EN24 is a high-strength, low-alloy steel (equivalent to 4340) known for its toughness, wear resistance, and good mechanical properties. It typically contains 0.35–0.45% Carbon, 1.3–1.8% Nickel, 0.45–0.7% Chromium, 0.2–0.35% Molybdenum, and very low Sulphur (S) and Phosphorus (P) levels (usually below 0.04%).
Effect of Increased Sulphur in EN24:
- Reduced Toughness & Impact Strength:
- Sulphur forms manganese sulfide (MnS) inclusions, which act as stress concentration points, leading to brittleness.
- This is especially detrimental to impact-loaded applications like shafts and gears.
- Improved Machinability (Up to a Limit):
- MnS inclusions help break chips during machining, improving surface finish and tool life.
- However, excessive Sulphur (>0.06%) can lead to reduced mechanical strength.
- Lower Ductility & Fatigue Resistance:
- The MnS inclusions create weak points that can initiate cracks under cyclic loading, reducing fatigue strength.
- This is problematic for aerospace and automotive applications where fatigue life is critical.
- Compromised Weldability:
- High Sulphur increases the risk of hot cracking during welding, making EN24 difficult to weld without preheating and post-weld treatments.
- Reduced Corrosion Resistance:
- Sulphur-rich inclusions can lead to localized corrosion, especially in humid or marine environments.
Recommended Sulphur Content:
For general applications, Sulphur is kept below 0.04% in EN24. In free-machining variants, Sulphur may be increased to 0.08–0.15%, but this compromises strength and toughness.
Comparison of standard EN24 vs. free-machining EN24 (EN24S)
Comparison: Standard EN24 vs. Free-Machining EN24 (EN24S)
EN24 is a high-strength alloy steel used in aerospace, automotive, and industrial applications. EN24S is a free-machining variant with higher Sulphur content to improve machinability. Below is a detailed comparison:
1. Chemical Composition
| Element | Standard EN24 | Free-Machining EN24S |
|---|---|---|
| Carbon (C) | 0.35–0.45% | 0.35–0.45% |
| Silicon (Si) | 0.10–0.35% | 0.10–0.35% |
| Manganese (Mn) | 0.45–0.70% | 0.45–0.70% |
| Chromium (Cr) | 1.00–1.40% | 1.00–1.40% |
| Molybdenum (Mo) | 0.20–0.35% | 0.20–0.35% |
| Nickel (Ni) | 1.30–1.80% | 1.30–1.80% |
| Sulphur (S) | ≤0.04% | 0.08–0.15% |
| Phosphorus (P) | ≤0.035% | ≤0.035% |
Key Difference:
- EN24S has higher Sulphur (S) content (0.08–0.15%), leading to better machinability but lower mechanical properties.
2. Mechanical Properties (After Heat Treatment, Typical Values)
| Property | Standard EN24 (T) | Free-Machining EN24S (T) |
|---|---|---|
| Tensile Strength (MPa) | 850–1000 | 750–900 |
| Yield Strength (MPa) | 650–800 | 550–700 |
| Elongation (%) | 13–16 | 10–14 |
| Impact Strength (Charpy V) | High | Lower |
| Hardness (HRC) | 28–36 | 25–32 |
| Fatigue Strength | High | Lower |
| Machinability | Moderate | High (due to MnS inclusions) |
Key Differences:
- EN24S has lower strength, hardness, and impact resistance.
- EN24S has better machinability due to MnS inclusions.
3. Applications
| Application | Standard EN24 | Free-Machining EN24S |
|---|---|---|
| High-Strength Shafts & Gears | ✅ Preferred | ❌ Not Recommended |
| Automotive Crankshafts & Axles | ✅ Best Choice | ❌ Lower Fatigue Life |
| Aerospace Components | ✅ Used | ❌ Rarely Used |
| High-Stress Bolts & Fasteners | ✅ Common | ❌ Weaker |
| Free-Machining Parts (Threads, Fittings) | ❌ Harder to Machine | ✅ Easier to Machine |
| Welded Structures | ✅ Possible (with preheat) | ❌ High Sulphur = Hot Cracking |
Key Differences:
- EN24 is used where high strength & fatigue resistance are needed.
- EN24S is used for applications where machinability is more important than toughness.
4. Pros & Cons
| Factor | Standard EN24 | Free-Machining EN24S |
|---|---|---|
| Machinability | Moderate | High (Easier to machine) |
| Toughness & Impact Strength | High | Lower |
| Fatigue Resistance | High | Lower |
| Weldability | Moderate (Preheat needed) | Poor (Hot Cracking) |
| Strength & Hardness | Higher | Lower |
| Chip Breaking (During Machining) | Long, Continuous | Short, Easy to Remove |
| Surface Finish (After Machining) | Moderate | Better Finish |
Conclusion: Which One to Choose?
