Alloy 20 seamless pipe is supplied to
ASME SB-729 under UNS N08020. SB-729 is
the seamless pipe and tube specification for UNS N08020, UNS N08026 and
UNS N08024, and its Scope 1.1 states that it covers those alloys in
seamless, cold-worked or hot-finished pipe and tube intended for general
corrosive service. The matching ASTM specification is B729.
The welded form of the same alloys is covered by a different
specification, ASME SB-464, which is the specification for
welded UNS N08020, N08024 and N08026 alloy pipe. That distinction matters
for procurement, and it is the single most common source of confusion in
Alloy 20 enquiries: a material certificate written to SB-729 and one written
to SB-464 are certificates for different products and cannot be substituted
for one another.
Alloy 20 and Its Neighbours
| Element, weight % | N08020 (Alloy 20) | N08026 (Alloy 26) | N08024 (Alloy 24) |
|---|---|---|---|
| Carbon, max | 0.07 | 0.03 | 0.03 |
| Manganese, max | 2.00 | 1.00 | 1.00 |
| Phosphorus, max | 0.045 | 0.03 | 0.035 |
| Sulfur, max | 0.035 | 0.03 | 0.035 |
| Silicon, max | 1.00 | 0.50 | 0.50 |
| Nickel | 32.00 – 38.00 | 33.00 – 37.20 | 35.00 – 40.00 |
| Chromium | 19.00 – 21.00 | 22.00 – 26.00 | 22.50 – 25.00 |
| Molybdenum | 2.00 – 3.00 | 5.00 – 6.70 | 3.50 – 5.00 |
| Copper | 3.00 – 4.00 | 2.00 – 4.00 | 0.50 – 1.50 |
| Columbium (Nb) + tantalum | 8 × carbon − 1.00 | — | 0.15 – 0.35 |
| Nitrogen | — | 0.10 – 0.16 | — |
| Iron | Remainder, by difference | Remainder, by difference | Remainder, by difference |
Source: ASME SB-729 Table 1. The same N08020 composition appears in
SB-463 for the plate, sheet and strip product form, which is a useful
cross-check: SB-463 Table 1 gives the same carbon 0.07 max, nickel
32.00–38.00 %, chromium 19.00–21.00 %, molybdenum
2.00–3.00 %, copper 3.00–4.00 % and columbium plus tantalum of
8 × carbon − 1.00 for N08020.
Alloy 20 is a nickel-iron-chromium alloy stabilised by columbium and
reinforced with copper. The copper is the reason for the alloy’s
characteristic behaviour in sulphuric acid, and the columbium is what ties
up carbon so that the alloy resists sensitisation and intergranular
corrosion in the hot, concentrated acid environment in which it is used.
The Scope of SB-729 is deliberately general — general corrosive
service — and the alloy’s reputation was built in sulphuric acid
and copper electro-refining service before it was used anywhere else.
Mechanical Requirements (ASME SB-729, Table 2)
| Property | Requirement |
|---|---|
| Tensile strength, min | 80 ksi (550 MPa) |
| Yield strength, min (0.2 % offset) | 35 ksi (240 MPa) |
| Elongation in 2 in. (50.8 mm) or 4D, min | 30.0 % |
Table 2 note A states that the yield strength shall be determined by the
offset method at 0.2 % limiting permanent set in accordance with Test
Methods E8. For comparison, the same properties apply to N08020 plate
under SB-463 Table 2, which additionally carries a hardness limit of 217
Brinell or 95 Rockwell B maximum.
Size Range
SB-729 4.1.4.2 requires the purchaser to specify the standard pipe size
and schedule for pipe ordered under this specification, and 4.1.4.1
requires the outside diameter and the average or minimum wall thickness for
tube. Section 3.1 makes the general requirements of Specification B829
applicable unless otherwise provided, and B829 Section 6 gives the pipe
dimensions in its Table 1, which is a reprint of Table 1 of ANSI B36.19.
Wall thickness and diameter tolerances for this product are therefore
handled under B829 rather than under a table inside SB-729 itself. Send us
the nominal pipe size, schedule and length and we will confirm the
dimensional standard, the tolerance basis and the delivery form.
