Low chromium semi-steel has high hardness and excellent wear resistance, but the carbides in its structure are distributed in a continuous network, and the toughness is poor, which limits its application. Considering that rare earths can be added to steel, it can change the morphology of carbides and inclusions, refine grains, and purify and strengthen the grain boundary. It can improve the properties of steel in many ways. Therefore, this paper studies the modification of rare earths in low-chromium semi-steels. The influence of rare earth on the performance of low chromium semi-steel provides a reference for improving the comprehensive mechanical properties of the material.
1 Test materials and methods 1.1 Test material The material was melted in a medium-frequency induction furnace of 250 kg. Raw iron, scrap steel and ferrochrome were used as raw materials. When the temperature of molten iron reached around 1 450°C, the material was deoxidized with a plug and then poured into the molten iron. The bag was modified by flushing. The modificator was a rare earth-based mixed rare earth metal. The blank was cast in a dry sand mold with the addition of rare earth (RE) and the chemical composition of the sample. Samples were cast and heat treated in two groups. The heat treatment process was 960°C for 3 h for normalizing, and then the blanks were processed into impact specimens (10 mm × No. 1.2 performance test for impact toughness and hardness in JB30B impact tester respectively. And HR150A Rockwell hardness measured on the machine, the impact work is 3. 5J wear test specimens in the MLD10 type dynamic load wear tester, abrasives for the 80 mesh Al hard abrasive, test time is 15 min, with one ten-thousand analytical balance measurement The weight loss is the normalized 45 steel as the standard sample, and the relative abrasion resistance (ε) of the material is equal to the ratio of the standard weight loss to the weightlessness of the sample.The bending strength (σ) is measured with a WE10A hydraulic universal measuring machine. The relative toughness is expressed in σ-special steel (f is the deflection).The above test data are taken as the average of the three results.
1.3 Metallographic and Electron Microscopy Observation The carbide morphology of the specimens was observed with an optical microscope and a KYKY1000B scanning electron microscope. The shape factor of the carbides was measured at the impact fracture and the wear surface (wherein Z = - array falling within the measured tissue. The number of points, P - - the number of intersection points between the measurement grid line and the measured tissue boundary line).
2 Test results and analysis 2.1 Effect of rare earth on carbide morphology Observation by metallographic observation showed that in the as-deposited as-cast specimens, the carbides in the structure were distributed in a continuous network. After the rare earth metamorphism, the network carbides appeared. Many necking joints partially broke the network. However, after the rare earth metamorphism of the network-like characteristics was maintained and the heat treatment was performed, the carbides were distributed in individual blocks, and the network characteristics disappeared. The shape factor of the carbides increases greatly, and at the same time there is the formation of particulate carbides in the matrix. result.
The non-metamorphic as-cast 0.25RE metamorphic heat treatment can be seen in Table 2. Reliance on rare earth metamorphism alone can only limitedly improve the morphology of carbides, and its effect is far inferior to the composite effect of rare earth metamorphism and heat treatment.
Rare-earth atoms are larger in radius than iron atoms and less dissolved in solid solution. Therefore, during the solidification process, they are likely to be enriched on the surface of new carbides, impeding the preferred growth rate of carbides, making it difficult to connect carbides into a closed net. , And necking connection or disconnection phenomenon.
The transformation of eutectic carbides from reticular to massive at high temperatures is an inevitable trend in the development of thermodynamics [6]. It is based on the dissolution of carbides and the diffusion and precipitation of carbon atoms. In contrast, the carbides in the weak parts of the mesh are easily broken first. The addition of rare earth elements, on the one hand, increases the weak parts of the networked carbides (necked connections). On the other hand, the rare earth atoms dissolve into the carbides to increase the lattice distortion of the carbides and reduce the stability of the carbides. In addition, the atomic radius of the rare earth element is larger than that of the iron atom 25 , which increases the distortion of the matrix around the rare earth atom, increases defects such as vacancies, and promotes the dissolution of carbides at high temperatures and the diffusion of carbon atoms, thereby accelerating the network. Dissolution and agglomeration of carbides and precipitation of matrix particulate carbides.
