Electric Furnace 1965 - The Application of Vacuum Degassing to...

18 Proceedings of Electric Furnace Conference, 1965

The Application of Vacuum Degassing to Bearing Steel

by C. P. Church, T. Ill. Krebs, and J. P. Rowe

The demand for cleaner bearing steels in quantity has been met a t Babcock & Wilcox by use of carbon deoxidation and vacuum degassing. An improvement of life ratings for anti-friction bearings has developed simultaneously with the application of steel made by this method. Be- cause of the demand for bearing steels made by this practice, conventional air melting of bearing steels has been replaced by a practice involving air melting followed by vacuum degassing.

Although still a relatively new product which can be expected to undergo further evolution in time, bearing steels made by the B & W practice have found general acceptance by bearing manufacturers. While quantitative data are still scarce, this product acceptance of- fers good qualitative evidence of the satisfactory performance of vacuum degassed bearing steels. llowever, quantitative data on the differences between air melt (AM) and ladle vacuum processed (VP) bearing steels a re being collected. Differences do exist and exert a beneficial effect in favor of vacuum processing. This paper discusses the results of various comparative studies and experiences with regard to both air melted and ladle vacuunl degassed bearing steels, particularly the 52100 through-hardening grade.

PROCESS AND EQUIPMENT Vacuum degassing a t B & W's

tubular products division has certain unique features; therefore, a brief description of the equipment and process is presented. Fig. 1 shows a drawing of the degassing facility located a t the steel plant in Koppel, Pa.

Bearing steels are melted in 100- ton electric-arc furnaces prior to treatment in vacuum. After conventional air melt refining, the molten

C. P. CHURCH and T. M. KREBS are, re- spectively, assistant to the chief metallurgist and plant metallurgist, tubular products division, Babcock and Wilcox Co., Beaver Falls, Pa. J. P. ROWE is chief, metallurgy and welding section, a t the company's research center in Alliance, Ohio.

metal is tapped into a 110-ton non- magnetic ladle lined with suitable refractory. The ladle is transferred by overhead crane from the furnace to a ladle-car located in front of the 21 x 17 x 16-ft degassing chamber. The ladle is positioned inside an induction coil built into the ladle- car. A heat shield is placed over the ladle and the ladle car is moved into the chamber. The chamber is equipped with sliding doors on two opposite sides. This allows the ladle-car to be charged into the chamber from the furnace side for degassing and afterwards, to be dis- charged through the opposite side into the teeming bay. Fig. 2 shows a cross sectional view of the ladle and car within the vacuum chamber.

For degassing, the ladle of hot metal is positioned in the chamber, the doors are closed and the stirring and vacuum systems are activated. The vacuum is obtained by a five- stage steam-ejector supplied with 160 psi steam from a 50,000 lb-per- hour B & W boiler. With present

ZuotiOGGER-

~ T H STAGE JET +

2~ C&lE N_S ER -

practice, the pressure is decreased to between 50 and 100 microns in the chamber. After the degassing cycle, which takes about 15 min, the chamber is purged with argon before breaking the vacuum. The entire cycle from tap to teem takes about 35 min.

Deoxidation results from the re- action of carbon with oxygen to form gaseous carbon monoxidc which is continuously withdrawn by the high capacity, vacuum system. Facilities are provided for making alloying additions under vacuum within the chamber as necessary to meet chemical specifications and to control grain size. Mixing of the additions is readily obtained by induction stirring.

About 2500 heats of bearing steel have been made by this process with good results. Most of the steel so treated has been through-hardening 52100 grade whose high carbon- content is ideally suited to vacuum carbon deoxidation practice; however, considerable quantities of car-

Isr HOGGER

Isr CONDENSER

3u0 STAGE JET

CONTROL HOUSE

SIGHTING GLASSES

ALLOY BINS

Fig. I-Babcock & Wilcox vacuum degassing unit a t Koppel, Pa.

