Carburization of austenitic and ferritic alloys in hydrocarbon environments at high temperature

A Serna,R A Rapp

doi:10.3989/revmetalm.2003.v39.iextra.1114

Abstract

The technical and industrial aspects of high temperature corrosion of materials exposed to a variety of aggressive environments have significant importance. These environments include combustion product gases and hydrocarbon gases with low oxygen potentials and high carbon potentials. In the refinery and petrochemical industries, austenitic and ferritic alloys are usually used for tubes in fired furnaces. The temperature range for exposure of austenitic alloys is 800-1100 °C, and for ferritic alloys 500-700 °C, with carbon activities a c > 1 in many cases. In both applications, the carburization process involves carbon (coke) deposition on the inner diameter, carbon absorption at the metal surface, diffusion of carbon inside the alloy, and precipitation and transformation of carbides to a depth increasing with service. The overall kinetics of the internal carburization are approximately parabolic, controlled by carbon diffusion and carbide precipitation. Ferritic alloys exhibit gross but uniform carburization while non-uniform intragranular and grain-boundary carburization is observed in austenitic alloys.

Highlights

In industrial environments such as in oil refineries and petrochemical plants, engineering alloys such as Fe-Cr-Mo and Fe-Cr-Ni often come into contact with corrosive gases containing oxygen, sulfur, carbon, and nitrogen at a relatively low oxygen activities and high total pressure
It is important to select the alloy composition or control the environment to minimize the damage produced by interactions with aggressive oxidants. These materials are selected because a scale rich in Cr203 is formed on the surface of these structural components which protects the material against high temperature corrosion
The main object of this paper is to compare the morphological differences in carburization occurred in austenitic and ferritic alloys exposed for a long time in environments with carbon activity over one in many cases

Summary

INTRODUCTION

In industrial environments such as in oil refineries and petrochemical plants, engineering alloys such as Fe-Cr-Mo and Fe-Cr-Ni often come into contact with corrosive gases containing oxygen, sulfur, carbon, and nitrogen at a relatively low oxygen activities and high total pressure. The alloys used in these furnaces are typically wrought stainless steels or Fe-Cr-Ni centrifugal cast alloys (HK, HP) In both cases, it is important to select the alloy composition or control the environment to minimize the damage produced by interactions with aggressive oxidants. It is important to select the alloy composition or control the environment to minimize the damage produced by interactions with aggressive oxidants These materials are selected because a scale rich in Cr203 is formed on the surface of these structural components which protects the material against high temperature corrosion. Metals or alloys are generally susceptible to carburization when exposed to an environment containing CO, CH4 or other hydrocarbon gases such as ethylene (C2H4) or propane (C3H8) at elevated temperatures. During thermal and steam cracking operation, carbon is deposited in the form of coke on the internal surfaces of the tubes. Carburization attack generally results in the formation of internal carbides that often cause the alloy to suffer embrittlement as well as other mechanical degradation'^', especially at low temperature

THERMODYNAMICS

FIELD CARBURIZATION

C Mn P S Si Cu Ti Co Sn Cr Mo V Ni AI Nb Fe