How to Select the Right Insert Grade for Different Steels
1. Why Insert Grade Matters When Turning Steel
Steel generates high cutting forces and heat during turning. Choosing the correct insert grade ensures proper wear resistance, toughness, and stability. Using the wrong grade can cause rapid flank wear, chipping, or poor surface finish.
2. Understanding ISO P Steel Insert Grades (P10–P40)
The ISO standard classifies steel machining inserts into P10–P40 based on hardness and cutting conditions.
| ISO Grade | Characteristics | Best For |
|---|---|---|
| P10 | High wear resistance, low toughness | High-speed finishing of mild steel |
| P20–P25 | Balanced toughness and wear resistance | General turning of all steel types |
| P30–P35 | High toughness, moderate wear resistance | Interrupted cuts, alloy steel, forged steel |
| P40 | Maximum toughness | Severe interruption, unstable setups |
3. Best Insert Grades for Each Steel Category
3.1 Mild Steel (1018 / A36)
- Recommended grade: P20–P25
- Ideal coating: TiN / TiCN
- Notes: Mild steel is soft but sticky—use sharper edges and light coatings.
3.2 Alloy Steel (4140 / 4340 / 4150)
- Recommended grade: P25–P30
- Ideal coating: TiAlN or Al₂O₃
- Notes: Higher heat generation—use coated inserts with good hot hardness.
3.3 Hardened Steel (48–60 HRC)
- Recommended grade: P05–P15 or CBN inserts
- Ideal coating: AlTiN, Al₂O₃
- Notes: Very high wear—CBN or ceramic insert is ideal for finishing.
4. Coating Selection for Steel Turning
- TiN: Low friction, good for general steel turning.
- TiCN: Higher hardness and wear resistance.
- TiAlN / AlTiN: Best for high-temperature alloy steel turning.
- Al₂O₃ ceramic layer: Maximum heat resistance for hard steel.
5. Recommended Cutting Conditions
| Steel Type | Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) |
|---|---|---|---|
| Mild Steel | 180–250 | 0.15–0.3 | 1–3 |
| Alloy Steel | 100–160 | 0.1–0.25 | 1–2.5 |
| Hardened Steel | 70–120 | 0.05–0.15 | 0.2–1 |
6. FAQ
What is the best insert grade for 4140 steel?
P25–P30 with TiAlN coating provides the best balance of heat resistance and toughness for 4140 steel.
Can I use the same insert grade for mild and alloy steel?
Yes, P25 is a general-purpose grade suitable for both, but coatings and geometry may differ.
Do hardened steels require special inserts?
Yes, hardened steels above 48–60 HRC require P05–P15 grades, CBN, or ceramic inserts.
