Home > Knowledge > Content
Effect of ferrite on DWTT performance of pipeline steel
- Jul 19, 2018 -

DWTT (Falling hammer tearing property) is an important performance parameter of pipeline steel.

For pipeline steels used for production of oil and gas pipelines, not only need certain strength, but also have some toughness, so the pipeline steels are required to be tested by DWTT test, and this parameter has close relationship with ferrite body.

1. The influence of ferrite type on DWTT

A multilateral ferrite or acicular ferrite is generally formed in pipeline steel.

The role of the multilateral ferrite is: 1) Coordinate deformation.

Because the multilateral ferrite belongs to the low strength and good plasticity phase, therefore, the existence of a proper polygon ferrite can improve the ability of coordination and deformation among various organizations, and the presence of pearlite in the grain boundary also restricts the coordination deformation of ferrite, increases the deformation resistance, the formation work and expansion work of the crack, and increases the toughness. 2) passivation crack tip. The multilateral ferrite, which belongs to the ductile microstructure and has low strength, can make the crack tip produce large plastic deformation, hinder the crack propagation, and relax the stress at the crack tip and the crack tip passivation.

In addition, the presence of a certain number of multilateral ferrite grains will force the cracks to continue to traverse between the pearlite and the multilateral ferrite, and a large number of interfaces inhibit the crack propagation and make the toughness improve.

Acicular ferrite Action: Acicular ferrite has irregular multilateral shape, grain boundary is fuzzy, there is no complete continuous grain boundary, grain size is inconsistencies, and has high dislocation density.

At the same time, it has dispersion distribution of precipitates, fine substructure and so on, thus, each unit length has a considerable number of grain boundaries, this compact microstructure hinders the initiation and expansion of crack, and thus has excellent strength and toughness.

2. The effect of ferrite content on DWTT Because of the coordinated deformation of the multilateral ferrite, the blunt edge crack tip, the micro-pressure structure and high density dislocation of acicular ferrite, the mixture of the two ferrite bodies can obviously hinder the initiation and propagation of the crack.

But the more the multilateral ferrite is not the better, the higher the content will lead to the reduction of toughness.

3. The influence of ferrite grain size on DWTT The grain orientations on both sides of the grain boundary are different and irregular, at the same time, when the grain boundary is a fairly disordered region of atoms, the grain boundary resistance is very large when the micro-cracks enter another grain through the grain boundary, and the crack propagation direction is changed after the crack passes through the grain boundary, and the energy is needed. Therefore, the finer the grains, the more the number of grains and the crack propagation direction, the more the crack growth function, the greater the fracture fiber area of the pipeline steel, the higher the DWTT performance.

Therefore, the finer the grains, the higher the fiber rate of dwtt fracture. In conclusion, the ferrite in high performance pipeline steel should be a mixture of multilateral ferrite and Acicular ferrite, and the multilateral ferrite should not be too high, and the smaller the ferrite grains the better.