Inconel 625 Heat Treatment: Annealing & Stress Relief (58 chars)

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: How Inconel 625 Should Be Heat Treated

Inconel 625 is solution annealed at about 1,090 to 1,205 °C and rapidly cooled to keep niobium in solution; it is not age hardened. Stress relief is a lower-temperature operation with a ductility trade-off. Slow cooling through 650 to 870 °C precipitates carbides and gamma double prime, reducing ductility and corrosion resistance.

Key Takeaways

• 625 is solid-solution strengthened, not precipitation hardened. Niobium and molybdenum strengthen the matrix in solution; deliberate ageing does not produce the controlled gamma double prime distribution that makes 718 strong, it produces embrittlement and lost corrosion resistance.
• The solution anneal is a specification requirement, not a mill preference. AMS 5599 for sheet, plate and strip and AMS 5662 for bar and forgings both define a solution treatment range, and the certificate must show the actual temperature and cooling method.
• Cooling rate is the hidden variable. Water quenching or rapid gas cooling preserves niobium supersaturation; slow furnace cooling through the 650 to 870 °C window precipitates carbides and gamma double prime at grain boundaries.
• Stress relief sits in a narrow, awkward window. Between about 870 and 980 °C the alloy relieves residual stress without significant precipitation, but the same treatment softens cold-worked product and must be justified by a ductility check.
• Every heat treatment leaves a surface that must be removed. Oxide and heat tint formed at annealing temperature are chromium-depleted and must be pickled or machined off before the part sees corrosive service.
• Furnace qualification and load thermocouples are part of the requirement. AMS 2750 pyrometry class, survey intervals and load thermocouple placement decide whether the recorded temperature is the metal temperature.

Why Inconel 625 Is Solid-Solution Strengthened and Not Age-Hardenable

Inconel 625 gains its strength from niobium and molybdenum held in solid solution in a nickel-chromium matrix, not from a controlled precipitation reaction, and that single fact governs every heat treatment decision made on the grade. The alloy was designed so that the slowly diffusing niobium remains supersaturated after rapid cooling, producing a stiff lattice and a high yield strength in the annealed condition without any ageing step. This is why 625 is supplied annealed and why a purchaser who asks for "aged 625" is asking for something the metallurgy does not support.

Compare the behaviour with Inconel 718, which contains similar nickel, chromium and iron levels but adds aluminium and titanium in controlled amounts. In 718, a two-step ageing treatment precipitates gamma double prime and gamma prime on a defined size distribution, and the yield strength roughly doubles. In 625, the niobium is present at a much higher level (nominally 3.15 to 4.15%) and there is no deliberate aluminium or titanium addition, so the precipitation reaction that does occur on ageing is coarse, grain-boundary dominated and detrimental. Heating 625 into the 650 to 870 °C range for long times produces niobium-rich carbides and gamma double prime, but the result is a loss of ductility and a loss of corrosion resistance rather than a useful strength increase. That is why the alloy is described as solid-solution strengthened and why the correct heat treatment is a solution anneal followed by rapid cooling — nothing more.

The practical consequence for buyers is that heat treatment of 625 is mostly about restoring or preserving a condition, not about developing one. Annealing after cold work restores ductility and removes the work-hardened structure. Annealing after welding dissolves the cast weld metal structure and the grain-boundary precipitates in the heat-affected zone. Stress relief reduces residual stress without changing the microstructure much. None of these operations is intended to raise strength, and a supplier who offers "heat treated to higher strength" on a 625 order is either describing cold work or misunderstanding the grade.

Element	UNS N06625 limit (wt %)	Per standard	Notes
Ni	58.0 min	ASTM B443 / AMS 5662	Balance including Co
Cr	20.0-23.0	ASTM B443 / AMS 5662	Passive film former
Mo	8.0-10.0	ASTM B443 / AMS 5662	Solid-solution strengthener, pitting resistance
Nb + Ta	3.15-4.15	ASTM B443 / AMS 5662	Principal solid-solution strengthener; supersaturated by rapid cooling
Fe	5.0 max	ASTM B443 / AMS 5662	Residual
C	0.10 max	ASTM B443 / AMS 5662	Lower carbon in 625LCF (N06626)
Mn	0.50 max	ASTM B443 / AMS 5662	—
Si	0.50 max	ASTM B443 / AMS 5662	—
Al	0.40 max	ASTM B443 / AMS 5662	Not a deliberate gamma prime former at this level
Ti	0.40 max	ASTM B443 / AMS 5662	Same
Co	1.0 max	ASTM B443 / AMS 5662	—
P / S	0.015 max each	ASTM B443 / AMS 5662	—

Table note: composition limits are from ASTM B443 and AMS 5662 (latest editions). The 625LCF variant (UNS N06626) carries a lower carbon limit and is specified where thermal fatigue resistance is required. Confirm the edition and the exact grade on the mill certificate before comparing suppliers.

