Machining Corrosion-resistant Metals

Companies serving the oil and gas sector are pushing the use of corrosion-resistant materials to new depths

INCONEL layer

Figure 1:
A thin INCONEL layer is welded to the inner walls of many oil and gas components and must be machined with properly matched carbide cutting tools.

The world’s energy demand has increased drastically throughout modern history; today, energy consumption is about 25 times the amount used 200 years ago.

According to recent demographic research, the global population is expected to reach 9 billion by 2050, meaning approximately 25 percent more than today’s population levels. The International Energy Agency (IEA) estimates that an unprecedented level of investment—$1.6 trillion per year on average—will be necessary to meet energy demand through 2035.

Large investments in R&D and manufacturing technology are crucial to maintain a long-term competitive advantage.

Exploration and production companies, known as E&P companies, are driven by continuous demand from international governments and are very much focused on exploration of deep-water and ultradeepwater reservoirs. These deep- and ultradeep-water projects continue pushing technology to the edge. Potential investors and multinational energy companies foresee long-term investments in the upstream sector.

Deep waters are among the most important and challenging exploration and production frontiers today, and they offer a unique opportunity for adding significantly to the world’s proven oil reserves. As the exploration expands to deeper waters, it reveals ever more technically and economically challenging environments, many of them frontier, with minimal local support infrastructure.

Going deeper and deeper into the sea means unprecedented technological challenges. The equipment involved in this hostile environment must be designed carefully using state-of-the-art technology and advanced engineering.

There’s no doubt that the oil and natural gas industries touch our lives in countless ways every day. They fuel our cars, heat our homes, and cook our food. The oil and gas industries provide the world’s 6.9 billion people with 60 percent of their daily energy needs. The other 40 percent comes from coal, nuclear, and hydroelectric power; renewables like wind, solar, and tidal power; and biomass products such as firewood. Global demand for energy continues to grow, especially in developing countries such as China and India, as the oil and gas industry continues to search for new sources of energy.

Increasingly, oil and gas are found in challenging areas, such as deep water, arctic regions, and politically challenged regions of the world.

To meet the worldwide, fast-growing energy demand, governments are approving more and more deep-water drilling permits. E&P companies foresee a strong growth in offshore activity, while the oil and gas deepwater exploration and extraction is expected to reach unprecedented rates.

Deep-water surface-to-seabed projects today typically are at depths of 1,500 to 2,000 meters, and the number of drilled wells at depths of up to 7,000 meters is increasing continuously. At these depths the hostile environment requires special equipment and exotic materials to meet the tough safety regulations, and that involves enormous investments.

Inserts

Figure 2:
Selecting inserts with a lack of radius and a small edge prep enables higher feeds and speeds to be used.

Exotic Material Usage

According to Iscar cutting tool engineers, there is a growing demand for exotic materials such as stainless steel, duplex and super-duplex steel, INCONEL® alloy, and titanium in these deep-water environments. These exotic and expensive materials are used widely in offshore projects becsuse of their mechanical and corrosion-resistant properties in most acid-alkaline solutions and chlorine bearing environments.

However, for metal cutting tool suppliers, these nonstandard materials normally are categorized as high-temperature resistant alloys. In general terms, medium carbon steel is easier to cut than a heat-resistant alloy.

When optimizing cutting conditions and choosing the correct tooling technology, it is important to consider the material properties of the workpiece and how they can affect machining. The four main properties to evaluate before choosing the right cutting conditions and tool are:

  • 1. Tensile strength.
  • 2. Hardness.
  • 3. Ductility.
  • 4. Thermal conductivity.

To keep in line with the tight security and environmental regulations in deep-water projects, E&P companies will choose materials suitable to withstand corrosive environments subjected to high pressure and temperatures.

For example, INCONEL 718 has become very popular among the companies related to the upstream exploration sector because of its excellent corrosion resistance. The austenitic microstructure of this nickelbased superalloy also provides high tensile and yield strength.

However, in the machining INCONEL 718, some major problems must be addressed. For instance, the very high temperatures on the cutting edge of the insert, due to the abrasive elements in the material composition (nickel content of 50 to 55 percent and chrome content of 17 to 21 percent), result in high wear rates, chipping, notching, and insert breakage.

In addition, INCONEL 718’s metallurgical sensitivity to residual stresses and self-hardening during the cutting operation may cause high deformation of the cutting edge, even at low cutting speeds, drastically reducing the expected tool life.

New grades, coatings, and postcoating treatments provide substantially improved tool life and better reliability.

Machining high-temperature alloys is not only a question of choosing the right substrate and coating, however; it also means choosing the correct cutting tool and geometry.

Through-spindle and through-tool coolants

Through-spindle and through-tool coolants are mandatory when machining high-temperature alloys.

Take, for example, INCONELcladded tubing hangers—a component used in the completion of oil and gas production wells (see Figure 1). A relatively thin INCONEL layer is welded to the inner walls of this component and must be machined with carbide cutting tools. This rough boring process involves critical machining setup, including a long tool overhang; unsteady material stock to be removed; and interrupted cutting.

These disadvantages can lead to chattering and, as a consequence, to poor carbide tool life. As a result, the INCONEL cladding machining operation becomes a bottleneck in the manufacturing process and eventually causes high manufacturing expenses.

For this application, inserts specially designed for machining high-temperature alloys should be used.

One of the most important features of these inserts is a lack of radius. Its 45-degree approaching angle reduces notch wear and allows increasing both cutting speed and feed, while achieving long tool life (see Figure 2).

A sharp cutting edge reinforced by a tiny edge preparation also should be used.

However, neither the coating nor the geometry of the insert can ensure efficiency during machining high-temperature alloys, because good chip control is hard to achieve.

High-temperature alloys produce a very high temperature as they are being cut. When heat is removed through coolant application, the chips become less ductile and thus easier to break. Through-spindle and through-tool coolants are mandatory when machining high-temperature alloys.

Today’s standard CNC machines normally are equipped with traditional coolant systems, which deliver coolant at low pressures. But when high-temperature alloys are machined, the heat rate generated is beyond the coolant’s boiling point, which turns to vapor and prevents the coolant from reaching the machined cutting zone. This causes thermal shock on the cutting edges and eventually reduces insert life.

Able to overcome this thermodynamic barrier, high-pressure coolant systems pressurize the coolant and deliver enough liquid volume through small outlet nozzles. At atmospheric pressure, the coolant flowing through the nozzle can reach a very high velocity. As a result, a considerable force is generated on the chips, lowering temperature and protecting the cutting edge from thermal shock, ensuring better carbide insert tool life and part surface finish.

With this high-pressure coolant tooling system, smaller chips also can be managed easily—they do not tangle around the workpiece or machine parts, so there is no need to stop the process frequently. This additional feature enhances productivity and cuts manufacturing costs.

Drilling

One application in which E&P companies must evaluate manufacturing efficiency is drilling.

To meet the challenges created by machining these exotic materials and to maintain economical productivity, E&P companies need to use innovative and advanced cutting tools suitable for machining metals that can withstand the deep-water hostile environment.

One example from Iscar is the SUMOGUN, a gundrill with an indexable drilling head that features two effective cutting edges. There is no need to remove the drill for head indexing, and the end user can employ different drilling head geometries that are suited to the material and application.

To be fully effective, drills need to enable high-feed drilling, provide high drilling rates, and deliver high accuracy and surface finish.

For machining deep cavities in oil and gas deep drilling parts, the drill must be a reliable tool that guarantees rigidity, repeatability, accuracy, and optimal tool life performance.

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