SAE 8720 is a low-carbon nickel-chromium-molybdenum alloy steel that belongs to the 8700 series of carburizing steels. It is specifically designed for case-hardening applications where a tough, wear-resistant surface must be combined with a strong, ductile core. This grade is widely used in the automotive, heavy equipment, and general engineering sectors for components that experience high contact stresses, impact loads, and fatigue. For CNC machining shops and design engineers, SAE 8720 offers an excellent balance of machinability in the annealed condition and exceptional mechanical performance after heat treatment. This article provides a comprehensive technical overview of SAE 8720, covering its chemical composition, mechanical properties, heat treatment practices, machining considerations, and typical applications, with practical guidance for precision manufacturing.
Chemical Composition of SAE 8720
The designation “8720” follows the AISI/SAE four-digit system. The “87” indicates the alloy family, which includes nickel, chromium, and molybdenum as primary alloying elements. The “20” indicates a nominal carbon content of 0.20%. This specific combination of alloying elements is what gives SAE 8720 its distinctive combination of case hardenability and core toughness.
The chemical composition is tightly controlled to ensure consistent response to heat treatment and predictable mechanical properties. Below is a table of the typical and specified ranges for SAE 8720.
Element Ranges and Their Roles
Carbon (C) is the primary hardening element. At approximately 0.20%, it provides sufficient carbon for a hardenable case after carburizing, while keeping the core tough and weldable. Nickel (Ni) is added to improve toughness, particularly in the core, and to increase hardenability. It also lowers the ductile-to-brittle transition temperature, which is beneficial for low-temperature applications. Chromium (Cr) enhances hardenability and contributes to wear resistance in the carburized case. Molybdenum (Mo) is a potent hardenability enhancer and also helps maintain strength at elevated temperatures, reducing temper embrittlement risks.
Typical Composition Table
| Element | Samenstellingsbereik (%) | Typische waarde (%) | Rol |
|---|---|---|---|
| Carbon (C) | 0.18 – 0.23 | 0.20 | Case hardenability, core strength |
| Manganese (Mn) | 0.70 – 0.90 | 0.80 | Hardenability, deoxidation |
| Phosphorus (P) | 0.035 max | 0.015 | Impurity – kept low |
| Sulfur (S) | maximaal 0,040 | 0.020 | Impurity – kept low |
| Silicon (Si) | 0.15 – 0.35 | 0.25 | Deoxidation, strength |
| Nickel (Ni) | 0.40 – 0.70 | 0.55 | Toughness, hardenability |
| Chromium (Cr) | 0.40 – 0.60 | 0.50 | Hardenability, wear resistance |
| Molybdenum (Mo) | 0.20 – 0.30 | 0.25 | Hardenability, temper resistance |
The combination of these elements results in a steel with a hardenability factor that is significantly higher than plain carbon steels like 1020. This allows larger sections to be through-hardened in the core and provides a more consistent case depth during carburizing.
Mechanische en fysische eigenschappen
SAE 8720 is almost always used in the heat-treated condition. As-supplied, it is typically annealed or normalized for machinability. After carburizing, quenching, and tempering, the material exhibits a hard, wear-resistant case and a tough core. The final properties depend heavily on the heat treatment parameters and the section size of the part.
Core Mechanical Properties (After Heat Treatment)
The core of SAE 8720 is designed to provide strength and toughness. After a typical quench and temper, the core achieves a tempered martensitic structure. The values below represent typical results for a 25 mm round bar, oil-quenched and tempered at 150°C.
| Property | Typical Value (Metric) | Typical Value (Imperial) |
|---|---|---|
| Tensile Strength (Ultimate) | 980 – 1180 MPa | 142 – 171 ksi |
| Vervormingssterkte (0,2%-offset) | 780 – 980 MPa | 113 – 142 ksi |
| Rek in 50 mm | 14 – 20% | 14 – 20% |
| Oppervlaktevermindering | 45 – 55% | 45 – 55% |
| Impact Toughness (Charpy V-notch, 20°C) | 55 – 80 J | 41 – 59 ft-lb |
| Hardness (Core) | 30 – 38 HRC | 30 – 38 HRC |
These are typical values, not guaranteed minimums. Actual properties vary with bar diameter, heat treatment specifics, and testing direction.
