Failures of Locomotive Axles

George F. Vander Voort, Carpenter Technology Corporation

FAILURES OF LOCOMOTIVE AXLES failed axle had tried to remove surface evidence Savannah and the Chicago & Northwestern caused by overheated traction-motor support of overheating. Also mentioned was earlier Railroads) with evidence of copper penetration. bearings are discussed in this article. These work that had been done on broken axles that The axle from the Macon, Dublin & Savannah failures are of interest because the analysis were overheated. In each of these cases, small Railroad had gross cracking with visible shows an example of what can be done when particles of bearing metal had penetrated the copper-colored material in the cracks. Spectro- the fracture face and origin are destroyed during axle in the overheated region. graphic analysis of samples from both axles the failure incident. In most failure analyses of In 1944, a review was published of railroad- containing copper penetration revealed that the broken components, it is generally assumed that car axle failures due to the absorption of molten major constituents were copper and lead with a conclusive results cannot be obtained if the copper (Ref 2). Most axle-journal failures oc minor amount of tin. These elements are the fracture and the fracture origin cannot be iden curred near the wheel hub, an area of high major constituents of the bronze friction bear tified and examined. In many failures this is stress and temperature. A broken axle was ing. The copper-penetration failure occurred in true. However, the failures described in this shown in which the fracture was not destroyed the following sequence: article possess some unique characteristics that after breakage had occurred. From the surface permit successful analysis despite the lack of a inward, the fracture surface was rough, indicat • The bearing surface was heated by friction preserved fracture face and origin. ing the depth of copper penetration. The central because of loss of lubrication Failures of locomotive axles due to over portion was smooth, indicative of a fatigue • The babbitt metal lining melts between heated friction bearings are rather common in fracture that ultimately led to failure. Color about 240 and 315 °C (465 and 600 °F) and the railroad industry and have been observed photomicrographs revealed a yellow grain- wets the surface, but penetration does not for more than 100 years. Because of this, such boundary copper phase. It was pointed out occur failures are usually diagnosed merely by visual that adequate lubrication is required to keep • The babbitt metal is displaced, possibly by inspection of the damage. Comprehensive me- the operating temperature of the contacting mechanical action or volatization tallographic studies, therefore, are not done and surface below the melting point of the bearing • The bronze backing is heated to its melting have not been accurately documented in the materials. point (900 to 925 °C, or 1650 to 1700 °F) open literature for many years. However, de Two reports were issued—one in 1947, the and penetrates the axle, causing failure tailed analysis of such failures reveals a number other in 1954—concerning copper-penetration of significant features. axle failures and the use of nondestructive In 1959, the New York Central Railroad testing to detect surface cracks in such failures Company studied copper penetration in over Background (Ref 3, 4). The 1947 paper (Ref 3) discusses heated journals (Ref 5). Two types of failures twist-off failures due to overheated bearings. were observed. The first type, referred to as a Friction bearings have been used for many This failure mode is referred to as a hot-box in burn-off, is indicative of a single continuous years and are perfectly adequate if they are railroad terminology. Almost all axle-journal heating to failure due to penetration of bearing lubricated. The bearing is essentially a bronze failures were claimed to be attributed to inter- metals into journals at elevated temperatures. cylinder lined with babbitt metal. Typically, the granular embrittlement of the steel by molten The second type, referred to as a cold break, bronze alloy composition is close to Cu-16Pb- brass or copper. The steel in contact with the also results from overheating, but is a two-stage 6Sn-3.5Zn, while the babbitt composition is bearing must be heated to a temperature above failure process. These fractures exhibit an outer usually Pb-3.5Sn-8-ll.5Sb. A window is cut the melting point of the brass journal bearing circumferential zone of irregular detail with into the bearing and packed with cotton waste, and must be under a stressed condition. Exper evidence of thermal checks and intergranular which trails down to an oil reservoir. Oil is iments were performed with 13-mm ('/2-in.) separation and an inner fracture zone typical of drawn up the wick during service to lubricate diam medium-carbon steel, loaded as cantilever a progressive-type fatigue fracture. In both the contacting surfaces. beams. Samples heated to 925 °C (1700 °F) ran cases, copper from the bronze-backed journal Friction-bearing failures due to overheating for hours without failure at 1750 rpm. How bearings was absorbed intergranularly into the have been examined by the metallurgist for ever, the instant they were wetted with molten hot steel journal. A surface analysis for copper many years. Perhaps the earliest example, doc brass, catastrophic failure occurred. These sam indicated that the copper content exceeded the umented in 1914, involved a failed Krupp ples were loaded above the yield point of the residual copper content of the steel to a depth of railroad axle (Ref 1). The study revealed a steel at 925 °C (1700 °F). Samples were also 1 mm (0.040 in.). A metallographic study rather complex crack pattern, evidence of ex treated in the same manner, but the load was confirmed the presence of grain-boundary cop posure to very high temperatures, and ruptures removed before complete rupture occurred. Mi per penetration in the affected surface layer. in the overheated region. Bronze bearing metal croscopic analysis showed that molten brass This work was subsequently published (Ref 6). was observed in the cracked surface region entered the steel in a narrow canyon at the An important source of information on located beneath the support bearing. The bear surface, then spread out in a delta pattern. copper-penetration failures is the reports of the ing metal was molten when it penetrated the The 1954 report (Ref 4) studied two failed Committee on Axle and Crank Pin Research, axle. In addition, the railroad that submitted the railroad axles (from the Macon, Dublin & formed at the 1949 annual meeting of the 716 / Manufactured Components and Assemblies

