Study on the Effect of Rare Earth Elements on the Microstructure and Performance

Abstract. Rare earth elements are able to improve the tenacity of materials and the nucleation rate of alloys in crystallization. Meanwhile, it can make the grain boundary purified and improve the strong tenacity of the interface. Therefore, the reduction of cladding layer’s tearing tendency and the increase of cladding layer’s strong tenacity were realized by adding rare earth elements in this paper. The result of the experiment showed that rare earth elements could be added into the cladding layer under the technological conditions of strictly controlling arc power density and shortening the retention of cladding materials in electric arc, and also could make grains refined and play a role in reducing the cracking tendency

Keywords: Electric Arc Cladding; Cracks; Rare Earth Elements

Introduction

Now, the role of rare earth elements in the alloy crystallization process has been understood by people in depth. It is thought that rare earth elements are able to improve the tenacity of materials and the nucleation rate of alloys in crystallization, and simultaneously make the grain boundary purified and the strong tenacity of the interface improved. Therefore, it is hopeful to realize the reduction of cladding layer’s tearing tendency and the increase of cladding layer’s strong tenacity by adding rare earth elements. The technological processes of sending-powder cladding and preset cladding are greatly different from each other: it is easy to add rare earth elements in the process of preset cladding, in which rare earth elements are burned to lose in spite of the very high heating temperature, but a part of them will be always preserved in the cladding layer. As for sending-powder cladding, cladding materials are heated to a very high temperature in the air so that rare earth elements are easily burned to lose. Therefore, if a part of rare earth elements can be preserved in the cladding layer is always a problem focused by people.

To solve the cracking problem in the cladding layer, the preheating before cladding and the slow cooling after cladding are often implemented currently using service parts or semi-finished products, so as to dry the cladding materials before cladding. However, the problem is not fundamentally solved although some effect is achieved. In this paper, the cracking problem of the cladding layer is well solved by adding rare earth elements into the cladding powders.

Experiment

5%~8% of rare earth element Re was added into cladding material Ni-Cr-B-WC and then fully mixed as sending-powder cladding material, 38CrSi was used as matrix material for the single-channel sending-powder cladding, and then the acquired cladding layer was analyzed and studied from microstructure, hardness and abrasive resistance, and cladding layer’s chemical composition. The chemical composition of the experimental materials were as shown in table 1, and the technological parameters of cladding were as shown in table 2; the obtained microstructure of the cladding layer was as shown in figure a. The microstructure of the cladding layer with no rare earth element

b. The microstructure of the cladding layer with rare earth element

Fig.1.The microstructures of the cladding layer with different composition under the same technological process

Analysis on the experimental result

The causes of the cracks in the cladding layer

Crack in the electric arc cladding layer is one of the largest technical barriers in the process of promoting and implementing electric arc cladding technology, and a solution to this problem has been greatly studied for a long time. There are many causes for the crack of cladding layer and also there are many factors to affect. To solve this problem, a detailed analysis was conducted in this paper for concluding the causes from the following several aspects:

First, crack occurred if the stress produced in the cladding heating and cooling process was higher than the ultimate strength of the corresponding material, while the stress sourced from thermal stress and phase transformation stress. At present, most of the materials for cladding are with high strength, hardness, and brittleness in terms of the mechanical industry. However, these materials are greatly different from each other in performance, properties, and basic components, mainly because their linear expansion coefficients are greatly different, and the solid phase transformation tendency and amplitude vary largely in the crystallization process and also the solid-state phase transformations are out of sync. Thus, crack occurred if the produced stress was higher than the scheduled strength, and then the location and direction of the crack were controlled by the direction of the stress and the anisotropy of materials.

Second, the mismatch between the internal properties and technologies of cladding material and base material was one of the important causes of the cladding layer’s crack. In terms of the current Fe-based self-melting alloy, Ni-based self-melting alloy, and Co-based self-melting alloy, the second two had almost no changes of solid-state phase transformation in the crystallization process of the cladding layer. If the hardening ability of the base material matching with the cladding material was getting very strong and also the base material had a martensite phase transformation, the tensile stress produced in the cladding layer increased accordingly, so that the cracking tendency was increased. Instead, the hardening tendency was very high in terms of Fe-based self-melting alloy and would often exceed that of the corresponding base materials, and also martensite point was high; advancing phase transformation and base material occurrence would produce a tensile stress in the cladding layer. Moreover, the tensile stress in the cladding layer was increased because the heating temperatures of the cladding layer and the substrate surface were different, and also the cooling crystallization was in a precedence order. Thus, the cracking tendency increased.

Third, the columnar crystal growing along the direction of heat flow at the direction of heat flow made the performance of the cladding layer featured with a significant anisotropy when the cladding layer was crystallized, and also impurities gathered at the interface between the adjacent columnar grains and also made an area of weak strength produced, so that the cracking tendency increased. The cracking tendency of the cladding layer was greater if the columnar crystal was more advanced.

