Morphology Of Lath Martensite Research Articles

Abnormal martensitic transformation and morphology induced by chemical heterogeneity in additively manufactured maraging steel

We report on the effect of chemical heterogeneity on martensitic transformation in a maraging steel produced through laser-based directed energy deposition. Unlike conventional maraging steels with lath martensitic microstructures, the maraging steel exhibited an abnormal morphology, which was refined and elongated in the building direction. The growth direction of the abnormal lath martensite did not occur along the habit plane, while the lath martensite morphology corresponded to the cellular dendritic microstructure of the maraging steel. Among the elements partitioned to the interdendritic region, Ni played a major role in the reduction of the martensitic transformation start temperature (Ms temperature), thereby interfering with the martensitic transformation across the interdendritic region. The interdendritic region acted as a new domain boundary of martensitic transformation, where incomplete martensitic transformation in the region resulted in the retention of austenite.

Microstructure and creep rupture strength of 2 kinds of P91 steels with abnormally low hardness

P91 samples were prepared by means of 2 kinds of heat treatments which involved low normalizing cooling rate &tempering temperature exceeding Ac1 followed by low cooling rate respectively. The correlation between low hardness, microstructure and creep rupture strength was discussed. Results showed that there was a clear microstructure disparity between samples enduring different heat treatments, even though low hardness emerged in both samples. The sample undergoing low normalizing cooling rate featured ferrite structure with coarse grains and continuously distributed precipitates at grain boundaries after normalizing, and intermittently distributed precipitates at grain boundaries after tempering. The sample witnessing tempering temperature exceeding Ac1 followed by low cooling rate represented precipitates inside grains and the morphology of lath martensite in the majority of areas, whereas other areas were characterized by fine blocks of ferrite. The creep rupture strength of the sample experiencing low normalizing cooling rate was higher than the one experiencing tempering temperature exceeding Ac1 followed by low cooling rate, although hardness of the former was lower than that of the latter. For P91 steel, hardness and creep rupture strength did not show a positive correlation, because the grain size and distribution of precipitate also mattered.

Analysis on microstructure and properties evolution and life prediction of P92 steel in high temperature service

P92 steel (10Cr9MoW2VNbBN) is widely used in high-temperature steam piping and high-temperature header fabrications of ultra-supercritical units. In this paper, a follow-up study on the microstructure, mechanical properties and life of P92 steel steam pipes which are virgin, 31,000 h in service, 49,000 h in service and 71,000 h in service was carried out. The results show that the mechanical properties of the P92 steel drop significantly after 31,000 h of service, then decrease slowly with the increase of the service time. When the service time reaches 71,000 h, the high temperature yield strength, percentage elongation, and impact absorbed energy are all close to the lower limit regulated by the ASME standard. Moreover, the morphology of lath martensite becomes fragmentary, the width of the martensitic lath increases and carbide particles cluster and grow at the grain boundary. M23C6 phase coarsens and local Laves phase precipitates. Meanwhile, the dislocation density decreases to some degree. Furthermore, there is a good corresponding relationship between the room temperature hardness and the high temperature tensile strength of the P92 steel after a long period of service. The prediction model of the remaining endurance life of P92 components is proposed on the basis of the room temperature hardness.

Tempering of deformed and as-quenched martensite in structural steel

Annealing of deformed martensite (high-temperature tempering) in St37 steel was studied. Different reductions in thickness were considered and compared with the behavior of as-quenched martensite during tempering. tempering of the asquenched martensite was accompanied by the formation of carbide particles, incomplete disappearance of the lath martensite morphology, and continuous decrease in hardness until reaching low values. However, during tempering of the cold rolled martensite, the precipitation of carbides in the lamellar structure, development of distinct equiaxed ultrafine grains through a continuous recrystallization mechanism, and a sudden hardness drop were characterized. The importance of cold rolling reduction and its amount were also discussed.

Magnetic and Kinetical Properties of Fe-27%Ni-4%Mn and Fe-27%Ni-4%Mn-2%Zn Alloys Investigated by VSM and DSC

We report on the structural, thermal, and magnetic properties of thermally induced martensite in Fe-27%Ni-4%Mn and Fe-27%Ni-4%Mn-2%Zn alloys. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and vibrating sample magnetometry (VSM) techniques were employed in the present research. SEM observations revealed the lath martensite morphology formation in both Fe-27%Ni-4%Mn and Fe-27%Ni-4%Mn-2%Zn alloys. As well, the thermally induced martensite start temperature (Ms) was determined to be − 27.18 °C for Fe-27%Ni-4%Mn and − 40.63 °C for Fe-27%Ni-4%Mn-2%Zn alloy from DSC measurements. Further, the Curie temperature was detected at 160 K for Fe-27%Ni-4%Mn and 196 K for Fe-27%Ni-4%Mn-2%Zn alloys after VSM measurements where magnetic saturation values were measured at 20 emu g− 1 for Fe-27%Ni-4%Mn alloy and 18 emu g− 1 for Fe-27%Ni-4%Mn-2%Zn alloy, respectively. The coercivity value of the alloys was also found to be 0.082 kOe for Fe-27%Ni-4%Mn and 0.050 kOe for Fe-27%Ni-4%Mn-2%Zn alloys. Finally, the effect of Zn addition to the austenite–martensite phase tarnsformations of Fe-27%Ni-4%Mn was discussed in detail according to structural, thermal, and magnetic properties of both alloys after a quantitative experimental comparison.

