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Mechanical Fatigue Failure of Stainless Steel Bolts

ENVIRONMENT:

Chemical Processing Plant

EQUIPMENT:

Agitator Blades

MATERIAL:

Type 316 Stainless Steel
SERVICE TIME: Approximately 3 years

FAILURE:

Mechanical Fatigue
 

Summary

The bolt failures are due to mechanical fatigue. Beach marks and striations, typical of fatigue fractures, are present on all of the fracture surfaces. Fatigue failure of threaded fasteners is most often associated with insufficient tightening of the fastener, resulting in flexing and subsequent fracture 

 

Background

Four (4) broken bolts were submitted for a failure analysis. The bolts were used to attach agitator blades to an agitator shaft in a process vessel used to process polyvinyl chloride (PVC). There are three blades on the agitator. Each blade was held with 3 bolts. The bolts had been in service approximately three years. Prior to installation, a cap was welded to the head of each bolt to facilitate installation. There was no bolt tightening procedure or torque requirements in place when the bolts were installed. The bolts were reported to be Type 316 stainless steel.

During a routine assessment, maintenance personnel observed that the agitator blades were loose. A closer examination revealed that four bolts were broken and the remaining bolts were loose. All of the bolts were replaced, the agitator re-assembled and returned to service.

 

Findings

Cleaning the Bolts

The four received bolts were coated with a white product reported to be PVC. The bolts were cleaned using a strong detergent in an ultrasonic bath. Although some deposits remained, the features of the fracture surfaces became clearly visible.

Visual and Macro-Examination

All of the bolts broke at the head-to-shank radius, Figure 1. Beach marks, characteristic of fatigue failure, are present on each of the fracture faces [A description of fatigue fracture].  Two of the bolts appear to have single crack origins and two of the bolts appear to have multiple crack origins.

Figure 2 is an optical fractograph of one of the fracture surfaces. The fracture initiates at one location and propagates across the bolt until final failure occurred. The final fracture area is very small indicating that the stresses present at final failure of the bolt were small.

 

Figure 1.  Broken bolts submitted for analysis.

Figure 2.  Optical fractograph of fracture surface on bolt head showing beach marks.

 

Microscopic Examination

A thin section (1/4) of the bolt head containing the fracture surface in Figure 2 was removed from the bolt head. Residual PVC was dissolved from the surface by soaking in tetrahydrofuran (THF) for a few minutes. The cleaned sample was then placed onto the goiniometer stage of a scanning electron microscope.

The surface of the fracture was imaged and then scanned for any unusual features that may be present. The features observed were typical of a fatigue failure of a bolt.  A photomicrograph of the surface shows a brittle fracture with indications of an advancing crack. Figure 3 is a close-up image showing the fatigue striations present.

 

Figure 3.  SEM image of fracture surface showing striations (1,000X Original Magnification).

 

Metallographic Examination

Both parts from one of the bolts were longitudinally cross-section in the area of the failure. The cross-sections were metallographically polished and electrolytically etched with ammonium persulfate.

The structure present in the threaded portion of the bolt is typical of an austenitic stainless steel.

The structure present in the head of the bolt is typical of a forged head. A noticeable change in grain size is present at the head to shank fillet. The failure has occurred along this change in microstructure.

 

DISCUSSION

These bolts failed due to mechanical fatigue. The presence of visual beach marks and microscopic striations characterize the progressions of fatigue cracks. These characteristics are present on the fracture surfaces of the bolts.

Additionally, fractures in threaded fasteners tend to occur at the head-to-shank fillet, as in these bolts, a stress concentration site for initiation of fatigue cracks. The microstructural change in grain size from the head to the shank, observed in these bolts, provides additional stress concentration and a preferred path for crack propagation.

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