If you need an alloy steel that excels in carburizing—perfect for high-wear, precision parts like automotive gears or machinery shafts—JIS SCM420 alloy steel is a top choice. As a Japanese standard chromium-molybdenum alloy, it balances great hardenability, toughness, and machinability. This guide breaks down everything you need to know, from its chemical makeup to real-world uses, to help you use it effectively.
1. Material Properties of JIS SCM420 Alloy Steel
JIS SCM420’s performance comes from its well-balanced properties, all compliant with Japan’s JIS G4105 standard. Let’s explore each key aspect clearly.
1.1 Chemical Composition
The elements in JIS SCM420 work together to boost carburizing performance, strength, and durability. Below is the standard composition range:
| Element | Symbol | Composition Range (%) | Key Role in the Alloy |
|---|---|---|---|
| Carbon (C) | C | 0.18 – 0.23 | Low carbon content, ideal for carburizing (forms a hard outer layer while keeping the core tough) |
| Chromium (Cr) | Cr | 0.90 – 1.20 | Enhances hardenability and wear resistance; improves carburizing depth and uniformity |
| Molybdenum (Mo) | Mo | 0.15 – 0.25 | Increases fatigue strength and high-temperature stability; prevents brittleness after carburizing |
| Manganese (Mn) | Mn | 0.60 – 0.85 | Boosts machinability and grain refinement; strengthens the alloy without losing ductility |
| Silicon (Si) | Si | 0.15 – 0.35 | Acts as a deoxidizer during steelmaking; enhances overall strength |
| Nickel (Ni) | Ni | ≤ 0.25 | Trace amounts slightly improve impact toughness without raising costs |
| Sulfur (S) | S | ≤ 0.030 | Kept low to avoid cracking in carburized parts and high-stress applications |
| Phosphorus (P) | P | ≤ 0.030 | Limited to prevent cold brittleness (fracture in low-temperature environments) |
| Vanadium (V) | V | ≤ 0.03 | Tiny amounts refine grain structure, ensuring uniform hardness across the part |
1.2 Physical Properties
These traits show how JIS SCM420 behaves in different working conditions—like temperature changes or magnetic fields:
- Density: 7.85 g/cm³ (same as most ferrous alloys, easy to integrate into existing designs)
- Melting point: 1420 – 1450°C (high enough for high-temperature applications like industrial rollers or engine parts)
- Thermal conductivity: 44 W/(m·K) at 20°C (retains heat well, suitable for parts that operate continuously)
- Specific heat capacity: 465 J/(kg·K) at 20°C (absorbs heat steadily, avoiding warping from sudden temperature swings)
- Thermal expansion coefficient: 12.2 μm/(m·K) (low expansion, critical for precision parts like gears)
- Magnetic properties: Ferromagnetic (attracts magnets, useful for tools like magnetic clamps or sensors)
1.3 Mechanical Properties
JIS SCM420’s full mechanical strength is unlocked after carburizing + quenching + tempering (its standard heat treatment). Below are typical values tested to Japanese standards:
| Property | Typical Value (Carburized) | Test Standard (JIS) |
|---|---|---|
| Tensile strength | ≥ 980 MPa | JIS Z2241 |
| Yield strength | ≥ 785 MPa | JIS Z2241 |
| Elongation | ≥ 12% | JIS Z2241 |
| Reduction of area | ≥ 45% | JIS Z2241 |
| Hardness (Brinell) | 240 – 300 HB | JIS Z2243 |
| Hardness (Rockwell C) | 25 – 32 HRC (core) / 58 – 62 HRC (surface) | JIS Z2245 |
| Hardness (Vickers) | 245 – 305 HV | JIS Z2244 |
| Impact toughness | ≥ 70 J | JIS Z2242 |
| Fatigue strength | ~530 MPa | JIS Z2273 |
1.4 Other Properties
- Corrosion resistance: Moderate (resists mild moisture and oils; use zinc plating or paint for outdoor/humid environments)
- Wear resistance: Excellent (thanks to carburizing and chromium (Cr); perfect for moving parts like gears or bearings)
- Machinability: Good (soft in annealed state; use high-speed steel (HSS) or carbide tools with cutting fluid to reduce wear)
- Weldability: Acceptable (preheat to 250 – 300°C and post-weld heat treat to avoid cracking; use low-hydrogen electrodes)
- Hardenability: Outstanding (carburizing penetrates deeply, creating a hard outer layer and tough core—ideal for high-wear parts)
2. Applications of JIS SCM420 Alloy Steel
JIS SCM420’s carburizing ability and toughness make it ideal for high-stress, high-wear applications. Here are its most common uses, with real-world examples:
2.1 Automotive Industry
Cars and trucks rely on its durability for key transmission parts:
- Gears: A Japanese automaker uses it for manual transmission gears—its carburized surface (HRC 58–62) gives wear resistance that extends gear life by 40% vs. carbon steel.
