Heat treated components are everywhere in deep hole drilling – transmission shafts, hydraulic cylinder rods, landing gear struts, and tooling components all go through the heat treater before or after the bore is machined.
The question of whether to drill before or after heat treatment comes up on almost every job. The answer depends on the tolerance requirement, the material hardness, and the part geometry. I have done it both ways and learned the hard way what works.
Drilling Before Heat Treatment
Drilling in the soft state (annealed or normalized) is the most common approach. The material is machinable, tool life is good, and the cycle time is short.
Pre-HT Drilling Parameters
| Material | Condition | Hardness | Cutting Speed | Feed Rate |
|---|---|---|---|---|
| 4140 | Annealed | 17-22 HRC | 90-120 m/min | 0.08-0.16 mm/rev |
| 4340 | Annealed | 20-25 HRC | 80-100 m/min | 0.08-0.14 mm/rev |
| 8620 | Annealed | 15-20 HRC | 90-110 m/min | 0.08-0.15 mm/rev |
| 1045 | Normalized | 18-22 HRC | 100-130 m/min | 0.10-0.18 mm/rev |
| 4150 | Annealed | 20-25 HRC | 70-90 m/min | 0.06-0.12 mm/rev |
The downside is distortion. When the part goes through heat treatment after drilling, the thermal cycle can move the bore. I have seen bores shift by 0.1-0.5mm depending on the material and the part geometry.
The shift is not random – it follows predictable patterns. In a long cylindrical part like a shaft, the bore tends to bow toward the surface that cools fastest during quenching. In a complex shape like a gear blank, the bore moves toward the thickest section.
Accounting for Distortion
| Part Shape | Typical Distortion | Direction |
|---|---|---|
| Solid shaft (round) | 0.1-0.3mm over 1m | Bow toward slow-cooling side |
| Tube/cylinder | 0.05-0.15mm over 1m | Ovalization more than bow |
| Gear blank | 0.2-0.5mm over 500mm | Moves toward heavy section |
| Plate (rectangular) | 0.3-0.8mm over 1m | Warps from uneven cooling |
I handle distortion by drilling with extra stock. If the final bore needs to be 50mm, I rough-drill at 48mm before heat treatment. After heat treatment, I finish-bore or hone to 50mm. The 2mm of stock allows for distortion and the final pass corrects the bore position.
Drilling After Heat Treatment
Drilling in the hardened state avoids distortion concerns but introduces tool life and process challenges. The material is 45-65 HRC, which is in the range where carbide tools wear quickly.
Post-HT Drilling Parameters
| Hardness | Material | Cutting Speed | Feed Rate | Tool Material | Coolant Pressure |
|---|---|---|---|---|---|
| 35-45 HRC | 4140/4340 Q&T | 50-70 m/min | 0.04-0.08 mm/rev | Carbide (TiAlN) | 800-1200 psi |
| 45-55 HRC | 4340/4150 Q&T | 40-60 m/min | 0.03-0.06 mm/rev | Carbide (AlTiN) | 1000-1500 psi |
| 55-62 HRC | Carburized 8620 | 25-40 m/min | 0.02-0.04 mm/rev | PCBN or carbide | 1200-1800 psi |
| 58-63 HRC | Tool steel (D2) | 15-25 m/min | 0.01-0.03 mm/rev | PCBN | 1500-2000 psi |
The biggest mistake I see is using the same tool for pre-HT and post-HT drilling. A standard carbide drill that works fine at 20 HRC will fail in minutes at 50 HRC. The tool needs a different geometry – negative rake, chamfered cutting edge, and a tougher substrate.
Tool Geometry for Hardened Material
| Feature | Standard (soft) | Hardened (45+ HRC) |
|---|---|---|
| Rake angle | +5 to +10 degrees | -5 to 0 degrees |
| Relief angle | 8-12 degrees | 5-8 degrees |
| Point angle | 120-130 degrees | 130-140 degrees |
| Edge preparation | Sharp | 0.05-0.10mm chamfer |
| Coating | TiN/TiCN | TiAlN/AlTiN |
The negative rake strengthens the cutting edge to withstand the higher cutting forces in hardened material. The chamfer distributes the load and prevents micro-chipping.
The Two-Step Approach
For critical components, I use a two-step approach that combines the best of both methods:
- Rough drill before HT – drill to within 2-3mm of final size in the soft state
- Finish drill after HT – finish-bore or ream to final size in the hardened state
The rough drilling removes 90% of the material in the soft state, where it is fast and cheap. The finish drilling corrects any distortion from heat treatment and achieves the final tolerance.
I use this approach on aircraft landing gear components, high-performance transmission shafts, and precision tooling where the bore tolerance is H6 (0.022mm for a 50mm bore) or tighter.
Case Hardened Components
Case hardened parts – carburized or nitrided – have a hard surface layer (0.5-2.0mm deep) over a softer core. Drilling through the case requires different parameters from drilling through the core.
For gun drilling through case-hardened material:
| Parameter | Case Layer (58-62 HRC) | Core (30-35 HRC) |
|---|---|---|
| Cutting speed | 30-40 m/min | 50-70 m/min |
| Feed rate | 0.02-0.04 mm/rev | 0.06-0.10 mm/rev |
| Tool material | PCBN or carbide | Carbide |
If the bore is drilled before carburizing, the case hardens evenly around the bore circumference. If it is drilled after carburizing, the bore wall will be at the case hardness and the entry surface will be extremely hard.
Key Takeaways
- Drill before heat treatment when possible – faster, cheaper, better tool life
- Account for 0.1-0.5mm of distortion after heat treatment – leave extra stock
- Drill after heat treatment for tight tolerances under 0.05mm
- Negative rake and chamfered edges are essential for hardened material over 45 HRC
- The two-step approach (rough before HT, finish after HT) gives the best results for critical parts
- Tool life drops by 50-80% when drilling hardened material vs annealed
For related topics, see my guides on casting and forging and gear and transmission drilling.