THE LIMITATION OF MATANSITIC INVARIANT HABIT PLANE AND MARTENSITE MORPHOLOGY

The strain tensor and displacement vector associated with lattice deformation were introduced to discuss the effect of self-accommodation and plastic accommodation in martensitic transformation,and the models for {557}_f and {3,10,15}_f martensite fomation were proposed, along with the experimental verification. It is considered that the invariant habit plane may result from the self-accommodation between different martensitic variants causing the internal twin structure. However, as the strength of parent phase is low enough for the plastic accommodation to occur the dislocation substructure in martensite be produced, in which case the invariant habit plane is difficult to form so the{225}_f and {557}_f habit planes are difficult to explain by the phenomenological crystallographic theory. So,there are some limitations for the martensitic invariant habit plane.In the present work the authors also suggest that the martensite morphology depends on the competition between self-accommodation and plastic accommodation including factors not only of strength but also of structure of the parent phase However, when the density of crystal defects is high dislocations rather than twinned martensite will form in spite of the higher strength of the parent phase.

Regular Pattern of Morphology Transition in Mixed Martensite Structures

The morphology and substructure of mixed martensites in ferrous alloys have been examined by using optical and transmission electron microscope. The results indicated that the main formation se- quence of martensitic morphology was butterfly→ plate→lath,with decreasing forming temperatures when the plastic accommodation takes place in the parent phase,which is affected by the transforma- tion strain fiélds.It was shown that the martensite morphology is not only decided by the forming temperature alone,but also by the dislocation struc- ture in austenite before the transformation.

NUCLEATION AND GROWTH OF FERROUS MARTENSITES ALONG BOUNDARIES

Martensites distributed along the austenite grain boundaries and twin boundaries have been examined in Fe-C,Fe-Ni-C and Fe-Cr-Mn-Mo-C alloys.The martensites may nucleate preferentially and grow easily along these boundaries.In the mixed martensites,the preferentially formed one is plate or butterfly martensite

The Acoustic Emission Effects of Martensite Transformation in Alloys Cr12MoV and Fe-33Ni-4Ti-10Co

1 Introduction The method of acoustic emission (AE)as a new technique to study the processesin metals and alloys in the solid phase hasbeen widely used and developed since thelast 20 years. Martensite transformationis diffusionless. Because of changing fromone crystal structure to another by coopera-tive shear process the interfacial coherency

Effect of Martensite Morphology on Impact Toughness of Ultra-High Strength 25CrMo48V Steel Seamless Tube Quenched at Different Temperatures

A 25CrMo48V steel for ultra-deep oil/gas well casings was quenched at 900-1 200 ℃ and tempered at 650 ℃. The lath martensitic structures were characterized by optical microscope （OM）, field emission scanning electron microscopy （FESEM）, electron backscattering diffraction （EBSD） and transmission electron microscopy （TEM）, and the transverse impact energy at 0 ℃ was measured from the as-quenched and tempered specimens. The results show that with the quenching temperature decreased, the prior austenite grain, martensitic packet and block are refined, while the lath width seems to remain unchanged. The enhancement of impact toughness with the decreasing quenching temperature can be attributed to refinement of the martensitic structure with high-angle boundaries, and the block is the minimum structure unit controlling impact toughness. The transverse impact energy [ECVN （0 ℃） ≥100 J] required for seamless casings with ultra-high strength （Rp0.2≥932 MPa） has been finally achieved with the experimental steel quenched at 900-1 000 ℃ and tempered at 650 ℃.

Experimental and simulated characterization of creep behavior of P92 steel with prior cyclic loading damage

The effect of prior cyclic loading on creep behavior of P92 steel was investigated. Creep tests on prior cyclic loading exposure specimens were performed at 650？C and 130 MPa. In order to clarify the influence of prior cyclic loading on creep behavior, optical microscope, scanning electron microscope and transmission electron microscope were used. Experimental results indicate that the prior cyclic loading degrades the creep strength significantly. However, the degradation tends to be saturated with further increase in prior cyclic loading. From the view of microstructural evolution, the recovery of martensite laths takes place during prior cyclic loading exposure. This facilitates the dislocation movement during the following creep process. Therefore, premature rupture of creep test occurs. Additionally, saturated behavior of degradation can be attributed to the near completed recovery of martensite laths. Based on the effect of prior cyclic loading, a newly modified Hayhurst creep damage model was proposed to consider the prior cyclic loading damage. The main advantage of the proposed model lies in its ability to directly predict creep behavior with different levels of prior cyclic loading damage. Comparison of the predicted and experimental results shows that the proposed model can give a reasonable prediction for creep behavior of P92 steel with different level of prior cyclic loading damage.