Manufacturing Route and Heat Treatment
SB-729 5.1 sets the delivery condition by alloy: the product of UNS
N08020 shall be furnished in the stabilised-annealed condition, the
product of UNS N08026 in the solution-annealed condition, and the product
of UNS N08024 in the annealed condition. NOTE 1 to that clause gives the
recommended annealing temperatures, all followed by quenching in water or
rapid cooling by other means: 1800 to 1850 °F (982 to 1010 °C) for
UNS N08020, 2050 to 2200 °F (1121 to 1204 °C) for UNS N08026, and
1925 to 1975 °F (1052 to 1079 °C) for UNS N08024. The section also
states that the pipe may be seamless, cold-worked or hot finished. The
stabilised-annealed condition for N08020 is not a formality: the
columbium addition only does its work if the carbon is tied up in the
columbium carbide, and that requires the anneal.
Inspection, Testing and Certification
- Chemical analysis: SB-729 6.1 requires the material to
conform to the composition limits of Table 1, with one test for each lot as
defined in B829; 6.2 allows the purchaser to perform a product analysis
subject to the product analysis tolerances of Table 1 of B829. - Mechanical properties: SB-729 7.1 requires the material
to conform to Table 2. - Hydrostatic or nondestructive electric test: SB-729 7.2
requires each pipe and tube to be subjected to either the hydrostatic test
or the nondestructive electric test, with the type of test at the option of
the manufacturer unless otherwise specified in the purchase order, and 9.3
requires the test to be made on each piece in each lot. - Hydrostatic test pressure: SB-729 10.1.1 requires each
pipe or tube to be hydrostatically tested in accordance with B829, with an
allowable fibre stress for the material in the condition furnished of
20 000 psi (138 MPa), and 10.1.3 permits the test at one and one-half times
that allowable fibre stress when so agreed between the manufacturer and the
purchaser. Section 10.1.2 requires visual examination of the full length of
the material while it is under pressure. - Nondestructive electric test: SB-729 10.2 requires each
pipe or tube to be examined with a nondestructive electric test in
accordance with B829, and 10.2.1 lists the imperfections whose signals may
be judged as injurious or non-injurious depending on visual observation of
their severity. - Certification: SB-729 11.1 requires a manufacturer’s
certification stating that the material was manufactured, tested and
inspected in accordance with the specification and that test results on
representative samples meet the specification requirements, together with a
report of the test results.
Packing & Delivery
Plastic end caps on all sizes; steel frames or wooden crates for large
diameters; VCI anti-rust wrapping. Beveled ends standard for welding; plain
ends available. Each length is marked with the standard, grade, size, heat
number and heat treatment condition.
Applications
Sulphuric and phosphoric acid plant piping, copper and brass
electrolytic refining and electrowinning installations, hydrochloric acid
and hydrofluoric acid service where the specific grade permits, pulp and
paper bleaching plant piping, hydrochloric acid pickling lines, chlorinated
hydrocarbons and organic acid process lines, and pharmaceutical and
specialty chemical process piping. The common thread is a chloride-bearing
acid environment in which a stabilised nickel-iron-chromium-copper alloy
outperforms both the nickel alloys and the stainless steels of the
previous generation.
Technical Notes
1. The columbium and the copper are both doing structural work,
and the anneal is what activates them. Alloy 20 gets its
sulphuric-acid resistance from the copper content of 3.00–4.00 %, and
its resistance to intergranular corrosion in hot concentrated acid from the
columbium, which is specified as 8 × carbon − 1.00. That
expression is a stabilisation requirement expressed as a function of the
actual carbon level, which is why it cannot be quoted as a simple
percentage: at a carbon of 0.05 % the columbium requirement computes to a
different figure than at 0.02 %. Section 5.1 requires N08020 product to be
furnished stabilised-annealed, with the recommended temperature band of
1800–1850 °F (982–1010 °C) followed by water quench or
rapid cooling. The anneal is what lets the columbium combine with the
carbon; without it the alloy is not in the condition the specification
intends, and the certificate is not worth the paper it is printed on if
the heat treatment record does not match.
