Research On Microstructure Evolution Of CF170 Ultra-high Strength Stainless Steel During Aging

CF170 ultra-high strength stainless steel is a 1700 MPa-grade Co-free maraging stainless steel,featuring excellent mechanical and corrosion properties.To fulfill the demands of lightweight design,CF170 steel had been chosen as the heavy-load gear material for a space manipulator,marking the first instance of such selection in China.Due to the prolonged nitriding time,the material was in an over-aged state during service,exhibiting significant differences from the previous peak-aged condition.Consequently,this paper systematically investigated the microstructure evolution during the under-aging stage,particularly focusing on the over-aging stage and its impact on properties.The research elucidated the precipitation behavior of the second phase and its impact on the strength through the adjustment of aging temperature parameters.The results showed that during the aging process at 450℃,the coherentηphase precipitated in the Ni/Ti/Al element-enriched region formed at the dislocation sites through an inhomogeneous nucleation mechanism,resulting in a rapid increase in tensile strength and a dramatic decrease in plasticity.When the aging temperature increased to 500℃,the interface between theηphase and the matrix transformed from coherency to semi-coherency,leading to the tensile strength reaching its peak and an improvement in plasticity.In the range of 500 to 600℃,theηphase continuously grew and coarsened,resulting in a consistent decrease in strength and a significant enhancement in plasticity.Employing a short-duration aging treatment（≤1 h）,the nucleation mechanism of reverted austenite in CF170 steel was revealed,and a phenomenon of reverted austenite forming aroundη-Ni₃Ti was discovered.The results indicated that the nucleation of austenite at the interface was part of theγ→α→γreversion process,controlled by a shear mechanism.When the aging temperature reached 593℃,non-intrinsic stacking faults formed inside theηphase.Fe diffused into theηphase through a pipe diffusion mechanism,resulting in the formation of Fe-containingγ’-Ni₃Ti.The relationship betweenγ’and martensite satisfied K-S orientation,and reverted austenite formed at theγ’/matrix interface.Austenite formed in both ways exhibited the"austenite memory"effect.The precipitation evolution behavior ofη-Ni₃Ti within the temperature range of300 to 600℃was as follows:formation of Ni/Ti/Al element-enriched region→coherent precipitation ofηphase（spherical）→growth ofηphase（spherical+rod-shaped）→coarsening ofηphase（rod-shaped）→dissolution ofηphase（transforming into sphericalγ’-Ni₃Ti）.The fitting results of non-isothermal experiments indicated that the activation energy E of the austenite reverse transformation was 255 k J·mol^-1,and the Avrami index n was 1.04608.By combining the formula of the pre-exponential factor K₀ and the maximum volume fraction of austenite f_max derived from isothermal experiments,a kinetic model for the reverse transformation of austenite during aging processes at 482–593℃/0–100 h was established.This model enabled the prediction and control of the microstructure and hardness of CF170 steel.