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Inconel 625 is a nickel-chromium-molybdenum alloy with a niobium addition, and the published answer to the question in the title is a sentence from the alloy bulletin: its strength comes from the stiffening effect of molybdenum and niobium on the nickel-chromium matrix, so precipitation-hardening treatments are not required. That is the identity of the material. Everything else about it, how it is bought, how it is welded and why it is chosen over the grade that sits next to it in most enquiry lists, follows from that one sentence.
The name itself needs unpacking first, because Inconel is a trade name and not a material designation. The material is UNS N06625, W.Nr. 2.4856, and ISO NW6625, and it is listed in NACE MR-01-75. A purchase order written against a trade name alone leaves the specification open; a purchase order written against the UNS number does not. Both names refer to the same alloy, and the alloy is available in all standard mill forms including rod, bar, wire, plate, sheet, strip, shapes and tubular products.
The specification limits below are the published ones, and the column on the right exists because three of these figures are commonly printed the wrong way round. The alloy is nickel based and the iron, aluminium and titanium figures are all maximums, not minimums. Read the other way round, they describe a different material.
| Element | Limit, % | A note on how it is often printed |
|---|---|---|
| Nickel | 58.0 minimum | A minimum, and the balance of the alloy is nickel |
| Chromium | 20.0 to 23.0 | A range, and the element that provides oxidation resistance |
| Molybdenum | 8.0 to 10.0 | A range with both ends specified. Published summaries sometimes give this as 8 minimum only, which drops the upper bound the specification sets |
| Niobium, plus tantalum | 3.15 to 4.15 | The addition that distinguishes this alloy in its family. Niobium is called columbium in the United States, which is why the same element appears under two names in different standards |
| Iron | 5.0 maximum | A maximum. Published pages do exist that give this as iron not less than 5 %, which is the reverse |
| Carbon | 0.10 maximum | A maximum, and lower than it looks: the low-cycle fatigue quality of this alloy controls carbon, silicon and nitrogen to tighter internal limits |
| Manganese | 0.50 maximum | A maximum |
| Silicon | 0.50 maximum | A maximum, and one of the three elements controlled in the low-cycle fatigue quality |
| Aluminium | 0.40 maximum | A maximum. This is the figure most often inverted in published summaries |
| Titanium | 0.40 maximum | A maximum, and the second figure most often inverted |
| Cobalt | 1.0 maximum | A maximum, determined if required |
| Phosphorus | 0.015 maximum | A maximum, held tight for welding |
| Sulfur | 0.015 maximum | A maximum, held tight for welding |
The strengthening mechanism is the reason this alloy is bought without a heat treatment schedule attached to it. Molybdenum and niobium dissolved in the matrix stiffen it, and because the strength is carried by elements in solution instead of by a precipitate that has to be grown, no ageing cycle is required and there is nothing to overage. The comparison below is the one that decides most specifications, because the two grades appear together on almost every high temperature enquiry.
| Inconel 625 | Inconel 718 | |
|---|---|---|
| How the strength is obtained | Molybdenum and niobium in solid solution, which stiffen the nickel-chromium matrix. No precipitation-hardening treatment is required | A precipitation reaction. The alloy is aged to develop the precipitate that carries its strength, so a defined heat treatment cycle is part of the material |
| What has to be on the purchase order | The form and the condition, which is annealed or solution treated. Both conditions are delivered without a further treatment by the customer | The form, the condition, and the ageing treatment. Material that has not been aged is not at its usable strength |
| What welding does to the schedule | Preheat, post heat and post weld heat treatment are published as not required. Argon shielding is used and the joint is finished | The aged condition is what the design is based on, so a welding sequence that disturbs the heat treatment has to be planned around the ageing cycle |
| Where the alloy sits in service | Selected where corrosion resistance and high temperature behaviour matter together with fabricability | Selected where the highest strength at temperature is the governing requirement |
| The consequence for a buyer | A simpler specification and a shorter route from plate to finished part | A higher strength and a specification with a thermal process attached to it |
The two are not rivals so much as answers to different questions, and the practical test is whether the design is limited by strength or by corrosion. Where it is limited by corrosion at temperature, this alloy is the one that can be welded and put into service without a heat treatment step in the middle of the fabrication sequence.
