Why Inconel 690 replaced 600 in nuclear steam generators

Both grades are nickel-chromium alloys with no molybdenum, so the datasheets differ in the one number that decides the duty: chromium, 14.0% against 27.0–31.0%. The rest of the comparison follows from that.

Inconel 690 exists because Inconel 600 did not survive one specific duty. Both are nickel-chromium alloys: no molybdenum, no age-hardening addition, supplied annealed and specified for corrosion resistance, not for load. What separates them is chromium. Our grade data lists 14.0% for 600 and 27.0–31.0% for 690, and almost everything else in this note follows from that single figure.

What the steam generator problem was

In a pressurised water reactor the steam generator tubing is the boundary between primary coolant and secondary water. 600 tubing was used widely in that duty, and in a number of units it developed primary-water stress corrosion cracking on the outside diameter — at the tube-to-tubesheet joint and in the U-bends. The mechanism is not general wastage. It is a localised crack that starts where the passive film breaks down and grows under sustained tensile stress, and it is therefore governed by the film rather than by bulk strength.

Raising chromium changes that film. At the 27.0–31.0% level the oxide is more stable and repassivates faster, so the same wall thickness gives a different service life. 690 was developed around that observation rather than around a new strengthening mechanism, which is why it is a substitution for 600 in one application and not a general upgrade of it.

Chemistry and designation, side by side

The two grades are not close relatives with a tweak. 690 holds nickel at 58.0–63.0% and iron at 7.0–11.0%; 600 is listed at 72.0 nickel, 14.0 chromium and 6.0 iron. Carbon in 690 is capped at 0.05%, which keeps grain-boundary chromium carbide out of welded tube, and the tube specification pair tells the same story: ASTM B167 appears for both, but 690 is called out with AMS 5581 and DIN 2.4642 where 600 carries AMS 5580 and DIN 17751.

	Inconel 690	Inconel 600
UNS	N06690	N06600
European designation	DIN 2.4642	DIN 2.4816
Nickel	58.0 – 63.0%	72.0%
Chromium	27.0 – 31.0%	14.0%
Iron	7.0 – 11.0%	6.0%
Carbon	≤ 0.05%	not stated
Manganese	≤ 0.50%	not stated
Silicon	≤ 0.50%	not stated
Sulfur	≤ 0.015%	not stated
Density	8.19 g/cm³	8.47 g/cm³
Melting range	1343 – 1377 °C	1354 – 1413 °C
Plate and sheet	ASTM B168, ASME SB-168	ASTM B168, AMS 5540, DIN 17750
Round bar	ASTM B166, ASME SB-166	ASTM B166, AMS 5580, DIN 17752
Seamless tube	ASTM B167, AMS 5581, DIN 2.4642	ASTM B167, AMS 5580, DIN 17751
Forgings	ASTM B564, ASME SB-564	ASTM B564, DIN 17754

Where a cell reads “not stated”, our grade sheet records only the three principal elements for that grade and gives them as single figures with no range. The heat certificate governs; ask for it before you freeze a drawing.

Where the extra chromium is spent

Chromium is what forms the passive film, and 690 carries nearly twice as much of it. That buys margin in two places a buyer actually feels. The first is high-temperature oxidising and sulfur-bearing service, where 690 is listed as excellent against both oxidation and sulfidation. The second is acidic attack, where it is listed for sulfuric and nitric acid exposure.

The trade is nickel. At 58.0–63.0% against 72.0%, 690 gives up part of the nickel margin that makes 600 comfortable in chloride stress-corrosion service and in carburising atmospheres. Iron moves the other way, 7.0–11.0% against 6.0%, and density follows it down to 8.19 g/cm³ from 8.47 g/cm³, so a tube ordered to the same dimensions weighs slightly less. The melting ranges overlap but do not coincide: 1343–1377 °C for 690, 1354–1413 °C for 600 — a difference for the welding procedure, not the service calculation.

Fabrication and welding

Neither grade hardens by heat treatment, so both are supplied annealed and both are welded in the annealed condition. Our grade sheets describe 600 as suitable for TIG, MIG and resistance welding, and 690 as easily fabricated and weldable; in the shop the difference is discipline, not difficulty. The higher-chromium grade leaves less tolerance for contaminated interpass surfaces, so clean and pickle between passes rather than at the end. For nuclear tube, the applicable document is AMS 5581 rather than AMS 5580, and that is a purchasing decision, not a metallurgical one.

Where 600 is still the right call

600 is not obsolete. Our grade sheet carries it to 1093 °C (2000 °F) without loss of mechanical properties, and credits it with resistance to oxidation, carburisation and chloride-ion stress-corrosion cracking. In furnace furniture, heat-treatment fixtures, thermocouple protection and chemical equipment handling carburising gas, that is still the conventional answer and the longer-established one. The point is narrower than it sounds: 600 is wrong for primary-water stress corrosion in a nuclear steam generator, and right for many hot, dirty, non-nuclear duties that are often specified as 690 out of habit.

Decision rule

• Nuclear steam generator tubing, primary-water SCC: 690 — N06690, seamless tube to ASTM B167, AMS 5581, DIN 2.4642.
• High-temperature oxidising or sulfidising service with acidic condensate: 690, on the strength of the chromium level alone.
• Carburising or oxidising furnace atmosphere up to 1093 °C: 600, unless the specification already names 690.
• Chloride stress-corrosion duty outside primary water: 600, because the nickel content is the protective variable there and not the chromium.
• Retubing an existing unit: match the original tube specification first, then confirm the tube-to-tubesheet joint procedure — a grade change made mid-campaign invalidates both.

What to send with the enquiry

Send the size with wall thickness and tolerance class, the specification you are buying to, the condition (annealed, pickled, ground) and the inspection document: 3.1 with chemistry and mechanical results per heat, or 3.2 with witnessed testing. Add the service medium and the metal temperature, because these two grades are separated by a corrosion mechanism rather than by strength, and state whether the part falls under a nuclear code. With those facts we will quote the grade, the heat size behind it and the delivery, or tell you plainly that the specification you have written will not do what the duty asks.

Values quoted here follow the ASTM / EN edition named in each paragraph. For a specific heat, the governing numbers are those printed on the EN 10204 certificate that ships with the material. Written by the technical desk at Shanghai Hangbo Alloy Group.