M7C3, M23C6 and M6C carbides in 13HMF steel


Table 1: Chemical composition of tested 13HMF steel (14MoV6-3).


Figure 1: Microstructure of 13HMF steel tube diameter of 273 32 mm, Magn. 1000x.


Figure 2: Carbides morphology in 13HMF steel, tube diameter of 273 32 mm. Observations performed at metallographical samples SEM, detector: a) BSE, b), c) SE. Scale bars: 20, 10 µm.


Figure 3: Carbides morphology in 13HMF steel, tube diameter of 27332 mm. Extraction replica, SEM, detector SE. Scale bars: 100, 10, 20 µm.


Figure 4: Carbides morphology in 13HMF steel microstructure, tube diamter of 273 32 mm. Extraction replica, SEM, detector SE. Scale bar: 10 µm.


Figure 5: EDS chemical microanalysis and morphology of carbides in 13HMF steel, tube diameter of 273 32 mm. Extraction replica, SEM, detector SE. Scale bars: 5, 10 µm.


Figure 6: a) Dispersed MC precipitates, TEM image b) Diffraction pattern of MC precipitates. Scale bar: 200 nm.


Figure 7: a) Dispersed M2C precipitates, TEM image b) Diffraction patern of M2C precipitates. Scale bar: 200 nm.


Figure 8: The diverse morphology of the precipitates. Arrow marked the precipitation where the diffraction was performed. Scale bar: 200 nm.


Figure 9: Diffraction pattern of particle shown by arrow in Fig. 8, M23C6 precipitate.


Figure 10: a) Dispersed M7C3 precipitates, TEM image b) Diffraction pattern of M7C3 precipitates. Scale bar: 500 nm.

Carbide name: M7C3, M23C6, M6C
Record No.: 899
Carbide formula: M7C3, M23C6, M6C
Carbide type: M7C3, M23C6, M6C
Carbide composition in weight %: See the figure 5.
Image type: No data
Steel name: 13HMF steel
Mat.No. (Wr.Nr.) designation: 1.7715
DIN designation: 14MoV6-3
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Structural steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: Examinations were conducted on one segment of .. 23732.0 mm fresh steam pipeline connecting the steam boiler with turbine Tube was made of 13HMF according to PN-75/H-84024 (14MoV6-3 according to PN-EN10216-2) steel grade and was operated for approximately 168 000 hours at 540C under 14 MPa pressure. The chemical composition of tested steel is given in Table 1.
Note: Purpose: of this paper is to reveal the microstructural changes in 13HMF steel exposed to long-term service at elevated temperatures. The degradation of bainite structure was determined and carbides morphology has been examined. The influence of carbides evolution was discussed in dependence of creep rupture strength and mechanical properties of the steel.
Design/methodology/approach: Examinations were conducted on 273 mm diameter, 32 mm wall thickness tube made of 13HMF (14MoV6-3) steel. The tube was a segment of stem pipeline used in power plant at 540C. The service time is 168,000 hours. Microstructure of the material has been examined with the use of light optical microscopy and scanning electron microscopy (SEM). The energy dispersive X-ray spectrometry (EDS) analysis was used for phase chemical composition identification. Transmission electron microscopy (TEM) of thin foils was used for carbides structure identification. The mechanical properties of the tube material were evaluated in static tensile tests at room temperature, hardness tests and impact Charpy U tests.
Findings: Microstructure of 13HMF steel tube shows an advanced level of degradation - coagulation of carbides at ferrite grain boundaries and inside bainitic grains. Precipitates of carbides decorated grain boundaries in chain forms. The presence of M7C3, M23C6, M6C phases were revealed. After extended service M23C6 and M3C carbides were replaced by more stable carbides. This transformation did not occur until the end. This indicates the presence of mainly Mo2C carbide, and only sporadic occurrence of carbide M6C.
Practical implications: Useability of the method for assessing the current degradation level and for predicting residual lifetime of creep-resistant tubes based on analysis of carbides morphology was confirmed for Cr-Mo-V steel.
Originality/value: Information available in literature does not clearly indicate the influence of microstructure and mechanical properties of Cr-Mo-V steel after long-term exploitation. The study shows such relations.

Tube wall thickness was uniform on the perimeter and averaged 31.8 mm. On the inner tube surface the uniform corrosion was observed in a form of compact layer, strictly adhering to the metal surface. There were no defects such as cracks or voids and the occurrence of pitting corrosion. The NDT magnetic tests confirmed good condition of tested tube. Inner and outer surfaces of tube were checked with the use of magnetic particle and ultrasonic testing. Not the type of discontinuity has been detected like outside and subsurface cracks (to a depth of about 2.5 mm). Metallographic studies were performed on specimens taken into transverse and longitudinal direction of tube axis to check the microstructure and its degradation level occurred during long term exploitation. The severity of non-metallic inclusions was defined according to PN-64/H-04510 standard. Contamination of non-metallic inclusions was on an acceptable level. The non-metallic particles occur in the form of separate oxides, no sulphides were detected.
The degradation level of tube microstructure was determined by comparison with the scale models. The extent of degradation of the carbide structure for steel 13HMF (14MoV6-3) was performed at magnification of 500 and 1000x. Observations was conducted on an light microscope Leica MEF4M. The structure of the tube material has an advanced degree of degradation. On Fig. 1 it can be seen separate coagulated cementite particles and carbides at the grain boundaries of ferrite and bainite areas. Visible are large single precipitates and precipitates occurring in the form of strings deployed along the grain borders. Assessment of structure degradation according to the criteria [10] corresponds to the model of SIII. There were no decohesion changes in the tube steel microstructure.

