Carbides in spray-formed AISI M3:2 high-speed steel

Figure 1: Microstructure of as-sprayed material (a) optical micrograph of polished sample demonstrating the presence of a carbide network, (b) optical micrograph of etched sample, (c) SEM image of etched sample. Scale bars: 50, 50, 5 µm.

Figure 2: Series of optical micrographs along compression axis of specimen deformed in experiment C. Positions corresponding to (a)(d) are indicated in Fig. 1. Scale bars: 50 µm.

Figure 3: SEM images of cross-sections perpendicular to compression axis of compression test samples (a) experiment B: nominal deformation temperature 900 C, (b) experiment H: nominal deformation temperature 1100 C. Note the difference in density of submicron-sized carbides and the presence of cracked carbides in (a). Scale bars: 20 µm.

Figure 4: Optical micrographs for specimens deformed at strain rates given and nominal temperatures T = 900 C (a), T = 1000 C (b) and T = 1100 C (c). Scale bars: 25 µm.

Table 1: Dimensions of specimens before and after deformation.

Carbide name: Carbides
Record No.: 733
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: LM, SEM
Steel name: ASP 2024
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: AISI M3:2
Other designation: No data
Steel group: PM High speed steels
Steel composition in weight %: 1.31% C, 6.24% W, 5.34% Mo, 4.12% Cr, 3.05% V, 0.56% Si, 0.24% Mn, 0.11% Ni, 0.22% Cu, 749 ppm N.
Heat treatment/condition: A 3-ton spray-formed high-speed steel billet was supplied by Danspray (Gregersenvej 8, 2630 Taastrup, Denmark) in the as-sprayed condition with approximate dimensions 0.5 wide by 2 m long. Axisymmetric compression tests at temperatures in the range of 9001100 C and strain rates between 0.1 and 10 s-1 were carried out in order to simulate the microstructure evolution during forging. The nominal maximum strain for all experiments was 0.69, which corresponds to a reduction of 50%. This is a typical height reduction for billets to be forged into work rolls. The load was applied by means of ceramic platens with an average temperature, as given in Table 1. There was a temperature gradient between upper and lower platen e.g. for a nominal test temperature of 1000 C, the temperatures of the platens were 990 and 1010 C, respectively. The temperature of the test specimen was measured by means of a thermocouple in the centre of the specimen. The specimen was heated by induction for 2 min to the required test temperature and then held at that temperature for 1 min before the compression test was started. After reduction of the specimen to 50% of the original height, the test specimen was removed from the furnace and water quenched. The time between the end of deformation and onset of water quench was about 1 s.
Note: Axisymmetric hot compression tests (9001100 C) on spray-formed AISI M3:2 high-speed steel were performed in order to establish suitable parameters for hot forging of this material. Special attention was paid to establish the deformation conditions that lead to the breakdown of the carbide network, present after spray forming, and to avoid fracture of the material as a result of deformation. By a combination of microstructural analysis and finite element modelling, values for fracture stresses in this temperature range and critical strains for the breakdown of the carbide network are given. The activation energy for hot deformation was also determined.

Influence of compression test parameters on microstructure: The initial microstructure of the as-sprayed material is shown in Fig. 1 .As far as the carbide distribution is concerned three different length scales were present: I. Carbide network around 70 µm consisting of irregular shaped carbides (highlighted by a black line in Fig. 1a), and close to elliptical or round carbides with average size of about 10 microns (marked by white lines in Fig. 1a); II. Close to elliptical or round carbides of micron size (examples marked by white circles in Fig. 1c); III. Rod shaped and rectangular carbides of sub micrometer size (Fig. 1c).
Carbides of category I and II were visible by optical microscopy of polished samples (Fig. 1a). Carbides of group III were only visible in SEM images (Fig. 1c). However, a darkening in optical micrographs of the nital etched material (Fig. 1b) is indicative of their presence. This can be seen by comparing Fig. 1b and c. From these figures it is also evident that the carbide density varies from cell to cell.
In order to investigate the influence of compression test parameters on microstructure, these different length scales have to be considered separately. The influence of strain on the carbide network structure is shown in Fig. 2, which shows a series of optical micrographs taken along the compression axis of the test specimen. No deformation of the cells is observed in Fig. 2a, taken from the area in contact with the tool during compression. Moving towards the centre of the specimen the carbide cell is first seen to be deformed in Fig. 2b, and then only local remnants are observed in Fig. 2c. There were no indications for the presence of a carbide network in the centre of the specimen (Fig. 2d). This behaviour was observed for all tests and indicates that the carbide network is broken down by deformation amounting to an equivalent plastic strain between 0.82 and 0.89, depending on deformation temperature.
A higher deformation temperature resulted in a slightly lower critical strain. The deformation temperature influences the carbide network in the direction perpendicular to the compression axis. The cracking of large irregular-shaped carbides was observed for deformation at 900 C (Fig. 3a) but not at 1100 C (Fig. 3b).
The number of carbides of category III strongly depends on the deformation temperature, as is evident by comparing Fig. 3a and b. In these figures, the small carbides are represented by the small white dots in the low-magnification SEM images. The low magnification was chosen to provide a better impression of the density distribution over a large area, whereas the insets demonstrate the actual shape and size of the type III carbides. The high density of carbides at the lower deformation temperature of 900 C in Fig. 3a is apparent. A low carbide density in conjunction with a strong local density variation was observed for deformation at 1100 C (Fig. 3b).
The deformation temperature also influences the structure of the matrix, as can be seen from Fig. 4. For deformation below 1100 C (Fig. 4a and b), the matrix grain boundaries were not revealed by nital etching. However, following deformation at 1100 C, a well-developed matrix grain structure, resembling a recrystallised structure, was observed (Fig. 4c). However, recrystallisation did not seem to have taken place homogenously, and was only observed in the bands of lower carbide density (represented by absence of small black dots) in Fig. 4c.
Links: No data
Reference: Not shown in this demo version.

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