Study by Differential Thermal Analysis of Reverse Spinodal Transformation in 15-5 PH Alloy.

Abstract

Alloy 15-5 PH is a stainless steel with 15 wt.% Cr and 5 wt.% Ni that is precipitation hardened by addition of Cu. In its semi-finished state, this alloy consists in Cu-supersaturated soft martensite; its high specific properties come from a final tempering consisting in a heating to 550-600°C, holding for 4 hours, and then air cooling. This treatment leads to nanometric Cu precipitation that hardens the material and to transformation of some martensite to reverted austenite which is then stable and provides ductility. While a' embrittlement of such steels is known to occur at temperature in the range 450-520°C, it has been reported that they can be sensitive to the same phenomenon after long term ageing at temperature as low as 300°C, with a significant loss of ductility and an increase of the ductile-to-brittle transition temperature. Atom probe studies showed that this degradation is related to demixtion of martensite into Fe-rich and Cr-rich phases. Depending on the ageing temperature, demixtion can proceed through a nucleation and growth precipitation or by spinodal decomposition of the martensitic matrix. The present study reports differential thermal analyses (DTA) performed upon heating samples of material held at various temperatures (290-525°C) for various times (410 h to 8500 h) that have been characterized by atom probe. A clear DTA signal is obtained upon the reverse spinodal transformation that is further found to depend on ageing conditions.

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

“…In this process, the matrix Cu‐content changes only marginally. In contrast to the explanation for peak 2 commonly reported in the literature, attributing the exothermal reaction peak to the growth of Cu precipitates, we propose that the compositional changes of the precipitates, which occurs around 500 °C (Figure ), are the main source of the signal . The thermo‐kinetic simulation in part II of this paper, showing the enthalpy evolution of matrix and precipitates separately as well as the overall system enthalpy, supports this conclusion.…”

“…This reaction has previously been assumed to originate from Cu cluster formation by some authors . The second, more pronounced reaction peak around 500 °C (peak 2), has been related to the growth of bcc Cu precipitates . Between these two main reaction peaks, an exothermal shoulder is visible.…”

“…A comparable peak around 200–400 °C has been attributed to Cu cluster formation in several studies . In other investigations, this peak during aging of a 15–5 PH specimen is visible, but the authors did not evaluate and describe it . This is possibly due to the fact that no rerun was carried out in these experiments, and, therefore, no suitable baselines were determined for analysis of the low‐temperature reaction.…”

“…The first one, in the range of 300 °C, has been attributed to Cu cluster formation followed by a second more pronounced peak around 500 °C, which has been related to the growth of (or transformation to) bcc Cu precipitates . Other studies only discuss the second reaction as the formation of bcc Cu precipitates, even though the first peak is clearly visible in their results …”

A comparative study of Fe-Cr unmixing using differential scanning calorimetry and small-angle scattering

“…While the isothermal unmixing kinetics is so slow that enthalpy changes cannot be measured in situ, this is not the case of the reversion of a phase separated microstructure being heated back to a homogeneous solid solution, since this reversion will take place at a temperature higher than that of the unmixing, where the atomic mobility is larger. Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA) has been occasionally used to assess the degree of phase separation in Fe-Cr based alloys [33,34]. Being a bulk technique, with easy data analysis, such experiments are promising candidates for establishing a routine analysis tool overcoming most of the drawbacks presented above.…”