Fig. 1 Fracture surface at the drive-wheel side of axle 1611 The 1952 AAR proceedings discussed stress measurements made on a new 140- x 250-mm (5'/2- X 10-in.) standard black-collar freight- car axle fabricated from AAR M-126-49, grade F, steel (Ref 9). Strain gages were placed at five locations, and the journal loads were 69 to 138 MPa (10 to 20 ksi), with speeds from 65 to 135 km/h (40 to 84 mph). Results showed that the dynamic stresses on the journals were very low, in most cases less than one-half the level of stress at the wheel seat. In this meeting, there was considerable discussion on the reuse of overheated axles. It was thought that if the bearing lining were melted out but surface cracks were not observed, the axle could be safely returned to service. However, when cracks were detected, it was thought that the axle surface could not be turned down to remove the crack with out going below the minimum allow able diameter. The Pennsylvania Railroad favored the scrapping of overheated axles when the babbitt metal was melted out. Experience indicated that about 90% of the overheated axles contained cracks that could not be turned out within the minimum diameter tolerance permitted. The expense of turning down all overheated axles when only 10% could be salvaged was con cluded to be unjustifiable. The Southern Rail way reported that 85% of its broken axles had been turned down previously, indicating that they had been overheated in earlier service. Removal of the cracking was concluded to be insufficient to guarantee that the axle was safe for additional service. The Atcheson, Topeka & Santa Fe Railroad reported on tests conducted using 19- to 25-mm (3/4- to 1-in.) diam minia ture journals that were heated, loaded, and subjected to melted bearing metal. The tests resulted in the type of break that 99 out of 100 burn-off journals show, and 99% of the journals exhibited copper penetration. Association of American Railroads (AAR). A copper are in contact, the steel is affected by The 1952 proceedings (Ref 10) contained major problem faced by this group concerned the copper and breaks sharply without a additional information on the Laudig iron- the decision to scrap or to recondition over reduction in diameter. The Atcheson, Topeka backed journal bearing that was developed to heated axles. Overheating can result from loss & Santa Fe Railroad reported that when an axle overcome the copper-penetration problem en of lubrication of either friction or roller that is necked down and elongated is inspected, countered with bronze-backed bearings. The bearings and possibly from other mechanical the brass is usually broken, and most of the Laudig journal bearing was recommended as an problems. Because the number of overheated journal brass is usually intact in the box. alternative to the bronze-backed journal (AAR axles per year was considerable, outright In 1951 and in 1953, the Delaware, M-501-48). In 1950, six railroads applied 600 scrapping of all overheated axles represented a Lackawana & Western Railroad reported a test bearings successfully. significant expense. revealing test (Ref 7, 8). A non-copper- In the 1956 AAR proceedings (Ref 11), the In the 1951 proceedings of the AAR, two containing (cast iron) bearing that was not Baltimore & Ohio Railroad stated that at least types of overheating failures were described lubricated was placed on one side of the axle, three factors must exist to cause copper- (Ref 7). One type of burn-off failure exhibited while a lubricated copper-base bearing was put penetration fractures: a necked-down, elongated fracture that gener on the other side. The car (a hopper car with a • A bearing that contains a metal or metals ally displayed large surface ruptures. The 63 500-kg, or 140000-lb, load) was run for that will diffuse into the axle material in the diameter may be necked down to half its 108 km (67 miles) without burning off the grain boundaries original size before the axle twists off. Copper journal, although it was red hot. The lubrica • Heat of the proper range penetration is not observed in these failures. In tion was then removed from the bronze bearing • Stress the second type of overheating failure, the at the other end of the axle. The car traveled 55 fracture is flat across the section with no km (34 miles) before the journal broke under Wetting of the axle surface was considered to reduction in diameter. Copper penetration and the bronze bearing due to copper penetration be a possible fourth prerequisite. A comprehen thermal cracks are usually found to be present (Ref 7). The journal under the nonlubricated sive, cooperative study was suggested to solve in this type of failure. The Union Pacific cast iron bearing was still red hot, but had not the problem of copper-penetration fractures. Railroad reported that when hot steel and broken. The Chesapeake & Ohio Railroad reported on Failures of Locomotive Axles / 717 an investigation that began in 1952 to determine Fig. 2 Fracture surface at the commutator side of axle 1611 what could be done to reduce or eliminate wrecks and hot-boxes due to defective journals. The railroad developed a journal-inspection car that used ultrasonic inspection. During prelim inary testing of this device, a cracked axle was found under a loaded car that was ready to be dispatched. The axle was removed and broken open. The crack had penetrated about an inch from the surface of the journal around approx imately one-half of the axle circumference. Subsequent use of this inspection car showed that about one defective axle was found for every 580 cars inspected, based on inspection of about 70 000 cars. Testing of cars owned by other railroads revealed that about 40% of the axles tested were defective. As a result of axle failures due to copper penetration, the AAR conducted a study to de termine the effects of refined copper or alloyed copper on axles under hot-box conditions (Ref 12). The study attempted to answer the often raised question of whether the copper is a pre cursor of axle failure or is a post-fracture phe nomenon, that is, does the molten copper flow into the grain boundaries and cause failure or does the molten copper flow into stress cracks after failure. In these tests, 19-mm (3/4-in.)diam centerless-ground AISI 1045 carbon steel sam ples were left bare or wrapped with 0.13-mm (0.005-in.) thick copper foil and held in place with fine copper wire. In some tests, wrapping was done with 70Cu-30Zn. Specimens that frac tured in the presence of brass failed suddenly and had sharp fracture faces; that is, no neckdown resulted. Copper penetration was found in these samples. The temperature above which failures occurred decreased as the applied bending stress increased. As the temperature increased, the probability of fracture increased. Two distinct fracture modes were observed The failure of railroad axles due to the embrittlement of a-brass by mercury (Ref 19). in these tests. When the failed samples exhib overheating of bronze-backed bearings has Since then, LME has been identified in ited reduced diameters (necking), evidence of been well documented. It is well recognized numerous failure analyses that include some twisting and the presence of cavernous cracks that the steel used in these axles is sensitive to notable cases: damage to aircraft by liquid- and fissures were due to high-temperature fa liquid-metal embrittlement (LME) by molten gallium embrittlement of aluminum and the tigue in an oxidizing atmosphere. When a copper when the steel surface is heated above embrittlement of stainless steel piping by brittle failure occurred, that is, a sharp, well- the austenitizing temperature by frictional heat molten zinc. defined fracture, microscopic examination due to loss of lubrication. Stress is present in References 13 to 19 have helped to eliminate showed copper or brass in the grain boundaries. the axle at this location due to the weight of the some of the confusion in the literature regarding This type of fracture occurred at temperatures traction motor, but the stress level is less than LME. This confusion is partly due to the above the melting point of the penetrating one-half that at the gear seat. existence of several different forms of LME. metal. The penetration followed grain-bound Overheating of friction bearings results in Four distinct forms have been identified (Ref ary paths, as in stress-corrosion cracking LME and failure of the axle. Liquid-metal 13): (SCC). Branching of the penetration was found embrittlement is a complex phenomenon; the in the grain structure of several specimens. example described in this article is only one • Instantaneous fracture of a particular metal Copper penetration was as deep as 10 mm (0.4 form of LME. Because of the different forms of under an applied or residual tensile stress in.) below the surface. LME that can occur, a brief review of the when in contact with particular liquid met There is a considerable body of published nature of LME is presented. als. This is the most common type of LME information that shows that steels will fail by Liquid-metal embrittlement is a phe • Delayed failure of a particular metal in grain-boundary penetration of molten copper. nomenon in which the ductility or fracture contact with a specific liquid metal after a The following conditions are required: stress of a solid metal is reduced by exposure certain time interval at a static load below of the surface to a liquid metal (Ref 13). The the ultimate tensile stress of the metal. This • Presence of applied tensile stress subject has been extensively reviewed (Ref form of LME is related to grain-boundary • Wetting of the steel surface by molten cop 13-18), and specific aspects have been reported penetration by the liquid metal and is less per in numerous scientific papers. The first re common than the first form • Heating of the steel substrate to tempera corded recognition of the problem was made • Grain-boundary penetration of a particular tures high enough to austenitize the steel in 1914 by Huntington, who observed the solid metal by a particular liquid metal such 718 / Manufactured Components and Assemblies

