Potential of maraging steel as a joining partner for cemented carbide

摘要

In industry, tools with cemented carbide/steel joints are often manufactured by brazing processes with a local heat input, such as by induction or laser brazing. Characteristic for these joining processes are the generally undesired microstructural changes of the steel in the vicinity of the brazed region. These undesired microstructural changes can lead to an embrittlement and a softening effect of the used tool steel depending on the heat exposure, inversely proportional to the distance from the brazing seam. Especially when cemented carbide-steel joints are manufactured by a vacuum brazing process, the steel is comprehensively exposed to the occurring process temperatures. Hence, integrating the steel heat treatment into the brazing process is recommended to supply controlled and improved mechanical properties of the steel component while simultaneously guaranteeing a high quality of the brazed joint. In this context, maraging steels are promising joining partner candidates for the manufacture of cemented carbidesteel brazements in terms of an increased joint quality and residual stress reduction. In this context, one advantage of maraging steels is the formation of lath martensite, which is almost independent of the cooling rate from solution annealing temperature. This publication mainly examines the integration of the heat treatment of the maraging steel grade 1.2709 into the vacuum brazing process for cemented carbide-steel joints by taking advantage of this mechanism. In this investigation, joint shear strength values of more than 150 MPa were achieved while supplying an elevated and homogenous hardness of around 420 HV1 in the steel component. The aimed steel hardness of 420 HV1 was selected based on industrial experiences 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 an insufficient low steel hardness. A good combination of joint shear strength and steel hardness was achieved by applying a relatively low cooling rate of 5 K/min from the brazing respectively solution annealing temperature of 815℃. The active filler metal Ag65Cu28Sn5Ti2 was used for the experiments to improve the wetting of the generally difficult to wet maraging steels. Moreover, an additional 5 μm thick nickel coating of the maraging steel joining surface was implemented to further improve the wettability of the maraging steel surface by the molten filler metal. Also, it was observed that the shear strength values of the brazements without the nickel-coated maraging steel joining surface are, on average, 28% higher than those with a nickel coating. Finally, a precise adjustment of the steel hardness of ca. 415 HV1 was realized by applying a dwell time of 10 min at a solution annealing temperature of 815℃ followed by a tempering process at 580℃ for 3 h.