Special to Pipeline & Gas Journal
Operating for nearly 30 years, Triple D Bending is considered Western Canada’s leading bender. With three Induction Benders, state-of-the art technology and a partnership with Prooftest Consulting Inc., research and development projects are a consistent priority for the company. Recently, Triple D Bending prepared an in-depth study on the influence of low-temperature tempering on mechanical properties of induction bends.
Induction bending is a largely automated, free-forming process in which the front part of the received pipe is clamped to a pivoted arm and pushed through an induction ring.
The induction ring heats up the entire cross section of the pipe above Ac3 temperature to guarantee that after bending, the microstructure will be homogenous. The main issue with some induction bends is that the mechanical properties in the bending zone, which was heated above Ac3 temperature, fall below the required minimum values (yield strength) or exceed them (hardness).
This is typical for micro-alloyed steels with grade X65 and higher which are made by using thermo-mechanical controlled process (TMCP). The purpose of this article is to improve the knowledge of post-bend heat treatment (PBHT) of induction-bended pipes which are made from high-grade steel (in our case API 5L X70). Different tempering temperatures were applied to investigate the changes of mechanical properties in the bending zone.
Thermomechanically Controlled Processing
The thermomechanically controlled processing (TMCP) produces material with superior characteristics by controlling the deformation and the temperature of deformation during the hot rolling process.
The purpose of thermomechanically controlled processing of steel is to exploit the effect of plastic deformation above and below the recrystallization stop temperatures on the microstructure of austenite in such a way as to develop a most favorable, fine-grained microstructure on transformation to achieve improved mechanical properties.
Grain refinement is a technique which improves toughness and maintains, if not improves strength level of the steel. However, after induction bending, these properties rapidly decrease because a new microstructure is obtained and also tensile strain, in extrados part, and compression strain in intrados part, is given to the material. Mainly, tensile strain on the extrados part causes a significant drop of yield strength in comparison to the original value before bending. To reach the desired values where the product could meet the minimum requirements, post-bend heat treatment is applied.
Tempering of HSLA Steels
Bends for oil and gas application should be heat-treated after bending, if required by the manufacture procedure specification (MPS) or code, to fulfill the high-quality level of general standards and requirements. In this case, the test bend must meet the requirements according to Canadian Standards Association (CSA) specification for pipe fittings, (Table 1).
However, in this specification, the temperature of post-bend heat treatment cannot go under 540°C with a minimum soaking time for 30 minutes. When applying tempering for HSLA steels, competing mechanisms such as Cottrell effect, recovery and secondary precipitation are leading to modified strength. Depending on the chemical composition of steel, applied temperature and soaking time, the increase of yield strength during tempering can differ.
At temperatures higher than 600°C the yield can also decrease. For example, when tempering HSLA steels that contain a mixture of Mo, Ni and Nb alloys, the secondary precipitation hardening is pronounced at temperatures from approximately 580°C to 620°C. On the other hand, while the yield strength increases, the tensile strength decreases.
Experimental Procedure, Material Analysis
The examined material is thermomechanically rolled HSLA API 5L grade X70 steel which mechanical properties are listed in Table 3 and chemical composition (Table 2).
After induction bending, the samples for destructive testing were cut in transverse orientation from six locations, (Fig.1). These locations were tested in as-received condition (no heat treatment) and after providing two experimental post-bend heat treatments, one at 540°C for 30 minutes and 500°C for 30 minutes.
After providing two experimental heat treatments the samples for tensile and toughness tests were machined according to ASMT A370. Metallographic samples were made according to ASTM E3-11. For hardness measurement nine indents were made in transverse orientation in the extrados, intrados and neutral axis location. Neutral axis weld consisted of 15 indents.
Heat treatment was provided in a draw (annealing or tempering) furnace Cress 162012. After finishing soaking, the samples were cooled in the furnace to 427°C, and then cooled on air.
In as-bent condition (no PBHT) the tensile strength meets the criteria for minimum tensile strength. After applying PBHT 540° C/30 min tensile strength of the received test bend in all locations decreases (Table 4). Here it is important to notice that in the extrados location the tensile strength decreases rapidly, below the minimum criteria.
