Search Results for austenitic grain size

The defects observed along weldings of stainless steel pipelines employed in marine environments were evidenced by metallographic and electrochemical examination. A compilation of cases on the effect of defective weldings, in addition to improper choice of stainless steel for water pipelines, lead to the conclusion that intercrystalline corrosion in steels involved precipitation of a surplus phase at grain boundaries. Intercrystalline corrosion in austenitic stainless steels due to precipitation of chromium carbides during conditions generated due to welding and ways to avoid the precipitation (including reduction of carbon content, appropriate heat treatment, cold work of steel, reduction of austenitic grain size and stabilizing elements) were described. The presence of microcracks due to highly localized heat concentrations with consequent thermal expansion and considerable shrinkages during cooling was investigated. The specimens were taken from various sources including transverse and longitudinal welding seam, sensitized areas and it was concluded appropriate material selection with respect to medium could control some corrosion processes.

In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, KID, is obviously lower than static fracture toughness, KIC, and has the same trend as KIC. The ratio of KID/sigma YD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.

Several crankshaft failures occurred in equipment that was being used in logging operations in subzero temperatures. Failure usually initiated at a cracked pin oil hole, and the failure origin was approximately 7.6 mm (0.3 in.) from the shaft surface. The holes were produced by gun drilling, giving rise to surface defects. The fracture surface was characteristic of fatigue in that it was flat, relatively shiny, and exhibited beach marks. The crack surface was at a 45 deg angle to the axis of the shaft, indicating dominant tensile stresses. The material was the French designation AFNOR 38CD4 (similar to AISI type 4140H) and was in the quenched-and-tempered condition, with a yield strength of about 760 MPa (110 ksi). It was treated to have compressive surface stresses, and the prior-austenite grain size was ASTM 8. Analysis (visual inspection, stress analyses, and macrographs) supported the conclusion that failure was caused by fatigue stress caused by surface defects in the oil holes. Recommendation includes drilling the oil holes by a technique that essentially eliminates surface defects.

A masonry type drill bit, designed for impact drilling in rock, fractured after a short time in service. Samples of the failed bit were analyzed using optical and scanning electron microscopy, quantitative metallography, and chemical analysis. The composition was found to be that of 18CrNi3Mo steel. Investigators also found evidence of inclusions and prior austenite grain size, although it was determined that neither played a role in the failure. Rather, according to test data, the failure occurred because of stress concentration (due to geometric discontinuities along the tooth profiles) and the cumulative effect of torque and force loading (the byproduct of continuous twisting and axial impact). Cracks readily initiate under these conditions then propagate quickly through what was found to be networks of tempered martensite, thus resulting in premature failure.

This article introduces some of the general sources of heat treating problems with particular emphasis on problems caused by the actual heat treating process and the significant thermal and transformation stresses within a heat treated part. It addresses the design and material factors that cause a part to fail during heat treatment. The article discusses the problems associated with heating and furnaces, quenching media, quenching stresses, hardenability, tempering, carburizing, carbonitriding, and nitriding as well as potential stainless steel problems and problems associated with nonferrous heat treatments. The processes involved in cold working of certain ferrous and nonferrous alloys are also covered.

This case study describes the failure analysis of a steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low-carbon CrMoV alloy steel that was subsequently single-pass butt welded using gas tungsten arc welding. Although no cracks were found when the welds were visually inspected, X-ray radiography showed small discontinuous surface cracks adjacent to the weld bead in the heat affected zone. Further investigation, including optical microscopy, microhardness testing, and residual stress measurements, revealed that the cracks were caused primarily by 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 high transformation stresses. These transformation stresses, working in combination with thermal stresses and constraint conditions, resulted in intergranular brittle fracture.

An electric transport vehicle, similar to an electric trolley or subway rail car, experienced frequent breakdowns due to in-service fractures of torsion springs that support the weight of an overhead electric pickup assembly. Scanning electron microscopy and metallographic examinations determined that the fractures stemmed from electric arc damage. Intergranular quench cracks in the transformed untempered martensite on the surface of the spring provided crack initiations that propagated during operation causing fatigue fracture.

During installation, a clamp-strap assembly, specified to be type 410 stainless steel-austenitized at 955 to 1010 deg C (1750 to 1850 deg F), oil quenched, and tempered at 565 deg C (1050 deg F) for 2 h to achieve a hardness of 30 to 35 HRC, and used for securing the caging mechanism on a star-tracking telescope, fractured transversely across two rivet holes closest to one edge of the pin retainer in a completely brittle manner. Comparison with a non-failed strap using microscopic examination, spectrographic analysis, and slow-bend tests showed that both fit the 410 stainless steel specs, but hardness and grain size were different. Reheat treatment of full-width specimens showed that coarse grain size (ASTM 2 to 3) was responsible for the brittle fracture, and excessively high temperature during austenitizing caused the large grain size in the failed strap. The fact that the hardness of the strap that failed was lower than the specified hardness of 30 to 35 HRC had no effect on the failure because that of the non-failed strap was even lower. Recommendation was that the strap should be heat treated as specified to maintain the required ductility and grain size.

Cracks were discovered in the cast 17-4 PH stainless steel outboard leading edge flap support of an aircraft wing during overhaul inspection. Failure analysis focused on an apparently intergranular area of fracture surface. It was determined that the original mode of crack growth was cleavage, probably caused by cast-in hydrogen. The intergranular appearance resulted from heat treatment of the already cracked part, which caused the formation of grain-boundary “growth figures” on the exposed crack surfaces. It was recommended that the castings be more closely inspected for defects before further processing and that foundry practices be reviewed to correct deficiencies leading to excessive hydrogen absorption during melting and casting.

