Yield strength, ageing and coating

Received a good question today:

Does anyone know how much particular coating types increase the yield strength? And the physics behind it?

There was extensive work in this area sponsored by the APIA Research and Standards Committee.  Members of the RSC are entitled to the report, which I think is now available through the Energy Pipelines CRC (will check and advise if anything different).  The report title is “High Y/T and low strain to failure effects in coated high strength pipe”.  But it’s nearly 200 pages so here is a very simplified answer to the question (and this is a lay interpretation from a non-metallurgist):

The phenomenon is called “ageing”, although it isn’t necessarily related to the steel getting older.  Plastic deformation of steel occurs through the movement of imperfections through the crystal lattice.  Under certain conditions those imperfections can become pinned and unable to move further, so plastic deformation ceases.  The practical effect of this is to increase the yield strength (ie. the stress at which plastic deformation begins).  Metallurgy texts should be able to explain in more detail; it’s a bit too technical for superficial Googling to help much.

One of the conditions that causes ageing is exposure to high temperature.  It doesn’t have to be extremely high, and the temperatures encountered in coating plants are sufficient to have an effect.  To apply fusion-bonded epoxy (FBE) coating the steel is heated to around 250ºC and stays above 200ºC for a couple of minutes.  That’s enough to cause a significant increase in the yield strength.  However the ultimate tensile strength is not affected.

Perhaps contrary to expectation, an increase in yield strength is not a good thing, especially if the tensile strength is only a bit higher and remains unchanged.  The effect is to increase the ratio of yield to tensile strength (Y/T, hence the title of the report mentioned above).   High Y/T means is that once the steel starts to yield there isn’t much scope for further strain before it reaches the tensile strength and starts to fail (ie. it is less ductile).

Low grade steels have quite low Y/T (maybe around 0.7) so ageing isn’t much of an issue.  But modern high grade steels (say X65 and up) may have Y/T above 0.9, and a 5-10% increase in yield strength due to ageing during coating means that the in-service Y/T may be close to 1.0 (ie. negligible strain capacity before failure).

In normal operation of a pipeline we never expect to approach yield so it might seem that this shouldn’t be a great problem.  But high Y/T is of concern for at least two reasons:

  • High level hydrostatic testing takes the steel close to or beyond yield, and if Y/T is too high there is a worry that the plastic strain capacity of the pipe might be exhausted before it is even put into service; similarly for cold field bending.
  • If high Y/T pipe is later subject to some sort of insult such as external interference damage or ground movement it may lack the ductility to yield gracefully and instead progress to failure after only minimal strain (ie. leak or rupture)

The original question was about how much different coating types affect the yield strength, and I don’t have a proper answer to that.  I’ve already mentioned FBE, and the same temperatures would apply to trilaminate coatings which also start with a layer of FBE.  I think extruded HDPE coatings (“Yellowjacket”) require a slightly lower pipe temperature – I’ll see if I can find out.

However I don’t think it would be appropriate to let the ageing problem have much influence on coating selection.  What is important is that if you are specifying high grade linepipe steel you should have a good understanding of steel properties, and seek specialist advice if necessary.

[Thanks to Kristy Brincat for the question.  The Y/T research program ran over a few years, capably directly by Leigh Fletcher as lead industry advisor guiding researchers Michael Law of ANSTO and Graham Bowie (now retired) of BlueScope Steel.  I contributed a little mechanical engineering input to the industry advisor team.  There is a follow-on project looking in a low-key way at the in-service properties of linepipe after both coating and field hydrostatic testing, but it is progressing only slowly after falling victim to resource constraints among both researchers and industry advisors.]

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4 Responses to Yield strength, ageing and coating

  1. Josh Wickham says:

    Hi Peter,
    Thanks for the informaiton, very interesting. I have a copy of a Onesteel report where this is discussed (Current Trends in Line Pipe Manufacture by John Piper) and had a couple of queries – although it seems I need to get a copy of the report you’ve listed. In the Onesteel report toughness was noted as one of the properties most likely to change with FBE coating (along with yeild strength & hardness) – in the bulk of the report it states the pipe body and weld charpy test showed little difference between coated and uncoated conditions (I couldn’t find toughness data listed for pre and post coated pipe). I would have thought that an increase in yeild strength would also result in a reduction in toughness – can you comment on this? Is this discussed further in the 200 page report?

    Also – regarding the yield ratio. How do you see this being mananged going forward – would it be a case of requesting a lower Y/T ratio from the steel supplier (i.e. well below the 0.93 limit for API5L PSL2 – or perhaps imposing a Y/T limit for PSL1 pipe)…or perhaps controlling composition (i.e. minimising free carbon / nitrogen – although I note known nitrogen content isn’t listed in API5L composition) to account for a predicted ageing affect…or do you think we’ll head down the path of requiring qualification of the affects of the coating process on a project by project basis (i.e. supplementary post coating mechanical testing for each pipe supplier on a project, or even heat?)

  2. John Piper says:

    Hello All,

    Intersting questions and well worth answering. Its regrettable that the ‘In-service pipe properties” project languished. Perhaps it can be resurected – we’ll see.

    To flesh out a couple of points and provide a counter view on others:

    Strain aging occurrs when the stress at which a previously strained piece of metal, like the wall of a pipe after forming, has an increase in strength because the dislocation structure is pinned by carbon and or nitogen atoms that diffuse to the dislocations using the energy available from the heating cycle, in this case during coating. Once pinned, they either can’t move and new dislocations must form, or additional stess is needed to break them away from their cloying atmospheres. The extra stress is, of course, peceived as extra strength.

    Data I’ve generated consistently demonstrates no change in toughness. The increase in yield strength does increase Y/T, however typically from levels from 0.85 to 0.90 before FBE coating to levels up to 0.93 after coating. These levels are considered acceptable by most designers.

    There are only two risks; issues with field hydrotest overstraining and exhaustion of ductility in the presence of gross deformation. The former is a potential issue with low Y/T material, as high Y/T pipe is too strong to yield. The jury is out on the latter concern – not enough experience. It is realy only an issue for construction dents and third party damage. Both have standard control measures which mitigate risk.


  3. petertuft says:

    I think John has pretty well answered Josh’s questions, certainly much better than I could. (For those who don’t know him, I don’t think John will mind me pointing out that he has been a pipe and metallurgy guru at OneSteel for a very long time, and also played a key industry advisor role in the Y/T research project.)

  4. Kristy says:

    Thanks Peter! This makes more sense (I was reading AS2885.5), with the Y/T higher for higher grades of line pipe, will this limit the strength we can go up to for new higher strength line pipe (in the States they use higher grades than we do)? Is there a way we can manufacture pipe with a higher Y but lower Y/T?

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