Complex bulk-surface hardening of materials using a highly concentrated heat source

Operational properties of tool steels after combined volume-and-surface strengthening

“…After plasma strengthening the hardness of the steel increases by a factor of 3.5-4 (regime 1 in Table 2). The impact toughness decreases simultaneously (due to reduction of a n ), as does Kl~ [3,9]. The zone of plasma hardening has a highly disperse martensite-austenite-carbide structure (Fig.…”

“…We have established in earlier dynamic tests [3,9,10] of specimens with a strengthened surface layer that the development of a trunk crack is hampered in the transition from the hardened layer into the initial metal because its trajectory bends. This is explained by the elevated ductility o f the initial metal and a stepwise reversal o f the sign o f residual stresses in the boundary zone from compressive to tensile, which first causes the crack to stop and then, under a dynamic load, leads to the appearance of secondary microcracks in the plane per-pendicular to the fracture surface.…”

“…The regimes of strengthening chosen with allowance for the previous studies and recommendations of [1,9] are presented in Table 1.…”

“…The hardened zone o f the specimens breaks under a dynamic load by the mechanism o f brittle quasi-spalling (Fig. 3) typical of tool steels with a structure of rapid quenching [3,9].…”

“…Plates strengthened by the given regimes were used for preparing specimens 10 • 10 • 55 mm in size for tests for dynamic crack resistance; the strengthened zone was positioned normally to one of the side faces; the notch was induced by the method of [9,10]. The dynamic fracture toughness K1a~ and the components of the impact toughness (the work of nucleation a n and the work of crack propagation ap) were calculated from the oscillograms obtained in impact tests in the "force-time" coordinates in accordance with the recommendations of [10,11].…”