Failure Analysis of HAZ Cracking in Low C–CrMoV Steel Weldment

This case study describes the failure analysis of steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low C–CrMoV alloy steel that was subsequently single pass butt welded using gas tungsten arc welding. No cracks were found in visual inspection of the welds; however, X-ray radiography showed small discontinuous cracks on the surface in the area adjacent to weld bead, i.e. heat affected zone. The welding of nozzle parts made of same material was a routine process and this type of cracking did not occur in the past. Therefore, it became essential to determine the root cause of the failure. A detailed investigation including visual examination, non-destructive testing, optical microscopy, microhardness measurements and residual stress measurements were carried out to find out the primary cause of failure and to identify actions required to avoid its reoccurrence in future. Results of the investigation revealed that the principal cause of failure was the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in the high transformation stresses. These transformation stresses combined with the thermal stresses and the constraint conditions to cause intergranular brittle fracture.     

Modulation of extraction variables of pectin from Manilkarazapota fruit peel,its bio characterization,and comparative study using dynamic light scattering studies

Influence of variables has a great impact upon yield of pectin. The present study was organized to harmonize these variables and examine the extracted pectin. In view of this, bio-characterizion and structure analysis of pectin through FTIR was performed. Additionally, a polydispersity and particle size study was also conducted using dynamic light scattering (DLS) of the Manilkarazapota fruit peel. The best extraction variables according to the response optimization predicted model were noted as pH of 5 at 61.11°C for 90 min of heating time, which lead to a 3.7% yield. Bio-characterization revealed that pectin extracted at pH 5 has characteristics similar to HM pectin influence of methoxyl content, degree of esterification, and Galacturonic acid content at 5.11%, 73. 63%, and 77.7%, respectively. The FTIR spectrum depicted a similar surface structure to food-grade pectin. DLS studies revealed that extracted pectin at pH 5 has a particle size in the range of 390.21–421.17 nm and polydispersity (?) of 28.2%-29.3%. These findings indicate that sapodilla is a potential source of pectin for food and pharmaceutical preparation.     

Effect of Cyclic Aging on Mechanical Properties and Microstructure of Maraging Steel 250

The effects of thermal cyclic aging on mechanical properties and microstructure of maraging steel 250 were studied using hardness tester, tensile testing machine, impact tester, optical, scanning electron, and stereo microscopy. Samples were solution annealed at 1093 K for 1 h followed by air cooling to form bcc martensite. Cyclic aging treatments were carried out at 753 and 773 K for varying time periods. Increase in hardness and strength with corresponding decrease in ductility and impact strength was observed with increasing aging cycles. Reverted austenite was detected by x-ray diffraction technique formed as a result of cyclic aging. The presence of reverted γ was also confirmed by EDX-SEM analysis and attributed to the formation of Mo- and Ni-rich regions which transformed to γ on cooling. Heterogeneity in composition and amount of reverted γ was found to increase with increase in aging cycles and aging time. Fractography reveals the change in fracture mode from ductile dimple-like to brittle cleavage with increase in hardness and strength due to cyclic aging.     

Investigation into Microstructures of Maraging Steel 250 Weldments and Effect of Post-Weld Heat Treatments

This study was undertaken to gain a better understanding of microstructures obtained by multipass gas tungsten arc welding in maraging steel grade 250. Metallography and microhardness measurements were carried out on sheet and welded joints in as-welded and post-weld aged conditions. It was found that there was a significant amount of reverted austenite formed on cell boundaries of weld metal after aging at 758-823 K for 3-5 h, and was stable at room temperature. Aging at higher temperatures led to an increase in the continuous network of patchy austenite along the cell boundaries. The reason for the above, in our opinion, is the concentrational heterogeneity which characterizes the microstructure of maraging steel welds. No reverted austenite was observed in as-welded specimens. Solution annealing at 1093 K for 1 h did not completely eliminate the chemical heterogeneity associated with weld structures. However, homogenizing at 1373 K produced homogenous structure that on subsequent aging produces austenite-free lath martensitic structure.     

