Influence of combined electromechanical processing modes of 40Kh steel on its structure and hardness

Abstract

The paper considers the effect of combined electromechanical processing in three different modes on the structure and hardness of the surface layers of 40Kh steel, which was in a normalized state (the original structure). The modes differ from each other by the different applied load and the number of pulses. The applied load in modes 1 and 2 (current strength 39 kA, pulse time 0.02 s, number of pulses 1) is 100 and 250 MPa, respectively. A distinctive feature of mode 3 compared to mode 2 is a greater number of pulses (two). Metallographically it was established that in all three cases a hardened surface layer of different thickness (from 300 to 1200 μm) with a hardness of 593 – 598 HV is formed, consisting of two zones (a surface zone with a structure of fine-needle martensite; a transition zone smoothly transitioning into the initial ferrite structure). The transition zone (treatment according to mode 1 ) in its structure contains martensite and ferrite. The transition zone (mode 2 processing) consists of a Widemannstett structure. A more substantial surface heating zone according to this mode (700 μm) in comparison with the processing according to mode 1 (300 μm) in combination with intensive heat removal contributed to the formation of a Widmanstett structure, which is defective and unacceptable for operation. The transition zone with the processing according to mode 3 has the structure of martensite and ferrite. The formation of a defective Widmanstett structure in the transition zone does not occur, since 2 times more pulses are used during processing than in mode 2 . This contributes to the heating of the surface layer to a greater depth (1200 μm), and, consequently, the structure formation in the transition zone occurs from the intercritical interval Ag 3 – Ag 1 .