Fig. 3 Fracture surface at the commutator side of axle 2028 Fig. 4 Hot acid etched longitudinal centerline section from the drive- wheel side of axle 1611

Fig. 5 Hot acid etched longitudinal centerline section from the commutator side of axle 1611

that the solid metal eventually disintegrates. • Formation of intermetallic compounds at the A large number of metals and alloys can fail Stress is not a prerequisite of this form of liquid/solid interface in this manner. Only specific liquid metals are LME in all observed cases • Intergranular penetration of the liquid metal known to embrittle each specific metal or alloy. • High-temperature corrosion of a solid metal into the solid metal without the presence of Indeed, this is one of the more perplexing by a liquid metal, causing embrittlement. applied or residual stress aspects of LME, and some investigators refer to This problem is entirely different from the • Brittle, premature failure of the solid metal this as specificity. Many of the studies of LME others due to intergranular penetration of the liquid couples have involved tests consisting of only metal into the solid metal under the influence one temperature and fixed conditions regarding In some respects, LME is similar to SCC. of applied or residual tensile stress. Failure stress, grain size, and so on (Ref 19). Until all However, SCC always requires a measurable occurs after a finite incubation period possible couples have been thoroughly studied incubation period before fracture occurs. • Brittle, instantaneous failure of a stressed using a wider range of test conditions, the Hence, SCC is more similar to the second form metal when a particular liquid metal is specificity feature is of questionable validity of LME than to any of the other types. Like applied. Grain-boundary penetration is not (Ref 20). In addition, the LME tests have SCC, LME is a rather perplexing subject since necessarily observed generally been "go, no-go" type tests rather embrittlement is restricted to only certain com than quantitative measurements of the degree of binations of solid and liquid metals. embrittlement. The following observations have been made The last three types of interactions are the The following conditions are required but are regarding liquid metal/solid metal interactions prime forms of LME. Most studies of LME not necessarily sufficient to cause embrittle (Ref 15): discuss the last type, the instantaneous form ment (Ref 13): of embrittlement, which is the most common • No apparent interaction form. Several authors refer to this form as • Wetting of the solid metal substrate by a • Simple dissolution of the solid metal into the adsorption-induced embrittlement. Other au liquid metal must be adequate liquid metal thors prefer to consider LME as merely a • An applied or residual tensile stress must be • Simple diffusion of the liquid metal into the special type of brittle fracture and view the present in the solid metal solid metal subject from a fracture mechanics approach. • A barrier to plastic flow must exist in the Failures of Locomotive Axles / 719

