Stress relieving before deep hole drilling removes residual stresses from the material. If these stresses are not removed before drilling, the part can distort when the material is removed from the bore area. I have seen this happen enough times that I now flag every new job for a stress relief review before I touch the machine.
The underlying mechanism is straightforward. A part that has internal residual stresses is in a state of equilibrium. When I remove material from the center – which is what deep hole drilling does – that equilibrium shifts. The remaining material redistributes the stress and the part moves. In extreme cases, the bore comes out straight but the part bends around it. I have had parts that looked fine on the bore gauge but were visibly curved when placed on a surface plate.
I pay attention to stress relieving for three types of parts: welded fabrications, large castings, and heat-treated parts that were quenched. Each category behaves differently and needs a different approach.
Welded Fabrications
Welded fabrications are the most common source of stress-related problems I deal with. The welding process introduces high residual stresses from localized heating and cooling. The weld bead shrinks as it cools, pulling the surrounding material with it. When I later drill through that zone, the stresses release and the part warps.
For welded cylinders, I recommend stress relieving before drilling if the wall thickness is less than 20mm or the length is over 1 meter. A welded barrel that is not stress relieved can distort by 0.5-1.0mm after drilling. The stress relief cycle is typically 550-650 degrees C for one hour per 25mm of thickness. I use the following guidelines for welded fabrications:
| Wall Thickness | Length | Stress Relief Recommended? | Expected Distortion Without Relief |
|---|---|---|---|
| Under 20mm | Any | Yes | 0.3-1.0mm |
| 20-40mm | Under 1m | Optional | 0.1-0.3mm |
| 20-40mm | Over 1m | Yes | 0.3-0.8mm |
| Over 40mm | Any | No | Under 0.1mm |
| Any | Under 500mm | No | Under 0.1mm |
I have seen a welded hydraulic cylinder barrel that was not stress relieved before drilling. After the bore was drilled, the barrel warped by 0.8mm over 2 meters. The part had to be scrapped. The stress relief cycle would have cost $50 and prevented the problem. That was an expensive lesson that I only needed to learn once.
For complex weldments with multiple weld passes, I recommend stress relieving between rough and finish machining as well. Rough the OD, stress relieve, then drill the bore. This sequence removes the bulk of the distortion before the final bore is cut.
Large Castings
Large castings present a different stress profile. Cast iron and steel castings have uneven cooling stresses from the casting process. The thicker sections cool slower than the thin sections, which creates a non-uniform stress distribution. I have seen cast pump housings that were perfectly straight before drilling and bowed by 0.4mm after the bore was completed.
For castings, stress relieving depends on the casting complexity. Simple castings with uniform wall sections usually do not need stress relief. Complex castings with uneven sections benefit from a stress relief cycle before drilling. The cycle for cast iron is typically 500-550 degrees C, lower than for steel, because cast iron starts to lose hardness above 600 degrees.
I have also found that aging – letting the casting sit for several months – can reduce residual stresses naturally. This was common practice in the past but modern production schedules do not allow for it. When I get a rush job on a complex casting, I insist on a stress relief cycle. The furnace time adds a few days but it beats scrapping a casting that cost thousands of dollars.
The table below shows my recommendations for castings based on geometry and material:
| Casting Type | Material | Stress Relief Temp | Hold Time | When Needed |
|---|---|---|---|---|
| Simple housing, uniform wall | Gray iron | 500-530 C | 1 hr/25mm | Optional |
| Complex housing, uneven wall | Ductile iron | 520-550 C | 1 hr/25mm | Recommended |
| Pump body, thick sections | Steel casting | 550-600 C | 1 hr/25mm | Required |
| Valve body, thin sections | Steel casting | 550-600 C | 1 hr/25mm | Recommended |
Heat-Treated Parts
For quenched and tempered parts, stress relieving before drilling is not usually needed. The tempering cycle already relieves most of the quench stresses. The tempering temperature is typically above 400 degrees C, which is high enough to relax the residual stresses from the quench.
The exception is parts that were quenched and then ground or machined heavily before drilling. The grinding process can reintroduce surface stresses. If a part was rough machined after heat treatment, then the machined surfaces have their own residual stress layer. In those cases, I recommend a low-temperature stress relief at 150-200 degrees C, which is enough to relieve machining stresses without affecting the base material properties.
I have also run into issues with case-hardened parts. The case has a different stress state than the core. When I drill through a case-hardened surface, the stress differential can cause the bore to drift. For case-hardened parts, I drill the bore before case hardening, then protect the bore during the hardening process. If that sequence was not followed, I have to account for bore drift in the drill setup.
I have seen surface-hardened rollers that were drilled post-hardening. The drill entered straight but exited 0.3mm off position because the case-core stress differential pulled the drill. The fix was to pre-drill 1mm undersize before hardening, then finish bore after.
Alternative Stress Relief Methods
Thermal stress relief in a furnace is the standard approach, but I have also used vibratory stress relief for large parts that do not fit in the furnace. Vibratory stress relief uses a mechanical oscillator attached to the part. The oscillator vibrates the part at its resonant frequency for 20-30 minutes. The vibration causes the material’s internal stresses to redistribute and stabilize.
I have used vibratory stress relief on large weldments up to 6 meters long that would not fit in any available furnace. The results are mixed. For simple welded structures like beams and frames, vibratory stress relief is effective. For complex structures with multiple weld zones, I have seen inconsistent results. Some parts responded well, others still warped during drilling.
My recommendation is to use thermal stress relief when the part fits in the furnace and vibratory stress relief only when the part cannot be furnace-treated. The thermal cycle is more reliable and the result is more predictable. I reserve vibratory treatment for parts that are too large for the furnace and for cases where the cost of a thermal cycle is prohibitive.
The table below compares the two methods:
| Method | Cost | Time | Reliability | Max Part Size |
|---|---|---|---|---|
| Thermal (furnace) | $50-200 | 4-8 hours | High | Limited by furnace |
| Vibratory | $100-300 | 1-2 hours | Moderate | No practical limit |
I have also used a combined approach – partial thermal stress relief followed by vibratory treatment for the remaining stress. This works for parts that are too long for the furnace but can have their welded zones heat-treated locally with induction heating before the vibratory cycle.
Cost-Benefit Analysis
Stress relief costs money and adds time to the production schedule. A typical furnace cycle costs $50-200 depending on part size and requires 4-8 hours. The cost of scrapping a part ranges from $200 for a simple welded barrel to several thousand dollars for a large casting. The math is usually clear.
I track distortion problems on my jobs. In the past year, I logged 12 distortion-related issues. Eight of them could have been prevented with a $50-100 stress relief cycle. The other four were in parts where stress relief would not have helped – they were thin-walled parts that deflected from cutting pressure, not from residual stress.
My rule of thumb is simple. If there is any doubt about residual stresses in the part, I run a stress relief cycle before drilling. The cost is low and the downside of not doing it is a scrapped part. I have never regretted stress relieving a part. I have regretted skipping it.
Key Takeaways
- Welded fabrications under 20mm wall or over 1m length need stress relief before drilling.
- Complex castings with uneven wall sections benefit from stress relief at 500-600 degrees C.
- Quenched-and-tempered parts usually do not need additional stress relief before drilling.
- The cost of a stress relief cycle ($50-200) is almost always less than the cost of a scrapped part.
- Aging castings for several months reduces natural stresses, but production schedules rarely allow it.
- If you are unsure whether a part needs stress relief, run the cycle. I have never regretted doing it.
- The distortion from residual stress is predictable. A 0.5-1.0mm warp in a 2m barrel is the classic sign.