Quick Answer: Composition Decides the Application
UNS N06600 and UNS N08825 are ordered on the same requisition far more often than they should be. They are not interchangeable. UNS N06600 is a solid solution nickel-chromium alloy with a minimum nickel content of 72 percent, which is why it keeps its strength and its oxidation resistance in hot, dry, high temperature service. UNS N08825 is a nickel-iron-chromium alloy with a much lower nickel level, balanced by deliberate additions of molybdenum, copper and titanium, which is why it survives reducing acids and chloride bearing water. In one line: select UNS N06600 for heat, select UNS N08825 for aggressive wet chemistry.
Chemical Composition Compared
Both grades are supplied against recognised wrought product specifications. UNS N06600 is covered by ASTM B166, B167 and B168 together with ASME SB-167, and its European designations are W.Nr. 2.4816, NiCr15Fe and NC15Fe. UNS N08825 is covered by ASTM B423, B424 and B425 with ASME SB-423, and its European designation is W.Nr. 2.4858, NiCr21Mo.
| Element | UNS N06600 | UNS N08825 |
|---|---|---|
| Nickel, Ni | 72.0 % min | 38.0 to 46.0 % |
| Chromium, Cr | 14.0 to 17.0 % | 19.5 to 23.5 % |
| Iron, Fe | 6.0 to 10.0 % | 22.0 % min, balance |
| Molybdenum, Mo | not specified | 2.5 to 3.5 % |
| Copper, Cu | 0.50 % max | 1.5 to 3.0 % |
| Titanium, Ti | not specified | 0.6 to 1.2 % |
| Carbon, C | 0.15 % max | 0.05 % max |
| Manganese, Mn | 1.0 % max | 1.0 % max |
| Silicon, Si | 0.50 % max | 0.50 % max |
| Sulphur, S | 0.015 % max | 0.030 % max |
The titanium addition in UNS N08825 is the key metallurgical difference. Titanium ties up carbon as a stable carbide during solidification, so the alloy resists sensitisation and intergranular corrosion after welding or long exposure in the 425 to 815 degree Celsius band. UNS N06600 reaches the same goal differently, using such a high nickel level that chromium carbides stay in solution instead of forming continuous grain boundary networks.
Mechanical Properties and High Temperature Behaviour
UNS N06600, annealed: tensile strength 550 MPa min, 0.2 percent proof strength 240 MPa min, elongation 30 percent min, hardness 100 HRB max.
UNS N08825, annealed: tensile strength 586 MPa min, 0.2 percent proof strength 241 MPa min, elongation 30 percent min.
Density: 8.47 g/cm3 for UNS N06600, 8.14 g/cm3 for UNS N08825.
Melting range: 1354 to 1413 degrees Celsius for UNS N06600, 1370 to 1400 degrees Celsius for UNS N08825.
Short term tensile data are close, so the real separation appears under load at temperature. UNS N06600 keeps useful creep and rupture strength to roughly 700 degrees Celsius and resists oxidation in air to about 1093 degrees Celsius, which is why it is specified for furnace internals, retorts, radiant tubes and heat treatment fixtures. UNS N08825 is normally limited to approximately 540 degrees Celsius in continuous service, because its creep strength falls away faster and its oxidation resistance is lower than that of the high nickel grade.
Corrosion Resistance: the Deciding Factor
UNS N06600 resists dry chlorine and hydrogen chloride, caustic alkalies and high purity water, and it is virtually immune to chloride ion stress corrosion cracking, the failure mode that destroys 300 series stainless steels in warm chloride service. Its weakness is reducing acid attack, and it performs poorly in hydrochloric acid and in many sulphuric acid solutions.
UNS N08825 covers exactly that gap. Its molybdenum and copper content gives it strong resistance to sulphuric and phosphoric acids across wide concentration ranges, and to pitting and crevice attack in chloride bearing water including seawater. The trade off is high temperature oxidation resistance: above about 600 degrees Celsius the lower nickel balance makes UNS N08825 the weaker of the two.
Applications and How to Choose
UNS N06600: nuclear steam generator tubing, shell and tube heat exchangers, thermocouple sheaths, catalyst regenerators, vinyl chloride monomer reactors, vacuum furnace retorts, alkaline digesters.
UNS N08825: phosphoric and sulphuric acid plant piping, pickling and acid regeneration equipment, nuclear fuel reprocessing, seawater cooling and desalination piping, sour gas handling.
Define the worst case environment before comparing price. If the service is hot, oxidising and essentially dry, UNS N06600 is usually the lower risk choice. If the service is wet and acidic, or contains chlorides at moderate temperature, UNS N08825 is the correct starting point and UNS N06600 should not be substituted simply because it contains more nickel. UNS N06600 work hardens quickly and should be machined in the annealed condition; UNS N08825 is easier to cold form but needs a matching filler on welded joints, because its titanium stabilised chemistry is sensitive to dilution.
Frequently Asked Questions
Q: What is the main difference between UNS N06600 and UNS N08825?
UNS N06600 is a high nickel solid solution chromium alloy for hot oxidising service, while UNS N08825 is a nickel-iron-chromium alloy with molybdenum and copper for acid and chloride service.
Q: Which alloy is better at high temperature?
UNS N06600. It retains creep strength to about 700 degrees Celsius and resists oxidation to roughly 1093 degrees Celsius, while UNS N08825 is generally limited to about 540 degrees Celsius.
Q: Which grade resists sulphuric acid better?
UNS N08825. Its molybdenum and copper additions suit it to sulphuric and phosphoric acid service, where UNS N06600 corrodes rapidly.
Q: Is UNS N06600 resistant to chloride stress corrosion cracking?
Yes. The high nickel content makes it virtually immune in chloride bearing environments, although stress relief is still advised for hot alkaline service.
Q: Are the two grades interchangeable in piping?
No. A substitution in either direction changes the corrosion and temperature design basis, so the line must be re qualified before any swap is approved.
Q: What filler metal is used for welding these alloys?
Nickel-chromium fillers for UNS N06600 and molybdenum bearing nickel-iron-chromium fillers for UNS N08825, matched to base metal corrosion resistance and to the service environment.