STAINLESS L A D L E

INDUCTION .COILS

TRANSFER CAR

Vacuum Steelmaking 1 19

burizing grade steels for bearings have been produced including 8620, 4620, 4720, 3310, and 3312. Typical oxygen contents for 52100 VP are in the range 10-25 ppm as compared to 25-60 ppm for air melt. Oxygen values for carburizing grade AISI 8620 are in the range 15-35 ppm for vacuum processed and 35-70 ppm for air melt.

PERFORMANCE CHARACTERISTICS

The transition from air melt to vacuum degassed bearing steel has been accomplished smoothly and without incidents for the various grades mentioned. No changes in processing other than in melting practice have been necessary for vacuum processed bearing. steels.

Machinability Machinability is a critical per-

formance characteristic for bearing steels. The conversion to vacuum degassed steel has caused no significant alteration in machining characteristics of tubing by comparison with similarly processed tubes made from air melt steel. No changes in heat treatment practices for either tubing or bearings have been necessary to accommodate VP steels.

Hardenability Vacuum degassing has no detri-

mental effect on the hardenability of bearing steel by comparison with air melting. This is shown by Jominy hardenability data in Fig. 3 and Table I which compares results for 80 heats each of air melt and vacuum processed 8620 carburizing grade. Inasmuch as the two lots were identical in size and average chemical analysis, note should be taken of the slight improvement in hardenability of the VP steel.

Rolling Contact Fatigue As noted earlier, the bearing com-

panies report satisfactory performance of bearings produced from VP steel. While long-time service performance has not yet been accumu- lated for vacuum degassed steel bearings, there is every reason to expect this to be satisfactory.

Since the introduction of VP steel the catalog ratings for many bearings have been increased by about 200-300% for both through-hardening and carburizing grades. These new performance ratings are the re- sult of rolling contact fatigue tests, i.e. bearing life tests, conducted by various bearing manufacturers. , The use of contact fatigue tests by users to qualify steelmaking practices, steel quality, and bearing life performance is a newly developing, but important trend for bearing steels. The increasing use of this highly specialized performance test stems from the inadequacy of current techniques for assessing bearing steel quality. In view of its significance and the potential problem

SIGHT GLASS

/

STOPPER ROD .- . - -

Fig. 2-Cross sectional view of ladle in vacuum chamber. Ladle sits on car within induction coil.

posed to specialty steelmakers by the development of this trend, some further brief remarks on this sub- j,ect are in order.

Interest in rolling contact fatigue results from its importance as the most common metallurgical cause of bearing failures. I t is known that re- sistance of steels to contact fatigue is affected by the size, kind, and distribution of inclusions.'-" Conse- quently, steel cleanliness has always been the primary quality criterion for bearing steels. While this is still the case for acceptance testing on a

influence of variations in melting practice and steel quality and for generating contact fatigue failures for laboratory studies. During the past five years, we have performed numerous contact fatigue tests on AM and VP 52100 bearing steel using the A 0 tester developed by a leading bearing company. In this test, five balls under springloading are rotated against a hardened, highly finished, 52100, washer-type specimen to produce rolling contact fatigue in the specimen. Fatigue

heat-to-heat basis, increasing em- Table I. Average Chemical Analyses and phasis and reliance are being placed Jominy Hardenability for 80 Heats by the bearing makers on contact fatigue tests. These have the advan- 8620 A M and 8 0 Heats 8620 VP.

tage of providing a more direct cor- ---

relation with performance. Chemical Analyses, % Jominy Hardenability

No attemDt will be made here to present a -detailed discussion of rolling contact fatigue testing of bearing steels. Briefly, it can be said that contact fatigue performance is governed by statistical principles and the tests must be conducted ac- cordingly to be meaningful. The results from one or a few specimens have no significance, and usually, a relatively large lot, frequently 30 specimens, must be tested to satisfy the statistical requirements. It may take months to perform the necessary tests with present techniques, thus making this method impractical for heat qualification on a day-to- day basis. The inherent difficulties in procurement of test bearings pre- clude extensive use of such tests for quality evaluation by steelmakers.

