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INCOLOY® alloy 27-7MO (UNS S31277; EP 1,263,999 B1) is an advanced 7% molybdenum super-austenitic stainless steel offering corrosion resistance in most environments superior to 6% molybdenum super-austenitic stainless steels. In many environments alloy 27-7MO offers resistance approaching that of much more highly alloyed materials such as INCONEL alloys 625, 22, and C-276.
  Alloy 27-7MO products typically contain 27% nickel, 22% chromium, 7.2% molybdenum, and 0.34% nitrogen. Its limiting chemical composition is presented in Table 1. The alloy's content of nickel and nitrogen produce a stable austenitic structure. By virtue of its content of molybdenum, chromium, and nitrogen, alloy 27-7MO offers excellent resistance to pitting and crevice corrosion. The nickel, nitrogen, and molybdenum provide resistance to reducing media while a high content of chromium offers resistance to oxidizing media. Alloy 27-7MO performs well in mixed acid environments, especially those containing oxidizing and reducing acids. The alloy's contents of nickel and nitrogen result in resistance to stress corrosion cracking and attack by caustic media. Alloy 27-7MO offers excellent resistance to corrosion in seawater and brine. It also resists stress corrosion cracking in all concentrations of sodium chloride up to saturation at the boiling point. It is resistant to the aggressive media encountered in air pollution control systems such as flue gas desulfurization equipment for high-sulfur coal-fired electric power utilities.
  Applications for INCOLOY alloy 27-7MO are found in the pollution control, power, marine, chemical processing, pulp and paper and oil and gas industries. The alloy offers a unique combination of corrosion resistance, high strength and ease of fabrication at an economical price. Alloy 27-7MO is available in standard product forms including plate, sheet, rod, bar, wire rod, pipe, tube, and forging stock.

Table 1 - Limiting Chemical Composition, wt %
Nickel ...................................................................26.0-28.0
Chromium.............................................................20.5-23.0
Molybdenum ............................................................6.5-8.0
Copper .....................................................................0.5-1.5
Nitrogen....................................................................0.3-0.4
Iron.......................................................................Balance*
Manganese .........................................................3.00 max.
Phosphorus .........................................................0.03 max.
Sulfur ..................................................................0.01 max.
Silicon ...................................................................0.5 max.
Carbon...............................................................0.020 max.
*Reference to the 'balance' of a composition does not guarantee this is exclusively of the element mentioned but that it predominates and others are present only in minimal quantities.

Physical and Thermal Properties

Table 2- Physical Properties
Density at room temperature, mill annealed conditon
 g/cm3................................................................... 8.02
 lb/in3.................................................................... 0.289
Permeability at 200 oersted (15.9 kA/m) at room temperature
 Annealed.............................................................. 1.004
Permeability at 200 oersted (15.9 kA/m) at -22°F (<-30°C)
 Annealed.............................................................. <1.01
 50% cold worked.................................................. <1.01
Specific Heat, J/kg °C.............................................. 454
 Btu/lb °F............................................................... 0.109
Electrical Resistivity at room temp., mill annealed condition
 ohm•circ mil/ft....................................................... 604
 μohm•cm.............................................................. 100
Temperature Young's Modulus Shear Modulus Poisson's
Ratio
°F °C 103 ksi GPa 103 ksi GPa
72 22 27.7 191 10.8 74 0.29
120 49 27.5 189 10.7 74 0.28
200 93 27.1 187 10.6 73 0.28
300 149 26.6 184 10.3 71 0.30
400 204 26.2 181 10.1 70 0.29
500 260 25.7 177 9.9 69 0.30
600 316 25.3 174 9.75 67 0.30
700 371 24.8 171 9.52 66 0.30
800 427 24.2 167 9.4 64 0.29
900 482 23.7 164 9.0 62 0.32
1000 538 23.3 160 8.8 61 0.32
1100 593 22.7 156 8.7 60 0.31
1200 649 22.1 153 8.3 57 0.33
1300 704 21.3 147 8.1 56 0.32
1400 760 20.7 143 7.8 54 0.32
1500 816 20.1 138 7.7 53 0.31
1600 871 19.6 135 7.5 52 0.30

