SAE 1120 is a low-carbon, resulfurized, and rephosphorized free-machining steel grade that occupies a specific niche in the world of CNC machining and precision manufacturing. While it is less common than its more popular siblings like SAE 12L14 or SAE 1215, SAE 1120 offers a distinct combination of machinability, mechanical strength, and cost-effectiveness that makes it a valuable choice for certain high-volume production components. Understanding the nuances of this grade—from its precise chemical composition to its response to various machining operations—is essential for engineers and procurement specialists looking to optimize both part performance and manufacturing efficiency. This comprehensive guide explores every facet of SAE 1120, providing the technical depth needed to make informed material selection decisions for your next CNC machining project.
Chemical Composition of SAE 1120
The designation “1120” follows the SAE (Society of Automotive Engineers) system for carbon steels, where the first two digits indicate the alloying family and the last two digits represent the nominal carbon content in hundredths of a percent. For SAE 1120, the “11” series denotes resulfurized and rephosphorized free-machining steels, while “20” indicates a nominal carbon content of 0.20%. This places SAE 1120 in a unique position: it offers the machinability benefits of the 11-series while maintaining a slightly higher carbon content than many other free-machining grades, which translates into improved mechanical properties.
Elemental Breakdown and Typical Ranges
The chemical composition of SAE 1120 is carefully balanced to achieve its characteristic free-machining properties. The key elements and their typical ranges are presented in the table below. These values represent standard industry specifications and may vary slightly depending on the producer and specific mill certifications. Understanding these elemental contributions is critical for predicting how the material will behave during both machining and service.
| 要素 | 組成範囲（%） | 合金における役割 |
|---|---|---|
| 炭素（C） | 0.18 – 0.23 | Provides core strength and hardness; defines the steel’s response to heat treatment |
| マンガン（Mn） | 0.70 – 1.00 | Improves hardenability, tensile strength, and acts as a deoxidizer |
| リン（P） | 0.04 – 0.09 | Enhances machinability by increasing chip brittleness; also increases strength |
| 硫黄（S） | 0.16 – 0.23 | Primary machinability enhancer; forms manganese sulfide inclusions that act as chip breakers |
| シリコン（Si） | 0.10 – 0.30 | Deoxidizer; contributes to strength and hardness |
| 鉄（Fe） | バランス | 母材金属 |
Typical values per SAE J403 and ASTM A29/A29M standards.
The manganese content in SAE 1120 deserves particular attention. At 0.70–1.00%, manganese serves a dual purpose. First, it acts as a deoxidizer during steelmaking, helping to remove dissolved oxygen that would otherwise create porosity and brittleness. Second, and more importantly for machinability, manganese combines with sulfur to form manganese sulfide (MnS) inclusions. The ratio of manganese to sulfur is carefully controlled—typically around 4:1 to 5:1—to ensure that virtually all sulfur is bound as MnS rather than existing as free sulfur, which would cause hot shortness (brittleness at elevated temperatures) during hot working operations.
How Sulfur and Phosphorus Influence Machinability
The elevated sulfur content in SAE 1120 is the defining characteristic of this grade. Sulfur combines with manganese to form manganese sulfide (MnS) inclusions. During machining, these inclusions act as internal stress concentrators that cause the chip to break more readily, producing short, manageable chips rather than long, stringy ones that can tangle around the tool and workpiece. This results in improved tool life, better surface finish, and higher achievable cutting speeds. The morphology of the MnS inclusions is also important; elongated, stringer-type inclusions aligned with the rolling direction provide the most effective chip-breaking action, while globular inclusions are less effective but improve transverse mechanical properties.
Phosphorus, while present in smaller quantities, serves a complementary role. It strengthens the ferrite phase of the steel through solid solution strengthening, which helps improve the overall mechanical properties without significantly compromising machinability. Phosphorus also increases the hardness and brittleness of the ferrite, which further promotes chip fracture during machining. The combination of sulfur and phosphorus creates a steel that is notably easier to machine than plain carbon steels of similar carbon content, such as SAE 1020. However, it is worth noting that the same phosphorus that aids machinability also contributes to a phenomenon known as “blue brittleness” in the 200–300°C temperature range, which can affect machining if cutting temperatures are not properly controlled.
