Metal-induced embrittlement is a phenomenon in which the ductility or fracture stress of a solid metal is reduced by surface contact with another metal in either liquid or solid form. This article summarizes the characteristics of solid metal induced embrittlement (SMIE) and liquid metal induced embrittlement (LMIE). It describes the unique features that assist in arriving at a clear conclusion whether SMIE or LMIE is the most probable cause of the problem. The article briefly reviews some commercial alloy systems where LMIE or SMIE has been documented. It also provides some examples of cracking due to these phenomena, either in manufacturing or in service.

Metal-induced embrittlement is a phenomenon in which the ductility or the fracture stress of a solid metal is reduced by surface contact with another metal in either the liquid or solid form. This article summarizes some of the characteristics of liquid-metal- and solid-metal-induced embrittlement. This phenomenon shares many of these characteristics with other modes of environmentally induced cracking, such as hydrogen embrittlement and stress-corrosion cracking. The discussion covers the occurrence, failure analysis, and service failures of the embrittlement. The article also briefly reviews some commercial alloy systems in which liquid-metal-induced embrittlement or solid-metal-induced embrittlement has been documented and describes some examples of cracking due to these phenomena, either in manufacturing or in service.

Failure of three C22000 commercial bronze rupture discs was caused by mercury embrittlement. The discs were part of flammable gas cylinder safety devices designed to fail in a ductile mode when cylinders experience higher than design pressures. The subject discs failed prematurely below design pressure in a brittle manner. Fractographic examination using SEM indicated that failure occurred intergranularly from the cylinder side. EDS analysis indicated the presence of mercury on the fracture surface and mercury was also detected using scanning auger microprobe (SAM) analysis. The mercury was accidentally introduced into the cylinders during a gas-blending operation through a contaminated blending manifold. Replacement of the contaminated manifold was recommended along with discontinued use of mercury manometers, the original source of mercury contamination.

Metallography is an important component of failure analysis. In the case of a liquid metal embrittlement (LME) failure it is usually conclusive if a third phase constituent can be formed inside of the cracks after failure. In the case where it is necessary to characterize the third phase material, one can use various x-ray spectrographic techniques in conjunction with a scanning electron microscope (SEM). This study describes those metallographic and SEM analysis techniques for determining the mode of failure for a locomotive traction motor by LME.

Four cadmium-plated ASTM A193 grade B studs from a steam line connector associated with a power turbine failed unexpectedly in a nil-ductility manner. Fracture surfaces were covered with a light-colored, lustrous deposit. Optical microscope, SEM, and EDS analyses were conducted on sections from one of the studs and revealed that the coating on the fracture surface was cadmium. The fracture had multiple origins, and secondary cracks also contained cadmium. The fracture topography was intergranular. The failures were attributed to liquid metal embrittlement caused by the presence of a cadmium plating and operating temperatures at approximately the melting point of cadmium. It was recommended that components exposed to the cadmium be replaced.

Several of the springs, made of 1.1 mm diam Inconel X-750 wire and used for tightening the interstage packing ring in a high-pressure turbine, were found broken after approximately seven years of operation. Intergranular cracks about 1.3 mm in depth and oriented at an angle of 45 deg to the axis of the wire were revealed by metallographic examination. A light-gray phase, which had the appearance of liquid-metal corrosion, was observed to have penetrated the grains on the fracture surfaces. The spring wires were found to fracture in a brittle manner characteristic of fracture from torsional loading (along a plane 45 deg to the wire axis). Liquid-metal embrittlement was expected to have been caused by metals (Sn, Zn, Pb) which melt much below maximum service temperature of the turbine. The springs were concluded to have fractured by intergranular stress-corrosion cracking promoted by the action of liquid zinc and tin in combination with static and torsional stresses on the spring wire. As a corrective measure, Na, Sn, and Zn which were present in pigmented oil used as a lubricant during spring winding was cleaned thoroughly by the spring manufacturer before shipment to remove all contaminants.

This case study demonstrates that Alloy 601 (UNS N06601) is susceptible to strain-age cracking. The observation illustrates the potential importance of post weld heat treatment to the successful utilization of this alloy in certain applications.

This article provides a background of friction-bearing failures due to overheating. The failures of locomotive axles caused by overheated traction-motor support bearings are discussed. The article also describes liquid-metal embrittlement (LME) in steel. It examines the results of various axle studies, with illustrations and concludes with information on the simulation of the LME mechanism.

This article discusses different types of mechanical fasteners, including threaded fasteners, rivets, blind fasteners, pin fasteners, special-purpose fasteners, and fasteners used with composite materials. It describes the origins and causes of fastener failures and with illustrative examples. Fatigue fracture in threaded fasteners and fretting in bolted machine parts are also discussed. The article provides a description of the different types of corrosion, such as atmospheric corrosion and liquid-immersion corrosion, in threaded fasteners. It also provides information on stress-corrosion cracking, hydrogen embrittlement, and liquid-metal embrittlement of bolts and nuts. The article explains the most commonly used protective metal coatings for ferrous metal fasteners. Zinc, cadmium, and aluminum are commonly used for such coatings. The article also illustrates the performance of the fasteners at elevated temperatures and concludes with a discussion on fastener failures in composites.