Case Properties After Carburizing
The carburized case is the defining feature of SAE 8720. After carburizing at 925°C and quenching, the case hardness typically reaches 58-62 HRC. The effective case depth is controllable and is usually specified by the application. A typical case depth for automotive gears is 0.8 – 1.2 mm, while for larger industrial gears it can be 2.0 – 3.0 mm.
The case microstructure is primarily martensite with fine carbides, providing high wear resistance and contact fatigue strength. The residual compressive stresses in the case, induced by the volume expansion during martensitic transformation, significantly improve bending and contact fatigue life.
Physical Properties
Physical properties are less dependent on heat treatment and are relevant for design calculations involving thermal expansion or mass.
| Property | Waarde |
|---|---|
| Density | 7.85 g/cm³ (0.284 lb/in³) |
| Modulus of Elasticity | 205 GPa (29,700 ksi) |
| Thermische geleidbaarheid (bij 100 °C) | 44.5 W/m·K |
| Specifieke warmtecapaciteit (bij 20°C) | 470 J/kg·K |
| Electrical Resistivity | 0.22 µΩ·m |
| Melting Point (Approximate) | 1420°C (2588°F) |
Heat Treatment Processes for SAE 8720
Proper heat treatment is essential to unlock the full potential of SAE 8720. The material is almost exclusively used in the carburized and hardened condition. Understanding the sequence of operations is critical for CNC machining planning, as the part must be machined to near-net shape before hardening, with only finishing operations (grinding, honing) after heat treatment.
Pre-Machining Annealing or Normalizing
As supplied by the mill, SAE 8720 is typically in the annealed or normalized condition. Annealing is performed to soften the steel for optimal machinability. A typical annealing cycle involves heating to 845-900°C, holding for sufficient time, and then cooling slowly in the furnace. The resulting hardness is typically below 200 HB, which allows for high-speed machining with good tool life.
Normalizing, which involves air cooling from the austenitizing temperature, results in a slightly higher hardness and a more uniform microstructure. It is often used as a preparation step before carburizing to refine the grain structure and ensure a more consistent case.
Carburizing Process
Carburizing is the process of diffusing carbon into the surface layer of the steel. For SAE 8720, gas carburizing is the most common method. The parts are heated to 900-950°C in an atmosphere rich in carbon (e.g., endothermic gas with methane or propane). The carbon diffuses into the austenite, creating a carbon gradient.
The depth of the case is controlled by time and temperature. A typical cycle for a 1.0 mm effective case depth might be 4-6 hours at 925°C. After carburizing, the parts are quenched. Direct quenching from the carburizing temperature is common, but some processes involve cooling, reheating, and then quenching to refine the core grain structure. A subsequent low-temperature temper at 150-200°C is always performed to relieve quenching stresses and temper the case martensite.
Post-Heat Treatment Operations
After quenching and tempering, the case hardness is very high (58-62 HRC), making conventional machining difficult. Therefore, any features that require tight tolerances or smooth surface finishes are finished by grinding, honing, or lapping. For CNC machining, this means the pre-heat-treatment operation must anticipate dimensional changes. Typically, parts grow slightly during hardening (on the order of 0.05-0.1%), and this must be accounted for in the machining allowances.
Machining SAE 8720: Best Practices
While SAE 8720 is not as free-machining as a resulfurized grade like 8620 with added sulfur, it is considered readily machinable in the annealed condition. Its machinability rating is approximately 65-70% of AISI 1112 (a free-machining standard), which is comparable to other low-carbon alloy steels.
Turning and Milling
In the annealed condition, SAE 8720 can be machined using standard high-speed steel (HSS) or, more commonly, carbide tooling. For turning, carbide inserts with a positive rake angle are recommended to reduce cutting forces and prevent built-up edge. Typical cutting parameters for carbide tooling include cutting speeds of 120-180 m/min (400-600 SFM) with feed rates of 0.2-0.4 mm/rev. For milling, similar speeds are used, with depth of cut dependent on the rigidity of the setup.
The material produces continuous, ductile chips. Chip breakers are essential to prevent long, stringy chips from tangling. Using a high-pressure coolant system helps with chip evacuation and improves surface finish.
Boren en tappen
Drilling SAE 8720 is straightforward. Standard HSS twist drills work well, but carbide drills offer higher productivity. For deep holes, peck drilling is recommended to break chips and prevent drill wandering. Tapping can be challenging due to the material’s toughness. Roll-form taps (thread forming) are often preferred over cutting taps because they produce stronger threads and are less prone to breakage. Adequate lubrication is critical.
