
| Carbon (C) | ≤ 0.09 | Strength control |
| Manganese (Mn) | ≤ 1.00 | Deoxidizer |
| Silicon (Si) | ≤ 1.00 | Improves oxidation resistance |
| Phosphorus (P) | ≤ 0.04 | Kept low for ductility |
| Sulfur (S) | ≤ 0.03 | Machinability |
| Iron (Fe) | Balance | Base element |
This carefully balanced composition directly influences both density and melting range, as nickel, chromium, and aluminum each have distinct atomic weights and thermal behaviors.
3. Density of 15-7PH Stainless Steel
The density of a metal defines its mass per unit volume and is a key parameter for:
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Weight calculations of components
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Structural design
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Fluid displacement and buoyancy calculations
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Conversion between mass and volume in material selection
For 15-7PH stainless steel, the typical density is approximately:
7.80 g/cm³ (7800 kg/m³)
This value is close to that of other martensitic and precipitation-hardening stainless steels such as 17-4PH (7.75 g/cm³) and 17-7PH (7.81 g/cm³).
3.1 Density Conversion Table
| Unit | Value |
|---|---|
| g/cm³ | 7.80 |
| kg/m³ | 7800 |
| lb/in³ | 0.282 |
| lb/ft³ | 487 |
This moderate density ensures that 15-7PH delivers a high strength-to-weight ratio, making it particularly attractive for aerospace and high-performance mechanical components where every gram matters.
4. Melting Range of 15-7PH Stainless Steel
The melting range of an alloy refers to the temperature interval between its solidus (start of melting) and liquidus (fully molten) points. Unlike pure metals, which have a single melting point, complex alloys such as stainless steels melt over a temperature range due to the varying melting temperatures of their constituent elements.
For 15-7PH stainless steel, the typical melting range is:
1400 – 1450 °C (2550 – 2640 °F)
This range is consistent with other high-chromium, nickel-bearing precipitation-hardening steels.
4.1 Melting Range Context
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17-4PH: 1400 – 1440 °C
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17-7PH: 1400 – 1455 °C
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15-5PH: 1400 – 1450 °C
Thus, 15-7PH fits within the same general range but may show slightly higher solidus due to aluminum’s effect, which raises melting temperature marginally.
5. Factors Influencing Density and Melting Range
5.1 Chemical Composition
The proportions of chromium, nickel, and aluminum determine how closely packed the atoms are and how much energy is needed to break atomic bonds during melting.
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Chromium increases hardness and melting temperature.
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Nickel improves toughness but slightly lowers density.
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Aluminum contributes to strength and slightly raises the melting point.
5.2 Heat Treatment
While density changes only slightly during heat treatment (less than 0.1%), thermal expansion and phase transformation may influence volume. Aging treatments (e.g., RH950 or TH1050) can cause small dimensional changes but do not significantly affect bulk density.
5.3 Impurities and Alloying Tolerances
Trace impurities or slight compositional deviations can shift the melting range by 10–20°C, but these variations are generally negligible for engineering purposes.
6. Importance of Density and Melting Range in Engineering Design
6.1 Weight and Structural Calculations
Knowing the exact density allows designers to calculate component weights accurately. For instance, in aircraft applications, replacing a 316L stainless part (density 8.0 g/cm³) with 15-7PH reduces overall weight while increasing strength, improving fuel efficiency and performance.
6.2 Casting and Forging Operations
The melting range determines furnace temperature, mold design, and solidification control. A narrower melting range helps minimize segregation and porosity during casting.
For 15-7PH, optimal melting control ensures consistent microstructure and uniform mechanical properties.
6.3 Welding and Brazing
When welding or joining 15-7PH, understanding the melting range is vital to prevent excessive grain growth or partial melting in the heat-affected zone. Proper control avoids distortion and ensures good joint integrity.
6.4 Thermal Expansion and Heat Treatment Design
Density and melting data help predict volume changes and thermal stresses during processing. Engineers use this data to optimize annealing, aging, or solution treatment cycles.
7. Physical Properties Summary of 15-7PH
| Property | Typical Value | Unit |
|---|---|---|
| Density | 7.80 | g/cm³ |
| Melting Range | 1400–1450 | °C |
| Elastic Modulus | 200 | GPa |
| Specific Heat Capacity | 460 | J/kg·K |
| Thermal Conductivity | 16.7 | W/m·K |
| Coefficient of Expansion | 10.9 × 10⁻⁶ | /°C |
| Electrical Resistivity | 0.00074 | Ω·cm |
These physical characteristics make 15-7PH both structurally efficient and thermally stable, suitable for service from subzero temperatures up to approximately 550 °C (1020 °F).
8. Comparison with 17-4PH Stainless Steel
| Property | 15-7PH | 17-4PH |
|---|---|---|
| Density (g/cm³) | 7.80 | 7.75 |
| Melting Range (°C) | 1400–1450 | 1400–1440 |
| Hardening Element | Aluminum | Copper |
| Max Service Temp (°C) | 550 | 315 |
| Strength (Aged) | Very High | High |
| Corrosion Resistance | Good | Very Good |
Although the densities are nearly identical, 15-7PH offers higher strength and greater high-temperature stability, while 17-4PH provides slightly better corrosion resistance and easier weldability.
9. Industrial Applications Where Density and Melting Range Matter
9.1 Aerospace
The excellent strength-to-weight ratio from its moderate density makes 15-7PH ideal for aerospace fasteners, bellows, and diaphragms. Engineers can design thinner, lighter parts without compromising load capacity.
