Search Results for Nil ductility transition temperature

A metallurgical failure analysis was performed on pieces of the cracked vent header pipe from the Edwin I. Hatch Unit 2 Nuclear power plant. The analysis consisted of optical microscopy, chemical analysis, mechanical Charpy impact testing, and fractography. It was found that the material of the vent header met the mechanical and chemical properties of ASTM A516 Grade 70 carbon-manganese steel material and microstructures were consistent with this material. Fracture faces of the cracked pipe were predominantly brittle in appearance with no evidence of fatigue contribution. The NDTT (Nil ductility Transition Temperature) for this material was approximately -51 deg C (-60 deg F). The fact that the material's NDTT was significantly out of the normal operating range of the pipe suggested an impingement of low temperature nitrogen (caused by a faulty torus inerting system) induced a thermal shock in the pipe which, when cooled below its NDTT, cracked in a brittle manner.

A spherical carbon steel fixed-catalyst bed reactor, fabricated from French steel A42C-3S, approximately equivalent to ASTM A201 grade B, failed after 20 years of service while in a standby condition. The unit was found to contain primarily hydrogen at the time of failure. The vessel had a type 304 stainless steel shroud around the catalyst bed as protection against the overheating that was possible if the gas bypassed the bed through the refractory material. The failure was observed to have begun at the toe of the shroud-support ring weld. The ring was found to have a number of small cracks at the root of the weld. The cleavage mode of fracture was confirmed by SEM. The presence of extensive secondary cracking and twinning (Neumann bands) where the fracture followed the line of the shroud-support ring was revealed by metallography. It was revealed by refinery maintenance records that the ring had been removed for hydrotest and welded without any postweld heat treatment. The final cause of failure was concluded to be cracking that developed during the installation of the new shroud ring. Stress-relief heat treatments were recommended to be performed to reduce residual-stress levels after welding.

A 460 mm (18 in.) diam suction line to the main feed water pump for a nuclear power plant failed in a violent, catastrophic manner. Samples of pipe, elbow, and weld materials (ASTM A106 grade B carbon steel, ASTM A234 grade WPB carbon steel, and E7018 carbon steel electrode, respectively) from the suction line were analyzed. Evidence of overall thinning of the elbow and pipe material and ductile tearing of fractures indicated that the feed water pipe failed as a result of an erosion corrosion mechanism, which thinned the wall sufficiently to cause rapid, ductile tearing of the material after its design stress had been exceeded. It was recommended that steel with a higher chromium content be used to mitigate the erosion corrosion potential in the lines and that more rigorous nondestructive (ultrasonic) examinations be performed.

A railway tank car developed a fracture in the region of the sill and shell attachment during operation at -34 deg C (-30 deg F). On either side of the sill-support member, cracking initiated at the weld between a 6.4 mm thick frontal cover plate and a 1.6 mm thick side support plate. The crack then propagated in a brittle manner upward through the side plate, through the welds attaching the side plate to a 25 mm (1 in.) thick shell plate (ASTM A212, grade B steel), and continued for several millimeters in the shell plate before terminating. Other plates involved were not positively identified but were generally classified as semi-killed carbon steels. Investigation (visual inspection, hardness testing, chemical analysis, Charpy V-notch testing, and drop-weight testing) supported the conclusions that the fracture was initiated by weld imperfections and propagated in a brittle manner as a result of service stresses acting on the plate having low toughness at the low service temperatures encountered. Recommendations included that the specifications for the steel plates be modified to include a toughness requirement and that improved welding and inspection practices be performed to reduce the incidence of weld imperfections.

The repeated failure of a welded ASTM A283 grade D pipe that was part of a 6 km (4 mi) line drawing and conducting river water to a water treatment plant was investigated. Failure analysis was conducted on sections of pipe from the third failure. Visual, macrofractographic, SEM fractographic, metallographic, chemical, and mechanical property (tension and impact toughness) analyses were conducted. On the basis of the tests and observations, it was concluded that the failure was the combined result of poor notch toughness (impact) properties of the steel, high stresses in the joint area, a possible stress raiser at the intersection of the spiral weld and girth weld, and sudden impact loading, probably due to water hammer. Use of a semi- or fully killed steel with a minimum Charpy V-notch impact value of 20 J (15 ft·lbf) at 0 deg C (32 deg F) was recommended for future water lines. Certified test results from the steel mill, procedure qualification tests of the welding, and design changes to reduce water hammer were also recommended.

A detailed fracture mechanics evaluation is the most accurate and reliable prediction of process equipment susceptibility to brittle fracture. This article provides an overview and discussion on brittle fracture. The discussion covers the reasons to evaluate brittle fracture, provides a brief summary of historical failures that were found to be a result of brittle fracture, and describes key components that drive susceptibility to a brittle fracture failure, namely stress, material toughness, and cracklike defect. It also presents industry codes and standards that assess susceptibility to brittle fracture. Additionally, a series of case study examples are presented that demonstrate assessment procedures used to mitigate the risk of brittle fracture in process equipment.

In 1979, during a routine bridge inspection, a fatigue crack was discovered in the top flange plate of one tie girder in a tied arch bridge crossing the Mississippi River. Metallographic analysis indicated a banding or segregation problem in the middle of the plate, where the carbon content was twice what it should have been. Based on this and results of ultrasonic testing, which revealed that the banding occurred in 24-ft lengths, it was decided to close the bridge and replace the defective steel. The steel used in the construction of this bridge was specified as ASTM A441, commonly used in structural applications. Testing showed an increase in hardness and weight percent carbon and manganese in the banded region. Further testing revealed that the area containing the segregation and coarse grain structure had a lower than expected toughness and a transition temperature 90 deg F higher than specified by the ASTM standards. The fatigue crack growth rate through this area was much faster than expected. All of these property changes resulted from increased carbon levels, higher yield strength, and larger than normal grain size.