The shafts on two centrifugal pumps failed during use in a petroleum refinery. Light optical microscopy and scanning electron microscopy were used to analyze the damaged materials to determine the cause of failure. The results showed that one shaft, made of duplex stainless steel, failed by fatigue fracture, and the other, made of 316 austenitic stainless steel, experienced a similar fracture, which was promoted by the presence of nonmetallic inclusion particles.

A steel valve spring meeting Steel-Iron-Test 1570 fractured during the high-stress condition of the operation of its valve. Metallographic examination of a transverse section adjacent to the fracture and a longitudinal section through the crack showed the steel was free of major defects and was of high purity, although a number of minor surface defects such as rolling laps were found. The spring was heat treated and its surface strengthened by shot-peening, but the surface was also decarburized to a depth of approximately 0.03 mm which resulted in a lowering of the surface hardness. The fracture of this valve spring is therefore primarily due to surface defects, and secondly perhaps also to weak surface decarburization. No recommendation resulted from the investigation except to note that comparatively minor effects suffice to cause fractures in highly stressed springs.

The accident at Three Mile Island Unit No. 2 on 28 March 1979 was the worst nuclear accident in US history. By Jan 1990, it was possible to electrochemically machine coupons from the lower head using a specially designed tool. The specimens contained the ER308L stainless steel cladding and the A533 Grade B plate material to a depth of about mid-wall. The microstructures of these specimens were compared to that of specimens cut from the Midland, Michigan reactor vessel, made from the same grade and thickness but never placed in service. These specimens were subjected to known thermal treatments between 800 and 1100 deg C for periods of 1 to 100 min. Microstructural parameters in the control specimens and in those from TMI-2 were quantified. Selective etchants were used to better discriminate desired microstructural features, particularly in the cladding. This report is a progress report on the quantification of changes in both the degree of carbide precipitation and delta ferrite content and shape in the cladding as a function of temperature and time to refine the estimates of the maximum temperatures experienced.

The US Army Research Laboratory performed a failure investigation on a broken main landing gear mount from an AH-64 Apache attack helicopter. A component had failed in flight, and initially prevented the helicopter from safely landing. In order to avoid a catastrophe, the pilot had to perform a low hover maneuver to the maintenance facility, where ground crews assembled concrete blocks at the appropriate height to allow the aircraft to safely touch down. The failed part was fabricated from maraging 300 grade steel (2,068 MPa [300 ksi] ultimate tensile strength), and was subjected to visual inspection/light optical microscopy, metallography, electron microscopy, energy dispersive spectroscopy, chemical analysis, and mechanical testing. It was observed that the vacuum cadmium coating adjacent to the fracture plane had worn off and corroded in service, thus allowing pitting corrosion to occur. The failure was hydrogen-assisted and was attributed to stress corrosion cracking (SCC) and/or corrosion fatigue (CF). Contributing to the failure was the fact that the material grain size was approximately double the required size, most likely caused from higher than nominal temperatures during thermal treatment. These large grains offered less resistance to fatigue and SCC. In addition, evidence of titanium-carbo-nitrides was detected at the grain boundaries of this material that was prohibited according to the governing specification. This phase is formed at higher thermal treatment temperatures (consistent with the large grains) and tends to embrittle the alloy. It is possible that this phase may have contributed to the intergranular attack. Recommendations were offered with respect to the use of a dry film lubricant over the cadmium coated region, and the possibility of choosing an alternative material with a lower notch sensitivity. In addition, the temperature at which this alloy is treated must be monitored to prevent coarse grain growth. As a result of this investigation and in an effort to eliminate future failures, ARL assisted in developing a cadmium brush plating procedure, and qualified two Army maintenance facilities for field repair of these components.

This article briefly reviews the various metallurgical or environmental factors that cause a weakening of the grain boundaries and, in turn, influence the occurrence of intergranular (IG) fractures. It discusses the mechanisms of IG fractures, including the dimpled IG fracture, the IG brittle fracture, and the IG fatigue fracture. The article describes some typical embrittlement mechanisms that cause the IG fracture of steels.

This article briefly reviews the factors that influence the occurrence of intergranular (IG) fractures. Because the appearance of IG fractures is often very similar, the principal focus is placed on the various metallurgical or environmental factors that cause grain boundaries to become the preferred path of crack growth. The article describes in more detail some typical mechanisms that cause IG fracture. It discusses the causes and effects of IG brittle cracking, dimpled IG fracture, IG fatigue, hydrogen embrittlement, and IG stress-corrosion cracking. The article presents a case history on IG fracture of steam generator tubes, where a lowering of the operating temperature was proposed to reduce failures.

A segment of a stainless steel clad bottom cone of an acid sulfite pulping batch digester failed from severe corrosion loss. The digester was fabricated of 19 mm ( 3 4 in.) low-carbon steel with 3.8 mm (0.15 in.) type 317L stainless steel cladding. The manufacturing method for the cladding was unknown. Visual and metallographic analyses indicated that the failure was from transgranular stress-corrosion cracking (TGSCC), which caused extensive cracking and spalling of the cladding and was localized in a segment of the bottom cone. The remainder of the digester cladding was unaffected. The TGSCC was attributed to high, locked-in residual stresses from the cladding process. It was recommended that the bottom cone replacement segment be stress relieved prior to installation.