9.2 Energy and Power Generation
In gas turbine or nuclear systems, components must resist both mechanical stress and high operating temperatures. The alloy’s melting range ensures thermal safety margins in environments near 600 °C.
9.3 Chemical and Process Equipment
For valves, springs, and high-pressure fittings, the combination of density, corrosion resistance, and thermal endurance enables stable performance under cycling conditions.
10. Weight Calculation Example Using Density
A practical example shows how engineers use density data.
Component: Rod of 15-7PH, diameter 50 mm, length 1 m
Volume = π × (25²) × 1000 mm³ = 1.963 × 10⁶ mm³ = 1.963 × 10⁻³ m³
Weight = 1.963 × 10⁻³ m³ × 7800 kg/m³ = 15.3 kg
This precise calculation is critical in aerospace and automotive design, where even small mass differences affect balance and efficiency.
11. Thermal Processing Guidelines Based on Melting Range
To maintain material integrity, industrial processors must heat treat below the alloy’s solidus temperature.
For 15-7PH, the following temperature limits are recommended:
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Solution Treatment: 1035–1050 °C
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Aging: 510–565 °C
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Forging Range: 1150–900 °C
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Avoid Overheating Above: 1350 °C
These parameters keep the alloy well below its melting onset (1400 °C), ensuring microstructural control and preventing incipient melting.
12. Advantages of 15-7PH from a Physical Property Perspective
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Balanced Density: Offers strong components without unnecessary mass.
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High Melting Range: Enables excellent thermal stability and processing versatility.
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Dimensional Stability: Minimal distortion during heat treatment.
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Reliable Strength-to-Weight Ratio: Ideal for structural and moving parts.
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Consistent Physical Properties: Maintained across wide service temperatures.
These benefits make 15-7PH a favorite for engineers balancing performance, weight, and cost.
13. Quality Control and Testing at SAKYSTEEL
SAKYSTEEL ensures that every batch of 15-7PH stainless steel meets strict physical and mechanical standards. Each lot undergoes:
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Density verification by hydrostatic or dimensional analysis
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Spectrometric composition analysis
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Melting point verification from controlled furnace tests
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Hardness and tensile property testing
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Ultrasonic and surface inspection
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Certification to ASTM A693 / AMS 5520 / AMS 5659 standards
(Logo SAKYSTEEL)
Every product is fully traceable by heat number and supplied with EN 10204 3.1 or 3.2 certificates for international compliance.
14. Product Forms and Supply Range
SAKYSTEEL supplies 15-7PH stainless steel in multiple forms:
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Round Bars & Rods: Hot-rolled, forged, or cold-drawn
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Plates & Sheets: Annealed or aged conditions
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Wires & Strips: For springs and diaphragms
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Forgings & Rings: Custom-made to drawing
All products are available in standard and custom sizes, precision-cut to client specifications, with global shipping support.
15. Practical Engineering Insight
When calculating thermal loads or designing pressurized systems, the density defines mass inertia, while the melting range defines safety limits.
For example:
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In aircraft engines, density determines rotational balance.
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In heat exchangers, knowing the melting range helps avoid structural degradation during overheating events.
Thus, these two values are not just material facts — they are engineering essentials for performance optimization.
16. Comparison with Conventional Stainless Steels
| Property | 15-7PH | 304 | 316L |
|---|---|---|---|
| Density (g/cm³) | 7.80 | 8.00 | 8.03 |
| Melting Range (°C) | 1400–1450 | 1400–1455 | 1370–1400 |
| Yield Strength (MPa) | 1100+ | 205 | 290 |
| Max Service Temp (°C) | 550 | 425 | 425 |
This comparison shows how 15-7PH delivers exceptional strength at nearly the same density and higher temperature capability than austenitic grades.
17. Environmental and Safety Considerations
The alloy’s moderate density and high melting point contribute to sustainability and safety:
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Lower density reduces overall structure weight, saving energy.
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High melting range provides a wide thermal safety margin.
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Stable oxide film minimizes maintenance and replacement cycles.
In long-term service, this translates to lower lifecycle costs and greater reliability.
18. Industry Standards Referenced
15-7PH stainless steel conforms to:
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ASTM A693 – Precipitation-Hardening Stainless Plate, Sheet, and Strip
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AMS 5659 / AMS 5520 – Aerospace Materials Specifications
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UNS S17700 – Unified Numbering System
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SAE 631 Alloy Designation
These standards define its physical, mechanical, and chemical requirements, including density and melting range parameters used in quality control.
19. Summary of Key Data
| Property | 15-7PH Stainless Steel |
|---|---|
| Density | 7.80 g/cm³ (7800 kg/m³) |
| Melting Range | 1400 – 1450 °C (2550 – 2640 °F) |
| Strength (Aged) | 1250–1450 MPa |
| Max Service Temp | 550 °C |
| Structure | Martensitic, precipitation-hardened |
| Hardening Phase | Ni₃Al |
| Applications | Aerospace, Chemical, Energy, Precision Components |
20. Conclusion
The density and melting range of 15-7PH stainless steel — 7.80 g/cm³ and 1400–1450 °C, respectively — define its reputation as a high-strength, thermally stable alloy suitable for the most demanding industries.
Its balanced weight, wide heat tolerance, and stable microstructure make it a trusted material for components exposed to both mechanical stress and elevated temperatures.
At SAKYSTEEL, we specialize in providing certified 15-7PH stainless steel in bars, plates, wires, and custom forms, all tested and traceable to international standards. Our focus is on delivering consistent quality, precision machining, and reliable performance for customers worldwide.
Post time: Oct-29-2025