Full-Thickness Decarburization of the Steel Shell of an Annealing Furnace

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1 Metallogr. Microstruct. Anal. (2012) 1:59 64 DOI /s TECHNICAL NOTE Full-Thickness Decarburization of the Steel Shell of an Annealing Furnace A. M. Dalley Received: 18 December 2011 / Accepted: 17 January 2012 / Published online: 28 February 2012 Ó Springer Science+Business Media, LLC and ASM International 2012 Abstract After 35 years of service, the steel shell of a continuous annealing furnace developed cracking problems, even after careful repairs. A metallurgical evaluation was performed on sections of the ASTM A36 steel furnace shell. Testing included chemical analysis, light metallography, mechanical testing, and hardness measurements. The evaluation found through-thickness decarburization of the 6 mm thick shell in the vicinity of open cracks. The strength of the shell decreased significantly in these areas. Laboratory decarburization tests performed on unaffected samples of ASTM A36 steel shell confirmed that field conditions existed that led to rapid through-thickness decarburization of the shell. High ambient humidity is believed to have contributed to rapid decarburization. Keywords Characterization Failure Metallography Microstructural examination Steels History The subject continuous annealing furnace was built in the 1960s in an area that experienced high seasonal humidity levels. The purpose of the furnace was to anneal or temper steel sheet to attain specific mechanical properties. The furnace atmosphere, which contained hydrogen and nitrogen, also reduced residual surface oxides and provided a clean surface prior to the immersion of the strip in the zinc galvanizing bath. A. M. Dalley (&) U.S. Steel Research & Technology Center, Munhall, PA, USA amdalley@uss.com The product mix required that the furnace cycle between zone temperatures of either 649 or 982 C. A small percentage of product was annealed at intermediate temperatures. Over time, this demanding thermal cycling damaged the refractory brick lining and eventually permitted hot furnace gases to contact the shell. The resulting thermo-mechanical fatigue generatedcracksintheastma36 steel shell, particularly at stress concentrators such as welds or at square corners of radiant heating tube openings. Large cracks were repaired by welding patch plates over them. To ensure that the quality of the product being annealed was maintained, the hydrogen content of the furnace atmosphere was increased to compensate for the amount lost through the cracks. The replacement of a large panel ( m) provided samples for the metallurgical evaluation described herein. Evaluation of Damaged Shell Material Figure 1 shows a large shell panel that was replaced. Multiple cracks were present, many associated with welds. A sample of the panel from an area away from the cracks was examined, where exposure to heat appeared minimal. Compositional analysis of the shell was performed using optical emission spectroscopy and total combustion techniques. The 6 mm thick shell met the compositional requirements of ASTM A36 carbon steel (Table 1). The microstructure consisted of fine ferrite and pearlite, a typical product of a normalizing heat treatment (Fig. 2). The hardness averaged 76 Rockwell B (HRB). Results of duplicate tensile tests from this area are shown in Table 2. This material appears to represent the original shell plate, having met the requirements of ASTM A36 steel plate. Cracked areas were then sectioned. Figure 3 shows a slice made through a large (12 mm) crack in the shell that

2 60 Metallogr. Microstruct. Anal. (2012) 1:59 64 was covered with a patch plate. Metallographic specimens were sectioned from the areas indicated with dashed lines and were mounted, polished, and etched (nital-picral etchant). The region between the combination weld (I-beam plus patch plate) and the crack edge had a heavily decarburized microstructure. Large columnar ferrite grains had nucleated at both interior and exterior surfaces and had grown inward, eventually impinging on each other (Fig. 4). The fine-grained ferrite (ASTM grain size 8) between the columnar grain fronts was heavily decarburized. A low concentration of spheroized carbides remained to pin grain boundaries in the fine-grained region (Fig. 5). Similar features were seen on the opposite side of the crack where through-thickness decarburization was achieved before the patch plate was welded onto the shell, as seen by the finer ferrite grains in the weld heat-affected zone (Fig. 6). Fig. 1 Steel shell panel removed from furnace after 35 years of service Hardness readings of the fine-grained decarburized ferrite in Fig. 4 averaged 60 HRB, lower than the hardness reading of 76 HRB in the original microstructure. The hardness of the columnar ferrite was even lower, measuring only 41 HRB. A value for the ultimate tensile strength (UTS) of the decarburized region was not available. It could not be measured by tensile testing. Also, the hardness was below the lower limit of tables that relate steel hardness to equivalent UTS. Chemical analysis of a decarburized region of the shell found that its carbon content Table 1 Composition of shell samples before and after decarburization (wt.%) Element Typical shell specimen Specified for ASTM A36 plain carbon steel Shell decarburized in service Typical IF steel composition C max Mn P max \0.01 S max Si max max Cu Ni Cr Mo V \0.002 \0.002 Ti \0.02 \ Al Nb \ Fig. 2 The typical shell microstructure consisted of finegrained ferrite and pearlite

