Introduction
440C stainless steel is the go-to material when you need high hardness and wear resistance in a corrosion-resistant package. With a hardness range of 58-60 HRC in the fully hardened condition, it’s one of the hardest stainless steels available. In this article, I’ll cover the hardness of 440C in detail, including how it’s achieved through heat treatment, its chemical composition, mechanical properties, and how to machine it in the annealed and hardened states.
Chemical Composition
440C is a high-carbon martensitic stainless steel. The high carbon content (0.95-1.20%) allows it to be hardened to high levels. Here’s the typical composition per ASTM A276:
| Element | Composition Range (%) |
|---|---|
| Carbon (C) | 0.95 – 1.20 |
| Manganese (Mn) | 1.00 max |
| Phosphorus (P) | 0.040 max |
| Sulfur (S) | 0.030 max |
| Silicon (Si) | 1.00 max |
| Chromium (Cr) | 16.0 – 18.0 |
| Molybdenum (Mo) | 0.75 max |
| Nickel (Ni) | 0.75 max |
| Vanadium (V) | 0.10 max (optional) |
The high carbon and chromium content form hard chromium carbides, which contribute to wear resistance. However, the high carbon also makes 440C more difficult to weld and machine compared to lower-carbon martensitic grades like 410 or 420.
Hardness of 440C in HRC
The hardness of 440C depends entirely on the heat treatment condition. Here are typical hardness values for different conditions:
| Condition | Hardness (HRC) | Notes |
|---|---|---|
| Annealed | 16 – 22 | Softest condition for machining |
| Hardened & Tempered (400°F) | 58 – 60 | Maximum hardness; high wear resistance |
| Hardened & Tempered (600°F) | 56 – 58 | Good balance of hardness and toughness |
| Hardened & Tempered (800°F) | 54 – 56 | Improved toughness; lower hardness |
| Hardened & Tempered (1000°F) | 48 – 52 | Best toughness; reduced wear resistance |
The typical hardening cycle involves austenitizing at 1850-1950°F (1010-1065°C), oil or air quenching, and tempering at the desired temperature. Double tempering is recommended for optimal toughness and dimensional stability. The achievable hardness is 58-60 HRC, which is exceptional for a stainless steel.
Mechanical Properties
In addition to hardness, 440C offers good strength and moderate toughness. Typical mechanical properties in the hardened and tempered condition (58 HRC) are:
| Property | Value |
|---|---|
| Tensile Strength (ksi) | 250 – 280 |
| Yield Strength (0.2% offset, ksi) | 220 – 250 |
| Elongation in 2″ (%) | 2 – 5 |
| Modulus of Elasticity (psi x 10^6) | 29 |
| Charpy V-notch Impact (ft-lb) | 5 – 10 |
Note that elongation and impact toughness are low at high hardness levels. For applications requiring impact resistance, temper at higher temperatures (600-800°F) to improve toughness at the expense of hardness.
Typical Applications
440C is used where high hardness and corrosion resistance are required. Common applications include:
- Cutting tools – Knife blades, scissors, and shear blades.
- Bearings – Ball bearings and races for corrosive environments.
- Valve components – Valve seats, stems, and balls for high-pressure applications.
- Medical instruments – Surgical scalpels, forceps, and dental tools.
- Mold components – Mold inserts and cores for plastic injection molding.
CNC Machining Guide
Machining 440C is best done in the annealed condition (16-22 HRC). After hardening, grinding or EDM is typically used. Here are my recommended parameters for machining annealed 440C:
| Operation | Tool Material | Speed (SFM) | Feed (IPR) | Depth of Cut (in) |
|---|---|---|---|---|
| Turning (rough) | Carbide (C2 grade, TiAlN coated) | 250 – 400 | 0.008 – 0.015 | 0.060 – 0.120 |
| Turning (finish) | Carbide (C2 grade, TiAlN coated) | 350 – 500 | 0.003 – 0.006 | 0.010 – 0.030 |
| Milling (rough) | Carbide (TiAlN coated) | 200 – 350 | 0.003 – 0.006 (IPT) | 0.030 – 0.080 |
| Milling (finish) | Carbide (TiAlN coated) | 300 – 450 | 0.002 – 0.004 (IPT) | 0.010 – 0.020 |
| Drilling | Carbide (with coolant-through) | 100 – 200 | 0.002 – 0.005 | N/A |
Critical considerations: Always machine in the annealed condition. Leave 0.010-0.020″ for finish grinding after heat treatment. Use rigid setups and high-pressure coolant. For drilling, peck cycles are essential to break chips and prevent work hardening.
Quality Control
At Dongguan Stirling Metal Products Co., Ltd., we ensure your 440C parts meet specifications. Our QC includes:
- Hardness testing – Rockwell C verification on every part after heat treatment.
- Chemical analysis – OES verification of carbon content (0.95-1.20%).
- Dimensional inspection – CMM and optical comparators for critical features.
- Microstructure examination – Metallographic analysis to verify carbide distribution and absence of retained austenite.
Conclusion
440C stainless steel offers exceptional hardness (58-60 HRC) and wear resistance, making it ideal for cutting tools, bearings, and wear components. While it requires careful machining in the annealed condition and post-hardening grinding, the performance in service is outstanding. If you need a stainless steel that can take a beating and stay sharp, 440C is the answer.
Dongguan Stirling Metal Products Co., Ltd. specializes in providing 440C material + CNC machining One-Stop Service:
- ✅ Material procurement: Genuine product guarantee, complete material certificates
- ✅ CNC machining: Precision ±0.01mm, smooth surface finish
- ✅ Sample production: Fast delivery in 3-5 days
- ✅ Batch production: Delivery time of 7-15 days
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