AL-6XN alloy is metallurgically stable to 1000° F (540° C) and has no phase transformation even after extensive deformation. When exposed to temperatures in the range of 1200-1800° F the opportunity for chi phase formations exists. Chi phase, a chromium-iron-molybdenum compound which is sometimes incorrectly referred to as sigma phase, forms along the grain boundaries depleting the area of molybdenum and chromium reducing the corrosion resistance of the material. In order to help combat chi phase formation, nitrogen was added to the AL-6XN alloy to minimize this formation improving corrosion resistance and increasing strength over a broad range of temperatures, while maintaining the good formability of austenitic stainless steel.

Example of chromium carbides formed along the grain boundaries depleting the surrounding areas of chromium and molybdenum

The physical properties of the AL-6XN alloy are similar to those of other austenitic stainless steels (table 1).

Table 1: Comparison of Physical Properties
Alloy Elastic Modulus Thermal Conductivity
at 212° F
Expansion Coefficient
from 77 to 212° F
psi x 106 MPa Btu-ft/hr-ft2-°F W/mK 10-6/°F 10-6/°C
Type 316L 29.0 200 9.2 16.0 8.5 17.3
C-276 29.8 205 6.4 9.9 6.2 11.2
C-22 29.9 206 6.5 10.2 6.9 12.4
Titanium 15.0 103 9.5 16.4 5.0 9.1
Alloy 904L 28.3 195 7.6 13.2 8.3 15.0
AL-6XN 28.3 195 6.8 11.8 8.5 15.3
Alloy 625 29.7 205 6.2 10.7 7.1 12.8
Nickel 200 30.0 207 38.8 67.1 7.4 13.4

The elastic modulus values of AL-6XN alloy are lower than those for type 316L stainless steel and Alloy 625. However, these moduli are higher in comparison to such non-ferrous alloys as titanium. The thermal conductivity and coefficient of expansion values are lower than those for type 316L stainless steel but are higher than Alloy 625. Typical physical properties of AL-6XN alloy are presented in table 2.

Table 2: Physical Properties of AL-6XN
Property Value Units
Density 0.291
8.06
lb/in3
g/cm3
Modulus of Elasticity 28.3 x 106
195
psi
GPa
Melting Range 2410 to 2550
1320 to 1400
°F
°C
Thermal Conductivity
68 to 212 °F
20 to 100 °C


6.8
11.8


Btu/hr · ft · °F
W/mK

Coefficient of Expansion
68 to 212 °F
20 to 100 °C

8.5
15.3

10-6/ °F
10-6/ °C
Specific Heat Capacity 0.11
500
Btu/lb·°F
J/kg·K
Electrical Resistivity 535
0.89
Ohm·circ mil/ft
µΩm
Magnetic Permeability
Fully annealed 0.5" plate
65% cold-worked plate

1.0028
1.0028

Oersted
(µ at 200H)
Sealing Temperature 1885
1030
°F
°C

Comparative chemistry for AL-6XN alloy vs. 316L stainless steel and other 6% Molybdenum containing products is as follows:

Table 3: Typical Comparative Chemistry
Chromium Nickel Molybdenum Nitrogen Copper PREN
AL-6XN 20% 24% 6.3% 0.22% 0.25-0.75% 44.3
316L 16% 10% 2% 0.05% 0.40% 23.4
254 SMO® 20% 18% 6.3% 0.20% 0.80% 44.0
25-6 MO® 20% 25% 6.5% 0.20% 1.20% 44.7
1925 hMO® 20% 25% 6.5% 0.18% 0.90% 44.3

Low copper content gives resistance against chloride corrosion. Cu above 0.75% tends to corrode in high chloride environments. For seawater pitting resistance PREN must be greater than 32.

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