Carbides in cast and hot-deformed S 6-5-2-5 high speed steel

Figure 1: As-cast microstructure of high-speed steel S 6-5-2. Scale bar: 100 µm.

Figure 2: Microstructure of the hot-deformed high-speed steel. Scale bar: 100 µm.

Carbide name: Carbide
Carbide No.: 955
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: LM
Steel name: High-speed steel S 6-5-2-5
Mat.No. (Wr.Nr.) designation: 1.3243
DIN designation: S 6-5-2-5
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: No data
Steel composition in weight %: 0.92% C, 0.42% Si, 0.27% Mn, 0.027% P, 3.93% Cr, 6.12% W, 4.77% Mo, 0.39% Ni, 1.74% V, 0.17% Cu, 4.59% Co, 0.018% Al, 0.018% N.
Heat treatment/condition: The investigated steel samples were cast as slightly conical ingots with the approximate dimensions of 300 x 300 mm in the sectional area. The ingots were hot formed in two heats down to 12.2 mm in diameter. The investigation was performed on material in two different fabrication stages: (i) after annealing, and (ii) after hardening and tempering.
Note: Depending on the steel composition several types of carbides are precipitated in highspeed steels: mainly MC, M2C, M6C and some others with minor importance. During hot working, the primary carbides formed during solidification change their as-cast structure to a more spherical one. They have an incoherent interface to the matrix and are a few micrometers in size. In the finished tool, the primary carbides together with the secondary, and the high hardness of the matrix are responsible for the high wear resistance. For the production of tools it is necessary that the steels can be machined, which is enabled by soft annealing. During this heat treatment some additional carbides of 50 to 300nm in size are precipitated as demonstrated by high-voltage electron microscopy (HVEM). After machining, the tools get their desired properties from hardening and tempering. The examination under these conditions shows the existence of nanometer-sized secondary hardening carbides, precipitated during this heat treatment and consisting mainly of vanadium and carbon as proven by energy filtered transmission electron microscopy (EFTEM). The high red hardness up to temperatures of approximately 550 C is caused by these nanometer-sized carbides. High resolution electron microscopy (HREM) revealed a completely coherent transition of the lattice planes from these carbides to the matrix-, without any irregularities.

The achievable properties strongly depend on the microstructure. In the solidified ingot of a S 6-5-2-6 steel the iron dendrites are surrounded by a carbide network. The solidification starts at about 1400 C with the formation of low-alloyed bcc delta-Fe. The remaining melt is enriched with the alloying elements, C, W, Mo, V, Cr, and Co. Therefore, 70100 C below the above temperature, the peritectic reaction starts, during which delta-Fe, already formed, together with the melt transforms into gamma-Fe. DTA-measurements often show a strong undercooling of this reaction. Nevertheless, the peritectic reaction always starts before the eutectic growth of the carbides. The eutectic temperature is as high as 1250 C. In the complex alloyed high-speed steel S 6-5-2-5, three types of primary carbides may form: fcc MC, hcp M2C, or fcc M6 C-carbides (M-metal), of a few micrometers in size. The type of the growing carbides depends on the fine adjustment of the melt, the enrichment of the alloying elements and the solidification rate.
The elements V, C, and N promote the formation of MC-carbides. More than 1% of V is needed to form MC-carbides in high speed steels. Other MC-forming elements are Ti and Nb, which are rarely used for producing conventional high-speed steels. M2C-carbides are favored by the presence of Mo, while W and Si encourage the formation of M6C. MC and M6C are thermodynamically stable carbides whereas M2C is metastable and immediately formed upon quick solidification. On slow cooling, the more stable M6C carbide is forming, though owing to its complicated structure, it is growing more slowly.
Figure 1 shows the as-cast structure of a high-speed steel solidified by M2C-carbides. The dark areas are the iron dendrites precipitated directly from the melt. The carbides appear as white phases within the interdendritic areas.
During annealing at higher temperatures for several hours the M2C-carbides decompose by the following reaction: M2C + gamma -> M6C + MC + alpha + C(gamma).
The metasabile M2C phase reacts with the austenitic matrix and forms the carbides of the type M6C and MC. Additionally in S 6-5-2 during this reaction, ferrite with a high content of W, Mo, and V is formed and there is a flow of carbon from the M2C into the austenite.
Moreover, the microstructure changes during hot and cold forming. The carbides of the high-speed steels are arranged within the socalled carbide stringers.
Figure 2 shows the typical micrograph of a longitudinal metallographic specimen of the high-speed steel bar with a diameter of 12.2 mm.
Depending on the reduction rate deformed carbide networks or carbide stringers occur. In small dimensioned steels with a high reduction rate only isolated carbides exist. Besides, the thermal and mechanical treatments during hot forming change the morphology of the carbides, i.e., rounding the edges and spheroidizing the carbides to minimize the surface energy.
Additional links: No. 955 and No. 958
Reference: Not shown in this demo version.

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