M23C6 carbides in E911 creep resisting steel

Figure 1: Microstructure of E911 pipe after normalizing and tempering by LM (a) and SEM (b). Scale bars: 100, 1 µm.

Figure 2: TEM image of extraction replicas of E911 as-treated pipe: a) M23C6 carbides located at subgrains; b) fine MX carbo-nitrides. Scale bar: 1, 0.2 µm.

Figure 3: TEM image of thin foils from E911 specimens: a) as-treated; b) aged at 575C/64,065h; c) aged at 650C/10,354h. Scale bars: 1 µm.

Figure 4: TEM image of MX carbo-nitrides (a); correspondent EDS microanalysis (b); evolution of chemical composition (c) and mean diameter (d) with ageing time and temperatures. Scale bar: 50 nm.

Figure 5: TEM image of M23C6 carbides (a); correspondent EDS microanalysis (b); evolution of chemical composition (c) and mean diameter (d) with ageing time and temperatures. Scale bar: 100 nm.

Figure 6: TEM image of Laves phase (a); correspondent EDS microanalysis (b); evolution of chemical composition (c) and mean diameter (d) with ageing time and temperatures. Scale bar: 250 nm.

Table 1: Chemical composition range of ASTM E911 and chemical analysis of CORUS industrial heat B2516 (weight %).

Carbide name: M23C6
Record No.: 689
Carbide formula: M23C6
Carbide type: M23C6
Carbide composition in weight %: No data
Image type: LM, SEM, TEM
Steel name: E911
Mat.No. (Wr.Nr.) designation: No data
DIN designation: G-X12CrMoWVNbN10-1-1
AISI/SAE/ASTM designation: No data
Other designation: Toshiba G-X12CrMoWVNbN 10 1 1
Steel group: Creep-resistant steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: Billets of about 2.5 tons, made by CORUS (UK), were hot rolled into pipes (285mmOD x 55mmWT) and tubes (48.3mmOD x 7.2mmWT) in TenarisDalmine mills. The chemical composition of the heat is shown in Table 1, together with chemical analysis range of ASTM A213. European Grade 911 directly results from Grade 91, with an addition of 1% Tungsten.
Samples of E911 pipe were long-term exposed at 625C and 650C in order to investigate the variation of tensile and impact properties at room temperature after 1,000, 3,000 and 10,000 hours. The ageing tests were performed by a muffle with a maximum temperature deviation of + 4 C.
Isothermal creep tests from pipes and tubes were performed at 575C, 600C, 625C and 650C. Creep ruptures passed 60,000 hours (longest test: 575C/140MPa/64,065h).
Note: The creep resistance of Grade E911 was demonstrated to be higher than that of Grade P91: 600C/101MPa/105h for Grade E911 and 600C/93MPa/105h for Grade P91. However, the differences are strongly reduced at 650C.
The higher creep resistance of E911 steel was related to the addition of W, which promotes the precipitation of Laves phase during creep service. No coarsening of MX carbides was observed, while the mean diameter of M23C6 remains below 200 nm and 400 nm, respectively, at 600C and 650C, even after a very long exposure. An excessive coarsening of Laves phase, which grows up to micrometric dimensions, was measured at 650C, with the result of a loss in creep resistance and ductility after 10,000 hours. Below 610C the microstructural stability of this steel during service is guaranteed for very long time.

In order to obtain a good compromise among creep resistance, toughness and ductility (i.e. cold formability), the tubes and pipes were finished in normalized (1060C) and tempered (760C) conditions. The normalizing gave the desired martensitic microstructure and provided a good carbide and nitride solubilisation into the matrix. The subsequent tempering softened the material and promoted the precipitation of M23C6 carbides and fine MX carbo-nitrides into the matrix.

AC1 and AC3 critical temperatures of Grades 91 and E911: Steel P91: Ac1=830 C, Ac3=915 C. Steel E911: Ac1=838, Ac3=980 C.

The microstructure of as-treated pipe of Grade E911 is shown by Light Microscopy (LM) and Scanning Electron Microscopy (SEM) in Figure 1. A predominant tempered martensitic matrix was obtained with air cooling after normalizing, with 225 HV30 average hardness value. A small amount of delta ferrite was observed too (F-delta <5%). The tempering heat treatment promoted the precipitation of M23C6 carbides and fine MX carbo-nitrides.
The largest M23C6 carbides are evident in SEM image. Examples of the precipitates in Grade E911 are shown by TEM in Figure 2: the M23C6 carbides are located on subgrain boundaries and MX carbo-nitrides are precipitated within subgrains.
Thin foils and extraction replicas from E911 samples in aged conditions were examined by TEM/STEMEDS, for investigating the evolution of tempered martensite and the formation of the different particles which precipitate and coarse during thermal exposure. Both the dimensional distribution of precipitates, distinguished in terms of type (MX, M23C6, Laves phase), and the evolution of their composition with ageing were assessed by quantitative analysis on extraction replicas by automatic image analysis, electron diffraction and EDS microanalysis.
In Figure 3 the microstructure of E911 as-treated material (a) and that from aged samples at 575C/64,065h (b) and 650C/10,354h (c) are compared. The recovery process is evident, because subgrains with a reduction of the dislocation density, gradually replaced the original martensite laths. Coarse M23C6 carbides are visible along grain boundaries and laths.
In all aged samples the MX carbides or carbo-nitrides were observed inside subgrains. These precipitates remained quite stable in terms of composition and size with increasing ageing time and temperature (Figure 4), but their number was progressively reduced in the steel. Only a slight reduction of Nb content, substituted by V during creep service, was observed.
The M23C6 particles were observed predominantly on subgrain boundaries and a tendency of coarsening with ageing was detected (Figure 5). Especially at 575C the M23C6 carbides showed very good stability and their growth was very limited: +50% mean diameter after 64,000 creep hours. The extensive quantitative analysis on precipitated phase showed an increase of Cr content and a reduction of Fe content in M23C6 carbides during creep service. The Cr content in M23C6 carbides increased from the value after tempering towards the value of the equilibrium composition at the ageing temperatures of 625C and 650C, calculated by Thermocalc software.
The most relevant change observed was the precipitation of the intermetallic Laves phase during ageing. The Laves phase was detected mainly at the subgrain boundaries and often was associated with carbides. At 625C the nucleation of the Laves phase was observed just in the first 1,000 hours. The rate of coarsening of this phase was very high: at 650C after 10,000 ageing hours their mean diameter reached micrometric dimensions. The chemical composition of Laves phase remained almost constant during creep service; only a slight reduction of Mo and Fe content, replaced by W and Cr, was measured (Figure 6).
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