2. The three alloys in SB-729 are not three versions of the
same material, and Alloy 24 is the one that gets confused with Alloy
20. Comparing Table 1 across the three columns: N08020 carries
chromium 19.00–21.00 % and molybdenum 2.00–3.00 % with copper
at 3.00–4.00 %, N08026 carries chromium 22.00–26.00 % and
molybdenum 5.00–6.70 % with no columbium limit and a nitrogen
requirement of 0.10–0.16 %, and N08024 carries chromium
22.50–25.00 % and molybdenum 3.50–5.00 % with copper reduced
to 0.50–1.50 % and a fixed columbium range of 0.15–0.35 %. So
N08024 is a higher-chromium, lower-copper alloy with a different
stabilisation mechanism, and it is a solution-annealed product (SB-729 5.1)
with a different recommended annealing band of 1925–1975 °F
(1052–1079 °C). The mechanical requirements of Table 2 are common
to all three. When a project calls for Alloy 20, the certificate has to name
N08020, and the heat treatment record has to show the stabilised anneal in
the right temperature range.
3. The hydrostatic test is the manufacturer’s choice of
method, and that choice should be fixed on the order. Section 7.2
allows either the hydrostatic test or the nondestructive electric test, with
the type of test at the option of the manufacturer unless otherwise
specified in the purchase order, and 9.3 requires the selected test to be
performed on each piece in each lot. Section 10.1.3 allows the hydrostatic
test at one and one-half times the 20 000 psi (138 MPa) allowable fibre
stress when the manufacturer and the purchaser have agreed it, and 10.2.1
lists the imperfections whose signals from the electric test may be judged
either injurious or non-injurious depending on their visual severity. For
a project with a defined inspection and test plan, the required test method
should be written into the purchase order, because the specification
explicitly hands that decision to the mill otherwise. A second point worth
settling in the same place is the certification level: 11.1 requires a
manufacturer’s certification and a report of test results, and a
third-party witnessed inspection with EN 10204 3.2 certification has to be
called for at the order stage, not requested afterwards.
Frequently Asked Questions
Which ASME specification covers Alloy 20 seamless pipe?
ASME SB-729, for UNS N08020, N08026 and N08024 seamless, cold-worked or
hot-finished pipe and tube for general corrosive service. The welded form
of the same alloys is covered by ASME SB-464, and the plate, sheet and
strip form by ASME SB-463. Quote the form you need, because the
certificates are not interchangeable between them.
What is the heat treatment condition for Alloy 20 pipe?
SB-729 5.1 requires the product of UNS N08020 to be furnished in the
stabilised-annealed condition, with a recommended annealing temperature of
1800 to 1850 °F (982 to 1010 °C) followed by quenching in water or
rapid cooling by other means. N08026 is solution-annealed at 2050 to
2200 °F, and N08024 is annealed at 1925 to 1975 °F.
What are the mechanical properties of Alloy 20 pipe?
SB-729 Table 2 requires a minimum tensile strength of 80 ksi (550 MPa), a
minimum yield strength of 35 ksi (240 MPa) by the 0.2 % offset method, and
a minimum elongation of 30.0 % in 2 in. (50.8 mm) or 4D. The plate form
under SB-463 additionally carries a hardness limit of 217 Brinell or 95
Rockwell B maximum.
Is the hydrostatic test or the electric test used?
Either, at the manufacturer’s option unless the purchase order
specifies otherwise, per SB-729 7.2. The hydrostatic test is run in
accordance with B829 at an allowable fibre stress of 20 000 psi (138 MPa),
and 10.1.3 permits one and one-half times that value by agreement.
How is Alloy 20 pipe marked?
Each length carries the standard, grade, size, heat number and heat
treatment condition. The pipe is manufactured in seamless, cold-worked or
hot-finished form and the delivery condition is stated on the certificate
together with the annealing temperature actually used.
Can Alloy 20 be used in sulphuric acid service?
That is the service the alloy was developed for, and the 3.00–4.00 %
copper content in SB-729 Table 1 is the reason. The concentration and
temperature of the acid, and the presence of chlorides, are what determine
whether the grade and the heat treatment condition are appropriate, and
those points should be checked against the material rather than assumed
from the alloy name.
