Research On The CTOD Fracture Toughness And The Fatigue Performance Of The High-strength Steel And High-ductility Steel And Their Relationship

Q690E and S355 steel plate are in great demand of steel in engineering equipment manufacturing field, as the key components in the equipment, the fracture toughness properties and fatigue performance of them is very important,hi this paper the content can be divided into three parts:first of all, the fracture toughness CTOD performance of Q690E(TMCP) and Q690E (hardened and tempered) are analyzed and compared, as for S355ML and S355NL steel, too. Second, the low cycle fatigue properties of Q690E(TMCP) and Q690E (hardened and tempered) are analyzed and compared, and the same to S355ML and S355NL steel. Third, the low cycle fatigue performance and CTOD toughness properties are compared for the steel of the same ingredient.The experiment shows that:The CTOD toughness assessment technology apply to the steel of Q690E and S355, and then the curve of CTOD are tested and fitted, it shows that all the CTOD curves of the steels above can be fitted with the index equation:8=α+β△aγ. The F-V curves of three point bending test of Q690E shows different characteristics.The curve of Q690E(hardened and tempered) steel presents very smooth shape, it shows the excellent toughness of this kind of steel; And the curve of Q690E(TMCP) steel presents multiple pop-in points. Meanwhile, the CTOD toughness properties of Q690E(hardened and tempered) steel is better than the CTOD toughness of Q690E(TMCP), and with the increase of the quantity of crack propagation, CTOD toughness values shows bigger differences. This content shows that:after tempering, the Q690E steel plate shows better crack resistance ability.The F-V curves of three point bending test of S355ML and S355NL steel shows similar characteristics. Both of them present very smooth shape. In the loading process, there is nearly no significantly pop-in point. And the CTOD curves of S355ML and S355NL is very close during the small quantity of crack propagation, but with the increase of the quantity of crack propagation, the gap of the two fitted curve tends to get big gradually. This content shows that:after normalizing, the S355 steel plate shows better crack resistance ability.Both of the Q690E(TMCP) and the Q690E (hardened and tempered) steels exhibit the cyclic softening properties in the high strain amplitudes (more than 0.54%), and the cyclic softening feature ofQ690E(TMCP) is less obvious than the Q690E(hardened and tempered). The S355ML exhibits the cyclic softening properties in the all the strain amplitudes in the fatigue process. This test shows that different heat treatment process has very big effect on cycle performance.The relations between the plastic strain amplitude, elastic strain and reversals to failure ofQ690E steel and S355ML,355 NL steel can be described as a line relationship. The fatigue life can be described by Manson-Coffin formula. In addition, a linear relationship between cyclic stress amplitude and plastic strain amplitude is also noted in the low cycle fatigue test.It is noted that the low cycle fatigue properties of Q690E(hardened and tempered) steel is better than Q690E(TMCP), while leads to a reduction in fatigue lives at a lower strain amplitudes. It is also noted that the low cycle fatigue properties of S355NL steel is better than S355ML, while leads to a reduction in fatigue lives at a lower strain amplitudes.During the fatigue loading, the fatigue cracks initiate at the surface of fatigue specimens; And move forward in a mode of the merger and divergent of the crack, in this area obvious secondary cracks and fatigue stripe can be observed. Instant rupture zone exists lots of dimples, it shows that the toughness of this two kinds of steel are both very excellent.For the above kinds of steel, when the steel has better CTOD toughness properties, it also reflects the better low cycle fatigue properties.