5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT Guwahati, Assam, India
Effect of Microstructure with Hardness on Heat Treatment of HP40Nb Microalloyed Reformer Tube Amitava Ghatak* and P.S.Robi Department of Mechanical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India -781039 *Email:
[email protected] Abstract Premature failure of service exposed centrifugally cast HP40Nb microalloyed steel are due to either creep, fatigue or corrosion and in few cases may be due to the combined effect of these phenomenon. Limited amount of information is available in the open literature regarding the effect of temperature and time on microstructure and mechanical properties of these materials. The present work was aimed at investigating the effect of heat treatment temperature and time on the microstructure and hardness of HP40Nb microalloyed steel. Heat treatment of the steel was carried out at temperatures in the range 1073 K - 1473 K for 8 hours and 72 hours. Presence of Cr rich and Nb rich carbides at grain boundary regions were observed. Image analysis of optical photomicrographs revealed the 8 vol.% and 2 vol.% of Cr carbide and Nb carbide respectively at the grain boundaries. The grain size of the HP40Nb steel increased from 64µm in the as-received condition to 91µm after heat treatment at 1073 K whereas the grain size increased to 96µm when heat treated at 1473 K for 72 hours. The hardness decreased only marginally when heat treated at 1073 K for 72 hours, whereas it decreased drastically when exposed at 1473K. The hardness of the steel decreased with increase in heat treatment time. Keywords: HP40Nb, heat treatment, Grain size, Hardness.
1 Introduction Centrifugally cast microalloyed HP40Nb steel is increasingly used in petrochemical industries as reformer tube at service temperature in the range of 1173 K to 1273 K and gas pressure around 2.3-2.8 MPa for prolonged period. Reformer tubes are the heart of the plant and any failure of this section results in premature shutdown leading to huge economic losses. Most of the reformer tubes fail due to creep deformation. The creep degradation process of the high temperature material is due to initiation, growth and propagation of grain boundary cavities at elevated temperature and stress [Mertingeret al. (2013)]. For the safety during performance, researchers assess the residual life using parametric models and tensile properties of reformer tubes [Jahromi and NaghiKhani(2004), Bonaccorsiet al. (2014), Rayet al.(2011), Swaminathanet al. (2008), Ghatakand Robi (2013), Alvinoetal. (2010)]. Hardness is effective indetermining the degradation the material since it is sensitive to the microstructural changes. Researchers found that the presence of carbides in microstructure has a decisive effect on mechanical properties [Ion et al. (1991), Mengket al. (1994), Andréset al. (1998), Yanet al.(2011)]. Liu and Che(2011) reportedan increase in the interdendritic carbide content with increase in ageing time at a constant temperature of 1173 K. However, the
effect of temperature on grain size and hardness of the material is not yet clear. The present study was aimed at investigating the effect of heat treatment time and temperature on the grain size and hardness of HP40Nb steel.Microstructure of the steel was studied by optical microscope and scanning electron microscope (SEM) attached with energydispersive x-ray analysis(EDX). Image analysis (IA) technique was used to quantify the interdendritic carbides content at grain boundaries.
2 Material and Experimental Procedure The reformer tubes used centrifugally cast HP40Nb micro-alloyed austenitic stainless steel obtained from Numaligarh Refineries Limited, Assam, India. The material was supplied in the form of tube with inside diameter of 106 mm and thickness 15.3 mm. The tube was supplied after exposure to 923 K for 11years. The chemical composition of the steel analyzed by optical emission spectrometeris shown in Table 1. Table 1 Chemical composition in weight percentage of the steel C
Si
Mo
Cr
Ni
Nb
Ti
Fe
0.4
1.3
0.037
23.6
34.9
0.8
0.037
Bal.