Temperature Conductivity Temperature Conductivity
°C W/m K °F BTU in/ft2 h °F
23 0.10114 73 70.17
100 0.12126 212 84.13
200 0.14148 392 98.16
300 0.16295 572 113.05
400 0.18003 752 124.91
500 0.20039 932 139.03
600 0.22631 1112 157.01
700 0.24069 1292 167
800 0.2516 472 174.56
900 0.25852 1652 179.36
1000 0.27377 1832 189.95
1100 0.28947 2012 200.84

INCOLOY® alloy 27-7MO

Table 3 - Coefficient of Expansion (CTE) and Linear Expansion (LE) Mill Annealed Condition

Temp. LE CTE Temp. LE CTE
°F in/in x 10-3 in/in•°F
x 10-6		 °C cm/cm x 10-3 cm/cm•°C
x 10-6
-300 -2.5445 6.84 -190 -2.5910 12.21
-200 -1.9889 7.31 -150 -2.2110 12.84
-100 -1.3154 7.65 -100 -1.6448 13.46
0 -0.5658 7.86 -50 -1.0057 13.93
200 1.024 8.33 100 1.127 15.03
300 1.890 8.49 150 1.911 15.29
400 2.774 8.59 200 2.705 15.46
500 3.668 8.67 250 3.505 15.58
600 4.575 8.75 300 4.320 15.71
700 5.509 8.84 350 5.151 15.85
800 6.450 8.92 400 5.996 15.99
900 7.383 8.97 450 6.838 16.09
1000 8.348 9.04 500 7.690 16.19
1100 9.416 9.2 550 8.573 16.33
1200 10.522 9.37 600 9.538 16.59
1300 11.629 9.51 650 10.537 16.86
1400 12.727 9.62 700 11.542 17.1

Figure 1. Thermal expansion of INCOLOY alloy 27-7MO in the mill annealed condition.

Mechanical Properties

INCOLOY alloy 27-7MO exhibits an optimum combination of strength and ductility. Typical mechanical properties of alloy 27-7MO products are compared with those of similar alloys in Table 4. Alloy 27-7MO is normally supplied in the annealed condition.

Table 4 - Typical Mechanical Properties at Room Temperature

Alloy Ultimate Tensile Strength, ksi / MPa Yield Strength (0.2%), ksi / MPa Elongation % Hardness Rockwell 'B'
INCOLOYalloy 25-6MO 95 / 655 45 / 310 42 90
INCOLOYalloy 27-7MO 120 / 827 60 / 414 50 95
INCONEL® alloy 625 125 / 862 68 / 469 50 95
INCONEL alloy C-276 105 / 724 50 / 345 60 88

Figure 2. Effect of temperature on mechanical properties of annealed INCOLOY alloy 27-7MO

Corrosion Resistance

Pitting Resistance Equivalency Number- A means of comparing the corrosion resistance of alloys is by their Pitting Resistance Equivalency Number or 'PREN'. Alloys exhibiting higher PREN values are generally found to be more resistant to localized corrosion than those with lower PREN values. The PREN can be calculated by several different equations based upon the chemical composition of the alloys. Some equations are applicable to stainless steels while others are better applied to nickel-based alloys. The equation used here is generally accepted as being applicable to a wide range of alloy compositions. When comparing alloys by their PREN it is absolutely necessary that the same equation be used for all materials to be compared. Comparing alloys by PRENs generated from different equations will give erroneous results. For more information on the use of PRENs to compare the corrosion resistance of materials, refer to the Special Metals publication, “High Performance Alloys for Resistance to Aqueous Corrosion" or visit the Special Metals website, www.specialmetals.com.
  PRENs for alloy 27-7MO and similar materials are seen in Table 5. Based on these values of PREN, alloy 27-7MO would be predicted to offer significantly better corrosion resistance than alloy 25-6MO (and similar grades of super-austenitic stainless steel) and even superior performance to alloy 625. Alloy 27-7MO's resistance approaches that of alloy C-276.

