More on strain ageing

A couple of updates after some offline inputs on the strain ageing post (maybe much more than a lot of you wanted to know, but perhaps of interest to some):

Peter Mayes of Bredero Shaw (pipe coaters) provided the following:

Taking typical 457.3 x 9.1 mm pipe:  Pipe is induction heated to 245ºC.  We now also use an infra red temperature gauge post powder booth and FBE coated pipe temperature is constantly in range 189-194ºC after approx 60 s (ie. distance ~7 metres from induction coil exit).  This is for a FBE thickness around 900 microns and this cooling is assisted by the use of cool (-30ºC) air through the 60+ application guns.  We do not start water cooling until 75-90 s after the powder booth.  The heat decay rate is varied by steel mass and FBE thickness (and number of guns).  The heat decay for bare steel pipe in the report does not include the cooling from the cold FBE and cold air. For FBE coated pipe I believe that the temperature would be above 200ºC for 60-90 seconds.

Michael Law’s modelling is more relevant for the 3LPE/trilaminate system.  As the FBE thickness is only 150-200 microns, having a lower cooling potential and the HDPE is applied at 245ºC, with immediate commencement of cooling water, a temperature above 200ºC may well extend from 120-180 seconds.

For Yellow Jacket coated pipe the steel is heated to approx 55-60ºC to assist in mastic sealant wet out/flow.  Although HDPE is again applied at around 245ºC external flood cooling starts immediately and the steel temperature would probably not exceed 100ºC (noting that the HDPE is solid and able to support pipe weight approximately 6 m after the coil – 30 s approx).

So that reconfirms the FBE temperatures and also provides some information on the much lower temperatures involved in HDPE coating.   However does HDPE coating still result in ageing?

Valerie Linton (CEO of the Energy Pipelines CRC and heavily involved in the in-service strength research project) also provided some additional information, including this extract from the draft report on the in-service strength project:

… in mild steel strain aging takes a few days at room temperature and 30 minutes at 100ºC. Zhao et al (2000) studying a ultra low carbon (20ppm) bake hardened steel showed that the increase in yield strength … was achieved within a minute for samples aged at 140ºC, 25 min at 100ºC and a couple of days at 50ºC. From these observations it is not unreasonable to conclude that strain aging would occur during the FBE coating process as this occurs at around 250ºC. However, the literature does not provide a clear view as to the extent to which strain aging occurs at each temperature for a given period of time.

This suggests that the short period at less than 100ºC in the HDPE coating line will not result in much ageing.  On the other hand, pipe almost always gets left in the sun for a few weeks …

Valerie also had a proper metallurgist’s explanation of how steel yields:

And the imperfections that move through the crystal structure are called dislocations.  During plastic deformation (say during pipe making) the stress applied to the steel gives the dislocations enough energy to break free from the local concentrations of solute atoms that prevented them from moving. It is the movement of the dislocations that allows the steel to deform without cracking/breaking. Heating the steel (say during pipe coating) provides the solute atoms with enough energy to diffuseback to the more ‘spacious’ regions in the crystal structure adjacent to dislocations, and so the dislocations are repinned. To get them moving again requires more energy (in the form of applied stress), which has the practical effect of increasing the yield strength.

And finally, the variability of steel properties is all complex stuff for the non-specialist and raises obvious questions about what the pipe specifier is supposed to do (but see John Piper’s comment on the previous post).  Because this is only one of several issues that affect pipe specification but are not addressed well (or at all) by API 5L/ISO 3183 there are suggestions that the industry should develop an Australian standard linepipe specification so that we can all have the benefit of the knowledge accumulated by the experts.  It would be a good thing, but if it happens at all it will take quite some time.  Be patient.


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