Fe3C, VC, M7C3 and M23C6 carbides in heat resistant 12Kh1MF steel

Figure 1: X-ray diffraction patterns of 12Kh1MF virgin (curve 1) steel and exploited 227000 h at 550 C temperature and pressure of 14 MPa (curve 2).

Table 1: Electrochemical parameters of 12Kh1MF steel in electrolytes solutions.

Figure 2: Optical microstructure of 12Kh1MF steel tempered at 700 C (hours) and etched by 10 % ferrous (III) chloride solution: 1 virgin steel, 2 24, 3 48, 4 144, 5 384 hours, 6 exploited 227000 hours at 550 C. Scale bars: 50 µm.

Figure 3: Microstructure (SEM) of 12Kh1MF steel tempered at 700 C: 1 virgin state, ferrite-pearlite; 2 384 h heated, partial disintegration of pearlitic structure; 3 227000 h exploited at 550 C, full disintegration of pearlitic structure. Scale bars: 5 µm.

Figure 4: X-ray diffraction patterns of 12Kh1MF steel tempered at 600 C. Curves: 1 virgin steel, 2 48 h, 3 216 h, 4 384 h, 5 854 h.

Figure 5: X-ray diffraction patterns of 12Kh1MF steel tempered at 650 C. Curves: 1 virgin steel, 2 24 h, 3 192 h, 4 288 h, 5 384 h, 6 854 h.

Figure 6: X-ray diffraction patterns of 12Kh1MF steel tempered at 700 C. Curves: 1 virgin steel, 2 24 h, 3 48 h, 4 192 h, 5 384 h, 6 576 h.

Carbide name: Fe3C, VC, M7C3, M23C6
Record No.: 849
Carbide formula: Fe3C, VC, M7C3, M23C6
Carbide type: M3C, MC, M7C3, M23C6
Carbide composition in weight %: No data
Image type: SEM, XRD
Steel name: 12Kh1MF
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: ASTM: F12C1.1
Other designation: EN: 13CrMo4-5
Steel group: Heat resistant steels
Steel composition in weight %: 0.12% C, 1.1% Cr, 0.54% Mn, 0.26% Mo, 0.26% Si, 0.17% V, 0.019% S, 0.015% P.
Heat treatment/condition: Investigated 12X1M. steel was taken from a thick section of superheated steam pipes originally supplied by SC Lietuvos elektrine. The samples were prepared from virgin and exploited for 227000 h at 14 MPa pressure and 550 C 560 C steel. Using a potentiostat PI-50-1 and a programmer PER-8 steel anodic polarization curves (e. g. dependence of operating electrode potential on current density) were recorded in solutions of different electrolytes, electrochemical etching solutions and the parameters (critical current density, critical potential) were determined.
The samples (60 mm 20 mm 20 mm, working area 2.2 cm2 2.5 cm2) of 12X1M. steel were soaked into 20 C electrolyte solution and current measurements were carried out according argentums chloride reference electrode (EVL 1M3.1), when the sample potential from the stationary value to 1.2 V varied at 1103 V/s or 5103 V/s rate. Performing anodic polarization of heat resistant steel, Fe ions, which forms insoluble bivalent and then trivalent ferrous hydroxides (Fe(OH)2, Fe(OH)3), enter the electrolyte solution. Anodic polarization curves were recorded in 0.05 % and 0.5 % hydrochloric acid solution and in hydrochloric solution with 0.01 % and 0.05 % oxalic or citric acid concentrations.
Note: Parameters of carbide phase electrochemical separation in electrolytes solutions of pearlitic 12Kh1MF heat resistant steel, used in thermal power plants facilities are presented in the article. The most relevant electrolyte is chosen and its concentration is specified. Operated and under laboratory conditions aged steel surface microstructure and morphology were evaluated by optic and scanning electron microscopy methods. Using electrochemical etching and XRD qualitative analysis of carbide compounds of 550 C exploited steel and aged under laboratory conditions at 600, 650, 700 C from 24 h up to 864 h was carried out. Kinetics of carbides formation is given in mathematical equations. It was determined that during exploitation and ageing samples at 700 C for 576 h the steel pearlite completely decomposes, whereas alloy elements diffuse into intergrain area thus forming special alloy carbides. Experiments revealed that the XRD analysis of electrochemically separated carbide phase is a rapid and informative method of evaluation the service condition of steel.

