how to produce inconel 600 tube and where to use it?

An Inconel 600 tube cannot be made by rolling a bar and then drawing it. That sequence appears in most short descriptions of the product, and it is the one thing a tube mill never does, because cold drawing reduces a hollow and a solid bar has no hollow to reduce.

The real sequence is decided by that fact. A billet is melted and cast, hot worked into a hollow shell, and only then cold reduced towards the finished wall. Every step after the hollow exists to undo the damage the previous step did to the metal: the cold reduction hardens it, the anneal softens it, and the anneal itself creates an oxide that has to be pickled off.

The applications follow the same logic in reverse. Alloy 600 is bought for two properties that are unusual together, and it is declined for several others that its reputation hides. This article covers the route and then the fit, in that order, because the route explains what the certificate is actually certifying.

Alloy 600 is a nickel-chromium-iron solid solution alloy with about 72 % nickel, and it is not precipitation hardenable. It is strengthened by cold work alone, which means the temper of a delivered tube is a process decision and not an alloy property.

Two numbers in that table drive the rest of the process. The carbon limit is generous for a modern nickel alloy, and it is the reason a 600 tube has to be cooled quickly through a particular temperature band. The melting range is narrow enough that hot working has to be done in a controlled window, which is why the hollow is usually made by extrusion and not by repeated rolling.

The sequence below is the standard one for seamless tube. A welded tube branches after the strip stage, and the two routes meet again at the final anneal and the tests.

The second row is the one that separates a real route from a plausible one. A cast billet is solid, and no amount of rolling turns a solid section into a tube. The hollow is created by forcing the billet through a die over a piercer or a mandrel, and the wall thickness of that shell sets the budget for every cold pass that follows. A shell that is eccentric at this stage cannot be corrected later, because cold reduction reduces the wall it is given.

The third row explains why high-alloy tube is pilgered and not drawn wherever the mill can do it. Pilger rolling works the wall between rolls and a tapered mandrel, so the metal is reduced in compression over a short arc, and the wall comes out more uniform than a plug draw achieves on an alloy this stiff. A drawn tube is not a defective product. It is a product with a different wall tolerance and a different surface, and the purchase order should say which one it expects.

The published guidance for this alloy is a soft anneal of about 1,850 °F for fifteen minutes, which is 1,010 °C, and a brief exposure to 1,040 °C gives a soft material without coarsening the grain. Grain growth is not the difficulty at those temperatures. The difficulty is what happens on the way down.

Alloy 600 precipitates carbides at the grain boundaries between about 540 and 760 °C, and the published instruction is to cool rapidly through that band if the tube is to be pickled or used where freedom from sensitisation matters. A slow cool leaves a continuous carbide network at the boundaries, and the metal immediately beside it is depleted of chromium. The pickle then attacks that depleted zone, and the tube leaves the works with a surface that looks acceptable and a grain boundary structure that is already damaged.

This is why the anneal is not a simple furnace setting on a certificate. The temperature is easy to record. The cooling rate through a 220 degree band is what separates a tube that will pass an intergranular corrosion test from one that will not, and on a thick wall it is the slowest part of the section that decides.

The common claim is that this alloy holds its mechanical properties up to 1,093 °C. The number is real and the sentence around it is wrong. Published guidance places the useful range of the alloy from cryogenic temperatures to above 1,095 °C, which is a statement about the top of the application envelope and about oxidation resistance at that temperature. It is not a statement that the strength is unchanged.

The short-time tensile data make the difference plain.

At 982 °C the alloy still carries 280 MPa, which is a respectable number for a metal at that temperature and is the reason the alloy is used in furnace furniture. It is also less than half its room-temperature strength, and the trend continues downward above that point. A design that takes the room-temperature allowable and applies it at 1,093 °C has misread the source.

The practical consequence is that a furnace application is designed on the strength at temperature, with a creep allowance, and not on the room-temperature table. A tube that carries a load at 900 °C will sag at 1,000 °C even though nothing has corroded and no number in the specification has been exceeded.

Alloy 600 earns its place in four situations, and each one uses a different property. Naming the property is what keeps the selection honest, because a tube chosen for the wrong reason will fail for the reason nobody checked.

The third row is worth a note, because a protection tube is often bought as an accessory and specified as an afterthought. In service it is the barrier that keeps the furnace atmosphere away from the thermocouple, and a 600 sheath is chosen because it forms a slow-growing oxide that stays in place. A thin wall saves money and shortens the life of the sensor inside it, and the failure appears as a thermocouple fault and not as a tube fault.

Three exclusions matter more than the benefits, because each of them is a case where the alloy reputation leads a buyer to the wrong material.

