Research On The Preparation Of Medium Thickness Hard Coatings

Enduring high temperature, thermal cycling and complicated loading, metallurgy rolls mainly fail in several ways such as breaking, thermal fatigue cracking, and being abraded. Since the quality of roll has direct influences on the rolling product’s quality and the production efficiency, improving the roll quality and increasing its serving life have an important significance.So far, engineers and researchers have paid much effort to increase the service life of rolls through improving the roll material and strengthening the roll surface. In this paper, a study of the medium thickness hard coating which prepared by high frequency induction cladding and electric contact strengthening on the surface of the42CrMo sample has been carried out. An appropriate binder and an effective protective layer were selected, then the pre-coating was prepared by free spraying. Through the induction melting of pre-coating, a special rule of high frequency induction melting was summarized. And a new experimental platform has been built. In order to prepare a strengthen layer of good quality, the composition and content of alloy powder were designed in this paper. And the microstructure, micro-hardness of induction cladding layer and electric contact strengthen layer were studied to analyze the strengthen mechanism.Some significant results have been achieved in this paper.Firstly, using the acetone solution of cellulose acetate as the binder, the uniform pre-coating was prepared by free spraying layer by layer. With graphite and asbestos cloth covering, the pre-coating was melted by high frequency induction heating. Then it was strengthened by the electric contact strengthening. So the medium thickness hard coating was prepared.Secondly, the experiment results showed that the quality of cladding layer was controlled by the composition of pre-coating. It is showed that WC reinforced Ni-based powder (Ni60-WC) is fit for the preparation of cladding layer. Compared with self-fluxing alloyed powder, metal based ceramic powder has no welding and poor wettability, so it is not suitable for the preparation of induction cladding layer. Fe-based eutectic alloy powders (Fe-Cr-C-H3BO3, Fe-Cr-WC-C-H3BO3) were prepared by mechanical milling. But Fe-based eutectic induction layer has poor formability, poor surface quality and a lot of defects such as half-baked regions, holes.Thirdly, the Ni60-WC cladding layer with good surface quality was prepared by free spraying high frequency induction sintering. Then the microstructure was studied by OM, SEM, EDS and XRD. The performance of cladding layer was studied by micro-hardness tester. Without apparent porosity and slag defects, the microstructure of even thickness cladding layer is dense. And cladding layer which consists of WC、W2C、Cr23C6、Cr2B、Ni3Fe、Cr7C3is metallurgical bonded to the substrate. Performance test shows that cladding layer has high red-hardness, and the average micro-hardness is about650HV0.2. According to the discussion, the reasonable thickness of the cladding layer is less than0.4mm, otherwise microstructure defects which include half-baked regions, holes and crackles will produced in the cladding layer. But the solutions of these defects were presented in the paper.Fourthly, the WC reinforced Ni-based composite cladding layer was strengthened by electric contact strengthening. With the half-baked region re-welded and the holes reduced, the strengthened layer grows denser. And the micro-hardness of strengthened layer was improved obviously compared to induction cladding layer.