Fig. 6 Hot acid etched longitudinal Fig. 7 Hot acid etched longitudinal rapid penetration of a particular liquid metal centerline section from the drive- centerline section from the along grain boundaries. Examples of this form of LME are much less common and not as well wheel side of axle 2028 commutator side of axle 2028 studied as the previously described form. Known examples include the wetting of alumi num by gallium (Ref 15), carbon steel by / ' copper (Ref 15), and copper by bismuth (Ref ' W ' 21). Failures Caused by LME of Steel. The first reported case of LME of steel, in this in stance by molten brass, was documented in 1927 (Ref 22). In 1931, tests were conducted on the influence of certain liquid metals on plain carbon steel, silicon steel, and chromium steel (Ref 23). These steels were embrittled at 1000 to 1200 °C (1830 to 2190 °F) by liquid tin, zinc, antimony, copper, 5% tin-bronze, and 10% zinc-brass. Liquid bismuth, cadmium, lead, and silver caused little or no embrittlement. In 1935, the bend strength of steel coated with a lead-tin solder at 250 °C (480 °F) and with a bearing metal at 350 °C (660 °F) was determined (Ref 24). Penetration of the liquid metals was ob Fig. 8 Macrographs of two polished sections from the failed axles served on the tension side only of the bend spec (a) Axle 1611. (b) Axle 2028. Each was positioned along the fracture at the outside-diameter surface. imens. In a similar study (Ref 25), the influence of liquid zinc at 425 to 500 °C (795 to 930 °F) on various steels was tested using bend, tensile, and creep-rupture specimens. Grain-boundary penetration of the zinc into the steels was ob served. Another study investigated intercrystal- line failures of a nickel-chromium steel airplane fracture axle (Ref 26). The fracture occurred in the mid dle of the tubular axle where a brass number Fracture plate was attached by soft solder. When 50Pb- 50Sn solder was applied to stressed rings cut (b) from the axle, the specimen ruptured rapidly. The intergranular crack paths exhibited some solder penetration. The same results were ob served when liquid tin, lead, zinc, cadmium, or solid metal at some point in contact with the • Thermal-mechanical history of the solid Lipowitz alloy (a tin-lead-bismuth-cadmium al liquid metal metal loy sometimes containing indium) was used. Obviously, for failure to occur the liquid Studies of specific LME couples have empir In 1968, studies were conducted on the metal must be of the correct composition to ically identified certain trends that are usually, influence of cold work on the LME of pure iron cause LME of the particular solid metal. but not always, obeyed. For example, most and Fe-2Si (Ref 27). Sheet samples were tested If the solid metal being embrittled is not embrittlement couples have very little mutual in the annealed state and after 10, 25, 50, and notch sensitive, as in the case of face-centered solid solubility. Solid metals that are highly 75% reduction in thickness. The degree of cubic (fee) metals, the crack will propagate soluble in the liquid metal and solid/liquid embrittlement increased with a small amount of only when the liquid metal feeds the crack. In a metal combinations that form intermetallic cold work, but further working reduced the notch-sensitive metal—for example, in a body- compounds are usually immune to LME. These degree of embrittlement. For pure iron, maxi centered cubic (bec) metal, such as iron—the studies have also demonstrated that good wet mum embrittlement occurred with the sample nucleated crack may become unstable and prop ting of the substrate is required in LME. Inter reduced 10% in thickness, while LME did not agate ahead of the liquid metal. estingly, the factors that promote the lowest occur with the sample reduced 75%. For the In cases of LME, crack-propagation rates can interfacial energy and therefore the best wetting Fe-2Si alloy, maximum embrittlement occurred be extremely rapid. Indeed, crack-propagation are high mutual solid solubility or intermetallic- with 25% reduction in thickness, and LME did rates between 50 and 500 cm/s2 (20 and 200 compound formation, both of which are gener not occur for either 50 or 75% reduction. The in./s2) have been measured. In most cases, the ally related to a low degree of LME. lack of LME at high levels of cold work was crack paths are intergranular; however, in The presence of notches or stress concentra attributed to the fragmentation of the grain stances of transgranular crack paths have been tors increases the severity of LME, and many boundaries. observed. current investigators have used a fracture me From 1970 to 1974, a series of studies was Factors influencing the fracture stress and chanics approach to understand LME. Since performed on the LME of iron (Ref 28-31). ductility of metals subject to LME include: stress raisers are known to influence brittle These tests were made under nonoxidizing failure detrimentally under ordinary (dry) situ conditions using notched tensile-creep test • Composition of the solid metal ations, the effect on LME is understandable. specimens. A thin copper coating (8.5 [xm • Composition of the liquid metal In the previously discussed adsorption- thick) was plated onto the notched surface using • Temperature induced mode of LME, grain-boundary pene a cyanide bath. The amount of copper was • Strain rate tration is not a prerequisite. There are a few increased by wrapping pure copper wire (0.18 • Grain size well-known cases in which LME occurs by the mm, or 0.007 in., in diameter) around the 720 / Manufactured Components and Assemblies

Fig. 9 X-ray elemental dot maps for copper and tin taken at three typical areas exhibiting penetration of the bearing elements The three regions shown in the specimen current images (left) were quantitatively analyzed for copper and tin; results are given under the concentration maps. All 320 X