- Choose Standard EN24 if you need high strength, impact resistance, and fatigue strength (e.g., aerospace, heavy-duty shafts, automotive axles).
- Choose EN24S (Free-Machining EN24) if machinability is a priority and strength is less critical (e.g., fasteners, general machining parts).
Details on heat treatment effects for both grades
Heat Treatment Effects on EN24 vs. EN24S
Heat treatment significantly affects the mechanical properties of EN24 and EN24S. Below is a detailed breakdown of the heat treatment processes and their effects on both grades.
1. Common Heat Treatment Processes
| Heat Treatment Process | Purpose |
|---|---|
| Annealing | Softens the material for machining, improves ductility. |
| Normalizing | Refines grain structure, improves strength & toughness. |
| Hardening (Quenching) | Increases hardness and strength by rapid cooling. |
| Tempering | Reduces brittleness, balances strength & toughness. |
| Case Hardening (Nitriding/Carburizing) | Improves surface hardness for wear resistance. |
2. Heat Treatment Comparison: EN24 vs. EN24S
| Process | Standard EN24 | Free-Machining EN24S |
|---|---|---|
| Annealing (Softening) | 830–850°C → Furnace Cool | 830–850°C → Furnace Cool |
| Normalized Condition | 850–880°C → Air Cool | 850–880°C → Air Cool |
| Hardening (Quenching) | 820–850°C → Oil Quench | 820–850°C → Oil Quench |
| Tempering | 400–650°C → Air Cool | 400–650°C → Air Cool |
| Achievable Hardness (HRC) | 28–36 HRC | 25–32 HRC |
| Impact Toughness (Charpy V) | High (~40–50J @ RT) | Lower (~25–35J @ RT) |
| Fatigue Strength | High | Lower (due to MnS inclusions) |
Key Differences:
- EN24 achieves higher hardness & toughness after heat treatment.
- EN24S does not respond as well to heat treatment due to MnS inclusions, reducing impact strength and fatigue life.
- Both grades require quenching & tempering for optimal performance.
- EN24 is preferred for case-hardening (nitriding) applications due to better wear resistance.
3. Effects of Heat Treatment on Mechanical Properties
| Property | As Rolled | Normalized | Hardened & Tempered (T Condition) |
|---|---|---|---|
| Tensile Strength (MPa) | 700–850 | 800–950 | 850–1000 (EN24), 750–900 (EN24S) |
| Yield Strength (MPa) | 500–600 | 600–700 | 650–800 (EN24), 550–700 (EN24S) |
| Elongation (%) | 15–18 | 14–16 | 13–16 (EN24), 10–14 (EN24S) |
| Hardness (HRC) | 20–25 | 25–30 | 28–36 (EN24), 25–32 (EN24S) |
| Impact Strength (J) | 25–30 | 30–40 | 40–50 (EN24), 25–35 (EN24S) |
Key Observations:
- EN24 retains high impact strength even after hardening.
- EN24S becomes brittle due to MnS inclusions, reducing its toughness.
- Both grades require tempering after hardening to prevent excessive brittleness.
4. Case Hardening (Nitriding & Carburizing)
- EN24: Can be nitrided (500–550°C) or carburized (900°C) to achieve surface hardness 55–60 HRC, improving wear resistance.
- EN24S: Not recommended for case hardening because Sulphur-rich inclusions create weak points, reducing wear life.
Conclusion: Best Heat Treatment for Applications
| Application | Best Heat Treatment | Recommended Grade |
|---|---|---|
| Heavy-Duty Shafts & Gears | Quenched & Tempered | EN24 |
| High-Strength Bolts & Fasteners | Quenched & Tempered | EN24 |
| Aerospace & Automotive Parts | Hardened & Tempered | EN24 |
| General Machined Components | Normalized or Annealed | EN24S |
| Surface Wear Resistance (Bearings, Rollers) | Case-Hardened (Nitriding) | EN24 |
| Precision Machining (Threads, Bushings) | Annealed | EN24S |
Details on specific heat treatment procedures for industrial applications.
Industrial Heat Treatment Procedures for EN24 & EN24S
Industrial heat treatment of EN24 and EN24S follows strict temperature controls to achieve optimal mechanical properties. Below are detailed procedures for different treatments.