The Standard Solution Anneal Range and What the Specifications Require

The solution anneal for 625 is specified in a range of approximately 1,090 to 1,205 °C, with the exact limits depending on the product form and the specification used. AMS 5599 covers sheet, strip and plate; AMS 5662 covers bar, forgings and rings; ASTM B443 covers plate, sheet and strip; ASTM B446 covers bar, rod and wire. The four documents do not use identical wording, and that difference is the first thing a buyer should check on a certificate.

The reason for the high temperature is niobium. To dissolve the niobium-rich carbides and secondary phases that form during solidification and processing, the metal must be held above roughly 1,050 °C, and industry practice settles in the 1,090 to 1,205 °C band to provide margin without approaching incipient melting. Within that band, a higher temperature dissolves more of the grain-boundary phase and produces a more homogeneous structure, but it also grows the grain. A lower temperature preserves a finer grain size but may leave undissolved carbides that reduce ductility and corrosion resistance. The specification range is therefore a compromise, and the mill's actual choice is part of the product it is selling.

Hold time is the second variable. The specification normally requires the metal to reach temperature throughout the section and to be held long enough to complete dissolution, typically expressed as a minimum time at temperature or a time per unit thickness for heavier sections. A large forging needs hours; a thin strip needs minutes. The certificate should state the hold time, not just the set-point temperature, because a part that was pulled out of the furnace as soon as the thermocouple reached temperature will not have dissolved its carbides.

Product form	Specification	Solution treatment range (standard)	Cooling requirement	What the buyer should see on the certificate
Sheet, strip, plate	AMS 5599 / ASTM B443	Approx. 1,090-1,205 °C	Rapid cooling, water or forced gas	Temperature, hold time, cooling medium
Bar, rod, wire	AMS 5662 / ASTM B446	Approx. 1,090-1,205 °C	Rapid cooling, water or forced gas	Same, plus test direction
Forgings, rings	AMS 5662	Approx. 1,090-1,205 °C	Rapid cooling after soak through section	Load thermocouple record for heavy sections
Welded tube	ASTM B704 / B705	Annealed after welding	Rapid cooling	Weld seam condition and anneal record
Seamless tube	ASTM B444	Annealed	Rapid cooling	Temperature and cooling medium
Additive or weld overlay	Procedure-qualified	Stress relief rather than full anneal is common	Controlled cooling	Procedure qualification record

Table note: the ranges quoted are the standard solution-treatment ranges in the specifications named; they are not mill recommendations for every section. Heavy sections require soak times sufficient to reach temperature at the centre, and the applicable specification governs. Verify the current edition before writing a purchase order.

A common purchasing error is to accept a certificate that reads "solution annealed" without a temperature. That wording is not evidence of conformity. The certificate should show the furnace, the set-point, the hold time, the load thermocouple reading for the part and the cooling method. Where the mill cannot produce that record, the material may still be correct, but the buyer has no way to demonstrate it to a notified body or an end user.

Cooling Rate: The Variable That Decides Niobium Supersaturation

Cooling rate determines how much niobium stays in solution, and it is the most frequently neglected part of the 625 heat treatment specification. The alloy relies on supersaturation, so the objective after the anneal is to get through the precipitation range quickly enough that the niobium does not have time to form coarse carbides or gamma double prime at the grain boundaries.

The critical window sits roughly between 650 and 870 °C. Above about 870 °C, diffusion is fast but the equilibrium phases are largely dissolved; below about 650 °C, diffusion is too slow for significant precipitation during a normal cool. In between, niobium and carbon have enough mobility to form M6C and M23C6 carbides at grain boundaries, and niobium can form the metastable gamma double prime phase. A slow furnace cool, a large furnace load cooled in a closed chamber, or a heavy section that cools from the centre over hours will all spend enough time in that window to precipitate.

Water quenching is the fastest practical route and is used for bar, rod and thinner sections. It produces the highest supersaturation and the best combination of strength and corrosion resistance, at the cost of distortion and residual stress. Forced-gas cooling is a compromise used for larger or complex shapes where water quenching would distort or crack the part. Air cooling is acceptable for thin sections that pass through the window quickly, but not for heavy sections. The choice must be stated on the order, because "cooled" without a medium is not a specification.

The consequences of slow cooling are measurable. Yield and tensile strength fall slightly, elongation usually falls more sharply, and the corrosion rate in oxidising acid media rises because chromium-depleted zones form adjacent to grain-boundary carbides. In chloride service the effect is worse, because the same boundaries become preferential pitting sites. A part that passed a corrosion test as annealed bar can fail the same test after a slow cool, and the certificate will still say "solution annealed".

