Attack of Hydrogen-Nitrogen Mixtures on Steels AT 13,000 TO 15,000 POUNDS PER SQLTAREINCH PRESSURE AND 204' TO 593" C. HARRY K. IHRIG Globe Steel Tubes Company, Milwuukee, W i s . Nitrogen and hydrogen attack steels a t high pressures and moderate temperatures. This attack a t 13,000 to 15,000 pounds per square inch and 204" to 593" C. was studied on carbon, nickel, chromium, and chromiumnickel steels. The carbon and low alloy steels lose carbon and absorb nitrogen throughout their cross sections. Numerous cracks and fissures were found which impair the original properties of the metal, particularly the duc-
tility. The austenitic stainless steels showed the best resistance but form high nitrogen layers on their surfaces. Carbide-forming elements added to intermediate chromium steels do not prevent attack as has been reported by previous investigators working a t lower pressures. Precautions should be taken in the design of very high pressure equipment to prevent deterioration of the metals t h a t are used.
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are certainly desirable for a complete understanding, more factual data are still needed. Definite information is desired that will enable distinction between a steel that will not embrittle (if there is any) and a potentially embrittling steel. Recently Evans (5') has described the attack of hydrogen on carbon steels a t 350 pounds per square inch and 300" C. He concludes that such steels should not be used even under such moderate conditions of temperature and pressure. Almost all the work reported in the literature has been done with pressures under 10,000 pounds per square inch with most of the experiments under 5000 pounds per square inch. The present paper reports on the effect of nitrogen and hydrogen on various steels for long periods at 13,000 to 15,000 pounds per square inch pressure.
INCE the development of the synthetic ammonia process by Haber and Bosch, it has been necessary to provide containers for mixtures of nitrogen and hydrogen at high pressures and temperatures. Metals, because of their high strength, ductility, and ease of fabrication, have been universally used. However, nitrogen and hydrogen attack many metals a t high pressures and temperatures and cause them to lose t h k r strength and ductility, although there is no visual evidence of such attack. Since the changes in original physical properties are not apparent to operators of high pressure equipment, serious failures of such vessels may occur unless proper precautions are taken. Schuyten ( 5 )has reviewed the literature on the effect of hydrogen on steels and has listed 27 of the most important papers on the subject. He gives detailed data from these and from his own investigations of metals used in synthetic ammonia plants operated a t 1600 to 2000 pounds per square inch and temperatures up to 475" C. He concludes that increased time, temperature, and pressures increase the attack on carbon steels and that steels containing carbide formers such as chromium, molybdenum, titanium, and columbium show much less attack. Steels with high carbon content are more susceptible to hydrogen attack than steels of low carbon content. Barta and Rawlins ( 1 ) state that while the mechanisms explaining embrittling are interesting, and satisfactory explanations
EXPERIMENTAL WORK
Catalyst Container Tube. A deoxidized low carbon steel tube used as a catalyst container for 2 months a t 500" C. and 15,000 pounds per square inch in an ammonia synthesis plant was found
Figure 2. Figure 1. Blistered Inside Surface of 1015 Steel Tube
blicrostructure of Tube in Figure 1 Talcen a t Edge of a Blister Nital etch, 2OOX
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
November 1949
TO HIGH PRESSURE HYDROGEN TABLE I. ANALYSES O F STEEL SAMPLES BEFORE EXPOSURE
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Steel Samples in Catalyst Bed. Because of this marked
NITROGEN
attack on low carbon steel, it seemed desirable to investigate Mo Ti Cb N Cu C Mn P. S Si Type Steel the effect of high pressure ni0.004 ... ... ... ... ... 0.05 0.24 0.015 0.026 0.01 Globe iron trogen and hydrogen on a series ... ... 0.006 ... ... 1015 0.14 0.39 0.021 0.032 0.09 0.006 .... *. ... 0.006 ,.. 0.20 0.45 0.023 0.042 0.09 of steels of different analyses. ... 0.33 0.74 0,019 0.033 0.23 0.005 ... 3:4? 0.024 0.17 0.14 0.40 0.019 Table I gives the analyses of ... 0.014 1.75 0 . 2 3 ... 0.14 0.46 0.015 0.021 0.19 ... the steel samples before expo0.54 0.47 ... 0.010 ... 0.08 0.48 0.016 0.016 0.47 5.35 0.033 ... ... ... 0.11 0.43 0.014 0.015 0.48 14.93 ... sure to high pressure nitrogen 1'. 'io 0.040 ... 22.41 . . . ... ... 0.014 0.34. 0.016 0.26 0.47 442 * . . ... 0.118 ... 0.12 0.36 0.014 0.012 0.30 25.77 446 and hydrogen. All samples 0.030 13 .'do 3: is ... 18.37 ... ... 0.57 0.011 0.017 0.08 1.49 317 were made from commercial 0.46 ... 0.040 ... 0.05 1.07 0.013 0.011 0.57 18.11 11.02 321 ... 0.78 0.035 0.06 1.56 0.014 0.012 0.48 19.46 11.72 ... 347 heats of deoxidized steels: the carbon and nickel steels were open hearth and the rest were electric furnace steels. The samples in bar or tubular form were inserted in the catalys to be badly blistered as shown in Figure 1. This experience was bed of an ammonia synthesis plant in which they were subjected unusual for this service, inasmuch as similar tubes are regularly to a pressure of 13,000 to 15,000 pounds per square inch and a used over periods of many years without sign of blister formatemperature range of 480" to 593" C. for 1788 hours. After tion. The pressure was substantially the same on both sides of exposure, sections of the above samples were etched. Figure 3 the catalyst container tube. Figure 2 shows a photomicrograph shows the etched cross sections o f these pieces. taken near the edge of a blister. Numerous voids can be seen Table I1 gives the thickness of the case and the hardness of the through the cross section in addition to the blister. outside surface and of the core after exposure. The analysis of the blistered steel was as follows: AND
Per Cent Ni Cr
7
. a .
