Failure Analysis of a fractured bolt

Case Study: Failure Analysis of a fractured bolt

R-Tech Materials received a fractured stainless steel bolt, grade 201, from a cargo line of a crude oil tanker with the request to determine the cause of the failure by conducting a fracture analysis (see Figure 1). The fracture surface exhibited an intergranular fracture morphology over the entire surface (see Figure 2).

Figure 2: Fracture surface of bolt showing intergrnular fracture on R-TECH’s Zeiss SEM.

Examination of a section taken through the fracture surface revealed extensive intergranular branched cracking throughout the section (see Figure 3). In some instances, entire grains were missing from the section. These cracks were associated with corrosion product which was found to consist predominantly of iron, oxygen and sulphur.

Figure 3: Extensive intergranular cracking in bolt section

Examination in the etched condition revealed chromium carbides along the austenite grain boundaries which can lead to a phenomenon known as sensitisation (see Figure 4). Sensitisation is the depletion of chromium in the matrix immediately adjacent to the austenite grain boundaries which locally increases susceptibility to corrosion. Precipitation of carbides occurs upon exposure to the temperature range 450-900°C which can occur upon exposure to high operating temperatures or by slowly cooling from elevated temperature such as welding or solution annealing during manufacture. R-TECH was informed that the bolt had not been exposed to temperatures significantly above ambient during service, therefore it was deduced that the microstructure had been produced during manufacture.

Figure 4: Austenite grains with chromium carbides at the boundaries

The morphology of the cracking and the presence of sensitised austenite grains indicated that the failure was attributable to polythionic stress corrosion cracking (PASCC). This phenomenon is due to the formation of sulphide scales in the presence of sulphur compounds which then react with air and moisture to form sulphur acids (polythionic acid). Polythionic acid then attacks sensitised austenitic stainless steels in the chromium depleted matrix directly adjacent to the grain boundaries, producing an intergranular fracture. The stress necessary to cause cracking can be applied or residual. Sulphur is likely to have originated from the products transferred in the cargo line.

In sulphur containing environments, it is vital that an adequate heat treatment is applied to austenitic stainless steels to ensure that there is no evidence of any carbide precipitation at the grain boundaries. However this was not the case for the bolt examined and therefore the bolt had been highly susceptible to PASCC.

A failure analysis of this type is a valuable and informative tool to reduce the likelihood of a repeat incident which improves safety and productivity and can enable evolution towards a better product. If you would like to know more about our Failure Analysis services? Contact us today: