L6 tool steel is a low-alloy hot work tool steel valued for its excellent toughness and good hardenability. It has a long history of use and remains a solid choice for specific tooling and component needs where impact resistance and strength are critical.
1. Applications
- Blanking Dies
- Punches
- Press-Brake Dies
- Cold Rolling Rolls
- Shear Blades
- Forming & Drawing Dies
- Massive Forging Dies
Machine part applications for L6 tool steel include:
- Arbors
- Cams and Cam Followers
- Chucks and Collets
- Spindles
- Gears and Pinions
- Clutch Parts and Ratchets
- Jigs and Drift Pins
2. L6 steel Composition
- Carbon (C): 0.65-0.75%
- Manganese (Mn): 0.25-0.80%
- Silicon (Si): 0.25%
- Chromium (Cr): 0.60-1.20%
- Nickel (Ni): 1.25-2.00%
- Molybdenum (Mo): Up to 0.50% (in some variants)
[Reference: Davis, J. R. (Ed.). (1998). Metals Handbook Desk Edition (2nd ed). ASM International.]
3. L6 Tool Steel Properties
L6 tool steel is categorized as a low-alloy special-purpose tool steel, belonging to the “L” series in the American Iron and Steel Institute (AISI) classification system. Its specific UNS (Unified Numbering System) designation is T61206.
3.1 Mechanical Properties
- Hardness
It has a flexible range of hardness depending on its condition and treatment:
The hardness range in the annealed state is between 183 and 212 HB, with good machinability. After heat treatment (working hardness), the hardness is 45-62 HRC, and after quenching, it can reach a hardness of approximately 64 HRC. The hardness for applications such as woodworking saws is 45-50 HRC, and the hardness of general-purpose tools after low-temperature tempering is 58-62 HRC.
- Toughness
L6 material has better toughness compared to many high-carbon oil-hardening grades. This high toughness makes it an excellent choice for tools and components subjected to shock loading or impact. While tensile ductility might be lower at lower tempering temperatures, it significantly improves when tempered above 315°C (600°F). L6 also has excellent torsional ductility after appropriate heat treatment.
- Wear Resistance
The wear resistance is moderate, lower than that of other tool steels, such as O1 or W1.
- Hot Hardness and Resistance to Softening:
L6 tool steel has low hot hardness and low resistance to softening when tempered. Consequently, it is generally not recommended for applications involving service at elevated temperatures. For high-heat environments, dedicated hot-work steels would be more suitable.
3.2 Machinability
The machinability is medium. The presence of nickel, while beneficial for toughness, makes it slightly less machinable than comparable grades that do not contain nickel, such as L2 tool steel.
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4. Heat Treatment
L6 tool steel heat treatment process involves four primary steps: preheating, austenitizing (hardening), quenching, and tempering.
4.1 Preheating
The preheating temperature for L6 tool steel is 1200°F (650°C), with a holding time of 10 to 15 minutes, or until the material is uniformly heated. Avoid prolonged soaking times, as this can disrupt molecular structure and cause adverse effects in subsequent processes. For large or complex L6 components, it is recommended to heat slowly within the range of 650–760°C (1200–1400°F). If placing the material directly into a furnace already heated to the preheating temperature, it is advisable to first position the material above the furnace to remove chill.
4.2 Hardening (Austenitizing)
The hardening temperature is 1525°F (830°C). Some sources list it as 790–870°C (1450–1600°F) or 790–845°C (1454–1553°F). This process is influenced by both time and temperature. The soaking time should be sufficiently long to ensure the workpiece is fully heated. During this process, the steel’s microstructure transitions from ferrite-pearlite to austenite, and the alloy carbides dissolve.
4.3 Quenching
The objective of quenching is to cool the austenite rapidly enough to transform it into a hardened martensitic structure. L6 tool steel is primarily an oil-hardened steel. It has good hardenability and achieves the required hardness through oil quenching. After quenching, parts should be cooled to a warm temperature of 125-150°F (52-65°C).
4.4 Tempering
When the part temperature drops to 125-150°F (52-65°C), tempering can be performed. Tempering reduces internal stress, improves the toughness of steel, and adjusts its hardness. L6 is often tempered once, but multiple tempering is recommended to reduce residual austenite and relieve stress. The holding time is 2 hours per inch (25mm) of cross-section.
The resulting hardness can be controlled by varying the tempering temperature:
5. How L6 tool steel compares to Other Steels
Selecting the right steel is crucial. Here’s how L6 compares:
- vs. O1 Tool Steel: L6 offers significantly higher toughness but exhibits lower abrasion resistance compared to O1.
- vs. L2 Tool Steel: L6 generally offers better hardenability due to its nickel content.
- vs. P-Type Mold Steels: For plastic injection molds, specialized P-type steels are usually the preferred choice.
- vs. 4140/4150 Alloy Steels: While 4140/4150 are sometimes used for simpler fixtures or short-run dies, L6 is a dedicated tool steel grade that offers superior performance for more demanding tooling.
- vs. 1095: L6 steel is an alloy tool steel engineered for a balance of hardness, toughness, and wear resistance due to its nickel, chromium, and molybdenum content, making it suitable for various dies and cutting tools. 1095 is a simpler, high-carbon plain carbon steel, primarily relying on its carbon for hardness, and may be superseded by alloy steels for more demanding applications.
6. Equivalent grades
7. Supply forms and dimensions
The L6 tool steel we supply is available in various shapes, including round bars, sheet plates, slabs, flat bars, square bars, and blocks. The dimensions of the flat bar range from: width 20–600 mm × thickness 20–400 mm × length 1,000–5,500 mm. The dimensions of the round bar range from a diameter of 20–400 mm × a length of 1,000–5,500 mm. The block dimensions are obtained by cutting the flat bar.