Specimen current 87.1% Cu 10.7% Sn notch. Testing was done at 1100 and 1130 °C surface cracks. The depth of surface cracking angle between liquid copper and steel revealed (2010 and 2065 °F) under stresses of 8.3 and 11 was found to be controlled by the depth of similar results: 35° at 1100 °C (2010 °F) and 28° MPa (1.2 and 1.6 ksi). Copper significantly copper penetration. Dihedral-angle measure at 1130 °C (2065 °F). altered the creep behavior of pure iron and ments (Ref 30) for liquid copper in steel at Five earlier studies regarding the dihedral caused premature failure. An increase in ap 1100 and 1130 °C (2010 and 2065 °F) revealed angle are significant. In the first, the dihedral plied stress or temperature reduced the time for that the most frequent angle was 34° for both angle of copper in steel was measured at 30° LME failure. Embrittlement was of the delayed temperatures. The liquid copper/austenite in- (Ref 32). In the second, a minimum value was type and occurred by diffusion-controlled terfacial energy was calculated as 444 ergs/cm2 observed for the dihedral angle in the region of grain-boundary penetration of copper. The liq at 1100 and 1130 °C (2010 and 2065 °F). the melting point of copper (Ref 33). The third uid copper appeared to be ahead of the growing Sessile drop measurements of the contact study claimed that stress had little effect on the Failures of Locomotive Axles / 721

Fig. 10 Structure of a specimen cut dihedral angle; however, stress was observed to Fig. 11 Structure of a specimen cut from the area near the center of promote spreading of the liquid metal along the from the area near the center of the fracture of axle 1611 grain boundaries (Ref 34). The fourth study the fracture of axle 2028 maintained that the dihedral angle decreased (a) Macrograph of specimen. Actual size, (b) Montage (a) Macrograph of specimen. Actual size, (b) Montage showing the microstructure from the fracture surface with increasing applied stress (Ref 35). In the showing the microstructure in regions 1 and 2. inward to just above region 3. 9X last study, it was found that in the presence of molten copper, the surface energy that must be applied to drive a crack along austenite grain boundaries is reduced by about a factor of 100, and the corresponding stress required is about one-tenth that required without the presence of copper (Ref 36). There have been numerous studies of LME by copper occurring during welding and joining processes applied to steel. One such study involved the influence of copper deposition by welding of austenitic and ferritic stainless steels (Ref 37). In stainless steels with mixed ferritic- austenitic structures, the presence of the stable ferrite phase in austenite reduced the penetra tion of copper. When the stable ferrite content was greater than 30%, penetration of copper was not observed. When a wholly ferritic stain less steel was deposited with copper, cracking did not occur. The wettability of molten copper on austenitic and ferritic stainless steel was measured. The contact angle was 92 to 100° for the ferritic stainless (no wetting) and 22 to 28° for the austenitic stainless steel (wetting) at 1100 °C (2010 °F). A series of reports has been published re garding the embrittlement of iron-base alloys by copper during welding (Ref 38-41). In a study of HY-80 steel welded with or without copper-nickel filler metal (Ref 38), infiltration of the grain boundaries by the copper-nickel deposit under an applied strain field was ob served. The attack occurred in the heat-affected zone (HAZ) while the steel in this area was austenitic. In heated areas that remained bcc, penetration did not occur. In another study involving copper deposition using a gas metal welding arc (Ref 39), alloy steels, such as AISI 4340 and 4140, and type 304 stainless steel were found to be extremely sensitive to copper penetration. In comparison, the penetration depths of AISI 1340, 1050, Armco iron, and carburized Armco iron were only about one-third those of the heat-treated alloy steels. A ferritic stainless steel, AISI 430, was almost completely immune to copper penetration. When stresses are absent, the ease of penetration by molten copper was concluded to be a function of the alloy content of the steel. When grain-boundary copper penetration oc curs under an applied stress, partially filled or open cracks are formed in the steel. Notch- sensitive heat-treated steels lose much of their strength and ductility when copper penetration occurs. Steels that remain ferritic at the melting point of copper do not lose strength as a result of exposure to molten copper. In a subsequent study (Ref 41), the Gleeble high strain rate hot-tensile test machine was used to determine the influence of temperature, atmosphere, stress, grain size, strain rate, and amount of copper on LME of iron- and cobalt- 722 / Manufactured Components and Assemblies

Fig. 12 Three views of the Fig. 13 Four views of the microstructure of axle 2028 microstructure of axle 1611 The specimens were taken from the area near the center at different distances from the fracture face, (a) At the The specimens were taken from the area near the fracture surface, (b) At the boundary between the edge structure and the heat-affected structure, (c) At the heat- center at different distances from the fracture face, affected area. Arrows in (b) and (c) show sulfide inclusions, (d) 25 mm (1 in.) from the fracture surface (a) Near the fracture surface, (b) 6.4 mm (0.25 in.) from the fracture face, (c) 38 mm (1.5 in.) from the fracture face. The longitudinal axle direction is vertically oriented in each micrograph. All 500 X

