Corrosion pitting resistant martensitic stainless steel

Abstract

A forged, martensitic, stainless steel alloy is disclosed. The alloy comprises, by weight: about 12.0 to about 16.0 percent chromium; greater than 16.0 to about 20.0 percent cobalt, about 6.0 to about 8.0 percent molybdenum, about 1.0 to about 3.0 percent nickel, about 0.020 to about 0.040 percent carbon; and the balance iron and incidental impurities. The forged, martensitic, stainless steel alloys are highly resistant to pitting corrosion and provide a combination of tensile strength, ductility, and fracture toughness suitable for use as turbine compressor airfoils.

Description

BACKGROUND OF THE INVENTION[0001]The subject matter disclosed herein generally relates to corrosion resistant stainless steels. More particularly, it relates to corrosion pitting resistant, martensitic, stainless steels, including those suitable for turbine rotating components.[0002]The metal alloys used for rotating components of a gas turbine, particularly the front stage compressor airfoils, including rotating and stationary blades, must have a combination of high strength, toughness, fatigue resistance and other physical and mechanical properties in order to provide the required operational properties of these machines. In addition, the alloys used must also have sufficient resistance to various forms of corrosion and corrosion mechanisms, particularly pitting corrosion, due to the extreme environments in which turbines are operated, including exposure to various ionic reactant species, such as various species that include chlorides, sulfates, nitrides and other corrosive specie...

AI Technical Summary

Benefits of technology

The patent describes a new stainless steel alloy that is resistant to pitting corrosion and forging. The alloy contains a specific amount of chromium, cobalt, molybdenum, nickel, and carbon. It is made by heating a pre-formed piece of the alloy to a high temperature to create a solutionized microstructure, then cooling it back down to room temperature to form a martensitic microstructure. The alloy is then tempered at a specific temperature for a specific amount of time to create a final product with improved resistance to pitting corrosion. The technical effect of this alloy is that it has improved resistance to pitting corrosion, making it better suited for use in various applications such as petrochemical equipment and piping.

Problems solved by technology

Corrosion can also diminish the other necessary physical and mechanical properties, such as the high cycle fatigue strength, by initiation of surface cracks that propagate under the cyclic thermal and stresses associated with operation of the turbine.

At present, there are no high-strength steels available that sufficiently resist corrosion pitting to survive harsh marine / industrial environments, such as coastal industrial power plants, for more than 2-3 years.

While these martensitic stainless steels have provided a combination of corrosion resistance, mechanical strength and fracture toughness properties sufficient to make them suitable for use in rotating steam and gas turbine components, these alloys are still known to be susceptible to corrosion pitting phenomena.

Electrochemically-induced corrosion pitting phenomena occurring at the airfoil surfaces can in turn result in cracking of the airfoils due to the cyclic thermal and operating stresses experienced by these components.

Corrosive contaminants usually result from the environments in which the turbines are operating because they are frequently placed in highly corrosive environments, such as those near chemical or petrochemical plants, where various chemical species may be found in the intake air, or those at or near ocean coastlines or other saltwater environments where various sea salts may be present in the intake air, or combinations of the above, or in other applications where the inlet air contains corrosive chemical species.

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