Inconel, titanium and superalloys – challenges in machining difficult-to-cut materials for the offshore sector

Machining Inconel, titanium and superalloys requires the use of ultra-rigid CNC machines, high-pressure cooling (HPC) and strategies such as trochoidal milling to handle extreme heating and hardening materials. Without precisely selected technology and experience, these heat-resistant alloys rapidly destroy cutting edges, generating costly downtime and material losses.

The offshore sector – encompassing drilling platforms, subsea pipelines and offshore wind farms – is one of the most demanding arenas of engineering in the world. Here, there is no room for compromise. Parts operate in an aggressive seawater environment, under pressure of hundreds of atmospheres at the ocean floor and often in extreme temperatures.

This is why engineers reach for “armor-plated” materials: Inconel, titanium and Super Duplex type steels. They are salvation for designers because they guarantee safety for decades, but a real nightmare for production technologists. Professional machining of these alloys is a dress rehearsal for the machine park and the operators’ skills.

In this article we will explain how to tame these materials, based on hard data and workshop practice.

Why does offshore require materials that “cannot be” machined?

Before we move on to chips, we must understand the “why”. The costs of extracting a damaged valve from a depth of 2000 meters run into millions of dollars daily. Materials must possess:

• A high PREN (Pitting Resistance Equivalent Number) index – it determines resistance to pitting corrosion. For offshore steels, its value must exceed 40.
• Heat resistance – maintaining mechanical properties at temperatures above 700°C (crucial in hydrocarbon extraction).
• Strength-to-weight ratio – key in floating structures (platforms, FPSO vessels).

The same properties that protect the material against the elements cause it to put up powerful resistance to cutting tools.

Inconel – the king of heat resistance and enemy of tools

Inconel (the most popular in offshore are Inconel 625 and 718) is a nickel-chrome based superalloy. It’s legendary in the industry. Its crystalline structure makes it hard, ductile and it does not lose its properties in fire.

The main challenge – work hardening

The biggest obstacle when machining Inconel is its tendency to rapidly harden under pressure.

How does this work in practice?

Imagine cutting soft butter that under the touch of a knife, in a fraction of a second, turns into granite. This is how Inconel behaves. If the CNC machine operator “hesitates” – that is, applies too little feed or the tool stops cutting and starts sliding – the surface will undergo immediate hardening (so-called glazing). The next tool pass will strike a layer hard as glass, which will lead to chipping of the insert in the blink of an eye.

For this reason, CNC milling of Inconel requires courage: aggressive, constant cutting into the material is a must. There is no room here for “caressing” the detail.

Titanium – a lightweight strongman with a thermal problem

Titanium (mainly Grade 2 and Grade 5 / Ti-6Al-4V for offshore) is 45% lighter than steel, yet equally strong. However, it is a peculiar material in terms of thermodynamics.

The main challenge – thermal conductivity

Titanium is a terrible heat conductor. This is catastrophic news for any process engineer.

• In steel – approximately 75-80% of heat generated during cutting is removed with the chip. The chip burns, but the detail and tool are relatively cool.
• In titanium – heat refuses to “enter” the chip. It stays in the material and accumulates in the cutting tool.

As a result, temperatures at the cutting edge can exceed 1000°C in a fraction of a second. This leads to plastic deformation of the tool and a chemical reaction – hot titanium becomes “sticky” and welds to the blade (built-up edge phenomenon), which destroys surface smoothness..

Super Duplex – steel that does not forgive vibrations

Steels from the Super Duplex group (e.g. UNS S32750/S32760) combine the features of austenitic and ferritic steels. They are incredibly hard (often >30 HRC in as-delivered condition) and have high yield strength.

Their machining requires machines with powerful torque and, most importantly, rigidity. Every vibration of the spindle or fixture, even microscopic, leads to chipping of the cemented carbide. Here, stable detail fixturing is absolutely key.

Difficulty table -machinability comparison 

To illustrate the scale of the challenge, let’s look at the data. We adopt AISI B1112 steel (easy to machine) as a reference point (100%). 