- Shafts: Compact SUVs use JIS SCM420 drive shafts; its fatigue strength (~530 MPa) handles constant torque without breaking.
- Axles: Commercial vans use it for rear axles—its impact toughness (≥70 J) prevents bending during heavy loading.
2.2 Mechanical Engineering
Industrial machinery benefits from its hardenability:
- Bearings: A manufacturing plant uses it for conveyor belt bearings—its carburized layer reduces maintenance downtime by 30%.
- Rollers: Printing machinery uses it for pressure rollers; its hardness (240–300 HB core) resists wear from paper friction.
- Bolts and fasteners: High-speed machine tools use it for critical bolts—its tensile strength (≥980 MPa) resists vibration loosening.
2.3 Heavy Machinery
For large-scale equipment, it provides reliable performance:
- Springs: Construction excavators use it for bucket springs; its elasticity (from tempering) withstands 10,000+ compression cycles.
- Structural components: Mining machinery uses it for crusher shafts—its hardenability ensures uniform strength in thick sections.
3. Manufacturing Techniques for JIS SCM420 Alloy Steel
To maximize JIS SCM420’s performance, follow these industry-proven manufacturing steps:
3.1 Steelmaking Processes
It’s typically produced using two methods:
- Electric Arc Furnace (EAF): Most common for medium batches. Scrap steel is melted with electrodes, then chromium (Cr) and molybdenum (Mo) are added to reach the target composition. EAF is flexible and reduces waste, ideal for custom parts like large gears.
- Basic Oxygen Furnace (BOF): Used for mass production. Molten iron is mixed with oxygen to remove impurities, then alloy elements are added. BOF is faster and cost-effective for standard parts like bolts.
3.2 Heat Treatment
Carburizing is the core heat treatment for JIS SCM420. The standard process is:
- Annealing: Heat to 820 – 850°C, cool slowly. Softens the alloy for easier machining (cuts tool wear by 35%).
- Carburizing: Heat to 900 – 950°C in a carbon-rich atmosphere (e.g., natural gas). Creates a 0.8–1.2 mm hard outer layer (carbon content 0.8–1.0%) for wear resistance.
- Quenching: Cool rapidly in oil (from 830 – 850°C). Hardens the carburized surface to HRC 58–62.
- Tempering: Heat to 180 – 220°C, cool in air. Reduces brittleness while keeping the surface hard.
3.3 Forming Processes
JIS SCM420 is shaped into parts using:
- Forging: Hammered or pressed at 1100 – 1200°C. Aligns the metal’s grain, increasing tensile strength by 15% vs. cast parts. Used for gears and shafts.
- Rolling: Passed through rollers to make bars or sheets. Used for basic shapes like bolt blanks.
- Extrusion: Pushed through a die to make complex shapes (e.g., hollow shafts). Ideal for precision parts like transmission components.
3.4 Machining Processes
After forming, parts are finished with:
- Turning: Uses a lathe to make cylindrical parts (e.g., shafts). Use cutting fluid to prevent overheating.