base superalloys. A copper contamination of low, the concentration of copper appears as only 0.08-mm (0.003-in.) thickness was found a layer of nearly pure copper between the to be sufficient to cause hot cracking. Suscep metal and the scale tibility to LME decreased with increasing tem • If the rolling or forging temperature is above perature above some transition temperature in the melting point of copper, the molten the HAZ. The intergranular LME cracks were copper on the surface of the steel will oriented perpendicular to the direction of the penetrate the steel along the grain bound principal stress. The hot cracks grew at high aries velocities after a critical amount of plastic strain was introduced in the HAZ. Elements such as tin, arsenic, and antimony decrease the melting point of copper and in It is well recognized that copper, as well as crease the sensitivity of steel to surface crack certain other elements in steel, can detrimen ing. Tin reduces the solubility of copper in tally influence hot workability (Ref 42-46). austenite by a factor as high as three (Ref 43). These studies have generally employed hot- Hence, tin additions can cause the precipitation torsion, bend, or cupping tests to assess the of a molten copper phase at a much lower level influence of the concentration of certain ele of copper enrichment than in the absence of tin. ments on hot workability. Much of the early It is quite clear that molten copper, or copper work on the influence of copper on hot work alloys, will embrittle steels under the proper ability can be summarized (Ref 42): conditions. The steel must be austenitic with • Because the solubility of copper in iron is tensile stresses present. Carbon steels contact- Failures of Locomotive Axles / 723

Fig. 14 Average angle of sulfide inclusions relative to the longitudinal ing liquid copper will be heated into the fully axle axis as a function of distance from the fracture face austenitic range. Liquid copper will penetrate Specimen was taken from the area near the centerline at the fracture face. Compare to the qualitative examples the austenitic grain boundaries. The speed of in Fig. 12 and 13. penetration depends on the magnitude of the applied tensile stresses. The depth of cracking Distance from fracture face, in. depends on the depth of copper penetration. 0.3 0.5 0.7 0.9 1.3 Thus, if the elements present in the friction- bearing sleeve are observed in the prior- austenite grain boundaries in failed axles, it can be safely concluded that the axle was intact and under normal loading when the bearing over heated and that frictional heating due to loss of lubrication heated the axle and the bearing to a temperature at which the axle surface was austenitic and the bearing surface was molten. If the axle was broken before the bearing overheated, it would not be stressed in tension at the failure location, and penetration of bear ing elements would not occur, irrespective of the crystallographic structure of the axle.

Results of Axle Studies Three axles were examined. Two, coded 1611 and 2028, were provided by the Seaboard 12 16 20 24 28 Coast Line Railroad. Visual examination of the Distance from fracture face, mm failures indicated that overheating of the traction-motor support bearings, caused pene tration of the bronze bearing material and fail Fig. 15 Examples of the normal microstructure of the axles ure. The third axle was the cause of a train Specimens were taken well away from the fracture surface, (a) Axle 2028. Etched with 2% nital. 100 X . derailment (Ref 47); a limited study was per (b) Axle 2028. Etched with 4% picral. 500 X . (c) Axle 1611. Same etchant and magnification as (a), (d) Axle formed on this specimen. 1611. Same etchant and magnification as (b) Views of the fracture faces of axles 1611 and 2028 are shown in Fig. 1 to 3. Both axles exhibit little, if any, necking in agreement with the general fractures of such failures. Axle 1611 was used in a yard engine, and little damage was done to the fracture after rupture. Axle 2028 experienced considerable deforma tion after fracture. Figures 1 and 2 of axle 1611 show that the outer portion of the fracture face experienced considerable tearing and rupture, while the central portion exhibits hot torsional rupture (note the swirl pattern of the fracture). This suggests that LME occurred to a depth of 4«$ nearly 25 mm (1 in.). The axle, which was heated to at least 925 °C (1700 °F), then ruptured to failure by twisting under the applied loads. The fracture features of axle 2028 are less distinct because of the rubbing action after fracture (Fig. 3). The outside-diameter surface of each of these axles in the area in contact with the traction-motor support bearing exhibited scale, and some of the friction-bearing material was present, as identified by x-ray fluorescence and diffraction. Figures 4 to 7 show longitudinal sections cut from each side of the axle fractures after hot- acid etching (1:1 HC1 in water at 70 °C, or 160 °F). This etching revealed a number of second ary cracks propagating from the outside- diameter surface inward along the portion of the axle that was in contact with the overheated bearing. The regions near the centerline of each axle behind the fracture face reveal evidence of the high temperatures and the metal flow from twisting to rupture. 724 / Manufactured Components and Assemblies

Table 1 Chemical analysis of axles 1611 and 2028(a)

Axle C(b) Mn(c) P(d) S(b) Si(el Ni(f» Cr(f) Mo(f) Sn(g) Cu(h) V(f) Nb(g) Ti(g) N(j) O(k)

2028 . . .0.57 0.70 0.012 0.042 0.16 0.01 0.08 0.004 0.004 0.002 0.057 0.003 0.01 0.007 0.0045 0.0057 1611 . . .0.54 0.79 0.012 0.026 0.19 0.01 0.09 0.005 0.004 0.002 0.023 0.003 0.01 0.007 0.0058 0.0056 M-126-F .0.45-0.59 0.60-0.90 0.045 0.050 >0.15 (a) Weight percent, (b) Analysis by high-temperature combustion; infrared detection, (c) Peroxydisulfate-arsenite titrimetric analysis, (d) Alkalimetric analysis, (e) Gravimetric analysis. (0 Atomic absorption spectro metry' analysis, (g) Plate spectrographs analysis, (h) Neocuproine photometric analysis, (j) Analysis by inert gas fusion; thermal conductivity detection, (k) Vacuum fusion analysis

