Study links process defects to creep failure in PBF-LB 316L stainless steel
Researchers from Università Politecnica Delle Marche, Ancona, Italy, have published a study in the Journal of Materials Research investigating the short-term creep behaviour of AISI 316L stainless steel produced by Laser Beam Powder Bed Fusion (PBF-LB) and subsequently solution-annealed at 1,050°C for two hours.
Dog-bone creep specimens were produced on a Print Sharp 250 Additive Manufacturing machine from Prima Additive, now AltForm, using commercial 316L stainless steel powder supplied by m4p. The specimens had a 25 mm gauge length and a 3 x 3 mm square cross-section.
The PBF-LB process used a laser power of 140 W, scan speed of 800 mm/s, layer thickness of 30 µm and hatch distance of 0.1 mm. The build plate was preheated to 60°C, with processing carried out under an inert nitrogen atmosphere. The researchers reported that these were the standard optimised process parameters provided by the machine manufacturer.
Following production, one set of samples was annealed at 1,050°C for two hours in air and then water quenched. These specimens were subjected to constant-load and variable-load creep testing in air at 600°C and 650°C. Applied nominal stresses ranged from 200-354 MPa at 600°C and from 120-270 MPa at 650°C.
The study examined the dependence of minimum creep rate on applied stress, as well as the mechanisms responsible for rupture. The researchers found that the dependence of minimum creep rate on stress and temperature was comparable to that observed in conventionally wrought material.
Annealing was found to increase both the minimum creep rate and total strain to rupture compared with as-built PBF-LB 316L. However, for a given minimum creep rate, the time to rupture of the annealed PBF-LB material was approximately half that of wrought steel.
Microstructural analysis showed that the annealed material consisted of equiaxed austenitic grains with a mean linear intercept of approximately 44 µm. Grain size was assessed using the Heyn linear intercept method in accordance with the ASTM E112 approach.
The researchers also quantified residual process-related porosity in polished, unetched cross-sections prior to creep testing. Nine optical micrographs covering a total area of 9.50 mm² were analysed using ImageJ to determine pore area fraction, pore number density and equivalent circular diameter. Although the overall level of residual porosity was very low, occasional larger isolated defects were identified.
Optical microscopy and scanning electron microscopy were used to characterise the microstructure and fracture behaviour, while hardness measurements were taken both before and after creep exposure to assess changes associated with deformation and thermal exposure.
Fractures were predominantly intergranular. The researchers attributed the premature damage to creep damage nucleating from pre-existing process-related cavities, with the presence of larger isolated defects supporting the proposed role of such cavities in the early development of intergranular creep damage.
The findings indicate that strict control of process-related defects is essential if annealed PBF-LB 316L stainless steel is to achieve a creep response comparable to conventionally wrought material.
The full open-access paper is available here.
