Martensite Packet Size Research Articles

Effects of high temperature austenitization on fracture, fatigue and stress corrosion cracking behavior of cast and forged high strength steels I: Monotonic fracture behavior

Abstract An investigation into the effects of high temperature austenitization (h.t.a.) on the monotonic fracture behavior of cast and forged high strength steels has been carried out. The h.t.a. increased the fracture toughness, the ductility and the impact toughness of the cast steel, whereas the ductility and impact toughness of the forged steel were reduced. Both the critical fracture stress and strain of the cast steel were enhanced by h.t.a., but the fracture strain of the forged steel decreased. Analysis of microstructural differences and fracture micromechanisms revealed that the higher ductility and impact toughness of h.t.a. cast steel than those of h.t.a. forged steel could be related to a smaller austenite grain, finer martensite packet and lath size, and the discontinuous nature of the interlath retained austenite film. The slight improvement in ductility and impact toughness of cast steel after h.t.a. compared with conventional temperature austenitization (c.t.a.) correlates with a reduction in the dendritic and impurity segregation. The fracture toughness of the cast and the forged steels austenitized at different temperatures has been analyzed using stress-controlled and stress-modified strain-controlled fracture models. The actual fracture stress and strain over the range of the fracture process zone are very different from those determined by notch bending or plane strain tensile tests, and the characteristic distances calculated from the above models were found to have no meaningful relation with the microstructural parameters important to fracture initiation. The improved fracture toughness is a result of a higher local fracture strain ahead of the crack tip for strain-controlled fracture and a larger characteristic distance for stress-controlled fracture in both the cast and forged steels.

Influence of nickel on toughness and ductile-brittle transition in low-carbon martensite steels

AbstractThe ductile-cleavage transition has been studied with respect to grain size, yield strength, and nickel content by impact testing of low-carbon < 0.1%C) lath martensite steels with 5%Mn, 5%Ni, and 9%Ni, respectively. The impact transition temperature (ITT) is found to be controlled by the martensite packet size and the yield strength analogously with the general case reported for ferritic steels. Regardless of packet size and yield strength, nickel additions imply powerful intrinsic toughness improvements and may decrease the ITT by as much as 20°C per percent nickel. At the same time, nickel moderated the effect of packet size and yield strength on ITT. These important effects of nickel additions are mainly attributed to a corresponding increase in cleavage fracture resistance and form the basis for the use of Ni steels in cryogenic applications. The effects of grain size, yield strength, and nickel additions on ITT are adequately described by a simple model, based on the assumption that the duct...

旧オーステナイト粒径の異なる18%Niマルエージング鋼の破壊靱性およびストレッチ・ゾーン

The effect of prior austenite (γ) grain or martensite (α') packet size on the plane strain fracture toughness (KIC) and the correlation between the stretched zone width and KIC of an 18% Ni maraging steel were studied by means of fracture toughness testing and fractography.KIC was not affected by γ grain size in the fine (6.7-15.7μ) or coarse (118μ) region, while it increased in the intermediate grain size range from 28.3 to 90.4μ. In the latter condition, the plastic zone spread over about one to tow α' packets ahead of the fatigue pre-crack before the onset of unstable fracture.Stretched zone was observed at the tip of the fatigue pre-crack, and the zone width related linearly to the critical crack opening displacement. The width of the stretched zone was increased by the enhanced relaxation of the stress concentration by the micro-cleavage nucleation after the spreading of the plastic zone over about one to two α' packets.It seemed that the enhanced blunting process of the fatigue crack led to the increment in KIC. On the other hand, KIC was independent of such a structural unit as γ grain or α' packet when the enhanced blunting effect was not introduced.

Design of Fe/4Cr/0.4C martensitic steels eliminating quench cracking

Inter-granular cracking in the as-quenched structures of vacuum melted Fe/4Cr/0.4C steel has been investigated in terms of the heat treatment conditions. It has been shown that amount of carbon in solution and martensite packet size are the two most important factors influencing cracking tendency. Multiple heat-treatments have been designed in order to take advantage of higher austenitizing temperature and fine austenite grain size. Elimination of quench cracking and good mechanical properties were obtained after these treatments.

Effect of transformation substructure on the strength and toughness of Fe−Mn alloys

Phase transformations in Fe−Mn alloys containing up to 9 pct Mn were studied by optical and electron transmission microscopy. Either equiaxed ferrite, massive ferrite, or massive martensite can form on cooling from austenite. The particular type of transformation product formed was found to depend on the alloy content, austenite grain size, and cooling rate. The mechanical properties of all the transformation products were evaluated using tensile and impact testing and are discussed in terms of the observed microstructural features. Yield strength and impact transition temperature were found to be relatively insensitive to manganese content but were strongly influenced by the transformation substructure and grain size of the transformed phase. In martensite it has been shown that the structural unit analogous to grain size in ferrite is the martensite packet size, which in turn is controlled by the prior austenite grain size. The fracture surface of broken impact specimens and the fracture profile were examined by means of electron and optical microscopy techniques. These fractographic observations were correlated with impact test data and microstructural observations of the various transformation products.