The initial microstructure of power plant alloy 12Kh1MF consists of ferrite-pearlite or ferrite-bainite as the major phases obtained following a hardening and normalizing heat treatment later on subjected to very severe tempering (~650 C700 C for several hours) generating the overall coarsening and the precipitation of ever more stable alloy carbides and intermetallic compounds (Fig. 1, curve 1), witch interfere with the progress of dislocations. These solid state reactions eventually determine the properties and mechanical stability of the power plant steels (for example, the resistance to creep deformation) and their useful design lifetimes.
In the heat and power generating plants, the pipelines are used to transport superheated steam in the temperature range 500 C 560 C and under a pressure, P = 10 MPa 15 MPa. During long time service in creep regime to such conditions the microstructure of steel changes, pearlite/bainite decomposes as well as carbides precipitation at the grain boundaries and carbides coarsening processes proceed (Fig. 1, curve 2). Structure changes cause formation of cavities and development of internal damages. It is well known that there is a close coherence between changes in microstructure and deterioration of mechanical properties, however, the accurate relation for creep rupture strength deterioration regarding the microstructural degradation is not yet determined.

Microstructure changes of 12Kh1MF steel surface during its ageing were observed using optic microscopy and scanning electron microscopy methods. In optical microscopy photos (Fig. 2, picture 1) of untreated steel pearlite and ferrite grains are observed, however it is hard to identify fine carbide compositions. High temperature ferrite and pearlite grains begin to decompose and in their limits the chains (Fig. 3, pictures 2 6) of alloyed carbides are formed, which are difficult to see in pictures since both the resolution, depth of field and range of magnifications (to about 550) of the light microscope are not sufficient.
The ongoing structure changes of samples and nucleation of voids may be analysed in an objective way with the scanning electron microscope. In SEM pictures (Fig. 3) the steel ferrite-pearlite structure can be clearly seen. After treating the sample for 384 h at 700 C, the pearlitic structure partially decomposes (Fig. 4, picture 2), whereas in the photo of the sample treated for 227000 h at 550 C, a full disintegration of pearlitic structure (Fig. 3, picture 3) is recorded.

XRD analysis was used to confirm carbides formation in steels during their ageing as well as to perform kinetics research. According to the steel critical current density, determined by anodic polarization method, steel etching operating current density was chosen. Its value corresponds to approximately half critical current density. Since for real samples due to approximately 10 times bigger surface area the analysed 0.05 % electrolyte concentration was too small (steel solution process ended very quickly, when a sufficient amount of carbide phase had not yet been released), it was increased up to 5 %.
Fig. 4 shows X-ray diffraction patterns of electrolytically extracted residues of specimens during the early stages of tempering at 600 C. After 48 h tempering exposure, intensity of diffraction peaks of Fe3C, VC and traces of M23C6 (M stands for metals: iron, chromium, molybdenum and vanadium) remains almost unchanged but small amount of M7C3 has been detected (Fig. 4, curve 2). After 216 h accelerated ageing the diffraction peak of carbide M23C6 considerably increases (Fig. 4, curve 3) while Fe3C diffraction peak marginally diminishes. The most significant changes have been identified after 654 h isothermal ageing (Fig. 4, curve 5). The intensity of M23C6, VC and M7C3 diffraction peaks considerably increase, while Fe3C significantly decreases. All the data showed that tempering causes the Fe-rich M3C carbide (the kinetically favoured phase in the pearlite) to transform to more thermodynamically favoured carbides, rich in Cr and Mo. The most thermodynamically stable carbide M6C was not yet identified. After further performing XRD analysis of the samples, it was determined that during ageing of 12Kh1MF steel at 650 C and 700 C (Fig. 5 and Fig. 6), the concentration of Fe3C reduces more rapidly and, at the same time, the rate of transformation to M23C6 alloy carbide is accelerated. No precipitation of stable carbide M6C was detected too. So, the same carbide precipitation sequence was observed at 650 C and 700 C as it was detected at 600 C, although the precipitation kinetics appears to be faster for all phases at that temperatures. Because the carbide M6C have been detected in 12Kh1MF steel exploited for 227000 h, thus it would be meaningful to continue experiments at elevated temperatures to obtain conditions of full carbide precipitation sequence. These results would be useful for evaluation of steel service time and for prediction remnant life.
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