UT testing: Sep 1921-84 D/d, E/e.
Surface Treatment: original black, peeled, machined/turned, polished, grounded, or milled surface finishes.
Inventory Status: We do not maintain a stock of L6 steel. We arrange production based on customer orders.
Delivery time: Electric Arc Furnace (EAF) materials are 30-45 days.
FAQ
1. What is L6 tool steel?
L6 tool steel is a versatile, oil-hardening tool steel classified as an L-type special-purpose tool steel under the ASTM A681 standard. It is known for its good toughness and is suitable for tools, dies, and machine parts that require a combination of hardness and toughness. It can also be referred to as L6 Bainite Steel, noted for its structure that balances superior toughness and wear resistance.
2. What are the key characteristics of L6 tool steel?
L6 tool steel’s key characteristics include good toughness and high shock-resistance due to its lower carbon content and relatively high nickel content. It also possesses remarkable wear resistance as a result of its elevated carbon levels and alloying elements.
3. Is L6 tool steel considered a stainless steel?
No, L6 tool steel is not stainless steel. Stainless steel requires a minimum chromium content of 10.5% for corrosion resistance, while L6 tool steel contains between 0.8% and 1.2% chromium.
4. Is L6 tool steel corrosion resistant?
L6 tool steel is not inherently corrosion resistant and will corrode when exposed to corrosive environments or moisture. Protective coatings or surface treatments can be applied to mitigate corrosion.
6. What is the machinability of L6 tool steel?
The machinability of L6 tool steel is considered very good. It rates at 90% compared to the W-group water-hardening low-alloy steels, which are rated at 100% as a baseline for machinability. Other sources indicate it is 75% of a 1% carbon steel.
7. What is the typical working hardness of L6 tool steel?
The typical working hardness for L6 tool steel ranges from 40 HRC to 54 HRC. When supplied, its delivery condition is a maximum of 250 HB. Pre-hardened conditions are often between 360 to 430 BHN, or 50-55 HRC and 38-42 HRC after hardening and tempering.
8. What are the common applications of L6 tool steel?
L6 tool steel is commonly used for general-purpose tools and dies that demand high toughness, sometimes at the expense of some abrasion resistance. Typical applications include spindles, forming rolls, punches, blanking and forming dies, trimmer dies, clutch parts, pawls, bearings, chuck parts, rollers, knuckle pins, clutch pins, and shear blades. It is also found in springs, cold forming tools, and coining dies.
9. Is L6 tool steel suitable for knife and sword making?
Yes, L6 tool steel is suitable for knife making due to its high carbon content, toughness, decent wear resistance, and its ability to hold an edge well while being relatively easy to sharpen. For swords, particularly Katana blades, L6 Bainite steel is considered ideal because of its exceptional combination of strength, flexibility, and shock absorption, providing a tough blade body and a very hard edge. However, as it is not stainless, knives made from L6 require maintenance to prevent corrosion.
10. What quenching methods are used for L6 tool steel?
L6 tool steel is typically oil quenched to 66-51°C (150-125°F). For cross-sections under 3 inches (76.2mm), a blast of air or pressurized gas can achieve moderate hardness. Air quenching is often preferred for intricate sections due to its safer hardening process and less distortion compared to oil quenching.
11. How is L6 tool steel tempered, and what are its benefits?
L6 steels should be tempered immediately after quenching. It is held at the tempering temperature for 1 hour per inch of thickness (with a minimum of 4 hours), then air-cooled. A key benefit is that L6 steel does not become brittle when tempered within the range of 232°C to 426°C (450 to 800°F). Higher tempering temperatures can be used to increase toughness, if some hardness can be sacrificed.
12. What is the annealing process for L6 tool steel?
Annealing L6 steel must be performed after hot working and before re-hardening. The steel is heated to 760°C (1400°F) and held for one hour per inch of maximum thickness (minimum 2 hours). It is then slowly cooled in the furnace at a rate not exceeding 28°C (50°F) per hour down to 538°C (1000°F), continuing to cool to ambient temperature in the furnace or air. The resulting maximum hardness should be 235 HBW.
13. Can L6 tool steel be normalized?
No, L6 tool steel should not be normalized. This is due to its inherent air-hardening ability.
14. Is L6 tool steel weldable?
Yes, L6 tool steel is generally weldable. Proper preparation, including cleaning the surface, preheating, using suitable fillers, and selecting the appropriate welding process, is crucial. A post-weld temper is necessary to relieve stresses and prevent issues such as cracking or loss of properties.
15. How is L6 tool steel forged?
L6 tool steel is forged by slowly and uniformly heating the workpiece to 982-1038°C (1800-1900°F). Forging should proceed down to 871°C (1600°F) and not below 843°C (1550°F). After forging, parts should be cooled slowly in materials like lime, dry ash, or in a furnace.
16. What is cryogenic treatment for L6 tool steel, and what are its benefits?
Cryogenic treatment for L6 tool steel involves cooling the material to very low temperatures. It can be performed as an extension of the quench from the austenitizing treatment or after tempering. This process can improve hardness, increase wear resistance and stability, and enhance toughness, potentially leading to a longer service life for tools.
17. Can L6 tool steel be surface treated, and how?
Yes, L6 tool steel can undergo various surface treatments. Nitriding introduces nitrogen to enhance surface hardness and corrosion resistance. Carburizing increases carbon content on the surface for improved wear resistance and hardness. Black oxide (bluing) coating provides an aesthetic finish that reduces light reflection. Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) deposit thin layers to boost wear resistance and reduce friction. Additionally, shot peening can strengthen the surface to prevent fatigue and optimize component form.
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