Developing Creep and Stress Relaxation Models to Assess the Service Life of an Additive Manufactured Industrial-Scale Recuperator Utilizing Inconel 625 and AISI 310S Materials.

This work is focused on the development of creep and stress relaxation models on Inconel 625 and Stainless Steel 310 materials for additive manufacturing. At the end, the operational lifespan of an industrial-scale additive manufactured recuperator is evaluated. An industrial-scale recuperator for burners with a highly complex geometry is manufactured using Continuous Wave SLM and Pulsed Wave Selective Laser Melting techniques. The recuperator operates under steady but high thermal loads, reaching temperatures of up to 875 °C. Therefore, its service life is assessed, considering creep and stress relaxation phenomena. Two different materials are evaluated: Inconel 625 and Stainless Steel 310. Tensile testing has been conducted on samples at various temperatures to acquire material parameters, incorporating appropriately the anisotropic nature of the materials. Creep parameters were determined through creep experiments and data from the literature, and the recuperator response was simulated by FEA modelling. Analytical creep and stress relaxation models were proposed based on the simulation results for each material to predict their creep response. The service life was determined by applying a custom failure criterion based on the creep testing data. The Inconel 625 recuperator exhibits a service life that is significantly higher compared to any burner's life, while the Stainless Steel 310 recuperator exhibits approximately 27 years of service life. Both materials are considered suitable; however, Inconel 625 offers higher resistance to creep according to creep tests, and due to its lower thermal expansion coefficient, the resulting thermal stresses are lower.

Thermodynamic Modelling and Microstructural Study of Z-Phase Formation in a Ta-Alloyed Martensitic Steel.

A thermokinetic computational framework for precipitate transformation simulations in Ta-containing martensitic Z-steels was developed, including Calphad thermodynamics, diffusion mobility data from the literature, and a kinetic parameter setup that considered precipitation sites, interfacial energies and dislocation density evolution. The thermodynamics of Ta-containing subsystems were assessed by atomic solubility data and enthalpies from the literature as well as from the experimental dissolution temperature of Ta-based Z-phase CrTaN obtained from differential scanning calorimetry. Accompanied by a comprehensive transmission electron microscopy analysis of the microstructure, thermokinetic precipitation simulations with a wide-ranging and well-documented set of input parameters were carried out in MatCalc for one sample alloy. A special focus was placed on modelling the transformation of MX into the Z-phase, which was driven by Cr diffusion. The simulation results showed excellent agreement with experimental data in regard to size, number density and chemical composition of the precipitates, showing the usability of the developed thermokinetic simulation framework.