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Effect of ferrite on DWTT performance of pipeline steel
- Sep 03, 2018 -

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

For the pipeline steel used in the production of oil and gas pipelines, not only need a certain strength, but also have a certain toughness, so the production of pipeline steel are required to carry out DWTT test testing, and this parameter and the body Ferrite has a close relationship.

1. The effect of ferrite type on DWTT

In pipeline steel, a multilateral ferrite or acicular ferrite is generally formed.

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

Because the multilateral ferrite belongs to the low strength, the plastic good phase, so the right amount of polygon ferrite can improve the ability of coordination between various organizations, in addition, the existence of grain boundary pearlite, the ferrite deformation is limited, deformation resistance increased, crack formation and expansion work increased, thus toughness also improved. 2) passivated crack tip. The multi-lateral ferrite is ductile and low in strength, which can cause large plastic deformation at the crack tip, hinder crack propagation, and loosen 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 growth, resulting in a higher toughness.

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

And it has dispersion distribution of precipitates, fine sub-structure, and so on, each unit length has a considerable number of grain boundaries, this compact microstructure hinders the initiation and expansion of crack, and thus has excellent toughness.

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

But the multilateral ferrite is not the more the better, the high content will lead to toughness decline.

3. Effect of ferrite grain size on DWTT The grain boundaries are different and irregular, and at the same time the grain boundary is the region of the atom permutation, the grain boundary resistance is very large when the micro-cracks pass through the grain boundary to another grain boundaries, and the crack propagation direction changes after the crack passes through the grain boundary, which will require additional energy. Therefore, the finer the grains, the number of cracks passing through the unit length and the change of crack growth direction more times, the crack expansion function is larger, the fracture fiber area of pipeline steel will be higher, and its DWTT performance is higher.

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