Fourth, the crystallization velocity of the cladding layer was very fast, and the crystallization was in a precedence order and sometimes got discontinuous; the crystallized area inside of the cladding layer would produce a stress and make the first cladding part crack because of repeated heating, and the causes of the crack were more complex. Experience has indicated that the main reason for the occurrence of crack is “after the first cladding layer was fused and covered, and the next cladding layer would cover a part of the first layer’s surface based on a certain overlap rate so that the crystallization process of this cladding layer was equivalent to being implemented on two kinds of base materials, and also the two materials were not in the same plane, making more complex forms of stress and strain produced within the lap joint area and crack more likely to occur”. At the same time, there were some columnar crystals growing along the heat flow direction on the interface combining with base materials as for a part of the first cladding layer being covered, while a new combination interface was formed at the lap joint after the next cladding lap joint was connected, and then there were columnar crystals growing along the heat flow direction near this interface; the two areas were superposed or linked so that a weak area to the microstructure easy to suffer crack emerged, and also the cracking tendency here increased.

From the experimental result in figure 1, it was seen that the microstructure of the cladding layer was refined to some extent after rare earth elements were added into the cladding material powder, and also the generation range of the columnar crystals was reduced; the micro-hardness of the cladding layer was accordingly improved, and the wear resistance of the cladding layer increased nearly once than that of the layer without rare earth element. This result showed that rare earth elements entered the cladding layer in the cladding process and also played an effect on the crystallization process of the cladding layer, so that the strong tenacity of the cladding layer was simultaneously improved. The mechanism of rare earth elements to improve the structure and performance of alloys has been not clear yet now. According to the experimental result of analyzing the cladding layer’s composition from points and lines using electron probe, the uniform distribution of rare earth element Re was seen in the cladding layer. It was deduced that adding Re made the nucleation rate of the cladding layer increased in the crystallization process, and the columnar crystals growing along the heat flow direction at the combined interface of the cladding layer and the base materials were shown: the refinement of the columnar crystals made the strong tenacity of the cladding layer increased and the brittleness reduced, so that the cracking tendency was lessened.

Conclusion

The columnar crystals growing along the heat flow direction at the combined interface of the cladding layer and the base materials were limited by adding rare earth element

The cracking tendency of the cladding layer was greater when the columnar crystals growing along the heat flow direction near the interface was more advanced, so measures were necessarily taken for limiting the crystals. The measures for limiting the growth of the columnar crystals included increasing the cooling rate and improving the composition: the atomic diffusion and migration in the crystallization process were restricted if the cooling velocity was very high, making too cold composition eliminated, the growth conditions of the branch crystals damaged, and the columnar crystals insignificant to grow. In the meantime, nucleation emerged inside the cladding layer because super-cooling was very big, and then the nucleation rate was greatly increased so that the crystals were refined and grew into the isometric crystals. For a certain work-piece, its cooling speed of the cladding layer was difficult to reach this level, and therefore the growth conditions of the columnar crystals were eliminated by improving the composition of the cladding layer. In this aspect, the study of “add rare earth elements for increasing the nucleation rate of the cladding layer’s crystallization” was mainly conducted.

The effect of rare earth elements on the microstructure and performance of the cladding layer

The role of rare earth elements in the cladding process has been reported in China and foreign countries, but all of them were studied in the preset cladding, while the studies of the role of rare earth elements in the cladding layer’s microstructure and performance in the sending-powder cladding process have been not reported yet. Now, the role of rare earth elements in the alloy crystallization process has been understood in depth, and it is thought that rare earth elements are able to improve the strong tenacity of materials and the nucleation rate in the alloy crystallization process. In the meantime, they can make the grain boundary purified and the strong tenacity of the interface improved. Therefore, it is hopeful to reduce the cracking tendency of the cladding layer and improve the strong tenacity of the cladding layer by adding rare earth elements. The technological processes of sending-powder cladding and preset cladding are greatly different from each other: it is easy to add rare earth elements in the process of preset cladding, in which rare earth elements are burned to lose in spite of the very high heating temperature, but a part of them will be always preserved in the cladding layer. As for sending-powder cladding, cladding materials are heated to a very high temperature in the air so that rare earth elements are easily burned to lose. Therefore, if a part of rare earth elements can be preserved in the cladding layer is always a problem focused by people.

Experiment showed that the microstructure of the cladding layer was refined to some extent after rare earth elements were added into the cladding material powder, and also the generation range of the columnar crystals was reduced; the micro-hardness of the cladding layer was accordingly improved, and the wear resistance of the cladding layer increased nearly once than that of the layer without rare earth element. As a result, the strong tenacity of the cladding layer was improved and the brittleness was lessened, making the cracking tendency effectively reduced.

References

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[3] WU Zijian, et al. Thermal Spraying Technology and Application. Beijing: Mechanical Industry Publishing House, 2005.