Material
All specifications call for the
common grades 304, 304L, 316 and 316L. Most specifications allow a number of
other stainless steel grades as well. AS1528.1 permits all grades of
austenitic and duplex stainless steel listed in ASTM A240, so the
possibilities are very extensive.
Manufacture
All specifications require
fusion welded tube without filler metal (in practice this permits standard
tube production using TIG or plasma welding). AS 1528, A269 and A270 also
cover seamless product, if requested, although this is rarely required.
Dimensional Tolerances
Wall Thickness
·
A249 requires ±10% of nominal - no nominal
thicknesses are stipulated.
·
A269 requires ±10% of nominal for sizes over ˝"
- no nominal thicknesses are stipulated.
·
A270 requires ±12.5% of nominal - no nominal
thicknesses are stipulated.
·
A554 requires ±10% of nominal - no nominal
thicknesses are stipulated.
·
AS 1528 specifies nominal thicknesses of 1.6mm for
all ODs except 2.0mm for 203.2mm OD; other thicknesses can be specified by
purchasers. Standard tolerance is +nil, -0.10mm. The all-minus tolerance
recognises the usual practice for tube, to all specifications, to be produced
towards the lower limit of the tolerance range. A range of between 1.52 and
1.58mm is typical. This tolerance also applies to tube fittings.
Outside Diameter
Table 1.
Spec requirements for standard inch series OD tube sizes
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25.4
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±0.15
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±0.13
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+0.05/-0.20
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±0.13
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±0.13
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38.1
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±0.15
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±0.25
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+0.05/-0.20
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±0.15
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±0.25
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50.8
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±0.25
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±0.25
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+0.05/-0.28
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±0.18
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±0.25
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63.5
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±0.3
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±0.25
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+0.05/-0.28
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±0.25
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±0.25
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76.2
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±0.38
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±0.25
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+0.08/-0.30
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±0.25
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±0.25
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101.6
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±0.38
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±0.38
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+0.08/-0.38
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±0.38
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±0.38
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* A554 tolerances for the
weld bead removed condition.
* AS1528 also covers OD
sizes 12.7, 19.0, 31.8, 127.0, 152.4 and 203.2mm
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All these tube specifications
give limits for OD and Wall Thickness – the Inside Diameter is not separately
specified, even although from the hygiene point of view there may be some
logic in doing so.
Surface Finish
·
A249 and A269 both require surfaces “free of scale” –
annealing of the tube is normally done in a controlled atmosphere and this
“bright annealed” finish is considered acceptable.
·
A270 requires selection of both internal and external
surfaces. The possible conditions range from a "mill finish" (i.e.
the strip's 2B finish, without any subsequent polishing), to abrasive
polishing with 80, 120, 180 or 240 grit, to special polishing and
electropolishing. Surface finishes may also be specified in terms of Ra
values, but no limits are given in the specification.
·
A554 requires only "free of scale" and
implies a "direct off mill" finish as standard. Clause 11.2 does
allow - "If special finishes are required, they shall be stated in the
order". Thus a large proportion of A554 tube used in the Australian
market is supplied in the externally polished or buffed conditions, in the
range of about 180 - 320#, or with a very highly polished surface, typically
stated as 600#.
·
AS1528 specifies the external surface "as
produced" or "buff polished", as agreed. The internal surface
is required to be 2B finish, quoted as typically 0.3µm Ra. Work done by Atlas
Steels indicates that for 1.6mm 2B coil (the starting material for welded
tube) the typical roughness is 0.10 - 0.20 µm Ra; this would be expected to
not be significantly degraded in the manufacture of tube. With weld bead
rolling it would be expected that the finish of the weld would be similar to
that of the parent tube.
Weld Bead
The food industry generally
requires a tube with no weld bead remnant on the inside surface if the
intended service is handling product.
·
A249 requires that at least the weld be cold worked
after welding and before final heat treatment.
·
A269 does not require any weld bead control or cold
working.
·
A270 makes no mention of weld bead.
·
A554 can be supplied with the weld bead left on, but
in recent years Australasian manufacturers of As Welded tube have all made
internal weld bead rolling a fairly routine procedure; this therefore
complies with the "Bead Removed" option of A554. (Weld bead rolling
is not generally possible in sizes below 31.8mm, although sizes down to 25.4
or even smaller can be hammer swaged if specifically ordered).
·
AS1528 requires removal of the weld bead (except in
the small sizes where the procedure is not possible). There is also a
requirement that the internal surface be smooth, with no lack of weld
penetration and no crevices adjacent to welds. This requirement addresses the
heart of the issue - freedom from sites for product or bacterial build-up.
Heat Treatment
·
A249, A269 and A270 all require that “all material
shall be furnished in the heat treated condition”. Heat treatment is
annealing (also referred to as solution treatment or solution annealing). In
practice this is not a common requirement for food industry tube unless it
requires significant bending or flaring.
·
A554 is normally supplied “as welded”, ie. no heat
treatment after tube forming (although the tube will be produced from strip
which has itself been annealed just prior to the final cold roll). There is
the possibility of calling for A554 tube in the annealed condition, but this
is never done - annealed tube (“As Welded Annealed” or AWA) is more usually
specified to ASTM A269.
·
AS1528 allows either annealed or un-annealed
conditions to be specified by the purchaser.
Mechanical Properties
·
A249 is intended for critical environments in boilers
or heat exchangers, and hence extensive mechanical testing is required. Full
tensile and hardness testing is standard, as are flattening, flange and
reverse bend.
·
A269 requires no tensile testing, but does require
hardness tests, plus flange, reverse flattening.
·
A270 requires a reverse flattening test only.
·
A554 requires no mechanical testing as standard.
·
AS1528 requires no tensile or hardness testing, but
does require the tube to be made from strip compliant with ASTM A240 - which
itself has tensile strength requirements.
Non-Destructive Inspection
·
A249, A269, A270 and AS1528.1 all require 100%
hydrostatic or eddy current testing.
·
A554 includes the possibility of non-destructive
testing as a supplementary requirement, but this is not usual for A554 tube.
Which Specification
·
ASTM A249 is written for a quite different
application area. It does specify weld bead removal, but this requirement can
be met from other standards, without unnecessarily calling up the stringent
mechanical properties of A249. The annealing mandatory in A249 will also not
be required in most food applications. A high cost option.
·
ASTM A269 again requires tube in the annealed
condition. Conversely, it does not specify internal weld bead removal, which
generally is a food industry requirement. A269’s main positive aspect is that
it is frequently a stock item. It will prove uncompetitive against
un-annealed tube.
·
ASTM A270 also has problems in that it requires the
tube in the annealed condition, and says nothing about weld bead. The finish
options available in this specification are very comprehensive.
·
ASTM A554 in its usual supply condition is intended
for mechanical applications, not for pressure containment and not for
sanitary use.
·
AS1528 is by far the safest option, and the most
cost-effective. It is specifically directed at food industry applications,
specifying the features necessary to ensure high integrity lines for hygienic
applications without requiring high cost additional mechanical testing.
Annealing is possible if required and surface finishes can be further
specified. Batch traceability marking (considered essential to validate many
food and pharmaceutical plants) is mandatory. Another key benefit is the
existence of matching specifications for associated tube fittings.
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