| Summary: | 碩士 === 國立中山大學 === 機械與機電工程學系研究所 === 95 === For the induction heating process of bi-metallic tubes, the inner tube of alloy-layer is much easier to cause a lot of defects of cavities due to the fact that heating power, maximum temperature value and the time frame of temperature retention were chosen improperly. This research focuses on the effect of maximum temperature value and the time frame of temperature retention on the micro-structure and defects of cavities of the Nickel-based alloy-layer.
The experiments of this study are divided into two parts. In the domain of the experiment in simulation fashion, Nickel-based alloy powders were put into the specimens of AISI 4140 steel. Radio Frequency (RF) oven were used to smelt Nickel-based alloy powders in the vacuum conditions over the maximum temperature range of 920~1180℃respectively. After that, the time frame of temperature retention was conducted from 0 to 10 minutes. Then, the furnace-cooling went down to 700℃ then air-cooling down to the room temperature. Nickel-based alloy-layer, microstructure, component analysis, defects of cavities of the interface between Nickel-based alloy-layer and steels, and diffusion of interfaces were analyzed using optical microscopes (OM) and scanning electron microscopes (SEM).
From the experiments, it was found that Nickel-based alloy-layer consisted of γ-Ni、CrB、Cr7C3 over the maximum temperature range of 920~1050℃whether temperature retention is performed or not. According to the findings of metallographic observation, the increase of coarsening and the reduction of the capacity of CrB and Cr7C3 become more obvious as maximum temperature value and the time frame of temperature retention become large. In addition, the whitening layer (diffusion zone) formed between the interface of alloy-layer and steels become much wider as maximum temperature value and the time frame of temperature retention become large.
Secondly, the field experiment method was also applied in this paper. The tube rich in Nickel-based alloy powders was heating to analyze induction coil in various conditions: heating power (200~285KW), maximum temperature value (1020~1040℃), the time frame of temperature retention (10, 30, 50sec), and the rotating speed (1000~1300rpm). The results of the experiments indicated that the surface of the alloy-layer cause defects of vermicular cavities since the volume of liquid cannot fill out the crack of cavities completely due to lower temperature when there is insufficient time; too long periods of the time frame of temperature retention lead to the tough and huge dendrites to obstacle the flowing of liquid and the solidification of shrinkage cavity.
According to the observation of the microstructure, the larger the maximum temperature value and the time frame of temperature retention were, the more the dendrites formed. The formation of dendrites causes not only the uneven distribution of hardening phase of CrB and Cr7C3 of the alloy-layer but also the reduction of hardness of the alloy-layer. The dendrites are typically formed from the interface to the surface of the alloy-layer. Besides that, the alloy-layer mainly consists of γ-Ni, Ni3B, Ni3Si, CrB, and Cr7C3 via X-ray Diffraction (XRD). Among them, the main hardening phases are CrB and Cr7C3 which is the main reason that the alloy-layer has high-level hardness.
As maximum temperature value and the time frame of temperature retention become large, the whitening layer (diffusion zone) was formed between the interface of alloy-layer become much wider because the faster the elements of the based materials (tube) diffused and the wider the intermetallic compound formed among the interfaces. After heated for 800 seconds over the temperature range of 750~1030℃, iron element was diffused all over the alloy-layer. The increase of coarsening and the reduction of the capacity near interface and interface become more obvious as maximum temperature value and the time frame of temperature retention become large.
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