L390, X56 and the Two Unit Systems of API 5L
L390 is the SI-unit designation for a line pipe grade whose specified minimum yield strength is 390 MPa. The same grade appears in the United States customary tables as X56, where the 56 stands for 56 000 psi. Both tables sit in API 5L and ISO 3183, they describe the same steel, and they are not arithmetically identical: 56 000 psi converts to about 386 MPa, so a pipe certified as X56 and a pipe certified as L390 are certified against different numbers in different tables. That difference matters at the point of rejection. A heat that yields at 388 MPa fails as L390 and passes as X56, and the only way to know which rule applies is to read the purchase order.
The second source of confusion is the suffix. L390N, L390Q and L390M are not three strength grades; they are one grade delivered in three different metallurgical conditions. The suffix letter describes how the plate or strip reached its properties, and it changes the chemistry limits, the toughness behaviour, the weldability and the price. Treating the three as interchangeable because they share the "L390" prefix is the single most common error in enquiries for this class of pipe.
Decoding the Suffix Letters: R, N, Q and M
Under the product specification level 2 route, the delivery condition is indicated by a suffix appended to the grade designation. Four letters are defined, and each carries a different set of consequences.
R - as rolled. The pipe is finished by controlled rolling without a separate normalising or quenching step. It is the cheapest condition, and it is difficult to combine with 390 MPa minimum yield and reliable low temperature toughness at the same time, which is why L390R is rarely offered.
N - normalised. The product is reheated above the upper critical temperature and air cooled, producing a uniform ferrite-pearlite structure with a consistent grain size along the length of the pipe. Properties are homogeneous and predictable, and the process is forgiving of variations in the rolling schedule.
Q - quenched and tempered. The product is austenitised, quenched, usually in water, and then tempered. This route produces tempered martensite or bainite, gives the highest strength per unit of alloy and the best low temperature toughness, but it consumes more alloy, more energy and more process control.
M - thermomechanically rolled. The rolling schedule is controlled so that the final passes occur in a temperature window that produces a fine grained ferrite or acicular ferrite structure without a separate heat treatment. It achieves high strength and toughness with lower carbon and a lower carbon equivalent than the Q route, which is why it welds more easily in the field.
Because the three conditions reach the same strength band by different metallurgical routes, a mill may offer any of them against a bare "L390" enquiry and quote three different prices. The specification of the delivery condition is therefore a commercial decision as much as a technical one, and it belongs in the order, not in the mill's discretion.
Chemistry by Delivery Route
The chemistry limits for the normalized and quenched and tempered routes differ in a way that a buyer can check on the mill certificate. The N route relies on solid solution strengthening and grain refinement plus a generous manganese addition, so it tolerates higher carbon. The Q route relies on the quench to develop strength, so carbon is cut back to protect weldability and toughness, while manganese is raised to maintain hardenability.
| Element, % max | L390N route | L390Q route | Practical meaning |
|---|---|---|---|
| C | 0.24 | 0.18 | Lower carbon in Q reduces hardness in the weld HAZ |
| Si | 0.45 | 0.45 | Deoxidation, identical in both routes |
| Mn | 1.40 | 1.50 | Higher manganese compensates for lower carbon in Q |
| P | 0.025 | 0.025 | Residual, capped for toughness |
| S | 0.015 | 0.015 | Residual, capped for weldability and through thickness ductility |
| V | 0.10 | 0.07 | Microalloying, tighter in Q to preserve toughness |
| Nb | 0.10 | 0.07 | Grain refinement, tighter in Q for the same reason |
| Ti | 0.04 | 0.04 | Nitrogen fixation in the weld heat affected zone |
Two notes belong under that table. First, the vanadium, niobium and titanium limits are not independent in practice: the combined addition is capped, so a heat that runs at the maximum on all three will be rejected even though each individual column is satisfied. Second, the table describes limits, not typicals. A competent mill will aim well inside the window to keep a margin for the mechanical test, and a certificate showing carbon at exactly 0.180 % on a Q route pipe is worth a question about how the analysis was rounded.
Strength, Ductility and the Yield to Tensile Ratio
In the SI tables, L390 requires a minimum yield strength of 390 MPa and a maximum of 545 MPa, with tensile strength between 490 MPa and 760 MPa. The corresponding United States customary figures for X56 are 56 000 psi minimum and 79 000 psi maximum yield, with 71 000 psi to 110 000 psi tensile. The upper limits are as binding as the lower ones, and they exist for a reason: an over-strength pipe raises the stress that the girth weld has to accommodate and makes the pipe less able to deform if the line is subjected to ground movement.
| Property | L390 / X56, PSL 2 | Comment |
|---|---|---|
| Yield strength, minimum | 390 MPa (56 000 psi) | Determined from the 0.5 % total extension or 0.2 % offset, as specified |
| Yield strength, maximum | 545 MPa (79 000 psi) | Over-strength pipe is rejected, not accepted |
| Tensile strength, minimum | 490 MPa (71 000 psi) | Governs the elongation calculation |
| Tensile strength, maximum | 760 MPa (110 000 psi) | Ceiling for the same reason as the yield ceiling |
| Yield to tensile ratio, maximum | 0.93 | Ensures reserve deformation capacity before fracture |
| Elongation, minimum | From the standard's formula | Calculated from cross sectional area and specified minimum tensile strength |
The elongation requirement is calculated rather than fixed, using the relationship published in the standard between the applicable cross sectional area of the tensile specimen and the specified minimum tensile strength of the grade, with the result rounded down to the nearest percent. That is why two certificates for the same grade can show different minimum elongation values: the specimens had different cross sectional areas. A buyer comparing elongation figures between mills should confirm that the specimen geometry was the same before drawing any conclusion.
