5 Werkstofftechnik
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Creep and creep damage behavior of stainless steel 316L manufactured by laser powder bed fusion
(2022)
This study presents a thorough characterization of the creep properties of austenitic stainless steel 316L produced by laser powder bed fusion (LPBF 316L) contributing to the sparse available data to date. Experimental results (mechanical tests, microscopy, X-ray computed tomography) concerning the creep deformation and damage mechanisms are presented and discussed. The tested LPBF material exhibits a low defect population, which allows for the isolation and improved understanding of the effect of other typical aspects of an LPBF microstructure on the creep behavior. As a benchmark to assess the material properties of the LPBF 316L, a conventionally manufactured variant of 316L was also tested. To characterize the creep properties, hot tensile tests and constant force creep tests at 600 °C and 650 °C are performed. The creep stress exponents of the LPBF material are smaller than that of the conventional variant. The primary and secondary creep stages and the times to rupture of the LPBF material are shorter than the hot rolled 316L. Overall the creep damage is more extensive in the LPBF material. The creep damage of the LPBF material is overall mainly intergranular. It is presumably caused and accelerated by both the appearance of precipitates at the grain boundaries and the unfavorable orientation of the grain boundaries. Neither the melt pool boundaries nor entrapped gas pores show a significant influence on the creep damage mechanism.
The refractory chemically complex alloy (rCCA) AlMo0.5NbTa0.5TiZr, with a density of 7.4 g/cm3, shows a compressive ultimate strength of 772 MPa at 1000 °C, comparatively surpassing Ni-base and other rCCAs. Its dual-phase microstructure, with a high volume fraction (≈ 62%) of cuboidal and plate-like particles coherently embedded in a continuous matrix, resembles the well-known pattern of the γ/γ" in Ni-base superalloys. Its developers have thus implied that it could stand as structural alloy for high temperature (HT) applications. Here, we report the HT creep properties and the underlying microstructural changes of the rCCA AlMo0.5NbTa0.5TiZr to propose deformation and degradation micromecanisms for this regime. The material was produced by arc-melting and subsequently heat treated in argon: at 1400 °C for 24 h plus a hot isostatic pressure treatment at 1370 °C and 170 MPa for 4 h, with a cooling rate of 10 K/min. Miniaturized tensile specimens (≈ 28 x 7 x 2 mm) were cut and polished to a quality of 1 μm. Creep tests were conducted in vacuum in the respective temperature and stress range 800-1200 °C and 30-120 MPa. For observation, thin slices were extracted from the gauge length, away from the fracture surface, grinded to a thickness of 100 μm, and electropolished to electron transparency. The microstructure was observed on the electropolished specimens using scanning (S) as well as transmission (T) electron microscopy (EM). The Norton plot gives Norton exponents of about 3.1 and 3.2 for temperatures of 1000 and 1100 °C, respectively. Curiously, creep rate minima are very close for a stress level of 30. The starting microstructure reflects a macroscopically lean coarse grain structure and a microscopically fine-meshed basketweave structure with coherency dislocations only around coarsened particles usually close to subgrain boundaries. Results are discussed on the base of variations of this starting microstructure after interrupted and ruptured creep tests.
The development of refractory CCAs has been explored for potential use in high temperature applications. An example of this is the AlMo0.5NbTa0.5TiZr alloy, which resembles the well-known γ/γ’ microstructure in Ni-Base superalloys with cuboidal particles embedded in a continuous matrix. The aim of this work is to evaluate the alloy’s mechanical behavior under tension in the temperature range 800-1000°C, by applying creep tests under vacuum (excluding oxidation effects). Some little temperature influence on minimum creep rate @ 1000 and 1100 °C was found and at a first glance, and Norton plots shows that deformation is probably both diffusion and dislocation controlled. However, further work is needed to stablish deformation and degradation micro mechanisms in the studied creep regime.
Creep and fracture behavior of conventionally and additively manufactured stainless steel 316L
(2020)
A critical task within the frame of establishing process-structure-property-performance relationships in additive manufacturing (AM) of metals is producing reliable and well-documented material behavior’s data and knowledge regarding the structure-property correlation, including the role of defects. After all, it represents the basis for developing more targeted process optimizations and more reliable predictions of performance in the future. Within this context, this contribution aims to close the actual gap of limited historical data and knowledge concerning the creep behavior of the widely used austenitic stainless steel 316L, manufactured by Laser-Powder-Bed-Fusion (L-PBF). To address this objective, specimens from conventional hot-rolled and AM material were tested under application-relevant conditions according to existing standards for conventional material, and microstructurally characterized before and after failure. The test specimens were machined from single blocks from the AM material. The blocks were manufactured using a standard scan and build-up strategy and were subsequently heat-treated. The creep behavior is described and comparatively assessed based on the creep lifetime and selected creep curves and characteristic values. The effect of defects and microstructure on the material’s behavior is analyzed based on destructive and non-destructive evaluations on selected specimens. The AM material shows shorter creep lives, reaches the secondary creep stage much faster and at a lower strain, and features lower creep ductility compared to its conventional counterpart. The creep damage behavior of the AM material is more microstructure than defect controlled and is characterized by the formation and accumulation of single intergranular damage along the whole volume. Critical features identified are the grain morphology and the grain-boundary as well as the dislocation’s density. Micro-computed tomography (µCT) proves to be an alternative to metallography to analyze the creep damage.