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The results of an investigation of high chromium hot work mill rolls revealed the failures related to certain manufacturing practices and/or quality control. Specifically, RJLG's analysis discovered that the rolls were not heat treated properly, as evidenced by the poor microstructure of the failed rolls, which contained small amounts of secondary carbide and large amounts retained austenite. To help define the problem, RJLG experts compared a similar roll from a different supplier that exhibited good service performance.
This other manufacturer's roll exhibited a microstructure consisting of eutectic carbide, secondary carbide, and only a very small percentage of retained austenite. Because of the large amounts of austenite in the failed rolls, RJLG specialists expected the rolls not to have the same level of beneficial residual compressive stresses in the roll barrel, which helps to resist cracking as evidenced by the properly heat treated roll. In addition to the high levels or retained austenite in the shell of the failed rolls, some of the rolls exhibited porosity on the shell side of the bondline and instead of a nodular iron, the core appeared to be more like a gray iron, as evidenced by the presence of poorly formed Type VIIB (rosette) flake graphite.
These factors and more allowed RJLG experts to arrive at the conclusion that original rolls were not heat treated properly. The roll user user was compensated for the poor roll performance and the manufacturer was able to correct the issue with confidence.
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