A U Q U 1852 ~~
IorrnsjOn Stainless steels with low carbon content take longer time at sensitizing temperatures to develop knife-line attack
P
RELIMINARY information on knifeline attack of welded austenitic stainless steels stabilized with niobium (columbium) was presented in this column for November 1950. This newly discovered type of corrosion has aroused widespread interest, not only from the theoretical standpoint but also because of the practical aspects involved. Type 347 (18-8s-Cb) was previously considered to be completely immune from intergranular corrosion under all conditions of fabrication and heat treatment. The writer feels it is desirable to report further here on knife-line attack because of the great interest in this phenomenon showed by readers of this column and others, Knife-line attack (KLA) is the intergranular corrosion of stabilized stainless steels in a narrow band of the plate or sheet material immediately adjacent to the weld. The metal in this narrow zone is heated to very high temperatures during welding and when cooled rapidly the columbium remains in solid solution and does not form columbium carbide. If the weldment is then sensitized or heated in the range of roughly 900" to 1400' F., chromium carbide precipitates and the well-known intergranular corrosion may occur. Columbium is added to 18-8 to combine preferentially nith carbon and thus avoid piecipitation of chromium cnrbide and intergranular corrosion. In other word$, knife-line attack occurs 11hen the columbium fails to do its dutjv. The cure for Type 347 stainless steel consists of heating at 1600" to 2100" F. after welding. This heat treatment dissolves chromium carbide and precipitates columbium carbide, which is the desirable form for the carbon.
bg Mars &. lhn#mua Additional studies have been made A, welded and then heated for 2 by M. L. Holzworth and others in the Corrosion Research Laboratory of The hours a t 1000" F., Ohio State University on Type 347 no knife-line atand other stainless steels. The condi- . tack: B, welded, tions required for knife-line attack to then heated 2 hours a t 1100" F., occur on these materials were determined. The compositions of the mateslight KLA; C, rials described are shown in Table I. welded, heated 2 hours st 1200" F., Corrosion tests consisted of five 48KLA; D, welded, 2 hours at 1300' F., hour periods in white fuming nitric KLA; E, welded, heakd 2 hours a t at 160" F. or boiling 65% nitric acid. 1400" F., slight KLA; F , welded, TABLEI. COMPOSIT~ONS O F STEELS STUDIED Material
Mo, %
Three Type 347 stainless steels were studied: one with a regular columbium to carbon ratio of 10; one with a higher ratio of 14; and one with extra low carbon (Type 347L). Figure 1 shows the appearance of regular Type 347 after exposure to white fuming nitric acid a t 160" F. Heat treatments and comments on these specimens are as follows:
C, %
Cb, %
Ti,%
Ratio Cb or Ti to C
heated 2 hours a t 1500" F., no KLA; and G, welded, heated 0.5 hour a t 1650" F., then for 2 hours at 1200" F., no KLA. Figure 2 (page 88R) is a photomicrograph of a section near the surface of specimen B showing intergranular corrosion of the metal near the weld. Figure 3 shows the apP&rance of Type 347 (Continued on page 88 A )
I
Figure 1. Corrosion of Welded Type 347 (Cb/C = 10) Stainless Steel Specimens by WFNA a t 160' F.
F G C D E A B Figure 3. Corrosion of Welded Type 347 (Cb/C =14) Stainless Steel Specimens by Boiling 65% Nitric Acid (Huey Test) August 1952
Ni, %
A
B C D E F Figure 4. Corrosion of Welded Type 347 (Cb/C = 14) Stainless Steel Specimens by WFNA at 160' F.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
G
87 h
~~
Temperatures *
d
&
y
&
d
e
e
with a columbium-carbon ratio of 14 after exposure to boiling 65y0 nitric acid. Heat treatments and comments on these specimens are as follows: A , melded, heated 2 hours at 1000" F., no knife-line attack; B, welded, heated 2 hours at 1100" F., slight KLA; C, welded, heated 2 hours at 1200" F., KLA; D, welded, heated 2 hours a t 1300' F., slight KLA; E , welded, heated 2 hours a t 1400' F., trace KLA; F , welded, heated 2 hours at 1500" F., no KLA; and G, welded, heated 0.5 hour a t 1650" F., then for 2 hours at 1200' E'., no KLA.
