Method for producing a tempered martensitic heat resistant steel for high temperature application

Abstract

A method for producing a tempered martensitic heat resistant steel for high temperature applications at an application temperature of up to 650° C. and to a steel produced by the method. The use of the steel in the production of components for high temperature applications such as turbine blades or casings, bolting and boiler tubes, heat exchangers or other elements in power generation systems.

1 . A method for producing a tempered martensitic heat resistant steel for high temperature applications at an application temperature of up to 650° C., wherein the steel comprises, on the basis of percent by weight:
 8.5 to 12% Cr, 
 up to 0.13% C, 
 0.15 to 0.5% Si, 
 up to 2.0% W, 
 up to 3.0%Co, 
 up to 2% Cu, 
 up to 0.8% Mn, 
 up to 1.0% Mo, 
 0.1 up to 0.7% Ni, 
 up to 0.04% Al, 
 between 0.001 and 0.015 B, 
 between 0.005 and 0.07 N, 
 up to 0.25%V, 
 at least one of from 0.01% to 0.09% Nb and/or from 0.01% to 0.14% Ta, 
 balance iron and inevitable impurities; 
 
       wherein the C:N ratio is below 1.3 to favor formation of nano-scale carbo-nitrides of the M(C,N) and/or M 2 (C,N)-type and to reduce the fraction of M 23 (C,B) 6  precipitates, the process comprising the steps of
   solution treating the steel in the austenite range at a temperature below the transformation temperature to delta-ferrite and between 1150 and 1250° C. to dissolve all precipitates including boron-nitrides and carbo-nitrides thereby bringing the precipitating elements in solid solution;   quenching the steel after solution treating to create a fully martensitic matrix and to suppress precipitation on cooling;   tempering the steel in one or more tempering treatments after quenching to precipitate of nano-scale particles M(C,N) or M 2 (C,N) particles, or mixtures thereof, at a tempering temperature between 10 to 50° C. higher than the application temperature, wherein the application temperature is up to 650° C.   
 
     
     
         2 . The method according to  claim 1 , wherein
 the solution treating is performed between 1150° C. and 1250° C., and/or   wherein the quenching is performed in oil.   
     
     
         3 . The method according to  claim 1 , wherein the steel comprises:
 between 8.5 and 11% Cr 8.5 11%Cr and/or   between 1.0 and 2.0% W, and/or   between 1 and 2% Co if Cr 10%, and/or   up to 1.5% Cu, and/or   up to 0.6% Mn, and/or   up to 0.8% Mo, and/or   up to 0.5% Ni, and/or   between 0.15 and 0.25%V, and/or   between 0.03 and 0.09% Nb, and/or   between 0.05 and 0.12% Ta, and/or   C:N<1.3.   
     
     
         4 . The method according to  claim 1 , wherein the steel comprises:
 8.5 to 9.5% Cr, and/or   between 0.07 and 0.13% C, and/or   between 1.5 and 2.0% W, and/or   between 0.30 and 0.60% Mn, and/or   between 0.3 and 0.6% Mo, and/or,   up to 0.4% Ni, and/or   between 0.001 and 0.006% B and/or   between 0.03 and 0.07% N and/or   between 0.18 and 0.25%V, and/or   between 0.04 and 0.07% Nb.   
     
     
         5 . The method according to  claim 1 , wherein the tempering treatment comprises at least two separate heat treatments. 
     
     
         6 . The method according to  claim 5 , wherein the at least two separate heat treatments are performed at substantially the same tempering temperature for substantially the same period of time. 
     
     
         7 . The method according to  claim 5 , wherein the period of time at the tempering temperature is between 1 and 5 hours. 
     
     
         8 . The method according to  claim 5 , wherein the first of the at least two separate heat treatments is at the a temperature range from 500° C. up to 10° C. higher than the application temperature and second is or the following are at the temperature range between 10 and 50° C. higher than the application temperature. 
     
     
         9 . A tempered martensitic heat resistant steel for high temperature applications wherein the steel comprises, on the basis of percent by weight:
 8.5 to 11.0% Cr,   up to 0.13% C,   0.15 to 0.5% Si,   up to 2.0% W,   up to 3.0%Co,   up to 2% Cu,   up to 0.8% Mn,   up to 1.0% Mo,   0.1 to 0.7% Ni,   up to 0.04% Al,   between 0.001 and 0.015% B,   between 0.005 and 0.07% N,   up to 0.25% V,   at least one of up to 0.09% Nb and/or up to 0.14% Ta,   balance iron and inevitable impurities;   wherein the C:N ratio is below 1.3 to favor formation of nano-scale carbo-nitrides of the M(C,N) and M 2 (C,N)-type and to reduce the fraction of M 23 (C,B) 6  precipitates, the martensitic steel made by a process comprising the steps of
 solution treating the steel in the austenite range at a temperature below the transformation temperature to delta-ferrite and between 1150 and 1250° C. to dissolve all precipitates including boron-nitrides and carbo-nitrides thereby bringing the precipitating elements in solid solution; 
 quenching the steel after solution treating to create a fully martensitic matrix and to suppress precipitation on cooling; 
 tempering the steel in one or more tempering treatments after quenching to precipitate nano-scale particles M(C,N) or M 2 (C,N) particles, or mixtures thereof, at a temperature between 10 to 50° C. higher than the application temperature, wherein the application temperature is up to 650° C., 
   wherein the microstructure of the steel after tempering comprises intragranular precipitates having a size of at most 70 nm of the M(C,N) and/or M 2 (C,N) type wherein M is one or more Nb, V, Ta or Cr and wherein the microstructure of the steel after tempering comprises M 23 (C,B) 6  precipitates wherein M is mainly composed of Cr and Fe on the lath, block, packets and/or prior austenite grain boundaries.   
     
     
         10 . The steel according to  claim 9  wherein the M(C,N) and/or M 2 (C,N) precipitates have a size of between 10 and 70 nm. 
     
     
         11 . The steel according to  claim 8 , wherein Ta and V are both present as an alloying element. 
     
     
         12 . The steel according to  claim 9 , in a form of a component for high temperature applications selected from the group consisting of turbine blades, turbine casings, bolting and boiler tubes, and heat exchangers. 
     
     
         13 . The steel according to  claim 9 , for use at an application temperature of up to 650° C. 
     
     
         14 . The steel according to  claim 9 , in a form of a component for high temperature applications in power generation systems. 
     
     
         15 . The method according to  claim 5 , wherein the period of time at the tempering temperature is between 2 and 4 hours. 
     
     
         16 . The method according to  claim 1 , wherein the quenching of the steel after solution treating is performed as fast as possible to create the fully martensitic matrix and to suppress precipitation on cooling. 
     
     
         17 . The steel according to  claim 9 , wherein the quenching of the steel after solution treating is performed as fast as possible to create the fully martensitic matrix and to suppress precipitation on cooling. 
     
     
         18 . The steel according to  claim 9 , wherein the M(C,N) and/or M 2 (C,N) precipitates have a size of between 10 and 50 nm. 
     
     
         19 . The steel according to  claim 9 , wherein the M(C,N) and/or M 2 (C,N) precipitates have a size of between 10 and 30 nm.