The yield to tensile ratio ceiling of 0.93 is the property most often overlooked in procurement. A pipe with high yield and moderate tensile can pass both strength limits and still fail the ratio, and in service that pipe has little capacity to redistribute strain at a wrinkle or a settlement. Where the line crosses unstable ground, buyers frequently tighten the ratio below the standard ceiling and specify it as a project requirement.
Carbon Equivalent and Weldability by Route
Weldability in this grade is governed by carbon equivalent rather than by carbon alone. The standard offers two expressions, the conventional carbon equivalent based on carbon, manganese, chromium, molybdenum, vanadium, nickel and copper, and the cracking parameter used for low carbon microalloyed steels. Which one applies depends on the grade and the analysis, and a purchase order that does not say which expression governs leaves the mill free to choose the more favourable one.
The practical ranking between the three delivery conditions is consistent. The M route produces the lowest carbon equivalent for a given strength, because thermomechanical processing does part of the strengthening work, and it is therefore the easiest to weld without preheat in moderate wall thickness. The N route sits in the middle, with a carbon equivalent driven mainly by its higher carbon and manganese. The Q route can meet the same carbon equivalent ceiling by cutting carbon, but the quenched structure itself is harder in the heat affected zone, so hardness control at the fusion line becomes the limiting factor rather than the analysis.
For field girth welding this translates into procedural differences that must be qualified, not assumed: preheat and interpass temperature ranges, heat input windows, consumable hydrogen class and, in the Q route, a check that the tempering treatment has not been locally destroyed. A welding procedure qualified on L390M is not automatically valid for L390Q even though both are L390.
Toughness Testing and Impact Requirements
Product specification level 2 requires Charpy V-notch testing, and the results are reported on the certificate. The requirement is expressed as a minimum average absorbed energy for a set of three specimens taken from the pipe body, at a stated test temperature, and the usual acceptance rule allows an individual specimen to fall below the specified average provided it stays above a defined fraction of that average. Test temperature is a project variable: ambient service lines are typically tested at 0 degrees Celsius, while low temperature or arctic service pushes the test temperature down and the energy requirement up.
Toughness is where the delivery condition shows its value most clearly. The Q route, with its tempered martensitic structure and fine prior austenite grain size, delivers the highest upper shelf energy and the lowest ductile to brittle transition temperature for a given strength. The M route with a fine ferrite or acicular ferrite structure runs close behind and often outperforms expectations at large wall thickness because the thermomechanical schedule suppresses segregation. The N route is the most variable: a well controlled normalising treatment gives good, uniform results, but a heavy wall normalized at a slow cooling rate can produce coarse pearlite bands that reduce both the absorbed energy and the consistency between specimens.
Two supplementary tests are worth specifying where the line is designed for arrest of a running fracture: drop weight tear testing, conducted according to API RP 5L3, and, for very large diameter high pressure lines, a CTOD or wide plate assessment. Neither is part of the base grade requirement, and both must be written into the order before production, because they cannot be retrofitted to finished pipe.
Manufacturing Routes and Dimensional Range
L390 is produced as seamless pipe, as welded pipe by electric resistance welding, and as submerged arc welded pipe in both longitudinal and helical configurations. The delivery condition interacts with the route: the Q condition is normally applied to the finished pipe after forming, whether seamless or welded, while the M condition is applied to the plate or strip during rolling and must be preserved through the forming operation, which in practice restricts the M route to welded pipe.
Typical size range: outside diameter from about 10 mm to 762 mm, wall thickness from 2 mm to 50 mm, length from 0.5 m to 12.2 m.
Plain end mass: calculated as 0.02466 x wall thickness x (outside diameter minus wall thickness), giving kilograms per metre when dimensions are entered in millimetres. This is the figure used for weighing bundles and for freight calculations.
End finishes: plain or square cut, saw cut, beveled for girth welding, or threaded where the pipe is used in a screwed application.
Coating: three layer polyethylene, three layer polypropylene, fusion bonded epoxy or an internal epoxy lining, each with its own thickness and holiday test requirements.
Hydrostatic testing is mandatory. The test pressure is calculated from the standard relationship between hoop stress, wall thickness and diameter, with the permitted hoop stress expressed as a percentage of the specified minimum yield strength. Buyers who need a test pressure above the standard value must state it, because mills size their test equipment around the default and may not have the capability for a higher figure at large diameter.
