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bars to 230 bars. This means that the final stages of the superheater and the pipelines delivering the turbine steam must be able to withstand these extreme conditions. In such a case, the high mechanical resistance of P91 and its fatigue strength makes it the right choice. By using P91 in such circumstances, the engineers were able to reduce the thickness of the pipelines while simultaneously increasing the operating temperature, all of which enhances the overall thermodynamic efficiency of a power plant. The high mechanical resistance of P91 steel means, however, that machining is difficult. Thus, the tool bits should be changed regularly to ensure their sharpness, and the cutting speeds should be kept slow. The feed rate can also be adjusted to increase the machining speed.
Duplex steel A Duplex stainless steel consists of stainless chromium steel with nickel added. The matrix contains both ferrite and austenite, hence the name Duplex. This alloy was designed to provide corrosion resistance and tensile strength. Duplex steel pipes are very commonly used in gas and petroleum offshore platforms where the pipelines are subjected to intense pressures and corrosive elements (salt water). Duplex steel tubes can also 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 the name of Super- Duplex or Hyper-Duplex. Duplex steel pipes are relatively difficult to machine due to their tensile strength and high yield strength. This can lead to very high cutting temperatures and to plastic deformation of the pipe. In any case, the tooling and clamping must be sufficiently rigid and stable in order to machine a Duplex steel pipe.
Stainless steels Just like standard steels, stainless steels are comprised of iron and carbon, to which chromium has been
3. Machining the inside of a P91 pipe with a PROTEM TT series beveling machine
added. Upon exceeding a certain proportion of chromium (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
improve the mechanical strength (nickel) or high-temperature performance (molybdenum, titanium, vanadium, tungsten). Although more expensive than
standard steel pipes, stainless steel pipes are widely used in many industries (chemical, petroleum, pharmaceutical, food, aerospace, shipbuilding, etc.). 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.) and for corrosive fluids (the chemical industry, in particular). The machinability of stainless steel
is highly dependent on the proportion of alloying elements. Specifically, a high proportion of chromium, nickel or titanium makes machining more difficult, whereas adding carbon or sulfur facilitates machining. The cutting edge must be sharp to facilitate chip detachment from the material and reduce the cutting forces.
The cutting tool must be sufficiently well assembled and the machine itself must be sufficiently rigid to support the forces caused by machining; as a rule of thumb, the forces deployed when cutting stainless steel can be more than 50% higher than with standard carbon steel.
Superalloys Most of the superalloys used to manufacture pipes belong to the range of nickel-based superalloys. This range includes Inconel and Austenite, named after the alloy manufacturer. Therefore, the alloy base is nickel which can be alloyed with chromium,
Machining a Duplex pipe at an angle of 30° with a PROTEM US 150 beveling machine
www.internationalmetaltube.com IMT June 2016 9
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