Hot-Twist Ductility of High-C Ferritic Steels

Abstract

The hot-twist ductility of AISI Type 446 and some other high-Cr ferritic steels, especially effects of metallurgical factors such as alloying elements, microstructure and grain size were investigated by hot-twist test rotated at 200r.p.m at a temperature ranging from 800 to 1250°C.
The hot-twist ductility of high-Cr steels was much influenced by its microstructure.
Some steels which had a single ferritic structure at these testing temperatures showed higher twist value and a convex type twist curve against the testing temperatures, while other steels which transformed at a testing temperature showed a lower twist value and had a concave-type twist curve. The temperature at which the specimen showed the minimum twist value nearly corrresponded to the transformation temperature.
As to the influence of the alloying elements in 25% Cr steels, C and N were the most detrimental ones which lowered these twist values. Addition of C+N up to about 0.2% greatly reduced a hot-twist ductility and about 0.2% of C+N was the inflection point of curve figure. Addition of Si, Ni, and Cr reduced a hot-twist value although Ni improved hottwist value above 1200°C.
The difference of hot-twist value could not be recognized between air- and vacuum-melted specimens which had similar chemical compositions.
Grain-coarsening owing to the elevation of annealing temperature lowered hot-twistductility, especially when single-phase specimens were annealed at a temperature higher than the temperature of maximum ductility. In the latter case intercrystalline fracture occurred at every test temperature and consequently twist values were much reduced.