M7C3, MC, M23C6 and M3C carbides in AISI H11 steel

Table 1: References and heat treatments of analysed samples.

Figure 1: Bright field TEM micrographs of dislocation structure in tempered steel at 550 C (2 hours) and 580 C (2 hours) (sample A4). (a) Low magnification, (b) high magnification: intralath dislocation structure ~g = [110]. Scales bar: 500, 50 nm.

Figure 2: Bright field TEM micrographs of dislocation structure in tempered steel at 550 C (2 hours) and 620 C (2 hours) (sample A6). (a) Low magnification, (b) high magnification : intralath dislocation structure ~g = [1 10]. Scale bars: 500, 50 nm.

Figure 3: Bright field TEM micrographs of dislocation structure in tempered steel at 550 C (2 hours) and 640 C (2 hours). (sample A7). (a) Low magnification, (b) high magnification: intralath dislocation structure. Scale bars: 500, 50 nm.

Figure 4: Bright field TEM micrographs of dislocation structure in tempered steel at 550 C(2 hours) and 640 C(2 hours) (sampleA7), pinning of dislocations on a large carbide. Scale bar: 100 nm.

Figure 5: Bright field TEM micrographs of the dislocation structure after a fatigue test at 550 C(delta epsilon = 2%) (sample A9). (a) Low magnification, (b) high magnification : intralath dislocation structure ~g = [110]. XX, scale bar: 10 µm. Scale bars: 500, 50 nm.

Figure 6: TEM micrograph of the extracted carbides from a tempered steel at 550 C(2 hours) and 620 C(2 hours) (sample A6). Scale bar: 100 nm.

Table 2: Statistics on carbide size.

Table 3: Statistics on carbide size.

Table 4: Theoretical maximal volume fraction of carbides.

Table 5: Estimation of the volume fraction of carbides.

Figure 7: Diffrent carbides. Scale bars: 50, 20, 100. 50 nm.

Figure 8: Precipitation sequence in a Mod. AISI H11 steel.

Carbide name: M7C3, MC, M23C6, M3C
Record No.: 769
Carbide formula: M7C3, MC, M23C6, M3C
Carbide type: M7C3, MC, M23C6, M3C
Carbide composition in weight %: No data
Image type: TEM, EDS, XRD
Steel name: X38CrMoV5
Mat.No. (Wr.Nr.) designation: 1.2343
DIN designation: No data
AISI/SAE/ASTM designation: Mod. AISI H11
Other designation: No data
Steel group: Hot work tool steels
Steel composition in weight %: 0.361% C, 0.35% Si, 0.36% Mn, 0.06% Ni, 5.06% Cr, 1.25% Mo, 0.49% V.
Heat treatment/condition: Heat treatment consists in austenitizing for one hour followed by air cooling, first tempering at 550 C for two hours, second tempering for two hours between 550 C and 640 C depending on desired hardness. A nital etch reveals a tempered martensitic structure with heterogeneous lath sizes. An electrolytic chromic acid etch reveals prior austenitic grain. The average diameter is 14 m for 50 grains measured.
Note: Usual mechanical properties of martensitic steels are strongly linked to their complex microstructure obtained after heat treatment. Heat treatments are generally performed in order to achieve a good hardness and/or tensile strength with a sufficiently acceptable ductility. Nevertheless, microstructural parameters (connected with carbides and dislocations) giving suitable mechanical properties at the initial state are never totally investigated. Our reference grade is the well-known X38CrMoV5 (AISI H11) steel. The main goal of this work deals with a quantitative identification of relevant microstructural parameters ensuring a good mechanical strength of these steels atworking temperatures. Particularly, influences of the tempering temperature and of the fatigue strain amplitude are discussed.

The Table 1 shows references and heat treatments of analysed samples.