The effect of alloying element Zn on magnetic properties of martensite phase in FeNiMn alloy

The effect of alloying element Zn on the characteristics of thermal induced martensite and magnetic properties of martensite phase were investigated with scanning electron microscopy (SEM) and Mössbauer spectroscopy characterization techniques. SEM observations revealed that the athermal martensitic transformation occurred with lath martensite morphology for different amount of Zn element. On the other hand Mössbauer spectroscopy showed that magnetic characteristic of martensite phase remarkably altered by increasing the amount of Zn element. The increased amount of Zn also causes the disappearance of the magnetization of the martensite phase.

ラスマルテンサイトの階層組織におよぼす固溶元素の影響

ラスマルテンサイトの階層組織におよぼす固溶元素の影響

Effects of carbon content on the formation of nano/ultrafine grained low-carbon steel treated by martensite process

Martensite process consists of simple cold rolling and annealing of martensite structure. This process can produce a nano/ultrafine grained structure in carbon steels. The key to the process is to start from martensite structure. Carbon can hardly affect the morphology of lath martensite. In this study, the effects of carbon content on required rolling reduction and formation mechanisms of nano/ultrafine ferrite grains are investigated. For this purpose, two low-carbon steels containing 0.04 and 0.08 wt% carbon are processed. Rolling reduction ranging from 40 to 80% is used. Specimens are annealed at various temperatures from 450 to 700 °C. After annealing at 500 °C, ultrafine ferrite grains are obtained in the 80% cold-rolled specimen containing 0.08 wt% C. The formation of these ultrafine ferrite grains is interpreted here in terms of the relationship between the size of the laths and the blocks of the lath martensite and required rolling reduction for dividing them into sub-micron scale sections. The required rolling reduction increases with decreasing the carbon content of steel.

Aspects of thermal martensite in a FeNiMnCo alloy

Thermal martensite characteristics in Fe–29%Ni–2%Mn–2%Co alloy were investigated with scanning electron microscopy (SEM) and Mössbauer spectroscopy characterization techniques. SEM observations obviously revealed the lath martensite morphology in the prior austenite phase of examined alloy. As well, the martensitic transformation kinetics was found to be as athermal type. On the other hand, Mössbauer spectroscopy offered the paramagnetic austenite phase and ferromagnetic martensite phase with their volume fractions. Also, the internal magnetic field of the martensite was measured as 32.9 T from the Mössbauer spectrometer.

Effect of Cooling Rate on Morphology and Crystallography of Lath Martensite in Fe-Ni Alloys

The development of blocks and subblocks in the lath martensite was observed with SEM/EBSD and TEM/Kikuchi diffraction patterns analyses. Morphology of lath martensite formed below Ms temperature was observed using step quenching method in 18 mass% Ni maraging steel. The development of lath martensite structure can be described as follow; lath groups with single K-S variant start to form at the austenite grain boundary. Next, other sub-blocks appear adjacent to the lath groups formed first and these lath groups form macroscopic blocks observed in optical microscopy. The morphologies of the lath martensite in Fe -23 mass% Ni alloy cooled at different cooling rates after austenitization were also observed. The packet size and block thickness decreases with increasing cooling rate, although the sub-block thickness do not change. A packet locally contains small packets with different relationship of close packed planes, and the amount of small packets increases with increasing the cooling rate.

Study of cast microalloyed steels

Improved grades of cast steels were prepared by microalloying C–Mn–Cr steels (0.15–0.22C, wt-%) with V (0.1)+Ti (0.01) or V (0.11)+Nb (0.034)+Ti (0.01) combinations. When quenched and tempered an ultimate tensile strength (UTS) of the order of 784–1078 MPa (80–110 kg mm2) and an elongation ranging from 10 to 35% were achieved. However, the YS/UTS ratio was usually of the order of 0.9, which indicates that these steels did not undergo enough plastic deformation before failure under tensile stress. Examination by TEM of the as quenched as well as the quenched and tempered samples yielded the following information. All the as quenched steels had lath martensite morphology with retained austenite entrapped in the interlath region. After water quenching from 950°C an appreciable volume fraction of undissolved precipitates remained in the steels. Retained austenite broke down during tempering at 400°C and above producing a chain of rod like carbides. During tempering partial recrystallisation of the laths into polygonal ferrite occurred. Fine precipitates were deposited on the dislocation substructure.

Influence of hot-deformation on morphology and effective grain size of lath martensite in 18Ni maraging steel

The changes in the morphology of lath martensite with hot-deformation of austenite were studied by using an 18Ni maraging steel. In the case of conventional quenched (undeformed) or completely recrystallized specimens, an austenite grain contains several packets and each packet is almost completely partitioned by parallel blocks. However, this morphology of packets and blocks was markedly changed when martensite was formed from hot-deformed (not recrystallized) austenite. The packet size increases but the block width decreases with increasing amount of deformation. The relationship between packet size (or block width) of martensite and the grain size of the prior austenite was discussed in detail. The effective grain size of martensite with lath morphology which decides the mechanical proporties of the steel has been discussed on the basis of the Hall-Petch formula.

Fe-Ni系ラスマルテンサイト組織におよぼす冷却速度の影響

Fe-Ni系ラスマルテンサイト組織におよぼす冷却速度の影響

Experiments on directional martensitic transformation in steel

This paper describes some preliminary experiments on the feasibility of producing steels with an aligned martensitic microstructure. The parent austenite is initially textured, following which the martensite habit plane “activity” is regulated by transforming under strain. An Fe-20 pct Ni-5 pct Mn alloy exhibiting a lowM s temperature and a lath mar-tensite morphology was found suitable for producing aligned martensite. On heavy defor-mation this alloy developed a typical (123) [412] fcc texture. Subsequent annealing then produced a strong (001) [100] cube texture. Nearly 80 pct of the martensite formed was aligned when such a cube-textured specimen was strained 2.5 pet at 45 deg to the rolling direction followed by cooling under strain in liquid nitrogen to produce martensite.