BS EN 10216-5:2021 Standard For Seamless Stainless Steel Tubes

The EN 10216 Series: Five Parts Explained

EN 10216 is published in five parts under the general title "Seamless steel tubes for pressure purposes - Technical delivery conditions". Part 5 is the endpoint of the series: where the fluid or the environment attacks carbon and low-alloy steel, none of Parts 1 to 4 offers a solution and Part 5 is the correct specification.

The parallel series for welded product is EN 10217, Welded steel tubes for pressure purposes - Technical delivery conditions. Part 5 also overlaps the other parts on temperature: its austenitic grades carry impact data at − 196 °C, matching the coldest grade in Part 4, and its creep-resisting grades carry creep rupture data to 1 000 °C, well beyond anything in Part 2. Where corrosion is not a factor, the carbon and low-alloy parts are almost always the cheaper answer.

Steel Classification and Designation

According to the classification system in EN 10020:2000, the grades in this standard are classified as austenitic steels (corrosion-resistant or creep-resistant steels) and austenitic-ferritic steels. EN 10088-1 gives further detail.

The steel designation consists of the number of this part of EN 10216 (EN 10216-5) plus either the steel name in accordance with EN 10027-1:2016 or the steel number allocated in accordance with EN 10027-2:2015. In practice, both appear in trade: X5CrNi18-10 and 1.4301 are the same grade, and the standard's own order example uses the number.

The wall-thickness ceilings are not a footnote. Above them, the mechanical properties are subject to agreement at the time of enquiry and order but must still meet the minimum requirements of European legislation for pressure equipment - covered by Option 7 for the corrosion-resisting grades above 60 mm and Option 8 for the creep-resisting grades above 50 mm.

BS EN 10216-5:2021 Steel Grades

Austenitic Corrosion-Resisting Steels (Table 2)

Twenty-one grades, spanning the standard 18/8 austenitics through the molybdenum-bearing 316 family to the high-alloy super austenitics and nickel-base-adjacent grades.

The familiar grades sit at the top of that list. X5CrNi18-10 (1.4301) is the grade the trade calls 304 and X2CrNi18-9 (1.4307) is 304L; X5CrNiMo17-12-2 (1.4401) is 316 and X2CrNiMo17-12-2 (1.4404) is 316L, with X6CrNiMoTi17-12-2 (1.4571) the titanium-stabilised 316Ti. At the other end, X1CrNiMoCuN20-18-7 (1.4547) and X1NiCrMoCuN25-20-7 (1.4529) are super austenitics carrying 6,0 to 7,0 % molybdenum with nitrogen additions, for aggressive chloride service.

Austenitic Creep-Resisting Steels (Table 3)

Twelve grades are characterised for service in the creep range. These are the only grades in the standard with creep rupture data in Annex A, and the standard states plainly that steel grades not mentioned in Table A.1 are not intended for use in the creep range.

X5NiCrAlTi31-20 appears twice, once in the solution-annealed condition and once as 1.4958+RA in the recrystallized-annealed condition, and the two carry different creep rupture values. X8NiCrAlTi32-21 (1.4959) is the outlier of the group: its Annex A data runs from 700 °C to 1 000 °C, the highest temperature range anywhere in the EN 10216 series.

Austenitic-Ferritic (Duplex) Steels (Table 4)

Six duplex grades. These deliver roughly twice the proof strength of a standard austenitic with better chloride stress corrosion cracking resistance, at the cost of a 30 mm wall thickness and a 250 °C limit temperature.

X2CrNiMoN22-5-3 (1.4462) is the grade the market calls duplex 2205, and X2CrNiMoN25-7-4 (1.4410) is super duplex 2507. Both carry a mandatory 40 J impact requirement at − 40 °C, which the other four grades share except X2CrNiMoSi18-5-3. For cold-finished and solution-annealed tubes in 1.4462, the maximum tensile strength is 920 MPa rather than the tabulated 880 MPa.