The ranges below are the published ones, and the published note attached to them is worth repeating with the table: they are composites for various product sizes up to 4 in and they are not suitable for specification purposes. A purchase order takes its minimums from the standard that covers the form.
| Form and condition | Tensile strength | Yield strength, 0.2 % offset | Elongation, % | Hardness, Brinell |
|---|---|---|---|---|
| Rod, bar and plate, as rolled | 120 to 160 ksi, or 827 to 1,103 MPa | 60 to 110 ksi, or 414 to 758 MPa | 60 to 30 | 175 to 240 |
| Rod, bar and plate, annealed | 120 to 150 ksi, or 827 to 1,034 MPa | 60 to 95 ksi, or 414 to 655 MPa | 60 to 30 | 145 to 220 |
| Rod, bar and plate, solution treated | 105 to 130 ksi, or 724 to 896 MPa | 42 to 60 ksi, or 290 to 414 MPa | 65 to 40 | 116 to 194 |
| Sheet and strip, annealed | 120 to 150 ksi, or 827 to 1,034 MPa | 60 to 90 ksi, or 414 to 621 MPa | 55 to 30 | 145 to 240 |
| Tube and pipe, cold drawn, annealed | 120 to 140 ksi, or 827 to 965 MPa | 60 to 75 ksi, or 414 to 517 MPa | 55 to 30 | Not specified |
| Tube and pipe, cold drawn, solution treated | 100 to 120 ksi, or 689 to 827 MPa | 40 to 60 ksi, or 276 to 414 MPa | 60 to 40 | Not specified |
Two things about that table are easy to miss. The solution-treated condition is weaker than the annealed condition at room temperature, and it is still the right choice for the hottest service because the published guidance recommends the solution-treated condition for components requiring optimum resistance to creep or rupture, while fine-grained annealed material is described as advantageous up to 1500 °F for fatigue strength, hardness and tensile strength. The choice between the two conditions is therefore a service decision and not a quality decision.
This is the behaviour that contradicts the usual description of the alloy as structurally simple, and it is published in the bulletin with measured figures. Samples of annealed bar were held for 2,000 hours at three temperatures, and both the room temperature and the at-temperature properties were measured afterwards.
| Exposure temperature, 2,000 hours | Tensile strength at room temperature | Yield strength at room temperature | Elongation at room temperature |
|---|---|---|---|
| No exposure | 140.0 ksi, or 965 MPa | 69.5 ksi, or 479 MPa | 54 % |
| 1200 °F, or 649 °C | 176.0 ksi, or 1,213 MPa | 126.5 ksi, or 872 MPa | 30 % |
| 1400 °F, or 760 °C | 163.0 ksi, or 1,124 MPa | 107.0 ksi, or 738 MPa | 26 % |
| 1600 °F, or 871 °C | 144.0 ksi, or 993 MPa | 76.7 ksi, or 529 MPa | 34 % |
Read the second row carefully, because it is the one that decides whether a long service life changes the material. Two thousand hours at 649 °C raises the room temperature yield strength from 479 MPa to 872 MPa, which is an increase of about eighty per cent, and it takes the elongation down from 54 % to 30 %. The alloy is described as hardening on exposure to intermediate temperatures, and the mechanism is the precipitation of a niobium rich phase that the same published data set describes as capable of causing embrittlement above about 650 °C over extended periods. The measurements also recorded a contraction of about 0.048 % in the samples exposed between 1,200 and 1,400 °F.
For a buyer the consequence is specific. A component that operates above roughly 650 °C for years will be a different material at the end of that period from the one that was installed: stronger, less ductile, and slightly smaller. A low-cycle-fatigue quality of the alloy exists for exactly this reason. It is produced with carbon, silicon and nitrogen controlled to low levels, and it is the grade supplied for bellows and similar cyclically loaded components. Where a long high temperature life and cyclic loading occur together, the standard quality is not the right purchase and the controlled quality is.
