Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation




Material Properties - Case Study No. 59: Rotary Kiln Lifter Welds' Lack of Fusion and Penetration


Finding the Unexpected

Lack of FusionEveryone assumed that the welding procedures used to attach the lifters provided adequate strength of the joint, as had been assumed in the initial analysis in 1988. However, when the samples were cut from the dryer many of them fell apart and revealed what appeared to be a very large amount of lack of fusion and lack of penetration (Fig.'s 1, left & 2, lower right, respectively). Could such a degree of poor welding found in the samples be representative of the rest of the weldments? If this were indeed the case the jolter and thermal stress failure theories would be compromised and would have to be revisited. There was a good possibility that additional aggravating failure causes existed and the search for these new causes would have to begin in earnest. To challenge this finding of bad welding technique, a certified NDE inspection company was brought in to ultrasonically check the welds using shear wave technology. The technique can differentiate between weld porosity, lack of fusion and lack of penetration based upon the returning shape of the waveform. The results showed a severe degree of lack of fusion and Lack of Penetration lack of penetration in nearly 60% of the total linear length of welds not yet visibly cracked.

Typical Lifter Crack into ShellThe Analysis Begin

Samples were sent to Dr. Becker with a well defined request. Using a combination of microstructural and fractographic information, characterize the fracture surface in terms of crack initiation site, crack propagation mechanism (cyclic, monotonic), and crack growth direction. Secondly, to determine any role of the microstructure in influencing any of the fractographic information.

To alter the validity of the initial assumption that cracking was initiated in the lifters due to thermal stresses, there would have to be fractographic evidence that cracking was initiated due to bending loading. The first sets of samples to be analyzed were taken midway between the feed-end and feed-end tire location (Fig. 3, above left). At this location they were far enough away from the tire and therefore could not be influenced by any conditions at the tire.

Fatigue Crack in ShellAll samples were usually cut at two locations, being either cut at the crack tip, or cut ahead of the crack in what usually appeared to be good material. Macroscopic examination showed the presence of beach marks (Fig. 4, left) and microscopic examination showed the presence of fatigue striations (Fig. 5, lower left). Initial findings indicated the presence of bending fatigue in this region (Fig. 4 & 5). Crack initiation always occurred at the toe of the weld, sometimes in a single location at the end of the lifter, and sometimes in multiple locations. In the latter case, cracks initiated not only at the end of the lifter, but also along the sides. Cracks at the end of the lifter propagated radially towards the outer diameter of the shell (Fig. 4). Extensive pitting attack in the weld metal provided microscale stress concentrations that were superimposed on the stress concentration associated with the weld joint (Fig. 6, lower right).
Fatigue Striations in 316L Pitting Attack on Surface of Weldment

Weld Cross Section

Metallographic examination of the majority of samples in other regions of the dryer revealed the same thing. The welding problems did not appear to be the cause for the cracks but they appeared to assist in crack propagation. Cracks initiating along the sides propagated towards a lack of fusion defect located at the juncture of the lifter and the shell, along the centerline of the lifter (Fig. 7, left). Lack of fusion was identified in every sample submitted. None of the samples between the feed-end and the midsection of the dryer gave any indication that crack initiation was due to net section yielding in the shell that might be caused by an overload condition. In addition, microstructural examination of the lifters showed no changes in microstructure that would encourage crack initiation or propagation (Fig. 8 lower right).Typical Microstructire of 309L  Stainless Steel

To be continued.....


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