A method for evaluating the crack resistance and predicting the preheating temperature of high hardness coating prepared by laser cladding
Introduction
As a type of surface strengthening and repair technology, laser cladding can improve the wear and corrosion resistance of the material surface [1]. However, the higher hardness of the coating, the worse toughness it is, resulting in that cracking is one of the main problems for high hardness coatings prepared by laser cladding [2]. Therefore, evaluating crack resistance is vital in the industrial cladding or repair field.
Traditionally, the crack resistance of a material is related to its elongation and toughness. Therefore, the tensile test, bending test, and impact test are usually performed to assess the crack resistance of materials [3], [4], [5]. Although these methods are effective, there exists enormous difficulty to be carried out on the coating due to the limitation of coating size. Generally, the thickness of the high hardness coating is no more than 1 mm [6], [7], so it is not convenient to obtain the test sample with full coating. Thus, the conventional tests are not appropriate for evaluating the crack resistance of high hardness coatings. To date, many novel tests have been proposed to overcome the above problems, such as miniatured tensile test or micro pillar compression test [8], [9]. Ahmadi et al. [10] successfully gained the strength and elongation of ZnAlMg coatings by miniatured tensile test and addressed the crack resistance as well as deformation mechanisms. Although the novel tests can overcome the limitation of coating size, they are prohibitive due to the requirement for special equipment [8], [11]. Besides, the results of the method are affected by the microstructure uniformity and thickness of the material. Tan et al. [12] carried out the micro pillar compression test on different microstructures in high-strength steel, and found that the FCC-rich region had better ductility than the BCC region. Kumar et al. [13] investigated the effect of thickness of miniature tensile specimens on mechanical properties. The average elongation of 0.15 mm thick specimens was only 50% of 0.4 mm thick ones. Therefore, this method is mainly used to study the fracture mechanism and is hardly used to evaluate the mechanical properties of materials in engineering. At present, there is no particular test to evaluate the crack resistance of the coating. It is an urgent problem to select a suitable method for evaluating the crack resistance of high hardness coatings.
There are special methods to evaluate the crack resistance of welded joints in the welding process. Among them, the self-restraint test is the most simple and effective during the arc welding process (ISO 17642-2: 2005, MOD). Lee et al. [14] used the self-restraint test to examine the crack resistance of weld metal at different preheating temperatures. The results suggested that the weld metal would not produce cold cracks when the preheating temperature was over 100 °C. Tawengi et al. [15] explored the risk of cold cracking for high strength low alloyed (HSLA) steel NIONIKRAL 70 during the welding by self-restraint test. The results demonstrated a probability of cold cracks in the welding process of NIONIKRAL 70. Generally, the welding cold crack is concerned with the restraint stress of the welded joint so that the self-restraint test displays high restraint at the root and near the weld. The stress concentration factor at the root is about 4.7 [16], which is too high to accurately evaluate the crack resistance of high hardness coating prepared by laser cladding [17]. Hence, it is necessary to broaden the traditional self-restraint test. On the other hand, it is well known that the restraint strength is affected by the size of the weld specimen, such as the depth and width of the groove [18]. Therefore, the restraint stress of the self-restraint test could be changed by adjusting the size of the groove to establish the crack resistance evaluation test for laser cladding.
In this work, a crack resistance test for high hardness coating prepared by laser cladding was designed by improving the self-restraint tests of the welding. The model between the crack resistance and strength plastic product had been built. The method was used to select the preheating temperature of the cladding process, proving the feasibility of this method in practical engineering applications.
Section snippets
Materials
Four kinds of high hardness and crack sensitivity Fe-based coatings, T1 (Fe-Cr-Mo), T2 (Fe-Cr-Ni), T3 (Fe-Cr-Mo-Ni), and T4 (Fe-Cr-Si-B), were prepared by laser cladding. These powders exhibited good sphericity, and the size was 53–150 μm. The substrates were 42CrMo steel plates with dimensions of 80 mm × 80 mm × 50 mm.
Laser cladding process
The cladding setup consisted of a Laserline LDF-8000 fiber laser. The spot was circular with a diameter of 5 mm, and the energy distribution in the spot was top-hat distribution.
Crack resistance test
Fig. 2 indicates the macro morphologies of the multilayer coating. It can be seen that all coatings were metallurgically bonded to the substrate, and no cracks were found after 5 layers were deposited. The results demonstrate that all the coatings presented excellent crack resistance in the cladding process of less than 5 layers, but the crack resistance of the coatings could not be compared. However, during the laser cladding process with 10 layers, there were still no cracks in T1, T2, and T3
Conclusion
In the present work, the crack resistance of different coatings was successfully evaluated by a proposed method, and the test was used to select the appropriate preheating temperature to avoid the appearance of cracks in high hardness coatings. Besides, the relationship between the product of strength and plasticity and the crack resistance of the coating was established. The conclusions are as follows:
- (1)
A crack resistance test method for high hardness coating prepared by laser cladding was
CRediT authorship contribution statement
Yulei Feng: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft. Xiaotong Pang: Investigation, Data curation. Kai Feng: Writing – review & editing, Project administration. Yueqiao Feng: Resources, Investigation, Writing – review & editing. Zhuguo Li: Validation, Funding acquisition, Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key R&D Program of China (No.2018YFB0407300).
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