Insensitive Munitions Fragment Impact Gun Testing Technology Challenges

The STANAG 4496 Ed. 1 Fragment Impact, Munitions Test Procedure, specifies a standard test of 2530±90 m s⁻¹, an alternate test of 1830±60 m s⁻¹ and a standard fragment (projectile) geometry. The standard can be challenging to achieve and has several loosely defined and undefined characteristics that can affect the test item response. In particular fragment velocity variation, projectile tilt upon impact and aimpoint variation are commonly observed challenges. Achieving 2530 m s⁻¹ consistently and cost effectively can be challenging. Additionally, there are some efforts where over testing to higher velocities are viewed as desirable. The aimpoint of impact of the fragment is chosen with the objective of obtaining the most violent reaction: one test is conducted with impact in the center of the largest presented area of energetic material and a second in the most shock sensitive region. No tolerance for aimpoint is specified, although it is shown through high rate continuum modelling that aimpoint variation can be a dominant source for IM response variation. Continuum modelling also shows fragment tilt on impact is to be a possible source for response variation and it is common to observe fragment tilts of 30° or even higher. The standard fragment is specified to be fabricated from mild, carbon steel with a Brinell Hardness (HB) less than 270. In the U.S., the fragment is often fabricated using ASTM1018 steel, which has a significant margin for mechanical properties that lay within the specification. These, as well as other gun testing issues, have significant implications to resulting IM response.     

Libra: a computational economy‐based job scheduling system for clusters

Clusters of computers have emerged as mainstream parallel and distributed platforms for high‐performance, high‐throughput and high‐availability computing. To enable effective resource management on clusters, numerous cluster management systems and schedulers have been designed. However, their focus has essentially been on maximizing CPU performance, but not on improving the value of utility delivered to the user and quality of services. This paper presents a new computational economy driven scheduling system called Libra, which has been designed to support allocation of resources based on the users' quality of service requirements. It is intended to work as an add‐on to the existing queuing and resource management system. The first version has been implemented as a plugin scheduler to the Portable Batch System. The scheduler offers market‐based economy driven service for managing batch jobs on clusters by scheduling CPU time according to user‐perceived value (utility), determined by their budget and deadline rather than system performance considerations. The Libra scheduler has been simulated using the GridSim toolkit to carry out a detailed performance analysis. Results show that the deadline and budget based proportional resource allocation strategy improves the utility of the system and user satisfaction as compared with system‐centric scheduling strategies. Copyright © 2004 John Wiley & Sons, Ltd.     

Characterization of Material Properties of 2xxx Series Al-Alloys by Non Destructive Testing Techniques

2xxx series Al-alloys are widely employed in structural applications due to their good mechanical properties. During heat treatment of these alloys, solution treated parts sometimes mixed with age hardened parts during handling. This result in difficulty in distinguishing between solution treated and aged parts of various grades. Moreover, it is also necessary to separate improper aged parts from properly treated parts. The traditional methods of characterization of different heat treated parts are hardness, tension testing and microscopy, however these are destructive in nature and sometimes not desired particularly for finished products. The main purpose of this paper is characterization of material properties of 2xxx series Al-alloys by eddy current and ultrasonic NDE techniques so that the inspection can be carried out effectively in the shortest possible time. Three wrought Al-alloys of 2xxx series (AA 2014, AA 2024 and AA 2219) were homogenized followed by solution heat treatment and age hardening treatments at specific temperatures for 1–16 h. The changes in hardness and microstructure during heat treatments were determined by traditional material characterization methods and then correlated with electrical conductivity, sound velocity and attenuation coefficient obtained through Nondestructive Evaluation (NDE) techniques. Results demonstrated an excellent correlation between hardness and sound velocity, whereas extend of aging can be easily predicted by electrical conductivity, and attenuation coefficient measurement. Investigation suggested a way towards the non-destructive detection and characterization of material properties when conventional testing methods are not applicable.     

