Key Conclusions:
Austenitic steels (300 series, including AISI 304 and 316L which are popular in the yacht and shipyard industries) are fundamental in combating difficult conditions at sea. They are extremely corrosion-resistant, but possess one technological flaw: they are enormously susceptible to work hardening.
Definition: Work hardening is a phenomenon in which metal becomes drastically harder and more brittle as a result of cold plastic deformation (e.g. under tool pressure).
How does this work in practice? Imagine an ordinary paper clip. Start bending it in one place back and forth. You’ll quickly notice that the bending point becomes hot, the metal becomes stiffer, until it finally breaks.
This exact same phenomenon occurs on a microscopic scale on the surface of your drive shaft if the CNC machine operator uses the wrong technology. Instead of smoothly cutting the metal, the tool “crushes” it.
Before we explain how work hardening destroys ship parts, it’s worth looking at this phenomenon from a broader engineering perspective. Work hardening is not always an error – in some materials it’s a deliberate procedure!
Excellent examples are carbon steels such as Hadfield steel (high-manganese) or the wear-resistant plates Hardox. The hardening mechanism in them works to the user’s advantage.
However, the difference is crucial: While in a mine or on tracks we do want the impacts to harden the steel, in precise stainless steel elements immersed in seawater, accidental work hardening caused by a dull lathe tool is a death sentence for the detail.
During processes such as turning or CNC milling, enormous forces are generated in the zone of contact between the tool and material. Austenitic steels are terrible heat conductors – it doesn’t escape into the depth of the material, accumulating on the very surface instead.
If machining proceeds incorrectly, a catastrophe occurs in the crystalline structure:
In the maritime industry, we work in one of the most aggressive environments on Earth – in an electrolyte full of chlorine ions (salt water).
When we combine microcracks and stresses (created by poor machining) with sea salt and continuous drive vibrations, we get stress corrosion cracking (SCC).
Fastening elements, pins or valves may look perfect from the outside only to suddenly, without any prior rust deposits, crack in half during a storm. This is not a metallurgical defect of the material – it’s an error of the machining workshop.
At EDBA, we know that to preserve the original corrosion resistance of marine steel, we must minimize friction and immediately remove heat.
Our technologists apply the “cut and run” principle. The golden rules for machining steel for the shipbuilding industry are:
The numbers speak for themselves. Let’s take a look at averaged data from metallographic research on AISI 316 marine steel, showing hardness on the Vickers scale (HV):
| Condition and Machining of AISI 316 Steel | Surface Hardness (HV) | Impact on Corrosion Risk in Seawater |
| Core (as-delivered from the mill) | ~200 HV | Elastic material, no risk |
| Correct machining (sharp tool) | ~240 HV | Acceptable norm, high resistance |
| Incorrect machining (sliding / dull tool) | ~480 HV | Critical hardening! Extreme risk of SCC cracks. |
Errors by the operator can raise surface hardness by over 100%, completely destroying the crystalline structure and steel passivity.
For elements requiring absolute precision (e.g. for propeller shaft seals), CNC grinding is necessary – it works excellently as a procedure removing the microscopic, damaged surface layer after turning. However, one must remember that poorly selected grinding wheels can lead to material burning, which in turn will cause new stresses. In our company, this process is subject to rigorous control.
When choosing a contractor for parts for yacht systems, offshore installations or ship elements, remember one thing: a 3.1 certificate for the material from the mill won’t protect you if the contractor destroys the steel on the lathe.
Commissioning machining to a random workshop without knowledge of metallurgy is a straight path to ship failure on the open sea. At EDBA, we understand the physics of cutting. We provide machining that respects the steel structure, guaranteeing that the parts you order will withstand the toughest storms.
1. How do I know if the workshop destroyed the structure of my stainless steel part?
The most visible symptom is thermal discoloration (temper colors: yellow, blue) on the machined surface. Unfortunately, in many cases strong work hardening shows no visual symptoms and can only be detected through microhardness measurements. Certainty comes only from choosing an experienced partner.
2. Will using more expensive steel e.g. Duplex, Super Duplex solve the problem?
No. Duplex type steels, commonly used in the offshore industry, are even more demanding in terms of machining than the classic 316L. Switching to more expensive material while maintaining poor cutting technology will only destroy tools faster and ruin the project budget.
3. When is surface work hardening of metal beneficial?
Only when the design assumes protection against abrasion and impacts, not against corrosion. A classic engineering example is Hadfield steel in track bed elements or Hardox steel in heavy machinery. In maritime applications (chloride resistance), work hardening created during turning is a disqualifying defect for the detail.
4. Does shot peening or passivation after machining save the situation?
Acid passivation restores the oxide protective layer, and shot peening uniformizes the surface, but neither of them will “remove” deep stresses from the hard, damaged surface layer. The key is preventing material overheating as early as at the turning and milling stage.