However, because of the importance of contact fatigue in bearing steel, B&W has been seeking for some time a simplified means of contact fatigue testing. Ideally, it should be suitable for evaluating the

Distance From Hardness, R,

Quenched - End Vacuum Air

VP AM ( 1 6 t h ~ ) Processed Melt

Table 11. Average J-K Cleanliness Index Ratings for 52100 Steel

~- -

Inclusion Type

A B C D

Air Melt* 21.7 12 0 21.6 Vacuum Processed* 24 11.6 0 9.6

Avg of 100 heats.

2 0 Proceedings of Electric Furnace Conference, 1965

6 0 1 A I R MELT .20 .80 , 0 1 1 .016 .27 . 5 4 .51 . 2 0 1 70 T Y P E * - - - C M n P - - - - S S i Ni C r &

0 2 4 6 8 10 12 1 4 16 18 20 22 2426 28 30 32 34 36 3 8 4 0

D I S T A N C E F R O M Q U E N C H E D E N D ( S I X T E E N T H S ) Fig. 3-Comparison of Jominy hardenability data for 8 0 heats each of air melt and vacuum processed 8620.

65

machines and test specimens are shown in Figs. 4 and 5. A typical pit- type fatigue failure is also shown in Fig. 5.

The time-to-failure is recorded for each specimen and the results are plotted as a Weibull distribution showing percent failed specimens versus times-to-failure. Fig. 6 sh0w.s a plot of fatigue failures occurring in the shortest times for a test lot of 112 specimens of VP 52100.

Comparison of 52100 AM and VP steel contact fatigue tested with the A 0 tester shows an improvem-ent in the LIo life* for 52100 VP over.air melt. However. there is some doubt

ZT .20 . 8 1 .009 .013 .27 . 5 5 .51 . 2 0

whether the results for this type of test are sensitive enough to steelmaking variables and steel quality variations to provide a true reflec- tion of performance potential or dif-' ferences between AM and VP steel. This question arises from the use of very high compressive stresses (mean stress-404,000 psi, maximum stress-606,000 psi) to accelerate failures and shorten testing time. Thus, at the present stage of development, the test leaves much to be desired as a tool for appraising the results of small changes in steel quality-the cumulative effects of which spell progress in steelmaking. Further investigations at lower load are cur- rently in progress. Incidentally, this test does fulfill the objective of providing materials with contact- fatigue induced microstructural changes, such as butterfly struc- tures, for laboratory studies.

QUALITY EVALUATION TESTING

There is no doubt that the chamber vacuum degassing process pro-

duces bearing steel of better quality than conventional air melt .practice. This is shown by cleanliness testing and confirmed by performance.

Ring Fracture and Etch Tests Our experience to date shows rel-

atively little difference between 52100 AM and 52100 VP in ring fracture and etch tests.

Magnaflux Tests Magnaflux testing is seldom used

as an acceptance test for bearing

steels, and relatively little informa- tion is available.

Ultrasonic Tests Ultrasonic testing shows promise

as a means for evaluating bearing steel quality. Use of ultrasonics for steel cleanliness rating is in an early stage of development. It is being ac- - tively examined by steelmakers and bearing manufacturers alike. Our UT investigations, coupled with careful metallographic searches, show a correlation between ultrasonic indications and inclusions.

Current efforts to evaluate vacuum degassed bearing steels by ultrasonics are handicapped by the fact that air melt bearing steel is no longer available for comparative studies. However, other investigators4 have shown significant differences in the ultrasonic trace for bearings made from AM and VP bearing steels. The VP steel reportedly shows smaller and fewer ultrasonic indications in comparisons with air melt. These same investigators also found a direct correlation between the ultrasonic trace and fatigue performance of groups of test bearings.

Microcleanliness and Microprobe Tests

Although leaving much to be desired as an acceptance test for bearing steels, microcleanliness testing is nevertheless the most widely used test for bearing steel quality. For 52100 and other through-hardening bearing steels, the J-K rating method (ASTM E45 Method A) is used. Since J-K ratings record only the worst inclusion fields, the test is not

-

L,o life: Time at which 10% of the test specimens have failed. Fig. &Type A 0 rolling contact fatigue testing machines.


