Martensitic Stainless Steel Strengthened by Copper-Nucleated Nitride Precipitates

Abstract

A martensitic stainless steel alloy is strengthened by copper-nucleated nitride precipitates. The alloy includes, in combination by weight percent, about 10.0 to about 12.5 Cr, about 2.0 to about 7.5 Ni, up to about 17.0 Co, about 0.6 to about 1.5 Mo, about 0.5 to about 2.3 Cu, up to about 0.6 Mn, up to about 0.4 Si, about 0.05 to about 0.15 V, up to about 0.10 N, up to about 0.035 C, up to about 0.01 W, and the balance Fe and incidental elements and impurities. The nitride precipitates may be enriched by one or more transition metals.

What is claimed is: 
     
         1 . A method comprising:
 providing a martensitic stainless steel strengthened by copper-nucleated nitride precipitates comprising, in combination by weight percent, about 10.0 to about 12.5 Cr, about 2.0 to about 7.5 Ni, up to about 17.0 Co, about 0.6 to about 1.5 Mo, about 0.5 to about 2.3 Cu, up to about 0.6 Mn, up to about 0.4 Si, about 0.05 to about 0.15 V, up to about 0.10 N, up to about 0.035 C, up to about 0.01 W, and the balance Fe and incidental elements and impurities; and   aging the alloy at a temperature between 420° C. and 496° C.,   wherein, after aging, the alloy has a tensile yield strength of about 1040 to 1360 MPa and an ultimate tensile strength of about 1210 to 1580 MPa.   
     
     
         2 . The method of  claim 1 , wherein the alloy has a martensite start temperature of at least about 50° C. 
     
     
         3 . The method of  claim 1 , further comprising, before the aging:
 subjecting the alloy to a solutionizing heat treatment; and   cooling the alloy in liquid nitrogen for a period of time.   
     
     
         4 . The method of  claim 1 , wherein, after aging, the alloy has an ambient impact toughness of at least about 10 ft·lb. 
     
     
         5 . The method of  claim 1 , wherein the alloy has a martensite start temperature above about 50° C. 
     
     
         6 . The method of  claim 1 , wherein the alloy has a ductile to brittle transition temperature below about 20° C. 
     
     
         7 . The method of  claim 1 , wherein, after aging, the alloy comprises precipitates of a copper-based phase and nitride precipitates enriched with transition metals. 
     
     
         8 . The method of  claim 7 , wherein, during aging, the nitride precipitates nucleate on the copper-based phase. 
     
     
         9 . The method of  claim 8 , wherein the copper-based phase comprises at least one alloying element selected from a group consisting of: iron, nickel, chromium, cobalt, and manganese, and is coherent with the martensite phase, and the nitride precipitates have a hexagonal structure and comprise at least one metal selected from a group consisting of: chromium, molybdenum, and vanadium. 
     
     
         10 . The method of  claim 1 , wherein the stainless steel comprises, in combination by weight percent, about 10.0 to about 12.0 Cr, about 6.5 to about 7.5 Ni, up to about 4.0 Co, about 0.7 to about 1.3 Mo, about 0.5 to about 1.0 Cu, about 0.2 to about 0.6 Mn, about 0.1 to about 0.4 Si, about 0.05 to about 0.15 V, up to about 0.09 N, about 0.005 to about 0.035 C, and the balance Fe and incidental elements and impurities. 
     
     
         11 . The method of  claim 9  wherein said alloy has a microstructure substantially absent cementite carbides and comprises a martensite matrix with nanoscale copper particles and alloy nitride precipitates selected from the group consisting of alloy nitride precipitates enriched with a transition metal nucleated on the copper precipitates, said alloy nitride precipitates having a hexagonal structure, said alloy nitride precipitates including one or more alloying elements selected from the group Fe, Ni, Cr, Co and Mn coherent with the matrix, and said alloy nitride precipitates having to dimensional coherency with the matrix.