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Metallurgy Failure Analysis Case Study

SGS MSi Investigative Summary

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.

SAMPLE IDENTIFICATION

Part DescriptionNominal Sheet ThicknessSteel GradeNo. 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

 

PERFORMED TESTING

  • Visual and Stereoscopic Examination
  • Scanning Electron Microscopy (SEM) Examination
  • Metallographic (Microstructural) Examination
  • Microhardness Testing
  • Chemical Analysis

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.
  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.
  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.
  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.
  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.
  6. Chemical analysis confirmed all submitted belt samples as Type 304 austenitic stainless steel.
  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.

SUMMARY of TEST RESULTS

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).
  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).
  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.

 

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.
    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).
    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.

 

      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 are as 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.

 

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.

 

Chemical Testing

      1. Chemical testing confirmed the belt sample as Type 304 steel
      2. The results are shown in Table 2 attached.

 

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

* 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
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

* Testing performed in accordance with ASTM E1086.

Typical Profile Microstructures in Belts #1 -- #4

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