Institute of Fundamental Technological Research

In this paper, an assessment of a topography measurement method for fracture surfaces of 10HNAP steel after bending-torsion fatigue tests was performed. Surface roughness was measured by using a non-contact Focus Variation Microscopy (FVM) technique in which the non-measured points (NMPs) and outliers (spikes) were removed by the application of general methods. The results revealed, that the optical measurement method introduced variations in the high-frequency errors, considered as noise within the selected bandwidth. Therefore, the minimization of the high-frequency noise (HFN) was proposed based on an extensive examination of ISO 25178 roughness parameters. Additionally, a general S-filter was applied, as recommended by international standards and commercial software. It was used to identify and remove noise from the measured data after pre-processing. Consequently, levelling and eliminating of NMPs and spikes was successfully performed. Subsequently, the results obtained by using various filters were compared to further assess the impact of different filtration bandwidths. Finally, the proposed procedure was validated by implementing different general functions, such as autocorrelation (ACF), power spectral densities (PSD), and texture direction (TD). It was concluded, that coupled characteristics, including profile and areal measurements, should be studied simultaneously since they are necessary to analyze the fracture surfaces comprehensively.

bending-torsion fatigue ,surface topography ,roughness ,non-measured points ,outliers ,high-frequency noise,measurement errors

In this paper, the entire fracture surface approach was used to assess an effect of 280,000 h of exploitation under internal pressure of 2.9 MPa and high temperature of 540 °C on the fatigue response of 10H2M (10CrMo9–10) power engineering steel. The mechanical testing was carried out on the hourglass specimens produced from the as-received, unused pipeline and the same material after long-time exploitation. The uniaxial tensile tests were performed to establish the stress amplitude for subsequent force controlled, fatigue testing in the range from ±250 MPa to ±400 MPa under the frequency of 20 Hz. Since the exploited 10H2M steel was characterized by significantly lower mechanical properties, different damage mechanisms responsible for specimen failure were revealed through fracture surface analysis. The fracture surface topography evaluation was performed with a 3D non-contact measuring system. It was found, that the exploitation state has a strong impact on the fatigue life and fracture characteristics since the significant drop in lifespan of about 300 %–400 % was found for the material after prolonged service. Finally, the proposed surface topography parameter was related to the stress amplitude in order to estimate the fatigue life for the steel in question.

fatigue , 10H2M steel , Mechanical properties , Surface topography

Background

Metal Laser Powder Bed Fusion Melting (LPBF-M) is considered economically viable and environmentally sustainable because of the possibility of reusing the residual powder feedstock leftover in the build chamber after a part build is completed. There is however limited information on the fatigue damage development of LPBF-M samples made from reused feedstock.
Objective

In this paper, the stainless steel 316 L (SS316L) powder feedstock was examined and characterised after 25 reuses, following which the fatigue damage development of material samples made from the reused powder was assessed.
Methods

The suitability of the powder to LPBF-M technology was evaluated by microstructural observations and measurements of Hall flow, apparent and tapped density as well as Carr’s Index and Hausner ratio. LPBF-M bar samples in three build orientations (Z – vertical, XY – horizontal, ZX – 45° from the build plate) were built for fatigue testing. They were then subjected to fatigue testing under load control using full tension and compression cyclic loading and stress asymmetry coefficient equal to -1 in the range of stress amplitude from ± 300 MPa to ± 500 MPa.
Results

Samples made from reused powder (25 times) in the LPBF-M process exhibited similar fatigue performance to fresh unused powder although a lower ductility for vertical samples was observed during tensile testing. Printing in horizontal (XY) and diagonal (ZX) directions, with reused powder, improved the service life of the SS316L alloy in comparison to the vertical (Z).
Conclusions

Over the 25 reuses of the powder feedstock there was no measurable difference in the flowability between the fresh (Hall Flow: 21.4 s/50 g) and reused powder (Hall Flow: 20.6 s/50 g). This confirms a uniform and stable powder feeding process during LPBF-M for both fresh and reused powder. The analysis of fatigue damage parameter, D, concluded cyclic plasticity and ratcheting to be the main mechanism of damage.

SS316L ,Stainless steel,Fatigue ,Additive manufacturing,Laser Powder Bed Fusion Melting (LPBF-M)