Bright field transmission electron photographs of samples A4, A6 and A7 are shown in Figs. 1 to 3. In Fig. 1 at low magnification, laths are generally clearly separated by elongated iron carbides. In addition, the observations of thin foils show a high density of intralath entangled dislocations even for high tempering temperatures (see Fig. 3). Consequently, the identification of the individual dislocations (Burgers vector and slip plane) and density evaluation becomes very difficult using the classical TEM method. Prior to fatigue testing, dislocation distribution is quite homogeneous on the whole even if, at a nanometric scale, a high density of dislocations was observed near lath boundaries and around carbides (see Fig. 4). To compare dislocation structures obtained at different tempering temperatures qualitatively, the observations were performed in the same crystallographic orientation conditions ~g = [110 ]. Lath lightening is observed when second tempering temperature increases. Therefore, recovery of themicrostructure is stated by a clear decrease of the dislocation density. This effect is strongly increased by the application of a cyclic strain (see Fig. 5)and a strong reduction of the intralath dislocation density is observed after a fatigue test. As the dislocation tangle is crushed during the fatigue test, such a configuration promotes a free dislocation movement between lath boundaries or carbides. The free slip distances are therefore probably increased by dislocation annihilation stated by lath lightening. This decrease of dislocation density seems to be one of the main mechanisms for cyclic softening. Nevertheless, dislocation annihilation is strongly heterogeneous and seems to take place in the bigger laths (breadth nearly > 0.3 m). In addition, dislocation cell development generally observed in tempered martensitic stainless steels after fatigue does not seem to take place in the 5% Cr steel even for the highest total strain amplitude investigated.

Carbides extracted from the martensitic matrix were observed at TEM (see Fig. 6). Crystal structure and chemical composition were analysed by electronic diffraction and EDX. For all tempering conditions (samples A3 to A7), four types of carbides were found depending on their morphology (see figures 7(a) 10(d)):
1) Angular and elongated intralath carbide: M7C3 Chromium iron carbide, Hexagonal structure
2) Globular intralath carbide: MC vanadium carbide, Face Centred Cubic (FCC) structure. Small size = secondary carbides, large size (100 nm 300 nm) = carbides not dissolved during the austenitization
3) Globular intralath carbide: M23C6 Chromium iron carbide, FCC structure, only important size (100 nm 300 nm) = carbides not dissolve during the austenitization
4) Elongated carbides situated at lath boundaries: M3C Iron chromium carbide, Orthorhombic structure (determined by XRD)
Statistics on carbide size are shown in tables 3 and 4. Three different populations were identified after tempering:
1) Small sized carbides MC and M7C3 types with an average size near 6 nm. This population is always found for all tempering conditions.
2) Middle sized carbides, mainly M7C3 type, with an average size near 30 40 nm. This population is found for tempering temperatures between 600 C and 640 C.
3) Scarce large M23C6 and MC type carbides with an average size >100 nm. This population is found for all tempering conditions and also in the as quenched sample (A2). These carbides are probably not dissolved during the austenitization. As these carbides were so scarcely encountered (less than 6/300), we did not take this population into account in the statistics.

Evolution of average carbide size: Increase of the average size is stated above 600 C with the formation of the second population. It is important to note that both populations have nearly a constant average size for all the tempering conditions. The carbide growth is mainly due to the increase of the amount of the second population carbides. After a fatigue test at 550 C, an increase of the average carbide size is observed. As no coalescence of carbides was observed during a second tempering at 580 C for 2h (A3) compared to a single tempering at 550 C (A2), we can obviously conclude that this coalescence is induced by cyclic strain.

In order to get an estimation of the volume fraction of carbides; XRD experiments on the bulk can be performed. Nevertheless, as major peaks coming from carbides are generally situated near peaks coming from the martensitic matrix, accurate calculations are difficult and time consuming. Therefore, a technique of dissolution of the martensitic matrix was used in order to assess carbide weight fractions and then carbide volume fractions (see Tables 4 and 5). (* In each case, we assumed that all the carbon (C) or all the alloying element (Cr, V) precipitates).
An increase of the weight fraction of carbides is observed for tempering temperatures above the secondary hardening peak situated near 550 C. So, these results clearly show that formation of precipitates still occurs during the second tempering. The formation seems to saturate for second tempering temperatures above 600 C (results at 640 C) are not available at that time).

Carbides X-Ray Analysis: Analysis of the X-Ray diffraction of carbides extracted from the martensitic matrix, confirm and complete the TEM results. Figure 8 shows the evolution of the carbide composition according to the heat treatment. So, the annealed steel contains Mo2C, Fe3C, M23C6 (M = Fe and Cr) and a small ratio of VC. After quenching, only the vanadium carbide (VC) and a small quantity of M23C6 which are probably not dissolved during the austenitisation were found. After the first and the second tempering, the X-ray analysis confirm the presence of Fe3C, Cr7C3, M23C6 (M = Fe and Cr) and trace of VC. Only the annealed steel contains molybdenum carbides.
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