Materiał	Machinability (according to AISI)	Main Risk	Cutting Speed (Vc)
Carbon Steel	100%	Standard wear	High
Stainless Steel (316 L)	45-50%	Built-up edge, work hardening	Medium
Tytan (Ti-6Al-4V)	20-25%	Overheating, chip fire	Low (40-60 m/min)
Inconel 718	10-15%	Extreme work hardening, cutting forces	Very low (25-40 m/min)

As shown, Inconel machining is almost 10 times harder and slower than ordinary steel.

Success strategies – how we do it at EDBA?

Theoretical knowledge is one thing, but in workshop practice, specific technological solutions are what counts. Here’s how we handle these materials::

1. HPC – high pressure coolant

Ordinary “pouring” of coolant is not enough for superalloys – the liquid cannot break through the vapor cushion around the heated tool. We use systems delivering coolant at a pressure of 70-80 bar (and even up to 100 bar) directly onto the cutting edge.

• Goal – physical chip breaking (Inconel creates long, dangerous ribbons) and drastic temperature reduction in the cutting zone.

2. Dynamic milling

This is a modern tool path strategy. Instead of entering the material deeply with the entire width of the mill (which generates enormous heat), the tool performs rapid, spiral movements, collecting a thin layer of material at high speed.

• Benefit –  the tool has time to cool down (for part of the rotation it “rests” in the air), and cutting forces are lower. This allows for effective turning and milling even of the hardest alloys.

3. Advanced tool materials

Standard carbide is sometimes not enough. At EDBA, we reach for:

• Ceramics (SiAlON) – for roughing Inconel. Allows working with speeds of 200-300 m/min (instead of 30 m/min for carbide), utilizing heat to soften the material.
• PVD coatings (e.g. TiAlN) – to provide a thermal barrier for titanium.

4. Surface finishing matters

In the offshore industry, surface roughness has a direct impact on corrosion resistance. Too rough a surface is a “nest” for pitting corrosion. Therefore, precise CNC grinding or lapping is often a necessary, final process stage, guaranteeing perfect smoothness.

Machining Inconel, titanium and superalloys is the “heavyweight” division of engineering. It requires machines of the highest rigidity, advanced CAM software and knowledge that cannot be gained from textbooks – it must be acquired at the machine. A poorly selected strategy in these materials ends with destruction of an expensive detail in a few seconds.

If you’re looking for a partner who understands the specifics of offshore materials and can deliver components compliant with the most rigorous norms – you’re in the right place.

FAQ – Knowledge base on superalloy machining

1. Can titanium be machined dry?

In most cases, this is very risky. Titanium has low thermal conductivity, which causes heat accumulation in the tool. Additionally, fine titanium chips are flammable – dry machining risks a fire hazard inside the CNC machine. Abundant cooling is a safety standard.

2. Why is Inconel machining significantly more expensive than stainless steel?

There are three factors influencing this:

• Time – the process is several times slower (lower cutting parameters).
• Tools – wear of expensive cutting inserts is very rapid (often one edge is sufficient for merely a dozen minutes of work in the material).
• Risk – the starting material is expensive, and the margin of error is zero.

3. What is the “Alpha Case” layer in titanium machining?

It’s a hard, brittle layer rich in oxygen that forms on the titanium surface when it is overheated during machining. It is susceptible to cracking. In the aviation and offshore industries, its presence is unacceptable, which is why the process must be cool and controlled.

4. What is the significance of “machine rigidity” with Super Duplex?

Crucial. Super Duplex generates enormous cutting resistance forces. If the machine tool is not sufficiently rigid, it enters vibrations (chatter). These vibrations rapidly destroy brittle carbide edges and degrade the detail’s surface quality, rendering it defective.

5. Do you undertake machining of customer-supplied material?

Yes. We understand that Inconel or specialized titanium alloys are costly and difficult to obtain. We fulfill orders using the client’s material, guaranteeing the highest care for the process and minimizing waste.

Do you need reliable components for work in harsh conditions?

Contact us! See how our experience in superalloy machining can help with your offshore project.