TPT January 2010

A rticle

How does the Bauschinger effect change the physical strength of welded steel pipe? By Lew Warren, P. Eng. Consultant, Warren Consulting Inc.

To explain the Bauschinger effect in simple terms, it can be defined as the phenomenon by which plastic deformation increases the yield strength in the direction of plastic flow, tensile, and decreases the yield strength in the opposite direction, compression. Figure 2 is a visual illustration of this statement. This is where the importance of the neutral axis comes to our attention and this article deals only with the neutral axis transverse to the rolling direction of the hot mill and not the neutral axis parallel to the hot mill rolling direction. The neutral axis will be visible, after an acid etch, when the flat material is formed into a round tube or pipe. Figure 2 illustrates the flat steel after being formed into a pipe with the neutral axis still very near to mid wall and the direction of forces above and below the axis. Fibres above the axis, ID surface, are in compression and fibres below the axis, OD surface, are in tension. In reality the transverse neutral axis does not remain at mid wall when flat plate is cold bent in a direction transverse to the hot mill rolling direction. The compressive force on the fibres above the axis (inside surface) tends to displace the axis more to the area below (outside surface). This amount of displacement is generally minimal. In the study of Strength of Materials 101 we learned the meaning and usage of the terms Elastic and Plastic as related to a relative ductile, polycrystalline material; such as steel. An elastic material is one that returns to its original shape when the applied load is removed. A plastically deformed material, such as steel, does not return to its original shape when the applied load is removed; there remains a degree of deformation within the fibres. The Bauschinger effect is more troublesome under the following three conditions: Final round size of the product is relatively small A. The product material has a heavy thickness B. The product material is high strength C. The author has been in the welded steel tube and pipe industry for over fifty years. He realised that there may be one individual in any pipe manufacturing facility which produces line pipe, by any of the forming processes listed above, that is knowledgeable of the Bauschinger effect on their forming method. This individual may not have recorded data as to the percentage of physical properties change in their finished product as compared to the strength level of the incoming material. Still this person is aware that most of the line pipe producers will beef-up their ordered chemistry to offset the strength level change due to the Bauschinger effect.

ALL flat steel products, plates and plates in coil form (skelp), have a neutral axis regardless of their thickness. This neutral axis may not be discernible, even with an acid etch, until the flat material is formed into a circular shape (see figure 1). When these steel products are formed into a round shaper, such as tube and pipe, a phenomenon occurs that causes a change in the physical properties from the flat profile of the incoming material as compared to the rounded profile of the finished product. This chance occurs with the following pipe forming equipment: U press and O press type forming 1. Continuous roll forming, ERW type pipe manufacturing 2. Pyramid type roll forming 3. Helical profile (spiral) type forming 4. Progressive forming and folding process 5. Circumferential sizing and or expansion 6. Making round pipe specimen flat for physical testing 7. Physical properties change in these forming operations is caused by the Bauschinger effect.

 Figure 1 : Flat steel through its thickness in the direction of rolling. Neutral axis is shown transverse to the rolling direction

Neutral axis

F

Neutral axis in red Fibres in black

E

E F + E

Neutral axis position =

x 100 = % of thickness

from ID surface

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J anuary 2010

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