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Metallurgy failure analysis of stainless steel belts In food manufacturing application |
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| BACKGROUND |
Four (4) rectangular samples removed from stainless steel belts with different service terms were submitted to our laboratory for a metallurgical investigation. All received parts were fabricated from corrugated, hot-rolled and cold-formed 304 stainless steel sheet. Reportedly, Belt #1 was brand-new and was included in the shipment for reference purposes. Belt #2 had been in service for ~2,000 hrs and appeared to be intact. Belt #3 contained a transverse crack that developed after an estimated service life of >5,000 hrs. The crack was located in the valley between the corrugations, and had been weld-repaired by maintenance personnel. Belt #4 contained an unrepaired transverse crack. Service history for Belt #4 was not readily available, but was estimated at ~1.5 years.
We were requested to compare the metallurgical properties of the submitted samples, determine the failure mechanism responsible for the failure of Belt #4, and suggest measures for increasing the service life of the belts. |
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| SAMPLE IDENTIFICATION |
| Part Description |
Nominal Sheet Thickness |
Steel Grade |
No. of Samples |
| Belt #1, new |
.65 mm |
Type 304 |
1 |
| Belt #2, used, intact |
1 |
| Belt #3, used, cracked and weld-repaired |
1 |
| Belt #4, used, cracked, not repaired |
1 |
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| PERFORMED TESTING |
Visual and Stereoscopic Examination
Scanning Electron Microscopy (SEM) Examination
Metallographic (Microstructural) Examination
Microhardness Testing
Chemical Analysis |
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| CONCLUSIONS |
1. Based upon the performed tests and examinations, it is our opinion that the failure of Belt #4 was caused by a combination of two failure mechanisms:
- First, the raised belt areas facing the stationary polymer glides became thinned through frictional wear. The sheet metal thickness in the affected area on the belt sample became reduced from ~.65mm to ~.13 mm.
- The reduction of the load-carrying cross-section led to a localized increase of service stresses, which triggered the second mechanism, identified as low-cycle, reverse-bending fatigue.
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2. Fatigue failures are progressive, beginning as small cracks that grow over time under the cyclic loads. Low-cycle fatigue is induced when the operational stresses are sufficiently high to induce plastic deformation in the affected components. |
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3. SEM examination of the crack surface in Belt #4 revealed coarse crack progression lines (striations) propagating from the opposite surfaces of the belt and meeting at the core of the steel sheet. A narrow ductile final fracture zone at the core was also observed. This observation positively identified the failure mechanism as low-cycle, reverse-bending fatigue, followed by a final, single-event ductile fracture, which occurred when the remaining metal ligament could not carry the applied service loads. |
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4. Comparative metallographic examination of the four belt samples at the corresponding locations on the cross-sections of the standoff dimples and corrugations revealed normal austenitic microstructures typical of hot-rolled and cold-formed Type 304 stainless steel sheet. No evidence was observed of pre-existing steel defects, excessive nonmetallic inclusions, or any other detrimental material conditions that could have contributed to the failure. |
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5. Microhardness measurements were made at the corresponding profile locations (peaks, slopes and valleys) of the standoff dimples and corrugations. The comparative data were provided at our client’s request for information purposes only, since no target profile hardness values were specified. |
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6. Chemical analysis confirmed all submitted belt samples as Type 304 austenitic stainless steel. |
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7. To increase the service life span of similar stainless steel belts, the following measures are respectfully recommended:
- Review friction properties of alternative materials for the polymer belt glides. A reduction of the belt-to-glide coefficient of friction will lower the friction wear rate of the contacting metal surfaces, and delay the thinning of the belt metal to the critical level.
- If practical, increase the thickness of the belt sheet metal, to further delay reaching of the critical thickness.
- Fabricate the belts from cold-rolled sheet with an increased overall hardness to improve wear resistance.