471-1
Effect of Microstructure with Hardness on Heat Treatment of HP40Nb Microalloyed Reformer Tube
Specimensof size 15.6 mm (length) x 10 mm (width) x 5 mm (height) were prepared by wirecutelectric discharge machining (EDM). The samples were heat treated at 1073 K, 1373 K and 1473 K in a resistance heated muffle furnace. After achieving the test temperature, the samples were further soaked for 8 h and 72 h before water quenching to room temperature.The heat treated specimens were polished for microscopic observation following the standard metallographic polishing techniques. The microstructure and elemental analysis of various phases were examined by using upright optical microscope and SEM coupled with EDX. The amount of various carbides was determined by quantitative IA technique using the optical photomicrographs whereas the grain size was measured by line intercept method. Hardness of the material before and after heat treatment was determined using a Vickers hardness tester. The testing was carried out by indenting the surface with a diamond indenter at a load of 30kgffor 18 s. The hardness values reported in the present paper are the average of nine readings.
3 Results and discussion The microstructure of the as-received specimen is shown in figure 1. Low magnification observation (Fig. 1(a)) reveals presence of second phase particles at grain
boundary regions. SEM image of the as-received specimen shown inFigure 1(b) indicates three types of second phase particles at austenitic grain boundary regions: (i) a dark grey phase (phase–A), (ii) a white phase (phase-B)with an average length of around 14µm, and (iii) a bright phase (phase-C)with an average size of around 2.23 µm. These phases were identified as Crrich, Nb-rich and (Nb,Ti) rich carbides respectively. Figure 2 show the EDX spectrum for the three phases. Volume fraction of phase-Awas observed to be more than that of phase-B and phase-C.
Figure 2 EDX spectrum of phase-A, phase-B and phase-C shown in figure 1(b) Figure 3plots thegrain size measured after heat treatment at different temperatureand time. The asreceived grainsize (at 300 K) was found to be 64 µm. For a constant soaking time, the grain size increased with increase in temperature. The grain size increaseddue to the heat treatment at higher temperatures. At 1473K, the grain size increased to 91µm and 96 µm when soaked for 8 hours and 72 hours, respectively.
(a)
(b) Figure 1 Optical photomicrographs of as-received steel under (a) optical micrograph and (b) SEM
Figure 3Grain size vs. temperature plot for 8 h and 72 h of heat-treatment Image processing of the digital images, obtained from the optical microscope, was carried out to separate the various carbides present in the microstructure and
471-2
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT Guwahati, Assam, India
subsequently determine its volume fraction. Figure 4(a) shows the digital images of the microstructure,whereasfigure 4(b) and (c) shows the same field after image processing indicating the total carbide and niobium carbide, respectively for sample heat-treated at 1173 K for 72 h. The volume fraction of Cr-rich carbide was estimated after deducting the volume of Nb-rich carbide from the total carbide content. Since, the volume fraction of (Ni,Ti) carbide was found to be very less this was not analyzed separately and was considered along with the Nb-rich carbides. The temperature-wise variation of Cr-rich carbide and Nb-rich carbide content are depicted in figure 5. The Cr-rich and Nb-rich carbide content in the as-received sample was 8.4 vol.% and 2.3 vol.% respectively. After heat treatment at 1473 K for 8 h, the amount of Cr-rich carbide was found to increase whereas the Nb-rich carbide remained almost the same.
up to 1073 K, it decreased by around 24% and 29% when heat-treated at 1473 K for 8 hours and 72 hours respectively.The decrease in hardness can be attributed to the increase in grain size as well as due to the decomposition of the Cr-carbide during the heat treatment at temperatures above 1073K.
Figure 5 Variation of Cr and Nb-rich carbides with test temperature for 8 h and 72 h
(a)
(b) (c) Figure 4 (a) Optical photomicrograph of the sample after heat-treatment for 72 h at 1173 K. The same area after image processing shows the distribution of (b) total carbide and (c) Nb-rich carbide
Figure 6 Effect of temperature on hardness on heat-treated samples
4 Conclusions However, when heat treated for 24 hours at the same temperature, the Cr-rich phase and Nb-rich phase was found to be 7.9 vol.% and 3.1 vol.% respectively. The resultsindicate that with prolonged soaking time, Crcarbide decomposes and the carbon combines with the Nb thereby increasing the Nb-rich carbides. These findings are consistent with Wang et al.(2011), where the carbide content was found to decrease at 1523 K. Figure 6 shows the variation of Vickers hardness of the steel in both as-received and heat-treated conditions. The hardness of the as-received steel was 225 HV. Though the hardness of the as-received steel decreased only marginally during heat-treatment at temperatures
The effect of heat-treatment on the evolution of microstructure and hardness of microalloyed HP40Nb steel was investigated. The conclusions are summarized as follows: (1) The microstructure of the as-received steel consists of Cr-rich carbide, Nb-rich carbide and a small amount of Nb-Ti-rich carbide precipitates at grain boundary of austenite matrix. (2) Heat treatment at elevated temperatures resulted in increase in austenite grain size. (3) The heat treatment also resulted in decomposition of Cr-carbides with concomitant increase in Nbrich carbides.
