Metallurgy Failure Analysis Case Study
SGS MSi Investigative Summary
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.
|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|
- Visual and Stereoscopic Examination
- Scanning Electron Microscopy (SEM) Examination
- Metallographic (Microstructural) Examination
- Microhardness Testing
- Chemical Analysis
- 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.
- 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.
SUMMARY of TEST RESULTS
Visual and Stereoscopic Examination
- 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).
- 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).
- 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.
Scanning Electron Microscopy (SEM)
- 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.
- 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).
- 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.
- 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 are as included cross-sections of the standoff dimples and corrugations. (See Photos 9 – 32 attached)
- 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.
- Microhardness testing was performed at our client’s request for information purposes. No target hardness profile values were specified.
- The results are shown in Table 1 attached.
- Chemical testing confirmed the belt sample as Type 304 steel
- The results are shown in Table 2 attached.
Table 1 – Microhardness Testing*
|Sample||Stand-off Dimple Profile||Corrugation Profile|
|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|
* 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.
Table 2 – Chemical Testing*
|Element||Belt #1||Belt #2||Belt #3||Belt #4|
* Testing performed in accordance with ASTM E1086.
Typical Profile Microstructures in Belts #1 -- #4
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