The original equipment manufacturer and/or designer should be consulted on the target hardness and strength levels suitable for the intended application. |
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| SUMMARY of TEST RESULTS |
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| Visual and Stereoscopic Examination |
1. Visual and stereoscopic examination of the submitted belt samples revealed no evidence of unusual discoloration, pitting, deposits, or any other signs of corrosion attack. Belt #3 contained a weld-repaired area in a valley between corrugations. Belt #4 exhibited an approximately 2.5”-long through-wall crack at a similar location (see Photos 1 – 3). |
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2. The gliding sides on Belts #2, #3 and #4 exhibited various degrees of frictional wear at the raised metal-to-glide contact points. Wear damage to Belt #4 appeared to be most severe, with localized sheet metal thickness reduction from ~.65 mm to ~.13 mm in the through-wall failure zone (see Photo 4). |
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3. The mating surfaces of the crack in Belt #4 were separated for further examination by scanning electron microscopy, described in the following section of this report. |
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| Photo 1: Belt samples 1 – 3, as received. |
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| Photo 3: A close-up view of the crack in Belt #4. |
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| Photo 2: Belt sample #4, as received. |
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| Photo 4: Worn area on the gliding side of Belt #4. The mating crack surface was separated for SEM evaluation |
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| Scanning Electron Microscopy (SEM) |
1. SEM examination of the exposed crack in Belt #4 revealed multiple microscopic ledges (ratchet marks) at the belt surface (see Photos 5 and 7, red arrows). The ratchet marks are associated with fatigue crack initiation sites. |
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2. The rest of the examined areas showed arrays of evenly-spaced, coarse, semi-circular crack progression lines (fatigue striations) propagating from the opposite surfaces of the belt and meeting at the core of the steel sheet. A narrow ductile fracture zone with a dimpled morphology was detected at several core locations (see Photos 5 – 7). Secondary cracking was detected on the wear-damaged gliding surface of the belt (see Photos 7 and 8). |
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3. The observed features positively identified the failure mechanism as low-cycle fatigue, followed by a final, single-event ductile fracture, which occurred when the remaining metal ligament could not carry the applied service loads. |
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| Photo 5:
SEM view of ratchet marks, fatigue striations and a
ductile fracture zone at the core of the sheet. |
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| Photo 7:
SEM view of ratchet marks (arrows), fatigue striations
and secondary cracking on the wear-damaged surface. |
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Photo 6:
SEM view of fatigue striations propagating from the
opposite sides of the sheet towards the core. |
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Photo 8:
A close-up SEM view of secondary cracking on the wear-
damaged surface (arrows).
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1. Comparative metallographic examination of the four belt samples at the corresponding peak, slope and valley locations revealed similar, normal austenitic microstructures typical of hot-rolled and cold-formed Type 304 stainless steel sheet. The examined areas included cross-sections of the standoff dimples and corrugations. (See Photos 9 – 32 attached)
2. No evidence was observed of pre-existing steel defects, excessive nonmetallic inclusions, or any other detrimental material conditions that could have contributed to the failures. |
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| Microhardness Testing |
1. Microhardness testing was performed at our client’s request for information purposes. No target hardness profile values were specified.
2. The results are shown in Table 1 attached. |
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| Chemical Testing |
1. Chemical testing confirmed the belt sample as Type 304 steel.
2. The results are shown in Table 2 attached. |
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| Table 1 – Microhardness Testing* |
| Sample |
Stand-off Dimple Profile |
Corrugation Profile |
| Peak |
Slope |
Valley |
Peak |
Slope |
Valley |
| Belt #1 |
24 HRC |
24 HRC |
78 HRB |
92 HRB |
78 HRB |
80 HRB |
| Belt #2 |
22 HRC |
25 HRC |
88 HRB |
91 HRB |
80 HRB |
81 HRB |
| Belt #3 |
33 HRC |
28 HRC |
95 HRB |
91 HRB |
87 HRB |
87 HRB |
| Belt #4 |
29 HRC |
29 HRC |
90 HRB |
98 HRB |
87 HRB |
86 HRB |
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* Microhardness testing performed using a Knoop diamond indenter and 500 gram load per ASTM E384.
Microhardness data converted to HRB/HRC values using ASTM E140. |
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| Table 2 – Chemical Testing* |
| Element |
Belt #1 |
Belt #2 |
Belt #3 |
Belt #4 |
| Carbon |
.04% |
.04% |
.04% |
.04% |
| Manganese |
1.21 |
1.20 |
1.52 |
1.49 |
| Phosphorus |
.032 |
.034 |
.032 |
.032 |
| Sulfur |
<.005 |
<.005 |
<.005 |
<.005 |
| Silicon |
.39 |
.39 |
.54 |
.53 |
| Nickel |
9.11 |
9.11 |
8.39 |
8.37 |
| Chromium |
18.09 |
18.05 |
18.11 |
18.21 |
| Molybdenum |
.20 |
.20 |
.15 |
.15 |
| Copper |
.26 |
.26 |
.23 |
.23 |
| Nitrogen |
.03 |
.03 |
.02 |
.02 |
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| * Testing performed in accordance with ASTM E1086. |
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| Typical Profile Microstructures in Belts #1 -- #4 |
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Photo 9: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #1, standoff dimple, peak. |
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Photo 11: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #1, standoff dimple, slope. |
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Photo 10: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #1, corrugation, peak. |
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Photo 12: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #1, corrugation, slope. |
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| Typical Profile Microstructures in Belts #1 -- #4 (cont.) |
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Photo 13: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #1, standoff dimple, valley. |
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Photo 15: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #2, standoff dimple, peak. |
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Photo 14: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #1, corrugation, valley. |
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Photo 16: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #2, corrugation, peak. |
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| Typical Profile Microstructures in Belts #1 -- #4 (cont.) |
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Photo 17: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #2, standoff dimple, slope. |
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Photo 19: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #2, standoff dimple, valley. |
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Photo 18: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #2, corrugation, slope. |
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Photo 20: Mag: 100X; Etchant: 10% oxalic acid (electrolytic)
Belt #2, corrugation, valley. |
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| Typical Profile Microstructures in Belts #1 -- #4 (cont.) |
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| Typical Profile Microstructures in Belts #1 -- #4 (cont.) |
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| Typical Profile Microstructures in Belts #1 -- #4 (cont.) |
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