Cooling route	Typical application	Effect on niobium supersaturation	Distortion risk	Comment
Water quench	Bar, rod, thin plate, small forgings	Highest; most niobium retained	High on asymmetric shapes	Standard for bar product
Forced gas / air blast	Plate, larger forgings, complex shapes	Good if section is thin enough	Moderate	Needs qualified cooling curve
Still air	Thin sheet and strip	Adequate for thin sections	Low	Not acceptable for heavy sections
Slow furnace cool	Not a specification-compliant route for 625	Low; grain-boundary precipitation likely	Low	Produces carbide networks and lost ductility
Controlled cool after stress relief	Fabricated components	Not applicable; precipitation already limited by temperature	Low	Must stay below the precipitation window

Table note: cooling routes are described as practice. The controlling requirement is the specification's cooling provision and, where the buyer needs evidence, a recorded cooling curve. No standard minimum cooling rate is implied by this table.

Stress Relief After Welding and Cold Work: Options and the Ductility Trade-off

Stress relief for 625 is a separate, lower-temperature operation than the solution anneal, and it exists because fabricated components cannot always be re-annealed. A pressure vessel, a large duct or a welded tube assembly may be too big for a solution treatment furnace, or may contain dissimilar joints that cannot tolerate 1,100 °C. In those cases the fabricator needs a treatment that reduces residual stress without destroying the properties that the solution anneal produced.

The practical window for stress relief is roughly 870 to 980 °C, followed by rapid cooling. At these temperatures the alloy relieves a large fraction of residual stress by creep relaxation, while the niobium remains largely in solution and the grain-boundary carbide reaction is limited. The treatment is not a full anneal: it does not recrystallise cold-worked material completely, and it does not dissolve carbides that were already present. What it does is reduce the tensile residual stress that drives stress corrosion cracking and dimensional movement during machining.

The trade-off is ductility. Cold-worked or welded 625 that is stress relieved in this window loses some of its work-hardened strength, and if the cooling after stress relief is slow, the part passes back through the 650 to 870 °C precipitation range and picks up the grain-boundary carbides that the stress relief was meant to avoid. Rapid cooling after stress relief is therefore as important as the hold temperature. A fabricator who stress relieves in a furnace and lets the load cool overnight in the chamber has undone most of the benefit.

Lower-temperature stress relief, in the 650 to 760 °C range, is sometimes proposed for dimensional stability before finish machining. It reduces stress less effectively and sits directly in the precipitation window, so the risk of embrittlement is higher. Where it is used, the procedure should be qualified by a ductility check — a room-temperature tensile test or a bend test on a coupon that saw the same cycle — rather than assumed. For most 625 fabrication, the 870 to 980 °C route with rapid cooling is the safer choice, and the solution anneal remains the reference treatment whenever the geometry allows it.

Operation	Temperature (typical)	Purpose	Effect on microstructure	Risk if done badly
Solution anneal	Approx. 1,090-1,205 °C	Dissolve carbides, restore homogeneity, maximise supersaturation	Recrystallised, single-phase matrix	Grain growth at top of range; distortion on quench
Stress relief (high)	Approx. 870-980 °C	Reduce residual stress after welding or machining	Limited precipitation if cooled rapidly	Slow cool re-enters precipitation window
Stress relief (low)	Approx. 650-760 °C	Dimensional stability before finish machining	Precipitation possible at grain boundaries	Loss of ductility and corrosion resistance
Post-weld anneal	Same as solution anneal where geometry allows	Dissolve weld and HAZ segregation	Recrystallised HAZ	Distortion; furnace capacity
Deliberate ageing	650-870 °C	Not a valid strengthening treatment for 625	Coarse grain-boundary precipitates	Embrittlement, reduced corrosion resistance

Table note: temperatures are typical practice for wrought 625 and are not standard minima. The governing requirement is the applicable product or process specification, and any stress-relief cycle on pressure equipment must be qualified under the design code and ASME Section IX where welding is involved.

How Thermal Condition Changes Mechanical Properties and Corrosion Resistance

Thermal condition changes the properties of 625 in ways that a room-temperature tensile test does not fully reveal, which is why certificates alone can be misleading. The annealed condition gives a good combination of strength and ductility; a slow-cooled or improperly stress-relieved condition can still meet a minimum tensile requirement while failing on elongation, toughness or corrosion resistance.

In the properly annealed and rapidly cooled condition, 625 typically shows a room-temperature tensile strength in the region of 830 to 930 MPa, a 0.2% yield strength around 415 to 500 MPa and an elongation of 30% or more, with hardness typically in the 150 to 220 HB range. These are typical values; the standard minima are lower, and the specification is what governs acceptance. At elevated temperature the alloy retains useful strength to about 650 °C and oxidation resistance well beyond that, but it is not a gamma-prime-strengthened superalloy and should not be selected for high-stress creep service at 700 °C and above where 718 or 263 would be used.

