Austenitic Alloy for High Temperature Equipment and Manufacturing Process

Abstract

The objective of this invention is to obtain an innovative austenitic alloy resistant to oxidation, carburization and creep, especially useful for high temperature applications, such as hydrocarbon pyrolysis furnaces, reformer tubes used in the direct reduction of iron (DRI) process and other equipment that operate under severe temperature and pressure conditions, including an alloy characterized by a high aluminum content and a Cr/Al weight percent ratio≤8.0, which ensures the formation of a protective layer of aluminum oxide (Al2O3), stable and adherent to the surface, instead of a layer of chromium oxide (Cr2O3). The aluminum oxide layer provides protection against oxidation and carburization at temperatures above 1150° C. The alloy composition was formulated to ensure resistance to creep and coke attack, and to provide good weldability, in addition to minimizing the formation of intermetallic phases and carbides, which impair performance at high temperatures compared to conventional and competing alloys. A specific manufacturing process was developed to control the content of impurities and active elements, which are very reactive to oxygen and are very difficult to incorporate, in order to guarantee the quality and compositional uniformity of the alloy.

1 ) An austenitic alloy for high temperature equipment, resistant to oxidation, carburization and creep, especially useful for applications at temperatures above 1150° C., such as hydrocarbon pyrolysis furnaces, reformer tubes used in the direct iron reduction process, and other equipment that operates under severe temperature and pressure conditions, wherein the alloy presents an austenitic matrix and has the following chemical composition (in % by weight):
 Al: 2.5% to 5.5% 
 Cr: 24% to 36% 
 C: 0.3% to 0.55% 
 Ni: 34% to 65% 
 W: 0.2% to 16% 
 Nb:  0 . 05  to 2.00% 
 Ti:  0 . 01  to 0.2% 
 Zr:  0 . 01  to 0.2% 
 Fe: Remainder with unavoidable impurities 
 V: 0.01 to 0.2% (optional) 
 N: <0.05% 
 O: <0.003% 
 S: <0.03% 
 P: <0.03% 
 Si: <1.5% 
 Mn: <1.5% 
 
       and wherein the weight ratio of Cr/Al in the alloy is ≤8.0. 
     
     
         2 . A manufacturing process, for producing an austenitic alloy according to  claim 1 , the process comprising the steps of:
 a) melting inactive element materials, including carbon, in a first stage to form an initial melt;   b) refining the initial melt to remove impurities;   c) adding a slag removal material to the melt to absorb and capture oxides, nitrides, sulfides, and inclusions;   d) alloying active elements, including at least aluminum, titanium, and zirconium, into the refined melt in a subsequent stage under an inert gas atmosphere;   e) controlling the temperature of the melt during at least one of the refining or active element alloying steps to be no lower than 1700°C; and   f) casting the alloy.   
     
     
         3 . The manufacturing process according to  claim 2 , wherein the inactive element materials melted in step (a) comprise at least one of Ni, Co, Fe, Cr, or W. 
     
     
         4 ) The manufacturing process according to  claim 2 , wherein the inactive element materials melted in step (a) comprise at least one of Ni, Co, Fe, Cr, or W. 
     
     
         5 ) The manufacturing process according to  claim 2 , wherein the slag material added in step (c) comprises Calcium Oxide (CaO). 
     
     
         6 ) The manufacturing process according to  claim 2 , wherein the inert gas atmosphere in step (d) comprises argon gas. 
     
     
         7 ) The manufacturing process according to  claim 2 , wherein the temperature of the melt during the refining step (b) is controlled to not be lower than 1700° C. 
     
     
         8 ) The manufacturing process according to  claim 2 , wherein the casting step (f) comprises centrifugal casting or gravity casting. 
     
     
         9 ) The manufacturing process according to  claim 8 , wherein when centrifugation casting is used, a centrifugation speed is controlled to ensure desired solidification characteristics and quality of the final product.