1. Annealing (Softening Treatment)
Purpose:
- Relieves internal stresses from manufacturing.
- Improves machinability by reducing hardness.
- Refines grain structure.
Procedure:
- Heat to 830–850°C.
- Hold at temperature for 1 hour per 25 mm thickness.
- Furnace cool at slow rate (~20°C per hour) down to 600°C.
- Allow to cool in air.
Expected Hardness:
- EN24: ~200–220 HB (~20 HRC).
- EN24S: ~180–200 HB (~18 HRC).
Industrial Use: Ideal for parts that require extensive machining before hardening.
2. Normalizing (Grain Refinement Treatment)
Purpose:
- Refines grain structure after forging or rolling.
- Improves strength and toughness before hardening.
Procedure:
- Heat to 850–880°C.
- Hold at temperature for 30–60 minutes depending on thickness.
- Air cool in still air to room temperature.
Expected Hardness:
- EN24: ~250–270 HB (~24–26 HRC).
- EN24S: ~220–240 HB (~20–22 HRC).
Industrial Use: Used before final heat treatment (hardening & tempering) for better response to quenching.
3. Hardening (Quenching Process for High Strength & Wear Resistance)
Purpose:
- Increases hardness and tensile strength.
- Essential for high-stress applications (shafts, gears).
Procedure:
- Heat to 820–850°C (ensure uniform heating).
- Hold at temperature for 30 minutes per 25 mm thickness.
- Quench in oil (preferred for EN24) or water (in special cases).
Post-Quenching Hardness:
- EN24: ~55–60 HRC (very hard, but brittle).
- EN24S: ~50–55 HRC (lower due to MnS inclusions).
Industrial Use: After quenching, tempering is mandatory to reduce brittleness.
4. Tempering (Balancing Hardness & Toughness)
Purpose:
- Reduces internal stresses from quenching.
- Improves toughness while maintaining strength.
Procedure:
- Reheat to 400–650°C depending on required hardness.
- Hold at temperature for 1 hour per 25 mm thickness.
- Air cool to room temperature.
Hardness After Tempering:
| Tempering Temp (°C) | EN24 Hardness (HRC) | EN24S Hardness (HRC) |
|---|---|---|
| 400°C | 45–48 | 40–45 |
| 500°C | 38–42 | 35–40 |
| 600°C | 30–35 | 28–32 |
| 650°C | 28–32 | 25–30 |
Industrial Use:
- 400–500°C: Used for high-strength applications (shafts, axles).
- 550–650°C: Used for high-impact parts (gears, bolts).
5. Case Hardening (For Surface Wear Resistance)
Used when high surface hardness is needed while maintaining a tough core.
(A) Nitriding (Best for EN24, Not for EN24S)
Procedure:
- Heat to 500–550°C in an ammonia gas atmosphere.
- Hold for 10–50 hours (longer = deeper case depth).
- Cool slowly in an inert atmosphere.
Results:
- Surface Hardness: ~55–60 HRC.
- Case Depth: 0.3–0.7 mm.
Industrial Use: Used for aerospace shafts, cams, and wear-resistant components.
(B) Carburizing (Not Common for EN24, More for Low-Alloy Steels)
Procedure:
- Heat to 900–920°C in a carbon-rich environment.
- Hold for 1–5 hours depending on case depth.
- Quench in oil for hard case and tough core.
Results:
- Surface Hardness: ~58–62 HRC.
- Case Depth: 0.5–1.5 mm.
Industrial Use: Used for bearings, gears, and rollers.
Summary of Best Heat Treatment for Different Applications
| Application | Recommended Heat Treatment | Best Grade |
|---|---|---|
| Heavy-Duty Shafts & Gears | Hardened (850°C Oil Quench) + Tempered (500°C) | EN24 |
| High-Stress Bolts & Fasteners | Hardened (850°C Oil Quench) + Tempered (550°C) | EN24 |
| Aerospace & Automotive Parts | Hardened (850°C Oil Quench) + Tempered (500°C) | EN24 |
| Bearings & Rollers | Nitrided (500°C) | EN24 |
| Precision Machined Parts | Annealed (850°C Furnace Cool) | EN24S |
| Gears & Components Requiring Machining | Normalized (880°C Air Cool) | EN24S |
Conclusion:
- EN24 is best for high-strength & fatigue-resistant applications after hardening & tempering.
- EN24S is preferred for machining-intensive parts, but not recommended for high-stress applications.
- Nitriding EN24 enhances wear resistance without excessive distortion.
- Avoid excessive Sulphur (in EN24S) if high toughness & fatigue life are needed.