471-3
Effect of Microstructure with Hardness on Heat Treatment of HP40Nb Microalloyed Reformer Tube
(4) The hardness of the steel above 1073 K, decreased with increase in heat treatment temperature and soaking time.
References Alvino, A. , Lega, D., Giacobbe, F., Mazzocchi, V. and Rinaldi, A. (2010), Damage characterization in two reformer heater tubes after nearly 10 years of service at different operative and maintenance conditions, Engineering Failure Analysis, Vol. 17, pp.1526-1541. Andrés, C.G., Caruana, G. and Alvarez, L.F. (1998), Control of M23C6 carbides in 0.45C-13Cr martensitic stainless steel by means of three representative heat treatment parameters, Materials Science and Engineering A, Vol. 241, pp.211 – 215. Bonaccorsi, L., Guglielmino, E., Pino, R., Servetto, C. and Sili (2014), A., Damage analysis in Fe–Cr–Ni centrifugally cast alloy tubes for reforming furnaces, Engineering Failure Analysis, Vol. 36, pp.65–74. Ghatak, A. and Robi, P.S. (2013), Effect of temperature on the tensile properties of HP40Nb microalloyed reformer steel, Proceedings of Twenty Second International Conference on Processing and Fabrication of Advanced Materials, December 18-20, 2013, Singapore. Ion, J.C., Moisio, T., Pedersen, T.F., Sorensen, B. and Hansson, C.M. (1991), Journal of Materials Science, Vol. 26, pp.43-48. Jahromi, S.A.J. and NaghiKhani, M. (2004), Creep life assessment of primary reformer HP40-Nb modified steel tube of an ammonia plant, International Journal of Engineering Transactions B, Vol. 17, pp.183-190. Liu, C.J. and Chen, Y. (2011), Variations of the microstructure and mechanical properties of HP40Nb hydrogen reformer tube with time at elevated temperature, Materials & Design, Vol. 32, pp.25072512. Mengk, F., Tagashira, K., Azuma, R. and Sohma, H. (1994), Role of eta-carbide precipitations in the wear resistance improvements of Fe-12Cr-Mo-V-1.4C tool steel by cryogenic treatment, ISIJ International, Vol. 34, pp.205-210. Mertinger, V., Benke, M., Kiss, G. and Réti, F. (2013), Degradation of a corrosion and heat resistant steel pipe, Engineering Failure Analysis, Vol. 29, pp.38–44. Ray, A.K., Kumar, S., Krishna, G., Gunjan, M., Goswami, B., Bose, S.C. (2011), Microstructural studies and remnant life assessment of eleven years service exposed reformer tube, Materials Science and Engineering: A, Vol. 529, pp.102–112. Swaminathan, J., Guguloth, K., Gunjan, M.K., Roy, P.K. and Ghosh, R. (2008), Failure analysis and remaining life assessment of service exposed primary reformer heater tubes, Engineering Failure Analysis, Vol. 15, pp.311–331.
Wang, W.Z., Xuan, F.Z., Wang, Z.D., Wang, B. and Liu, C.J. (2011), Effect of overheating temperature on the microstructure and creep behavior of HP40Nb alloy, Materials & Design, Vol. 32, pp.4010–4016. Yan, J., Gao, Y., Yang, F., Yao, C., Ye, Z., Yi, D. and Ma, S. (2011), Effect of tungsten on the microstructure evolution and mechanical properties of yttrium modified HP40Nb alloy, Materials Science and Engineering A, Vol. 529, pp.361-369.
471-4