- Milling: Uses a rotating cutter to shape gear teeth. Carbide tools work best for precision (e.g., gear tooth tolerance ±0.02 mm).
- Drilling: Creates holes for bolts. High-speed drills (1000–1500 RPM) avoid cracking.
- Grinding: Smooths the carburized surface to tight tolerances. Improves wear resistance by reducing surface friction.
4. Case Study: JIS SCM420 in Automotive Transmission Gears
A Japanese automotive factory faced a problem: their carbon steel transmission gears failed after 150,000 km, causing costly recalls. They switched to JIS SCM420—and solved the issue.
4.1 Challenge
The factory’s compact cars were used in urban areas with frequent start-stop cycles. Carbon steel gears had low wear resistance and fatigue strength (400 MPa), leading to tooth wear and transmission slippage. The failure rate was 5% per year, hurting brand trust.
4.2 Solution
They switched to JIS SCM420 gears, using:
- Carburizing (920°C for 4 hours) to create a 1.0 mm hard outer layer.
- Quenching + tempering (200°C) to reach HRC 60 on the surface and 30 HRC in the core.
- Precision grinding to smooth gear teeth, reducing friction and wear.
4.3 Results
- Service life: Gears now last 300,000 km—double the previous lifespan.
- Cost savings: Cut recall costs by ¥280,000 per year.
- Performance: Transmission efficiency improved by 5%, reducing fuel consumption.
5. Comparative Analysis: JIS SCM420 vs. Other Materials
How does JIS SCM420 stack up against common alternatives? Here’s a side-by-side comparison:
| Material | Tensile Strength | Wear Resistance | Impact Toughness | Cost (vs. JIS SCM420) | Best For |
|---|---|---|---|---|---|
| JIS SCM420 | ≥980 MPa | Excellent | ≥70 J | 100% (base) | High-wear, precision parts (gears) |
| Stainless Steel (SUS304) | 515 MPa | Good | 60 J | 180% | Food/chemical equipment |
| Carbon Steel (S45C) | 600 MPa | Low | 35 J | 50% | Low-stress parts (brackets) |
| Alloy Steel (SCM415) | 800 MPa | Good | 60 J | 80% | General machinery parts |
| Titanium (Ti-6Al-4V) | 1100 MPa | Good | 110 J | 800% | Lightweight aerospace parts |
Key takeaway: JIS SCM420 has better wear resistance than SCM415 or carbon steel, thanks to carburizing. It’s cheaper than stainless steel and titanium, making it the best value for high-wear, precision applications.
Yigu Technology’s Perspective on JIS SCM420 Alloy Steel
At Yigu Technology, JIS SCM420 is our top choice for carburized parts like automotive gears. We’ve supplied it to clients for 12+ years, and its hardenability and molybdenum-enhanced fatigue strength consistently meet strict standards. We optimize the carburizing process to ensure a uniform hard layer, helping clients extend part life by 2–3x. For outdoor use, we pair it with anti-rust coatings. For manufacturers needing durable, cost-effective high-wear parts, JIS SCM420 is unmatched.
FAQ About JIS SCM420 Alloy Steel
1. Is JIS SCM420 the same as AISI 4120?
Yes—JIS SCM420 is nearly identical to AISI 4120 (American standard) in chemical composition and performance. They can be used interchangeably, making JIS SCM420 a great choice for global supply chains.
2. Can JIS SCM420 be used without carburizing?
It can, but carburizing is recommended for high-wear parts. Without carburizing, its wear resistance is lower (similar to mild alloy steel), so it’s only suitable for low-stress parts like non-critical shafts or brackets.
3. How to improve JIS SCM420’s corrosion resistance?
Add a protective coating: zinc plating (for outdoor parts like bolts), powder coating (for structural components), or chrome plating (for moving parts like bearings). These treatments extend service life in humid environments by 5–10 years.