The more sensitive indicators are elongation, reduction of area, impact toughness and corrosion rate. Slow cooling through the precipitation window reduces elongation before it reduces tensile strength, so a certificate that reports only UTS and yield can hide a ductility problem. In corrosive service the effect appears as preferential attack at grain boundaries, which is detected by ASTM G28 testing rather than by a tensile test. A buyer who specifies G28 on the certificate gets a direct check on thermal condition; a buyer who specifies only tensile properties gets an indirect and weaker one.

Condition	UTS (typical, MPa)	0.2% YS (typical, MPa)	Elongation (typical, %)	Hardness (typical, HB)	Corrosion behaviour
Solution annealed, rapid cooled	830-930	415-500	30-45	150-220	Reference condition; best resistance
Annealed, slow cooled	800-880	380-450	20-30	150-200	Grain-boundary attack risk in oxidising acid
Stress relieved 870-980 °C, rapid cool	800-900	380-470	25-40	150-210	Close to annealed if cooling is rapid
Stress relieved 650-760 °C	850-950	420-520	15-25	170-230	Reduced ductility; pitting at boundaries
Cold worked, not annealed	950-1,200	600-900	5-20	220-300	Susceptible to SCC and distortion
Weld metal, as welded	700-830	350-450	20-35	150-200	Segregated; requires anneal for critical service

Table note: property ranges are typical values for wrought product and are not standard minima. Standard minimum values are stated in ASTM B443, ASTM B446, AMS 5599 and AMS 5662 and vary by product form, size and test direction. Corrosion behaviour is described qualitatively; quantify it with ASTM G28 where the duty requires evidence.

Furnace Control, AMS 2750 and Load Thermocouples

A heat treatment is only as good as the temperature the metal actually reached, and that is what AMS 2750 pyrometry requirements are designed to establish. The specification sets furnace class, instrument accuracy, calibration intervals, survey frequency and the use of load thermocouples, and a certificate that reports a set-point without a survey status and a load thermocouple record is not evidence that the part was treated correctly.

For 625 the stakes are higher than for a simple anneal because the acceptable window is narrow relative to the soaking temperature. A furnace with a large uniform zone and a well-maintained control system will hold the load within a few degrees; a furnace with a failed zone or a badly placed control thermocouple can show the set-point while the load sits 30 °C low at one end. The consequence is undissolved carbides at the cold end and grain growth at the hot end, both of which affect ductility and corrosion resistance without necessarily failing a tensile test.

The practical requirements are straightforward to write into a purchase order. State the AMS 2750 pyrometry class required, require a current furnace survey report, require load thermocouples for sections above a stated thickness, require the actual temperature chart or electronic record to be retained and supplied on request, and require the cooling medium to be recorded. For critical components, require the heat treater to be Nadcap accredited or equivalent, because the accreditation is the evidence that the pyrometry system is audited rather than self-declared.

Verification after heat treatment should include hardness at a defined location, a tensile test where the specification requires it, and a grain size check to ASTM E112 where grain growth is a concern. For service in corrosive media, add ASTM G28. These checks are cheap relative to the cost of a rejected pressure component, and they close the gap between a certificate that says "annealed" and a part that is actually annealed. Buyers comparing suppliers can review the supplier qualification audit guide for the furnace and pyrometry questions worth asking, and the heat treatment knowledge notes for the acceptance records that should accompany each lot.

Heat Treatment Failures: Over-Ageing, Sensitisation and Distortion

Most 625 heat treatment failures fall into three groups, and all three are visible in the certificate or the part before the customer sees a corrosion problem. Understanding them is the difference between accepting a lot and rejecting a future failure.

The first is over-ageing through slow cooling. The part is annealed correctly, then cooled in the furnace or in a closed chamber over several hours. The certificate reports the correct annealing temperature, the tensile test passes, and the part is shipped. In service, grain-boundary carbides act as pitting initiation sites, and the first leaks appear at welds and heat-affected zones. The diagnostic evidence is a G28 result that is worse than expected and a metallographic section showing continuous grain-boundary carbide networks.

The second is sensitisation of the heat-affected zone during welding without a post-weld anneal. Welding takes the HAZ through the precipitation range and leaves it there as the weld cools, so the HAZ can be sensitised even though the parent metal is correct. The remedy is a post-weld anneal where geometry allows, or a weld procedure with low heat input and rapid cooling where it does not. For thin-wall tube and small fittings, the practical answer is often to specify annealed tube and to qualify the weld procedure with a G28 test on a coupon.

The third is distortion and residual stress from quenching. Water quenching a asymmetric forging or a machined component can warp it beyond the machining allowance, and the residual stress left by the quench can move the part during subsequent machining. The remedies are to quench symmetrically, to use forced gas for complex shapes, to rough machine before the final anneal and finish machine after, and to stress relieve before finish machining where the geometry permits.

