Application of nickel plating by galvanization on steel surface of brazed cemented carbide-maraging steel joints

Abstract

Recently, the integration of a maraging steel heat treatment into a vacuum brazing has been conducted successfully to manufacture high-strength and sound cemented carbide-steel joints with elevated mechanical properties of the steel component. Besides the use of low-melting silver-based active filler alloys and cost-intensive palladium-containing filler alloys, the application of a nickel layer on the maraging steel surface by an arc-PVD process is an adequate approach to overcome the low wettability of those steels, which is caused by high fractions of elements with a high oxygen affinity like titanium and molybdenum. Though nickel plating by arc-PVD is an elaborate and time-consuming coating process, it is not suitable for industrial applications since a high vacuum is required in the specimen chamber and only a small number of specimens can be processed at the same time.Therefore, this publication evaluates the application of nickel galvanization by chemical plating and electroplating as an alternative nickel-coating method to manufacture a brazed joint between a cemented carbide and maraging steel (1.2709) component by using the copper filler metal Cu 110 (TM = 1085 °C). The electroplated and PVD-coated reference specimens featured a sound joint from a minimum nickel layer thickness of 7.0 µm with a similar microstructure consisting mainly of a copper-based fillet and a nickel-rich phase band at the maraging steel-fillet interface. The chemical-plated specimens showed excessive diffusion between the joining partners due to the presence of melting point depressant phosphor between 10.5 and 12.0 wt.-% in the applied characteristic nickel-phosphorus layer. Consequently, titanium migration occurred from the maraging steel surface to the cemented carbide-fillet interface, and columnar iron-cobalt phases formed originating from the cemented carbide into the copper-rich fillet. Except for the specimens coated with no nickel and a 2.5-µm PVD layer, all brazed joints featured a shear strength of at least 150.0 MPa. The maximum shear strength of 344.8 MPa was achieved by electroplating the maraging steel joining surface with a 20.0-µm-thick nickel layer. Moreover, the steel heat treatment was carried out successfully since an elevated and homogenous hardness of at least 648 HV1 was measured in all steel specimens after brazing.