INCOLOY® alloy 27-7MO

Table 5 - Pitting Resistance Equivalency Numbers (PREN)*

Alloy % Ni % Cr % Mo % Nb % W % N PREN
INCOLOY alloy 25-6MO 25 20 6.5 0 0 0.20 35.8
INCOLOY alloy 27-7MO 27 22 7.2 0 0 0.34 43.0
INCONEL alloy 625 62 22 9 3.5 0 0 40.8
INCONEL alloy C-276 58 16 16 0 3.5 0 45.2

*PREN = %Cr + 1.5 (%Mo + %W + %Nb) + 30 (%N)

Critical Pitting Temperatures and Critical Crevice Corrosion Temperatures

Alloys may also be ranked by the threshold temperature at which they begin to be attacked in a given medium. Samples may be directly exposed to the medium which may induce pitting, or a crevice device may be attached which may induce crevice corrosion. The samples are exposed at increasing temperatures until corrosive attack occurs. The lowest temperature at which measurable corrosion takes place is defined as the Critical Pitting Temperature (CPT) or Critical Crevice Temperature (CCT), depending on whether or not a crevice device is attached to the sample. One test method is ASTM G48. Method C is a pitting test while Method D is a crevice corrosion test. The maximum test temperature is 85°C (185°F) as the test solution becomes unstable at higher temperatures. For procedural details, the reader is directed to the test procedure published by ASTM.
  CPT and CCT values for some alloys are presented in Table 6. It is seen that alloy 27-7MO exhibits higher values than alloys 25-6MO and 625 and approaches those of alloy C-276.


Table 6 - CPT and CCT per ASTM G48 Test Methods C and D

Alloy Critical Pitting
Temperature		 Critical Crevice
Temperature
°C °F °C °F
INCOLOY alloy 27-7MO >85 >185 45 113
INCOLOY alloy 25-6MO 70 158 35 95
INCONEL alloy 625 >85 >185 35 95
INCONEL alloy C-276 >85 >185 50 122
UNS N08031 75 167 45 113

Critical corrosion temperatures (CPT and CCT) for an alloy can be determined in essentially any corrosive aqueous medium. "Green Death" is a well known aggressive medium that is often used to evaluate the corrosion resistance of nickel-chromium-molybdenum alloys. It is composed of 11.9% H2SO4 + 1.3% HCl + 1% FeCl3 + 1% CuCl2.

Table 7 - CPT and CCT per ASTM G48 Test Methods C and D

Alloy Critical Pitting
Temperature		 Critical Crevice
Temperature
°C °F °C °F
INCOLOY alloy 25-6MO 60 140 45 113
INCOLOY alloy 27-7MO 75 167 60 140
INCONEL alloy 625 75 167 55 131
INCONEL alloy C-276 >Boiling >Boiling 90 194
6% superaustenitic
stainless steel		 65 149 50 122
UNS N08031 55 131 50 122

Nickel-chromium alloys and austenitic and super-austenitic

When tested by ASTM G 28, Method A, annealed INCOLOY alloy 27-7MO wrought products exhibit a typical corrosion rate of 15 mpy.

Resistance to Corrosion by Seawater

Nickel-chromium alloys and austenitic and super-austenitic stainless steels containing molybdenum are well known for their resistance to corrosion in seawater and marine environments. A test program conducted at the LaQue Center for Corrosion Technology in Wrightsville Beach, North Carolina (a laboratory well known for marine corrosion testing) produced data showing that INCOLOY alloy 27-7MO offers corrosion-resistance superior to many of the corrosion-resistant alloys commonly used in marine service. Samples of sheet to which Teflon crevice devices were attached were exposed to flowing natural seawater at 30°C for 60 days. Results of the tests are reported in Table 8.

