Effect Of Si On The Low Temperature Toughness Of Pressure Vessel Steels

The low temperature toughness of the Nb-Ti microalloyed cryogenic pressure vessel steeland the Cr-Mo alloyed medium temperature pressure vessel steels containing different levels ofsilicon was investigated. It was found that the low temperature toughness was improved by thereduction of the silicon content, which was attributed to the pre-existing cementite formationbefore the precipitation of alloy carbides during tempering.The results of the Nb-Ti microalloyed steels during a normalizing (830oC) and tempering(630oC) heat treatment are as follows. The microstructure of bainite and equiaxed ferrite wasobtained in the steels containing0.05%and0.47%Si. The ratio of (Nb,Ti)C carbide particles(<10nm) decreased from53.5to26.2%, and that of coarse particles (>30nm) increased from~13.0to54.1%, resulting in an average size of the particles increasing from~11.2to25.1nm, asthe silicon content increased from0.05to0.47%. High resolution TEM revealed that (Nb,Ti)Ccarbides precipitated on these iron carbides in the0.05%Si steel. The average Charpy absorbedenergy decreased continuously as the test temperature was decreased from-90to-130oC withthe lower Si steel having a higher absorbed energy at all test temperatures, as the silicon contentincreased from0.05to0.47%. The ductile fracture surface features showed that the quantity offine dimples in the facture surface was sharply decreased as the silicon content increased from0.05to0.47%. The Boltzmann function fitting analysis of the Charpy absorbed energy showedthat the ductile to brittle transition temperature (DBTT) increased from-115.1to-105.4oC as thesilicon content increased from0.05to0.47%.The results of the Cr-Mo alloyed steels during a normalizing (1000oC) and tempering (650oC) heat treatment are as follows. The microstructure of bainite and ferrite was obtained in thesteels containing0.47%and1.55%Si. The average Charpy absorbed energy decreased from158to83J as the silicon content increased from0.54to1.55%. Finer M6C and M23C6carbideswere precipitated in the0.54%Si steel, while coarser M7C3and M23C6carbides wereprecipitated in the1.55%Si steel. The average size of alloy carbides increased from~50to200nm as the silicon content increased from0.54to1.55%.