This article aims to identify and illustrate the types of overload failures, which are categorized as failures due to insufficient material strength and underdesign, failures due to stress concentration and material defects, and failures due to material alteration. It describes the general aspects of fracture modes and mechanisms. The article briefly reviews some mechanistic aspects of ductile and brittle crack propagation, including discussion on mixed-mode cracking. Factors associated with overload failures are discussed, and, where appropriate, preventive steps for reducing the likelihood of overload fractures are included. The article focuses primarily on the contribution of embrittlement to overload failure. The embrittling phenomena are described and differentiated by their causes, effects, and remedial methods, so that failure characteristics can be directly compared during practical failure investigation. The article describes the effects of mechanical loading on a part in service and provides information on laboratory fracture examination.

Mechanical testing is an evaluative tool used by the failure analyst to collect data regarding the macro- and micromechanical properties of the materials being examined. This article provides information on a few important considerations regarding mechanical testing that the failure analyst must keep in mind. These considerations include the test location and orientation, the use of raw material certifications, the certifications potentially not representing the hardware, and the determination of valid test results. The article introduces the concepts of various mechanical testing techniques and discusses the advantages and limitations of each technique when used in failure analysis. The focus is on various types of static load testing, hardness testing, and impact testing. The testing types covered include uniaxial tension testing, uniaxial compression testing, bend testing, hardness testing, macroindentation hardness, microindentation hardness, and the impact toughness test.

During the routine hydraulic pressure test of a boiler following modification, failure by leakage from the drum took place and was traced to a region where extensive multiple cracking had occurred. Catastrophic rupture or fragmentation of the vessel fortunately did not take place. Prior to the test, cracking was present already, extending up to 90% of the wall thickness. Analyses of brownish deposit material did not reveal the presence of any substances likely to cause stress-corrosion cracking of a Ni-Cu-Mo low-alloy steel.

Overload failures refer to the ductile or brittle fracture of a material when stresses exceed the load-bearing capacity of a material. This article reviews some mechanistic aspects of ductile and brittle crack propagation, including a discussion on mixed-mode cracking, which may also occur when an overload failure is caused by a combination of ductile and brittle cracking mechanisms. It describes the general aspects of fracture modes and mechanisms. The article discusses some of the material, mechanical, and environmental factors that may be involved in determining the root cause of an overload failure. It also presents examples of thermally and environmentally induced embrittlement effects that can alter the overload fracture behavior of metals.

Alloy 430 stainless steel tube-to-header welds failed in a heat recovery steam generator (HRSG) within one year of commissioning. The HRSG was in a combined cycle, gas-fired, combustion turbine electric power plant. Alloy 430, a 17% Cr ferritic stainless steel, was selected because of its resistance to chloride and sulfuric acid dewpoint corrosion under conditions potentially present in the HRSG low-pressure feedwater economizer. Intergranular corrosion and cracking were found in the weld metal and heat-affected zones. The hardness in these regions was up to 35 HRC, and the weld had received a postweld heat treatment (PWHT). Metallographic examination revealed that the corroded areas contained undertempered martensite. Fully tempered weld areas with a hardness of 93 HRB were not attacked. No evidence of corrosion fatigue was found. Uneven temperature control during PWHT was the most likely cause of failure.

This article discusses the effect of using unsuitable alloys, metallurgical discontinuities, fabrication practices, and stress raisers on the failure of a pressure vessel. It provides information on pressure vessels made of composite materials and their welding practices. The article explains the failure of pressure vessels with emphasis on stress-corrosion cracking, hydrogen embrittlement, brittle and ductile fractures, creep and stress rupture, and fatigue with examples.

This article provides an overview of the structural design process and discusses the life-limiting factors, including material defects, fabrication practices, and stress. It details the role of a failure investigator in performing nondestructive inspection. The article provides information on fatigue life assessment, elevated-temperature life assessment, and fitness-for-service life assessment.

This article discusses pressure vessels, piping, and associated pressure-boundary items of the types used in nuclear and conventional power plants, refineries, and chemical-processing plants. It begins by explaining the necessity of conducting a failure analysis, followed by the objectives of a failure analysis. Then, the article discusses the processes involved in failure analysis, including codes and standards. Next, fabrication flaws that can develop into failures of in-service pressure vessels and piping are covered. This is followed by sections discussing in-service mechanical and metallurgical failures, environment-assisted cracking failures, and other damage mechanisms that induce cracking failures. Finally, the article provides information on inspection practices.

This article describes some of the welding discontinuities and flaws characterized by nondestructive examinations. It focuses on nondestructive inspection methods used in the welding industry. The sources of weld discontinuities and defects as they relate to service failures or rejection in new construction inspection are also discussed. The article discusses the types of base metal cracks and metallurgical weld cracking. The article discusses the processes involved in the analysis of in-service weld failures. It briefly reviews the general types of process-related discontinuities of arc welds. Mechanical and environmental failure origins related to other types of welding processes are also described. The article explains the cause and effects of process-related discontinuities including weld porosity, inclusions, incomplete fusion, and incomplete penetration. Different fitness-for-service assessment methodologies for calculating allowable or critical flaw sizes are also discussed.