Several cases of embrittlement failure are analyzed, including liquid-metal embrittlement (LME) of an aluminum alloy pipe in a natural gas plant, solid metal-induced embrittlement (SMIE) of a brass valve in an aircraft engine oil cooler, LME of a cadmium-plated steel screw from a crashed helicopter, and LME of a steel gear by a copper alloy from an overheated bearing. The case histories illustrate how LME and SMIE failures can be diagnosed and distinguished from other failure modes, and shed light on the underlying causes of failure and how they might be prevented. The application of LME as a failure analysis tool is also discussed.

Parts of 21Cr-6Ni-9Mn stainless steel that had been forged at about 815 deg C (1500 deg F) were gas tungsten arc welded. During postweld inspection, cracks were found in the HAZs of the welds. Welding had been done using a copper fixture that contacted the steel in the area of the HAZ on each side of the weld but did not extend under the tungsten arc. In SEM examination, the cracks appeared to be intergranular and extended to a depth of approximately 1.3 mm (0.05 in.). The crack appearance suggested that the surface temperature of the HAZ could have melted a film of copper on the fixture surface and that this could have penetrated the stainless steel in the presence of tensile thermal-contraction stresses. The cracks in the weldments were a form of liquid-metal embrittlement caused by contact with superficially melted copper from the fixture and subsequent grain-boundary attack of the stainless steel in an area under residual tensile stress. The copper for the fixtures was replaced by aluminum. No further cracking was encountered.

The failure mode of through-wall cracking of a butt weld in a 5083-O aluminum alloy piping system in an ethylene plant was identified as mercury liquid metal embrittlement. As a result of this finding, 226 of the more than 400 butt welds in the system were ultrasonically inspected for cracking. One additional weld was found that had been degraded by mercury. A welding team experienced in repairing mercury contaminated piping was recruited to make the repairs. Corrective action included the installation of a sulfur-impregnated charcoal mercury-removal bed and replacement of the aluminum equipment that was in operation prior to the installation of the mercury-removal bed.

This article focuses on common failures of the components associated with the flow path of industrial gas turbines. Examples of steam turbine blade failures are also discussed, because these components share some similarities with gas turbine blading. Some of the analytical methods used in the laboratory portion of the failure investigation are mentioned in the failure examples. The topics covered are creep, localized overheating, thermal-mechanical fatigue, high-cycle fatigue, fretting wear, erosive wear, high-temperature oxidation, hot corrosion, liquid metal embrittlement, and manufacturing and repair deficiencies.

One tube in a watertube boiler developed leakage from a perforation. The external surface was covered with a dark deposit indicative of local fusion. Perforation resulted from the development of a crack from the internal surface. Microscopic examination revealed extensive intergranular penetration by molten copper. Particles of copper were seen in scale deposits on the bore of the tube. The tube in general showed a ferritic structure with partially spheroidized carbide. The fact that fusion of the copper had occurred indicated temperatures of 1100 deg C (2012 deg F) had been experienced locally, and the structural condition suggested that the tube in general had been heated at a lower temperature of the order of 600 deg C (1112 deg F) for some appreciable time. In this instance, overheating of the tube in the absence of the copper deposits may not have led to failure.

Penetration by molten copper occurred in the economizer of a large water-tube boiler. A cross section through a weld and the crack in the tube revealed a crack was an intergranular fissure. Small fissures of the same type also extended from its flanks. The main fissure was filled with an oxide scale in which were embedded particles having the appearance of metallic copper. It was concluded that the cracking that occurred at the time of re-welding was due to intergranular penetration by copper present in the deposit within the tubes, which had not been completely removed prior to welding. Subsequently, it was ascertained that trouble had been experienced with the centrifugal feed pumps, resulting in scuffing of some bronze rings. The presumption is that bronze particles had been carried in mechanical suspension in the feed water and deposited in the economizer tubes.

A steel bolt had been used to join the copper connecting strips between the poles of a 10-pole, series-connected, rotating field rotor of a synchronous motor. The exciting current was 155 amps. Failure of the bolt resulted in severe damage to the stator windings by the loosened ends of the strips. The bolt had fractured near the head, a location which probably coincided with the junction of the strips. A portion of the fracture surface was covered with copper that had been deposited in the molten state, while some was also present along the shank of the bolt, having apparently run in between the bolt and the hole in the strip. The bolt end adjacent to the fracture had been subjected to intense local heating. The extent of the grain-growth indicating that the temperature had been in the region of 1200 deg C (2192 deg F). When the temperature reached the melting-point of copper, 1083 deg C (1981 deg F), molten metal came into contact with the bolt, into which it penetrated along the grain boundaries, culminating in rupture.

Several ruptures took place in the front wall tubes of a water tube boiler. Some rupture samples showed ductile failure while others showed brittle failure. Specimens taken from the rupture where a thick edge had been produced, i.e., with little evidence of prior plastic deformation, showed a coarse microstructure indicative of gross overheating. The examination indicated that failure in the main resulted from gross overheating arising from water starvation as could have been due to a number of causes. The ruptures in some tubes were of the type commonly found in overheated tubes, the material being drawn out to a feather edge at the time of rupture. Other ruptures in the same and other tubes were of a more brittle type, this being associated with penetration of material by molten copper derived from scale.