機械的・物理的特性
SAE 1120 delivers mechanical properties that are modest but entirely adequate for a wide range of non-critical structural and mechanical components. Its strength is lower than medium-carbon steels but higher than very low-carbon grades, making it a balanced choice for applications where moderate load-bearing capacity is required alongside excellent machinability. The material’s response to cold working and heat treatment further expands its utility, as discussed below.
Mechanical Properties in the As-Rolled and Cold-Drawn Conditions
The mechanical properties of SAE 1120 vary depending on the condition in which it is supplied. The two most common conditions for CNC machining are hot-rolled and cold-drawn. Cold-drawn material, in particular, offers improved dimensional accuracy, better surface finish, and increased strength due to strain hardening. The table below summarizes the typical mechanical properties for both conditions, along with the practical implications for component design.
| 特性 | Hot-Rolled (Typical) | Cold-Drawn (Typical) |
|---|---|---|
| 引張強度（MPa） | 450 – 550 | 520 – 620 |
| 降伏強度（MPa） | 280 – 350 | 350 – 420 |
| 50 mm における伸び率（%） | 20 – 25 | 15 – 20 |
| 断面収縮率（%） | 40 – 50 | 35 – 45 |
| 硬度（HB） | 130 – 160 | 150 – 180 |
| 弾性係数（GPa） | 190 – 210 | 190 – 210 |
Typical values; actual properties depend on bar size, processing history, and heat treatment.
It is important to understand that the cold-drawn condition offers a significant strength advantage—approximately 15-20% higher tensile and yield strength compared to hot-rolled material—at the cost of reduced ductility. For CNC machining applications, cold-drawn bar stock is often preferred because its tighter dimensional tolerances reduce the amount of material that must be removed to achieve final part dimensions, and its improved surface finish can eliminate the need for a roughing pass in some operations. However, the residual stresses introduced by cold drawing can cause distortion when material is removed asymmetrically, so machinists should consider stress-relieving operations for parts with complex geometries or tight tolerances.
Physical Properties and Thermal Characteristics
The physical properties of SAE 1120 are similar to other plain carbon and resulfurized steels. Its density is approximately 7.87 g/cm³ (0.284 lb/in³). The coefficient of thermal expansion is around 11.7 µm/m·°C (6.5 µin/in·°F) in the range of 20–100°C, which is important to consider when designing parts that will experience temperature fluctuations or when holding tight tolerances during machining. This thermal expansion behavior is particularly relevant for precision components that must maintain dimensional accuracy across a range of operating temperatures, such as those found in precision assemblies.
The thermal conductivity is approximately 51.9 W/m·K at room temperature, which is typical for low-carbon steels and facilitates heat dissipation during cutting operations. Good thermal conductivity is beneficial in machining because it helps carry heat away from the cutting zone, reducing tool tip temperatures and extending tool life. However, the presence of MnS inclusions creates slight discontinuities in the thermal path, resulting in marginally lower thermal conductivity compared to clean steels of similar composition.
The material’s melting point is approximately 1480–1520°C (2696–2768°F). SAE 1120 is magnetic, which is a consideration for any application involving electromagnetic interference or where magnetic properties are undesirable. It is also important to note that the manganese sulfide inclusions, while beneficial for machinability, can slightly reduce the material’s transverse ductility and impact toughness compared to a clean steel of equivalent carbon content. This anisotropy means that the material’s properties are directionally dependent—longitudinal properties (aligned with the rolling direction) are typically 10-15% better than transverse properties. Designers should account for this when orienting parts within the raw material.
主要な特性と利点
SAE 1120 is selected for manufacturing primarily because of its exceptional machinability, which translates directly into lower production costs and higher throughput. However, it also possesses other characteristics that make it suitable for specific applications, including its response to case hardening and its favorable cost structure for high-volume production.
