Hastelloy C276 tube for shipment

A batch of Hastelloy C276 tube left the DLX works in Changzhou at the end of March, packed for export and tested against the order. The tube is UNS N10276, supplied in the mill annealed condition, and the size range quoted from 3 mm outside diameter upward.

A shipment notice is a short document, and this one keeps to that. What follows is the part a buyer usually has to ask for: what condition the tube is in when it is crated, what travels with it, and what can damage it between the works and the site. All three decide whether the tube that arrives can be installed as delivered.

C276 tube is supplied mill annealed unless the order says otherwise, and the anneal is the step that sets the corrosion resistance. The published treatment for this alloy is a soak at 1,121 °C, a hold of 10 to 30 minutes depending on section, and a water quench. Rapid air cooling is accepted for sections thinner than 10 mm, which covers most tube.

The last row is the one that catches fabricated orders. Cold work affects the resistance of this alloy to stress corrosion cracking, even where it leaves general corrosion resistance intact, so a tube that has been rolled, bent, or straightened past that strain limit needs the anneal repeated before it goes into service. A tube delivered straight from the mill is in the condition the certificate describes. A tube that was formed after the mill anneal is not.

Annealing forms an oxide on the surface, and on this alloy the oxide is not the passive film that a stainless steel develops. It is a thermally formed layer that is richer in some of the alloying elements than the metal beneath it, and it has to be removed and not left in place.

The removal is a pickle in a nitric and hydrofluoric acid mixture, and the published guidance is that a heavy oxide is broken up first by abrasive blasting or fine grinding. A pickle that is asked to do the whole job on a thick annealed scale will leave patches, and a patch of unremoved oxide is a starting point for attack in service because the layer under it is depleted.

This is why the surface condition belongs on the order and on the inspection record. A tube that is bright, uniform, and free of scale is a tube that has been through the sequence. A tube with a grey band along one side has been through it unevenly, and the band is where the first pitting will appear.

The alloy is chosen for aggressive chemistry, and it is not immune to everything. Its weakest point in a clean environment is a crevice in the presence of chlorides, and both of those conditions can be created by packaging.

The published critical crevice temperature for C276 in acidified 6 % ferric chloride, tested to ASTM G48, is 55 °C, against a critical pitting temperature above 150 °C in the same solution. The gap between those two numbers is the whole story: this alloy resists pitting to a very high temperature and resists crevice attack to a much lower one.

None of these items is expensive, and all of them are decided at the packing bench. A tube that arrives with a chloride pit is not a warranty argument about the alloy. It is a packing decision that was made three weeks earlier.

For seamless tube the governing specification is ASTM B622, with B983 for the general requirements, and the welded equivalent is B626 with B619. The certificate and the test record are what convert a delivered tube into an accepted one.

Small tube is where a purchase order most often becomes ambiguous. Three millimetres is 0.118 in, and 1/8 in is 3.175 mm, so a 3.00 mm tube sits just below the size at which the standard tube tables begin. The alloy and the heat treatment are not in question at that size. The applicable specification is.

The practical answer is to put both on the order: the nominal size in millimetres, and the specification or drawing that governs it. A tube ordered as 3 mm to B622 and a tube ordered as 3 mm to a mill drawing may both be correct, and they are not the same purchase.

DLX supplies C276 tube and the wider nickel alloy range from Changzhou, Jiangsu, in the annealed condition and with the certificate set described above. The same works cover the fittings and the plate that a C276 exchanger or reactor needs alongside the tube, so the heat treatment and the documentation stay consistent across the order.

Two questions settle most enquiries. The first is the service, because a chloride environment and a sour gas line lead to different specifications and different documentation. The second is the size and the governing standard together, because at the small end the size alone does not identify the purchase.

Annealed, unless the order states otherwise. The mill annealed condition is the default for wrought C276, and the anneal is the step that restores corrosion resistance after forming. A hard drawn tube is available and has to be specified, because the condition changes both the mechanical properties and the resistance to stress corrosion cracking.

Because the annealed surface is an oxide layer that is not passive, and it sits over metal that has been depleted of some alloying elements. Left in place, it becomes the place where attack starts. The pickle removes it, and the surface record is what shows the removal was uniform.

It can, and the packing has to be chloride-free and dry. The critical crevice temperature of this alloy in acidified ferric chloride is 55 °C, which is far below its pitting resistance, so a chloride-bearing wrap plus moisture plus a tight contact is the combination that causes damage on a long voyage.

B622 covers seamless tube, and B626 covers welded tube, with B619 for welded pipe and B983 for the general requirements that apply across them. The alloy and the anneal are the same. The difference is the seam, and the seam decides the inspection and the applications the tube can be used in.

A C276 tube shipment is a delivery of a heat treatment and a surface as much as a delivery of metal. The tube leaves mill annealed at 1,121 °C, pickled to remove the annealed oxide, and documented against the specification named on the order. Those three facts are what the buyer is actually accepting.

The risk between the works and the site is chloride and moisture in a crevice, which is a packing question and not an alloy question. The risk in the order is an ambiguous size at the small end, which is settled by naming the standard as well as the millimetre.

Haynes International, HASTELLOY C-276 alloy data sheet, for the nominal composition, the 1,121 °C anneal, the 10 to 30 minute hold, the cooling guidance, and the 7 % outer fibre elongation rule for re-annealing after cold work.

ASTM B622, seamless nickel and nickel-cobalt alloy pipe and tube, for the tube specification and the test requirements.

ASTM B626 and B619, welded nickel and nickel-cobalt alloy tube and pipe, and ASTM B983 for the general requirements.

ASTM G48, pitting and crevice corrosion resistance in ferric chloride solutions, for the critical crevice temperature of 55 °C and the critical pitting temperature above 150 °C quoted for this alloy.

NACE MR0175 / ISO 15156, for the sour service requirement where it is specified on the order.

VDM Metals, Alloy C-276 data sheet, for the guidance that annealed oxide is broken up before pickling in nitric and hydrofluoric acid mixtures.

The heat treatment figures are the published recommendations for this alloy and a mill may work inside a narrower band; the certificate governs for a delivered lot. The crevice and pitting temperatures are laboratory values from the test solution named, and they rank materials against each other and do not predict a service life. Actual resistance depends on the environment, the surface condition, and the geometry of the crevice. The size observation is arithmetic on the inch and millimetre values, and the specification applicable to a given size is settled by the standard or the drawing named on the order. Standards are revised, and the edition named on the order governs. Nothing here is a substitute for an engineering release or for the inspection agreed between buyer and supplier.