Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes

Metallogr. Microstruct. Anal. (2015) 4:273–285 DOI 10.1007/s13632-015-0210-8 TECHNICAL ARTICLE Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes A. C. McLeod1 • C. M. Bishop1 • K. J. Stevens2 • M. V. Kral1 Received: 22 March 2015 / Revised: 4 June 2015 / Accepted: 7 June 2015 / Published online: 25 June 2015  Springer Science+Business Media New York and ASM International 2015 Abstract A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M23C6 carbides transformed to M7C3 at the inner wall region and NbC carbides partially transformed to the chromium-rich g-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M23C6/M7C3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response. & M. V. Kral 1 Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand 2 Quest Integrity NZL Ltd, Part of the Quest Integrity Group, Gracefield Research Centre, 69 Gracefield Road, PO Box 38-096, Lower Hutt 5045, New Zealand Keywords Scanning Electron Microscopy
Steels
Phase transformations
Metallography Introduction Ethylene is used extensively in the production of plastics, cabling, and automotive products, and is typically produced by the thermal cracking (pyrolysis) of more complex hydrocarbons, such as naphtha or ethane, at 700–1100 C inside a tube in a pyrolysis furnace. In order to provide the corrosion resistance and creep strength necessary to withstand such harsh operating conditions, the heat resisting (H-series) centrifugally cast, austenitic stainless steels are typically used for pyrolysis tubes [1, 2]. Carburization is one of the major degradation mechanisms of pyrolysis tubes [3]. High carbon potentials at the inner diameter due to the cracking reaction and high tube temperatures result in the diffusion of carbon into the tubes, where it combines with chromium and other carbideforming elements to grow existing carbides and create new ones [4, 5]. The carbides undergo chemical and morphological changes, initially coarsening and coalescing, followed by phase transformation [4]. Internal carbide precipitation results in considerable volume increases, changes in thermal expansion coefficient of the carburized zone, and embrittlement. The result is a reduction in ductility as well as increase in internal stresses, adversely affecting the creep properties and the ability of the tube to withstand thermal cycling [2, 5–8]. Consequently, there is interest in non-destructive testing (NDT) methods to evaluate the level of carburization of tubes in situ. Carbide precipitation and the segregation of elements in HP alloy tubes result in a change in the magnetic properties of the carburized region, allowing 123 274 Metallogr. Microstruct. Anal. (2015) 4:273–285 carburization to be detected non-destructively using eddy current techniques [6, 7, 9]. However, the eddy current systems require calibration using ex-service pyrolysis tubes that have had their microstructure, mechanical properties, and magnetic response characterized [6, 9]. This paper presents the results of detailed microstructural characterization of a highly carburized ex-service ethylene pyrolysis tube. The purpose of the paper is to determine whether there are certain microstructural features or compositional variations (e.g., chromium depletion) that dictate the eddy current NDT response of ex-service tubes. Materials and Methods An ex-service HP40-Mod alloy ethylene pyrolysis tube was examined. It had spent approximately 4 years in service, at 20 psig and 1300 F (138 kPag and 704 C). The typical as-cast composition of an HP40-Mod alloy is given in Table 1 [10]. The tube had an outer diameter of 100 mm, and an average wall thickness of 7.7 mm. A ring approximately 10-mm thick was cut from the tube, and a metallographic sample approximately 8 9 10 mm was sectioned and mounted in order to observe the through-wall face. The sample was ground on SiC paper at 180, 240, 320, 400, and 600 grit, then polished with 9 and 3 lm diamond suspension. A final polish to a 0.02-lm finish was achieved using colloidal silica. Prior to electron back-scattered diffraction (EBSD) analysis, the sample was etched in glyceregia for 10 s, in order to increase the visibility of the precipitates. The composition of the glyceregia was 30 ml of glycerol, 30 ml of hydrochloric acid, and 10 ml of nitric acid. Phase identification was carried out using a JEOL JSM 7000 field emission scanning electron microscope (FESEM), equipped with a JEOL JED-2300 energy dispersive x-ray spectroscopy (EDS) system, and a JEOL 6100 SEM, equipped with an Oxford HKL Channel 5 EBSD system. A combination of back scatter electron (BSE) imaging, EDS, and EBSD was used to identify the precipitate phases present in the tubes and determine their locations across the tube wall. BSE images were taken at set intervals across the tube wall, with multiple EDS spectra taken for each phase at each location. The appearance of each phase in the BSE images and the composition determined by EDS were used to produce a list of candidates for each phase. Crystal structure was then confirmed using EBSD. The chromium depletion of the matrix was measured using EDS. The spectra were taken at a distance of Table 1 Typical Composition of an HP40-Mod alloy [10] Wt% C Cr Ni Nb Si Mn Fe HP-40-Mod 0.4 25 35 1.5 1.5 1.5 Bal. 123 approximately 10 lm from primary chromium carbides. In the highly carburized region of the sample, 10 lm was approximately the distance at which a point in the matrix was equidistant from surrounding chromium carbides. Progressive deep etching was used in order to gain an understanding of the three-dimensional structure of the carbide network. The sample was imaged in BSE mode at multiple locations in the as-polished condition, and the same locations were imaged in SEI mode after 15, 30, 60, 120, and 180 min etching using glyceregia. Glyceregia was chosen as the etchant as in HP alloys it only etches away austenite, leaving the carbide network intact [10]. The etching method described in the NACE Internatio (...truncated)