% Carbon Manganese Phosphorus Sulfur Silicon Nitrogen
0.001 0.75 0 005 0.036 0.09 0.024
The steel before use contained about 0.15% carbon and 0.001% nitrogen. The carbon had been almost entirely removed by conversion to methane (6). This has been reported by Evans (3)at pressures as low as 350 pounds per square inch. The nitrogen content of the steel had increased twentyfold. The blistering was probably caused by an abnormal release of the pressure from one side of the tube.
Figure 3. Transverse Etched Cross Sections of Samples after Exposure to High Pressures of Hydrogen and Nitrogen
TABLE 11. CASE THICKNESS AND HARDNESS OF EXPOSURE
STEELS AFTER
Case Rockwell Hardness Thicknesa, Inch Outside surface Center of sample None B 42-44 B 42-44 None B 41-43 B 42-45 None B 64-65 B 64-66 None B 76-78 B 74-76 0.105 c 37-37a B 67-68 0.047 C 39-40 B 85-87 0.036 c 35-37 B 87-87 0.033 C 40-41 B 85-86 0.007 C 51-61 B 85-87 0.011 C 60-62 B 86-87 0.009 C 51-52 B 84-85 All Rockwell, C readings taken on Rockwell Superficial 15N and oonverted to C scale. Type Steel
The straight carbon and nickel steels show no cases on etching. Thick cases were found on the chrome steels and thin cases on the chrome-nickel steels. The thin cases were harder than the thicker ones, and also harder than the carbon and nickel steels without cases. Table I11 shows the analyses of the samples after removal from the catalyst container tube. Cuts were turned from the outside surfaces of the bar samples and these were analyzed for carbon and nitrogen. The center or core was drilled and analyzed for comparison. I n the straight carbon steels, the nitrogen was generally distributed throughout the cross section with slight concentration in the surface layers. The more carbon the steel originally contained, the more nitrogen it absorbed during exposure. The nickel steels (4615 and 2315) showed low carbon and high nitrogen content similar to the straight carbon steels. They had no cases. The chromium steels, 4 to 6% chromium-molybdenumtitanium, Types 430, 442, and 446, developed thick cases which became thinner with increasing chromium content. The hardness of all these samples was about the same. The core hardness is much lower than that of the case. The chromium-nickel stainless steels, Types 317, 321, and 347, all had thin, hard cases after exposure. The carbon content of the case was lower than that of the core, although the nitrogen was markedly higher in the case, and there waB some increase in the core. Figure 4 shows photomicrographs of the carbon steels, The
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Globe Iron, 250 x
Figure 4.