Examination of the microstructure along the Fig. 16 Optical micrograph of Figure 13 shows similar micrographs from surface of the axle from the fracture face copper penetration (arrows) near axle 2028 at regions 1 to 4 shown in Fig. 11. outward reveals extensive penetration of the the outside-diameter surface of the The microstructure in region D shows that the bearing elements. Macrographs of two of the third axle temperature was in the correct range to microsamples are shown in Fig. 8, which illus spheroidize the pearlitic structure to some Etched with 2% nital. 55 X trates some of the gross crack patterns. Figure 9 extent (near the lower critical temperature). shows electron microprobe views of three typ This same type of behavior was observed in ical regions from axle 1611. Specimen current samples from the third axle studied. Figure 14 images and scans for copper and tin are illus shows measurements of the average angle of trated. The three indicated areas were analyzed the sulfides with respect to the longitudinal quantitatively; results appear under the appro axis as a function of the distance from the priate dot maps. Minor amounts of lead, zinc, fracture face. and antimony were also detected, but were not Table 1 shows the results of chemical anal analyzed quantitatively. ysis of axles 1611 and 2028 taken in unaffected Figure 10 shows an etched section from the regions compared with the specified range for central region of axle 1611 and a montage of M-126, grade F, axle steel. Figure 15 shows the the microstructure. The temperature in regions normal microstructure of axles 1611 and 2028, 1 and 2 was high enough to reaustenitize the taken well away from the affected regions. steel. Region 3 was heated into the two-phase These axles are double normalized and tem region, while the lower portion of the sample, pered. The microstructure is fine lamellar below region 3, was not heated above the lower pearlite with grain-boundary ferrite. critical temperature. The montage shows that Penetration of bearing elements in the third the original longitudinal fiber of the forged axis axle is shown in Fig. 16. Color metallography was removed by the high temperature. The reveals the characteristic copper color of the twisting action produced a metal-flow pattern penetrating material. Figure 17 shows scanning perpendicular to the original hot-working axis electron microscope views of the area shown in and subsequent rupture. Around the larger Fig. 16, further confirming the presence of crack, recrystallization of the grain structure vertical in each illustration. At a distance of 38 copper. More extensive elemental mapping of occurred. mm (l'/2 in.) from the fracture face, the sulfide penetrating bearing material is shown in Fig. Figure 11 shows a similar microsample cut inclusions are parallel to the hot-working axis, 18. Again, all of the typical bearing elements from along the centerline of axle 2028 at the as normally encountered. However, at 6.4 mm are observed. fracture face. The microstructure demonstrates (0.25 in.) from the fracture, the sulfides are perpendicular to the longitudinal axis. In the influence of the very high temperature and Simulation of torsional rupture metal flow. addition, they are much smaller, and many fine The influence of the high temperatures near globular sulfides are observed. This indicates LME Mechanism the fracture and the rotational forces on axle that the temperature was high enough at this To provide additional information on the 1611 are further illustrated in Fig. 12, which location to dissolve most of the sulfides, which embrittlement mechanism, laboratory tests shows views of the microstructure taken at precipitated upon cooling as small globules. were conducted using material from one of the three locations with respect to the fracture Near the edge of the fracture, only very fine axles. Tensile specimens measuring 12.8 mm along the centerline. All are oriented in the spherical sulfides are observed, and consider (0.505 in.) in diameter X 250 mm (10 in.) long same manner; that is, the longitudinal axis is able decarburization is evident. were prepared. A 60° V-notch was machined

Fig. 17 SEM micrographs of the area from the third axle shown in Fig. 16 (a) Secondary electron image, (b) Backscattered electron image, (c) Copper x-ray dot map. All 55 X Failures of Locomotive Axles / 725

Fig. 18 X-ray elemental composition maps made with the electron microprobe using a region of copper penetration shown in Fig. 16 and 17 All 270 X

Copper Antimony into the periphery at midlength of each sample. The tests showed, however, that fracture fracture pattern is indicative of LME. Two The diameter at the root of the notch was 9.1 occurred after 18 s when a 129-kg (285-lb) load views of the microstructure of a partially bro mm (0.357 in.). The notch surface, as well as was applied; this corresponds to a crack-growth ken specimen tested at 65-kg (143-lb) load some of the bar surface on each side, was rate of 907 mm/h (35.7 in./h). At a load of (25% of normal tensile strength) are also electroplated with copper. The V-notch region 97 kg (214 lb, or 37% of the normal tensile shown. The white grain-boundary films are was then wound with fine copper wire. One strength), complete rupture occurred in 24 s; copper, showing the intergranular nature of the tensile specimen was left bare to determine the this is a crack-growth rate of 680 mm/h penetration, also shown by the crack path. tensile strength of the steel at 1100 °C (2010 °F) (26.8 in./h). At 30% of the normal tensile For comparison, a similar copper-filled without the influence of copper. All tests were strength (78 kg, or 171 lb), rupture occurred in V-notch specimen was heated to 1100 °C (2010 performed using a Gleeble. The uncoated, 79 s. A number of tests were conducted at 32 kg CF) in a laboratory furnace without any applied notched sample was heated to 1100 °C (71 lb) and 65 kg (143 lb). These samples were load. Figure 22 shows that no cracking oc (20!0 °F) and pulled in tension with a cross- held for different lengths of time, cooled to curred at the root of the V-notch. The high- head speed of 2.5 mm/s (6 in./min). A maxi room temperature, sectioned, and examined to magnification micrograph shows some dark- mum load of 259 kg (570 lb) was observed; that measure the crack depths. All of the test data are etching grain boundaries at the extreme surface is, the ultimate tensile strength was 39 MPa plotted in Fig. 19, which reveals a consistent where copper diffused into the specimen. As (5.7 ksi). relationship between the test load and crack- predicted by the literature, no evidence of LME Samples with copper in the notch were then growth rate. was detected because there was no applied heated to 1100 °C (2010 °F), high enough to The microstructure of the sample that was stress. melt pure copper, and held at constant loads of not coated with copper when tested at 1100 °C 32, 65, 78, 97, and 129 kg (71, 143, 171,214, (2010 °F) is shown in Fig. 20. The spherical and 285 lb), that is, 12.5, 25, 30, 37, and 50% cavities found at the grain boundaries are indic Conclusions of the 1100 °C (2010 °F) tensile strength with ative of ductile overload rupture. Figure 21 The carbon steel railroad axles examined out copper. If the liquid copper had no influ shows examples of the fracture surface of a failed because of LME due to friction-bearing ence on the steel, the tensile samples should specimen tested to rupture when copper was overheating. After the babbitt metal had melted have stayed intact for a long time. present at 1100 °C (2010 °F). The intergranular away, the bronze sleeve rubbed against the steel 726 / Manufactured Components and Assemblies

