Microstructural control of maraging steel C300

Abstract

The microstructure evolution and precipitation kinetics of maraging steel C300 have been studied in the aging temperature range from 400 to 600°C. The relation between mechanical properties and precipitation hardening response is explained, and modelling is used to optimise the properties. Ultrafine needle shaped Ni3Ti phase is the main strengthening precipitate in maraging C300, and it shows very high resistance to coarsening. A spherically shaped Fe2Mo phase is formed at higher temperatures and in the overaged condition. Inter- and intralath reverted austenite nucleates at higher temperature (∼600°C). Rolling and aging treatment can produce the highest hardness by a combination of work hardening and precipitation strengthening. Microstructural evolution simulation using Monte Carlo modelling has been applied to this alloy, and the modelling has been validated by the experimental results.

Predicting microstructure and strength of maraging steels: Elemental optimisation

“…For a prior-austenite grain size of 20 μm, σ Mart = 450 MPa, whereas for the alloys tested, σ ss values range between 300 and 500 MPa. This gives the initial hardness to be in the range 250-330 H v , being these predictions in agreement with experimental estimations for the hardness in as-quenched conditions [3,20,25,26].…”

“…For NiAl, γ p has been estimated to be 0.02 J m −2 in Fe-Ni-Al-Mo [72]; however, the kinetics of NiAl and variants increase with Mn content [25]; hence, based on the effect of Mn in NiMn and austenite reversion, the factor exp(7x Mn ) is added to the interfacial energy: γ NiAl = 0.02exp(7x Mn ) J m −2 ; For Ni 2 AlMn and Ni 2 AlTi, the interfacial energy is assumed to be γ Ni2AlMn = γ Ni2AlTi = 0.1exp(7x Mn ) J m −2 . The interfacial energy of Ni 3 Ti is estimated to be [20] These results provide descriptions with a direct link between the microstructure evolving during ageing and chemical composition. This will allow us to integrate microstructure-based models for yield stress in different steels.…”

“…However, small variations in d lath have been observed in a variety of maraging steels with different Ni and Mn content, even for tempered conditions [14,20,25,26,39]; this behaviour can be due to the relatively low lattice distortion and high solubility of substitutional atoms in Fe with respect to C atoms; hence, based on these experimental results, it is considered that d lath = 250 nm remains constant for all steels tested. For this case, ρ ≈ 3.6 × 10 14 m −2 , being this prediction consistent with experimental estimations in a stainless maraging steel [40].…”

“…Nevertheless, Schintzer et al [14,19] and Leitner et al [3] did not report carbide formation in PH13 8Mo for the ageing conditions employed in this work. Similarly, Zhu et al [20] did not measure any carbon content in C300. Additionally, no carbide formation was reported in M350 [21], 5Mn [22], Fe12Ni6Mn [15], LeanLAl and LeanLAl [23], and the Mar6-13 grades [7].…”

“…However, they require introducing a number of fitting parameters, thus they are not included in the models to simplify the descriptions. For the non-spherical intermetallics tested (NiMn and Ni 3 Ti), the average aspect ratio (a r ) between their length (l p ) and diameter (2r p ) has been measured to be almost constant: a r for Ni 3 Ti has been found to be ≈4.5 [20], whereas for NiMn a r is approximately 3 [71]. Hence, the relation l p = 2a r r p is adopted.…”