Deep hole drilling removes 10–40% of the workpiece cross-section along the entire length of the hole. This material removal releases the locked-in stresses from prior manufacturing steps — forging, rolling, welding, or rough machining. If stress relief is not performed at the correct point in the process sequence, the part moves during or after drilling and the finished hole fails to meet straightness and dimensional requirements.
Why Stress Relief Matters
Material Removal and Stress Redistribution
| Starting Stock Condition | Internal Stress State | Effect of Drilling |
|---|
| Hot-rolled bar | Compressive skin, tensile core | Bar bends toward the removed material side |
| Forged blank | Complex, direction-dependent stress | Uneven distortion, unpredictable |
| Welded assembly | High localized stress at weld zone | Weld zone distortion, cracking risk |
| Pre-hardened (quenched and tempered) | Relatively stable | Minimal distortion if stress-relieved before Q&T |
| Rough-machined (prior ops) | Surface compressive stress | Re-distribution causes fine movement |
Consequences of Skipping Stress Relief
| Problem | Mechanism | Cost Impact |
|---|
| Hole straightness drift | Part bows as stress rebalances | Scrap or rework |
| Diameter variation after final boring | Part moves during finishing | Out-of-tolerance bore |
| Cracking at thin wall sections | Residual stress exceeds material strength | Scrap |
| Delayed distortion (after final inspection) | Stress slowly relaxes over days/weeks | Warranty claims |
| Dimensional shift during subsequent machining | Stress relief from later operations | Reject at final inspection |
Stress Relief Before Drilling
When to Stress Relieve Before Drilling
| Condition | Recommendation | Reason |
|---|
| Hot-rolled bar stock > 100 mm diameter | Stress relieve before drilling | High locked-in stresses from rolling |
| Forged blanks | Stress relieve before drilling | Complex stress state from forging |
| Welded assemblies | Stress relieve before drilling | Weld shrinkage stresses |
| L/D ratio > 20:1 | Stress relieve before drilling | Long holes magnify distortion |
| Wall thickness-to-diameter ratio < 0.1 | Stress relieve before drilling | Thin walls distort easily |
| Pre-hardened materials (30+ HRC) | Stress relieve before drilling | Material already stable, but verify |
Stress Relief Parameters by Material
| Material | Temperature | Hold Time | Cooling Method | Expected Stress Reduction |
|---|
| Low-carbon steel (1018, 1026) | 600–650°C | 1 hour per 25 mm thickness | Furnace cool to 300°C, then air | 70–85% |
| Medium-carbon steel (1045, 4140) | 600–650°C | 1 hour per 25 mm thickness | Furnace cool to 300°C, then air | 70–85% |
| Alloy steel (4340, 8620) | 620–680°C | 1–2 hours per 25 mm thickness | Slow furnace cool | 75–90% |
| Stainless steel (304, 316) | 350–450°C | 2 hours | Slow cool | 50–70% |
| Stainless steel (416, 17-4 PH) | 600–750°C | 1–2 hours | Air cool | 70–85% |
| Cast iron | 500–550°C | 1 hour per 25 mm thickness | Furnace cool | 60–80% |
| Aluminum (6061, 7075) | 300–350°C | 1–2 hours | Slow cool (25°C/hour max) | 40–60% |
| Titanium (Ti-6Al-4V) | 600–700°C | 1–2 hours | Furnace cool | 60–75% |
Stress Relief After Drilling
When to Stress Relieve After Drilling
| Condition | Recommendation | Reason |
|---|
| Finish boring or honing follows drilling | Stress relieve between rough and finish | Remove drilling-induced stress before final sizing |
| Tight straightness tolerance (< 0.05 mm/300 mm) | Stress relieve after drilling | Drilling itself induces stress |
| Welded assembly drilled and then welded again | Stress relieve after each major operation | Multiple stress introduction points |
| Case hardening or nitriding follows drilling | Stress relieve after drilling | Prevent distortion during case hardening |
| Part geometry includes thin sections near hole | Stress relieve after drilling | Thin sections distort from drilling heat and pressure |
Drilling-Induced Stress
| Drilling Factor | Stress Effect | Magnitude |
|---|
| Feed force (axial) | Compressive stress ahead of drill | 50–200 MPa |
| Coolant pressure | Expansive stress on bore wall | 5–25 MPa (at 100–250 bar) |
| Cutting temperature | Thermal stress at surface | 300–600°C at cutting zone |
| Material plastic deformation | Work-hardened layer | 0.02–0.10 mm deep |
| Chip evacuation | Friction stress on bore wall | Moderate |
Tip: Drilling-induced stresses are typically shallow (0.05–0.20 mm deep) and are removed by subsequent boring or reaming stock removal. If finish boring removes 0.25 mm per side, the drilling-induced stress layer is completely removed and post-drilling stress relief may not be needed for the final hole.
