Carbides in X70, X80 and Grade 100 steels

Table 1: Microalloyed steels analyzed.

Figure 1: Fine precipitates dispersed in ferrite matrix. a) BF image; b) DF image. Scale bars: 100 nm.

Figure 2: Nano-precipitates extracted via carbon replicas from grade 100 steel. Scale bars: 100 nm.

Figure 3: Size distributions of nano-precipitates in grade 100 steel by df imaging.

Figure 4: Cumulative distribution of nano-precipitates in steels.

Figure 5: Residue yield by hcl acid dissolution.

Figure 6: TEM BF image and EDX spectra from fine particles extracted from grade 100 steel using HCl solution. Scale bars: 20 nm.

Figure 7: XRD patterns for residues extracted from different steels.

Figure 8: Profile fitting of XRD pattern by Rietveld refinement for grade 100 steel electrolytically dissolved using 10% AA dissolution. a) Overall XRD pattern profile fitting. b) Calculated diffraction pattern for Ti0.9(Nb)0.1N. c) Calculated diffraction pattern for Ti0.5(Nb)0.5C(N). d) Calculated diffraction pattern for Nb0.7(Ti)0.3C(N). e) Calculated diffraction pattern for Nb0.48(Mo)0.28(Ti)0.21(V)0.03C.

Table 2: Lattice parameters of different phases in grade 100 steel electrolytically dissolved with 10% AA solution.

Figure 8: Relative amount of nano-precipitates (.5nm) compared with the original weight of the steel.

Carbide name: See the text
Record No.: 777
Carbide formula: See the text
Carbide type: See the text
Carbide composition in weight %: No data
Image type: TEM, EDS, XRD
Steel name: X70, X80 and Grade 100 steels
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Microalloyed steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: X70, X80 and Grade 100 microalloyed steels, supplied by IPSCO Inc., were used in this study. The composition, normalized finish rolling temperature (FRT) and coiling temperature (CT) are given in Table 1.
Note: Microalloyed steels are widely used in oil and gas pipelines. They are a class of high strength, low carbon steels containing small additions (in amounts less than 0.1 wt%) of Nb, Ti and/or V. The steels may contain other alloying elements, such as Mo, in amounts exceeding 0.1wt%. Precipitation in these steels can be controlled through thermomechanical controlled processing, leading to precipitates with sizes ranging from several microns to a few nanometers. The larger precipitates are essentially TiN, with partial substitution of Nb for Ti, while the smaller precipitates are based on NbC, with Ti, Mo and V partially substituting for Nb and N partially substituting for C. Microalloyed steels have good strength, good toughness and excellent weldability, which are attributed in part to the presence of the nano-sized carbides and carbonitrides. Because of their fine sizes and low volume fraction, conventional microscopic methods are not satisfactory for quantifying these precipitates. Matrix dissolution is a promising alternative to extract the precipitates for quantification. Relatively large volumes of material can be analyzed, so that statistically significant quantities of precipitates of different sizes are collected. In this paper, matrix dissolution techniques have been developed to extract the precipitates from a series of microalloyed steels. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) are combined to analyze the chemical speciation of these precipitates. Rietveld refinement of the XRD pattern is used to fully quantify the relative amounts of the precipitates. The size distribution of the nano-sized precipitates is quantified using dark field imaging in the TEM.

Fine precipitates dispersed in the steel matrix: A thin foil TEM sample prepared from the as-processed Grade 100 steel is shown in Figure 3. A bright field (BF) image, and corresponding selected area diffraction (SAD) pattern from the field of view shown, are given in Figure 1a. Several intermittent rings are visible (indexed as {111}, {200}, {220}) and correspond to fine precipitates (which have a NaCl-type crystal structure), which are not visible in the BF image. The other four diffraction spots, indexed as 200,002, 310 and 013, are from the ferrite matrix. The SAD pattern also indicates evidence of preferred orientation. A DF image, taken using part of the {111} and {200} rings, is shown in Figure 1b; some of the fine precipitates (<20 nm) are clearly visible as white spots.

Size distribution by DF imaging: Because of the difficulty in analyzing fine precipitates, the size distribution of nano-precipitates was investigated using dark field imaging. Figure 2a shows an example of a TEM BF image from a Grade 100 carbon replica. The inset shows the corresponding SAD pattern from the field of view. Several intermittent rings are visible and correspond to fine precipitates with a NaCl-type crystal structure. Figure 2b shows a DF image, taken using part of the {111} and {200} diffraction rings. Several DF images were taken by varying the position of the objective aperture around the diffraction rings, so that all particles in the field of view were selected. Image processing software (ImageTool) was used to analyze the DF images and to quantify the size distribution of nano-sized precipitates. Figure 2c shows an EDX spectrum from some of the nano-sized precipitates. They are Nb/Mo-rich with smaller amounts Ti and V. Although there is C present in the precipitates, the large C peak arises primarily from the carbon replica. The Cu peaks are an artifact arising from the Cu support grid. The small Fe peak is from the matrix. The relative frequency versus diameter of nano-precipitates in the Grade 100 steel is shown in Figure 3. The relative frequency (n/N) is defined as the ratio of the number of particles (n) within a given size range to the total number of particles counted in that region (N). The size distribution of nano-precipitates in X70-564, X80-462, X80-A4B, X80-B4F and Grade 100 steels are very similar and a comparison of the cumulative distribution of nano-precipitates in the X70, X80 and Grade 100 steels is shown in Figure 4. The largest number of precipitates is in the 3-5 nm size range for all the steels.

