Deep Hole Drilling Hardened Steels and Tool Steels

Hardened steels (HRC 40–60) and tool steels present a unique challenge for deep hole drilling: the material is extremely hard and abrasive, but does not work-harden as aggressively as stainless steel or superalloys. The primary failure mode is edge chipping and abrasive wear rather than work-hardening or built-up edge.

This guide covers parameters, tooling, and strategies for deep hole drilling in hardened and tool steels.

Material Categories

Hardened Engineering Steels

Material	Typical Hardness	Applications
4140 PH (pre-hardened)	28–35 HRC	Hydraulic components, shafts
4340 (heat-treated)	35–45 HRC	Landing gear, heavy shafts
300M	48–55 HRC	Aerospace landing gear
H13 (hardened)	45–52 HRC	Die casting, hot work
D2	58–62 HRC	Cold work dies, cutting tools

Tool Steels

Type	Examples	Hardness	Deep Hole Drilling
Hot work	H11, H13, H21	40–52 HRC	Moderate difficulty — similar to alloy steel at same hardness
Cold work	D2, A2, O1	55–62 HRC	Very difficult — high abrasive wear
High-speed	M2, M42	60–67 HRC	Extremely difficult — EDM preferred
Mold steel	P20, 718	30–38 HRC	Similar to alloy steel

Cutting Parameters

Gun Drilling

Hardness	Speed (m/min)	Feed (mm/rev, Ø12 mm)	Coolant Pressure
30–35 HRC	60–90	0.020–0.040	Standard
35–40 HRC	50–75	0.018–0.035	+10%
40–45 HRC	35–55	0.015–0.028	+15%
45–50 HRC	25–40	0.012–0.022	+20%
50–55 HRC	15–25	0.008–0.015	+30%
55–60 HRC	10–20	0.006–0.012	+40%

BTA Drilling

Hardness	Speed (m/min)	Feed (mm/rev, Ø40 mm)	Coolant Pressure
30–35 HRC	45–70	0.12–0.25	Standard
35–40 HRC	40–60	0.10–0.20	+10%
40–45 HRC	30–45	0.08–0.18	+15%
45–50 HRC	20–35	0.06–0.15	+25%

Tool Selection

Carbide Grade

Hardness	Recommended Grade	Cobalt	Grain
30–40 HRC	K30–K35	6–8%	Medium (1–2 µm)
40–50 HRC	K20–K30	4–6%	Fine (0.5–1 µm)
50–60 HRC	K10–K20	3–5%	Ultra-fine (0.2–0.5 µm)
> 60 HRC	PCBN recommended	—	—

Coating

Hardness	Recommended Coating
30–40 HRC	TiAlN
40–50 HRC	AlTiN nano
50–60 HRC	AlTiN nano or AlCrN
> 60 HRC	PCBN (no coating needed)

Edge Preparation

Edge preparation is critical for hardened steels. Unlike soft materials where a sharp edge is best, hardened steels require a reinforced edge to prevent chipping:

Hardness	Edge Preparation	Why
< 35 HRC	Sharp or light hone (< 0.02 mm)	Lower forces, less chipping risk
35–45 HRC	Light hone (0.02–0.05 mm)	Protects edge from micro-chipping
45–55 HRC	Hone (0.03–0.08 mm)	Required to prevent edge chipping
> 55 HRC	Heavy hone (0.05–0.10 mm) or chamfer	Reinforces the cutting edge

Coolant Strategy

Parameter	Recommendation
Coolant type	Neat oil with EP additives
Coolant pressure	Increase 10–40% based on hardness
Coolant temperature	30–40°C
Filtration	10–20 micron

Depth Ratio Limitations

Hardened steels become significantly more challenging at depth. The combination of high cutting forces and reduced tool rigidity at depth makes tool breakage more likely.

Hardness	Maximum Recommended L/D (gun drilling)
30–35 HRC	100:1
35–40 HRC	80:1
40–45 HRC	60:1
45–50 HRC	40:1
> 50 HRC	30:1

Common Problems

Problem	Cause	Solution
Edge chipping	Feed too high or edge prep insufficient	Reduce feed; increase edge hone
Tool breakage at depth	Depth ratio too high for hardness	Reduce depth ratio; add whip guide
Oversize hole	Tool deflection from high cutting forces	Reduce feed; ensure rigid setup
Poor surface finish	Worn edge or built-up edge	Regrind earlier; check coating
Excessive tool wear	Grade too soft for hardness	Switch to harder grade (lower cobalt)
Chatter	High cutting forces exciting resonance	Reduce speed; increase feed

When to Switch to Non-Conventional Methods

Hardness	Recommended Deep Hole Drilling Method
< 45 HRC	Gun drilling or BTA (standard tooling)
45–55 HRC	Gun drilling or BTA (specialized tooling)
55–60 HRC	Gun drilling (specialized), BTA (difficult), EDM
> 60 HRC	EDM is preferred — mechanical drilling very difficult

For materials above 55 HRC, consider EDM deep hole drilling (see EDM drilling guide).

Summary

Hardened steels require progressively lower speeds, reinforced cutting edges, and harder carbide grades as hardness increases. Below 45 HRC, standard deep hole drilling with AlTiN-coated carbide is feasible with parameter reductions. Above 55 HRC, consider EDM drilling as an alternative. Edge preparation becomes critical above 40 HRC — a small hone (0.02–0.08 mm) prevents the micro-chipping that occurs when a sharp edge encounters hard material.

For the most commonly drilled prehardened alloy — 4140HT — see gun drilling 4140. For cutting tool materials, see tool materials guide. For EDM and non-conventional methods, see EDM and laser drilling guide. For a complete overview, visit the materials-specific drilling guide.