Cracked turbine blades from a 600 megawatt steam turbine were submitted to Metallurgical Associates for analysis by an electric power utility. The main turbine steam pressure is 2400 psi at 1000 F° but only 250 F° at crack locations in the blade roots at the attachment to the hub. The unit had been subjected to 300 start/stops cycles over its service life, a high stress procedure in turbine operations.
Additional previously unidentified cracks were found in the blades after magnetic particle inspection. Opening of the cracks revealed dark oxidized fracture features indicating the blades had been cracked for a long time before removal from the turbine hub. Extensive corrosion pitting was present on the original blade surface adjacent to the crack.
Analysis by EDS indicated the dark deposits contained sulfur, chlorine and carbon. Examination of the opened crack features using a Scanning Electron Microscope revealed intergranular fracture features that are consistent with Stress Corrosion Cracking.
Metallographic sections were prepared through the cracks to confirm Stress Corrosion Cracking (SCC) as the fracture mechanism. These sections would also identify any other potential materials defects affecting the initiation of the cracks. This examination revealed branching crack features that are characteristic of SCC. Extensive corrosion pitting is also associated with these cracks.
Cause and Prevention
Metallurgical Associates analyses confirmed that the tensile strength, impact strength, chemical composition and heat treatment of the turbine blades were all as specified and none of these characteristics contributed to the cracking. Metallographic examination of the microstructure indicated the blades were heat treated by quench and tempering, also to specification.
The results of our evaluation identified Stress Corrosion Cracking (SCC) as the cause of the cracking. SCC occurs when metals alloys are exposed to a combination of stress and specific corrosive environments. The material used for these turbine blades is susceptible to caustic environments, which result in intergranular fracture, as we identified on the opened cracks by SEM. The high rotational speed of the turbine provided the required stresses to generate the SCC cracking in combination with these corrodants.
Evaluation of the service history of this steam turbine indicated low levels of sulfur and chlorine were present in the feed water used to produce the steam. Alternate wetting and drying of the feed water at the relatively low temperature (250 F°) root of the blades resulted in highly concentrated localized corrosive deposits at this location. Our recommendation for more stringent control of feed water chemistry prevented cracking in replacement blades which have now been in service for several years.