Thermal Fatigue /Sootblower Nozzle

Two failed sootblower nozzles were submitted for analysis. All other nozzles, regardless of elevation and location are experiencing failures. The nozzles were about 5 year old, RA-330 (UNS N08330) heat-resistant austenitic alloy material.

A few other retractable wall blowers had become stuck, exposing them to furnace conditions and burned. These sootblowers have been operating from 25 (typical) to as high as 50 times per day. The plant is currently dealing with delayed combustion, which is causing higher superheater/reheater temperatures. The sootblower media is high temperature steam at about 600F, while the nose arch temperature is about 2500F. Actual wall temperatures could be much lower.  

 

 

Failure Analysis: Sootblower nozzle tip B-12 failed due to thermal fatigue likely from repeated quenching when the high temperature sootblower nozzle tip was exposed to the relatively cold steam/condensate in the blowing media. Higher operating temperatures were likely due to location of sootblower and/or poor shielding/retractable mechanism. Higher carbon was detected than specified maximum. Higher carbon content likely played a role in the failure to propagate the cracks at a faster rate. The cracks may have followed the inter-dendrites due to lack of ductility. Higher carbon in the steel likely diminished the fatigue strength due to lack of ductility. Relatively higher hardness was observed than typical range.            

 

Material Verification: Sample did not meet the chemical requirements for UNS N08330 (RA330) heat-and corrosion-resistant alloy (Ni-Fe-Cr alloy). Higher carbon was detected, perhaps due to diffusion of carbon into the steel during operation. However, away from the nozzle tip where it was not exposed to high radiant heat, the carbon content was also measured at 0.41%. This suggests that the nozzle may have contained higher carbon when it was new. Higher carbon content than specified maximum is detrimental in oxidation and corrosion resistance. Over a period of time, the higher carbon in the steel may have combined with chromium and reduced its ductility, resulting in reduced fatigue strength.

   

Severe cracks were visible at both the OD and ID surfaces of the nozzle

At higher magnification, the cracking appears to be thermal fatigue. At any point in time, a sudden temperature gradient over 200F between the hot steel and blowing media induces significant stresses. Those thermal stresses would be closer to the yield strength of the steel. These repeated thermal cycles initiate thermal fatigue cracks both at the ID and OD surfaces, dependent upon stress levels. The appearance of the cracks suggests that these may have followed the inter-dendrites due to lack of ductility. Higher carbon in the steel may have diminished fatigue strength due to reduced ductility. Higher carbon content likely played a role in the failure to propagate the cracks at a faster rate.

 

   

Severe cracks were visible at both the OD and ID surfaces of the nozzle