Figure 5 shows a comparison between the relative amount of residue determined by weight measurements (precipitates and SiO2) and the amount of residue estimated via ICP analysis. The solid line corresponds to a direct correlation between the weighed residue and the ICP results. Two types of points are shown. The square points are based on the assumption that only carbides form from Nb, Ti and Mo, while the diamond points represent nitride formation. In the reality, carbonitrides form, but because C and N have similar atomic weights, there is virtually no difference for the two assumptions. Figure 5 also shows that there is good correlation between the two types of measurements.

TEM analysis of extracted nano-precipitates: Figure 6 shows TEM images of extracted fine precipitates from the Grade 100 steel. The finest precipitates extracted by the HCl method are less than 5nm in size (Figure 6b). EDX (Figure 8c) reveals that they are Nb- and Mo-rich, and contain small amounts of Ti and V, which confirms the results from the extraction replicas. An amorphous Si-O phase (Figure 6d), seen as spherical particles in Figure 8a, was also present in the residue. This is consistent with the ICP analysis of the supernatant where most of the Si in the steel matrix was present in the residue. The amorphous phase is SiO2, which was confirmed through XRD analysis (presented in the next section). Silicon is present in the steel, either in solid solution in the Fe matrix or as oxide/silicate inclusions. During steel dissolution, dissolved Si combines with dissolved O to precipitate out as SiO2. The source of this dissolved O is likely to be oxide inclusions in the steel dissolving in the HCl solution.

XRD analysis of the extracted residue: Because the crystal structure (NaCl-type) and lattice parameters of the precipitates are similar and the lattice parameters vary with composition, it is difficult to differentiate between the various carbides, nitrides and carbonitrides. Figure 7 shows XRD patterns for residues extracted from three different steels. For the Grade 100 steel, results were obtained through chemical (HCl) and electrochemical (10%AA) dissolution. For the other steels, only chemical (HCl) dissolution was utilized.
The main difference between the two matrix dissolution techniques is the presence of a broad peak for chemical dissolution (HCl) between the 2theta angles of 20 and 30, which is due to the presence of amorphous silica. This is consistent with the TEM analysis, which showed spherical Si-O particles (20-30nm in size) from the extracted residue by HCl dissolution. The same broad peak was observed for the X70 and X80 steels. The broad peak was not found in the XRD spectrum of the Grade 100 steel that was electrolytically (10% AA) dissolved. The absence of the broad amorphous peak in the sample allows for a better signal-to-noise ratio and peak resolution in the diffraction pattern, making Rietveld refinement less error prone. However, electrolytic dissolution is time consuming, making it impractical to implement.
Preliminary analysis of the diffraction patterns shows two crystalline phases with similar structures. One set of peaks (indicated by squares in Figure 8) represents the smaller NbC-based precipitates and the other set (circles in Figure 7) corresponds to the larger TiN-based precipitates. Both types of precipitates have a NaCl-type structure. More detailed analysis of the patterns reveals differences among the samples.

Rietveld refinement of the XRD data: Detailed analysis of the XRD patterns was obtained via Rietveld refinement. An example for the Grade 100 steel is shown in Figure 8. Four phases are superimposed on the observed diffraction pattern. Figure 8b, c, d and e show calculated diffraction patterns for Ti0.9(Nb)0.1N, Ti0.5(Nb)0.5C(N), Nb0.7(Ti)0.3C(N) and Nb0.48(Mo)0.28(Ti)0.21(V)0.03C, respectively. The lattice parameters for the four types of precipitates are very similar and are listed in Table III. The smallest precipitates (3-5nm in size) are attributed to be Nb0.48(Mo)0.28(Ti)0.21(V)0.03C as described next. Table IV shows the composition of the nano-precipitates (<=5nm) for the different steels. They are Nb/Mo-rich carbides. The relative amount of nano-precipitates (<=5nm) in different steels can be determined by two ways, with the results presented in Figure 9. One way is based on Rietveld profile fitting of XRD pattern. Because of the strong correlation of nano-precipitates with amorphous SiO2, SiO2 was used as an internal standard in the refinement and the resulting residual amorphous content is ascribed to the nano-precipitates (<=5nm). Another way to determine the amount of nano-precipitates (<=5nm) is based on the amount of Mo. Most of the Mo is present in solid solution in the ferrite. The remaining Mo is only present in the nano-precipitates, as confirmed by TEM EDX analysis (see Figure 6). The amount of Mo in particle form, presented in Table II, is then attributed to the nano-precipitates (. 5nm). The results are compared in Figure 8. They indicate that the amounts of nano-precipitates (<=5nm) determined based on Rietveld refinement and Mo amount are within 17% of each other for the above steels. For the Grade 100 steel, the amounts of nano-precipitate (.5nm) by HCl and 10% AA dissolution are within 17%. Figure 8 also indicates that the amount of nano-precipitates (<=5nm) increases with increasing steel grade.
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