Delivery Conditions and Surface Finish

This is where Part 5 differs most visibly from the carbon steel parts. Tubes are manufactured by a seamless process and may be hot finished or cold finished, but they are always supplied in the solution annealed condition over their full length, and the standard tabulates five distinct delivery conditions distinguished by surface finish.

The symbols are abbreviations of the condition, so CFD reads as Cold Finished Descaled. Combinations of the different conditions may be agreed upon at enquiry and order. For the ground and polished conditions, the enquiry and order must state whether the roughness requirement applies to the internal or external tube surface, or to both - an easy detail to omit and an expensive one to get wrong.

Unless Option 2 is specified, the type of delivery condition is at the manufacturer's discretion. On any order where surface finish matters - hygienic service, polished handrail-grade appearance, or a tube that will be electropolished later - Option 2 should be invoked.

Solution Annealing

Tubes are supplied solution annealed over their full length in either reference heat treatment conditions or solution annealed conditions obtained directly by extrusion and subsequent cooling, provided the mechanical properties, corrosion resistance and other properties conform to the standard. In the second case, all specified mechanical properties have to be met even after a subsequent reference heat treatment.

Solution treatment consists of heating the tubes uniformly to a temperature within the range given for the steel grade in Tables 6, 7 and 8, and cooling rapidly. The cooling medium is tabulated per grade as w (water) or a (air, cooling sufficiently rapid).

Two of the six duplex grades - 1.4507 and 1.4501 - specify water cooling only, with no air option. In the duplex grades, the cooling rate controls the ferrite-austenite balance and suppresses intermetallic precipitation, so this is a functional requirement, not a processing preference.

Handling and packaging carry a stainless-specific rule of their own: the tubes shall be protected from carbon steel strapping, which shall not come into contact with the tubes. Carbon steel in contact with a passivated stainless surface leaves iron contamination that initiates rust staining and pitting. Option 25 covers any special protection agreed at enquiry and order.

Test Category 1 vs Test Category 2

The test category is a classification indicating the extent and level of inspection and testing. It is mandatory order information for every grade in this standard - unlike Parts 2, 4 and 5's carbon steel counterparts, there is no family of grades exempted from stating it - and it appears in the tube marking.

Two features of the test category 2 regime are specific to the 2021 edition. The frequency of the room temperature tensile test in test category 2 was modified against the 2013 edition and now calls for two tests per test unit taken from different tubes. And the technological test in test category 2 is set at 10 % per test unit with a minimum of one, rather than the single test per unit that test category 1 requires.

The mandatory NDT threshold is dimensional rather than categorical. A test category 2 tube larger than 101,6 mm outside diameter or thicker than 5,6 mm wall gets NDT for longitudinal imperfections automatically; smaller and thinner tubes do not, unless Option 14 is invoked. On small-bore instrument tubing, this is the single most commonly missed option in the standard.

Chemical Composition of BS EN 10216-5:2021

Chemical composition is specified as cast analysis in Table 2 for austenitic corrosion-resisting steels, Table 3 for austenitic creep-resisting steels, and Table 4 for austenitic-ferritic steels, expressed in % by mass. Elements not listed shall not be intentionally added without the agreement of the purchaser, except for elements added to finish the cast, and all appropriate precautions are to be taken to avoid the addition of such elements from scrap or other materials that would impair mechanical properties or the suitability of the steel.

Key Austenitic Corrosion-Resisting Grades

Five footnotes govern Table 2. Option 3 allows a controlled sulphur content of 0,015 % to 0,030 % by agreement for products to be machined, provided resistance to corrosion is still satisfied for the intended purpose. Three further footnotes permit the maximum nickel content to be increased - by 0,50 %, 1,00 % or 1,50 % depending on the grade - where hot workability or low magnetic permeability requires deltaferrite to be minimised. Titanium-stabilised grades carry a 5 × C minimum, and niobium-stabilised grades a 10 × C minimum, tying the stabiliser content to the actual carbon level of the cast rather than to a fixed floor.

