INSIGHT TO THE MICROSTRUCTURE CHARACTERIZATION OF A HP AUSTENITIC HEAT RESISTANT STEEL AFTER LONG-TERM SERVICE EXPOSURE

Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used in high temperature structural applications. Their composition include Nb as strong carbide former. Electron Backscatter Diffraction (EBSD) investigations revealed that, in the as-cast condition, alloys exhibit austenitic matrix with intergranular primary carbides such as MC, M23C6 and/or M7C3. During exposure at a high temperature, phase transformations occurred: chromium carbides of M7C3 type transform into the more stable M23C6 type, intergranular M23C6 carbides precipitate, and Lave phase due to increase of Niobium activity with temperature increase, as thermodynamic simulation confirmed. Therefore, combination of EBSD-EDS technique with thermodynamic calculation is one of the novel and most accurate method to investigation of phase transformation, as the precipitations are identified on the basis of their crystal structure, chemical composition and their thermodynamic features.


Introduction
Heat resistant cast steels are widely used in various high temperature industrial applications thanks to their excellent creep strength, oxidation and abrasion resistance at elevated temperatures.The HP (Fe-25Cr-35Ni) steel which is one of the most important austenitic heat resistant alloy that has a combination of creep strength and resistance to oxidizing and carburizing atmospheres at service temperature around 1000°C [1].As regards the austenitic microstructure, HP heat resistant steel type is being developed for different applications vary from pyrolysis in petrochemical industries to reformer furnaces in steel making factories.These grades of steel have replaced the traditional nickel based super alloys with a substantial reduction of cost and have equivalent properties under conditions of creep, with excellent resistances to high temperature oxidation and metal dusting [2].DOI  Although this alloy steel castings are designed for use at elevated temperatures, metallurgical degradation, aging and phase transformations can occur after extended service exposure above 800 °C, potentially resulting in embrittlement.The typical microstructure of as-cast alloy is an austenite matrix with intergranular eutectic-like primary chromium-rich carbides (M7C3 and/or M23C6 types) and niobium carbides (MC type) [3][4].Long-term service at high temperature of heat-resistant steels can bring about decomposition of austenite and forming of carbides and intermetallic compounds such as sigma, chi, Laves, and epsilon phases [5].In addition, all the primary chromium carbides eventually transform into M23C6; intragranular secondary M23C6 carbides also precipitate [6].These changes usually result in higher strength but also causes a loss of ductility at ambient temperature, leading to potential repair problems during shutdown, so it is important to be able to fully characterize the microstructure of both as-cast and aged alloys [1,6].
Although a number of studies, e.g.[1][2][3][4][5][6][7][8][9], have been conducted subsequently to evaluate of phase changes during service and the influence of these changes on the failure mechanisms on 25Cr-35Ni-type heat-resistant cast steel, there is still a lacking of detailed investigations on the microstructural evolution after long-term service by using EBSD, such as phase orientation, and phase map.In this paper, the microstructural evolution of two HP steels in as cast condition and after long time service exposure have been characterized by means of EBSD techniques.Our objective, in this work, is to address the phase transformation during aging (service condition) in high temperature in depth.

Experimental Procedures
In this work, two alloys of a HP heat resistant steel were studied: an as-cast alloy (A), and an exservice alloy (X).The alloy B has stayed in service at approximately 900°C under a stress of 12 MPa for more than 10 years.Average chemical composition is presented in Table 1.The materials examined were pieces with the 10×20×40 mm dimensions.Microstructural investigation was carried out using a Hitachi SU6600 Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray Spectroscope unit (EDS) and an Electron Backscatter Diffraction (EBSD).The diamond polished samples were subsequently polished with 50 nm colloidal silica slurry for 6 h using Vibro Met 2 Vibratory polisher and etched in Marble's reagent.For obtaining orientation maps a voltage of 20 kV, working distance of 15 mm, and a step size of 50 nm were used.The HKL CHANNEL5 software was used to perform EBSD analysis and post-processing.In addition, thermodynamic simulation of the phases present in the HP alloy was performed with Thermo-Calc software V.2016b.