SEM examinations of carbides morphology and chemical composition: Carbide morphology was performed on scanning electron microscope (SEM) Hitachi S3400N using SE and BSE detectors. SE detector creates an image in secondary electron reflecting surface topography. BSE detector forming the image in backscattered electrons and further highlights the contrast depends on differences in chemical composition of the different areas of the image. Quantitative chemical analysis in microareas (carbides) was performed on the scanning electron microscope with an attachment to the X-ray microanalysis EDS. The study was conducted on two types of samples: a.) metallographic samples,b.)extraction replicas removed from the surface of metallographic cross-sections. Morphology of carbides observed on metallographic samples is shown in Fig. 2. Many small carbides are visible - dispersed inside the degradated bainite grains and relatively large carbides at grain boundaries, which often form a continuous net.
Carbides morphology disclosed on a replica is shown in Figs. 3,4. Example results of chemical composition investigations of carbides are presented in Fig. 5. It was found that carbide content by weight is about: 14% Mo, 8% Mn, 7% Cr and 1% V. It should be noted that the EDS analysis of low volume carbide precipitates is a qualitative analysis and does not give the exact chemical composition of the phase only by informing the elements are present.

TEM examinations of carbides morphology and chemical composition: Structural studies, including analysis of phase composition was performed using a JEOL 100B transmission electron microscopy operating at 100 kV accelerating voltage. Research was conducted on extraction replicas and thin foils. Diffraction studies were performed using selective diffraction. Constant microscope (lambda L) was determined from the standard sample. In order to analyze the phase composition, the lattice crystallographic system was generated by entering the number of atoms in the unit cell and their x, y, z positions. After loading the data described above, unit cell parameters was described (lattice parameters a, b, c, angles between them of alpha, beta and gamma, and a minimum interplanar distance d). After indexing diffraction lines the B direction was determined.
Figures 6-10 show the shape of precipitates and diffraction patterns of determined carbides. TEM examinations revealed the presence of the following phases in 13HMF steel microstructure: MX, M2X, M7C3, M23C6, M6C, where X is a non-metal such as C or N. The presence of M6C carbide is disclosed a single case. There is a simultaneous occurrence of partial areas of the MX M2X in the matrix, but there are other areas with only the MX precipitates. M7C3 precipitates occur at grain boundaries and inside grains as well as precipitates of M23C6 carbides. M23C6 phase observed in the areas of grain boundaries are often defected and have irregular shapes. They are rarely seen inside the grains. Local areas free of dislocations and precipitates, and showing the effects of high-angle grain boundary migration were no detected. In many areas, the process of forming cellular structures were found (built with dislocation tangles). Dislocations uniformly distributed in the matrix are blocked by the fine particles of MX and M2X phases.

Mechanical properties of tube material after service period of 168,000 hours at 540C are only at the acceptable level. In comparison to the as received material, both tensile strength and hardness decreased considerably when elongation and impact strength remain as satisfactory.
13HMF (14MoV6-3) steel tube microstructure shows an advanced level of degradation. Carbides coagulation process occurs at ferrite grain boundaries and in bainite areas. Precipitates are in the form of chains and large, separate particles along the grain boundaries. Evaluation of degradation of the tubes structure according to the criteria based on light microscopy observations corresponds to the model SIII. Morphology, structure and chemical composition of carbides determined in SEM and TEM investigations confirmed the advanced degradation of the microstructure, The presence of M7C3, M23C6, M6C phases were revealed. After extended service M23C6 and M3C carbides were replaced by more stable carbides. This transformation did not reach the end, i.e. degradation process is not in final stage which is characterized by occurrence of creep micro-voids. This indicates the presence of mainly Mo2C carbide, and only a few of M6C carbides.

Determination of a Cr-Mo-V steel microstructure degradation, after long-term service at elevated temperature, requires conduction of wide range investigations including mechanical, metallographic, structural, and chemical analysis tests [13-15]. The tested 13HMF (14MoV6-3) steel steam pipeline shows an advanced level of degradation after service period of 168,000 hours at 540..C. Carbides coagulation process occurs at ferrite grain boundaries and in bainite areas. The presence of M7C3, M23C6, M6C phases were revealed. But degradation process is not in final stage yet, which has been proofed by presence of mainly Mo2C carbides, and only a few of M6C carbides. There were no creep micro-voids in the tube.
Links: No data
Reference: Not shown in this demo version.

Copyright © 2018 by Steel Data. All Rights Reserved.