The second row is the one that most often appears in a short product description as a selling point, usually phrased as use in reactor components. The history is more useful than the slogan. Alloy 600 was the standard steam generator tube material, it cracked under primary water chemistry over years of operation, and the replacement material for new equipment was alloy 690. A buyer specifying 600 today for that service is specifying a material that the industry has already moved away from, and the specification should say why if that is the intent.

The first row is a subtler trap. A datasheet that states immunity to chloride stress corrosion cracking is accurate and easy to read as immunity to chlorides in general. A tube in brackish cooling water sits in a crevice under a tube sheet, and that geometry is exactly where pitting and crevice attack start on an alloy whose pitting resistance is ordinary.

The alloy is one decision and the product form is another, and the standard quoted on the order decides the tests.

A tube ordered to B167 and a tube ordered to B163 can be the same alloy from the same heat and are not the same purchase. The second carries a testing regime written for heat exchanger service, and the difference shows up in the inspection report and not in the metal.

Six items decide whether the tube that arrives can be installed, and the first two are the ones a mill cannot infer.

Name the product form and the standard, because seamless and welded are different purchases with different inspection. Name the condition, annealed or cold worked, because this alloy hardens only by cold work and a temper that is not stated will be supplied annealed. Give the dimensions with the tolerances that matter, which for a heat exchanger tube means the wall and the concentricity as well as the outside diameter. State the test regime, including the eddy current or ultrasonic inspection and the pressure test. State the surface, because a pickled tube and a bright annealed tube are different products and bright annealing requires very dry hydrogen or a vacuum. And state whether an intergranular corrosion test or a grain size is required, because those are the two results that show whether the cooling through the carbide band was done properly.

DLX produces Inconel 600 tube and the wider nickel alloy tube range in Changzhou, Jiangsu, in seamless and welded forms, with the anneal, the pickle, and the inspection reported against the order. The same works cover the fittings, the plate, and the thermocouple sheaths that a 600 installation 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 property being bought, because chloride cracking, oxidation, and caustic service lead to different specifications and sometimes to a different alloy. The second is the test that has to be passed, because the anneal and the cooling rate are what that test is really checking.

Both are made. The seamless route pierces or extrudes a hollow and cold reduces it, and the welded route forms strip into a tube and welds the seam. They meet at the final anneal and the tests, and they are covered by different standards, B167 and B163 for seamless and B516 and B517 for welded. The purchase order has to name which one.

Because cold reduction hardens the alloy, and this alloy hardens only by cold work. After a pass or two the metal is too strong to reduce further and too brittle to form, so it is softened at about 1,010 °C and pickled before the next pass. The final anneal sets the temper that the certificate reports.

The cooling rate, not the temperature. Carbides precipitate at the grain boundaries between about 540 and 760 °C, and a slow cool through that band leaves a chromium-depleted zone beside each boundary. The pickle then attacks it, and the tube can pass a dimensional inspection while carrying a sensitised structure into service.

It can be exposed to that temperature, and it does not retain its room-temperature mechanical properties there. The published figure is the top of the application envelope. Typical short-time tensile strength falls from about 660 MPa at room temperature to about 280 MPa at 982 °C, and the trend continues above that point, so a load-bearing design at that temperature needs the strength at temperature and a creep allowance.

Because alloy 600 steam generator tubing cracked by primary water stress corrosion cracking in service, and the cost of that failure was measured in plugged tubes and replaced generators. Alloy 690 carries roughly twice the chromium and has performed better in that chemistry. Alloy 600 remains a legitimate material for the chemical and heat treating applications described here.

It resists chloride stress corrosion cracking, which is the mode that destroys austenitic stainless tube in that service. It does not resist pitting and crevice attack in chlorinated seawater, and a tube sheet crevice is the geometry where that shows up. A super-austenitic stainless or a higher molybdenum nickel alloy is the usual answer for seawater cooling.

An Inconel 600 tube starts as a hollow that was created hot, and that hollow is then reduced cold towards the finished wall. Between passes the metal is softened and pickled, and the final anneal sets the temper before the tests. The route exists because a solid billet cannot be drawn into a tube. The anneal exists because the cold work has to be undone without precipitating carbides at the grain boundaries.

The applications come down to two properties, resistance to chloride stress corrosion cracking and resistance to oxidation at temperature, with a third in alkaline service. The exclusions are just as specific: pitting in chlorinated seawater, caustic cracking, and the nuclear steam generator service that moved to alloy 690. A tube ordered with the property and the test named is a tube that can be accepted on arrival.

The mechanical values quoted are typical short-time tensile results for annealed material and are not suitable for design; the allowable at temperature comes from the applicable code and includes a creep allowance. The processing description is the standard seamless route and a mill may arrange the passes and the intermediates differently. Carbide precipitation depends on time as well as temperature, and the band quoted is the range within which the risk exists. The alloy 690 substitution applies to new steam generator tubing in pressurised water reactors, and it does not make alloy 600 unsuitable for the chemical and heat treating services described here. 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.