68Cr17 (7Cr17) Martensitic Stainless Steel
68Cr17 (7Cr17) Martensitic Stainless Steel: High Hardness & Wear Resistance for Cutlery
In-depth analysis of 68Cr17 (7Cr17) martensitic stainless steel under GB/T 4237, covering chemical composition, mechanical and thermal properties, and equivalent grades.
Hot rolling, Cold rolling, Annealing, Quenching, Tempering, Grinding
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68Cr17 Martensitic Stainless Steel Introduction
68Cr17 (also designated as 7Cr17) is a high-carbon, medium-chromium martensitic stainless steel standardized under GB/T 4237 (Hot rolled stainless steel plate, sheet and strip). This grade is characterized by its high hardness and excellent wear resistance after heat treatment, achieved through a quenching and tempering process that transforms its microstructure to martensite. While its chromium content provides moderate corrosion resistance, it is inferior to austenitic grades like 304. Key features include:
- High attainable hardness (up to 58 HRC).
- Excellent wear and abrasion resistance.
- Good strength and edge retention.
- Low toughness and weldability.
Its primary application is in manufacturing high-quality cutlery, surgical instruments, bearings, valves, and other wear-resistant mechanical components.
68Cr17 Martensitic Stainless Steel Chemical Composition
The chemical composition for 68Cr17 (7Cr17) stainless steel is specified per GB/T 4237, which aligns with GB/T 20878. High carbon content promotes martensitic transformation and high hardness upon quenching, while chromium provides essential corrosion resistance. Phosphorus and sulfur are strictly controlled to ensure material integrity.
| Element | Standard Value (Mass %) | Remarks |
|---|---|---|
| Carbon (C) | 0.60 - 0.75 | Primary hardening element. |
| Silicon (Si) | ≤ 1.00 | Deoxidizer and strength contributor. |
| Manganese (Mn) | ≤ 1.00 | Improves hardenability. |
| Phosphorus (P) | ≤ 0.040 | Impurity, controlled for toughness. |
| Sulfur (S) | ≤ 0.030 | Impurity, controlled for machinability. |
| Chromium (Cr) | 16.00 - 18.00 | Provides corrosion and oxidation resistance. |
| Nickel (Ni) | ≤ 0.60 | Residual element, not intentionally added. |
68Cr17 Martensitic Stainless Steel Thermal and Electrical Physical Properties
Physical properties for 68Cr17 (7Cr17) martensitic stainless steel are representative of high-chromium alloys. The data provided are typical values at room temperature unless otherwise specified. Thermal expansion and conductivity are critical for applications involving thermal cycling. Electrical resistivity is relatively high, typical for stainless steels.
| Property Item | Typical Value/Requirement | Unit | Test Condition / Remarks |
|---|---|---|---|
| Density (ρ) | 7.73 | g/cm³ | At 20°C |
| Modulus of Elasticity (E) | 200 | GPa | At 20°C |
| Shear Modulus (G) | 77 | GPa | Calculated from E and ν |
| Poisson's Ratio (ν) | 0.30 | - | Typical value |
| Coefficient of Thermal Expansion (α) | 10.2 (20-100°C); 10.7 (20-200°C); 11.4 (20-400°C) | 10<sup>-6</sup>/K | Linear expansion between stated temperatures |
| Thermal Conductivity (λ) | 24.2 (at 20°C); 25.1 (at 100°C); 25.8 (at 200°C) | W/(m·K) | Conductivity at elevated temperature |
| Specific Heat Capacity (c) | 460 | J/(kg·K) | At 20°C |
| Electrical Resistivity (ρ_e) | 0.60 | Ω·mm²/m | At 20°C |
68Cr17 Martensitic Stainless Steel Mechanical Properties
Mechanical properties for 68Cr17 (7Cr17) plate/coil as per GB/T 4237 are specified in the solution-annealed delivery condition. The final application properties are achieved after hardening and tempering. Elongation and impact values are typically low in the hardened condition, reflecting the intrinsically brittle nature of high-carbon martensite.
