Improving Fatigue Limit and Rendering Defects Harmless through Laser Peening in Additive-Manufactured Maraging Steel

Additive-manufactured metals have a low fatigue limit due to the defects formed during the manufacturing process. Surface defects, in particular, considerably degrade the fatigue limit. In order to expand the application range of additive-manufactured metals, it is necessary to improve the fatigue limit and render the surface defects harmless. This study aims to investigate the effect of laser peening (LP) on the fatigue strength of additive-manufactured maraging steel with crack-like surface defects. Semicircular surface slits with depths of 0.2 and 0.6 mm are introduced on the specimen surface, and plane bending-fatigue tests are performed. On LP application, compressive residual stress is introduced from the specimen surface to a depth of 0.7 mm and the fatigue limit increases by 114%. In a specimen with a 0.2 mm deep slit, LP results in a high-fatigue-limit equivalent to that of a smooth specimen. Therefore, a semicircular slit with a depth of 0.2 mm can be rendered harmless by LP in terms of the fatigue limit. The defect size of a 0.2 mm deep semicircular slit is greater than that of the largest defect induced by additive manufacturing (AM). Thus, the LP process can contribute to improving the reliability of additive-manufactured metals. Compressive residual stress is the dominant factor in improving fatigue strength and rendering surface defects harmless. Moreover, the trend of the defect size that can be rendered harmless, estimated based on fracture mechanics, is consistent with the experimental results.

Effect of pack carburizing with chicken egg shell powder agent and vibrator quenching on the mechanical properties of AISI 9310 steel

The effects of vibrator quenching (QV) on the carbon content, microstructure, and mechanical properties (surface hardness number, wear resistance) in the pack carburizing of AISI 9310 steel were studied. The aim of this research is to increase the surface hardness and improve the wear resistance of AISI 9310 steel. The problem that often occurs in the quenching treatment after pack carburizing is that the thick cooling medium does not evenly wet the surface of the specimen, so that the cooling rate is not uniform, the impact is the distribution of the specimen surface hardness is not the same. Therefore, it is necessary to research the implementation of the vibrator in the quenching treatment. The specimens were treated with pack carburizing at a temperature of 875 °C, soaking time for 3 hours. The carburizing agent consisted of chicken egg shell powder (CESP) and rice husk charcoal (RHC) with various weight ratios of 5 %:95 %, 15 %:85 %, and 30 %:70 %. Followed by quenching treatment using a 10 % cane molasses cooling medium and vibrator. Hardness testing was carried out using a Vickers microhardness tester, wear resistance test using the pin-on-disc method, and a scanning electron microscope (SEM-EDX) was used to observe changes in the microstructure and carbon elemental content on the specimen surface. The results showed that the application of VQ caused the formation of a small martensite microstructure while without VQ it was large martensite and a few of residual ferrite. The highest surface hardness number is 685 kg/mm2, the wear resistance is 0.32 cm/mg for pack carburizing, using carburizing agent 70 % RHC, 30 % CESP and VQ. VQ causes a more even distribution of the thick cane molasses cooling medium so that the cooling rate of the specimens is uniform.

Evaluation of Sulfides as Solid Lubricant: Lubricity of Compounded Sulfides

Sulfide was used as solid lubricant, and MoS2 was popular. It was used industry as powder shape. Sulfur as oil additive; ZDDP, MoTDC were also well known. These composites make tribofilm which prevent seizure, scoring and some tribological troubles. In this paper, sulfides were synthesized by powder metallurgy technics. In addition, these sulfides were mixed with bronze powders and sintered as cylindrical specimen. As a result of the friction test in the lubricated condition, tribofilm were covered with specimen surface. By XPS observation, sulfides and oxides were detected on the specimen surface. In the test, additive was not contained in the lubricant because PAO as base oil was adapted. However, sulfide in the specimen affect the making the tribofilm, especially when bornite (Cu5FeS4) was used.

Influence of Specimen Surface Roughness on Hydrogen Embrittlement Induced in Austenitic Steels during In-Situ Small Punch Testing in High-Pressure Hydrogen Environments

An in-situ small punch (SP) test method has recently been developed as a simple screening technique for evaluating the properties of metallic materials used in high-pressure hydrogen environments. With this method, the test conditions including temperature and gas pressure can easily be adjusted to those used in practice. In this study, specimens of STS316L steel and 18 wt% Mn steel were prepared at two different surface roughness, fabricated using wire-cutting and mechanical polishing. Their effects on hydrogen embrittlement (HE) were evaluated using in-situ SP testing at both room temperature and a lower temperature where HE was shown to occur under 10 MPa hydrogen. Both steels were evaluated using two variables obtained from in-situ SP testing, the SP energy, and the relative reduction of thickness (RRT), to quantitatively determine the effect of specimen surface roughness on HE susceptibility. Their fracture characteristics due to HE under 10 MPa hydrogen showed little difference with surface finish. Surface roughness had a negligible influence on these quantitative factors describing HE, indicating that it is not a dominant factor to be considered in in-situ SP testing when it is used to screen for HE compatibility in steels used in high-pressure hydrogen environments.

