The effects of microstructure and precipitates on mechanical properties in Fe-Cr-Ni maraging steels have been studied. The crystallographic characteristics of martensitic laths in Fe-Cr-Ni maraging steel have been investigated using electron diffraction patterns. The microstructure of the solution-treated and subsequently quenched maraging steel consists of complete lath martensite with high dislocation density. The martensite laths contain screw dislocations with Burgers vector $a/2 <111>_{a'}$. The twin relationship between some adjacent martensitic laths is observed. It is identified that the twinned laths are transformed from the austenite of two specific variants having Kurdjumov-Sachs(K-S) orientation relationship.
It is also found that the hexagonal $Fe_2W$ Laves phase is formed during solution treatment. The amount of the $Fe_2W$ precipitate increases with decreasing solution treatment temperature below solvus temperature of the $Fe_2W$ Laves phase, and with increasing W content. A new orientation relationship between $Fe_2W$ precipitate and martensitic matrix is identified by using electron diffraction pattern.
The precipitation characteristics and reversion behavior in Fe-10Cr-10Ni maraging steel during aging treatment were investigated. The fine rod-shaped $η-Ni_3Ti$ phase is observed to be precipitated having two specific orientation relationships, named as type I orientation relationship and type II orientation relationship, with martensitic matrix during aging. The reverted austenite phases are observed to be formed, in addition to $η-Ni_3Ti$ precipitates, also having two specific orientation relationships, known as K-S orientation relationship and N orientation relationship, with martensitic matrix during aging above 550℃. Analysing the observed electron diffraction patterns and the computer-simulated electron diffraction patterns, the orientation relationships among the $η-Ni_3Ti$ precipitate, reverted austenite phase and martensitic matrix are unified and two types of unified orientation relationships, named as K-S type and N type, are identified to be co-existed as follows:
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The morphology and crystallographic features of reverted austenite formed during aging of the Fe-10Cr-10Ni maraging steel have been investigated. Reverted austenite is formed having both the K-S orientation relationship and the N orientation relationship with the martensitic matrix. However, the observed orientation relationships are dependent upon the zone axis. A twinned austenite is found to have a specific orientation relationship not only with the martensitic matrix but also between adjacent austenites. Furthermore, the twinned austenite is observed to have a very sharp twin boundary, and the observed longitudinal direction of twinned austenite is dependent upon the beam direction and the orientation relationships between austenite and martensite.
Precipitation behavior and strengthening mechanisms in Fe-10Cr-10Ni maraging steel have been investigated by means of tensile test and transmission electron microscopy. The microstructure of the 0 W and 2 W alloys is composed of lath martensite after solution treatment. While, the $Fe_2W$ Laves phase, besides the lath martensitic matrix, exists in solution-treated 4 W alloy. The strengthening of the Fe-10Cr-10Ni maraging steel is caused by a distribution of the fine $η-Ni_3Ti$ precipitates. High strength of the maraging steel is maintained at higher temperature from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. Loss of strength at higher temperature is associated with overaging and mainly from the larger volume fraction of reverted austenite. The strengthening mechanism by means of $η-Ni_3Ti$ phase in Fe-10Cr-10Ni maraging steel has been analyzed in terms of the cutting mechanism by dislocations. This interpretation is made base on the observation of the microstructures of samples after tensile test.
Low cycle fatigue properties and dislocation structures in solutiontreated and aged 2 W alloy have been investigated. The tensile peak stress during fatigue shows rapid softening followed by saturation in solution-treated specimen. During cyclic saturation, the cell structure is derived from the ladder structure formed at the lath martensite structure. While, the tensile peak stress of the fatigue shows rapid softening followed by continuous softening without saturation in aged specimen. During cyclic softening, the dislocation arrangement is converted to a low-energy dislocation structure by cutting mechanism of the precipitate in aged specimen. In case of solution-treated specimen, good fatigue resistance can be achieved by homogenous slip deformation because of the presence of the cell structures formed during fatigue.
