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Metal Failure Analysis
Lab Tests to Identify the Causes of Metal Failure |
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| BACKGROUND |
Four (4) fractured shoe samples were submitted to our laboratory for a metal failure analysis. As metallurgical experts in the railroad industry we were requested to visually identify the fracture origin and characteristics. We were also requested to perform a full metallurgical evaluation of the case and core regions including determination of the effective case depth for the four samples. |
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| SAMPLE IDENTIFICATION |
| Specimen No. |
Sample Description |
| 1 |
2-98 28822E MF MGE |
| 2 |
9-92 28622C L |
| 3 |
9-92 28622C L |
| 4 |
12-93 28822C MF AVD |
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| PERFORMED TESTING |
Visual & Stereoscopic Examination
Metallographic (Case Depth & Microstructure) Examination
Microhardness Testing
Chemical Analysis |
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| COMMENTS |
1. Based upon the results of the performed tests and examinations, our metallurgical expert opinion identified the fractures originated from the flat surface where overheating occurred due to abrasive or frictional contact. The overheated surface microstructure and associated secondary cracks served as severe stress risers and contributed to the brittle fractures of the shoe.
2. The case microstructure and total case depth of each sample conformed to the requirements of MES 1.3K. The case carbon content in Samples 2 and 4 conformed to the required range of .75/.90%. However, the case carbon in Samples 1 and 3 were below the minimum requirement of .75%. All four samples conform to the .030” minimum effective case depth requirement.
3. No evidence was observed of a pre-existing crack or material defect that could have contributed to the initiation of the fractures. |
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| SUMMARY of TEST RESULTS |
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| Visual and Stereoscopic Examination |
1. The initial step of the metal failure analysis evaluation is a visual examination to identify the fracture features and identify the initiation site for further evaluation. The examination revealed all the failures originated from the flat surface (denoted by the arrows in Photo 1). The flat surfaces exhibited some heat discoloration, which indicated severe frictional wear and possible overheating
2. No evidence was observed of a pre-existing crack or material defect that could have contributed to the initiation of the fractures. |
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| Photo 1: Submitted fractured shoe samples.
(origin at arrow)
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| Metallographic Examination |
1. A transverse section was removed from each sample through the origin for a microstructure evaluation. Examination of Samples 1 - 4 on the flat surface adjacent to the origin revealed a total case depth of .074”, .060”, .060” and .066”, respectively, which conformed to the requirement of .060/.080”.
2. The examination also revealed the fractures initiated from localized surface metal flow consisting of untempered martensite (see Photo 2 on the following page). As metallurgical experts it was apparent that the localized overheating occurred at the shoe surface from abrasive or frictional contact. The overheated microstructure was observed to depths ranging .001 to .003”. Secondary cracks were also observed within the overheated surface layer. The overheated surface microstructure and secondary cracks served as severe stress risers and contributed to the brittle fracture of the shoe.
3. The case microstructure consisted of primarily tempered martensite with less than 5% bainite (see Photos 2 – 5). No significant evidence was observed of retained austenite, intergranular surface oxidation, or grain boundary cementite networks.
4. The core of each sample exhibited a mixed microstructure that consisted of tempered martensite, bainite, and some acicular ferrite, typical of a carburizing alloy steel grade in the hardened (quenched and tempered) condition. Sample 3 also showed a coarse grain core microstructure (see Photo 6). Samples 1, 2 and 4 showed fine grain core microstructures. |
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| Photo 2: Mag: 500X; Etchant: 3% Nital
Sample 1 – Surface/Case Microstructure |
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| Photo 4: Mag: 500X; Etchant: 3% Nital
Sample 3 – Case Microstructure |
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| Photo 6: Mag: 50X; Etchant: 3% Nital Sample 3 – Core Microstructure
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Photo 3: Mag: 500X; Etchant: 3% Nital
Sample 2 – Case Microstructure
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Photo 5:Mag: 500X; Etchant: 3% Nital
Sample 4 – Case Microstructure
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| Microhardness Testing |
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1. Microhardness testing was performed on all four samples on the flat face directly adjacent to the fracture origin to determine the effective case depth and hardness profile.
2. The results are shown in Table 1 attached.
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| Chemical Testing |
1. Chemical testing identified the samples as 8630 steel. The case carbon content in Samples 2 and 4 conformed to the required range of .75/.90%. However, the case carbon in Samples 1 and 3 were below the minimum requirement of .75%.
2. The results are shown in Table 2 attached.
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| JJF/tt E-document |
Respectfully Submitted,
MSi Testing & Engineering, Inc.
John J. Fruscione
John J. Fruscione
Senior Metallurgical Engineer
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| Table 1 – Microhardness Testing* |
| Depth Below Surface, in. |
1 |
2 |
3 |
4 |
| .002 |
41 |
52 |
53 |
59 |
| .005 |
51 |
54 |
55 |
54 |
| .010 |
52 |
54 |
54 |
54 |
| .020 |
52 |
51 |
54 |
52 |
| .030 |
52 |
51 |
53 |
50 |
| .031 |
-- |
-- |
-- |
48 |
| .032 |
-- |
-- |
-- |
**50** |
| .033 |
-- |
-- |
-- |
49 |
| .040 |
50 |
50 |
52 |
46 |
| .041 |
51 |
51 |
-- |
41 |
| .042 |
51 |
50 |
-- |
-- |
| .043 |
50 |
**50** |
-- |
-- |
| .044 |
**50** |
49 |
-- |
-- |
| .045 |
49 |
-- |
-- |
-- |
| .050 |
48 |
47 |
50 |
-- |
| .051 |
-- |
-- |
50 |
-- |
| .052 |
-- |
-- |
49 |
-- |
| .053 |
-- |
-- |
50 |
-- |
| .054 |
-- |
-- |
50 |
-- |
| .055 |
-- |
-- |
**50** |
-- |
| .056 |
-- |
-- |
48 |
-- |
| Core |
34 |
42 |
41 |
32 |
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* Microhardness testing performed using a Knoop diamond indenter and 500 gram load per ASTM E384.
Microhardness data converted to HRC values using ASTM E140.
** Effective Case Depth at 50 HRC.
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| Table 2 – Chemical Testing* |
| Element |
1 |
2 |
3 |
4 |
| Carbon (Case) |
.57 % |
.79 % |
.69 % |
.79 % |
| Carbon (Core) |
.29 |
.30 |
.29 |
.30 |
| Manganese |
.80 |
.94 |
.79 |
.82 |
| Phosphorus |
.012 |
.022 |
.014 |
.011 |
| Sulfur |
.019 |
.037 |
.027 |
.020 |
| Silicon |
.19 |
.29 |
.24 |
.19 |
| Nickel |
.45 |
.54 |
.42 |
.46 |
| Chromium |
.51 |
.57 |
.51 |
.50 |
| Molybdenum |
.16 |
.21 |
.22 |
.17 |
| Copper |
.12 |
.16 |
.16 |
.10 |
| Aluminum |
.03 |
.02 |
.01 |
.02 |
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| * Chemical testing performed in accordance with ASTM E415. |
