Microstructure and continuous cooling transformation behaviour of C-Mn-Ni and C-Mn-Ni-Mo weld metals

Fourteen systematic CCT diagrams were produced which revealed that the γ-α transformation behaviour of the weld metals was controlled by the cooling rate, type and content of the alloying element, prior austenite grain size and the oxygen content. It was shown that progressive addition of nickel depresses the transformation temperature which has the effect of increasing the proportion of acicular ferrite at lower cooling rates and promoting martensite at higher cooling rates. High nickel contents (e.g. 5.5% ) have been shown to shift the martensite region into the `welding widow'. Manganese was shown to have a similar but stronger effect to nickel. Molybdenum was found to not only depress the ferrite transformation temperature but change the microconstituents. Low oxygen contents were shown to result in an unfavourable condition for intragranular nucleation and encourage ferrite sideplates or martensite. Increasing the [Al]/[O] ratio in low oxygen level welds was seen to have similar effect to reducing oxygen content, but the corresponding increased soluble aluminium is the controlling factor. The examination of the microstructure and properties of the as-deposited weld metals provided practical data, consistent with the CCT results. The columnar grain size of the as-deposited welds was found to be controlled by the alloying contents and the addition of nickel indirectly influenced weld microstructure by changing the prior γ grain size. The presence of inclusions was shown important to the nucleation of acicular ferrite. For the intermediate oxygen level weld, inclusions were revealed to nucleate acicular ferrite by acting as an inert substrate. The morphology of the acicular ferrite laths was found to be influenced by the alloying and oxygen contents and hypotheseses have been proposed to explain this phenomenon. A good understanding of the transformation characteristics and microstructural development of the weld metals has been obtained. Thiswill help the design of advanced consumables which can produce high toughness weld metals with an optimum balance of microstructure.(DX182206)