The Study of Heat Treatment Effects on Chromium Carbide Precipitation of 35Cr-45Ni-Nb Alloy for Repairing Furnace Tubes

Abstract

This paper presents a specific kind of failure in ethylene pyrolysis furnace tubes. It considers the case in which the tubes made of 35Cr-45Ni-Nb high temperature alloy failed to carburization, causing creep damage. The investigation found that used tubes became difficult to weld repair due to internal carburized layers of the tube. The microstructure and geochemical component of crystallized carbide at grain boundary of tube specimens were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) with back-scattered electrons mode (BSE), and energy dispersive X-ray spectroscopy (EDS). Micro-hardness tests was performed to determine the hardness of the matrix and the compounds of new and used tube material. The testing result indicated that used tubes exhibited a higher hardness and higher degree of carburization compared to those of new tubes. The microstructure of used tubes also revealed coarse chromium carbide precipitation and a continuous carbide lattice at austenite grain boundaries. However, thermal heat treatment applied for developing tube weld repair could result in dissolving or breaking up chromium carbide with a decrease in hardness value. This procedure is recommended to improve the weldability of the 35Cr-45Ni-Nb used tubes alloy.

Investigation of the 0.4C-35Cr-45Ni-Nb Alloy after Service in High-Temperature Steam and Hydrocarbons Environment

“…Elemental composition (weight percentage) of the service exposed sample obtained analysis at three spots on the material's surface (the places marked 1-3 in Figure 5). The results of the investigation of the non-exposed reference sample are in ance with the literature data [6][7][8]. The metallographic tests revealed carbides pr the form of branched lines (Figure 3a) [9,10].…”

“…The obtained results reveal st differences in the material corresponding to the phases of various nobility. The results of the investigation of the non-exposed reference sample are in accordance with the literature data [6][7][8]. The metallographic tests revealed carbides present in the form of branched lines (Figure 3a) [9,10].…”

The Electrochemical Investigation of the Corrosion Rates of Welded Pipe ASTM A106 Grade B

“…On the other hand, the size of the γ-phase increased when the welding current was raised. The XRD results indicated that the heating from the welding current could change the metallurgical and crystallite properties of the γ-phase which have an effect on the mechanical and corrosive properties [24]. A possible explanation was that the austenite specimens exhibited properties different than those with ferrite phase.…”

Evaluation of creep rupture behaviour for weld joint and base metal of the Cr35Ni45Nb alloy

“…Owing to appropriate weldability and ductility, an improved ethylene pyrolysis furnace tube material involving about 35 mass% Cr, 45 mass% Ni and a total quantity of around 2 mass% Nb, Ti, Zr and W heatresistant cast steel is now extensively used in ethylene pyrolysis furnaces [10,11]. Because of the high carbon content of this type of heat-resistant cast steel, it generally contains a lot of carbides, such as low melting point γ + eutectic carbides, NbC, M 7 C 3 and M 23 C 6 [12][13][14]. For this reason, the weld joint produced from this steel is reported as having a higher weld cracking susceptibility, and most notably a higher hightemperature weld cracking susceptibility than general forged steels [15][16][17].…”