Mechanical Stability of Retained Austenite in Quenched and Tempered AISI 4340 Steel
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specimens loaded uniaxially under plane stress and plane strain conditions.
II. CHERUVU N. SASTRY, KHALID H. KHAN, and WILLIAM E. WOOD I.
INTRODUCTION
In several recent studies, the presence of retained austenite films has been reported on martensite lath boundaries, 1'2 packet boundaries, 2 and prior austenite grain boundaries 3 in low alloy steels, especially when higher than conventional temperatures for austenitization were used. It has been reported ~'4 that the presence of this retained austenite has a beneficial effect on the plane strain fracture toughness (Klc) of ultrahigh strength steels. Two mechanisms have been suggested I as explanations for the beneficial effect of retained austenite on toughness: (a) Localized Trip: the mechanically induced phase transformation of austenite to martensite in the plastic zone absorbs additional energy, thus effectively enhancing the fracture toughness. (b) Crack Tip Branching and Blunting: the advancing crack is diverted and branched when it meets an area of retained austenite, resulting in an increased energy absorption. In this case, the retained austenite within the plastic zone, ahead of the crack front, should be mechanically stable for the crack to branch or blunt. Horn and Ritchie 5 have discussed the significance of the mechanical stability of retained austenite on the fracture toughness and tempered martensite embrittlement of ultrahigh strength steels. They have measured the amount of retained austenite by the magnetic saturation technique in 4340 and 300M steels during uniaxial tensile tests to assess the mechanical stability of retained austenite. They demonstrated that retained austenite in quenched, and quenched and tempered martensitic structures displays very little mechanical stability. However, the material in front of an advancing crack deforms under plane strain conditions during a plane strain fracture toughness (K~c) test. Thus, it is necessary to determine whether or not retained austenite transforms during deformation under plane strain conditions in order to discuss its beneficial effect on the plane strain fracture toughness. The goal of the present investigation was to obtain more exact information using transmission electron microscopy (TEM) about the mechanical stability of retained austenite in: (a) the plastic zone in front of an advancing crack tip in specimens loaded monotonically and cyclically to the load corresponding to K~c, and (b) deformed tensile
The material used in this investigation was aircraft quality AISI 4340 steel with a chemical composition given in Table I. Standard ASTM compact tension specimens and tensile plate specimens with four-inch (10 cm) gage leng~ were machined. These specimens were austenitized at 1200 ~ for one hour in a salt bath, quenched in agitated oil, and tempered at 180 ~ for one hour in a neutral salt bath. Compact tension specimens were tested in accordance with ASTM E399-72. Fatigue precracking was done on a closed loop Instron test system. Fracture toughness testing was carded out at a cross
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