Superior Machinability and Chip Control
The primary advantage of SAE 1120 is its excellent machinability rating. Compared to a baseline of 100% for a standard AISI B1112 steel, SAE 1120 typically has a machinability rating of around 70-80%. This is significantly higher than plain carbon steels like SAE 1020, which often rate below 60%. The practical implications are substantial: faster cutting speeds, longer tool life, reduced machine downtime for tool changes, and better surface finishes. For high-volume production runs, these factors can lead to cost savings of 20-30% or more compared to machining a non-free-machining grade. To put this in perspective, consider a typical CNC turning operation producing 10,000 identical parts. With SAE 1120, a machinist might achieve a cutting speed of 200 m/min and a tool life of 30 minutes per edge. With SAE 1020, the cutting speed might need to be reduced to 120 m/min, and tool life might drop to 15 minutes per edge. The result is a cycle time increase of roughly 40% and a tooling cost increase of 100%—a dramatic difference in total manufacturing cost.
The MnS inclusions also contribute to superior chip control. In CNC turning operations, this means fewer issues with chip entanglement and better chip evacuation, which is critical for automated machining centers and lights-out manufacturing. The short, broken chips produced by SAE 1120 are also safer for operators and easier to handle in chip management systems. For Swiss-type CNC lathes and multi-spindle screw machines, where chip control is paramount to prevent machine stoppages, SAE 1120’s chip-breaking characteristics are particularly valuable.
費用対効果と入手可能性
SAE 1120 is a commodity steel grade, which means it is widely available in various forms, including round bar, hex bar, square bar, and flat stock. Its cost is only marginally higher than plain carbon steel grades due to the addition of sulfur and phosphorus. This makes it an economical choice for high-volume parts where material cost is a significant factor. Furthermore, because it is not a specialty alloy, lead times from suppliers are typically short, allowing for flexible scheduling in manufacturing operations. Most major steel service centers stock SAE 1120 in common diameters ranging from 1/8 inch to 6 inches, and larger quantities can be sourced directly from mills with minimal lead time.
When evaluating the total cost of ownership for a machined component, material cost is only one factor. The machinability advantages of SAE 1120 often result in lower total manufacturing cost even when the raw material price per pound is slightly higher than a plain carbon steel. This is because machining labor, machine time, and tooling costs frequently dominate the total part cost, especially for complex or high-precision components. For parts with significant material removal, the cost savings from faster machining and longer tool life can easily offset a 5-10% premium in material cost.
Weldability and Formability Considerations
While SAE 1120 is primarily intended for machining, it can be welded and formed if necessary. However, the high sulfur content reduces weldability compared to plain carbon steels. The sulfur can cause porosity and hot cracking in weldments. If welding is required, low-hydrogen welding processes and preheating are often recommended. Specifically, preheating to 150–200°C (300–400°F) and using low-hydrogen electrodes (E7018 or equivalent) can help mitigate the risk of cracking. Post-weld heat treatment to relieve residual stresses is also advisable, particularly for load-bearing weldments. The use of austenitic stainless steel filler metals can help accommodate the sulfur content, as they are less susceptible to hot cracking.
Similarly, the material’s formability is somewhat reduced by the sulfide inclusions, making it less suitable for severe cold forming operations like deep drawing or severe bending. The MnS inclusions act as stress concentrators that can initiate cracks during severe deformation. For cold heading operations, such as those used to produce bolts and screws, the material can be used but with reduced reduction ratios compared to clean steels. For applications primarily requiring forming and welding rather than machining, a grade like SAE 1020 would be a better choice.
Typical Applications of SAE 1120
The combination of moderate strength and excellent machinability makes SAE 1120 ideal for a wide range of components that require intricate machining but do not demand high tensile or fatigue strength. It is a workhorse material for the production of fasteners, fittings, and various automotive and industrial parts. Its versatility extends across multiple industries, from automotive manufacturing to consumer goods production.
Fasteners and Hardware Components
SAE 1120 is commonly used to manufacture bolts, screws, nuts, and studs that are not subjected to high stress. The material’s machinability allows for the efficient production of threaded components with precise thread forms and good surface finish. It is particularly well-suited for producing custom fasteners on CNC lathes and screw machines. These fasteners are often used in non-critical applications where the cost of a higher-grade alloy steel cannot be justified. For example, SAE 1120 is frequently specified for set screws, thumb screws, and other hardware items where thread quality and consistency are important but ultimate tensile strength is not a primary concern.
