T’ol. 22,
I S D U S T R I A L A S D ESGILVEERISG CHEMISTRY
776
experiments, the authors were unable to find a definite simple proportionality between these constants and the degree of yellowing of single oils and their combinations. The fact that the synthetic triglyceride of the linolenic acid yellows far less than Tvould be expected on the basis of Eibner’s theory lends further support to the belief that there are other factors entering into this reaction. From all the data so far collected on the subject it appears rather certain that the final solution will not be found through a study of commercial oils, because the variables and unknowns are far too numerous. The reactions causing yellowing are probably purely chemical, and the final solution of this problem must, therefore, be expected to come from a fundamental study of the chemistry of the drying and aging processes of oils under various conditions. J. S. Long has supplied the writers with a quantity of pure trilinolenic glyceride for this work and an extensive study of these reactions has been started, the results of which will be reported as the work progresses.
so. 7
Literature Cited Amsel, Farben-ZLg., 34, 23, 1372 (1929). Brosel, Dissertation, Munich, 1927. Eibner, Farbi Lack, 1914, 310. Eibner, “ u b e r fette Ole,’’ p. 69, hlunich, 1929. Emery, Paint Oil Chem. Rev., 87, 3 (January 17, 19291. Fahrion, Z . angew. Chem., ZS, 722 (1910). Farbe L a c k , 1929, 166. Gdrdner, Paint hffrs. Assocn. S.,Tech. Circ. 162 (19222). Gardner, I b i d . , 191, 86 (1923). Holde and hlarcusson, Ber., 36, 2637 (1903). hlorrell and Marks, J . Oil Colour Chem. Assocn., 12, 183 (1929). Munzert, Dissertation, Munich, 1925; Farben-Ztg., 33, 46, 2849 (1928). S a u r o y , “Peintures, Pigments, Vernis,” p. 997 (1929). Pfund, Proc. .Am. Soc. Tei!ing A\!~le7ialr, 2 0 , 11, 440 (1920). Pickard, Monograph: “Driers,” Am. Paiizt J . , p. 79 (1925). Rebs, Farben-Ztg., 26, 242 (1921). Reithler, Dissertation, Nunich, 1926. Scheiber, “Lacke und ihre Rohstoffe,” p. 173. Leipzig, 1926. Smith, “Paint and Painting Defects,” London, 1912. Thompson, “Painting Defects. Their Causes a n d Prevention” (1914). Tach, “hlaterials for Permanent Painting,” p. 32 (19111.
u
Pressure-Aging of Duralumin’ Leopold Pessel ”01
xI.%XHEIM
S T , PHIL4DELPHIA. Pa.
HE prevention of intergranular corrosion of duralumin is still one of the main problems of the practical metallurgy of this alloy. A considerable amount of work
T
has been done in studying this question from many angles ( I ) , the most recent and comprehensive being that done by Rawdon (3). The \+-eight of opinion indicates that the best method for decreasing the susceptibility of duralumin towards intergranular embrittlement lies in cold quenching and aging at room temperature. Although the duralumin treated by this method s h o m a better corrosion resistance than hot-quenched and hot-aged material. there is enough of this undesirable tendency left to warrant further attempts to decrease it by treatment of the material itself without resorting t o protective coatings, such as sprayed molten a l ~ m i n u m the . beneficial effects of which are well known. Theory of Corrosion of Duralumin
The structural causes of the susceptibility of duralumin and siniilar alloys to intergranular corrosion are not exactly known. As it is fairly well established that the copper content of these alloys has much to do with the intergranular failure under corrosion attack, the prevailing opinion is that the loosening of the crystal bond is effected by electrolytic action between the aluminum-copper compound particles and the grains of the alloy itself. Without any definite proof-on account of the submicroscopic dimension of the hardening particles-it is assumed that these particles accumulate along the grain boundaries and open the path for progressive electrolytic action. It is contended that the improvement of the resistance against this form of corrosion by cold-water quenching is due to the fact that the quickness of cooling prevents the migration of the particles towards the grain boundaries ( 2 ) . As a consequence of the theory that the maximum of corrosion resistance would be obtained in a material that has the hardening particles distributed as evenly as possible and shows a minimum of accumulation along the grain boundaries, the question arose whether there could not be found 1
Received March 12, 1930.
