Evaluation of toughness in AISI 4340 alloy steel austenitized at low and high temperatures

Abstract

It has been reported for as-quenched AISI 4340 s~eel that high temperature austenitizing treatments at 1200°C, instead of conventional heattreatment at 870°C, result in a twofold increase in fracture toughness,. ~Ic'• but a decrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference between Kic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of ' notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of h;i.gh temperature austenitizing b~ questioned as a practical heat-treatment procedure for ultra-high strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely on Kic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required.

Effect of Carbide Size Distribution on the Impact Toughness of Tempered Martensitic Steels with Two Different Prior Austenite Grain Sizes Evaluated by Instrumented Charpy Test

“…The specimens that showed grain size dependence of the impact toughness arranged by Hui 7) were tempered at 673 K, which is not inconsistent with the present results. It is reported by Ritchie et al 8,9) that for the as-quenched and 473 K tempered martensitic steels, the impact Charpy energy decreases by about 78 J as PAG size increases from about 20 to 200 µm. Therefore, it might be difficult to observe the PAG size effect on the impact toughness in the present plain martensitic steels.…”

“…Hui et al 7) investigated the effect of prior austenite grain (PAG) size on impact toughness, where they reported the enhancement in toughness and ductility of fine-grained 1.08Cr0.39C martensitic steel tempered at 673 K. Decrease in the Charpy impact energy in accordance with the increase in PAG size was observed in the as-quenched and 473 K tempered martensitic steel. 8,9) Kawasaki et al 10) used an induction heating method, which makes use of the rapid heating process in grain growth or carbide forming kinetics, 11) in austenitizing and tempering of 0.58%C steel to investigate the mechanical properties in relation to the microstructures, and they reported that the finer microstructure gives rise to enhancement in ductility, impact toughness and so forth. However, in tempered martensitic steels, the grain size effect could be diminished by the carbide precipitation.…”

Statistical analysis of cleavage fracture ahead of sharp cracks and rounded notches

“…For example, in low alloy 4340-type steels, it has been shown that"sharp crack" Krc toughness can be increased as the coarseness of the microstructure is increased by raising the austenitizing temperature, while the "rounded-notch" Charpy values actually decreased (6,7), a result 1 independent of loading rate and fracture mode (S-7). Another example involves wrought and cast steels, for which Charpy V-notch measurements imp 1 i ed an inferior toughness of cast a 11 oys, whereas Krc values remained unchanged (8).…”

Simple and accurate fracture toughness testing methods for pyrolytic carbon/graphite composites used in heart‐valve prostheses

“…Composites made from PyC-coated graphite, however, exhibit slightly higher toughness values, ranging from ϳ1.3 to 1.6 MPa͌m. 5,7 Because the fracture resistance is a critical property of these materials, there is a often a 12 and PyC-coated graphite 5 using saw-cut notches (notch root radii unreported), in place of precracks, yielded erroneously high toughness values of 2.5 and 3.9 MPa͌m, respectively, a common trend seen in most ductile 13 and brittle 14 materials. Due to its high fatigue resistance, however, fatigue precracking is a difficult and costly procedure for PyC and its composites.…”