The material is also used for pipe fittings, hydraulic fittings, and other fluid-handling components. The machinability of SAE 1120 allows for the efficient production of complex internal geometries, such as tapered threads and sealing surfaces, which are critical for leak-free connections. When combined with appropriate surface treatments like zinc plating or black oxide, SAE 1120 fittings can provide adequate corrosion resistance for many industrial applications.
Automotive and Industrial Parts
In the automotive industry, SAE 1120 is used for components such as oil pump gears, small shafts, spacers, and various brackets. Its moderate strength is sufficient for these applications, and its machinability allows for high-volume production at competitive costs. In industrial machinery, it is used for similar non-critical parts, including bushings, collars, and simple gears. The material can also be case-hardened to improve surface wear resistance for applications like cam followers and small pins. When case-hardened to a depth of 0.5–1.0 mm, SAE 1120 components can achieve surface hardnesses of 58–62 HRC, making them suitable for applications that require wear resistance combined with a tough core.
For the production of precision shift knobs and similar consumer-oriented components, SAE 1120 offers an excellent balance of machinability and surface finish quality. The material’s ability to achieve fine surface finishes without secondary operations makes it cost-effective for parts that will be visible in the final product. Components such as 精密シフトノブ benefit from SAE 1120’s combination of machinability and the ability to accept decorative finishes like chrome plating or powder coating.
Shafts, Spindles, and Precision Turned Parts
Because of its free-machining properties, SAE 1120 is an excellent choice for long, slender turned parts like shafts and spindles. The short chip formation reduces the risk of vibration and chatter during turning operations, allowing for better dimensional accuracy and surface finish on parts with high length-to-diameter ratios. This makes it a popular choice for applications where precise, intricate turned parts are needed, such as in the production of components for 精密CNCカメラ部品. The material’s ability to be machined to tight tolerances and fine surface finishes is a key advantage in such applications.
For example, a camera barrel assembly might require a shaft with a diameter tolerance of ±0.005 mm and a surface finish of 0.4 µm Ra. SAE 1120, with its excellent machinability, can achieve these specifications in a single pass with appropriate cutting parameters, whereas a plain carbon steel might require multiple passes and additional finishing operations. The dimensional stability of SAE 1120 during machining is also advantageous—the material does not exhibit significant spring-back or distortion when material is removed, provided that cutting forces are kept within reasonable limits.
加工・製造上の留意点
To fully exploit the machinability benefits of SAE 1120, machinists must select appropriate tooling, cutting parameters, and processes. While the material is forgiving, following best practices will ensure optimal results. The following guidance is based on practical experience and industry standards, and should be adjusted based on specific machine capabilities and part requirements.
Recommended Cutting Tools and Parameters
SAE 1120 can be machined effectively with high-speed steel (HSS) tools, but carbide tools are recommended for high-volume production to maximize cutting speeds and tool life. For turning operations, carbide inserts with a sharp edge and a positive rake angle are ideal. A positive rake angle (typically 5–10°) reduces cutting forces and promotes clean shearing of the material, which is important for achieving good surface finishes. Coated carbide inserts, particularly those with titanium nitride (TiN) or titanium carbonitride (TiCN) coatings, can further extend tool life by reducing friction and heat buildup at the cutting edge.
The table below provides typical starting parameters for common machining operations. These values should be treated as starting points and adjusted based on the specific machine, tooling, and desired results.
| 作業工程 | 切削速度（m/min） | 送り速度（mm/回転） | 切り込み深さ（mm） |
|---|---|---|---|
| Turning (Carbide) | 150 – 250 | 0.15 – 0.30 | 1.0 – 4.0 |
| Turning (HSS) | 60 – 90 | 0.10 – 0.20 | 0.5 – 2.0 |
| Drilling (HSS) | 30 – 45 | 0.10 – 0.20 | N/A |
| Milling (Carbide) | 120 – 200 | 0.10 – 0.20 (mm/tooth) | 1.0 – 3.0 |
| Threading (Die) | 10 – 15 | N/A | N/A |
Starting parameters; adjust based on machine rigidity, tool geometry, and desired surface finish.
