Martensite Packet Research Articles

Electron microscopic study of the structure of steel U8A after LTMT

As the result of LTMT at 300° a ferrite — carbide mixture is formed in steel U8A that increases the dislocation density, reduces the size of martensite packets and crystals of acicular internally twinned martensite, and increases the volume percentage of packet martensite.

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.

The relation between microstructure and yield strength in tempered low-carbon lath martensite with 5% nickel

AbstractThe tensile yield strength of quenched and tempered low-carbon (< 0.10%) lath martensite with 5% nickel is analysed with respect to grain size and substructural parameters and at the same time effects from an addition of 0.3% molybdenum are investigated. Microstructures are studied in detail by transmission electron microscopy. The initial martensite packets control the high-angle boundary grain structure after normal tempering, thus contributing to the yield strength through an ordinary Petch-Hall relation for the packet size. The Petch-Hall coefficient for packets seems to decrease on tempering if extensive recovery and subgrain formation occur. Subgrain boundaries and dislocations within subgrains contribute substantially to the yield strength and account for the high strength of this type of martensitic steel. Softening on tempering is mainly a result of recovery through which the original lath structure is destroyed and the initially high dislocation density is reduced. The addition of 0.3%Mo...

Orientation relationships between individual bundles in FeCrNi alloy with austenite and ferrite phases

Orientation relationships between adjacent bundles within a packet martensite have been investigated in the FeCrNi alloy with austenite and ferrite. All bundles has a twin relationship at −50°C, while there occurs a deviation from the twin relationship by 0,5,9,15,19,29 and 35 degrees at −196°C, −150°C and −102°. At −196°C, −150°C and −102°C the percentage of approximately twin-related bundles, which are 0 and 5 degrees away from the twin relationship, is 70 %. The fact that the fracture elongation is in the maximum at −50°C may be due to the twin-related bundles. The presence of such bundles is substantiated by the twin-operation on the stereographic projection figure and twin-related variants expected from the Nishiyama variants.

The non-cubic lattice of rapidly quenched packet martensite

The positions of 200, 020 and 002 peaks of ferrous martensite have been measured individually by a precision X-ray diffractometer technique. Martensite with 18 wt pct Ni and 0.1, 0.2, 0.3, and 0.4 wt pct C was formed by salt-water quenching from preoriented austenite single-crystal slices about 0.5 mm thick. The variations in X-ray peak positions interpreted as changes in lattice parameter yield to the first approximation:Δa/a 0 = −0.005 [wt pct C],Δb/a 0 = −0.016 [wt pct C],Δc/a 0 = −0.037 [wt pct C] for carbon contents between 0.1 and 0.4. For such carbon contents the martensite has packet or mixed packet-lenticular morphology; consequently packet martensite is not cubic as sometimes claimed. Instead its lattice is nearly like that of lenticular martensite with the"a " and"c " parameters varying with carbon content as previously observed for tetragonal martensite, and the" b " parameter unequal to the" a" . The significance of the inequality of the" a " and" b " parameters is unclear, but that inequality appears to be characteristic of some plate and lenticular martensite.

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.

Packet microstructure in Fe-0.2 pct C martensite

The arrangement, size, and boundary area of the laths that make up a packet of martensite in a coarse-grained Fe-0.2 pct C alloy were studied by replica and thin foil electron microscopy. Frequently laths of two habit plane variants coupled to a single {111} A plane of the parent austenite are observed in a packet. The width distribution of the laths is log normal, with the most frequently observed lath width being 0.15 microns. Larger laths between 1 and 2 microns are distributed throughout a packet. The total lath boundary area per unit volume of martensite obtained by analysis of micrographs taken from thin foils is quite high, 65,000 cm-1, and analysis of packet structure by selected area diffraction and precision dark field techniques show that there may be five times as much low angle boundary area as high angle boundary area in a packet.

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.

Austenite ferromagnetism and martensite morphology

The Curie temperature of the austenite, the martensite-start temperature, and martensite morphology have been determined in a series of nil-carbon Fe−Ni and Fe−Ni−Co alloys. For these alloys, austenite ferromagnetism aboveMs is a necessary, but not sufficient, condition for the formation of lenticular rather than packet martensite. In contrast to Fe−Ni alloys where lenticular martensite only forms below ≈O°C, some of the Fe−Ni−Co alloys transform to this structure at temperatures up to ≈200°C. The results support the hypothesis that the resistance of austenite to plastic deformation affects the habit plane and thus morphology of the martensite which forms.