Fig. 15: Fatigue crack growth in TMCP, G8, 10kN, 5Hz in air

It was observed that the crack-tip diversion and branching in Fig. 15 were caused by the crack tip following path of least resistance created mainly by the α_HA or theα_LR/α_HR boundaries - but mostly by the α_HA ribbons. The crack bifurcation is quite extensive with long arms as can be seen in Fig. 15(a to e) and the diversion of the main active crack front as shown in Fig. 15(f) was measured to be about 43^ᵒ to the horizontal plane. The branched arms are measured to be in the range 38^ᵒ - 46^ᵒ. It is interesting to see that the path of least resistant caused many of the branched arms to deviate almost at an angle within this range. This simply implies that both tensile and shear stresses continuously acted on the crack as it propagated in theTMCP , even in the Paris Region! The metal crumbs as noted in Fig. 15 are by far more in the TMCP than the NR steel. It was observed also that the α_HA does not only have the potency to divert and branch out crack front, it can as well facilitate re-joining of branched crack fronts into one main crack front resulting to the crumb formation. This crumb is found to be a tore-off of the steel grain or block of grains. In other words, the fatigue crack front in α-P steel appears not to have a horizontal crack-tip edge, but rather series of fluctuating wavelike crack front, whose nature depends on the steel grains and the phases surrounding the grain surfaces. It is important to note that this metal crumb along the crack path has not been reported elsewhere in the literatures or considered in any theoretical formation or treatment. The P- colonies in theNR steel are large, blocky and dense. In the TMCP steels, the P appears to grow in two forms – small-blocky colony and a thin, elongated or needle-like morphology colony along what may be described as the α grain boundaries as shown, in some cases, by the space enclosed with the red dotted line in Fig. 15. The effect of thermomechanical control rolling process appears to be the disruption of the P banding and blocky morphology into randomly distributed thin P needles. From the observation, it appears that the interface between the α and P is an additional effective preferable path for the crack growth if the P -nodule is elongated and aligns favourably to the crack front. In Fig. 15, the branched cracks did not just move about linearly but tends to trace theα_HA phase. This suggests that the energy needed to propagate through the α_HA is much less than that for traversing the α_LR grain. The influence of the phase morphology persisted to a length of 9 mm and at 32 MPa√m.

The existing theory says that the crack path in the Stage II of theda/dN vs. ∆K sigmoidal curve is generally across the grains - transgranular [17] and the material microstructure has little or negligible effect in this region [3][5][6][18]. It can be seen that the crack path in theNR in Fig. 14 and that of TMCP in Fig. 15 are not the same. The extensive crack diversion, bifurcation and metal crumbs formation retarded the FCGR and explains why the FCGR in the TMCP is lower than that of the NR in Fig. 13. This finding appears to show that microstructure has a strong influence on the rate at which fatigue crack grows in α-P steel in air. This assertion is contrary to the existing theory that microstructure of a steel has little or negligible effect on the FCGR in air. Again, both transgranular and intergranular modes of propagation were observed in air. What appears as the intergranular cracking mode predominated in the TMCP steel. However, what appears as an intergranular mode is actually the growth of the crack through the thin layer ofα_HA in between the grains ofα_LR or the P . Hence this mode is identified here as the quasi-intergranular mode. The morphology and chemistry of the microstructural phases and load level appear to determine which mode the crack growth will adopt.

Another factor is that the angle the crack front makes with the phases ahead of it tends to determine if it would propagate in transgranular or quasi-intergranular mode. However, in Fig. 10(f) ductile striations were found inside the secondary crack – which actually appears as a branched crack when viewd from the crack line. This clearly shows that the mode is not strictly intergranular, but the crack moved in the α_HA phase ribbon immediately adjacent to the α_LR or the P interface, justifying the use of word - quasi-intergranular . In general, TMCP steel is more resistant to crack propagation than NR steel of the same steel grade. This is made possible by its refined microstructure that branched (with long arms) and diverted the crack front at large angles as compared to the small angle path diversion found in the NRsteel. It must be noted that the ability of the α_HA to deviate crack front appears to lie in its alloying composition relative to the surrounding phases. Theα_HA clearly has more of carbon, silicon and oxygen contents than other phases (see Table 3). The increase in the amount of these elements might have embrittled the phase, making it the least resistant path.