9Cr18 Stainless Steel Properties and Applications

9Cr18 Stainless Steel is a high-carbon, high-chromium martensitic stainless steel. It’s known for its high hardness, wear resistance, and good corrosion resistance after proper heat treatment. Here’s a breakdown of what you need to know.

1.  9Cr18 Stainless Steel Chemical Composition

The specific composition of 9Cr18 gives it its distinct properties. According to the GB/T 1220—2007 standard, the typical ranges (by weight percentage) are:

• Carbon (C): 0.90% – 1.00%
• Chromium (Cr): 17.00% – 19.00%
• Silicon (Si): ≤0.80%
• Manganese (Mn): ≤0.80%
• Phosphorus (P): ≤0.040%
• Sulfur (S): ≤0.030%

You may also see this grade referred to as 95Cr18 or find similar specifications under international standards, such as ASTM A959 (as 440B) or JIS G4303 (as SUS440B). While there can be slight variations in ranges (e.g., C: 0.75-0.95%, Cr: 16.00-18.00% for 440B), the core characteristics remain consistent.

A key point: The high carbon and chromium content necessitate careful control during production to manage carbide formation and ensure uniform properties. This is a key focus area for Aobo Steel.

2. 9Cr18 Stainless Steel Physical and Mechanical Properties

2.1 Key Physical Data

• Density: Approximately 7.7 t/m³
• Elastic Modulus (E at 20°C): 203.89 GPa
• Thermal Conductivity (λ at 20°C): 29.3 W/(m·K)
• Specific Heat Capacity (cₚ at 20°C): 459.8 J/(kg·K)
• Electrical Resistivity (ρ at 20°C): 0.60 × 10⁻⁶ Ω·m
• Linear Expansion Coefficient (×10⁻⁶ /°C): Varies with temperature (e.g., 10.5 from 20-100°C, 11.5 from 20-400°C)

2.2 Mechanical Properties & Hardness

Mechanical properties, especially hardness, are heavily dependent on the heat treatment applied.

1. Hardness (Annealed): Soft annealing typically results in hardness ≤255 HBW.
2. Hardness (Quenched): Quenching from 1030-1070°C (some sources specify 1050-1075°C) yields a hardness of≥56 HRC, potentially reaching up to 60.5 HRC, depending on the exact temperature.
3. Hardness (Tempered):
   
   • Tempering at 200-300°C typically results in a hardness of 56-60 HRC.
   • Tempering at around 160-200°C is often preferred for achieving the best balance of hardness and corrosion resistance, typically yielding a hardness of 55-59 HRC.
   • Higher tempering temperatures (500-600°C) reduce hardness to the 40-53 HRC range but increase toughness.
4. Cold Treatment: Applying cold treatment (e.g., -75°C) after quenching can increase hardness by another 1-1.5 HRC.

3. 9Cr18 Stainless Steel Heat Treatment

Achieving the desired performance from 9Cr18 relies heavily on correct heat treatment.

• Soft Annealing: Heat to 800-840°C, hold, furnace cool to below 500°C, then air cool. Target hardness: ≤255 HBW.
• Quenching: Heat to 1030-1070°C (or 1050-1075°C), then oil quench. Target hardness: ≥56 HRC.
• Tempering: Temper according to the required balance of hardness, toughness, and corrosion resistance (see hardness values above). Common ranges are 160-300°C.
• Critical Temperatures (Approximate): Ac1: 830°C, Ac3: 810°C, Ms: 145°C.

4. Corrosion Resistance

As a martensitic stainless steel, 9Cr18 offers good corrosion resistance, which is superior to that of Cr17 type steels.

• Resistant to atmosphere, water, and certain mild acids and salts.
• Performance varies in different chemical media (like nitric, acetic, and phosphoric acids) depending on concentration and temperature.
• Shows good resistance to seawater.
• For optimal corrosion resistance, the structure requires at least 11-12% chromium dissolved in the martensite, which can be achieved through proper heat treatment.

5. Forging Process

Correct forging is essential for quality. We have extensive experience in this area.

• Initial Forging Temperature: 1050-1100°C
• Final Forging Temperature: >850°C
• Cooling: Slow cooling after forging (e.g., furnace cooling) is critical to prevent issues. Careful control of the heating rate is also important.

6.  9Cr18 Stainless Steel Applications

The combination of high hardness, wear resistance, and corrosion resistance makes 9Cr18 suitable for demanding applications:

• Cutting Tools: High-quality knives, surgical instruments, and scissors require sharp edges and resistance to staining.
• Molds: Plastic injection molds, particularly for corrosive resins or optical parts that require high polish and durability.
• Bearings: Rolling element bearings need wear and corrosion resistance.
• Components: Shafts, pump parts, valves, fasteners, and springs operating in mildly corrosive or high-wear environments.

7. Considerations

• Carbide Distribution: As mentioned, controlling carbide formation during steelmaking and forging is crucial for consistent performance. Aobo Steel manages this through strict process control.
• Mold Making: Ensure proper heat treatment achieves the necessary dissolved chromium for corrosion resistance in mold applications. Adding Molybdenum (like in 9Cr18Mo) can enhance corrosion resistance and secondary hardening if needed for specific high-carbon applications.
• Surface Enhancements: Treatments such as nitrogen ion implantation can further enhance surface hardness and wear resistance, as needed.

Summary

9Cr18 is a reliable martensitic stainless steel offering a strong combination of hardness, wear resistance, and corrosion resistance. Its performance hinges on precise control of the chemical composition and proper heat treatment – areas where Aobo Steel’s experience provides real value. It’s a solid choice for tools, molds, bearings, and various industrial components.