H21 Tool Steel: Properties and Key Applications
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Table Of Content
Table Of Content
H21 Tool Steel is classified as a high-speed tool steel, specifically designed for applications requiring high wear resistance and toughness. It is primarily alloyed with tungsten, molybdenum, and chromium, which significantly enhance its hardness and heat resistance. This steel grade is known for its excellent performance in high-temperature applications, making it suitable for manufacturing cutting tools, dies, and molds.
Comprehensive Overview
H21 Tool Steel is part of the AISI/SAE high-speed tool steel category, characterized by its ability to maintain hardness and strength at elevated temperatures. The primary alloying elements include tungsten (W), molybdenum (Mo), chromium (Cr), and vanadium (V). These elements contribute to the steel's overall toughness, wear resistance, and ability to withstand thermal fatigue.
The significant characteristics of H21 Tool Steel include:
- High Hardness: Achieves hardness levels of 60 HRC or higher after heat treatment.
- Excellent Wear Resistance: Suitable for high-stress applications where abrasion is a concern.
- Good Toughness: Maintains structural integrity under impact loads.
Advantages:
- Exceptional performance in high-temperature environments.
- Retains sharp edges and resists deformation during cutting operations.
- Versatile applications across various industries.
Limitations:
- More challenging to machine compared to lower alloy steels.
- Requires precise heat treatment to achieve optimal properties.
- Higher cost compared to standard tool steels.
Historically, H21 has been a preferred choice for toolmakers due to its balance of hardness and toughness, making it a staple in the manufacturing of precision tools and dies.
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | T20821 | USA | Closest equivalent to AISI H21 |
| AISI/SAE | H21 | USA | Commonly used designation |
| ASTM | A681 | USA | Specification for tool steels |
| EN | 1.2561 | Europe | Similar properties, minor compositional differences |
| JIS | SKD6 | Japan | Comparable grade with slight variations |
The differences between these equivalent grades can affect performance in specific applications. For instance, while H21 and SKD6 may exhibit similar hardness, their toughness and wear resistance can vary due to differences in alloying elements and heat treatment processes.
Key Properties
Chemical Composition
| Element (Symbol and Name) | Percentage Range (%) |
|---|---|
| C (Carbon) | 0.30 - 0.50 |
| Cr (Chromium) | 4.00 - 5.00 |
| Mo (Molybdenum) | 1.00 - 1.50 |
| W (Tungsten) | 11.00 - 13.00 |
| V (Vanadium) | 0.50 - 1.00 |
The primary role of tungsten in H21 Tool Steel is to enhance hardness and wear resistance, particularly at elevated temperatures. Molybdenum contributes to strength and toughness, while chromium improves corrosion resistance and hardenability. Vanadium aids in refining the grain structure, which enhances toughness.
Mechanical Properties
| Property | Condition/Temper | Test Temperature | Typical Value/Range (Metric) | Typical Value/Range (Imperial) | Reference Standard for Test Method |
|---|---|---|---|---|---|
| Tensile Strength | Quenched & Tempered | Room Temp | 1800 - 2200 MPa | 261 - 319 ksi | ASTM E8 |
| Yield Strength (0.2% offset) | Quenched & Tempered | Room Temp | 1500 - 1900 MPa | 217 - 276 ksi | ASTM E8 |
| Elongation | Quenched & Tempered | Room Temp | 5 - 10% | 5 - 10% | ASTM E8 |
| Hardness | Quenched & Tempered | Room Temp | 58 - 62 HRC | 58 - 62 HRC | ASTM E18 |
| Impact Strength | Quenched & Tempered | -20 °C | 20 - 30 J | 15 - 22 ft-lbf | ASTM E23 |
The combination of high tensile and yield strength, along with significant hardness, makes H21 Tool Steel suitable for applications involving high mechanical loads and wear, such as cutting tools and dies. Its impact strength ensures that it can withstand sudden shocks without fracturing.
