Precipitation hardening (PH) stainless steels processed using additive manufacturing (AM) display variations in heat treatment responses and mechanical properties. These variations have been primarily attributed to changes in interstitial alloying element compositions within the allowable ranges, which then influence the stability of the primary phases. The standards for composition and heat treatments are predicated on the wrought condition thus making it difficult to predict the heat treatment response of AM PH stainless steel alloys. With the introduction of AM processing of these alloys, nontypical composition variations of both interstitial and primary alloying elements combined with microstructures that differ from wrought, impact the thermodynamic stability of important phases such as copper, austenite, delta-ferrite, nitrides, and oxides. This work aims to connect compositional changes in interstitial alloying elements like nitrogen as well as other primary elements such as chromium, nickel, copper, manganese, and silicon in AM PH stainless steels to carbonitride development and aging response after different solution heat-treatments. A predictable solution heat-treatment response can be established by investigating key aspects of solutionizing such as the thermodynamic stability of delta-ferrite and copper as well as the influence high temperature phases like carbonitrides have on development of other phases and final properties.
Computational thermodynamic calculations using the CALculation of PHAse Diagrams (CALPHAD) method were used to predict the equilibrium mass fraction of each phase as a function of temperature for materials of various compositions within the allowable ranges. This tool enables direct evaluation of bulk thermodynamic stability of both primary and secondary phases. The variations in elemental compositions were used as input for these calculations within the Thermo-Calc software package with the TCFE10 database. Samples were then created from various alloy compositions to connect experimental observations of changes in microstructure and mechanical properties with changes in solutionizing to the computational results. These samples, produced using power bed fusion, enable an evaluation of how various compositions react to changes in solutionizing time and temperature to help develop predictable solution heat-treatments dependent on compositional variations.
To connect changes in composition with changes in solutionizing response, X-Ray Diffraction (XRD) and Microhardness were used in combination with evaluation of microstructures using electron microscopy techniques. The XRD was done using PANalytical X’Pert Pro MPD with an Empyrean Co radiation source (λ=1.78899 Å) at 40kV and 40mA. A 2ϴ range between 48° and 126° was used with a fixed slit size. A Qness Q60 A+ automated microhardness indenter was used to measure Vickers (HV 0.3) microhardness. To provide structural information on phases observed in TEM as well as to detect any phases that may not have been captured in the TEM, high energy X-ray diffraction (HEXRD) was conducted using beam line 11-ID-B at the Advanced Photon Source (APS) of Argonne National Laboratory. The beam and data collection parameters include a beam energy of 58.59 keV, a wavelength of 0.2113 Å, a 2θ range from 1.0° to 11.2° (q-spacing from 2 Å-1 to 6.5 Å-1), a step size of 0.004°, and a beam diameter of 0.3mm.
Solution heat treatments are used to uniformly disperse the precipitating elements and establish a uniform microstructure. To adequately spread the elements, the solution heat treatment must be at a high enough temperature to enable dissolution of secondary phases, such that one phase is present within the microstructure. The single-phase region should indicate adequate solution heat treatment temperatures. Elimination of the delta-ferrite phase in nitrogen atomized material allows for a larger range of solutionizing temperatures. Furthermore, shifts in the delta-ferrite dissolution temperature, as well as shifts in the copper formation temperature can heavily influence the necessary solutionizing conditions.
As well as evaluating the impact of solutionizing temperature with respect to phase stability, the impact of solutionizing time on the uniformity of the elemental distribution and the microstructure was also investigated. Solutionizing time can significantly impact martensite morphology as well as possibly impacting the retained austenite fraction or morphology. Carbonitrides also tend to form at these higher temperatures, so it is possible longer solutionizing times can heavily influence nitride formation and growth thus manipulating the concentration of elements like nitrogen, niobium, and chromium in the alloy matrix. The impact of nitride formation could help explain large changes in hardness observed with increased solutionizing time. The argon atomized material does not change hardness significantly with changes in solutionizing time, but the nitrogen atomized material had a large increase in hardness after solutionizing for one hour which could be due to the formation of the aforementioned carbonitrides after ample time for nucleation and growth. Since aging behavior is heavily dependent on the preceding elemental distribution as well as the microstructure, control of these factors through solutionizing is critical to sufficiently age these alloys.
Solution heat-treatments are often overlooked in favor of investigations of the as-deposited or aged conditions. Microstructures preceding the aging heat-treatments have a large impact on the ability to form precipitates as well as develop the desired martensitic microstructure. By evaluating the impact of composition on solutionizing AM PH 17-4 PH stainless steel both computationally and experimentally, predictable solution heat-treatment responses can be developed to more readily facilitate tailoring of properties through aging of these alloys. Investigations of the role of solutionizing time, temperature and alloy composition have led to several conclusions including:
Solutionizing temperature is heavily influenced by the formation temperature of the copper phase as well as the dissolution temperature for delta-ferrite.
Solutionizing time yields a large variation in microhardness for high nitrogen material, but not for argon atomized material which is most likely linked to the development of high temperature nitrides and oxides.
Manipulating the microstructure using various solution heat-treatment conditions enables a more predictable aging response which enables more control over the final mechanical properties.