Adsorption of bromo-phenol blue from an aqueous solution onto thermally modified granular charcoal

The adsorption phenomenon of bromo-phenol blue onto pristine and thermally evacuated granular charcoal (GC) was studied via a batch technique at 25 °C. The effect of evacuation temperature on the GC surface and pore structure (e.g. pore volume and diameter) was studied by Fourier transform infrared spectroscopy (FT-IR), point of zero charge (PZC), proximate analysis, Brunauer, Emmett and Teller (BET) method and field emission scanning electron microscopy (FE-SEM). The FT-IR spectra of the samples after evacuation showed considerable decrease in the acidic functional groups. PZC showed that the surface of the evacuated charcoal became basic as the evacuation temperature was increased from 300 to 800 °C. Volatile matter decreased while ash and fixed carbon contents increased during evacuation, which led to an increase in the micro-pore volume from 0.25 to 0.42 cm³ g⁻¹, meso-pore volume from 0.04 to 0.13 cm³ g⁻¹, pore diameter from 5.01 to 6.21 nm, and specific surface from 150.32 to 254.70 m² g⁻¹. Adsorption of the bromo-phenol blue onto charcoal, increased as the evacuation temperature was increased from 300 to 800 °C. The interaction of bromo-phenol blue with charcoal was proposed to have occurred via hydrogen bonding. The adsorption data fitted well with the Langmuir equation, which indicated that the monolayer adsorption has occurred at specific sites within the adsorbent.     

Preparation of the chitosan containing nanofibers by electrospinning chitosan–gelatin complexes

Electrospinning is an interesting technique, which provides a facile and an effective mean in producing nonwoven fibrous materials; however, for producing nanofibers, investigation of the electrospinning conditions is very important. In this study, chitosan, gelatin, and their polyelectrolyte complexes (PECs) were electrospun to prepare nonwoven nanofibrous mats. The concentrations of chitosan and gelatin solutions and electric field (kV/cm) were optimized. The solutions were then blended in different ratios (0–100%) to get electrospun nanofibrous mats. Solution concentration and electric field showed pronounced effect on the electrospinnability and fiber diameter of these systems. Mostly large beads coexisted with the fibers were observed for chitosan at 1 wt% solution concentration, which then showed good electrospinnability at 2 wt% (nanofiber diameter was 145 and 122 nm at 15 and 20 kV/10 cm, respectively), whereas gelatin showed no electrospinnability below 15 wt% solution concentration and a homogenous fibers network at 15 wt% (149 nm at 20 kV/10 cm). The morphology and diameter of chitosan–gelatin PEC nanofibers varied with the chitosan/gelatin ratio. The crystallinity of chitosan was also observed to reduce with electrospinning and addition of gelatin. POLYM. ENG. SCI. 50:1887–1893, 2010. © 2010 Society of Plastics Engineers     

Failure Analysis of High Density Polyethylene Butt Weld Joint

This article discusses a detailed failure analysis conducted on high density polyethylene which acts as the liner of composite compresses natural gas cylinder. Leakage from the cylinder was observed after about 2000 cycles of hydrostatic pressure testing at 250 bars. Visual inspection revealed that the leakage occurred from the circumferential fusion joint between the cylinder and dome section. The cylinder and dome sections were produced from different techniques and joined together by using a heated tool butt welding process conducted by a local manufacturer. The joined components work as an integral part. The investigation was carried out using various techniques including mechanical, thermal and metallurgical examination. Fractography of the failed joint surface showed stepwise marks typical of a fatigue failure. Mechanical testing results showed that the strength of dome section was significantly lower than that of the cylinder section. Moreover, both the tensile and fatigue strength of joint was also almost half that of the cylinder. The fracture surface of the broken welded joint showed brittle failure. The melting points of both the cylinder and the dome sections were found almost same by differential scanning calorimeter but large difference was found between the melt point indexes of both sections. The results suggested that the failure was actually a stress cracking via a process of slow crack growth, which occurred due to use of a dome material having inferior properties and very high melt flow index. These properties inhibited proper fusion and resulted in a poor weld joint. Consequently, the weld joint of lower strength eventually failed in macroscopically brittle manner upon cyclic loading.