Failure mode	Root cause	How it shows up	Detection method	Prevention
Grain-boundary carbide network	Slow cooling through 650-870 °C	Reduced ductility, preferential corrosion at boundaries	ASTM G28, metallography	Rapid cooling after anneal; recorded cooling curve
Sensitised HAZ	Welding without post-weld anneal	Weld decay, pitting beside the weld	G28 on procedure coupon, SEM	Low heat input, PWHT where possible
Distortion after quench	Asymmetric section, water quench	Out-of-tolerance dimensions, machining stock lost	Dimensional inspection after heat treatment	Forced gas cooling, symmetrical loading, rough machine first
Residual stress movement	Quench stress not relieved	Dimensions move during finish machining	Dimensional drift between operations	Stress relief before finish machining
Grain growth	Top of anneal range, long soak	Lower toughness, coarser structure	ASTM E112 grain size	Control temperature at lower half of range
Surface contamination	Sulphur or chlorine in furnace atmosphere	Green/black staining, corrosion initiation	Visual, surface analysis	Clean furnace, no sulphur-bearing lubricants

Table note: failure modes are described from mill and field practice. Detection methods are named as test techniques; acceptance criteria must be stated in the purchase order with the standard revision.

Buying 625 on the Right Thermal Condition: Cross-Reference and Price

Buying 625 on the right thermal condition means writing four things into the order: the product specification, the solution treatment range and cooling method, the acceptance tests that prove the condition, and the documentation that records it. The grade designation alone does not carry any of that information, and the cross-reference between national systems does not either.

The five standard systems describe 625 differently. In the US system the grade is UNS N06625 under ASTM B443 for flat-rolled product, ASTM B446 for bar and rod, ASTM B444 for seamless tube and ASTM B704/B705 for welded tube, with AMS 5599 and AMS 5662 controlling aerospace-grade material. In China the nearest designation is GH3625 under the GB system, with composition and properties specified in the GB/T 14992 family for superalloys; the designation is close but the permitted ranges and the test requirements are not identical, so a cross-reference is a starting point for discussion rather than an equivalence. In Japan, JIS G 4901 and G 4902 cover superalloy bar and plate respectively. In Europe, DIN 17744 covers composition and DIN 17750 covers wrought product properties, with the workshop designation 2.4856 widely used. In Russia and the CIS, GOST 5632 lists the grade family and the associated GOST specifications control delivery.

System	Grade designation	Product specification	Heat treatment requirement	Notes for buyers
US (UNS/ASTM)	N06625	ASTM B443, B446, B444, B704/B705	Solution anneal per product spec; AMS 5599 / AMS 5662 for aerospace	Most widely traded basis; certificates usually complete
US (AMS)	N06625	AMS 5599 (flat-rolled), AMS 5662 (bar, forgings)	Solution treatment with defined range and cooling	Requires AMS 2750 pyrometry evidence
China (GB)	GH3625	GB/T 14992 family for superalloy grades	Annealed condition; Chinese test requirements apply	Composition ranges close to 625 but not identical
Japan (JIS)	NCF 625	JIS G 4901 (bar), JIS G 4902 (plate)	Solution annealed	Often supplied to JIS with Japanese-language certification
Europe (DIN/EN)	2.4856 / NiCr22Mo9Nb	DIN 17744 (composition), DIN 17750 (wrought product)	Solution annealed	EN 10204 3.1 certification standard for EU projects
Russia (GOST)	KhN22Mo9Nb-type designation	GOST 5632 grade list plus delivery specification	Annealed	Availability limited; confirm mill route in writing

Table note: national designations are not interchangeable. Composition ranges, test requirements and heat treatment wording differ between systems. Where a project requires cross-system acceptance, state the primary specification and the additional requirement separately, and obtain the mill's written confirmation of the actual values.

Price for 625 is driven by nickel, molybdenum and niobium content, by the product form, and by the thermal condition and test package. A solution-annealed bar with a full certificate and G28 testing is not the same product as a commercial annealed bar with a chemistry certificate only, and the difference shows up in the quotation. The ranges below are indicative only and move with the alloy surcharge.

Product form	Condition	Indicative price, 2026, EXW Shanghai, USD/kg	Price drivers	Typical lead time
Round bar, 20-100 mm	Solution annealed, pickled	38-52	Ni, Mo, Nb content; test package	2-4 weeks from stock
Round bar, 100-250 mm	Solution annealed	42-58	Section size, forge route	4-8 weeks
Plate, 5-20 mm	Solution annealed, pickled	40-55	Width, flatness class, edge condition	3-6 weeks
Plate, 20-60 mm	Solution annealed	43-60	Thickness tolerance, UT requirement	4-8 weeks
Seamless tube	Annealed	45-65	Size, wall tolerance, test pressure	6-10 weeks
Welded tube	Annealed after welding	35-50	Weld seam testing, size	3-6 weeks
Forging / ring	Solution annealed	50-75	Machining allowance, UT, grain size	6-12 weeks
Wire, 1-8 mm	Annealed	42-60	Surface finish, coil weight	3-6 weeks

Table note: 2026, EXW Shanghai, USD/kg, indicative ranges only, subject to nickel, molybdenum and niobium price movement and to the test and documentation package. They are not offers. Confirm against a current quotation before budgeting.

