Effects of heat treatment and testing temperature on fracture mechanics behavior of low-Si CA-15 stainless steel

This research studied the effects of heat treatment and testing temperature on fracture mechanics behavior of Si-modified CA-15 martensitic stainless steel (MSS), which is similar to AISI 403 grade stainless steel, which has been widely used in wall and blanket structures and in the pipe of nuclear power plant reactors, turbine blades, and nozzles. The results indicated that fracture toughness of low-Si CA-15 MSS is better than that of AISI 403. The specimens of the low-Si CA-15 MSS after austenitization at 1010 °C and then tempering at 300 °C have higher plane-strain fracture toughness (K _IC ) values for both 25 °C and −150 °C testing temperatures. However, the specimens tested at 150 °C cannot satisfy the plane-strain fracture toughness criteria. The fatigue crack growth rate is the slowest after austenitization at 1010 °C for 2 hours and tempering at 400 °C. Observing the crack propagation paths using a metallographic test, it was found that the cracking paths preferred orientation and branched along ferrite phase, owing to martensite-phase strengthening and grain-boundary-carbide retarding after 300 °C to 400 °C tempering. Also, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.     

The Dipeptidyl Peptidase-4 Inhibitor Linagliptin Ameliorates Endothelial Inflammation and Microvascular Thrombosis in a Sepsis Mouse Model

The pathophysiology of sepsis involves inflammation and hypercoagulability, which lead to microvascular thrombosis and compromised organ perfusion. Dipeptidyl peptidase (DPP)-4 inhibitors, e.g., linagliptin, are commonly used anti-diabetic drugs known to exert anti-inflammatory effects. However, whether these drugs confer an anti-thrombotic effect that preserves organ perfusion in sepsis remains to be investigated. In the present study, human umbilical vein endothelial cells (HUVECs) were treated with linagliptin to examine its anti-inflammatory and anti-thrombotic effects under tumor necrosis factor (TNF)-α treatment. To validate findings from in vitro experiments and provide in vivo evidence for the identified mechanism, a mouse model of lipopolysaccharide (LPS)-induced systemic inflammatory response syndrome was used, and pulmonary microcirculatory thrombosis was measured. In TNF-α-treated HUVECs and LPS-injected mice, linagliptin suppressed expressions of interleukin-1β (IL-1β) and intercellular adhesion molecule 1 (ICAM-1) via a nuclear factor-κB (NF-κB)–dependent pathway. Linagliptin attenuated tissue factor expression via the Akt/endothelial nitric oxide synthase pathway. In LPS-injected mice, linagliptin pretreatment significantly reduced thrombosis in the pulmonary microcirculation. These anti-inflammatory and anti-thrombotic effects were independent of blood glucose level. Together the present results suggest that linagliptin exerts protective effects against endothelial inflammation and microvascular thrombosis in a mouse model of sepsis.     

Influence of casting size and graphite nodule refinement on fracture toughness of austempered ductile iron

Casting size affects the solidification cooling rate and microstructure of casting materials. Graphite nodules existing in the structure of ductile iron are an inherent and inert second phase that cannot be modified in subsequent heat-treatment processing. The matrix and the fineness of the second phase undoubtedly have some impact on the fracture toughness of the as-cast material, as does the subsequent heat treatment, as it alters the microstructure. This research applied austempering heat treatment to ductile iron of different section sizes and graphite nodule finenesses. The influence of these variables on the plane strain fracture toughness (K _IC ) of the castings so treated was compared to that of the as-cast state. Metallography, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.     

Ultrasonic nondestructive evaluation of matrix structures and nodularity in cast irons

The primary purpose of this research was to investigate the nondestructive ultrasonic wave response, in terms of acoustic velocities and attenuation of sound energy, in cast irons with different nodularities and matrix structures and its correlation with mechanical properties. The results indicated that the influences of matrix structures on the acoustic velocities were not apparent in the cast irons investigated. As to the nodularity, when graphites were largely spheroidal in shape (i. e., nodularity over 80 pct), the velocity of longitudinal waves propagation was about 5300 to 5500 m/s. The velocities seemed to decrease linearly down to nodularity of 25 pct, where velocity was approximately 4800 m/s. Below 25 pct nodularity, the values of acoustic velocity dropped rapidly to about 4000 to 4200 m/s. This represented the velocity of longitudinal waves propagation in gray cast iron, in which the graphites appeared in flake form. The analysis of the attenuation of ultrasonic amplitude indicated that when the nodularity of cast irons is low, the echo sound amplitude will decay more rapidly with respect to distance of echo sound travel. As to the matrix structures, ferritic, bainitic, ferritic-pearlitic (low pearlite content) and tempered martensitic matrix structures were found to have similar ultrasonic attenuation characteristics at the testing frequency of 2 MHz. A higher amount of pearlite (over 90 pct) or fresh martensite in the matrix of cast irons has resulted in faster attenuation of ultrasonic energy, with the fresh martensitic matrix being the fastest. At a testing frequency of 4 MHz, the attenuation of the ultrasonic amplitude in pearlitic and fresh martensitic matrices was found to be even greater than that of 2 MHz. However, other matrices exhibited similar attenuation behavior at both 2 and 4 MHz frequencies. The relationship between the mechanical properties of various cast irons and ultrasonic characteristics was also examined. JIA-MING SUEN, formerly Graduate Student, Department of Materials Engineering, Tatung Institute of Technology     

Low-temperature fracture toughness study of Fe-7Al-27Mn-C alloys

This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents: 0.5% C, Fl alloy: 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, – 50, – 100 and – 150 °C. It was found that plane-stress,K _C, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highestK _C value at all temperatures among the three alloys. TheK _C values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively,K _IC values of the F2 alloy at – 150 °C were ca, 60% less than at 25 °C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at –150°C only alloy F2 satisfied the requirement of plane-strain fracture toughness with aK _C value of 106 MPa m^1/2. The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the low toughness value observed.     

