Impact Property of Case Hardening Steel Subjected to Combined Heat Treatment with Excess Vacuum Carburizing and Subsequent Induction Hardening

Abstract

Synopsis : Charpy impact value of the case hardening steel subjected to combined heat treatment with excess vacuum carburizing and subsequent induction hardening was evaluated. The purpose of this study is to clarify the effect of retained austenite and cementite on the impact property. The characteristic of combined heat treatment is that the initial microstructure can be designed easily. The initial microstructure is designed by carburizing and annealing at the hyper-eutectoid composition of 1.3 mass% C and subsequent induction heating temperature is chosen between A cm and A 1 to obtain different amounts of retained austenite and cementite. The impact value improves by the induction hardening with increasing heating temperature and the tempering. The steel treated at the low heating temperature shows intra-granular fracture irrespective of the presence of pro-eutectoid cementite. As the heating temperature increases, the formation of C solid solution progresses by the decomposition of cementite and increased retained austenite transforms into the deformation-induced martensite by the impact energy, thereby increases intra-granular strength. Hence critical fracture strength transits to grain boundary strength and showed inter-granular fracture at the interface of cementite and matrix in this study. The impact value showed the correlation with the amounts of retained austenite before the test and the decrement in retained austenite before and after the test. The effect of retained austenite is due to the plastic deformation of austenite, the increase of the compressive residual stress generated by deformation-induced martensite transformation, and the consumption of the impact energy as the driving force for deformation-induced martensite transformation.

Crack Propagation Behavior of Impact Fracture in Case Hardening Steel Subjected to Combined Heat Treatment with Excess Vacuum Carburizing and Subsequent Induction Hardening

“…The specimens used in this experiment were made of the same material and subjected to the same heat treatment as in our previous report. 3) The chemical composition of the case hardening steel used in this study was 0.21 C, 1.12 Si, 0.81 Mn, 0.020 P, 0.017 S, 0.32 Cr, 0.17 Mo [mass%]. 3) It is similar to JIS SCM 420, but Si was added to improve temper softening resistance.…”

“…3) The chemical composition of the case hardening steel used in this study was 0.21 C, 1.12 Si, 0.81 Mn, 0.020 P, 0.017 S, 0.32 Cr, 0.17 Mo [mass%]. 3) It is similar to JIS SCM 420, but Si was added to improve temper softening resistance. The steel bar was normalized at 1 223 K for 7.2 ks and machined into the shape with the notch.…”

“…The heat treatment was performed under the conditions shown in Table 1. 3) A subsequent heat treatment was performed with or without tempering and with or without sub-zero treatment prior to tempering. A vacuum carburizing furnace was used for the carburizing process.…”

“…Although case hardening steel subjected to excess vacuum carburizing exhibits improved wear resistance and rolling contact fatigue strength due to θ, toughness may decrease. In our previous report, 3) the relation between the impact property, the undissolved θ and the retained γ formed by controlling the induction heating temperature of two-phase (γ, θ) for the case hardening steel subjected to this process was investigated. It was shown that the amount of retained γ increased with the induction heating temperature, and that crack initiation and crack propagation energies showed a further increase by tempering.…”

“…Figure 1 shows the definition of impact properties obtained from the load-displacement curve of the instru-mented Charpy impact test. In our previous report, 3) the discussion of the crack propagation behavior focused on the propagation energy (E p ) which is calculated through integration using the maximum load (P max ) as the dividing point in the load-displacement curve. Kobayashi et al 4) studied a method for evaluating brittle fracture using the Charpy impact test method.…”