Creep Strengthening Mechanism of Boron Containing 9-12 Chromium Heat Resistant Steel

摘要

Boron effectively increases the creep rupture strength of ferritic heat resistant steel. But the strengthening mechanism of boron containing steel has not yet been well researched. In this report, the contribution of boron to tungsten containing ferritic heat resistant steels is discussed through creep rupture and creep strain test results. Boron in ferritic steels extends the creep rupture life and transient creep stage on the creep strain rate measurement plot This fact implies the increase of precipitation strengthening effect in the steels and resulted in increase of creep resistance. Precise α-autoradiographical investigation of boron distribution revealed boron segregation along the prior austenite grain boundary. Boron hardly dissolves into the ferrite matrix, therefore most boron at the grain boundary must replace carbon of M_(23)C_6 type carbides at the prior austenite grain boundaries or lath boundaries. Based on the experimental results and their discussion, the following two hypotheses explain the creep strengthening mechanism of boron. (1) Grain boundary strengthening by a small amount of dissolved free boron. (2) Intergranular precipitates stabilizing effect. Transmission Electron Microscopy (TEM), Electron Diffraction Pattern Analysis, Auger Electron Spectroscopy (AES), Electron Dispersive X-ray Analysis (EDX), Electron Energy Loss Spectroscopy (EELS) and Atom Probe-Field Ion Microscopy (AP-FIM) identified precipitates and dissolved boron.