Carbides in 0.57C-1.96Si-0.16Mn-1.02Cr steel

Carbides in 0.57C-1.96Si-0.16Mn-1.02Cr steel

Figure 1: Carbide microstructures of the samples that were tempformed at 500 (a), 600 (b) and 700°C (c) and were conventionally quenched and tempered at 500 (d), 600 (e) and 700°C (f). Scale bars: 0.5, 1, 2 µm.

Carbide name: No data
Record No.: 1077
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: SEM, TEM
Steel name: 0.57C-1.96Si-0.16Mn-1.02Cr
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Structural steels
Steel composition in weight %: 0.57% C, 1.96% Si, 0.16% Mn, 0.001% P, 0.001% S, 0.041% Al, 1.02% Cr, 0.002% Mo, 0.0018% N, 0.0005% O.
Heat treatment/condition: A 100 kg ingot was prepared by vacuum melting and casting, homoghomogenized at 1200°C, and then hot-rolled to a plate with a thickness of 4 cm. A 12 by 4 by 4 cm block was cut out of the plate, heated to 1200°C, hot-rolled to a square bar with a cross section area of 10 cm2, solution-treated at 1200°C for 60 min, and water quenched to obtain a martensitic structure without any coarse undissolved carbides. The average prior-austenite grain size of the quenched bars was 210m m and the hardness was HV 850. The quenched bars were tempered at 500, 600 and 700°C for 90 min, subjected to multi-pass caliber-rolling at the respective temperatures to square bars with a cross section area of 2 cm2, and air cooled (tempformed (TF) samples). The accumulative reduction in area through tempforming was 80% in ten passes, which corresponds to an equivalent strain of 1.8. Note that the samples were held for 5 min in a furnace after every three passes during the rolling and passed through twice for the final groove to control the cross sectional shape of the bars, namely the pass schedule was 3-3-4 at each temperature. To obtain conventional quenched and tempered samples with the same tensile strengths for comparison, normalized bars were austenitized at 880°C for 30 min, followed by oil quenching, tempered at 500, 600 and 700°C for 90 min, and then water cooled (QT samples). The QT samples had an average prior-austenite grain size of 26 µm and showed random textures.
Note: The deformation of tempered martensitic structures, namely tempforming treatments, were applied to a 0.6C–2Si–1Cr steel at 500, 600 and 700°C using multi-pass caliber-rolling with an accumulated area reduction of 80%. The tensile and Charpy impact properties were investigated to make clear the relation between the microstructure and the delamination behavior of the tempformed (TF) samples. The tempforming treatments resulted in the evolution of ultrafine grain structures with strong <110>//rolling direction (RD) fiber deformation textures and fine spheroidized cementite particles distributions. In contrast to the ductile-to-brittle transition of the conventional quenched and tempered (QT) samples, the TF samples exhibited inverse temperature dependences of the impact toughness due to the delaminations, where the cracks branched in the longitudinal direction (//RD) of the impact test bars. As a result, high strength with excellent toughness was achieved in the TF samples. A yield strength of 1364 MPa and a V-notch Charpy absorbed energy of 125 J were obtained at room temperature in the sample that was tempformed at 500°C. The delamination was shown to occur due to the microstructural anisotropy of the TF samples, and the dominating factors controlling the delamination toughening were the transverse grain size, the grain shape and the .110.//RD fiber deformation texture. The discussion also indicated that the ultra refinement of the transverse grain structure was the key to enhancing both the yield strength and the toughness of the TF steel while lowering the ductile- to-brittle transition temperature.

Figure 1 are TEM and SEM images showing morphologies and distributions of carbide particles of the TF and QT samples. Spheroidized cementite particles are dispersed in all of the TF samples. Relatively large cementite particles exist on the (sub) boundaries of the matrix ferrite grains, while finer spherical cementite particles are homogeneously dispersed inside the matrix grains. In particular, the cementite particles on the (sub) boundaries may play an important role in retarding the grain migration through their pinning effect during tempering and tempforming. The average aspect ratios of cementite particles were measured to be about 1.3 inside the grains and 1.5 on the (sub) grain boundaries, respectively. The average lengths of the long axis for the cementite particles inside the grains and on the (sub) boundaries in tempforming at 500°C are 28 and 64 nm, respectively, and increased to 177 and 370 nm in tempforming at 700°C. It appears that the number of cementite particles inside the grains tends to decrease with increasing tempforming temperature. When compared at the same tempering temperature, there is no significant difference in the cementite particle size distribution between the QT and TF samples. This indicates that such bimodal distributions for the cementite particles in the TF samples might be inherited from those of prior tempered martensitic structures.
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