Figure 2. Specimen B in Figure 1; Electrolytic Sodium Cyanide Etch; 500 Diameters
Large, readable scales . . . durable, all-metal construction . . , stainless steel stems . . . accuracy within 1% of thermometer range. Available in types, sizes, stem lengths and ranges for all applications. W'rite for literature. WESTON Electrical Instrument Corporation, 617 Frelinghuysen Avenue, Newark 5 , N. J. . . . manufacturers of Weston and TAG instruments. n
WESTON 88A
Figure 4 (page 87 A) shows the appearance of specimens identical in composition and heat treatment to those in Figure 3 escept that they were exposed to white fuming nitric acid a t 160' F. instead of boiling 6570 nitric acid. The degree of KLA in Figure 4 is much more Severe than in Figure 3. This shows that the white fuming nitric acid is much more aggressive and selective in connection with intergranular corrosion than acid of lower concentrations. The beneficial effect of a heat treatment a t 1650" F. to precipitate columbium carbide prior to sensitization at 1200' F. is exhibited by specimens G in Figures 1, 3, and 4. Compare these with specimens C in these figures. Figure 5 shows the results on Type 3471, with O . O Z ~ ocarbon. Heat treatments and comments on these specimens are as follows: A , as melded, no KLA; B , welded, heated 2 hours at 1100" F., no KLA; C, welded, heated 4 hours a t 1100" F., no KLA; D, welded, heated 8 hours at 1100" F., slight KLA; E , welded, heated 16 hours a t 1100" F., severe KLA; and F, welded, heated 24 hours a t 1100" F., severe KLA. (Conttnued on page 90 A )
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
with
LINERS
m
6200
RIGID, NON-PLASTICIZED POLYVINYL-CHLORIDE Liners of Boltaron 6200 can be accurately made to provide complete protection for heavy industrial equipment used in handling highly corrosive materials. Machinability - Boltaron 6200 is unquestionably the best machinable plastic on the market today. Can be machined to any tolerance within the range of common metals. Will take any type of finish. Welds easier than steel -easily fu te Ure hot air torch. Exfrusions available -solid bar stock, 10' lengths, 1%''in diameter. Pipe in standard iron pipe sizes X" to 4" i.d., 10' and 20' lengths . . . can be threaded with ordinary pipe dies. Sheet stock -in sizes approximately 30" x 6O"-%" to 1" thick. AVAILABILITY-Distribution is limited to selected fabricators trained by the Hartwell Company. This ensures that your fabricator will make efficient and economical use of Boltaron 6200. Trained fabricators have already been licensed and established on West Coast, East Coast and in Central States area. All inquiries are processed by the Hartwell Company to ensure that the right fabricator is assigned to a given job. For further details, write to our homeoffice, address below.
&No
=AR?EWE&L Industrial Plastics Division
August 1932
PARTIAL LIST OF USES
i
''
I1
I I I I I
f
I
'
Anodizing Dyeing Bleaching Tanning Brewing Food Processing Dairy Products Processing Textile Processing Photographic Processing Industrial Plumbing
1
0
I I 1
Boltaron 6200 i s
maniq%rtzrred by BOLTA, Lawrence, Mass,
ON-1
Park Squore Building, Boston 76, Mars
INDUSTRIAL AND ENGINEERING CHEMISTRY
89A
The important point here is that longer time at sensitizing temperatures is required t o develop KLA when the alloy has a very low carbon content.' I n this case approximately 8 hours or longer a t 1100" F. are needed. A similar picture develops for 1200" F. sensitizing treatments wherein specimens heated for 2 hours show no KLA. Eight hours at 1200" F. produces heavy KLA. This indicates that Type 347L should be satisfactory as far as most fabricating techniques and heat treatments are concerned. Metallographic examination also indicates that only the very selective corrosives would cause intergranular corrosion of type 347L because of the discontinuous nature of the precipitate at the grain boundaries.
A B C D E F Figure 5. Corrosion of Welded Type 347L (Extra Low Carbon) Stainless Steel by WFNA at 160" F. after Varying Sensitization Times a t 1100" F.
Incidentally, the holes in the center of the welds of some of the specimens in Figure 5 are primarily due to defective welds and not corrosion. Tests on specimens welded with and without filler rod showed no differences in behavior with regard to knife-line attack. Type 304L also sensitizes with long times at 1100" or 1200" F.: 5 hours a t 1100" F. shows severe corrosion by white fuming nitric acid. In 304L the attack is all over (not KLA) the plate or sheet material because this alloy does not contain columbium or titanium. TestsonweldedType318 (18-8s-MoCb or Type 316-Cb) also showed susceptibilitv t o knife-line attack somewhat similar to Type 347. It should be emphasized that KLA will not occur in all cases when stabilized stainless steels are used. Environments aggressive toward sensitized steels are needed and the metal must be in susceptible condition. However, knowledge of the causes and cures for KLA may avoid costly failures of equipment.
20005 WEST LAKE ROAD
9O:A
CLEVELAND 16, OHIO
Correspondence concerning this column will be forwarded promptly if addressed to the author, YoEditor, INDUSTRIAL A N D E N U I N E E R I N Q CHINISTRY, 1155--16th S t . , N.W., Washinaton6,.D. C .
INDUSTRIAL AND ENGINEERING CHEMISTRY
VoI. 44, No. 8