Physical Properties
| Property | Condition/Temperature | Value (Metric) | Value (Imperial) |
|---|---|---|---|
| Density | Room Temp | 7.85 g/cm³ | 0.284 lb/in³ |
| Melting Point/Range | - | 1425 - 1450 °C | 2600 - 2642 °F |
| Thermal Conductivity | Room Temp | 25 W/m·K | 14.5 BTU·in/h·ft²·°F |
| Specific Heat Capacity | Room Temp | 460 J/kg·K | 0.11 BTU/lb·°F |
| Electrical Resistivity | Room Temp | 0.0005 Ω·m | 0.0003 Ω·in |
The density of H21 Tool Steel contributes to its overall weight and stability in applications. Its high melting point allows it to maintain structural integrity under extreme thermal conditions, while thermal conductivity is crucial for heat dissipation during machining processes.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C) | Resistance Rating | Notes |
|---|---|---|---|---|
| Chlorides | 5% | 25 °C | Fair | Risk of pitting corrosion |
| Sulfuric Acid | 10% | 60 °C | Poor | Not recommended |
| Sodium Hydroxide | 50% | 25 °C | Good | Moderate resistance |
H21 Tool Steel exhibits fair resistance to chlorides, which can lead to pitting, particularly in marine environments. Its performance in acidic conditions is poor, making it unsuitable for applications involving strong acids. Compared to other tool steels, such as D2 or M2, H21's corrosion resistance is generally lower, necessitating protective coatings or treatments in corrosive environments.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 540 °C | 1000 °F | Suitable for high-temperature applications |
| Max Intermittent Service Temp | 600 °C | 1112 °F | Short-term exposure only |
| Scaling Temperature | 700 °C | 1292 °F | Risk of oxidation beyond this point |
H21 Tool Steel maintains its hardness and strength at elevated temperatures, making it ideal for applications such as hot work tooling. However, care must be taken to avoid prolonged exposure to temperatures above 600 °C, as this can lead to oxidation and degradation of the material.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| MIG | ER70S-6 | Argon/CO2 mix | Preheat recommended |
| TIG | ER70S-2 | Argon | Requires post-weld heat treatment |
H21 Tool Steel can be welded, but it requires careful consideration of the welding process and filler materials. Preheating is often necessary to prevent cracking, and post-weld heat treatment is recommended to restore mechanical properties.
Machinability
| Machining Parameter | H21 Tool Steel | AISI 1212 | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 50% | 100% | More challenging to machine |
| Typical Cutting Speed | 20 m/min | 30 m/min | Use carbide tools for best results |
H21 Tool Steel is more difficult to machine than lower alloy steels due to its hardness. Using high-speed steel or carbide tools and optimizing cutting speeds can improve machinability.
Formability
H21 Tool Steel is not particularly formable due to its high hardness and strength. Cold forming is generally not recommended, while hot forming can be performed with care to avoid cracking. The minimum bend radius should be considered during fabrication.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Annealing | 800 - 850 °C / 1472 - 1562 °F | 1 - 2 hours | Air | Reduce hardness, improve machinability |
| Hardening | 1000 - 1100 °C / 1832 - 2012 °F | 30 - 60 minutes | Oil | Achieve high hardness |
| Tempering | 500 - 600 °C / 932 - 1112 °F | 1 - 2 hours | Air | Reduce brittleness, enhance toughness |
The heat treatment processes significantly affect the microstructure of H21 Tool Steel. Hardening transforms the steel into a martensitic structure, while tempering reduces residual stresses and brittleness, enhancing toughness.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection |
|---|---|---|---|
| Aerospace | Cutting tools | High hardness, wear resistance | Precision and durability |
| Automotive | Molds for plastic injection | Toughness, heat resistance | High-volume production |
| Manufacturing | Dies for stamping | Wear resistance, impact strength | Long tool life |
Other applications include:
- Metal forming tools
- Cold work tooling
- High-speed cutting tools
H21 Tool Steel is chosen for these applications due to its ability to maintain hardness and toughness under demanding conditions, ensuring longevity and reliability.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | H21 Tool Steel | AISI D2 | M2 High-Speed Steel | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Hardness | High | High | Very High | H21 offers a balance of toughness and hardness |
| Corrosion Resistance | Fair | Good | Fair | D2 has better corrosion resistance |
| Weldability | Moderate | Poor | Poor | H21 can be welded with care |
| Machinability | Moderate | Good | Poor | D2 is easier to machine than H21 |
| Approx. Relative Cost | Moderate | Moderate | High | H21 is cost-effective for high-performance tools |
| Typical Availability | Common | Common | Less common | H21 is widely available in tool steel markets |
When selecting H21 Tool Steel, considerations include its mechanical properties, cost-effectiveness, and availability. While it offers excellent performance in high-temperature applications, its machinability and weldability require careful handling. The choice between H21 and alternative grades like D2 or M2 depends on specific application requirements, including wear resistance, toughness, and corrosion resistance.
In summary, H21 Tool Steel is a versatile material that excels in demanding applications, making it a preferred choice for toolmakers and manufacturers seeking durability and performance.