Product Analysis Deviations (Option 4)

Option 4 calls for a product analysis. Table 5 gives the permissible deviations from the cast analysis limits of Tables 2, 3 and 4:

A footnote to Table 5 sets a rule worth knowing before disputing a certificate. If several product analyses are carried out on one cast and an individual element falls outside the permissible range for cast analysis, it is only allowed to exceed the permissible maximum value or to fall short of the permissible minimum value, but not both for one cast.

Mechanical Properties of BS EN 10216-5:2021

Mechanical properties at room temperature apply irrespective of whether they are verified or not. Austenitic stainless steels do not show a sharp yield point, so the standard specifies both the 0,2 % proof strength Rp0,2 and the 1,0 % proof strength Rp1,0 for the austenitic grades. Duplex grades carry Rp0,2 only.

Austenitic Corrosion-Resisting Steels (Table 6)

For wall thicknesses up to 60 mm, in the solution-annealed (+AT) condition. Impact energy is the minimum average absorbed energy KV2 on three test pieces.

Proof strength and tensile strength in MPa, elongation and impact energy as marked; l = longitudinal, t = transverse. Two grades appear twice because the cold-finished and hot-finished conditions carry different strength values - X6CrNiTi18-10 and X6CrNiMoTi17-12-2 both gain roughly 20 MPa of proof strength in the cold-finished condition.

The − 196 °C impact column is the quiet headline of this table. Almost every austenitic grade in the standard returns 60 J transverse at liquid nitrogen temperature in the as-supplied solution annealed condition, with no special quality level, no separate grade and no alloying premium. That is the metallurgical reason austenitic stainless displaces 9 % nickel steel on many cryogenic jobs.

Austenitic Creep-Resisting Steels (Table 7)

For wall thicknesses up to 50 mm, in the solution-annealed (+AT) condition. Note that these grades carry impact requirements at room temperature only - no − 196 °C column - and that all of them are marked "no" for intergranular corrosion resistance in the delivery condition.

For X5NiCrAlTi31-20 + RA (1.4958+RA), the grain size after solution annealing shall be 1 to 5 according to EN ISO 643:2019. For X8NiCrAlTi32-21 (1.4959), it shall be 1 to 5 for cold-worked tube and 5 or coarser for hot-extruded tube. And X8CrNiMoVNb16-13 (1.4988) is additionally tempered at 750 °C to 800 °C for 1 h to 5 h and air-cooled.

Austenitic-Ferritic Duplex Steels (Table 8)

For wall thicknesses up to 30 mm, in the solution-annealed (+AT) condition. Duplex grades are the only family in the standard with a mandatory low-temperature impact requirement at − 40 °C rather than − 196 °C.

The strength of duplex is visible by comparison. X2CrNiMoN25-7-4 offers 550 MPa minimum proof strength against 205 MPa for X5CrNiMo17-12-2 - close to three times - and it holds 150 J longitudinal impact energy at room temperature while doing so. The costs are the 30 mm wall ceiling, the 250 °C limit temperature and the reduced elongation of 20 % against 40 %.

Elevated Temperature and Creep Properties

Proof Strength at Elevated Temperature

Minimum proof strength Rp0,2 and Rp1,0 at elevated temperatures are specified in Table 9 for austenitic corrosion-resisting steels, Table 10 for austenitic creep-resisting steels, and Table 11 for austenitic-ferritic steels. Verification is optional under Option 9, with the test temperature agreed at enquiry and order; for the duplex grades in Table 11, only Rp0,2 applies.

Minimum Rp0,2 in MPa for selected austenitic corrosion-resistant grades (Table 9):

Minimum Rp0,2 in MPa for the austenitic-ferritic steels (Table 11). The duplex grades are tabulated only to 250 °C, and 250 °C is also their limit temperature for intergranular corrosion susceptibility:

The 250 °C ceiling on duplex is a hard design constraint, not a data gap. Above roughly that temperature, prolonged exposure embrittles the ferrite phase, and the standard simply stops characterising the grades rather than extrapolating.