Table 8 - Crevice Corrosion in Seawater

Alloy PREN* of
Test
Material		 Maximum
Area
Attacked
(sq. mm)		 Maximum
Depth of
Attack
(mm)
316L Stainless Steel 21.2 1745 2.84
6% Mo super-austenitic stainless steel 37.0 80 0.01
INCONEL alloy 625 40.2 0 0
INCOLOY alloy 27-7MO 41.6 0 0
INCONEL alloy C-276 45.2 1 0.02

*PREN = %Cr + 1.5 (%Mo + %W + %Nb) + 30 (%N)

INCOLOY® alloy 27-7MO

Resistance to Corrosion by Acids

The molybdenum content of alloy 27-7MO imparts resistance to reducing acids while the alloy's content of chromium results in resistance to oxidizing acids. With its balanced composition the alloy offers resistance to mixed acid environments. Thus, alloy 27-7MO is particularly useful for service in chemical processing and wet scrubbing

systems for air pollution control.
  Alloy 27-7MO and other materials were tested in hydrochloric acid (HCl), sulfuric acid (H2SO4), and a mixed acid solution. Results of those tests are reported in Table 9.
  Alloy 27-7MO also offers excellent resistance to attack by phosphoric, sulfuric and hydrochloric acids, as shown in Figures 3, 4 and 5. Figure 6 shows the CPT of several alloys in green death solution.

Table 9 - Corrosion Resistance in Acids, mpy(mm/a)

Test Media Temperature,
°F (°C)		 INCOLOY
alloy 25-6MO		 INCONEL
alloy 625		 INCOLOY
alloy 27-7MO		 INCONEL
alloy C-276
0.5% HCl Boiling - 0.2 (0.005) 0.7 (0.018) 0.8 (0.020)
1% HCl Boiling 218 (5.54) 15 (0.38) 1.3 (0.033) 6.5 (0.165)
5% HCl 122 (50) 45 (1.14) <0.1 (<0.0025) <0.1 (<0.0025) 0.5 (0.013)
95% H2SO4 122 (50) 18 (0.46) 48 (1.22) 14 (0.36) 0.1 (0.0025)
10% H2SO4 + 2%HCl 122 (50) 29 (0.74) <0.1 (<0.0025) <0.1 (<0.0025) <0.1 (<0.0025)
10% H2SO4 194 (90) 33 (0.84) 1.5 (0.04) 1 (0.025) <0.1 (<0.0025)
10% H2SO4+10,000 ppm Cl- 149 (65) 26 (0.66) - <0.5 (<0.01) 0.5 (0.01)
10% H2SO4+1,000 ppm Cl- 149 (65) 26 (0.66) - 0 (0) 1 (0.025)
85% H3PO4 Boiling 30 (0.76) 180 (4.57) 27 (0.69) 13 (0.33)

Figure 3. Isocorrosion chart for INCOLOY alloy 27-7MO in phosphoric acid.

Figure 5. Isocorrosion chart for INCOLOY alloy 27-7MO in hydrochloric acid.

Figure 4. Isocorrosion chart for INCOLOY alloy 27-7MO in sulfuric acid.

Figure 4. CPT of several FGD alloys in green death.

INCOLOY® alloy 27-7MO

Resistance to Corrosion in Flue Gas Desulfurization Service

Components for service in flue gas desulfurization (FGD) systems for removal of sulfur dioxide from high sulfur coalfired electric power utilities are exposed to very aggressive conditions. The environment inside the scrubbers is typically sulfuric acid at very low pH (1 or less) containing significant concentrations of halide ions (chlorides and fluorides) at temperatures up to 80°C (176°F). To determine the performance of alloy 27-7MO and similar alloys, test specimens were exposed to a test solution that has been used to rank materials for FGD service. The solution contained 60% H2SO4 + 2.5% HCl + 0.2% HF + 0.5% flyash at 80°C (176°F). The test duration was one week. The resulting corrosion rates of the materials tested are reported in Table 10.