The effect of testing temperature on fracture toughness of austempered ductile iron

The effect of testing temperature (− 150 °C, 25 °C, and + 150 °C) on the fracture toughness of austempered ductile iron (ADI) was studied. Specimens were first austenitized at 900 °C for 1.5 hours and then salt-bath quenched to 360 °C or 300 °C, for 1, 2, or 3 hours of isothermal holding before cooling to room temperature. The resulting matrices of the iron were of upper-ausferrite and lower-ausferrite. It was found that raising the testing temperature to 150 °C from ambient improved the fracture toughness by 18, 30, and 7 pct for the as-cast/lower-ausferrite ADI/upper-ausferrite ADI, respectively. Lowering the testing temperature to −150 °C produced a decrease of −15, −35, and −48 pct. Optical microscopy, X-ray diffraction analysis, and scanning electron microscopy (SEM) fractography were applied to correlate the toughness variation with testing temperatures.     

Fracture toughness and crack growth rate of ferritic and pearlitic compacted graphite cast irons at 25 ‡C and 150 ‡C

This research studied the ambient (25 ‡C) and intermediate (150 ‡C) temperatures plane strain fracture toughness(K _Ic ) and crack growth rateda/dN vs stress-intensity variation (δK) behaviors of compacted graphite (CG) cast irons in an atmospheric environment. As-cast ferritic irons with different percentages of compacted graphite (vermicularity) were produced by using insufficient amounts of spheroidizer. Irons with pearlitic matrix were obtained by heat treating the as-cast structure. The results of fracture toughness testing indicated that (1) for the same matrix, CG irons with higher vermicularity yielded lowerK _Ic values, but their values were still much higher than those of gray (flake graphite) cast iron; (2) for the same vermicularity, CG irons with pearlitic matrix exhibited higher fracture toughness values than those of ferritic matrix; (3) at intermediate temperature (150 ‡C), the influence of vermicularity and matrix on fracture toughness is the same as at ambient temperature, except that theK _Ic values were all a bit lower (1 to 8 pet). From crack growth ratevs stress-intensity variation experiments, the Paris equationda/dN = C(δK) _n was derived, where a smaller value of indicates better crack growth resistance of materials. Compacted graphite cast irons with pearlitic matrix and/or greater vermicularity rendered highern values and, thus, inferior crack growth resistance. At elevated temperature, then values were all lower, indicating that the crack growth resistance was improved. Formely a Graduate Student, Department of Materials Engineering, Tatung Institute of Technology.     

On thermal shock resistance of austenitic cast irons

This research applied rapid induction heating and water quenching cycles to study the thermal shock resistance of high-nickel austenitic cast irons. Both flake graphite (FG) and compacted graphite (CG) cast irons were evaluated. In addition, alloying of cobalt and (Cr + Al) to the high-nickel base materials was performed to produce ultralow thermal expansion and hightemperature specialty irons. Microstructural analysis, as well as mechanical and thermal properties testings, was performed to find out the relationship among microstructures, properties, and thermal shock resistance of the materials under investigation. LIN-CHAO WENG, formerly Graduate Student, Department of Materials Engineering, Tatung Institute of Technology.     

Thermal fracture endurance of cast irons with application study of pig iron ingot molds

Pig iron ingot molds manufactured with flake, compacted graphite cast iron, and spheroidal graphite cast iron were installed on a pig iron casting machine and subjected to thermal cycling for studying thermal fracture endurance of the three cast irons. The effects of graphite morphology on the fracture mechanism were analyzed by examining the fracture patterns, microstructures, and microcracks in the failed molds. The determining factors of thermal fracture endurance were elucidated with thermal fracture resistance indices. Compacted graphite cast iron exhibited better thermal fracture endurance than flake and spheroidal graphite cast irons because of its higher strength-to-thermal stress ratio.     

The effects of surface hardening on fracture toughness of carburized steel

This research program was carried out to evaluate the effects of surface hardening on the fracture toughness of carburized steel. The materials AISI 8620 steel was machined into compact-tension (CT) specimens. The specimens were pack carburized at 930°C (1706°F) for different periods of time, cooled to ambient temperature and subsequently tempered at various temperatures for one hour. The fractured specimens were examined by hardness tests, metallography, X-ray diffraction analysis for retained austenite in the case, and scanning electron microscope fractographic analysis of the fracture surfaces. The experimental results revealed that theKIC values of the carburized, AISI 8620 steels were improved by the increase in case depth. Martensitic/tempered-martensitic structure in the case was the major constituent contributing to the improved toughness. The amount of retained austenite at the case increased as the thickness of the hardened layer increased. But retained austenite as well as large grain size were found to have adverse effects on fracture toughness of the carburized steel. The tempering temperature of 500°C (932°F) provided maximumKIC values. Higher tempering temperatures resulted in sharp decrease of fracture, toughness values. W{upeio}-Y{upoue} H{upo}, formerly a Graduate Student, in the Department of Materials Engineering Tatung Institute of Technology, is in compulsory 0 ROTC military service of Republic of China.