Matching Thermal Condition to Application

The right thermal condition depends on the application, not on a preference for "more heat treatment". The table maps common 625 applications to the condition that is normally specified, and to the condition that causes problems in that application.

Application	Recommended condition	Why it fits	Condition to avoid
General corrosion-resistant components	Solution annealed, rapid cooled	Maximum ductility and corrosion resistance	Slow-cooled annealed
High-pressure tubing and fittings	Solution annealed, rapid cooled	Supersaturated niobium, consistent properties	Stress relieved only
Large welded fabrications	Stress relief 870-980 °C, rapid cool	Relieves stress without full re-anneal	Low-temperature relief without ductility check
Machined parts with tight tolerances	Stress relief before finish machining	Dimensional stability	Quench stress left in
Aerospace sheet and plate	AMS 5599 anneal	Specification compliance	Non-certified anneal
Fasteners and springs	Cold worked plus stress relief	Strength retention	Full anneal, which removes the strength
Bellows and expansion joints	Solution annealed, rapid cooled	Fatigue and ductility	Cold worked without relief

Table note: condition recommendations reflect common practice and the requirements of the applicable product specification. Structural welds on pressure equipment must also satisfy the design code and ASME Section IX.

Standard Index

Standard	Title or scope	Covers	Product form
ASTM B443	Nickel-chromium alloy 625 plate, sheet and strip	Chemistry, mechanicals, heat treatment	Plate, sheet, strip
ASTM B446	Nickel-chromium alloy 625 bar, rod and wire	Chemistry, mechanicals, condition	Bar, rod, wire
ASTM B444	Nickel-chromium alloy 625 seamless tube and pipe	Chemistry, mechanicals, pressure	Seamless tube, pipe
ASTM B704	Nickel-chromium alloy 625 welded tube	Chemistry, mechanicals, weld	Welded tube
ASTM B705	Nickel-chromium alloy 625 welded pipe	Chemistry, mechanicals, weld	Welded pipe
AMS 5599	625 sheet, strip and plate	Solution treatment, properties	Flat-rolled
AMS 5662	625 bar, forgings and rings	Solution treatment, properties	Bar, forging, ring
AMS 2750	Pyrometry	Furnace class, surveys, load thermocouples	Heat treatment
ASTM E8/E8M	Tension testing of metallic materials	Tensile, yield, elongation	All forms
ASTM E112	Determining average grain size	Grain size	All forms
ASTM E18	Rockwell hardness	Hardness verification	All forms
ASTM G28	Intergranular corrosion susceptibility, nickel-rich Cr-bearing alloys	Practice A and B	Wrought products
ASTM E1476	Metals identification and sorting	PMI verification	All forms
GB/T 14992	Superalloy grades and properties	Chinese designation and properties	All forms
JIS G 4901 / G 4902	Superalloy bar / plate	Properties and condition	Bar, plate
DIN 17744 / DIN 17750	Nickel alloys composition / wrought products	Composition and properties	All forms
GOST 5632	Corrosion- and heat-resistant alloy grades	Grade designation	All forms

Table note: the table lists the documents that control 625 supply. Product specifications set chemistry, mechanicals and heat treatment; AMS 2750 and the ASTM test methods set the evidence. Quote editions and, for G28, the practice letter and acceptance limit.

FAQ: Inconel 625 Heat Treatment

Q1: What is the correct annealing temperature for Inconel 625?

The correct annealing temperature is the solution treatment range given in the applicable product specification, which for 625 sits at approximately 1,090 to 1,205 °C. AMS 5599 applies that range to sheet, strip and plate, while AMS 5662 applies it to bar, forgings and rings; ASTM B443, B446, B444 and B704/B705 carry their own wording for the corresponding forms. Within the range, a mill will choose a temperature that dissolves the niobium-rich carbides without growing the grain excessively, so the actual value is part of the product being purchased. The important purchasing point is that the certificate must state the temperature, the hold time at temperature and the cooling medium — not merely the words "solution annealed". A certificate without those three items cannot be checked against the specification, and a buyer who accepts it has no evidence if the part later cracks or corrodes at grain boundaries. Confirm the edition of the specification as well, because the range and the cooling provision are revised from time to time.

Q2: Can Inconel 625 be age hardened?

