Materials, machinability, and successful weld prep for pipe

In industrial applications, the material used to manufacture products is never selected by chance. During the design phase, engineers study each material’s characteristics carefully and choose the material deliberately. Careful selection prevents subsequent complications when the product is put into use and helps to prevent incurring unnecessary costs.

While such considerations are critical for many applications, they are especially important regarding pipe. This is because some pipes are subjected to considerable mechanical, thermal, or chemical stresses, depending on the type of fluid they convey. Operating pressures and temperatures also play determining roles.

The material used to manufacture the pipe has an influence on all the manufacturing operations, including machining. The machinability of the pipe depends directly on the material used to manufacture it, and for each given material, equipment operators must follow the process instructions carefully to produce a quality end preparation.

Low-carbon Steel

Low-carbon steel—iron plus a small amount of carbon, from 0.008 to 2.14 percent—is the most common pipe material. It is favored for its low cost and mechanical properties that make it suitable for innumerable applications. While formable and durable, low-carbon steel is resistant to mechanical stresses. This means it can be used for applications with significant temperature or pressure variations and applications involving impacts or vibrations (underneath roads, for example). In addition, steel pipes are fairly easy to manufacture, bend, and cut.

The downside is that steel pipes are susceptible to corrosion if the pipe producer doesn’t use a corrosion-prevention treatment. Galvanization—an application of a zinc coating—is a common corrosion-control treatment. This coating oxidizes in the place of the steel that it protects. The chief difference between steel and zinc is that zinc oxidizes very slowly.

Low-carbon steel can be machined easily. As the carbon rate increases, its hardness and mechanical resistance tend to improve significantly. This makes the process of machining high-carbon steels more difficult.

Stainless Steel

Just like standard steel, stainless steel is composed of iron and carbon. Unlike standard steel, stainless steel has some chromium content. If the chromium content exceeds 10.5 percent, a chromium oxide layer forms on the steel’s surface. This so-called passive layer is chemically inert, corrosion-resistant, and stable. Adding other elements, such as nickel, improves the mechanical strength. Elements such as molybdenum, titanium, vanadium, and tungsten improve the material’s high-temperature performance. Although more expensive than standard steel pipes, stainless steel pipes are used widely in many industries, such as chemical processing, petroleum refining and processing, pharmaceuticals, food and beverage, aerospace, and shipbuilding.

The alloying elements and their proportions determine the material’s machineability. Specifically, as the proportion of chromium, nickel, or titanium increases, so does the machining difficulty. Increasing the amount of carbon or sulfur makes machining easier. Effectively machining these tough alloys requires a well-assembled cutting tool and a sturdy machine; as a rule of thumb, the forces deployed when cutting stainless steel can be more than 50 percent higher than with standard carbon steel. Furthermore, maintaining the sharpness of the tool’s cutting edges helps to facilitate chip detachment and to reduce the cutting forces.

P91 Steel

Alloy P91 is steel material with high chromium and molybdenum content—9 percent and 1 percent, respectively. The chromium content increases high-temperature mechanical resistance and corrosion resistance; the molybdenum improves creep resistance. Adding small amounts of nickel and manganese enhances the material’s overall hardness.

While the material has excellent mechanical properties at elevated temperatures, P91 does have its limits. The material is very sensitive to excessive heating, which makes cold machining the preferred method for cutting this material.

Initially developed for manufacturing pipelines in power plants—conventional or nuclear—it is subjected to the steam leaving a boiler’s superheater at temperatures from 1,060 to 1,110 degrees F and pressures from 2,465 to 3,335 pounds per square inch. By using P91 in such circumstances, power-plant designers can reduce the wall thickness while increasing the operating temperature, both of which improve the thermodynamic efficiency of the power-generation process.

The drawback is that the material’s high strength makes it difficult to machine. Changing tool bits frequently and keeping the cutting speeds low both help to machine this material successfully. Increasing the feed rate is an option for increasing the machining speed.

Duplex Stainless Steel

A duplex stainless steel contains two structures: ferrite and austenite (hence the name duplex). Duplex stainless steel provides corrosion resistance and tensile strength. In piping applications, common applications for duplex stainless steel are gas and petroleum offshore platforms, where the pipelines are subjected to intense pressures and corrosive elements (salt water). Duplex steel tubes also can be found in industries with chlorinated products and acids, such as in the chemical or pharmaceutical industries. In recent years, more strongly alloyed duplex steels have emerged under such names as superduplex and hyperduplex.

Duplex steel is a high-strength material. Its high minimum yield strength and ultimate tensile strength make it relatively difficult to machine, leading to very high cutting temperatures that can cause plastic deformation of the pipe. The tooling and clamping must be sufficiently rigid and stable.

Superalloys

Most of the superalloys used to manufacture pipes are nickel-based materials, including the INCONEL® family of alloys. Using nickel as a base and alloying it with chromium, iron, titanium, or aluminum results in a material with many of the advantages of stainless steels, but to greater extent. Specifically, superalloys’ heat resistance is higher, up to 1,650 degrees F, as is their corrosion resistance. They also are more expensive than standard alloys, but this can be justified for applications that require these characteristics.

Pipes made from nickel-based superalloys are used in aerospace (in combustion chambers for heat resistance, for example); in the chemical industry (in the presence of caustic media); nuclear engineering; and, to a lesser extent, in the food industry.

Superalloys are considered very difficult to machine, a characteristic attributed to several factors. First, about 70 percent of the heat generated is returned directly to the cutting tool (as opposed to 15 percent for carbon steel). Therefore, it is essential to keep the cutting-edge cooled during machining. Second, these alloys are extremely hard. The service life of a cutting tool used to machine a superalloy can be reduced to just a few minutes if the tool does not have the necessary power or if the cutting speeds and tools are not suitable.

Titanium

Titanium pipe is lightweight, highly resistant to corrosion, and can withstand temperatures up to 1,110 degrees F. Its mechanical properties (resistance, fatigue, and ductility) make it a favored material for many applications, although its cost restricts its use. Common applications are in the aerospace sector, where its low density combined with its attractive mechanical properties make it an essential material.

Since the thermal conductivity of titanium is very low (about 10 percent that of steel), the heat dissipation during machining is relatively poor. Therefore, the cutting edge needs to be properly cooled to prevent machining defects. Also, it’s necessary to keep tools sharp to facilitate chip detachment and thus reduce the cutting force.

Note that treated titanium—whether treated by precipitation hardening, a chromium coating, or alloying—makes these alloys even more difficult to machine.

Aluminum

Aluminum is used widely in the industry. Aluminum pipe is inexpensive, easy to form, and easy to assemble. It is also lightweight and corrosion-resistant, making it a good choice in the aeronautics, transport, and construction sectors. It’s also used often for building compressed-air pipelines.

Aluminum is not a hard material, so it’s relatively easy to machine. However, the material’s malleability can cause problems. Shavings can cause a machine to jam, for example. In such a case, the best remedy is to increase the cutting speed, the feeding speed, and the depth of each pass. The high thermal conductivity of aluminum allows for good heat dissipation; therefore, the cutting speed can be increased without reducing the cutting tool’s service life.

Risks include deformation and surface damage by the machine tool’s clamping jaws. Taking care in choosing the appropriate jaws and setting the clamping pressure can help to prevent deformation.

Nadia Reicher is executive sales manager for Protem USA LLC, 29340 Industrial Way, Ste. 402, Evergreen, CO 80439, 303-955-4862, www.protemusa.com.