INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1924
611
Prevention of Case-Hardening by Copper Plating’ By James C. McCullough and Orland M. Reiff O B B R t I N COLLGGB, OBBRLIN, OHIO
analysis showing C 0.10, ELECTIVE case-hardSelective case-hardening by the carbon pack method m a y be seM n 0.55, P 0.014, S 0.042 ening has been occured by protecting the desired parts with a copper plating. The per cent. Samples w e r e casionally p r a c t i c e d method can always be relied upon if the copper plating i s perfect. cleaned for plating with for many years, but has Flaws in the copper m a y be due to excessive current density during hydrochloric acid containonly recently come into plating, to nonconducting impurities i n the surface of the steel, or to ing 1 per cent by volume of prominence through the devaporization of copper from areas that were insuficiently covered. 40 per cent formaldehyde velopment of the automoA test is given that will show which pieces can be successfully carsolution (Holmes’ patent), bile industry. The literaburized without further d a t i n g with copper. or by fling, grinding, and ture contains few references polishing. to the subject. Copper plating was in all cases from a cyanide solution of It is now quite generally believed that case-hardening by the carbon pack method is due to the reaction of carbon the following composition: monoxide gas with iron heated above its critical range. Water 10 liters Sodium carbonate crystals 250 grams 2CO 3Fe -+ Fed2 COa Sodium hydrogen sulfite crystals 200 grams 200 gramq Copper carbonate The liberated carbon dioxide diffuses out of the iron and rePotassium cyanide (98 per cent) 250 grams acts with the charcoal of the pack to keep up the supply of The carburizing pack was three parts wood charcoal carbon monoxide. and two parts barium carbonate ground together, or the finely The various methods of restricting case-hardening t o certain parts of the pieces are, therefore, methods of exposing to ground “Rapid Mixture,” supplied by the Park Chemical carbon monoxide gas only those surfaces that are to be Company, Detroit. The results were the same with either hardened and so covering the other surfaces that the gas mixture. I n all cases the samples to be carburized were packed cannot reach them. Three methods of covering the surfaces in a sheet-iron box 8 x 10 x 15 cm., held a t a temperature of 925’ to 950” C. for 75 minutes in an electric furnace, and then to be protected are in use: allowed to cool with the furnace. To insure accurate tem1-Covering with iron. This consists of so arranging the pieces perature control, one pyrometer couple was kept in the furthat the surfaces to be protected are in contact, one protecting another, or by covering certain parts with thin plates or sleeves nace chamber and another was inserted to the center of the packing box. of iron that may be discarded later. 2-Covering with a silicate cement or other nonmetallic One end of each test piece was left unplated and the depth coating. Wood and McMullan2 have reported on the effective- of case developed on this end was compared with the more or ness of such coatings and have found that sodium silicate mixed with finely ground asbestos or aluminium oxide is the best cover- less complete protection furnished by various copper platings. The methods of preparing sections for microscopic ing of this class. 3-Covering with a coating of copper or other metal de- examination were the same as reported in a previous paper13 posited electrolytically, or by the Schoop spray. Copper plated except where indicated to the contrary. from a cyanide bath has been found satisfactory under many Thicknesses of copper platings were measured on the preconditions and is quite commonly used, but cannot always be relied upon, as i t fails to prevent carburization in some pieces pared sections by means of a calibrated micrometer eyepiece, while it protects others that have been treated in exactly the same or in some cases by polishing off the copper from the end of a way. This method has been under investigation in this labora- rod and measuring the change in length with micrometer calitory for some time, and a preliminary report by McCullough and Pray3 showed that copper may be depended upon to prevent pers. These measurements were checked by calculations based carburization if the copper plating is free from holes, but that the on electric current used in plating on known surfaces. Copcondition of the plating is affected by several factors, and that it per gave far better protection on this good grade of steel than is not always easy to secure good deposits. It was shown that was found by Pray in his work with a more slaggy metal. slag and oxide impurities in the surface of the iron cause openings in the copper plating through which the carburizing gases may Complete protection was given in most cases by platings 0.002 enter, anti also that copper deposited a t high current densities is ’mm. thick deposited a t a current density of 0.2 ampere per likely to be too porous to afford good protection unless very square decimeter, and as thin a plating as 0.0005 mm., when thick deposits are used. deposited a t this same current density, gave complete protecThe writers’ continuation of the study has supported these tion on some samples. But platings 0.002 mm. thick depospreliminary claims and has also shown t h a t volatilization ited a t current densities of 0.4 and 0.6 ampere afforded of the copper may leave bare spots, especially where non- very little protection t o the iron, and the copper appeared conducting impurities in the surface of the steel have been quite porous when examined a t high magnification. Iron cleaned by grinding and polishing, as for microscopic but thinly covered with copper. They have also developed a potassium ferricyanide test that is a good indicator of the examination, seemed t o plate over somewhat better than condition of the copper-plating and will not interfere with samples cleaned with acid or with caustic soda followed by further plating if needed, or with the later carburization of the acid. Slag spots especially appeared to be more easily covered with copper after this mechanical cleaning. The piece tested. difference in degree of carburization would not, however, METHODS OF STUDY seem to warrant the extra labor of grinding the surfaces beThe steel used in these experiments was in the form of fore plating them. I n response to an inquiry directed to the 13-mm. rods, supplied by the Carnegie Steel Company, their U.S. Bureau of Standards it was learned that they hope to publish in the near future the results of some tests upon the 1 Received Nwember 28, 1923. thickness of copper required for protecting steel against case* Chem. Met. Eng., 26, 1077 (1922). hardening. They also suggested that wherever practicable * I b i d . , 27, 1119 (1922).
