Investigations on carburizing mechanisms of Cr35Ni45Nb subjected to different service conditions in a high-temperature v
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Chenyang Du Department of Pressure Vessel, China Special Equipment Inspection and Research Institute, Beijing 100013, China (Received 31 August 2014; accepted 21 January 2015)
The carburizing behaviors and mechanisms for Cr35Ni45Nb alloy subjected to different service conditions were studied in a high-temperature vacuum environment. Generally, the carburizing process of an alloy is always accompanied by diffusional heterogeneous reactions regardless of the service condition of the alloy. For a carburized original tube, there is a layered structure at the inner wall of the tube, which is comprised of a M7C3 zone, a M7C3–M23C6 mixed zone, and a M23C6 zone with different morphologies. However, for a 6-year tube (short for a tube serviced for 6 years), the composite oxide layers formed previously act as effective barriers to carbon infiltration. Moreover, the Cr2O3 scale tended to be carbonized to form carbide scale to spall from the surface in a reducing environment, while the SiO2 kept stable all along. Once the oxide layers were removed or carbonized enough, inconceivable internal carburization occurred widely.
I. INTRODUCTION
Pyrolysis is the most important process to produce ethylene and other products in the petrochemical industry.1,2 In recent years, the temperature of radiant core tubes (RCTs) for pyrolysis has been continuously increased for the purposes of increasing yield of ethylene and reducing cost. Typically, the RCTs serviced in an oxidizing/carburizing environment for a long time tend to be damaged by serious oxidation and carburization attack.3 Thus, high-temperature alloys cater to the demand for better anticorrosion performance.4 In the 1960s, to replace previous 304 stainless steel, high-strength centrifugally cast tube made of HK40 (25Cr–20Ni) was applied to pyrolysis facilities and was widely used in the petrochemical industry due to its outstanding creep properties.5 However, as lower level of Cr and Ni contents cannot ensure the antioxidizing and anticarburizing capability of the tube, massive corrosive failures were incurred. Thus, heatresistant alloys with both better mechanical and anticorrosion performances, such as HP40 (25Cr–35Ni) and 35Cr–45Ni alloys, were developed. In general, chromium and silicon oxide scales may be formed on the inner surface of RCTs to prevent carbon atoms from penetrating into the tubes.6–8 Thus, higher Cr content is undoubtedly necessary to form continuous Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.46 J. Mater. Res., Vol. 30, No. 6, Mar 28, 2015
chromium oxide scale, while higher Ni content is necessary to reduce the solubility of C in Ni–Cr–Fe solid solution and to stabilize the c-phase region. Normally, the Cr content beneath the surface is required to exceed 10 (wt%) all the time to guarantee the continuity of the scale.9,10 Nevertheless, many RCTs fail in service life due to carburization caused by failure of the oxide scale.11 Generally, if chromium oxide scale is damag
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