There are three principal process parameters for induction bending which affect the material properties – these are: speed, peak temperature and rate of cooling. Secondary process parameters,which are very specific from machine to machine and depend on the sophistication of the control process for each machine, are the start and stop procedures. Once qualified, these parameters must be set as the target parameters for all subsequent production bends.
High Strength HFW Linepipe
Modern HFW line pipe steels are relatively low carbon micro-alloyed steels. Induction bending isgenerally carried out in the temperature range 875C to 1075C which is above the austenitizingtemperature where re-crystallisation takes place. Over this temperature range the dissolution of micro-alloyed elements increases with temperature. For a given starting chemistry, the peak temperature achieved during induction heating and the rate of cooling determine the resultingmaterial properties. The established relationship of increasing strength and hardness withincreasing temperature and/or cooling rate is complex and is not the point of detailed discussionhere – suffice to say that the strengthening mechanism is a combination of grain size effects, thesolution and re-precipitation of micro-alloying constituents and the formation of low temperature transformation products.
To confidently achieve high strength and toughness directly off the induction bending machine, the peak temperature and cooling rate needs to be carefully controlled and this process must bedetermined and supported by physical testing. For a fixed speed and constant cooling rate, the peak temperature is controlled by the level of induction power applied during the bending process. Thecooling is rate determined by the speed of bending and the cooling water spray system comprising pressure, volume and apertures etc.
Higher processing temperatures and rates of cooling give higher strength and hardness; but lower elongation and impact properties. For high strength HFW linepipe it tends to be the yield strengthafter bending which dictates the necessary induction bending process parameters. Obtaining highstrength directly off the induction bending machine becomes more difficult when:
. the size of the pipe limits the bending speed;
. the pipe has inadequate chemistry (as indicated by a low carbon equivalent CEQ);
. low hardness is required for sour service.
The pipe wall thickness has a major bearing on the success or otherwise of the induction bending process in achieving high strength. For a given induction heating frequency, as the pipe wallthickness increases the induction bending speed slows to allow sufficient soak time to through-heatthe pipe wall. At slower speeds the cooling rate reduces since the pipe bend emerges more slowlyfrom the heating coil into the cooling water spray ring. In addition, for thicker wall pipe thedistance increases for heat conduction from the bore of the pipe to the water spray heat sink at theoutside surface. Slower cooling rates result in lower hardness and strength toward the bore of the pipe.
In recognition of the effects of the induction bending process and the technical requirements for pipeline maximum hardness, The Pipeline Induction Bending Standard ISO 15590-1 limits thehardness for induction bends for PSL1&2 class of pipe to a maximum value of 300 HV10; and for PSL2S (Sour Service) a maximum allowable hardness of 250HV10 at the bore and 275HV10 at theoutside surface.
An understanding of the induction bending process can be gained by an evaluation of how variousstarting conditions and process parameters affect the material properties. Some general trends areclearly evident; in particular strength and hardness increase with higher processing temperaturesand chemistries (CEQ).
The graphs below have been derived from preliminary test results performed over the past severalyears as part of the qualification testing programme for a select sample of HFW linepipe induction bends. The selected sample of motherpipes were typically high yield strength (413 – 482 MPa)with wall thicknesses ranging from 7.9 to 14.3mm. The induction bending process parameters didvary between projects but were typically: speed 40 to 65mm/minute and peak induction bendingtemperatures between 875C to 1,150C. All bends were water spray cooled, but at various pressuresand spray nozzle configurations. Details have not been shown for individual projects - rather theintent is to illustrate the trends which are evident despite the variety in pipe diameter, chemistry,wall thickness and processing parameters.
More subtle trends have been observed over time, these are:
•An increase in strength and hardness with an increase in the cooling rates – either by faster processing speeds or higher cooling water pressure.
•For a given chemistry (CEQ) and processing temperature and cooling rate; a lower overall pipe strength results as the pipe wall thickness increases. This is due to the proportionatelyincreasing contribution of material which has undergone slower and slower rates of coolingas the wall thickness increases and cooling rates diminish with distance from the coolingsurface.
•Lower toughness and elongation with increasing temperature or cooling rates.
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