S
+
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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the steel surfaces would be machined or sand-blasted b e fore trying to copper-plate them.
Vol. 16, No. 6
tics used. The top section of each rod has the thickest plating and the bottom section is unplated. ON SBCTZONS
0s v*nrous
5
6
7
8
0.6
0.6
0.6
0.4
.,
E
Fro. ~--PGRRICYANIOB TPSI.SHOWING DSFHCTS ZN C O P P EPLATINCE ~
TESTING THE COPPER PLATIKO Since the writers were so sure that ease-hardening cannot occur under copper plating unless there are openings in the copper to permit passage of tlie gases, they sought a method for finding these holes easily. The folloiring test proved very satisfactory: The samples to be tested were dipped into a solution of 5 grams potassium ferricyanide, 5 cc. concentrated hydrochloric acid, water to make 1 liter. The hydrochloric acid penetrating through any breaks in the copper would react n,ith the iron to form ferrons chloride, and this in turn would react with the ferricyanide to form Turnbull’s blue. 3FeC12 f 2&Fe(CN)s
-
Fea(l7eCcNs)r f GKCl
Of course, the bottom, unplated section (E) of each rod was immcdiatcly covered v i t h tlre blue complex and shows black in the photograph. The tliiirner coverings toward the bottom of each rod show large, dark areas, while the npper ends show inuelr more uniform copper deposits. Several of the rods show the darkest deposit in lines tlie entirc length of the rod, indicating that tlie slag or oxide strcaks are not easily covered even hy long plating. Since these impurities are nooconductors of the elcetric current, they could only be corered hy tlre copper bridging over from the sides, and this woiild be a slow process. Tire dark spots in the photographs arc many times larger than the breaks in the copper that they represent, because of spreading of the blue componnd. The rods shown in Fig. 1 were later carburized and examined for penetration t,hrough each section. The results agreed exactly with the writers’ nxpectat,ions. Rod 2 showed almost no carburization in Sections A and B, but much in Sections C and U. Rod 4 showed little penetration in Sections A, B, and C, except at tho line of dark spots in the photograph corresponding to a slag line, where the penetration \vas marked. Rod 5, Section A, with a copper plate 0.015 mm. thick, showed carburization only a t the point of the large dark spot. Tests were also made to show that the test solution has little or no tendency to open or enlarge break? in %hecopper. Plated rods were dipped half their length into the test solution and later carburized. The same amounts of penetration were noted on sections from each end of these rods.
The rapidity of formation and apparent size of the hlue spots forming around any holes in the copper depend on the amouiit of hydrochloric acid used in the solution. Hence the sensitiveness of the test may be varied by this means. The formula recommended will show b r e a k in the copper that are too small to permit objectionable Carburization under most eonditions. I n fact, the writers found few copper platings of the thicknesses they used that were pcrfect enough to show no blue spots with this test, hut it was easy to recognize the extent of the blue markings that would indicate objectionable defects in the copper. While this test was a n adequate i n d i d i o n of the nature of tho copper deposit, there was a tendency for the blue’ compound to flow over the surface of the copper as the piece was withdrawn from the solution. Ordinarily this would do no harm, but time to study and even photograph the marks was desired, and for this i t was found that the acid and ferricyanide could be added to collodion or could be thickcued with agar agar so that the solution \iould “set” quickly on the surfaces to which it w&s applied. Fig. 1 shows the results of some of these tests. Rods of iron 13 mm. in diameter and 100 mm. long were divided into five sections, one of which was left clean and the Fzc. Z-SLAG SPOTIN smsr THAT BASNo7 BBENCOVBRBD BY A others were plated with various thicknesses of copper dePLATING OP COPPSR0.005 Mr. THICK.M ~ w r s r a o600 X. (Prior0 posited at several different current densities. Rod l is so T ~ ~ R O C CRBD I~ Ihrsnl prepared. Rods 2 to S are similar rods set in test tubes and Of course, the t.est solution must he kept in a wooden vat surrounded with the warm tost solution made up with agar agar so that a firm jelly would form very quickly. The white or some other container free from iron. Many samples were examined to see if all blue spots deshowing in the bottoms of the tubes is paraffin used to support the rods. Table I shows the thicknesses of copper deposited veloped hy thc tcst solution indicated breaks in the copper on the sections of the various rods and also the current densi- that would permit Carburization. For this purpose sections