For drilling operations, it is important to use appropriate coolant delivery to flush chips and control temperature. Through-tool coolant is particularly effective when drilling deeper holes, as it ensures that cutting fluid reaches the cutting zone and helps evacuate chips. For tapping operations, spiral-flute taps are recommended for through-holes, while spiral-point taps are better suited for blind holes. The use of tapping fluid or a water-soluble cutting fluid at the correct concentration is essential to prevent tap breakage and achieve clean threads.
熱処理と表面仕上げ
SAE 1120 can be case-hardened through carburizing or carbonitriding to produce a hard, wear-resistant surface while maintaining a tough, ductile core. A typical case-hardening process involves carburizing at 870–925°C (1600–1700°F), followed by quenching and low-temperature tempering. This treatment can achieve surface hardnesses of 58-62 HRC, depending on the case depth and process parameters. The core hardness remains relatively low, typically in the range of 25-35 HRC, which provides toughness and impact resistance.
The case depth can be controlled by adjusting the carburizing time and temperature. A typical cycle might produce a case depth of 0.5–1.5 mm, with deeper cases requiring longer cycle times. For applications requiring a shallower case, carbonitriding at lower temperatures (820–870°C) can produce a case depth of 0.1–0.5 mm with good wear resistance. After case hardening, components should be tempered at 150–200°C to relieve quenching stresses and achieve the desired final hardness.
For surface finishing, SAE 1120 responds well to all conventional methods, including grinding, polishing, and plating. The material can be easily zinc-plated, nickel-plated, or painted to provide corrosion resistance and improve aesthetics. However, the machinist should be aware that the sulfide inclusions can sometimes cause slight surface tearing if the cutting parameters are too aggressive, so a final light pass or a polishing operation may be necessary for the finest surface finishes. When grinding SAE 1120, the use of a coolant is essential to prevent heat buildup, which can cause localized hardening and surface cracking.
関連鋼種との比較
To make an informed material selection, it is helpful to compare SAE 1120 with other common low-carbon and free-machining steel grades. The choice between these materials often comes down to a trade-off between machinability, mechanical properties, and cost. Understanding these trade-offs is essential for selecting the optimal material for a specific application.
SAE 1120 vs. SAE 1215 and SAE 12L14
SAE 1215 and SAE 12L14 are other popular free-machining grades. SAE 1215 has a similar carbon content but contains no phosphorus, resulting in slightly lower strength and hardness but even better machinability due to a more uniform distribution of sulfides. SAE 12L14 is a leaded version of 1215, offering the highest machinability of all carbon steels. However, leaded steels are subject to increasing environmental regulations, and some markets restrict their use in certain applications. SAE 1120 offers a compromise, providing better mechanical properties than 1215 and 12L14 while still offering excellent machinability. The table below summarizes these differences.
| グレード | Carbon (%) | Machinability Index | Tensile Strength (MPa)* | Key Advantage |
|---|---|---|---|---|
| SAE 1120 | 0.18 – 0.23 | 70 – 80 | 450 – 550 | Good strength with high machinability |
| SAE 1215 | 0.09 max | 80 – 90 | 400 – 480 | Superior machinability |
| SAE 12L14 | 最大0.15 | 90 – 100 | 420 – 520 | Best machinability (contains lead) |
*Typical values for hot-rolled condition.
The choice between these grades depends on the specific requirements of the application. If maximum machinability is the primary concern and environmental regulations permit the use of leaded steels, SAE 12L14 may be the best choice. If lead content is a concern but machinability is still paramount, SAE 1215 offers excellent performance. However, if the application requires higher strength or the ability to be case-hardened, SAE 1120 is the superior choice. Its higher carbon content also provides better response to heat treatment, allowing for surface hardening that is not practical with 1215 or 12L14.