Electrochemical Testing for Pitting Corrosion Above Ambient Temperatures Using the Syringe Cell

The syringe cell method has been further developed to evaluate the temperature dependence of pitting corrosion in passive alloys having critical pitting temperature above ambient without artifacts associated with crevice corrosion. The pitting potential of commercially available duplex stainless steel type 2205 was measured at different temperatures by using a hot plate to heat the specimen. Breakdown potentials decreased by about 1 V at test temperatures above 50 °C. The critical pitting temperature (CPT) of the alloy was determined to be between 54 °C and 59 °C by scanning the temperature of the specimen surface during a constant potential hold. In all experiments, pits were observed in the area defined by the electrolyte droplet in contact with the specimen surface. The CPT of the alloy determined using a more conventional approach mentioned in ASTM Standard G48 Method C was 55 °C.

Analysis of fracture deformation field and energy evolution of granite after high confining pressure cyclic load pre-damage

Considering the recent developments of deep mining, investigating the rock properties under high ground stress periodic load is highly demanded. Studies show that these characteristics are important factors affecting the long-term steadiness of rock. However, the mechanical properties of rock mass without macro failure after cyclic load should be studied. In the present study, granite in a mine is considered as the research object. A rock pre-damage experiment is conducted with the same cycles under different confining pressures and constant cycle upper and lower limit loads. The pre-damaged rock sample is subjected to a uniaxial compression test, and a high-speed charge couple device camera is used to record the speckle field image of the sample surface during the whole loading process. The digital speckle techniques are used to analyse the image of the pre-damaged sample, the deformation field of the specimen surface, the displacement dislocation value of the localized deformation area and the deformation energy value of the specimen surface. The results show that for the same cycle times, the confining pressure is less than 80 MPa, which has a weakening effect on the rock's axial strength. As the confining pressure approaches 120 MPa, the pre-damaged rock uniaxial peak strength increases. The characteristics of displacement dislocation energy evolution of the localized deformation bound are divided into three stages (pre-peak stage, peak point and post-peak stage). After pre-damage under the same cycle times and different confining pressure conditions, the deformation field evolution of rock is relatively consistent.

Cool, Dry, Nano-scale DIC Patterning of Delicate, Heterogeneous, Non-planar Specimens by Micro-mist Nebulization

Background: Application of patterns to enable high-resolution Digital Image Correlation (DIC) at the small scale (μm/nm) is known to be very challenging as techniques developed for the macro- and mesoscale, such as spray painting, cannot be scaled down directly. Moreover, existing nano-patterning techniques all rely on harsh processing steps, based on high temperature, chemicals, physical contact, liquids, and/or high vacuum, that can easily damage fragile, small-scale, free-standing and/or hygro-sensitive specimens, such as MEMS or biological samples. Objective: To present a straightforward, inexpensive technique specially designed for nano-patterning highly delicate specimens for high-resolution DIC. Methods: The technique consists in a well-controlled nebulized micro-mist, containing predominantly no more than one nanoparticle per mist droplet. The micro-mist is subsequently dried, resulting in a flow of individual nanoparticles that are deposited on the specimen surface at near-room temperature. By having single nanoparticles falling on the specimen surface, the notoriously challenging task of controlling nanoparticle-nanoparticle and nanoparticle-surface interactions as a result of the complex droplet drying dynamics, e.g., in drop-casting, is circumvented. Results: High-quality patterns are demonstrated for a number of challenging cases of physically and chemically sensitive specimens with nanoparticles from 1 μm down to 50 nm in diameter. It is shown that the pattern can easily be scaled within (and probably beyond) this range, which is of special interest for micromechanical testing using in-situ microscopic imaging techniques, such as high-magnification optical microscopy, optical profilometry, atomic force microscopy, and scanning electron microscopy, etc. Conclusions: Delicate specimens can conveniently be patterned at near-room temperature ($\sim $ ∼ 37 ∘C), without exposure to chemicals, physical contact or vacuum, while the pattern density and speckle size can be easily tuned.

Influence of Light-curing Distances on Microflexural Strength of Two Resin-based Composites

SUMMARY Objective: Our objective was to investigate the influence of different curing distances on microflexural strength and the microflexural modulus of two resin-based composites. Methods: Two nanohybrid composites were used; Filtek Z250 (Z250) and Tetric EvoCeram (TEC). Rectangular specimens were prepared (2-mm wide × 1-mm deep × 6-mm long) light cured according to the manufacturer's instructions at 0-mm, 2-mm, and 8-mm distances (n=10) and were stored wet at 37°C for 24 hours. A microflexural strength test was performed using a universal testing machine at a crosshead speed of 1 mm/min. The microflexural strength and microflexural modulus data were analyzed using a two-way analysis of variance followed by a Tukey multiple comparison post hoc test (α=0.05). Results: The TEC composite had a significantly higher microflexural strength at an 8-mm distance compared with the 0-mm distance. The Z250 composite expressed significantly higher microflexural strength, at 2-mm and 8-mm compared with the 0-mm distance. TEC showed a significantly higher microflexural modulus at an 8-mm distance compared with the 0-mm and 2-mm distances. Z250 also exhibited a significantly higher microflexural modulus at the 2-mm distance, compared with the 8-mm distance. In total, Z250 presented a significantly higher microflexural strength and modulus compared with TEC. Conclusion: Curing the explored composites at 2-mm or 8-mm distances from the specimen surface did not have a significant influence on microflexural strength but did significantly affect the microflexural modulus.