dune, 1'324
INDUSTRIAL AND ENGINEERING CHEMISTRY
GI3
FLO.~ - C A R B O = ~ Z A I I OONc o m n i s o TXIOUCW RRRAT *N COPPHX Sxoww IN Frs.2. SLACswows AT CBNTEQOP P ~ o r o . M m n m m 500 X . (B'rcnro W I ~ H Arco~orrcPLCSZC ACID)
of iron were ground flat, polished, etched, and tlie exact location of slag spots noted under tlie niicroscope before tlirse surfacos were plated with copper. (Fig. 2) The copper plntinp was next examined, and any uncovered slag spots that could be seen under the microscope wcre noted. Then t,he test reagent was applied and tlie exact location of the blue spots recorded. ?Text the samples were carburized in the usual v a y , and, after cooling in the furnace, the copper was removed by careful polishing. Idhliing now revealed the carburized areas, and subsequent polishing and etching showed how deep the carburization had extended. On samples so examined the the larger breaks in the copper u'ere easily visible under tlie microscope, and always corresponded with the reoorded slag spots. Visible breaks in the copper could not always be found t o correspond with tlie smallest slag spots. The test solution always marked tl~esevisible breaks and showed other smaller ones which were not otherwise suspect.ed. Sometimes the hlue marks of the t.est were removed with sodium hydroxide solution, and the sample u-as returned to the plating tank. In this way the sp0t.s could finally he covered, the copper bridging over Srom the sides, hut the test solution would continue to mark the spots unless the plating was continued for sorue time after the hole appeared closed. This is one very dcsirahle characterist~icof this test; it perInits returning a defective plating for further treatment, or a good plating is not spoilcd by it for later carburization. Carburization u-as not generally found radiating symmet,rically from the slag spots that had caused the breaks in the copper, as shown in Fig. 3, but was always apparent somewhat near these spots and not 'elsewhere. Since iron is seldom homogeneous, this would be expected. Some of thesamples slrowed masses of divorced cementite in slag areas rat,her than the more common pearlitic structure of annealed steel. The essential point is the fact that little or no penetrat,ion occurs through a copper p1at.e that gives little blue color with the test solution, and that the amount of blue color appearing is always in direct proportion to the amount of hardening that u-ill occur if the pioee is carburized. The writers expect t o continue the study of copper plating on iron with special refereiiee to mettmds of cleaning the iron, or of coating over t.lie slag spots with some conducting material such as graphite, which will insure uniform capper deposits.
~r.4POIlIZATIOS O F CoFPElI
Several writers liave noted a "flow" of copper at the edrburizing temperature, but have not studied it. This "iiow" is probably due to vaporization of the copper arid condeiisadion at another point. 9feu- tests weremade to determine the effect of this vaporisatinn on case-hardening. Pieces of iron were copper-plated and heated to 025" C. for 3 hours in an atmosphere of hydrogen. A copper plating 0.004 mm. thick shoved no indication of the copper condensing on bare slag spots in tlie iron or on areas OS iron filed free froin copper. One sample with copper plating 0.0004 mm. thick, heated in hydrogen for 3 hours at 800" C., showed numerous bare spots which were not apparent before heating. Fig. 4 is a pliotograph of this sample magnified 500 X. The dark slag spots were not visible before the heating, and there seemed to he no bare spots except where slag appeared. Since tliesc slag spots were probably very poorly plat,ed, owing to tlieir nonconducting nature, they would bo the first to lose all their corjper. Higher temperatures would probably increase the vaporization of copper, and hence heavier copper platings would be needed to prevent carburization uiider such conditions. This is common experienee. The writers liope to continue the study of vaporization of copper.
Coscznsrrm Frim an investigation of t.he protective action of copper in case-hardeninp tlie following conclusions liave been drawn: 1-Penetration occurs only through lioles in the copper plate. 2-Breaks in the copper are due to: ( a ) Eonconducting impurities that fail to plate over properly. ( b ) Iiigh current density giving a porous deposit, thus requiring a much thicker plating for completc protection. (c) Vaporization of copper leaving bare spots, especially over noncoilducting impurities which are not so heavily covered as the pure iron.
3-The potassium ferricyanide test is a reliable indicator of the condition of the coppcr plating on any sample, and will show which pieces to return for heavier platings, or even for cleaning and replating.