Grinding and Finishing
Grinding is the primary finishing operation after heat treatment. The hard case (58-62 HRC) requires aluminum oxide or CBN (cubic boron nitride) grinding wheels. CBN wheels are particularly effective for high-production grinding of hardened steel. Surface finishes of 0.4 µm Ra or better are achievable with proper grinding parameters.
For CNC machining operations that must be performed after hardening, such as precision hole finishing, the process is limited to grinding or electrical discharge machining (EDM). This is a key consideration when designing parts for SAE 8720.
Comparison with Related Alloy Steels
SAE 8720 is part of a family of carburizing steels. It is often compared to its more common cousin, SAE 8620, and to higher-alloy grades like 8822 or 4320. Understanding these differences helps in material selection.
SAE 8720 vs. SAE 8620
SAE 8620 (0.20% C, 0.55% Ni, 0.50% Cr, 0.20% Mo) is the most widely used carburizing steel. The key difference is the slightly higher carbon content in 8720 (0.20% vs. 0.18-0.23% for 8620). This gives 8720 slightly higher core hardness and strength after heat treatment. The hardenability of the two grades is very similar. In practice, 8720 is often specified when a slightly higher core strength is required, while 8620 is preferred for its slightly better machinability and lower cost.
SAE 8720 vs. SAE 4320
SAE 4320 has a significantly higher nickel content (1.65-2.00%) and molybdenum (0.20-0.30%) compared to 8720. This gives 4320 superior core toughness and fatigue resistance, especially in larger sections. However, 4320 is more expensive and slightly more difficult to machine. SAE 8720 is often chosen when the cost premium of 4320 cannot be justified, and the component is not excessively large or highly stressed.
Selection Guide Table
| Kwaliteit | Kernsterkte | Kerntaaiheid | Bewerkbaarheid | Relatieve kosten | Typisch gebruik |
|---|---|---|---|---|---|
| SAE 8620 | Good | Good | Excellent | Low | General gears, shafts, cams |
| SAE 8720 | Beter | Good | Zeer goed | Low-Medium | Higher-strength gears, pinions, spindles |
| SAE 4320 | Best | Excellent | Good | High | Large, heavily loaded gears, aircraft parts |
Typical Applications of SAE 8720
SAE 8720 is chosen for components that need a hard, wear-resistant surface to resist sliding wear and contact fatigue, combined with a tough core to withstand impact and bending loads. The combination of nickel, chromium, and molybdenum provides a good balance of these properties at a moderate cost.
Automotive and Heavy-Duty Drivetrain
The most common application for SAE 8720 is in the drivetrain of vehicles and industrial machinery. Transmission gears, differential gears, pinions, and ring gears are frequently manufactured from this grade. The case provides the hardness needed to resist pitting and scuffing on the gear tooth flanks, while the tough core supports the tooth roots and resists tooth breakage under shock loads. It is also used for camshafts, crankshafts (in some applications), and universal joint crosses.
Industrial Machinery and Tooling
Beyond vehicles, SAE 8720 is used in a wide range of industrial components. This includes spindles, shafts, heavy-duty bearings, and rollers. It is also used for certain types of tooling, such as collets, arbors, and mandrels, where a hard wearing surface is required on a tough body. These components are often machined by CNC turning and milling centers before being sent for heat treatment. For components like precision shift knobs, which require a durable and aesthetically pleasing surface, the material’s ability to be carburized and polished is an advantage, although softer materials are often preferred for purely cosmetic parts. You can see examples of how precision machined components are crafted in our article on CNC-bewerkte schakelknoppen.
Other Engineering Applications
The material is also found in agricultural equipment, mining machinery, and material handling systems. Anywhere a rotating or sliding component experiences heavy wear and moderate to high impact loads is a candidate for SAE 8720. It is also used in the oil and gas industry for downhole tools and pump components. The material’s good fatigue strength makes it suitable for axles and spindles in various machinery.
CNC Machining Considerations and Design for Manufacturability
When designing parts for SAE 8720, it is crucial to consider the entire manufacturing process, from raw material to finished, heat-treated component. The CNC machining strategy must account for the material’s behavior in both the annealed and hardened states.