| Property Item | Standard Requirement Value | Unit | Test Condition / Remarks |
|---|---|---|---|
| Yield Strength (Rp0.2) | ≥ 245 | MPa | Solution Annealed Condition |
| Tensile Strength (Rm) | ≥ 590 | MPa | Solution Annealed Condition |
| Elongation after Fracture (A) | ≥ 15 | % | Solution Annealed Condition, gauge length 50mm |
| Hardness (HBW) | ≤ 255 | HBW | Solution Annealed Condition |
| Hardness (HRC) | ≥ 54 | HRC | Hardened Condition (Quenched & Tempered) |
68Cr17 Martensitic Stainless Steel Fully Equivalent Material Standards & Replaceable Grades
| Country/Region | Standard | Grade Designation | Remarks |
|---|---|---|---|
| China | GB/T 4237 | 68Cr17 (7Cr17) | Original specification. 7Cr17 is the old designation. |
| USA | ASTM A240/A240M | 440A (UNS S44002) | Closest direct equivalent, very similar composition. |
| Japan | JIS G4304 | SUS 440A | Functionally identical to grade 440A. |
| Europe | EN 10088-2 | X70CrMo15 (1.4109) | Contains a small molybdenum addition for enhanced corrosion resistance, considered a direct functional replacement. |
68Cr17 Martensitic Stainless Steel Application Introduction
68Cr17 (7Cr17) material is designed for applications that require a combination of high hardness, good wear resistance, and moderate corrosion protection. Its properties are optimized through a hardening and tempering heat treatment, making it unsuitable for welding or cold forming in its final state. It is a premium choice for cutlery, medical tools, and high-load mechanical components.
Product Applications: High-quality kitchen knives and scissors, Surgical scalpels, forceps, and dental instruments, Ball bearings and roller bearings, High-pressure valve seats and stems, Pump shafts and wear rings, Precision measuring tools like gauges and ruler tapes
Processed into products: Knife blades and edges, Bearing balls, races, and cages, Valve spools and glands, Injection molds for abrasive plastics, Wear-resistant nozzles and bushings
Application industries: Cutlery & Kitchenware Industry, Medical & Surgical Instrument Manufacturing, Bearing & Valve Manufacturing, General Mechanical Engineering, Petrochemical Industry (for wear components)
68Cr17 Martensitic Stainless Steel Recommendations for Similar/Alternative Materials
| Country/Region | Standard | Grade Designation | Remarks / Guidance on Suitability |
|---|---|---|---|
| China | GB/T 4237 | 85Cr17 (8Cr17/440B) | Higher carbon (0.75-0.95%) for slightly higher achievable hardness and wear resistance, but lower toughness than 68Cr17. |
| China | GB/T 4237 | 108Cr17 (11Cr17/440C) | Highest carbon content in the 440 series (0.95-1.20%). Provides maximum hardness and wear resistance, primarily for bearing applications. |
| USA | ASTM A240/A240M | 440B (UNS S44003) | Higher carbon alternative to 68Cr17, offering enhanced hardness and wear characteristics. |
| USA | ASTM A240/A240M | 440C (UNS S44004) | Maximum hardness and wear resistance option. Used for critical bearing and valve parts where corrosion resistance is secondary. |
Notes:
- Heat Treatment: Austenitizing temperature is critical; overheating can cause excessive grain growth and retained austenite. Tempering should generally be performed immediately after quenching to avoid cracking. A sub-zero treatment (deep freezing) between quenching and tempering can maximize hardness by transforming retained austenite.
- Corrosion Resistance: Best corrosion resistance is obtained in the hardened and tempered condition. The solution-annealed state has lower corrosion resistance. This grade is not suitable for use in non-oxidizing acids (e.g., sulfuric, hydrochloric) or strong alkaline environments.
- Processing: Material is typically supplied in annealed coil or plate form for blanking and grinding. Final machining to tight tolerances is performed on the hardened part by grinding.
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