Table 10 - Corrosion Rates in a Simulated FGD Environment
(60% H2SO4 + 2.5% HCl + 0.2% HF + 0.5% Flyash at 80°C)

Alloy Corrosion Rate, mpy (mm/a)
INCOLOY alloy 25-6MO 199 (5.08)
UNS N08031 177 (4.50)
INCOLOY alloy 27-7MO 153 (3.91)
Ni-Cr-Mo alloy UNS N06059 47 (1.20)
INCONEL alloy 622 40 (1.02)
INCONEL alloy C-276 28 (0.71)
INCONEL alloy 686 23 (0.58)

Resistance to Corrosion in Sour Gas Service

INCOLOY® alloy 27-7MO is the next-generation super austenitic material for sour gas, seawater, brine, and high chloride environments in the oil & gas sector. Alloy 27-7MO U-bend corrosion test specimens exposed for two months in boiling saturated sodium chloride were fully resistant to stress corrosion cracking and pitting corrosion. Alloy 27-7MO has improved corrosion resistance and higher strength compared to INCOLOY® alloy 25-6MO and similar 6% molybdenum alloys. Corrosion resistance of alloy 27-7MO is compared with other alloys in Table 11. Effect of cold work on hardness of alloy 27- 7MO in the as-cold drawn condition is shown in Figure 6. Applications in the oil and gas sector include wireline, armor wire, screens, and subsea banding.

Table 11 - Testing for INCOLOY alloy 27-7MO Sour Gas Armor Wire Service
Tested wire was in cold-worked condition on 0.031-in wire. Samples were stressed by wrapping wire upon itself.

Alloy Test 1 Test 2 Test 3 Test 3b Test 4 Test 5 Test 6
25-6MO No cracking No cracking No cracking No cracking No cracking None <60
36MO No cracking No cracking No cracking No cracking No cracking All attacked 75
27-7MO No cracking No cracking No cracking No cracking No cracking None 80
MP35N No cracking No cracking No cracking No cracking No cracking None >80

Test 1:   Saturated NaCl + 2.5% NH4HSO3 boiling for 1008 hours.
Test 2:   23.5% MgCl2 + 6% KCl + 0.3% CaO boiling for 1008 hours.
Test 3:   5% NaCl + 0.5% acetic acid purged with H2S room temperature for 1008 hours (no coupling to steel).
Test 3b:   5% NaCl + 0.5% acetic acid purged with H2S room temperature for 1008 hours (coupled to steel).
Test 4:   Saturated NaCl + 5% MgCl2 + 5% H2S at 350°F (177°C) and 5 ksi (35 MPa) for 336 hours.
Test 5:   G48-D (6% FeCl3 + 1% HCl) Crevice test at 77°F (25°C).
Test 6:   G48-C (6% FeCl3 + 1% HCl) Critical pitting temperature evaluation.

Fabrication

INCOLOY alloy 27-7MO is readily fabricated, formed, and joined utilizing conventional equipment, techniques, and products. Forming can either be hot or cold.
 Alloy 27-7MO is best hot worked in the temperature range of 1800 to 2100°F (980 to 1150°C). Annealing is done at 2050- 2150°F (1121-1177°C). Stagnant oxidizing conditions must be avoided, particularly when heating the alloy above 1700°F (925°C), to prevent catastrophic oxidation. For example, attack can occur at crevice points when flat rolled products are stacked or laid against each other in a heat treating furnace. Forced gas and air flow are recommended. Cooling after heat treatment should be by rapid air cool or water quench. Prolonged exposure to temperatures between 1100 and 1700°F (600 to 930°C) should be avoided, as undesirable phases such as sigma can form. It can also cause sensitization which may lead to intergranular attack or increased susceptibility to stress corrosion cracking.

