Cr7C3 and NbC carbides in 2.50C-3.30Si-6.20Cr-1.42Ni-2.20Mo-2.60V-0.98W steel

Figure 1: Microstructure of the steel after soft annealing, scale bar: 10 µm.

Figure 2: Distribution of alloying elements in the investigated steel (X-ray maps).

Figure 3: Microstructure of the hardened steel (austenitizing temperature of 1050C), scale bar: 10 µm.

Figure 4: Primary Cr7C3 carbide particle in the hardened steel (TEM-bright field), scale bar: 200 nm.

Figure 5: Primary NbC carbide particle in the hardened steel (TEM-dark field), scale bar: 200 nm.

Carbide name: Cr7C3, NbC
Record No.: 745
Carbide formula: Cr7C3, NbC
Carbide type: M7C3, NbC
Carbide composition in weight %: No data
Image type: SEM, TEM, X-ray
Steel name: 2.50C-3.30Si-6.20Cr-1.42Ni-2.20Mo-2.60V-0.98W steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: PM tool steels
Steel composition in weight %: 2.50% C, 3.30% Si, 6.20% Cr, 1.42% Ni, 2.20% Mo, 2.60% V, 2.60% Nb, 0.98% W, 0.03% N, 0.04% O, 0.03% P, 0.02% S.
Heat treatment/condition: The investigated tool steel was prepared using a powder-metallurgy process. The powder was produced by atomizing the melt using a nitrogen flow at a pressure of 400600 kPa.
Chromium, vanadium, molybdenum and niobium are the main carbide-forming elements present in steel. The contents of oxygen and nitrogen, which are of the order of hundreds of ppm, are in accordance with the method used for the powder preparation.
The powder was compacted using a hot isostatic pressing (HIP) method. The steel capsule (length 200 mm and diameter 40 mm) containing the powder was evacuated. The HIP processing was performed at 1120C under an argon pressure of 147 MPa for 4 h. The residual porosity of the hot isostatically pressed product was negligible.
The heat treatment of the compacted semi-product consisted of soft annealing, austenitizing, hardening and tempering. The soft annealing was performed at 780 C for 8 h with subsequent slow cooling to 680 C, and then holding at this temperature for 2 h. Next, the steel was slowly furnace cooled to room temperature. The conditions of soft annealing were selected with the aim of obtaining a desirable growth of carbide particles. After this treatment, the hot isostatically pressed semi-product was cut into samples of size approximately 5 mm 10 mm 10 mm. To characterize the influence of the heat treatment on its properties, the steel was austenitized for 30 min at temperatures ranging from 1000 C to 1150 C, after which it was cooled in nitrogen with a pressure of 500 kPa to 600 kPa in a vacuum furnace. As a final step, the hardened steel was tempered three times at various temperatures in the range 150700 C with intermediate air-cooling.
Note: The microstructure of a P/M tool steel with 2.5% C, 3.3% Si, 6.2%Cr, 2.2%Mo, 2.6%V, 2.6%Nb and 1.0%W was investigated for various heat-treatment conditions, with the aim of achieving an alloy with the maximum hardness. The heat treatment of the compacted semi-product consisted of soft annealing, austenitizing, hardening and tempering.

The microstructure of the investigated alloy is shown in Figure 1. It consists of carbide particles that are and uniformly distributed in the matrix and surrounded with the matrix. The carbide particles are of two different sizes.

EDAX microanalysis was used for to determine theation of chemical composition of the carbides. The X-ray maps in Figure 2 show that chromium is the main constituent of the coarse carbides. The size of these particles ranges from approximately 5 µm to 10 µm. Vanadium, niobium and molybdenum are present mainly in the smaller carbide particles of with diameters less than 3 µm. Vanadium and niobium preferably tend to form MC carbides. The number of smaller V-, Nb- and Mo-rich particles is much higher than that of the coarse Cr-rich particles.

The microstructure of the hardened samples consists of particles uniformly distributed in the matrix of the solid solution, as can be seen in Figure 3.

The microstructure of the hardened steel is similar to that of the soft annealed steel with two types of particles. The coarse particles of irregular shape, see Figure 4, were identified using TEM as Cr7C3, and the smaller particles (Figure 5) as NbC. The volume fraction of the primary carbide particles in the steel depends on the austenitizing temperature, and the volume fraction of the carbides decreases with the increasing temperature of austenitizing, due to their dissolution in the austenite. The finest carbides were present in the steel hardened at 1050 C.

The final properties of many highly alloyed tool steels are adjusted by tempering at relatively high temperatures (500600 C), where the peak of the secondary hardness occurs. To find the optimum tempering conditions for our material, the hardened samples were tempered (tempering time: 3 1 h) at temperatures from 150 C to 700 C and detailed investigations of the microstructure, the phase composition and the mechanical properties after tempering were performed.
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