No. 625 is a solid-solution strengthened alloy, and there is no valid ageing treatment that raises its strength in the way that ageing raises the strength of 718. The alloy contains about 3.15 to 4.15% niobium plus tantalum, which stays in supersaturated solution after rapid cooling and strains the lattice, giving the annealed product its strength. It does not contain the deliberate aluminium and titanium additions that form the fine gamma prime dispersion in precipitation-hardened grades, so heating it to the 650 to 870 °C ageing range produces coarse grain-boundary carbides and gamma double prime rather than a useful strengthening precipitate. The result is a loss of ductility, a loss of toughness and a loss of corrosion resistance at the grain boundaries, with little or no gain in yield strength. A supplier who offers "aged 625" or "heat treated to higher strength" is either describing cold-worked material or does not understand the grade, and the certificate should be examined closely before the material is accepted.

Q3: Why does slow cooling damage Inconel 625?

Slow cooling damages 625 because it allows the niobium and carbon that were dissolved at the annealing temperature to precipitate at grain boundaries while the metal passes through roughly 650 to 870 °C. In a fast cool, the metal spends only seconds or a few minutes in that window, so diffusion is too limited for significant precipitation and the niobium stays in solution, which is the condition that gives the alloy its strength and corrosion resistance. In a slow cool — a furnace cool, a large load cooling in a closed chamber, or a heavy section cooling from the centre over hours — the metal spends enough time in the window for M6C and M23C6 carbides to form at the boundaries, together with niobium-rich gamma double prime. The chromium consumed by the carbides leaves narrow depleted zones beside the boundaries, which become preferential corrosion paths in oxidising acid and preferential pitting sites in chloride service. Elongation falls before tensile strength does, so a certificate that reports only UTS and yield can look satisfactory while the part is already compromised.

Q4: Should I stress relieve Inconel 625 after welding?

Stress relieve after welding when the component will see chloride-bearing service, when dimensional stability during machining matters, or when the design code requires it for the section thickness and restraint level. The practical window is roughly 870 to 980 °C with rapid cooling, which relaxes residual stress without dissolving the whole structure or precipitating coarse carbides. A full solution anneal at 1,090 to 1,205 °C is better metallurgically because it also dissolves the cast weld structure and the HAZ segregation, but it is often impossible for large fabricated assemblies and it carries a distortion penalty. Where neither is practical, a low-temperature stress relief in the 650 to 760 °C range can be used for dimensional stability, but it sits in the precipitation window and should be qualified by a ductility check on a coupon that saw the same cycle. The decision should be made by the welding engineer against the design code, not by the purchasing department against the price list.

Q5: How do I verify the heat treatment on a mill certificate?

Verify it by checking four items. First, the certificate must name the specification and edition, not just the grade. Second, it must state the solution treatment temperature and the hold time, with the furnace identification. Third, it must state the cooling medium — water quench, forced gas or air — because the cooling route is part of the treatment. Fourth, it must carry the mechanical and corrosion test results that the specification requires, with the test standard and the acceptance limits. For aerospace or critical service, add the AMS 2750 pyrometry class and the load thermocouple record. If the certificate carries only a chemistry table and a room-temperature tensile result, it does not demonstrate that the heat treatment was correct. Hangbo Alloy issues certificates with the furnace, temperature, hold time and cooling medium stated for every lot, and can add G28 intergranular corrosion testing where the duty requires evidence of thermal condition.

Q6: Does heat treatment change the corrosion resistance of Inconel 625?

Yes, substantially, and the effect runs in one direction: a correct solution anneal and rapid cool give the best corrosion resistance, while slow cooling or low-temperature ageing degrade it. The mechanism is chromium depletion at grain boundaries where carbides precipitate. In the annealed and quenched condition the chromium is uniformly distributed and the passive film is homogeneous. After slow cooling through the precipitation range, the boundaries are surrounded by chromium-poor zones that cannot form a protective film, and attack concentrates there. In oxidising acid service the result is intergranular corrosion; in chloride service it is pitting and crevice corrosion at the boundaries. The bulk corrosion rate measured on a polished coupon may barely change, which is why the effect is often missed, while the penetration rate at a sensitised weld HAZ can be orders of magnitude higher. ASTM G28 is the practical test for this, and it should be specified whenever the part will see acid or chloride service.

Q7: What hardness should Inconel 625 have after annealing?

Annealed 625 typically falls in the 150 to 220 HB range, depending on product form, section size and the exact annealing and cooling route. Hardness is a useful but blunt indicator: a reading inside the typical band does not prove that the cooling was fast enough, and a reading at the top of the band does not prove that the part is bad. The value of hardness testing is as a consistency check across a lot and as a screen for cold-worked or improperly annealed material, not as a substitute for tensile or corrosion testing. Where hardness is used for acceptance, the purchase order should state the method (ASTM E18 for Rockwell, ASTM E10 for Brinell), the location of the test, the number of readings and the acceptance range. A single reading on a forged bar end is not representative of the section. For critical parts, pair the hardness check with a tensile test and, where corrosion resistance matters, with G28.