| Property or limit | Value | Note |
|---|---|---|
| Density | 8.44 g/cm³ | Common to both the standard and the low-cycle-fatigue quality |
| Melting range | 1,290 to 1,350 °C, or 2,350 to 2,460 °F | This is the figure to hold against any claim about the alloy resisting attack at temperature |
| Service temperature range | Cryogenic to 1,800 °F, which is 982 °C | The published service range. Published pages do exist that quote 1,093 °C as the high temperature capability, which is above the published limit |
| Pressure vessel qualification | Pre-qualified for use in pressure vessels up to 815 °C | A different and lower figure than the service range, because it is a design qualification instead of a material capability |
| Thermal conductivity at 21 °C | 9.8 W/m·°C | Rising to 25.2 W/m·°C at 982 °C. The low room temperature figure is why heat concentrates in a weld pool on this alloy |
| Mean coefficient of expansion | 12.8 × 10−6 per °C from 21 °C to 93 °C | Rising to 16.2 × 10−6 per °C at 927 °C |
| Specific heat at 21 °C | 410 J/kg·°C | Rising to 645 J/kg·°C at 982 °C |
| Electrical resistivity at 21 °C | 1.29 µΩ·m | Rises slightly with temperature, to 1.34 at 982 °C in the same units |
| Modulus of elasticity in tension | 208 GPa annealed, 205 GPa solution treated | Also reported as 30.1 and 29.7 × 10³ ksi. The modulus falls with temperature, to about 147 GPa at 871 °C |
| Poisson ratio | 0.278 annealed, 0.312 solution treated | The two conditions differ, which matters if the value is being used in a calculation and not quoted |
| Magnetic permeability | 1.0006 at 200 oersted, with a Curie temperature below −196 °C | Effectively non-magnetic over the whole service range, with no ambiguity about temperature |
The thermal treatments exist to soften and to relieve, not to strengthen, and each has a published window.
| Operation | The published range | What it is for |
|---|---|---|
| Annealing | 925 to 1,040 °C, with the cooling rate described as not critical | Softens the material and gives the fine-grained condition that is advantageous for fatigue strength, hardness and tensile strength up to 1,500 °F |
| Solution annealing | 1,090 to 1,200 °C | The condition recommended for components requiring optimum resistance to creep or rupture |
| Stress relief | 590 to 760 °C, or up to 870 °C for complete relief | Reduces residual stress. It is not a strengthening treatment and the alloy has no ageing cycle to confuse it with |
| Welding | Gas metal arc, gas tungsten arc, shielded metal arc and submerged arc, with argon shielding | Preheat, post heat and post weld heat treatment are published as not required, which is the practical advantage of a solid solution alloy. The area around the weld has to be clean to prevent contamination of the pool |
| Cold work | Increases the tensile properties for service at moderate temperature | The published data for cold-rolled sheet shows the effect: 20 % reduction raises the yield strength at room temperature to 101 ksi and holds it up through the temperature range |
Which standard applies depends on the product form, and the three ASTM specifications that cover the common forms share one composition but not one set of mechanical requirements. For plate, sheet and strip it is ASTM B443 with ASME SB443, alongside SAE/AMS 5599 and 5869, ISO 6208, BS 3072NA21, DIN 17750 and EN 10095. For rod and bar it is ASTM B446 with ASME SB446, alongside SAE/AMS 5666, ISO 9723, BS 3076NA21, DIN 17752, EN 10095 and ASME Code Case 1935.
The tubular forms split further than the flat products. Seamless pipe and tube come under ASTM B444 with B829 for general requirements and their ASME equivalents, with SAE/AMS 5581, ISO 6207 and BS 3074NA21 alongside. Welded tube comes under ASTM B704 with B751 for general requirements, and welded pipe under ASTM B705 with B775. Forging stock comes under ASTM B564 with ASME SB564, ISO 9725, DIN 17754 and ASME Code Case 1935, wire under SAE/AMS 5837 with ISO 9724 and DIN 17753, and fittings under ASTM B366 with ASME SB366. ISO 4955A and DIN 17744 cover chemical composition across all product forms.
The point of the list is that an order written against one of these numbers is not interchangeable with an order written against another, because the mechanical requirements and the permissible variations differ between them even though the composition does not. A purchase order that names the alloy and the form but not the standard has left the applicable minimums open.
A nickel-chromium-molybdenum alloy with a niobium addition, UNS N06625 and W.Nr. 2.4856, in which the strength comes from molybdenum and niobium held in solid solution in the nickel-chromium matrix. Because the strength is carried by elements in solution instead of by a precipitate, precipitation-hardening treatments are not required, and the alloy is delivered in an annealed or a solution-treated condition with no further thermal treatment needed.