a way to influence this submicroscopic structure by treatment after quenching. It seems clear that the precipitation, growth, and migration of the particles is going on long after quenching and that the sum of these phenomena constitutes what is known as aging. If the particles have a tendency to migrate towards the grain boundaries, their mobility will be accelerated by an increase in temperature. This holds true also during the period of aging. As mentioned above, experience has shown that aging a t increased temperatures decreases the resistance against corrosion embrittlement. TThile it seems that aging at lower temperatures improves this resistance. there is set a practical lower limit on account of the slowness of aging a t very low temperatures. Aging at room temperature will not completely prevent accumulation of the particles along the grain boundaries. I t seems that there is little hope in attempting any form of thermic treatment during the aging. The possibility occurred t o the writer that another way of influencing the migration of the hardening particles might lie in the application of pressure during the period of aging. Khile pressure might have a retarding effect upon the precipitation itself, it was held to be more plausible that the increased rigidity of the compressed material would tend to keep the particles in their place of precipitation and counteract their migration. It was also thought that lines of strain might be set up within the grains, which might tend to attract the particles and prevent them from reaching the grain boundaries. The experiments described below are an outcome of these theoretical considerations. Their undertaking mas stimulated by the fact that the influence of pressure during aging upon the corrosion properties of duralumin has not yet been investigated. Experimental
The duralumin used was obtained from the iiluniinum Company of America and was in form of 17SO sheet, l / inch (1.6 mm.) thick, fully annealed. The alloy had the following approximate analysis: copper 4.0, magnesium 0.5, silicon 0.35, manganese 0.6, iron 0.5, balance aluminum.
~
Standard tensile specimens of 2-inch (50.8 -nim.) gage length were milled out of the same sheet, following the direction of rolling. The heat treatment consisted in heating the specimens in an electric furnace at 510" C. for 20 minutes and quenchiiig in cold water. Immediately after quenching they nere transferred to ice water to retard aging. After approxiniately 30 niinutes they were put under the press plates of a screw press and subjected to a pressure estimated a t GOO pounds per square inch (42 kg. per sq. em.), which was iiiaintained for 9 hours a t room temperature. The specmiens n-ere then removed from the press and permitted to age coinpletely a t room temperature, which required approxnnately one weelr. Some of these specimens were subjected to a salt-spray (20 per cent S a C l solution) corrosion test for 30 days, together with other material that had the same composition and history but had been aged without application of pressure. The relative physical qualities of duralumin treated according to this method and in the heretoforr customary manner, before and after exposure to corrosion, are shown in Tahle I. Properties of Duralumin Aged w i t h and w i t h o u t Pressure ELONGATIOS HARDNESS I X 2 IN. TENSILE STRESCTH (50.8 MM.) Lbs./s(I. in. I i g . / s q . cm. Pev cent
Table I-Physical
~~~~
A-0.
"
~
A G E D V:ITH A P P L I C A T I O N Oli P R E S S I - R E
1 1
3
! 6 Average
63,890 63,510 63,920 63,940 63,820 63.930 63,640
61,970 D 10n 11 12
.%verage
---
I-I-DCSTRIAL A S D E S G I S E E R I S G CHEMISTRY
July, 1930
62,400 62,470 58,040
62,780 62,770 62,480
Before Corrosion 4492 4465 4494 4495 4487 4494 4488 After Corrosion 4357 4387 4392 4080 4413 4413 4392
A G E D IVITIIOUT APPLIC.4TIOX
20.1 19 6 20 1 20.3 20.4 20 1 20.1 15 16 16 16 16 16 16
3 4 6 0 6 6 3
61 61 61 61 61 61
54 50
51
48 49 50 51
OF P R E S S U R E
Before Corrosion 64,160 4510 13 14 64,160 4510 4517 64,390 1.5 4509 64,140 16 64,050 4503 17 64,070 4504 18 64,160 4510 .Iverage After Corrosion 60,170 4230 19 59,420 4177 20 60,630 4261 21 L', 4269 60,730 4268 L'3 60,710 24 60,270 4237 60,320 4240 Averare '1 S o t included in average calculation.