SAE 1120 vs. Plain Carbon Steel SAE 1020
SAE 1020 is a general-purpose low-carbon steel without the sulfur and phosphorus additions. It has slightly lower strength and hardness than SAE 1120 but offers superior weldability and formability. The key trade-off is machinability: SAE 1120 will machine significantly faster and with better chip control, leading to lower production costs. For parts that are primarily machined, SAE 1120 is often the more economical choice despite the slightly higher cost per pound of material. For parts that require significant forming or welding, SAE 1020 is the safer option.
In practical terms, the machinability difference between SAE 1120 and SAE 1020 is substantial. A CNC lathe that can produce a SAE 1120 part in 45 seconds might require 70 seconds for the same part in SAE 1020, representing a 55% increase in cycle time. Over a production run of 50,000 parts, this translates to an additional 347 hours of machine time—a significant cost that far outweighs any material cost savings from using the cheaper grade. This is why SAE 1120 is often the preferred choice for high-volume machined components, even when the raw material cost is slightly higher.
Tuofa CNC: Your Partner for Machining SAE 1120 Components
At Tuofa CNC, we understand the unique properties of free-machining steels like SAE 1120 and have the expertise to turn them into high-quality, precision components. Our state-of-the-art CNC machining facilities are equipped to handle everything from small, intricate parts to large-scale production runs. With years of experience machining a wide range of steel grades, we have developed optimized processes that maximize the benefits of SAE 1120 while ensuring consistent part quality.
精密加工能力
Tuofa CNC Germany specializes in precision CNC turning, milling, and drilling. Our team of experienced engineers and machinists is adept at optimizing cutting parameters for SAE 1120 to maximize efficiency and part quality. We utilize advanced multi-axis CNC lathes and machining centers that can hold tight tolerances and produce complex geometries with excellent surface finishes. Whether you need simple turned pins or complex machined components, we have the capability to deliver. Our expertise extends to producing custom components that might be used in various assemblies, similar to the precision required for CNC machined mounting blocks.
Our machining capabilities include the ability to handle bar stock up to 200 mm in diameter and parts up to 1000 mm in length. We offer both single-spindle and multi-spindle CNC lathes, allowing us to efficiently produce both small batches and high-volume runs. For complex parts requiring multiple operations, our machining centers can perform milling, drilling, tapping, and boring in a single setup, reducing handling time and improving dimensional accuracy. We also offer Swiss-type CNC turning for small-diameter, high-precision components, which is particularly well-suited to SAE 1120’s excellent machinability.
品質保証とサポート
We are committed to delivering parts that meet the highest standards of quality. Our quality control processes include in-process inspection and final dimensional verification using precision measuring equipment, including coordinate measuring machines (CMMs) and surface profilometers. We provide full material traceability and can supply material certifications upon request. From the initial design review to final delivery, our team works closely with you to ensure your SAE 1120 components are manufactured to your exact specifications.
We also offer a range of secondary services, including heat treatment, surface finishing, and assembly. Our in-house heat treatment capabilities include carburizing and carbonitriding, allowing us to provide case-hardened SAE 1120 components with the required surface hardness and case depth. For surface finishing, we offer zinc plating, nickel plating, black oxide, and powder coating, among other options. For projects requiring a deep understanding of material performance, our engineers can provide guidance on material selection and design for manufacturability, ensuring you get the most cost-effective and reliable parts for your application. Whether you are developing a new product or optimizing an existing design, our team is ready to support you with technical expertise and responsive service.
結論
SAE 1120 is a versatile and cost-effective free-machining steel that offers an excellent balance of machinability and mechanical properties. Its resulfurized and rephosphorized composition ensures efficient chip control and long tool life, making it a top choice for high-volume production of fasteners, shafts, and various precision-machined components. While it cannot match the strength of medium-carbon alloys or the weldability of plain carbon steels, its advantages in CNC machining are clear. By understanding its composition, properties, and best machining practices, engineers can leverage SAE 1120 to reduce manufacturing costs and improve throughput. For your next project requiring efficient, high-quality machining of SAE 1120, Tuofa CNC offers the expertise and capabilities to bring your designs to life.