Detailed analysis of materials and properties of Incoloy Alloy Rod

A rod order is three decisions, and most catalogue pages answer only the first. The grade is one decision. The condition of supply is the second. The specification that the certificate will be written against is the third. A buyer who settles the grade and leaves the other two to the mill has not bought a material, and the invoice will prove it.

The clearest demonstration is alloy 800 in rod form. The same chemistry, tested at room temperature, gives a yield strength of 250 MPa when annealed at 980 °C, 445 MPa when hot rolled, and 690 MPa when cold drawn. The tensile strength moves from 590 to 665 to 771 MPa across the same three conditions, and the hardness moves from 138 to 198 HB between the first two. Nothing in the composition changed. The processing did.

That spread is why a page titled materials and properties of Incoloy alloy rod has to begin with a warning about its own subject. There is no single set of properties for a grade of rod. There is a set of properties for a grade in a named condition at a named test temperature, and anything else is an average of things that should not be averaged.

This page covers the three Incoloy families that are supplied as rod, the conditions each one is bought in, the temperature at which each one stops being the right answer, and the four items that have to appear on a purchase order before the certificate means anything.

Incoloy is a trademark of the Special Metals group covering a family of nickel-iron-chromium alloys. It is not a grade. Three members of that family are commonly supplied in rod and bar form, and they are used for three different jobs.

The 800 series is an austenitic nickel-iron-chromium alloy with about 32 % nickel and 21 % chromium, and it is the heat-resistant member of the group. Alloy 825 raises the nickel to the 38 to 46 % range and adds molybdenum and copper, which turns it into an aqueous corrosion alloy for sulfuric acid, phosphoric acid, and seawater. Alloy 925 keeps the 825 base and raises the titanium and aluminium, which makes it precipitation hardenable and takes the yield strength from 271 MPa to 832 MPa after a two-stage heat treatment.

Two things follow from that table before any property is discussed. The first is that an enquiry for Incoloy rod has named a family and nothing else, so the first reply has to be a question about which grade. The second is that the 800 line contains three UNS numbers with almost the same chemistry, and the difference between them is a carbon range, an aluminium and titanium total, and a grain size. Those are not cosmetic differences, and they are not visible in a certificate that reports chemistry only.

The table below puts six rod and bar products side by side at room temperature. Five of the six are the same three families already described. The sixth is the same 800 chemistry in a third condition, which is the point of the exercise.

The range of yield strength in that table runs from 250 to 832 MPa, a factor of 3.3, and only part of that span comes from changing the alloy. The 800 column alone covers 250 to 690 MPa with the chemistry held constant. A buyer who compares the yield strength of 800 with the yield strength of 925 from two data sheets is comparing an annealed rod with an aged bar, and will conclude that 925 is a stronger alloy when most of the difference is a furnace schedule.

The practical reading is straightforward. Where a design needs 700 MPa from 800, the answer is cold-drawn rod, and the same purchase brings an elongation of 17 % and a hardness that is not published for that condition. Where the design needs ductility for a formed or welded part, the answer is annealed rod at 250 MPa with 138 HB, and the drawing has to accept the lower strength. The two are the same grade and they are not interchangeable.

The same logic explains the 825 line. Annealed bar sits at 324 MPa yield, which is low for a corrosion alloy, and the bulletin notes that the alloy can be strengthened substantially by cold work. Cold-drawn tubing of the same grade reaches 889 MPa yield with 15 % elongation. A bar order that needs more than the annealed strength has to ask for a cold-drawn condition and accept the same loss of ductility that the tubing data shows.

Alloy 800 was released in the 1950s as a heat-resistant alloy with a relatively low nickel content, and the reason for that last detail was a strategic designation on nickel at the time. It was approved for pressure vessels in 1963, and the approval introduced the distinction that still governs the series: grade 1 annealed at approximately 980 °C, and grade 2 annealed at approximately 1,150 °C. The second grade is what became 800H.

The reason for the second grade is creep. Alloy 800 is intended for service up to about 816 °C, and at those temperatures design is normally based on tensile properties. Above that temperature the design is governed by rupture and creep, and both respond to grain size and to carbon content. A coarse grain structure carries grain boundary sliding better than a fine one, and a higher carbon level supports the carbide network that pins the boundaries. Alloy 800H was written to require both, and its specification asks for a carbon range of 0.05 to 0.10 % together with an average grain size of ASTM 5 or coarser.

Alloy 800HT is a further restriction inside the same envelope, introduced to hold the aluminium and titanium in the upper part of the range, where creep and rupture properties are higher. Its chemistry is within the 800H limits, its aluminium plus titanium is 0.85 to 1.20 % against 0.30 to 1.20 % for 800H, and its heat treatment is 2,100 °F (1,149 °C) minimum.