Q8: Can Inconel 625 be heat treated after cold working?

Yes, and it usually should be. Cold working — drawing, rolling, bending, swaging — raises strength and hardness while reducing ductility and leaving residual stress, and a solution anneal at approximately 1,090 to 1,205 °C with rapid cooling restores the annealed condition. The alternative, a stress relief at 870 to 980 °C, reduces residual stress but does not recrystallise the cold-worked structure, so the part retains much of its increased strength and reduced ductility. The choice depends on the application: a cold-drawn bar that needs maximum ductility for machining or forming should be fully annealed, while a formed component that needs dimensional stability but can keep some strength may be stress relieved. The trap is to specify "annealed" and receive stress-relieved material, which looks the same on a certificate that omits the temperature. If ductility matters, specify the annealing temperature range and require an elongation result to confirm it.

Q9: What is the difference between AMS 5599 and AMS 5662?

AMS 5599 and AMS 5662 are both aerospace material specifications for 625, but they cover different product forms. AMS 5599 applies to sheet, strip and plate, and AMS 5662 applies to bar, forgings and rings. Both specify the solution treatment range and the resulting mechanical properties, and both require the heat treatment to be performed under an approved pyrometry system, but the test requirements, the specimen orientation and the property limits differ because the product forms differ. A purchaser buying plate should cite AMS 5599; a purchaser buying bar or a forging should cite AMS 5662. Citing the wrong one is a common error in aerospace subcontract packages and usually results in a certificate that cannot be accepted, because the mill's certification is written against the specification for the form it produced. Where a project requires both forms, list both specifications against the relevant line items rather than quoting one for the whole order.

Q10: Why does Inconel 625 distort after quenching?

625 distorts after quenching because the thermal gradient between the surface and the centre produces differential contraction and residual stress, and because the alloy has a relatively high coefficient of thermal expansion for a nickel alloy. A symmetrical round bar quenched uniformly distorts little; an asymmetric forging, a thin-walled tube or a machined component with unequal sections will bend, twist or change dimension as the cooler surface contracts against the hotter core. The residual stress that remains after the quench can also cause the part to move during subsequent machining, which is often misdiagnosed as a machining problem. The practical controls are to design the heat treatment sequence rather than the part alone: rough machine, anneal, finish machine, and stress relieve before the final finishing cuts where the geometry allows. For complex shapes, forced-gas cooling reduces the thermal gradient at some cost in cooling rate, and a fixtures or support during quenching helps maintain shape.

Q11: How long should a heavy forging soak at temperature?

The soak time must be long enough for the centre of the section to reach the solution treatment temperature and for the niobium-rich carbides to dissolve, and it is normally specified as a minimum time at temperature or as a time per unit of section thickness in the applicable specification or the mill's approved procedure. For a heavy forging, the heating rate also matters: a fast heat into the furnace can create a large thermal gradient and internal stress, so the load is usually charged at a controlled rate, equalised at an intermediate temperature, then raised to the solution treatment temperature and held. The only reliable evidence is a load thermocouple reading taken at or near the thickest section, recorded against time, showing that the metal — not the furnace atmosphere — held the required temperature for the required period. A certificate that reports a soak time without saying where it was measured is weak evidence. For sections above roughly 100 mm, the load thermocouple requirement should be written into the purchase order.

Conclusion: Specify the Thermal Condition, Then Prove It

Heat treatment of Inconel 625 is not complicated, but it is unforgiving. The alloy is solid-solution strengthened, so the only correct treatments are a full solution anneal at approximately 1,090 to 1,205 °C with rapid cooling, or a stress relief at roughly 870 to 980 °C with rapid cooling where a full anneal is impractical. Ageing has no place in the specification, slow cooling has no place in the procedure, and a certificate that reports a temperature without a hold time, a cooling medium and the applicable test results has no evidentiary value.

The purchasing programme follows directly. Name the product specification and edition for the form being bought — AMS 5599 for flat-rolled, AMS 5662 for bar and forgings, ASTM B443 or B446 for commercial product. State the solution treatment range, the cooling medium and the acceptance tests, including G28 where the part will see acid or chloride service. Require the AMS 2750 pyrometry class and a load thermocouple record for heavy sections. Require hardness, tensile and grain size results as applicable, and require the surface oxide to be removed by pickling or machining before dispatch. Then check the certificate against those four items before the material enters the warehouse.

Buyers who need 625 bar, plate, tube, forging or wire on a specified thermal condition can review the Inconel product line, the Hastelloy and nickel product ranges and the alloy knowledge centre, or send the specification and the service conditions to the sales engineering desk. Shanghai Hangbo Alloy Group supplies 625 with the furnace, temperature, hold time and cooling medium recorded on the certificate, with G28 testing available for acid and chloride duties.

Related Reading

1. melting routes: VIM, ESR and VAR compared
2. the supplier qualification audit checklist

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