Not for strength. The published windows are an anneal at 925 to 1,040 °C with the cooling rate described as not critical, a solution anneal at 1,090 to 1,200 °C, and stress relief at 590 to 760 °C or up to 870 °C for complete relief. The solution-treated condition is recommended where optimum resistance to creep or rupture is wanted, and fine-grained annealed material is advantageous up to 1,500 °F for fatigue strength and tensile properties. The choice is a service decision, and none of these treatments is an ageing cycle.
In how the strength is obtained, and that difference runs through the whole specification. This alloy is strengthened by solid solution and needs no ageing treatment; 718 is strengthened by a precipitation reaction and has to be aged to reach its usable strength, which puts a defined thermal process inside the fabrication sequence. The published guidance for this alloy states that preheat, post heat and post weld heat treatment are not required. Where the design is limited by strength, 718 will be stronger. Where it is limited by corrosion at temperature and the part has to be fabricated and welded, this alloy is the simpler route.
The published service range is cryogenic to 1,800 °F, which is 982 °C. There is also a separate pressure vessel qualification at 815 °C, which is lower because it is a design qualification instead of a material capability. Published pages do exist that quote 1,093 °C as the high temperature capability, which is above the published service range. And any claim about resistance at a temperature above the melting range of 1,290 to 1,350 °C is describing a liquid instead of an alloy.
Yes, and the published measurements quantify it. Annealed bar held for 2,000 hours at 649 °C came back with a room temperature yield strength of 872 MPa against 479 MPa before exposure, with the elongation down from 54 % to 30 %, and the samples contracted by about 0.048 %. The hardening is attributed to precipitation of a niobium rich phase, and the published data notes that this phase can cause embrittlement above about 650 °C over extended periods. A long high temperature life therefore changes the material, and where cyclic loading occurs with it, the low-cycle-fatigue quality with controlled carbon, silicon and nitrogen is the grade to specify.
Because the low-cycle-fatigue quality of the alloy is produced with carbon, silicon and nitrogen held to low levels, and that quality is the one supplied for bellows and similar cyclically loaded components. The published composition limits for the standard grade are unchanged at carbon 0.10 maximum and silicon 0.50 maximum, so the tighter control in the fatigue quality is a purchase specification instead of a different alloy table. Where a part is cyclically loaded at temperature, the standard quality and the controlled quality are not interchangeable.
Inconel 625 is a solid solution nickel-chromium-molybdenum alloy with niobium, UNS N06625 and W.Nr. 2.4856, whose strength comes from molybdenum and niobium stiffening the matrix, so no precipitation-hardening treatment is required and nothing can be overaged. It is delivered annealed or solution treated, welded with no preheat and no post weld heat treatment, and used from cryogenic temperature to 982 °C, with a separate pressure vessel qualification at 815 °C. Its composition is nickel at 58.0 % minimum, chromium at 20.0 to 23.0 %, molybdenum at 8.0 to 10.0 %, niobium plus tantalum at 3.15 to 4.15 %, and iron, aluminium and titanium all as maximums instead of minimums. Long service above about 650 °C changes it: strength rises and ductility falls through precipitation of a niobium rich phase, which is why a low-cycle-fatigue quality with controlled carbon, silicon and nitrogen exists for bellows and cyclically loaded parts.
The alloy is supplied from Changzhou, Jiangsu in rod, bar, wire, plate, sheet, strip, shapes, forgings and tubular products, in the annealed and solution-treated conditions, against the standard covering each form, from a plant that holds ISO 9001 certification and is audited by SGS. Send the form, the condition, the service temperature and whether the duty is cyclic, and the specification follows from those four answers.
Every figure above is a published value for the alloy and the condition named, and the composite ranges in the mechanical property table are not suitable for specification purposes. Properties depend on the form, the condition and the section size together, and the behaviour of a component in long service depends on the temperature and the time as well. Acceptance of a delivered lot rests on the mill test certificate issued against the standard named on the order, and a design for elevated temperature service should be checked against the current edition of the applicable code instead of against a bulletin.
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