Technical knowledge
Inconel 690 for steam generator tubing and caustic service
Why Inconel 690 replaced Inconel 600 in steam generator tubing: stress corrosion cracking resistance, chromium content, thermal treatment and specification notes for seamless tube and fittings.
Why the industry moved from 600 to 690
Inconel 600 tubing gave decades of service before operators began seeing primary water stress corrosion cracking and outside diameter stress corrosion cracking in the crevices of tube support plates. Laboratory and plant data showed that raising chromium from about 15 % to about 30 % increased the time to cracking by more than an order of magnitude, which led to the widespread adoption of 690 for replacement generators and retubing campaigns.
Composition and the thermal treatment that matters
| Item | Inconel 600 | Inconel 690 |
|---|---|---|
| UNS number | N06600 | N06690 |
| Chromium | 14.0 - 17.0 % | 27.0 - 31.0 % |
| Nickel plus cobalt | 72 % minimum | 58 % minimum |
| Iron | 6.0 - 10.0 % | 7.0 - 11.0 % |
| SCC resistance in primary water | Susceptible | Very high |
| Typical specification | ASTM B163 / B167 | ASTM B163 / B167, SB-163 |
Tip Specify thermal treatment explicitly. Mill annealed 690 tubing performs differently from tubing given a thermal treatment that develops semicontinuous grain boundary carbides, and that treatment is a recognised requirement for steam generator tubing.
Beyond steam generators: caustic and nitric service
- Caustic evaporators. 690 resists caustic stress corrosion cracking better than 600 at high concentration and temperature.
- Nitric acid service. The high chromium content gives excellent resistance to oxidising acids where molybdenum bearing alloys are not needed.
- Waste incineration and flue gas treatment. Useful where both oxidising salts and chlorides are present.
Buying notes for tube and fittings
Order seamless tube to ASTM B163 or B167 as applicable and butt weld fittings in the same UNS number so the weld metal and parent metal share a corrosion mechanism. Require the certificate to quote actual chromium and carbon values, the thermal treatment temperature and time, grain size where specified, and the results of intergranular attack testing when the purchase specification calls for it.
Questions about this topic
Is Inconel 690 always better than 600?
For stress corrosion cracking, yes. For very high temperature strength and for reducing acid service the two are much closer, and 690 is not a general purpose substitute for 600 in every duty.
Can 690 be welded with 600 filler metal?
It should not be. Use a matching high chromium nickel filler so the weld metal retains the SCC resistance of the parent material. A mismatched weld becomes the preferred crack path.
What is thermal treatment in this context?
A low temperature heat treatment, typically in the region of 700 to 730 C, applied after mill annealing to develop grain boundary carbides that improve intergranular stress corrosion cracking resistance. It is distinct from stress relieving and should be named separately on the order.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email demi@nickel-alloy.com or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
Share your duty conditions and we will shortlist candidate alloys within one working day.