Fig. 19 Graph showing the agated to final rupture by torsion. This is Fig. 21 Examples of axle steel influence of load on the LME crack- consistent with the observed depth of copper tensile specimens tested at 1100 °C growth rate for the experimental penetration and the observed metal flow in the (2010 °F) with the Gleeble, with conditions described in text central region of the axle. copper present Although the fracture face and origins were Copper-free tensile strength at 1100 °C (2010 °F), % The liquid copper has penetrated prior-austenite grain destroyed, there was evidence of LME along boundaries, producing intergranular fracture, (a) the entire surface in contact with the bearing Bottom of notch of a specimen held for 10 min at ! I surface. Fracture occurred in the axle under the temperature at 65 kg, or 143 lb, load (25% of the % approximate center of the bearing, where tem normal tensile strength). 70 X . (b) Below crack in the peratures and stresses were highest. same specimen as (a). 70 X . (c) Fracture surface of a _ specimen tested to rupture. 35 X \ REFERENCES \ 1. B. Straub, Mikroskopische Stahlunter- suchung, Stahl Eisen, Vol 34 (No. 50), - Dec 1914, p 1814-1820 2. F.H. Williams, Car Axle Failures Traced \ to Absorption of Bearing Metal, Prod. °\ Eng., Vol 15, Aug 1944, p 505-508 \ 3. "The Mechanism of Copper Penetration 0 _ Failure," Magnaflux Corp., Chicago, 1947 \ 4. "Copper Penetration—New and Revealing \ Information Concerning the Analysis of the Penetrating Material Itself," Research - Laboratory Technical Memo No. 220, . (1C Project 125, Magnaflux Corp., Chicago, psi) I i) uni. ps Feb 1954 1000 500 200 100 50 20 10 5 5. E.A. Unstich and E.J. Palinkas, "A Study of Copper Penetration in Overheated Jour Load, lb nals," Report 69222, New York Central Railroad Company, New York, July 1959 Fig. 20 Micrograph of the 6. New York Central Research Shows . . . Turning Removes Copper Penetration, fractured end of the specimen Rail. Locom. Cars, Feb 1961, p 34-35 tested with the Gleeble at 1T 00 °C 7. Report of the Committee on Axle and (2010 °F) without copper present Crank Pin Research, in Proceedings of the The fracture is transgranular. Etched with 4% picral. Association of American Railroads, Me 55 X chanical Division, 1951, p 158-166 8. J.J. Laudig, Hot-Box Research Re sults ... in the Iron-Back Journal Bear ing, Rail. Age, Vol 134 (No. 12), March 1953, p 65-69 9. Report of the Committee on Axle and Crank Pin Research, in Proceedings of the Association of American Railroads, Me chanical Division, 1952, p 116-129 10. Laudig Iron Back Journal Bearing, in Pro ceedings of the Association of American Railroads, Mechanical Division, 1952, p 358-359 11. Report of Committee on Axles, in Pro ceedings of the Association of American surface. Both overheated because of the fac Railroads, Mechanical Division, 1956, p tional stresses, to a point where the bearing 94-104 melted and the contacting steel surface became 12. "The Failure of Railroad Axle-Journal austenitic. In this region the axle is under stress Steel Including the Effects of Copper- because of the weight of the traction motor; Bearing Metals in Contact with Stressed hence, the liquid bearing material penetrated Steel Rods at Elevated Temperatures," the axle. Further heating of the axle in the Report MR-434, Association of American contact region continued while the copper pen Railroads, Research Department, Wash etrated the axle. Cracking around the periphery ington, Dec 1963 reduced the effective cross-sectional area of the 13. N.S. Stoloff, Liquid Metal Embrittlement, axle. Because the axle was quite hot, its in Surfaces and Interfaces, II, Proceedings strength decreased substantially, and twisting of 14th Sagamore Army Materials Re began under the action of the rotational forces. search Conference, Syracuse University Thus, fracture was initiated by LME and prop Press, Syracuse, NY, 1968, p 157-182 Failures of Locomotive Axles / 727

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