Abstract:
The aim of the work presented was to investigate the effects of upper bainite on impact
toughness in quench and tempered low alloy plate steels. The experimental research included
construction of CCT diagrams by dilatometry, verification of phases by optical microscopy
(OM), Vickers hardness, scanning electron microscopy (SEM), transmission electron
microscopy (TEM) on precipitates extracted by carbon replica and by electrolytic means and
finally impact testing of Charpy specimens with mixed bainite:martensite microstructures.
Bainite was formed in High Chromium Low Molybdenum (HCrLMo) and in High
Molybdenum Low Chromium (HMoLCr) steel samples by isothermal annealing within the
bainite C-curve of the respective CCT diagrams. The isothermal kinetics of the upper bainite
transformation was modelled with the Johnson Mehl Avrami Kolmogorov (JMAK) model.
Avrami exponents of 1.4 and 1.3 were obtained for the HCrLMo and HMoLCr steels
respectively which indicated linear growth with a considerable lengthening rate of laths and
negligible thickening.
The measurably slower growth kinetics in the HMoLCr steel as observed in the JMAK model
and the higher hardenability with reference to its CCT diagram, suggested a strong Mo
alloying element effect. The stronger effect of Mo compared to Cr was attributed to a solute
drag like effect.
The effect of upper bainite in a tempered martensitic matrix was investigated for the
following amounts of bainite; 0%, 10%, 25%, 60%, 75%, 90% and 100%. The impact
toughness of the mixed bainite:martensite samples was evaluated against the toughness of 100% bainite and 100% martensite. It was demonstrated that upper bainite reduces the total
absorbed impact energy by an adverse effect on crack nucleation energy and crack
propagation energy.
