In an industrial setting, a material is never chosen at random. During the design phase, the characteristics of the materials must be carefully studied and determined in order to avoid complications later on in the process and to avoid incurring unnecessary costs.

This is especially true when selecting a material to be used in a pipe. This is due to the fact that some pipes are subjected to significant mechanical, thermal, or chemical stresses depending on the type of fluid they carry, with pressure and temperature playing a significant role in determining how long they last.

The material that is used to manufacture the pipe has an impact on all aspects of the manufacturing process, including machining. The machinability of the pipe is directly related to the material that was used in its manufacture, and for each specific material, specific precautions must be taken in order to ensure high-quality machining. Machining is a common operation when preparing a piece for welding. For example, the aluminium tube end must be machined at specific angles in order for the weld to penetrate the entire thickness of the pipe material.

STANDARD STEEL
Standard steel pipes are the most commonly used types of pipes because of their low cost and mechanical properties, which make them suitable for a wide range of applications. Steel pipes are durable, long-lasting, and deformable. This means that they can be used in applications where there are significant temperature or pressure variations. Standard steel pipes are also very commonly used in situations where impacts or vibrations can have an impact on the pipeline (for example, beneath roads). In addition, steel pipes are relatively simple to manufacture, bend, and cut. Steel pipes, on the other hand, are extremely susceptible to corrosion if no protective treatment is applied. Galvanization is a common corrosion-control treatment that involves coating steel pipes with a zinc coating. This coat then oxidizes in place of the steel that it has been protecting, with the critically important distinction being that the zinc oxidizes at a very slow rate.

ALUMINUM
Aluminum is a metal that is widely used in the industrial sector. Aluminum pipes are inexpensive, lightweight, and simple to form and assemble. Their light weight and corrosion resistance make aluminium tubes an excellent choice for applications in the aerospace, transportation, and construction industries. Aluminum pipes are also used in the construction of compressed-air pipelines. Aluminum pipes have a very low level of hardness and are therefore relatively easy to machine. The malleability of aluminum, on the other hand, can cause problems (for example, shavings can cause machine jamming). In this case, increasing the cutting speed, the depth of the pass, and the feeding speed are the best options. Aluminum pipes can also be deformed during machining if the machine tool, and in particular the clamping jaws, are not selected properly. Aluminum's high thermal conductivity allows for efficient heat dissipation. As a result, the cutting speed can be increased without affecting the tool's lifetime.

P91 STEEL
P91 steel is an alloy steel with a high chromium (9%) and molybdenum (1%) content. Adding chromium improves mechanical resistance at high temperatures as well as corrosion resistance, and adding molybdenum improves creep resistance. Small amounts of nickel and manganese are used to increase the overall hardness of the material. P91 steel is extremely sensitive to changes in its microstructure that can occur as a result of excessive heating. These microstructure variations have a tendency to weaken the material. This is why cold machining is frequently used to cut this material.

P91 was initially developed for the fabrication of pipelines in conventional or nuclear thermal power plants, where steam leaves the superheater of a boiler in a modern conventional/thermal plant at a temperature ranging from 570°C to 600°C and a pressure ranging from 170 bar to 230 bar. This means that the final stages of the superheater, as well as the pipelines that deliver the turbine steam, must be capable of withstanding these extreme conditions. In such a situation, the high mechanical resistance of P91, which remains constant over time, makes it the best choice. The high mechanical resistance of P91 steel, on the other hand, makes machining difficult. As a result, the tools should be changed on a regular basis to maintain sharpness, and the cutting speeds should be kept to a minimum. The pass depth can also be changed in order to increase the machining speed.

DUPLEX STEEL is a type of steel that has two layers of steel welded together.
A Duplex stainless steel is a combination of stainless chromium steel and nickel. Due to the presence of both ferrite and austenite in the matrix, it has been given the name Duplex. This alloy was created to provide corrosion resistance as well as tensile strength. Duplex steel pipes are very commonly used in gas and petroleum offshore platforms, where the pipelines are subjected to extreme pressures and saline elements. Duplex steel tubes can also be found in industries that use chlorinated products and acids, such as the chemical and pharmaceutical industries. Over the last few years, more heavily alloyed Duplex steels have emerged under the names Super-Duplex and Hyper-Duplex. Because of their high tensile strength and low yield strength, duplex steel pipes are relatively difficult to machine. This can result in extremely high cutting temperatures as well as plastic deformation of the pipe. In any case, the tooling and clamping used to machine Duplex steel pipe must be sufficiently rigid and stable.

STAINLESS STEELS
Stainless steels, like standard steels, are composed of iron and carbon, to which chromium has been added. When the chromium content of steel exceeds a certain threshold (10.5%), a chromium oxide layer is formed on the steel surface. This so-called "passive layer" is chemically inert, corrosion resistant, and stable. Other elements can be added to increase mechanical strength (nickel) or high-temperature performance (molybdenum, titanium, vanadium, tungsten). Their popularity stems from their corrosion resistance and chemical stability, which make stainless steel piping suitable for fluids that must not be contaminated (pharmaceutical industry, food industry, etc.) as well as for corrosive fluids (particularly in the chemical industry). The machinability of stainless steel is highly dependent on the proportion of alloying elements present. SUPERALLOYS
The majority of the superalloys used in the fabrication of pipes are nickel-based superalloys. Inconel and Austenite are two alloys in this category, each named after the company that manufactures them. As a result, nickel serves as the alloy base, which can be alloyed with chromium, iron, titanium, or aluminum. These alloys offer the same advantages as stainless steels, but to a greater extent. To be more specific, their heat resistance (approximately 900°C) and corrosion resistance (corrosion in chlorine ion, pure water, and caustic medium) are both higher. They are also significantly more expensive than standard alloys, but this is justified in applications where operator safety is an essential criterion. Pipes made of nickel-based superalloys are used in a variety of applications, including aeronautics (in combustion chambers, for example), the chemical industry (because of their corrosion resistance), nuclear engineering, and, to a lesser extent, the food industry.

TITANIUM is a metal that is used in the production of weapons.
Titanium is a metal that has a lot of potential in the industrial world. Titanium can be used to manufacture pipes that are lightweight while also being highly resistant to corrosion and able to withstand extremely high temperatures (600°C). Its mechanical properties (such as resistance, fatigue, and deductibility) are also highly regarded. Titanium, on the other hand, is prohibitively expensive, limiting its use to a few specific applications. Titanium is most commonly found in the aeronautics industry, where its low density, combined with its desirable mechanical properties, make it an indispensable material. Because titanium has a very low thermal conductivity (about ten times lower than steel), the heat dissipation during machining is poor. As a result, the cutting edge must be properly cooled in order to avoid machining defects. Sharp tools should be used to facilitate the detachment of the material and, as a result, reduce the cutting force. Machining titanium that has been treated (e. g., by precipitation or with chromium) is even more difficult.