317LXN

Allegheny Ludlum's 317LXN™ is molybdenum-bearing austenitic stainless steel with greatly increased resistance to chemical attack as compared to the conventional chromium-nickel austenitic stainless steels such as Type 304. AL 317LXN™ alloy also offers higher creep, stress-to-rupture, and tensile strength at elevated temperatures than conventional stainless steels. The alloy has low carbon to provide resistance to sensitization during welding and other thermal processes. The combination of increased levels of molybdenum and nitrogen enhance resistance to pitting and crevice corrosion especially in process steams containing acids, chlorides and sulfur compounds at elevated temperatures. Nitrogen also serves to increase the strength of this alloy. This alloy is intended for severe service conditions such as flue gas desulfurization (FGD) systems.Information provided by Allegheny Ludlum Corporation.

Table 1 ASTM A240 Chemical composition (wt%)

Component Elements Properties	Metric	English	Comments
Carbon, C	<= 0.030 %	<= 0.030 %
Chromium, Cr	17 - 20 %	17 - 20 %
Iron, Fe	60 %	60 %	as balance
Manganese, Mn	<= 2.0 %	<= 2.0 %
Molybdenum, Mo	4.0 - 5.0 %	4.0 - 5.0 %
Nickel, Ni	13.5 - 17.5 %	13.5 - 17.5 %
Nitrogen, N	0.10 - 0.20 %	0.10 - 0.20 %
Phosphorous, P	<= 0.040 %	<= 0.040 %
Silicon, Si	<= 0.75 %	<= 0.75 %
Sulfur, S	<= 0.030 %	<= 0.030 %

Corrosion resistance: 317LXN, 317LX and 317L stainless steel in the medium corrosive media in the more ordinary 304 stainless steel has a higher corrosion resistance, usually, 18-8 stainless steel does not produce corrosion of the environment, the stainless steel is not Mo Will produce corrosion, but strong oxidizing acids, such as nitric acid.

In the sulfuric acid solution, 317LXN, 317LX and 317L are more resistant to corrosion than 304 stainless steel, and as the Mo content increases, the corrosion resistance increases. These three stainless steels can withstand corrosion at 49 ° C and 5% sulfuric acid. When the temperature is below 38 ° C, these three stainless steels also exhibit excellent corrosion resistance in higher concentrations of sulfuric acid. However, field tests are recommended to better assess the effect of specific operating conditions on corrosion behavior of the alloy. In the process environment containing sulphide-containing furnace gas condensate, these three stainless steels are more resistant to corrosion than the 316 stainless steel. In this environment, the concentration of acid will significantly affect the corrosion rate of corrosion and corrosion of the alloy, which should be carefully tested by field test corrosion of various alloys.

Table 2 shows the uniform corrosion rates for 317LXN, 317LX and 317L strip samples in ASTM standard test solutions, and the values for 316L, AL-6XN and AL-276 alloys are also listed in the table for comparison.

Test solution	Corrosion rate MPy（mm/y）
	316L	3317L	317LXTM	317LXNTM	AL-6XN?	AL276TM
20% Acetic acid	0.12 (<0.01)	0.48 (0.01)	0.24 (<0.01)	0.12 (<0.01)	0.12 (0.01)	0.48 (0.01)
45% Formic acid	23.41 (0.60)	18.37 (0.47)	11.40 (0.29)	11.76 (0.30)	4.56 (0.12)	2.76 (0.07)
10% Oxalic acid	48.03 (1.23)	44.90 (1.14)	46.68 (1.19)	35.76 (0.91)	10.92 (0.28)	11.24 (0.28)
20% Phosphoric acid	0.60 (0.02)	0.72 (0.02)	0.72 (0.02)	0.24 (<0.01)	0.24 (<0.01)	0.36 (0.01)
10% Sulfuric acid	635.7 (16.15)	298.28 (7.58)	255.00 (6.48)	157.80 (4.01)	84.36 (2.14)	13.93 (0.35)
10% Sulfite	71.57 (1.82)	55.76 (1.42)	25.08 (0.64)	15.60 (0.40)	24.00 (0.61)	2.64 (0.07)
50% Sodium hydroxide	77.69 (1.92)	32.78 (0.83)	69.18 (1.76)	85.68 (2.18)	15.96 (0.41)	17.77 (0.45)
ASTM A262 B（FeSO2·H2SO4）	26.04 (0.66)	20.76 (0.53)	23.52 (0.60)	17.28 (0.44)	21.36 (0.54)	264.5 (6.72)
ASTM A262 C (65%HNO3)	22.31 (0.56)	19.68 (0.50)	48.24 (1.23)	16.32 (0.42)	29.04 (0.74)	908.0 (23.06)
ASTM A262E (Cu·CuSO4·H2SO4)	pass	pass	pass	pass	pass	pass

The carbon content of these three alloys is less than 0.03%, low carbon content effectively improve their resistance to intergranular corrosion during welding or forging. 317LXN, 317LX and 317L stainless steel contain higher Cr, with better resistance to intergranular corrosion. However, it should be noted that prolonged exposure to the temperature range of 417-816 ℃, due to the precipitation of δ phase, will greatly reduce the corrosion resistance of these alloys, while the material may also become brittle. The higher N content of 317LXN delays the precipitation of δ and carbide precipitates.

