Fire-Cracking Characteristics of Cr-Mo Steel

Abstract

Fire-cracking characteristics of Cr-Mo steel which was widely used for blooming and stabbing mill rolls were investigated by means of the cyclic thermal stress test developed by the author.
The test results were as follows:
1) The maximum crack depth of Cr-Mo steels of various carbon contents and microstructures had the linear relationship on the log-log coordinates with the U-notched Charpy impact value.
2) The relation between hardness and fire-cracking characteristics depended upon the carbon content, and the lower the carbon content the shallower the crack.
3) Among the Cr-Mo steels which had been normalized and annealed at the various temperatures the spheroidized structure had the best characteristics. The structure containing the marked network of the primary cementite was relatively inferior.
4) Longitudinal characteristics were improved by increasing forging reduction, but transverse property showed its best at the rather small forging rate and the excessive forging injured the transverse fire-cracking property.
5) Porosities of cast steel became the starting point of the crack, and with the transverse specimen the crack took its path in the crystalline segregation.