The last column is the one that causes commercial trouble. Alloy 800HT meets the requirements of both N08811 and N08810 and can be certified to either or to both. The reverse does not hold. Material produced to N08810 cannot be certified as N08811 unless it meets the additional aluminium, titanium, and heat treatment requirements, and a supplier who offers 800H as a substitute for 800HT has offered a different design stress.

Grain size belongs on the purchase order for the same reason. It is a result of the final anneal and of the amount of hot work before it, and it is not recoverable from a chemistry certificate. A buyer who orders 800H by composition alone can receive a fine-grained bar that meets every element of the chemistry and does not deliver the creep life the design assumed.

The 825 grade is a different proposition. It is a nickel-iron-chromium alloy with molybdenum, copper, and titanium, and the additions are aimed at aqueous corrosion. Chromium covers oxidising media such as nitric acid and nitrates. Molybdenum and copper cover reducing media such as sulfuric and phosphoric acid. Molybdenum also raises resistance to pitting and crevice corrosion. Titanium, with the right heat treatment, stabilises the alloy against sensitisation to intergranular corrosion.

The temperature limit is the part that the catalogue pages leave out. The alloy bulletin states that exposure above about 1,000 °F (540 °C) can cause microstructural changes and phase formation that significantly lower ductility and impact strength, and that for this reason the alloy is not normally used at temperatures where creep-rupture properties are design factors. That is not a derating for a specific product form. It is a statement about the alloy.

For a rod buyer the consequence is that 825 belongs to the corrosion side of a plant and not to the hot side. A heat exchanger tube, a valve body, or a pump shaft in sulfuric acid service is its application. A support that runs at 700 °C is not, and no amount of nickel in the analysis changes that.

Heat treatment matters here too. Mill products are treated to develop the combination of stabilisation, corrosion resistance, mechanical properties, and formability. Subsequent anneals should be between 1,700 and 1,800 °F (930 and 980 °C) followed by rapid air cooling or water quenching. Heavy sections may become sensitised while cooling from the hot working temperature and can then be subject to intergranular corrosion in certain media, in which case a stabilising anneal restores the resistance. A bar that is reworked in a fabricator shop and left to cool in air is a bar that may have lost the property it was bought for.

Alloy 925 is the 825 base with the titanium raised from 0.6 to 1.2 % into the 1.9 to 2.4 % range and with aluminium added at 0.1 to 0.5 %. Those two elements are the strengthening addition, not a corrosion addition. The heat treatment precipitates gamma prime, Ni3(Al, Ti), and that precipitate is what takes the yield strength from 271 MPa in the solution annealed condition to 832 MPa after aging.

That distinction corrects a common description. The 925 grade is often presented as 825 with extra alloying for corrosion and pitting resistance. The molybdenum and copper are the corrosion additions and they are the same as in 825. The titanium and aluminium are there for strength. The corrosion resistance of the two grades is close, which the sour service data in the alloy bulletin shows: in a sour autoclave test with free sulfur, 925 corroded at 1.1 and 1.2 mpy in two solutions and 825 at 1.1 and 1.6 mpy, against 3.9 and 4.5 mpy for type 316 stainless steel.

The heat treatment has two stages and both have tolerances that matter to a rod order. Solution annealing is done at 1,800 to 1,900 °F (980 to 1,040 °C) for a minimum of 30 minutes and a maximum of 4 hours, with cooling equivalent to air cooling or faster for sections of 1 inch (25 mm) and under, and a water quench for all sizes over 1 inch. Aging follows at 1,350 to 1,380 °F (732 to 749 °C) for 6 to 9 hours, furnace cooling to 1,150 °F (621 °C), then holding at 621 °C plus or minus 8 °C for a total aging time of 18 hours.

The diameter column is the part that surprises buyers. The property limits are written by product form and by size, and the hot worked route is held to a higher minimum yield than the cold worked route because the test bar comes from a different part of the process. A purchase order that quotes the minimum yield without the diameter range has quoted a number that may not apply to the bar being cut.

Machining sequence is the second practical consequence. Rough machining is done before age hardening and finishing after, because the alloy machines well in the solution annealed and in the aged condition and poorly in between. The bulletin goes further on distortion: after trepanning a bar to a wall thickness below about 25 mm, turning produced distortion of up to 0.010 inch (0.25 mm), and the runout was eliminated or substantially reduced by letting the material stand for several weeks or by reheating it at 1,150 °F (621 °C) for 2 hours before turning. A shop that trepans and finishes in one setup will hold a tolerance that the material will not hold.

Welding carries its own restriction. Gas tungsten arc and gas metal arc welding with the matching 725NDUR filler are the recommended routes, and submerged arc and shielded metal arc welding are not recommended for this grade. A rod that becomes a welded assembly has to be designed around that.