Pre-Machining Design Rules
First, always machine to near-net shape in the annealed condition. Leave a uniform grinding allowance of 0.3-0.5 mm per side on surfaces that require tight tolerances or a fine finish after hardening. Avoid sharp internal corners, as these can be stress concentrators and lead to quench cracking. Use generous fillet radii wherever possible. Also, consider the hardenability of the section. Very thick sections may not achieve full core hardness, while very thin sections may become fully hard, making them brittle. This is similar to the design considerations for other alloy steels, such as those discussed in our guide on soorten ijzermetalen.
Tooling and Process Optimization
For efficient machining of annealed SAE 8720, use modern CNC equipment with rigid setups. Carbide inserts with TiN or TiAlN coatings are recommended for their wear resistance and ability to handle the material’s ductility. Always use coolant to manage heat and improve chip control. For high-volume production, consider using specialized tooling designed for alloy steels.
When it comes to secondary operations, remember that the material will be very hard after heat treatment. Plan for grinding or EDM for any features that cannot be machined before hardening. Threads, for example, are almost always cut before hardening, and any damage to the threads during heat treatment must be avoided.
Kwaliteitscontrole en inspectie
Dimensional distortion is the biggest challenge in heat treating SAE 8720. The geometry of the part, the quenching method, and the fixturing all influence distortion. For precision components, it is common to perform a “pilot” heat treatment run on a sample part to measure distortion and adjust the pre-machining allowances accordingly. This is a standard practice in high-precision CNC machining environments.
Surface Treatments and Alternatives
While carburizing is the standard surface treatment for SAE 8720, other options exist depending on the application. Understanding these alternatives helps in selecting the right material and process.
Alternative Case Hardening Methods
Carbonitriding is a variation of carburizing where nitrogen is added to the atmosphere. This is done at a lower temperature (around 850°C) and produces a shallower, harder case with better tempering resistance. It is often used for smaller parts where a case depth of less than 0.5 mm is sufficient. Nitriding, which introduces nitrogen without carbon, is not typically used for SAE 8720, as it is more suited for steels with strong nitride-forming elements like aluminum.
Material Alternatives for Specific Needs
If a component requires higher core strength than SAE 8720 can provide, a through-hardening steel like SAE 4140 or 4340 might be considered, although they would not offer the same surface wear resistance. If the need is for maximum toughness in a large section, SAE 4320 is a better choice. For applications requiring corrosion resistance, a stainless steel like 17-4 PH would be used, although it cannot be carburized in the same way.
Tuofa CNC: Precision Machining of SAE 8720 Components
At Tuofa CNC Germany, we have extensive experience machining SAE 8720 and other low-carbon alloy steels for a wide range of industries. Our CNC machining services are tailored to meet the demanding requirements of components that require subsequent case hardening.
Our Capabilities with Alloy Steels
We understand the nuances of machining SAE 8720 in the annealed condition. Our state-of-the-art CNC turning and milling centers are equipped to handle the material’s ductility, ensuring tight tolerances and excellent surface finishes before heat treatment. We work closely with our customers to determine the correct pre-machining allowances, taking into account the expected distortion during carburizing and quenching. Our goal is to deliver parts that are ready for final grinding with minimal material removal.
From Prototype to Production
Whether you need a single prototype for a new gear design or high-volume production of pinions and shafts, Tuofa CNC has the capacity and expertise to deliver. We offer a range of services, including complex 5-axis machining, deep hole drilling, and precision grinding. Our quality assurance team uses advanced metrology equipment to verify dimensions and surface finish, ensuring that your SAE 8720 components meet the most stringent specifications. We also provide guidance on material selection and heat treatment, helping you optimize your design for manufacturability and performance. For more insights into how we manage precision projects, you can read about our approach to sourcing and manufacturing in our article on sourcing manufacturers in Mexico.
Conclusion
SAE 8720 is a versatile and reliable carburizing steel that offers an excellent balance of core toughness, surface hardness, and fatigue resistance. Its moderate cost and good machinability make it a popular choice for a wide array of drivetrain and industrial components. While it requires careful planning regarding heat treatment distortion and post-hardening finishing operations, the performance benefits are substantial. By understanding its composition, properties, and machining requirements, engineers and manufacturers can effectively leverage SAE 8720 to produce durable, high-performance parts. For precision CNC machining of SAE 8720 components, partnering with an experienced shop like Tuofa CNC ensures that the material’s potential is fully realized, from the first cut to the final inspection.