No 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.

Cemented carbides are commonly brazed to transformation hardening tool steels without taking a proper and adequate steel heat treatment into account. This publication shows the limits and possibilities of integrating a steel heat treatment, including a quenching process, into a vacuum brazing process. Therefore, copper-based filler metals are selected to ensure the steel component’s high and homogenous hardness and supply a high joint quality. In this context, the aimed steel hardness was chosen in the range between 400 and 440 HV1 based on industrial experiences. This specific hardness range for the steel component was set to avoid wear of machining tools in subsequent machining steps if the steel hardness is too high and to prevent wear and deformation of the tool itself in case of a steel hardness too low. When using the transformation hardening tool steel 1.2344, the obtained shear strength values did not exceed a threshold of 20 MPa which can be attributed to the required N2-quenching from brazing respectively solution annealing temperature. However, the steel components featured a hardness of 527.1 HV1 for the specimens brazed with pure copper at 1100 °C and 494.0 HV1 for those brazed with a CuGeNi filler metal at 1040 °C. This publication also shows an alternative route to manufacture long-lasting tools with a cemented carbide/steel joint by applying the difficult to wet and not well researched, but for many other reasons very suitable precipitation hardening maraging steel. Especially, the comparable low coefficient of thermal expansion (CTE) and the capability of the lath martensite to compensate large amounts of externally imposed stresses during the austenite-to-martensite transformation as well as the cooling rate independent of the hardening mechanism of the maraging steel and a pre-applied nickel coating including the corresponding diffusion processes are responsible for a sound joint with a shear strength > 300 MPa. Moreover, the subsequent tempering process at 580 °C for 3 h provides the maraging steel joining partner with a hardness of 426.6 ± 6.0 HV1.