Date

September 1979

Document Type

Thesis

Degree Name

Ph.D.

Department

Dept. of Materials Science

Institution

Oregon Graduate Center

Abstract

The effects of notch root radii on the toughness of quenched and tempered 4340 steel were studied in greater details by instrumented and slow-bend Charpy tests. Also studied was the effect of tempering after both high and conventional austenitizing temperatures by fracture toughness, slow-bend Charpy and instrumented Charpy tests. The effects of bainitic isothermal transformations from high temperature austenitization of 4340 steel were studied by fracture toughness and tensile tests. Microstructural investigation was carried out by optical, transmission and scanning electron microscopy as well as X-ray diffraction. The effects of notch root radii on the toughness results showed that initially the toughness increased as the notch-root radius increased and then after a critical notch-root radius was reached, the toughness dropped. The loss in toughness was coincident with an intergranular fracture initiation mode. Also, the critical root radius at which the drop in toughness was noticed was strongly temperature and strain rate dependent but independent of the prior austenitic grain size. The critical stress or strain model applies for the initial rise of the toughness with notch root radius. Beyond the critical notch root radius, fracture criteria based on 'strain energy density' is consistent with the observed drop in toughness. A physical model involving double slip band for the intergranular fracture initiation mode is also discussed. A new explanation for the differences in toughness behavior between the sharp crack and blunt notch behavior for the two heat-treatments is provided. ε-carbide precipitation in as-quenched martensite from high temperature austenitization is proposed to improve the toughness in 'sharp crack' testing. In blunt notch testing, the importance of grain size is stressed. Here, larger grain size resulted in inferior toughness. Finally the results indicated that the limited bainitic heat-treatments from high austenitizing temperature studied in this investigation were not conducive to either toughness or tensile strength. The impairment of mechanical properties is attributed to large bainitic ferrite grain size. The tempering behavior from high austenitizing temperature showed that up to a tempering temperature of about 175°C, toughness increased continuously; however, toughness dropped at a tempering temperature as low as 225°C for the high austenitizing case, whereas no such drop was observed in that tempering temperature range for the low austenitizing temperature case. SEM, TEM and X-ray studies have indicated that the loss in toughness was associated with a change in fracture mode, i.e. from microvoid coalescence to intergranular fracture, and was also coincident with cementite precipitation. It is believed that the prior austenite grain boundary segregation coupled with cementite precipitation triggered the above embrittlement.

Identifier

doi:10.6083/M4SB43P4

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