However, after applying 500°C/30 min regime the drop of tensile strength is less significant. According to the values (Table 4), it is possible to assume that tempering below 540°C has a smaller negative impact on tensile strength.
The yield strength (0.5% extension under load) in as-bent condition compared with the values (Table 3) decrease significantly, below the minimum requirements. After applying 540°C/30 min regime the yield strength in the bend zone increases, especially in the extrados and intrados location and slightly decreases in neutral axis location.
However, this increase of yield strength isn’t sufficient enough and extrados location doesn’t meet the minimum requirements. When applying the 500°C/30 min regime the yield strength in all locations increases. According to the values (Table 4), it is possible to assume that tempering below 540°C has positive impact on yield strength. All the locations after applying 500°C/30 min regime meet the minimum requirements for yield strength. Yield strength of neutral axis weld isn’t recorded because, according to the specification, it is not recommended.
After applying PBHT the Y/T ratio of the received test bend in all locations increases (Table 4). Y/T ratio from neutral axis weld location wasn’t measured because it is not required in the specification. The values of Y/T ratio in as-bent and PBHT condition meet the requirements (Table 1). The elongation of the received test bend in all locations is approximately the same, and also meets the requirements (Table 4). Elongation from neutral axis weld location wasn’t provided because it was not required.
Induction bends also must meet the minimum requirements for toughness. That’s why an appropriate temperature, and amount of quenching, must be chosen to provide the induction bend with good toughness properties. According to Table 1, the minimum absorbed energy for this grade is 27 J (seamless locations), and 18 J for locations containing welds. In as-bend condition the toughness values (Table 5) meet these minimum requirements.
After applying PBHT the toughness in all locations increases. This means that tempering at temperatures below 540°C has positive effect on toughness properties for steel with the chemical composition (Table 2).
Hardness values of induction test bend do not exceed 300 HV10 and therefore meet the requirements for sweet condition. Sweet condition means that the pipe can be used only for transportation of natural gas that does not contain significant amounts of hydrogen sulfide. Again, due to increased toughness properties, the hardness after PBHT decreases, thanks to the softening processes during tempering; the average hardness value of each location (Table 6).
As mentioned above, temperature used in the induction bending process exceeds the Ac3 temperature, and therefore a new microstructure is obtained. Instead of elongated grains and upper bainite (or pearlite), the microstructure (Figure 2) contains a mixture of ferritic grains with (probably) small islands of martensite.
Figure 3 represents the microstructure of extrados location after applying PBHT 540°C/30 min. As one can see during tempering on this temperature, the grains get coarser, which can explain the loss of tensile strength (Table 5). The increase of yield strength can be due to precipitation strengthening mechanism, mostly because of niobium carbide (NbC) particles or free nitrogen contained in the steel.
Figure 4 represents microstructure of the same location after applying PBHT 500°C/30min where the grain coarsening is less pronounced due to lower temperature. That’s why the loss of tensile strength is not as significant as in the previous heat treatment. Also, the secondary precipitation takes place, which can explain the increase of yield strength.
Because the requirements for heat treatment of induction bends have not been revised for years, and the procedures for thermomechanical controlled rolling as well as the chemical composition of the material have improved, the purpose of this article was to suggest that temperatures below 540°C could also be considered as a part of post-bend heat treatment in the future.
After destructive testing of induction-bended pipe and provided experimental heat treatments, it is possible to assume that both regimes increase the yield strength and decrease the tensile strength, decrease hardness, and increase toughness. However, when applying lower tempering temperature (500°C), the decrease of tensile strength is not as significant as it is when applying 540°C.
The most important thing is that when tempering at 500°C the yield strength increases more than at 540°C. After taking into account the measured values after applying 540°C tempering temperature, the test bend would not meet the minimum requirements because it would fail in the extrados location on low yield.
But after applying 500°C tempering temperature, all locations in the bend zone would meet the minimum requirements. Of course, these conditions can be related only to the material with such (or approximate) chemical composition (Table 2).
However, the CSA specification for pipe fittings also requires PBHT for tangent body and weld location. In this case, additional tests must be done to assure that PBHT below the minimum recommended temperature stated in CSA specifications doesn’t have negative influence on mechanical properties of induction bends.
Acknowledgements: Assistance provided by Prooftest Consulting Inc. was greatly appreciated.
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