Effect of Annealing Temperature on Microstructure and Hardness of Cold-Worked Low Cr-Mo-Ball Iron Rolls
Abstract
In this paper, the effects of annealing temperature on hardness and microstructure of cold-worked low Cr-Mo-ball iron rolls were studied. Metallographic analysis and nanoindentation technology were used. The results show that with the increase of annealing temperature, the hardness of the rolls increases obviously. After annealing, the grains become finer, the grain size and the average grain size of the samples decrease, spheroidization of ferrite and the dispersion of carbides increase. The hardness and microstructure change of cold-worked low Cr-Mo-ball iron rolls are related to recrystallization, carbide dispersion, and grain refinement which are positively correlated with annealing temperature, which increases with the increase of annealing temperature.
Keywords: Annealing; Low Cr-Mo-Ball Iron Rolls; Hardness; Microstructure
1. Introduction
Rolls are main parts of rolling mills and the quality of rolls is directly related to the quality of products and their processing conditions [1-2]. In particular, cold-worked alloy rolls greatly improve the forming quality of long steel products and reduce the manufacturing cost of rolled products [3-4]. Therefore, the microstructure and hardness of cold-worked alloy rolls have been the research hotspot of metallurgical industry. There are many kinds of cold-worked alloy rolls, and some of them are made of low cobalt-molybdenum alloy [5-6]. As the mainstream of low alloy steel rolls in Chinese market, Low Cr-Mo-ball iron rolls based on low cobalt-molybdenum alloy have good economic performance, high processing efficiency and good comprehensive properties [7-8]. Therefore, it has broad application prospects in the production of long steel products.
At present, most studies adopt annealing treatment before and after rolling of low alloy metal rolls, in order to improve their comprehensive performance [7]. However, there are few studies on the effect of annealing temperature on their microstructure and hardness. In this paper, the effects of annealing temperature on hardness and microstructure of low Cr-Mo-ball iron rolls were studied.
2. Experiments
2.1 Materials
The chemical composition of the material used in the experiments was (wt.%,): C≤0.10, Si 0.17, Mn 0.52, Cr 0.95, Mo 0.59, V 0.35, P 0.019, S 0.008, The material has been completely decarburized and can be treated by annealing.
2.2 Processing
Annealing experiments of Low Cr-Mo-ball iron rolls were conducted in a box-type electric furnace with 1500 W heating method. Four different annealing temperatures, i.e., 900℃, 950℃, 1000℃ and 1050℃ respectively, were used in the experiments. Before annealing, the rolls were preheated at 850℃ for 1 h. Then the samples annealed at different temperatures for 1 h and the results of four kinds of annealing temperatures were separately analyzed.The cold working of the samples was realized by dynamic load rolling machine.
2.3 test
Metallographs of the four samples were prepared by electrolytic polishing, etching with Nital 2% solution, and observed by optical microscope. The average grain size was measured by measuring software. The hardness was characterized by nanoindentation technology.
3. Results and Discussion
3.1 Microstructure
Figure 1 shows the microstructure of Low Cr-Mo-ball iron roll samples before and after annealing. Before annealing, the microstructure mainly includes granular ferrite and M23C6 carbides (Fig.1a). With the increase of annealing temperature, the grains become finer, the grain size and the average grain size of the samples decrease, spheroidization of ferrite and dispersion of carbides increase (Fig.1b-d).
Figure 1 Microstructure of Low Cr-Mo-ball iron rolls before and after annealing. (a) before annealing; (b) 900℃; (c)950℃; (d)1000℃; (e) 1050℃.
3.2 Hardness
Figure 2 shows the variation of hardness of Low Cr-Mo-ball iron roll samples before and after annealing. It can be seen that before annealing, the hardness of the roll is low and increases with the increase of annealing temperature. After annealing at 1000℃, the hardness of the roll reaches the maximum and the hardness increases by more than 1.6 times. With the increase of annealing temperature, the hardness of the roll increases obviously.
Figure 2. Hardness of Low Cr-Mo-ball iron roll before and after annealing.
4. Conclusion
In this paper, the effects of annealing temperature on hardness and microstructure of cold-worked low Cr-Mo-ball iron rolls were studied. Metallographic analysis and nanoindentation technology were used. The results show that with the increase of annealing temperature, the hardness of the rolls increases obviously. After annealing, the grains become finer, the grain size and the average grain size of the samples decrease, spheroidization of ferrite and the dispersion of carbides increase. The hardness and microstructure change of cold-worked low Cr-Mo-ball iron rolls are related to recrystallization, carbide dispersion, and grain refinement which are positively correlated with annealing temperature, which increases with the increase of annealing temperature.