Result and Discussion
Fig. 1 illustrates the SEM microstructure of the as-cast condition (Alloy A) in two different magnifications; It has been clearly observed that microstructure consists of an austenitic matrix (almost 87% relative area fraction) and a continuous network of primary precipitation (around 13%), located at the boundaries of grains and dendrites.The typical microstructure of HP alloys is an austenite matrix with intergranular eutectic-like primary chromium carbides with the stoichiometric composition of M7C3 and/or, M23C6 and MC type of niobium carbides [3,8].The SEM micrograph of the long-term (around 10 years) serviced sample (Alloy X) is presented in Fig. 3.The microstructure has significantly changed during the aging where the secondary precipitation decorates the austenitic matrix and is clearly defined at higher magnification.The secondary carbides strikingly increased and slightly grew in the matrix.The secondary carbides (mostly M23C6) precipitate inside the grains, due to the high solute supersaturation resulting from the casting process.In addition, primary carbides also coalesced and coarsened to form continuous films along grain boundaries.Backscattered imaging contrast and/or EDS analysis can be used to make a difference between chromium carbides, MC carbides, and some other phases like G, Z and Laves phases, but they cannot differentiate M23C6 from M7C3 and M6C due to low atomic number of carbon which causes difficulties to have a precise distinguish between those carbides.Therefore, a combination of EDS and EBSD would be an excellent tool to identify the various precipitates in the studied samples.Moreover, Fig. 5 shows the EBSD phase color in the as cast (A) and after aging (X) condition.
In the as-cast condition, three types of carbides can be detected as primary carbide: Cr7C3 and Cr3C as chromium carbides and NbC as niobium carbide [9].J. Liu et al. [3] reported that M7C3 carbides were clearly identified only in the as-cast sample but not in the aged and/or in the serviced sample, they suggested that the M7C3 carbides formed during the solidification and transformed into other phases after aging, this phenomenon also can be seen in Fig. 5(b).It has been obvious that integrated EBSD and EDS technique helps to a very accurate phase identification.Indeed, the EDS spectrum along with diffraction pattern of each phase which are used to find all of matching phases and distinguishing the different type of chromium carbides that not possible to identify with other common methods.These results were also in agreement with Thermodynamic Scheil calculation results which could be considered to represent the sample microstructure constituents during solidification or in the as-cast condition, see Fig. 6.
As observed, primary eutectic carbides are, thermodynamically, in nonequilibrium state, since they are formed due to carbon and chromium segregation to the dendritic cell boundaries.At elevated temperatures, these eutectic carbides are prone to the interaction with the FCC matrix towards equilibrium state.It has been also found that NbC forms at temperature higher than the temperature for Cr23C6 formation, it means that a significant amount of the total carbon is consumes by Nb before formation of Cr23C6.It could be the reason why of a very low amount of this type of chromium carbide in the as-cast condition [6].As the Cr7C3 carbide has an orthorhombic crystal lattice and is instable at high temperatures; it, therefore, transforms to Cr23C6 according to the following reaction: where M is Cr combined with Fe [10] In addition, under high temperature and time conditions, Cr23C6 partially transforms to Cr6C according to the following reaction [10]: Fig. 6 Scheil calculation results for HP alloy with Thermo-Calc In the sample X (Fig. 5(b)), due to service temperature many fine secondary Cr23C6 and Cr6C carbides have been observed.As expected, the aging treatment resulted in considerable secondary or interdendritic carbides precipitation.Moreover, the coarsening of primary Cr23C6 carbide and agglomeration of secondary Cr23C6 carbide have been obviously occurred in the sample.Additionally, niobium carbide particles are detected where it could neither be identified by XRD due to its small amount nor by the TEM possibly due to their fall-off during electro polishing for preparing TEM foils.It can clearly see from the phase map that NbC particle being stable at the age temperature.The phase identification and image analysis data were schematically plotted in Fig. 7 to illustrate the microstructure evolution during service exposure.It shows that almost all of the Cr7C3 in the as-cast condition appeared to be replaced by Cr23C6 and/or Cr6C [11].Besides, a small quantity of M6C carbide formed bordering to M23C6.Some literatures [3,[12][13][14] also reported that M6C carbide transformed from M23C6 kept twin OR with M23C6 /austenite, but M6C carbide directly precipitated from the austenite kept a cube-cube OR with M23C6 /austenite.It may be explained that the 3 to 1 relation between M23C6 and austenite in lattice parameter, while the nearly 1 to 1 relation between M6C and M23C6 have led to the twin OR between the two phases.These two types of carbides present similar morphology with fine particle, and they are not easy to be distinguished using OM or SEM.In the other word, the remarkable microstructural evolution of the heat-resistant samples is attributed to their chemical composition and service condition.As the aging time and/or service time were prolonged, the precipitated carbide markedly increased, but their size hardly increased.Moreover, 10 years of aging at 900•C lead to the nucleation of fine intermetallic Fe2Nb (Laves) phase, evenly dispersed needle-shaped precipitates in the size range of 3 µm and below throughout the austenitic matrix (Figs.5(b), 6) and the grain boundaries.Laves phase is an intermetallic component of iron and other metallic elements like Nb, Mo and Ti.It has a hexagonal unit cell which necessitates close atomic size of its constituent elements.From the literature [15], Nb is known to be the most potent Laves phase former and its strong effect can be enhanced by combining it with Si [16].precipitate in intergranular regions and almost all Cr 7 C 3 carbides transformed into Cr23C6 carbides.Moreover, some strong evidences related to formation of Laves phase during aging have been found.3. Combination of EBSD-EDS technique with thermodynamic calculation is one of the novel and most accurate method to investigation of phase transformation, as the precipitations are identified on the basis of their crystal structure, chemical composition and their thermodynamic features.

Fig. 1
Fig. 1 SEM microstructure of the as-cast condition in two different magnifications

Fig. 2
Fig. 2 EDS analysis results at two locations of alloy A, a) Point 1 and, b) Point 2

Fig. 3
Fig. 3 SEM microstructure of the ex-serviced sample in two magnifications

Fig. 4
illustrates EBSD IPF color map image of precipitates for the sample A and X.In the as cast condition, crystallographic orientation of all identified precipitates, as a network of primary carbide, in variety of colors is presented where M7C3 carbides have an orthorhombic structure with lattice parameter a = 4.49 Å, b = 6.93 Å and c = 12.02 Å, M23C6 carbides have a FCC structure with a lattice parameter a = 10.64 Å, M6C carbides and MC have also a FCC structure with a lattice parameter a = 10.95Å and a = 4.43 Å, respectively.

Fig. 4
Fig. 4 IPF (Inverse Pole Figures) color map of a) Alloy A and b) Alloy X

Fig. 5
Fig. 5 EBSD phase color map of a) Alloy A and b) Alloy X

Fig. 7 9 .
Fig. 7 Schematic illustration of phase transformation during aging in HP alloy

Fig. 8 Fig. 9
Fig. 8 Thermodynamic simulation results for a) NbC stability and activity of Nb, and b) the amount of Cr23C6 and Cr7C3 vs Temperature with Thermo-Calc