Type 1015, 250 X
Type 1015, 1000 X
Vol. 41, No. 11
Type 1030, 250 X
Type 1020, 250X
Microscopic Structures of Carbon Steels after Exposure t o High Pressures of Ilydrogen and Nitropeit l'ransveree scutions, nital ctcli
phase piaselit in the coiiici3 oi tho gi.rms The phutoi graphs a t the interface show that the grain boundaries i t point have been more heavily attacked Type 347 uppc:uq to have 1eacted sirnilai ly to Type 321 Forging. A forged flange made f i om 5% chromiuni-niolg bdenurri atcel developed C Y & after several years' service 111 a n :tmmoni:L plant a t a temperature of 450' to 510' in an atmosphere of hydrogen, nitlogen and several per cent ammonia by volume, and with the entire foiging under 14,000 p o u n h pcr squarc inch pressure. At the outer edge there is a reccss oscd for making a coppel gasket joint. Bolts in the bolt circle I ~ I P used to compress the gasket and t h n b s i r w the outer edgv oi the forging. Fiyuie 8 is a photogiaph 01 thc foiging Cracks are concmtiated along the outer edge nenr the bolt circle and also ncw o m of the bolt holes on the under sick'. Figure 0 is a crow section of the forging that was etched with 6% nitaP. This had darkened the case, probably because of tlie lowered corrosion resistancbc of the nitiided layer. The case liad a Rock\~ellB hardness of 100 to 102 although the core was oril3 R 70 to 72. The case and w t ' e showed the following analyses: "0 I
e.,
Typr
4615
Type 2315
Figtire 5.
Microstructure of Nickel Steels after Exposurc. to High Pressures of' Llydrogen mid Ni troperi TraorTrrsr q e r t i o n R , 100X
black areas arc voids where the cwbon has been removed. KO erace of the pearlite, which was in the samples before exposure, can be seen. The IO30 sample shows the greatest number of voids because of its original high carbon content. The photomicrograph at IOOOX of the 1015 steel shows evidence of a new Case C 0.028 N 0.83 phase present in the grains. This may be nitrides. Figure 5 s h o w the nickel steels. Tlic 2315 sainple shows voids about the same as the 1015 s t e d of similar caibon content, but tlie 4615 sample shows considerably iiiore. Figure 6 shows the chromium steels. The cases etch dark b ~ laause of the reduced corrosion resistance of the high nitrogenehroniium alloy. The 4 t o 6% chromium-molybdenuni-titanium -toel has a very dark etched cme and it transition zone between Tbci c t w ia much lower in caibon and higher in nitrogen thim thcl the case and the core. The structure of the core is normal for cole. this alloy. The structure or Type 430 has been con+iei:ibl> Mici oscopic examinatioii of longitudiiial sections of the sampE(> changed near the caqe and to some exteiit in the core. Type 442 showrtl numerous small cracks in the case and underneath Et has developed a very complex structure which varies from thc Those in the case are shown in Figure 10, Cooling Coil Tubing. Samples taken from 1.8125 inches o u t d e vase to tho core. Type 446 shows a dark etching case. Tho grains have grown under the case and some grain boundaries rlianieier by 0.8125inside diameter chromium-molybdenum waitihave etched more than others. At higher magnification, the thick grain boundaries appear OF Curs TAKEN IWX OUTERLAX~JERR .\SI> Taxr.: 111. ANALYSESFOE GARBONAND NITROGBN to be a new phase. CORESOF SAMPLES AFTER E x P o S i i R E Figure 7 shows the microCase 1)eptlt I>epth structure of the chromiumThickIYL 2nd Cut, 0 . Chro _ _ _ _ nickel stainless steels, Types ness, Cut, Inoil N % C I;{ a Type Steel Inch Inch N 317, 321, 347, after exposure. Sone 0,020 0.012 0.067 0.020 0. 8 0.066 0.010 0.045 Globe iroti ... ... ,.. ... ... . ,. 0.012" 0.0 Type 317 steel has a dark etch1015 on? . .. 0.020a 1J.l ... ... ... 1020 one , . ... ing case and a core structure Oil0 1030 one o:Oio o',Ui3 o.'i$o 6.020 0:i)i8 o'.ibs o.oici o.18 which is normal, except that ' -3313 0.016 0.212 0.013 0.195 0.184 0.020 0,012 0.020 4618 i ton^ 0.280 0.076 0.034 0.080 0.056 the grain boundaries are 4-676 Cy-I\Io-Ti 0,870 0,034 0.10,i 0,020 0.086 0.03U 0,070 0.280 0.930 0.027 0.024 0,020 0.04: 430 thickened. This indicates car(1.200 0.336 0.240 0.041 0.880 0.020 0,060 0,020 0.030 442 0,033 0,020 0.025 0.672 0.020 0,110 0.245 0.118 0,lIfj bide precipitation which occurs 446 ... 0.oi4 0.04b 317 0.007 0.010 0.045 1.281 in this steel when i t is heated 521 0.011 0,010 0.023 2.320 0:OiO , , , (31640 0.046 0.043 ... . .. 0.060 0.0310 347 0.009 0.010 0.040 0.830 ... in the range of 800 to 1500' F. a Drillinga taken through entire orom section of the piece. The structure of the core of i h e 321 sample shows a da,rk
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