Process Sequencing Strategies
Sequence Options
| Sequence | Process Flow | Best For |
|---|
| A — Minimal stress concerns | Rough machine OD → Drill → Finish bore → Finish OD | Low-carbon steel, thick walls |
| B — Standard stress relief | Stress relieve billet → Drill → Stress relieve → Finish bore | Alloy steel, moderate tolerances |
| C — Precision sequence | Stress relieve → Rough bore → Stress relieve → Finish bore → Hone | Tight tolerance, thin walls |
| D — Welded assembly | Stress relieve weldment → Drill → Stress relieve → Finish bore | Fabricated parts, hydraulic cylinders |
| E — Case-hardened part | Drill → Stress relieve → Case harden → Finish bore (if needed) | Parts requiring surface hardness |
Recommended Sequence by Part Type
| Part Type | Material | Recommended Sequence | Rationale |
|---|
| Hydraulic cylinder | 1026 or 4140 | C | Tight seal bore tolerance, thin wall |
| Gun barrel | 4140 or 4340 | B | Straightness critical |
| Structural tube | Low-carbon steel | A | Generous tolerances |
| Mold core cooling hole | P20 or H13 | C | Thermal cycling application |
| Aerospace actuator | 17-4 PH or Ti-6Al-4V | B or C | Tight tolerances, critical application |
| Oilfield component | 4140 or 4340 | B | Heavy sections, moderate tolerances |
Distortion Control During Heat Treatment
Fixturing and Positioning
| Method | Application | Effectiveness |
|---|
| Vertical suspension | Long, slender parts | Excellent — gravity maintains straightness |
| Horizontal support on V-blocks | Medium-length parts | Good — supports evenly |
| Ceramic or refractory supports | High-temperature treatment | Excellent — no reaction with part |
| Plugging the hole | Prevent ID scaling or decarburization | Good — protects bore surface |
| Weighted or restrained fixture | Thin-walled parts | Moderate — may introduce new stress if uneven |
Heating and Cooling Rate Control
| Section Thickness | Maximum Heating Rate | Maximum Cooling Rate |
|---|
| < 25 mm | 200°C/hour | 100°C/hour |
| 25–75 mm | 150°C/hour | 75°C/hour |
| 75–150 mm | 100°C/hour | 50°C/hour |
| > 150 mm | 75°C/hour | 35°C/hour |
Inspection After Stress Relief
Verification Checks
| Check | Method | Acceptable Change |
|---|
| Straightness | Straightedge or laser alignment | < 0.03 mm/300 mm change from pre-treatment |
| Hole diameter | Bore gauge | < 0.02 mm change |
| Surface condition | Visual | No scaling, pitting, or decarburization |
| Hardness | Hardness test | Within specification ±2 HRC |
| Cracking | Dye penetrant or magnetic particle | No cracks |
FAQ
Should I stress relieve before or after deep hole drilling?
It depends on the material and tolerance requirements. For most alloy steel parts with tight straightness requirements, stress relieve both before and after drilling — once before to stabilize the raw material, and once after to relieve drilling-induced stress before finish boring. For low-carbon steel with generous tolerances, stress relief is often not required at all.
What temperature is needed for stress relief of deep hole drilled parts?
For carbon and alloy steels, the standard stress relief temperature is 600–650°C. This is below the transformation temperature (Ac1), so the material's mechanical properties are not significantly changed. Hold for 1 hour per 25 mm of cross-section thickness, then furnace cool slowly. Higher temperatures remove more stress but may reduce hardness.
Does deep hole drilling itself induce stress in the part?
Yes — the feed force, coolant pressure, and cutting temperature all induce stress in the part. The feed force creates axial compressive stress ahead of the drill. The cutting temperature creates thermal stress at the bore surface. These drilling-induced stresses are typically confined to a shallow layer (0.05–0.20 mm deep) and are removed by subsequent boring or reaming stock removal.
How do I prevent distortion during stress relief heat treatment?
Suspend long parts vertically during heat treatment to prevent sagging. Use slow heating and cooling rates (75–150°C/hour depending on section thickness). Support thin-walled parts on evenly spaced V-blocks or ceramic supports. Plug the bore to prevent internal scaling. Never quench stress-relieved parts — always furnace cool or slow cool.
Can stress relief be combined with other heat treatment operations?
Yes — stress relief is often combined with normalizing (for carbon steel) or annealing cycles. For quenched and tempered parts, stress relief before final machining is incorporated into the process sequence. However, stress relief should never be combined with hardening or tempering if the temperature and cooling rate requirements are different — use separate operations with controlled parameters for each.
Stress relief is an investment in dimensional stability. The cost of one additional heat treatment cycle is far less than the cost of scrapping a deep hole drilled part that moved out of tolerance. This article reflects industry practice as of 2026.