3 Metallogr. Microstruct. Anal. (2012) 1: dropped to wt.%, very similar to a soft, interstitialfree (IF) steel (Communications with B. M. Hance, 2003, Table 1). Without the addition of grain-refining elements found in IF steels, the ferrite grains in the decarburized furnace shell measured as large as 3 mm long and 1 mm wide. The cause of decarburization was hypothesized to be related to localized atmospheric conditions in the vicinity of the cracks, which differed significantly from conditions within the heating zone adjacent to the steel strip. Open cracks allowed the hydrogen-rich atmosphere to combine with moisture in the ambient air. Where a crack or leak was present, dissociated oxygen from humid air can combine with carbon in the steel to form CO/CO 2. This created a gradient of carbon in the steel from almost zero at the surface to 0.23 wt.% in the non-decarburized areas. As the carbon level fell below a critical level, there was insufficient carbon or other alloying elements to prevent rapid growth of high-purity columnar ferrite grains. Table 2 Mechanical properties of shell samples before decarburization Property Typical shell specimen Specified for ASTM A36 plain carbon steel Yield strength, ksi min UTS, ksi Fig. 5 The large ferrite grains grew into the fine decarburized ferrite grains Fig. 3 Section containing a 12 mm crack and patch plate Fig. 4 Severe decarburization and inward growth of columnar ferrite grains from the surface (arrows) were discovered along crack edges. The boxed area is shown in Fig. 5

4 62 Metallogr. Microstruct. Anal. (2012) 1:59 64 Laboratory Decarburization Studies Studies were conducted to determine conditions and rates at which decarburization can occur in this 6 mm thick ASTM A36 steel. For the laboratory studies, specimens were sectioned from actual shell panel areas that were free from decarburization. Sample bars measured approximately mm. To replicate actual performance of the cracked furnace shell in service, no surface cleaning was done. Specimens were supported on a steel mesh boat that permitted contact with flowing gases on all surfaces (Fig. 7). The boat was placed into a tube furnace (Fig. 8) with a high-purity hydrogen nitrogen gas mixture flowing over the specimen. Dew point was controlled by a dew point monitor equipped with a refrigerated constant temperature circulator and an auxiliary heater. This system was limited to operating at dew points greater than -4 C. Each test was conducted for 64 h. Table 3 lists the eight Fig. 6 Through-thickness decarburization was found more than 200 mm from a crack Fig. 7 Decarburization sample taken from unaffected area of the shell Table 3 Laboratory furnace environment for decarburization study (run time for each test = 64 h) Sample designation Temperature ( C) Hydrogen in furnace environment, % Dew point, C A B C D E F G H Average decarburization depth from single side, mm Fig. 8 Laboratory equipment used for the decarburization study

5 Metallogr. Microstruct. Anal. (2012) 1: Fig. 9 Decarburization condition A shows impingement of ferrite grains at edges Fig. 10 Decarburization condition C illustrates growth of ferrite grains from outer edges decarburization parameter combinations that were tested. Metallographic examination was performed on specimens from the centers of the bars to avoid end effects. Depth of decarburization was determined by measuring the average depth of penetration of the columnar ferrite grains. Results In the first four tests, the temperature was set at either 760 or 816 C, dew point at 4 or 16 C, and hydrogen at 15 or 25% based on annealing furnace conditions and information in the literature [1]. The matrix of conditions and the resulting depth of decarburization are listed in Table 3. Figures 9 and 10 illustrate the results of test conditions A and C, respectively. In the laboratory decarburization samples, the formation of columnar grains and their impingement at the corners was similar to the columnar grains found near the shell cracks. Temperature was identified as having the greatest influence. Decarburization progressed slightly more rapidly at 760 C and generated larger ferrite grains than at 816 C. The next two tests, E and F, were designed to gain information on the effect of decreasing temperature on decarburization. These were performed at 704 and 649 C, holding constant hydrogen (25% H 2 ) and dew point (16 C) levels. Note that decarburization was nearly as severe at 704 C as it was at 816 C. The primary microstructural difference was that the diameter of the columnar grains increased as the temperature decreased. At 649 C, decarburization was markedly slowed. The final two tests, G and H, evaluated the effects of decreasing the dew point, subject to constraints of the equipment. It was hypothesized that a very low dew point may arrest the decarburization reaction even if temperature and hydrogen concentrations were high. This hypothesis was based on the actual situation inside non-cracked areas of the furnace where the temperature was high, the atmosphere had high hydrogen

6 64 Metallogr. Microstruct. Anal. (2012) 1:59 64 but the dew point was low (about -29 C), yet the exposed furnace shell did not decarburize. Indeed, the decarburization was less severe. Conclusions This evaluation confirmed that conditions required for shell decarburization can arise in service for a continuous annealing furnace located in high humidity areas. The localized shell temperature would need to exceed 649 Cin the presence of an open crack. During the high temperature process with heater tubes firing at 982 C, the active temperature range could be attained at the shell once damage to refractory linings allowed heated gas to reach the shell. Cracks eventually opened in the shell due to thermo-mechanical fatigue cycling. Humid, ambient air then mixed with dry, hydrogen-rich furnace gas. Oxygen from dissociated moisture in the air could combine with carbon that diffused out of the steel to form gaseous CO/CO 2. Although lower limits were not determined experimentally, a dew point of -3 C, a hydrogen concentration of 15%, and a temperature of 704 C appeared to be sufficient to cause through-thickness decarburization within a week. The resultant loss in mechanical strength affected the rate at which the cracks propagated. This information was used to improve the repair procedures for this furnace. Reference 1. A.R. Marder, S.M. Perpetua, J.A. Kowalik, E.T. Stephenson, The effect of carbon content on the kinetics of decarburization in Fe C alloys. Metall. Trans. A 16A, (1985)