A common claim for the group is that these rods retain strength at temperatures often exceeding 1,000 °C, depending on grade. That sentence does not survive contact with the three alloy bulletins. Alloy 825 is limited to about 540 °C. Alloy 925 holds strength to about 650 °C. Alloy 800 is designed to 816 °C, and above that the answer is a different heat treatment inside the same family, not the same rod at a higher temperature.

Where the 1,000 °C figure comes from is the creep and rupture testing of the 800H and 800HT grades, which is reported at temperatures well above the design range. A test temperature is not a service temperature. A loaded rod at 1,000 °C is outside the design basis of every grade in this family, and a specification that quotes such a figure has confused the laboratory with the furnace.

Five items decide whether the certificate matches the part. The first is the grade with its UNS number, because the trade name alone does not fix the chemistry. The second is the condition of supply, written as annealed, hot rolled, cold drawn, or solution annealed and aged, with the aging treatment if the last one applies. The third is the grain size, for 800H and 800HT only, because it is a result of the final anneal and is not in the chemistry. The fourth is the property basis, meaning the test temperature and whether the values are minima or typical. The fifth is the inspection document.

The 925 line is worth a note. The alloy bulletin names three documents for the product and does not name a separate ASTM rod and bar specification, so a purchase order that invents one has invented a document. The property limits for the grade come from the producer specification HA 46, and the pressure vessel basis comes from Code Case 2218. A buyer who needs an ASTM reference for 925 should have the supplier state which of the two it is certifying against.

The certificate itself should be an EN 10204 3.1 document, which carries actual test results from the lot and is endorsed by the manufacturer independent inspection representative. A 2.2 certificate reports the results of non-specific inspection and is not the same document, and the difference matters when the part is going into a pressure boundary.

DLX supplies the Incoloy grades in rod and bar from Changzhou, Jiangsu, together with the same families in wire from 0.025 mm and strip from 0.50 mm to 2.5 mm where the application moves to a different form. Orders are produced against the standard named on the purchase order, with an EN 10204 3.1 certificate on request and the heat number traceable to the test results.

Two questions decide most quotations. The first is the condition, because a price for annealed 800 is not a price for cold-drawn 800. The second is whether the grade needs a grain size requirement, because 800H and 800HT cannot be quoted without one.

The answer depends on the environment and not on the trade name. For a furnace part or a heat exchanger above 600 °C, the 800 series is the candidate, with 800H or 800HT above 816 °C. For sulfuric acid, phosphoric acid, or seawater below about 540 °C, 825 is the candidate. For a downhole or wellhead part that needs 724 MPa minimum yield with sour service resistance, 925 is the candidate.

Because creep and rupture depend on grain boundaries and on carbides, and 800H requires a carbon range of 0.05 to 0.10 % together with an average grain size of ASTM 5 or coarser. The coarse grain reduces boundary sliding and the carbon supports the carbide network that pins the boundaries. Alloy 800 has neither requirement, so its design basis above 816 °C is not the same.

No. The alloy bulletin limits it to about 540 °C, above which phase formation significantly lowers ductility and impact strength, and states that the alloy is not normally used where creep-rupture properties are design factors. The limit is a property of the alloy and applies to rod, bar, tube, and sheet alike.

Aging precipitates gamma prime, Ni3(Al, Ti), which raises the yield strength from 271 MPa in the solution annealed condition to 832 MPa. It is done after solution annealing at 980 to 1,040 °C, at 732 to 749 °C for 6 to 9 hours, then furnace cooled to 621 °C and held for a total aging time of 18 hours. Rough machining is done before aging and finishing after.

It is a stronger 825, and the corrosion resistance of the two is close. The titanium and aluminium additions in 925 are there for the gamma prime reaction. The molybdenum and copper, which carry the resistance to reducing acids, are the same in both grades. In the sour autoclave test with free sulfur the two alloys recorded similar rates, and both were well below type 316 stainless steel.

Only when the condition and the test temperature match. The same 800 chemistry gives a yield strength of 250 MPa annealed, 445 MPa hot rolled, and 690 MPa cold drawn. A comparison between an annealed 800 sheet value and a cold-drawn 800 rod value is a comparison of two processes, and the alloy disappears from the comparison.

The Incoloy rod family covers three jobs. The 800 series is the heat-resistant member, and its three UNS numbers differ by carbon, by aluminium and titanium, and by grain size, with the grain size carrying the creep design stress above 816 °C. Alloy 825 is the aqueous corrosion member, with a ceiling near 540 °C that the catalogue pages tend to omit. Alloy 925 is the age-hardenable member, with a yield strength of 832 MPa after a two-stage treatment and a property table that is written by product form and diameter.

The property that ties them together is that none of these grades has a strength without a condition. The spread from 250 to 832 MPa across the family, and from 250 to 690 MPa within one grade, is larger than the difference between most of the grades. A purchase order that names the grade, the condition, the grain size where it applies, and the certificate has specified a material. One that names a trade name has specified a family.