Is the tissue of the HK-40 transformed tube austenitic? Is it still ferrite?

The tissue of the HK-40 transformed tube is austenite

HK-40 and HP-Nb furnace tubes are austenitic steels, both containing high levels of C, Cr, and Ni elements. The converter tube sections of the hydrogen production unit are made by centrifugal casting. Based on the characteristics of the crystallization process in centrifugal casting, it can be seen that under the action of centrifugal force, impurities such as gases and slag in the metal accumulate on the inner surface due to their lower density. The metal crystallizes in a directional manner from the outside inward, resulting in fine-grained crystals without defects such as shrinkage cavities, porosity, gas holes, or slag inclusions. Furthermore, the cast conversion tube requires internal boring to remove slag and loose material from the inner wall surface, thereby ensuring good mechanical properties. In centrifugal casting, the high-speed rotation of the mold causes the casting material to develop large columnar and equiaxed crystals; such large grains possess a high resistance to high-temperature creep. The macroscopic metallographic structure of equiaxed and columnar grains should be controlled such that columnar grains account for 60%–70%, while equiaxed grains account for 30%–40%

With austenite (γ) as the matrix, along with a skeletal eutectic structure (γ+Cr7C3), it is therefore austenite, not ferrite