Inspection, Documentation and PSL 1 versus PSL 2
The two product specification levels differ in ways that go far beyond chemistry, and the difference is the first thing a buyer should settle. The comparison below summarises what actually changes on the certificate and in the inspection scope.
| Item | PSL 1 | PSL 2 |
|---|---|---|
| Chemical composition | Wider limits, fewer restrictions on residuals | Tighter limits, specified for each delivery condition |
| Carbon equivalent | Not required unless agreed | Required, with limits dependent on grade |
| Charpy impact test | Not required in the base specification | Required and reported |
| Non-destructive inspection of welds | By agreement | Required for welded pipe |
| Heat treatment | Not always required | Specified by the delivery condition suffix |
| Traceability | Heat number to pipe | Heat number plus full manufacturing record |
| Certificate | Inspection certificate | Inspection certificate with complete test data |
Non-destructive testing on welded pipe follows the ISO 10893 series for the relevant method, with ultrasonic examination of the seam as the usual requirement and acceptance limits stated as a severity level. On seamless pipe the body is examined ultrasonically to ISO 10893-10 or ASTM E213. A purchase order that asks for "ultrasonic testing" without a level or an acceptance criterion has not asked for anything enforceable.
Sour Service and Other Service Restrictions
Where the pipe will carry wet gas or oil containing hydrogen sulphide, ISO 15156 with its NACE MR0175 counterpart governs material selection. Carbon steel line pipe can be used in sour service, but only if hardness is controlled, and the usual ceiling for carbon steels is 250 HV10. That ceiling is achievable in the M and N conditions with careful welding, and it is achievable in the Q condition only if the tempering treatment and the weld procedure are both controlled to avoid local hard spots. Where sour service is expected, the hardness survey on the weld and heat affected zone becomes the governing acceptance test, and it should be specified as such.
Two other restrictions belong in the same conversation. Low temperature service below the Charpy test temperature chosen for the project requires a fresh toughness assessment rather than an extrapolation. And where the pipe will be cold field bent, the permitted bending strain must be limited and the bend subsequently checked for ovality, because bending a quenched and tempered pipe consumes ductility that the design may be relying on.
Procurement Checklist and Common Mistakes
A complete enquiry for L390 pipe states six things: the grade with its delivery condition suffix, the product specification level, the size with tolerances, the impact test temperature and energy, the non-destructive examination level, and the certificate type. Omitting the suffix is the mistake that costs most, because it allows three different products to be quoted as if they were one.
Do not specify "X56" and "L390" interchangeably without saying which unit table governs acceptance.
Do not accept a certificate that reports only yield and tensile without carbon equivalent and Charpy results on a PSL 2 order.
Do not assume that a welding procedure qualified on one delivery condition covers another.
Do not accept a hydrostatic test pressure below the standard calculation for the grade and wall.
Do check that the mass per metre on the packing list matches the calculated plain end mass, since weight discrepancies usually indicate a wall thickness deviation.
Technical literature on this grade also carries a persistent error worth correcting: several secondary descriptions list potassium among the elements in the chemical composition of L390Q. Potassium is not an alloying element in line pipe steel and does not appear in any composition table in the standard. It is a transcription fault, and its presence in a supplier's technical page is a fair indicator of how carefully the rest of the data was assembled.
FAQ
Q: Is L390 the same grade as X56?
They designate the same strength class in two unit systems. L390 means 390 MPa minimum yield in the SI tables of API 5L and ISO 3183, while X56 means 56 000 psi minimum yield in the United States customary tables. The converted values differ slightly, so the purchase order should state which table governs acceptance.
Q: What does the N, Q or M suffix change?
It changes the metallurgical route used to reach the properties. N means normalised, with a ferrite-pearlite structure. Q means quenched and tempered, giving tempered martensite with the best toughness per unit strength. M means thermomechanically rolled, giving a fine ferrite structure with the lowest carbon equivalent of the three and therefore the easiest field welding.
Q: Why is there a maximum yield strength as well as a minimum?
The 545 MPa ceiling exists to keep the pipe deformable and to protect the girth welds from over-matching. A pipe that is stronger than specified is rejected because it transfers strain into the weld and reduces the line's ability to accommodate ground movement without fracture.
Q: Which delivery condition should be chosen for a cold climate pipeline?
The quenched and tempered or thermomechanically rolled condition, because both give a lower ductile to brittle transition temperature than the normalized route at the same strength. The impact test temperature and energy must be stated explicitly, since the base grade requirement does not fix them for a cold climate application.
Q: Is impact testing required on L390 pipe?
It is required at product specification level 2 and is not part of the base requirement at level 1. Since level 2 also brings tighter chemistry, carbon equivalent limits and mandatory weld inspection on welded pipe, most transmission projects specify level 2 as a matter of course.
Q: How is the hydrostatic test pressure determined?
It is calculated from the hoop stress relationship in the standard, using wall thickness, outside diameter and a permitted hoop stress expressed as a percentage of the specified minimum yield strength. If the project needs a higher pressure than the standard value, the requirement must be stated in the order so the mill can confirm its test capability.
