Contact Interaction Peculiarities at the Boundary of Layers of Structural Steel–High-Speed Steel Hot-Forged Powder Bimetal

The main problem in the production of bimetals (BMs) is the necessity to ensure adhesive interaction at the contact boundary of the layers, preventing their delamination during the operation. Hot forging of porous preforms (HFPP) offers the possibility of fabricating high-density powder BMs with a minimal amount of pores both in the bulk of the layer material and at the boundary between the layers, which promotes an increase in the adhesion strength. When fabricating hot-forged powder BMs, it is possible to mix materials of charges of a working layer and a substrate, which can lead to uncontrollable interface “spread.” In this work, to fabricate porous BM performs of the structural steel–high-speed steel type, the previously proposed method foreseeing the preliminary prepressing of the powder of the hard-to-deform material is used. In order to determine mechanical properties and perform a structural analysis, bilayer cylindrical samples 20 mm in diameter and 30 mm in length are prepared. The BM base material is steel PK40 and that of the working layer is atomised high-speed steel M2 powder with satisfactory compressibility characteristics. Porous preforms of BM samples are pressed in a specially designed mold using a hydraulic press, enabling one to perform two-side pressing of bilayer powder moldings with the specified distribution of density and strength of layers. Cold-pressed BM preforms were sintered in a protective atmosphere and then subjected to hot repressing using a laboratory drop hammer. Some preforms are examined as sintered. In addition, hot repressing of cold-pressed green preforms is performed. The satisfactory process strength of the working layer material is observed at its porosity of 34% < P_wl < 45%. The powder is not molded at P_wl > 45% and delaminates at P_wl < 34%. It is established that the maximal layer bonding strength and BM thermal-shock resistance is provided by the application of a flowsheet foreseeing the preliminary sintering of cold-pressed preforms and subsequent hot forging. The optimal repressing pressure of the working layer is 145 MPa.