2.2. Effect of Rare Earth on Properties As can be seen from the test results, the impact toughness (a), wear resistance (ε), and bending strength (σ) increase with the increase of rare earth content, when the rare earth content is 0.25. Within the range of ~ 0.45, a? f decreases with increasing rare earth content, while ε does not change significantly. After heat treatment, the impact toughness (a), wear resistance (ε), bending strength (σ), and relative toughness (σf) are further improved. The variation is generally consistent with the change of the rare earth without heat treatment. Only slight decrease in hardness value. As a result of the 0.25 rare earth addition amount, the impact value was found to be 3.58 J? The increase of the bending strength of cm to 75, the improvement of the wear resistance of 25 cm, and the effect of rare earths on the low chromium semi-steel a are shown in Fig. 1. It can be seen that the composite effect of rare earth modification and heat treatment can improve its comprehensive performance, and is particularly effective for toughness.
Liu Jianhua et al.: Effects of rare earths on the morphology and properties of carbides in low chromium semi-steels. As a result of rare earth metamorphism in low chromium semi-steels, the continuity of the networked carbides is destroyed, and the segmentation effect of carbides on the matrix is ​​reduced. The grains reduce the inclusions of the grain boundary and the phase interface, increase the strength of the grain boundary and the phase interface, and increase the toughness and relative toughness. At the same time, the discontinuous carbides protect the continuity of the matrix, giving full play to its plastic properties, and to a certain extent, it can co-deform with the matrix, which will hinder the initiation and propagation of cracks at the carbide interface at the bottom of the groove. The effect is to greatly reduce the weight loss due to flaking and improve the surface's resistance to spalling. Combined with heat treatment, the network carbides are broken more completely. Furthermore, the precipitated particulate carbides in the matrix play a protective role for the matrix. Therefore, after the material is degraded and then heat-treated, its comprehensive performance is further improved.
From the fracture morphology (Fig. 2), it can be seen that the as-cast specimen has larger fracture grains, and the typical brittle fracture metamorphism has relatively smaller grains, some of which show quasi-cleaving characteristics, a torn ridge, and a small amount of dimples. The number of dimples in the fracture surface after heat treatment further increases, the wear surface of the undegraded as-cast sample has a tendency to spall off, the tendency to exfoliate after heat treatment decreases, and there is a significant deformation feature at the bottom of the cutting furrow.
Further illustrating that the rare earth has the effect of improving the performance of the low-chromium semi-steel, the effect of the joint action with the heat treatment is more significant, and its performance is in good correspondence with the organization. The further increase of rare earth elements and the decline of various performance indicators may be related to the increase of rare earth inclusions.
3 Conclusions (1) Rare-earth elements can not only improve the morphology of carbides in the solidification process of low-chromium semi-steels, but also accelerate the breaking and agglomeration of network carbides during heat treatment and promote the precipitation of particulate carbides in the matrix.
(2) After the rare-earth metamorphism of low-chromium semi-steel, its comprehensive performance can be improved. When combined with heat treatment, the effect is more significant, especially impact toughness, its performance and organization have a good correspondence.
(3) Low-chromium semi-steel is tempered with 0. 25 rare earth and then tempered at 960°C for 3 h for normalizing. The impact toughness is increased 158, the relative toughness is improved, the bending strength is increased by 75, and the wear resistance is improved by 1; Mo, Shen Paul. Effect of Rare Earth on Cr-Ni-W-Mo Heat-Resistant Cast Steel.
2 Roddy, Xing Guohua. The segregation of SP on the grain boundary of high speed steel and the net of rare earth elements Jiang Qichuan, Wang Shuqi, Cui Xianghong. Effect of Rare Earths on the Morphology and Properties of Casting Cr12MoV Die Steels . Chinese Journal of Rare Earths, 1993, 14(1):43 6 Ma Qian, Liu Baicheng, Wang Zhaochang. Study on Continuous Change of Carbide Surface During High Temperature Insulation of FeC Alloys . Steel, 1994, 29(5):47 Liu Jianhua, male, 38 years old, senior engineer. He graduated from Harbin University of Science and Technology in 1985. Engaged in research on wear-resistant materials and nanocrystalline materials, won the provincial and municipal scientific and technological progress awards, and published more than 40 papers.
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