Early Stages of Cu Precipitation in 15-5 PH Maraging Steel Revisited − Part I: Experimental Analysis

Abstract

Even though Cu precipitation during isothermal aging of binary Fe-Cu alloys and of Cu-alloyed maraging steel has been carefully studied in the past, no detailed investigations of the early stages of Cu clustering upon continuous aging have been carried out so far. During continuous aging with a heating rate around 15 K min À1 of as-quenched 15-5 PH maraging steel in a differential scanning calorimeter (DSC), two exothermal reactions are observed, one at approximately 300 8C and the other one around 500 8C. These DSC signals are attributed to the nucleation and growth of Fe-rich Cu clusters in the lower temperature range, followed by Cu-enrichment of these clusters and seamless transformation into bcc-Cu precipitates at higher temperatures. Part I of this investigation focuses on the experimental characterization of the mechanisms governing these reactions in a comprehensive manner, where the authors first present the results of this analysis based on dilatometry, differential scanning calorimetry, hardness testing, and 3D atom probe tomography. The latter method is capable of detecting early clusters, which are not yet accessible by transmission electron microscopy. In the companion paper, part II, thermo-kinetic simulations are presented, which support the interpretation of the observed reactions and substantially aid in understanding the underlying mechanisms.

Early Stages of Cu Precipitation in 15-5 PH Maraging Steel Revisited - Part II: Thermokinetic Simulation

“…In part I of this work, the first exothermic DSC peak has been attributed to the nucleation and growth processes of Cu‐rich clusters forming in the supersaturated Fe‐rich matrix. Experimentally, a slight increase in the segregation parameter between 150 and 300 °C has been observed, which would indicate some initial phase separation process taking place in this temperature range. However, no clear and distinct interpretation could be derived purely from the experimental results, since the expected and modeled cluster sizes are too close to the detection limit of the atom probe tomography (APT) device.…”

“…In the present work, we study the early stages of precipitation responsible for the two exothermic DSC reactions by thermokinetic simulation. To support the findings obtained in part I, and to aid in interpretation of the experimental results, we apply computational simulation of the full‐process heat treatments depicting the DSC experiments from solution annealing over quenching to annealing with a constant heating rate. The results comprise of an analysis of thermodynamic properties, such as Gibbs energy of phases, driving force for precipitation, as well as molar enthalpy of phases and thermokinetic properties, such as phase fraction, radius, or number density of the precipitate phase.…”

“…The experimental analysis consists of DSC heating experiments, atom probe tomography, as well as mechanical testing, and it is discussed in detail in part I of this work . Only parts of it are briefly recapitulated in this section as far as they are related to the DSC measurements relevant for our thermokinetic analysis.…”

“…The heat treatment used in the simulation reproduces the experimental DSC cycle applied in part I, consisting of solution annealing at 1050 °C for 30 min and a quench to room temperature leading to a fully martensitic microstructure. The martensite start temperature shows a weak dependency from the quenching rate and a representative value of 150 °C is used in the present work. In the actual DSC run, the specimens are heated from room temperature to 600 °C with a heating rate of 15 K min −1 .…”

“…The 15‐5 PH steel investigated in the present work is a maraging steel with low carbon content and Cu as major precipitate‐forming element. The experimental analysis with continuous heating DSC, dilatometry, hardness testing, and atom probe tomography has been topic of part I of this paper . The relevant literature on PH steels is outlined there and, for this reason, not duplicated in this second part.…”