INCOLOY® alloy 27-7MO

Fabrication, continued

The work hardening rate of alloy 27-7MO is similar to that of INCOLOY alloy 25-6MO (Figure 7). Equipment used to form austenitic (300 series) stainless steels and nickel-based alloys will normally be suitable for forming alloy 27-7MO. The effect of cold work on the mechanical properties of INCOLOY alloy 27-7MO is shown in Figure 8. Additional information on fabrication is available in the Special Metals publication, "Fabricating", on the Special Metals website, www.specialmetals.com.


Figure 7. Effect of cold work on hardness.

Figure 8. Typical room temperature mechanical properties of cold worked sheet.

INCOLOY® alloy 27-7MO

Welding

INCOLOY alloy 27-7MO products are readily joined using conventional welding processes such as gas tungsten-arc (GTAW), gas metal-arc (GMAW), shielded metal-arc (SMAW), submerged arc (SAW), and plasma arc (PAW). Alloy 27-7MO (like other super-austenitic alloys) suffers a loss of corrosion resistance when autogenously welded or when welded with matching composition filler metal. Thus, overmatching composition welding products (i.e., those with higher molybdenum content) such as INCONEL Filler Metals 622, and 686CPT® and INCONEL Welding Electrodes 122 and INCO-WELD® Welding Electrode 686CPT are used. INCONEL Filler Metal 622 and INCONEL Welding Electrode 122 are considered optimum for joining the alloy to itself or to dissimilar materials. Weld procedures which minimize dilution by the base metal will result in the most corrosion-resistant weldments. Maintaining low heat input will minimize elemental segregation in the fusion zone and optimize corrosion resistance. The interpass weld temperature should be limited to 300°F (150°C).
  Post-weld heat treatment is not required when overmatching composition welding products are used. However, autogenous weldments should be post weld heat treated at a minimum temperature of 2000°F (1100°C) for 5 minutes and air cooled or water quenched. This heat treatment will improve the corrosion resistance of products welded with filler metal as well. Pickling after welding or heat treatment is also generally effective in optimizing corrosion resistance. Removal of heat tint on the back of the welded component is often beneficial in improving corrosion resistance.
  Additional information on joining nickel alloys and stainless steels is available in the Special Metals publication, "Joining", on the Special Metals Corporation website, www.specialmetals.com.

Machining

The machining characteristics of alloy 27-7MO are similar to those of other austenitic alloys. Products are readily machined in the annealed condition. Detailed information on machining alloy 27-7MO is available in the Special Metals publication, "Machining", on the Special Metals website, www.specialmetals.com.

Applicable Specifications

INCOLOY alloy 27-7MO is designated as UNS S31277. Allowable design stresses for ASME Boiler and Pressure Vessel Code construction are defined in ASME Code Case 2458.

Rod, Bar, Wire, and Forging Stock - ASTM A 479/ASME SA 479 (Bar)

Plate, Sheet, and Strip - ASTM A 240/ASME SA 240

Pipe and Tube - ASTM A 213/ASME SA 213 (Tube), ASTM A 249/ASME SA 249 (Tube), ASTM A 312/ASME SA 312 (Pipe)

Other - ASTM A 182/ASME SA 182

Publication Number SMC-092

Copyright © Special Metals Corporation, 2005 (Dec 05)

INCOLOY, INCONEL, INCO-WELD and 686CPT are trademarks of the Special Metals Corporation group of companies.

The data contained in this publication is for informational purposes only and may be revised at any time without prior notice. The data is believed to be accurate and reliable, but Special Metals makes no representation or warranty of any kind (express or implied) and assumes no liability with respect to the accuracy or completeness of the information contained herein. Although the data is believed to be representative of the product, the actual characteristics or performance of the product may vary from what is shown in this publication. Nothing contained in this publication should be construed as guaranteeing the product for a particular use or application.