Creep Rupture Strength (Annex A)

Annex A is informative and gives mean values as preliminary data about creep rupture strength for the austenitic creep-resisting steels only. Values are given at 10 000 h, 100 000 h, 200 000 h and 250 000 h. The standard notes that steel grades not mentioned in Table A.1 are not intended for use in the creep range, which excludes every corrosion-resistant and duplex grade in the document.

The annex carries three qualifications worth passing to a designer. Scattering of data between batches is about ± 20 % for values around 100 000 h at temperatures up to 800 °C, rising to 35 % to 40 % at 1 000 °C. Values in parentheses involve time and/or stress extrapolation, and values marked with an asterisk involve time extrapolation. And the strength values given up to the listed temperatures do not mean the steels can be used in continuous duty up to those temperatures - the governing factor is the total stressing during operation, and oxidation conditions should also be taken into account.

Corrosion Resistance and Intergranular Corrosion Testing

Clause 8.4 has no counterpart anywhere else in the EN 10216 series. Tables 6, 7 and 8 give the method - A, B or C - for testing against intergranular corrosion according to EN ISO 3651-2:1998, and Tables 9, 10 and 11 give the limit temperature for susceptibility to intergranular corrosion for each grade.

Two points follow. First, the intergranular corrosion test is invoked by Option 12; it does not happen by default, and a mill certificate will not show it unless it was ordered. Second, the limit temperature in Tables 9, 10 and 11 has a precise meaning: up to that temperature, the material should, within 100 000 h, not have changed so as to show susceptibility to intergranular corrosion when tested in conformity with EN ISO 3651-2. It is a sensitisation limit, not a strength limit.

The creep-resisting grades of Table 7 are all marked "no" for resistance to intergranular corrosion in the delivery condition, with the footnote noting that it is normally not fulfilled in the sensitized condition. Specifying a Table 7 grade for a corrosive duty is therefore a category error - those grades are selected for creep strength, and Table 2 is where corrosion resistance lives.

If other specific corrosion tests are required - pitting resistance, chloride stress corrosion cracking, a ferric chloride test - they shall be agreed upon at the time of enquiry and order. The standard provides no default beyond the intergranular test.

Dimensions, Masses and Tolerance Classes

Tubes are ordered by outside diameter D and wall thickness T. Preferred outside diameters and wall thicknesses are given in EN ISO 1127:1992 - not EN 10220, which the carbon steel parts of the series use. For the calculation of mass per unit length, the density values given in EN 10088-1:2014 and EN 10028-7:2016 shall be used, which matters because stainless steel densities differ from the 7,85 kg/dm³ assumed for carbon steel.

Tolerance Classes

Part 5 uses a tolerance class system taken from EN ISO 1127 - classes D1 to D4 on diameter and T1 to T4 on wall thickness - rather than the single tolerance table of the carbon steel parts. Out-of-roundness is included in the tolerances on diameter, and eccentricity is included in the tolerances on wall thickness.

Hot finished tubes (Table 12):

Cold finished tubes (Table 13), for D ≤ 219,1 mm:

Classes D 3 and T 3 apply to cold-finished tube by default. Option 20 specifies the tighter classes D 4 and T 4, which bring diameter control to ± 0,1 mm and wall to ± 7,5 % - the specification for instrumentation tubing, heat exchanger tube and any application relying on compression fittings. Option 19 covers sized ends on hot-finished tube of D > 219,1 mm, giving ± 0,6 % on the outside diameter over approximately 100 mm at the tube ends.

Lengths, Straightness and End Preparation

Unless Option 18 is specified, tubes are delivered in random lengths with the delivery range agreed at the time of enquiry and order. Option 18 calls for exact lengths, and the tolerances are tighter than in any other part of the series:

Deviation from straightness shall not exceed 0,0015 L over any tube length L, and deviation over any one metre length shall not exceed 3 mm.

Tubes are delivered with square-cut ends, free from excessive burrs. Option 17 calls for bevelled ends on tubes with wall thickness of 3,2 mm and above: a bevel angle of 37,5° (+ 2,5° / − 0°) with a root face of 1,6 mm ± 0,8 mm, except that for wall thickness greater than 20 mm an agreed alternative bevel may be specified.