Table 3 pitting index of various alloys

Alloy	PRE
316 317L 317LXTM 317LXNTM AL-904LTM AL-6XN AL TEMP 625 AL276TM	25 30 34 38 36 48 52 69

As shown in Table 3, the high Mo high N significantly improved the pitting index (PRE) of the alloy, that is, the corrosion resistance of the alloy, the PRE value was based on the corrosion test results. The corrosion test found that CI-point corrosion performance, N is 30 times the effectiveness of Cr, is 9 times the Mo.

The critical crevice corrosion temperature of stainless steels and nickel-based alloys measured according to the modified ASTM G48 B test method is significant (see Table 4). It can be seen from Table 4, with the austenitic stainless steel Mo and N content increased, the alloy critical crevice corrosion temperature also greatly improved.

Table 4 the critical crevice corrosion temperature of various alloys

Alloy	℃
316 825TM 317L 317LXTM 317LXNTM AL-904LTM AL-6XN AL TEMP? 625 AL276TM	－3 －3 2 18 24 20 43 45 55

317LXN, 317LX, 317L stainless steel in the acidic solution containing CI- or other halogen ions in the 316,304 type stainless steel has better resistance to corrosion and crevice corrosion, so the three kinds of stainless steel is often used for flue gas Desulfurization system (FGD). Table 5 shows the crevice corrosion test data for the various alloys in the simulated FGD solution.

Table 5 Corrosion data of various alloys in simulated FGD test solutions

Alloy	Weightlessness（g/cm2）
	24℃ （75°F）	50℃ （122°F）	70℃ （158°F）
317L 317LXTM 317LXNTM AL-6XN AL TEMP 625 AL276TM	0.0007 0.0000 0.0000 0.0000 0.0000 0.0000	0.0377 0.0319 0.0129 0.0000 0.0000 0.0001	0.0500 0.0462 0.0462 0.0266 0.0149 0.0004

According to ASTM G 48 B operating procedures, the test time: 72 hours

Solution Ingredients: 7 vol% H2SO4, 3 vol% HCI, 1 wt% CuCI2, 1 wt% FeCI3

Antioxidant: In the atmospheric environment, Cr-Ni-Mo stainless steel has excellent oxidation resistance and low descending speed when the temperature is lower than 871 ~ 899 ℃.

Manufacturing: 317LXN, 317LX, 317L stainless steel physical properties and mechanical properties similar to the conventional 316 and 304 stainless steel, which can be used in the same way as ordinary stainless steel manufacturing.

Forging: the three kinds of stainless steel recommended forging temperature is 1150 ~ 1205 ℃, the final forging temperature of 927 ~ 955 ℃.

Heat treatment: 317LXN, 317LX, 317L stainless steel can be in the temperature range of 1080 ~ 1175 ℃ heat treatment, and then according to the thickness of water or air cooling. The plates should be heat treated at 1150 ° C and 1170 ° C and then allowed to cool the material from red and white at an annealing temperature to black in 3 minutes.

Hardness: These three kinds of stainless steel can not improve the hardness by heat treatment.

Welding: welding with high alloy welding material to maintain the corrosion resistance after welding. When welding 317L stainless steel, the welding material should contain at least 6% Mo; welding 317LX and 317LXN stainless steel, the welding material should contain at least 8% Mo, such as the use of 625 alloy as a welding material. In the actual welding operation, if you can not use high matching welding materials, and can not be post-weld heat treatment and pickling, you need to carefully consider whether the self-welding weldment to meet the requirements of the process conditions. 317LXN, 317LX, 317L Self-welded welders are heat treated and pickled to obtain the best corrosion resistance. ASTM A-380 "Recommended procedure for descaling and cleaning of steel surfaces" can provide more information on pickling information.

Table 6 ASTM A-240 provides the minimum mechanical properties

Performance	317L	317LXTM	317LXNTM
Tensile Strength MPa	515	515	550
Yield Strength （0.2%）MPa	205	205	240
Elongation (5%)	40	40	40
Hardness (Max)	217BHN	/	/
UNS NO.	S31703	S31725	S31726

Table 7 317LXN, 317LX, 317L stainless steel physical properties

Density	8.0	g/cm3
Elastic Modulus	200	GPa
Melting point	1320—1400	℃
Thermal conductivity 20~100℃	14.6	Walts/m °K
Thermal expansion coefficient  100℃ 500℃ 1000℃	16.5 18.2 19.5	10-5/℃ 10-5/℃ 10-5/℃
Specific heat	0.46	J/G °K
Conductivity	0.79	μ-ohm-m

Source: China Stainless Steel Pipe Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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