reaching a well-defined end point. For practical purposes duralumin can be considered completely aged after 5 days at room temperature. As there is little known about the phenomena of growth and migration of the hardening particles, it is uncertain just when the migration ceases. I t is obvious that the pressure should be exerted as long as such a migration takes place. This quest'ion will require detailed study, which will probably indicate an optirnuin time during which the pressure should be kept up in order to produce certain physical characteristics. I t seeiiis rather certain that the pressure period should start imiiiediately after quenching. If this should be impractical-for instance, on account of remoteness of pressure equipment from the furnace or on account of the necessity of treating a large number of pieces simultaneously-the material should be kept in ice water or otherwise at a low temperature to retard aging until the pressure can be applied. While it is not very likely that the extension of the period of pressure beyond the time required will adversely affect the qualities of the material-although this too will have to be invest,igated-there is a definite limit to the amount of pressure that can be exerted without harm. I n some specimens, which mere exposed to much higher (not exactly determined) pressures than those described above, a distinctly higher tensile strmgth was obtained, x\liich 1%as follomed by a xery large decrease of strength and elongation after corrosion. I t is assumed that this mas caused by the coldworking effect of the high pressure, 15hich had an influence upon the metal grain sufficiently harmful to cover the beneficial effect of the pressure upon the migration of the partlcles. Several specimens nere quenched iii hot nater and aged under pressure, but it seems that in this case the resistaiice against corrosion embrittlement 1s not iniprox ed to the same extent as with cold-water-quenched material. The migration of the particles has probably made too much progress to be influenced by subsequent pressure, although here too a slight mprovenient n as noted.
~
~
~
~
"
Practical Aspects
19 3 19 3 18 6 18 4 18 6 18 6 18 b
63 62 62 63
9 12.0 12 s 13 4 13 0 13 3 12.9
54
1'
4 / /
63
63 51
52 56 52 47 52
Discussion of Results
It can be clearly seen that the duralumin treated by this method has a distinctly higher elongation than that aged without application of pressure. The tensile si rength and hardness show a practically insignificant decrease. After exposure to corrosion, however, the decrease in tensile strength and elongation is much smaller in the duralumin that has been aged under pressure. The decrease of hardness, due to corrosion, is about the same with both methods. KOessential difference in structure could be (detected by nieaiis of the iiiicroscope a t 1000 X magnification. I t does not seem probable that even very high magnifications will reveal the accumulation of hardening particles along the grain boundaries; possibly x-ray investigations will advance our knowledge in this respect. The aging of dura,luiiiin makes very rapid progress iminediately after quenching but soon becomes slower without
As this method is still in its experimental stage, many points will have to be investigated and cleared up before its full practical value can be ascertained. The experlments h a l e been made, so far. on duralumin onlv, but It 1s w r y likely that this ;nethod can be applied o n any light alloy that can be strengthened by a heat treatment which also iiicludes a period of aging. The amount of pressure, the length of time during which it is to be applied, and t'he temperature during the pressure aging will prohahly vary with the different alloys. The choice of the mechanical equipment required to exert this pressure on a production basis in conjunction with a heat-treating installation will offer some difficulties. Mechanical presses will probably be found less convenient than high-pressure vessels. in which the niaterial will be exposed t o coinpressed liquids or gases and which will probably also contain arrangements t o niaintaiii a desired temperature. There can be little doubt, however, that, if this process actually reduces the susceptibility of certain light' alloys towards intergranular corrosion embrittlement, there will be sufficient incentive to overcome these mechanical difficulties. Literature Cited (1) "Light hfetals a n d Alloys," Bur. Standards, Circ. 346; gives good compilation of t h e literature. (2) "Protecting Aircraft against Corrosion," Bur. Standards, T e c h . S e w s Buil. 122. ( 3 ) Rawdon, S a t l . Advisory Comm. Aeronautics, Tech. S o t e s 282, 283, 284, 285, 304, 305.
