Crystallization of High Alloy White Iron LTF Zone
Abstract
High alloy white iron LTF zone (LTF zone) is a kind of cast iron alloy with high chromium and high carbon content, which is mainly used for high wear-resistant parts. In this paper, the crystallization process of high alloy white iron LTF zone was studied by choosing molten steel and monotonic cooling, and the heat treatment process was simulated by using Pro/E to obtain the microstructure information. The results show that the microstructure of high alloy white iron LTF zone mainly manifests as a dispersed nodular graphite structure, and the microstructure change of each part is very obvious. The main changes are that the grain size gradually becomes smaller from the center of the casting, the microstructure of the interlayer gradually changes from local proeutectoid ferrite to nested proeutectoid ferrite, and the morphology of graphite changes from acicular graphite to flake graphite. These findings can help foundries to develop a suitable process route and testing methods for controlling the crystal structure of high alloy white iron LTF zone, so as to improve the comprehensive mechanical properties of the parts after processing.
Keywords: high alloy white iron; LTF zone; crystallization; microstructure
1. Introduction
High alloy white iron LTF zone (LTF zone) is characterized by high chromium content and high carbon content. It is mainly used as wear-resistant parts, such as high strength rollers, roll rings, roller linings and other components in metallurgical industry [1]. Due to its special properties, the structure and properties of LTF Zone are different from ordinary cast iron and steel. The microstructure of this kind of alloy mainly contains ferrite, pearlite and graphite. In order to meet the required performance indicators, it is necessary to understand the crystallization and heat treatment process of high alloy white iron LTF zone and reason about the corresponding microstructure.
2. Experiment
2.1 Materials and experiment preparation
The main component of the tested high alloy white iron LTF zone is as follows: C: 3.9%, Si: 1.3%, Mn: 0.6%, P: 0.1 ％。Sex:0.2% ；Cr: 2.6 %，Ni: 0.5%,Mo: 0.5%, Cu: 0.5%, V: 0.3%. The alloy was melted in an electric arc furnace and casted into cylindrical specimens with a diameter of 30mm and a length of 80mm. After the specimens were normalizing heat treated and machined, they were polished and etched by KOH solution to observe the microstructure.
2.2 Crystallization and heat treatment simulation
Crystallization and heat treatment simulation of high alloy white iron LTF zone were carried out by using Pro/E software. The simulation process selected molten steel as the initial state and monotonic cooling mode. The cooling process of the experimental materials was simulated, and the temperature was changed from 1350℃ to 950℃ according to the crystallization curve. The total cooling time was 120S.
3. Result and discussion
3.1 Microstructure of high alloy white iron LTF Zone
Figure 1 is the microstructure of high alloy white iron LTF zone after KOH etching. It can be observed that the microstructure of high alloy white iron LTF zone mainly manifested as dispersed nodular graphite structure. The microstructure photos of different parts of the specimen are shown in Figure 2, it can be seen that the microstructure change of each part is obvious.
Figure 1. Microstructure of high alloy white iron LTF zone
Figure 2. The microstructure of different parts of the specimen
From the center to the surface of the casting, the grain gradually becomes smaller, and the interlayer gradually changes from local proeutectoid ferrite to nested proeutectoid ferrite. Figure 2 is the comparison of the microstructure of different parts of the experiment specimen. From the graph, it can be seen that the grain size and interlayer structure change significantly with the position location.
3.2 Graphite morphology change
The graphite morphology of high alloy white iron LTF zone changes from acicular graphite to flake graphite with the increase of radial distance (Figure 3). From the graph, it can be seen that the shape of graphite changes from acicular to flocculent near the welding line, and then gradually changes from flocculent to uneven flake along the radial direction. The graphite size gradually decreases from the center to the surface of the casting, which may be related to the rapid cooling caused by the liquid surface.
Figure 3. Graphite morphology of high alloy white iron LTF zone at different position
4. Conclusion
In summary, the crystallization process of high alloy white iron LTF zone was studied by using the molten steel and monotonic cooling method. The heat treatment process was simulated by using the software Pro/E to obtain the microstructure information. The results show that the microstructure of high alloy white iron LTF zone mainly manifests as dispersed nodular graphite structure, and the grain size gradually becomes smaller from the center of the casting, the microstructure of the interlayer gradually changes from local proeutectoid ferrite to nested proeutectoid ferrite, and the morphology of graphite changes from acicular graphite to flake graphite. The research findings can help foundries to develop a suitable process route and testing methods for controlling the crystal structure of high alloy white iron LTF zone, so as to improve the comprehensive mechanical properties of the parts after processing.
