The morphology of upper bainite (temperature range 550-400°C) bears a close resemblance to Widmanstätten ferrite, as it is composed of long ferrite laths free from internal precipitation.

Two-surface optical micrography decisively reveals that the ferrite component of upper bainite is composed of groups of thin parallel laths with a well-defined crystallographic habit. Like Widmanstätten ferrite, the bainitic ferrite laths exhibit the Kurdjumov-Sachs relationship with the parent austenite, but the relationship is less precise as the transformation temperature is lowered.

A widely-accepted view is that the crystallography of upper bainite is very similar to that of low-carbon lath martensite. However, a detailed examination of the crystallography reveals that there are significant differences, and that upper bainite ferrite formation cannot be understood in terms of the crystallographic theory of martensite-formation.

Electron microscopy shows that upper bainite laths have a fine structure comprising smaller laths about 0.5 μm wide. These laths all possess the same variant of the Kurdjumov-Sachs relationship, so they are only slightly disoriented from each other. The longitudinal boundaries are, therefore, low angle boundaries.

A typical austenite grain will have numerous sheaves of bainitic ferrite exhibiting the several variants of the Kurdjumov-Sachs orientation relationship, so large angle boundaries will occur between sheaves. The dislocation density of the laths increases with decreasing transformation temperature, but even at the highest transformation temperatures the density is greater than that in Widmanstätten ferrite.

The upper bainitic ferrite has a much lower carbon concentration (<0.03% C) than the austenite from which it forms, consequently as the bainitic laths grow, the remaining austenite is enriched in carbon. This is an essential feature of upper bainite which forms in the range 550-400°C when the diffusivity of carbon is still high enough to allow partition between ferrite and austenite. Consequently, carbide precipitation does not occur within the laths, but in the austenite at the lath boundaries when a critical carbon concentration is reached.

The morphology of the cementite formed at the lath boundaries is dependent on the carbon content of the steel. In low carbon steels, the carbide will be present as discontinuous stringers and isolated particles along the lath boundaries, while at higher carbon levels the stringers may become continuous. With some steels, the enriched austenite does not precipitate carbide, but remains as a film of retained austenite. Alternatively, on cooling it may transform to high carbon martensite with an adverse effect on the ductility. This type of bainite is often referred to as granular bainite.