Fig. 5-Washer specimen used in contact typical fatigue failure.

sensitive to small differences in steel quality. However, J -K ratings show that 52100 VP steel is mark- edly cleaner from the standpoint of frequency and size of D-type inclusions than 52100 AM. This may be shown by means of an index which weighs and averages the J-K microcleanliness ratings for the six standard heat locations (top and bottom of first, middle, and last ingots). This index may be expressed for a given inclusion type as follows:

where z T = Summation of thin ratings for six standard

heat locations XH = Summation of heavy ratings for six standard heat locations I = Index value

"Heavies are weighted as two times thins in this index. Example: A heat showing D- type J-K ratings of 1.0 thin and 1.0 heavy a t all six locations would have an Index Value of 18.0 for D-type inclusions.

fatigue testing. Small pit on ball-trock is a AM steeis showed that they were characteristically complex and con-

0 - REPRESENTS FAILED SPECIMENS

2 0 .I .2 .5 I 2 5 10 20 50 ICQ

SPECIMEN LIFE-MILLIONS OF REVOLUTIONS

Fig. G W e i b u l l plot showing the earliest contact fatigue failures in 112 specimen test lot of 52100 VP steel. Test conditions involve thrust load a t 404,000 psi mean compressive stress and 606,000 psi maximum compressive stress.


melt is completely encased by sulfide. These sulfides are not readily identified as such, and hence are not counted in microcleanliness evaluation.

Study of the D-type oxides found in V P heats of 52100 steel showed that the particles not only are fewer and smaller than in AM heats, but that their composition is different from those in AM heats. Fig. 10 shows a series of scans for a typical D-type oxide from a VP heat. This inclusion is shown to be a particle of MgA1,0, surrounded by a matrix of (Al, Ca, Si) oxide. This is essentially the same total chemistry shown for the inclusion in Figs. 8 and 9, but the particle morphology is considerably less complex. This relative simplicity is typical of the few oxides found in VP heats. These particles seem to have formed by the deposition of the (Al, Si, Ca) oxide on a single nucleus of MgA1,0,. The scan for sulfur further shows that the oxides in the VP heats are completely free from sulfides; those in the AM heats were shown to be associated with sulfides. Thus, although the sulfur contents of VP heats are somewhat lower on the average, virtually all of the sulfides which form in the steel are rated as Type A inclusions. As mentioned

0 6 12 18 2 4 30 36 4 2 48 54 60 previously, these differences in the

SIZE OF INCLUSIONS (INCHES x 10-41 inclusions contained in air melt and VP steels stem basically from the

Fig. 7-Globular inclusion diometer 52100 VP vs. 52100 AM. difference in deoxidation practice made possible by vacuum processing. From the standpoint of bearing steel quality the -decreased amount

tained several distinct phases. An represent the most continuous phase of globular 'oxides in 52100 VP is optical micrograph of a typical air present; and (3 ) rather angular. more important by the melt globular inclusion is shown in well-defined pafticles of MgA1,0, bearing manufacturers than the sul- Fig. 8 together with an X-ray scan occurring throughout the matrix. fide for the major elements in the Par- The significant aspect of these re- A stud; of the Type A sulfide in- ticle. ~ h e s e scans indicate the dis- sults is the fact that the so-called clusions in the two types of steel in- tribution of each element relative globular oxide inclusion in 52100 air dicates that there is essentially no to the optical micrograph shown. Five important elements were de- tected: manganese, sulfur, magnesium, calc~um, and aluminum. The scans show that the most generally distributed element is aluminum, whlch 1s present in all of the darker areas of the sample. Calclum and magnesium are also present in large amounts throughout the core of the particle. The scan for manganese, calcium, and similarly for sulfur, demonstrates that the rim of the en- OPT APH MANGANESE SULFUR

distribution indicated in these scans with the microscopically observed

of the pLases in the particle as shown in Fig. 9. Three phases com- m prise the entire particie: (1) the (Mn, Ca) S which forms the rim and which is also present as discrete particles within the inclusion; ( 2 ) (Al, Ca, Si) oxides which seem to Fig. 8--Electron microprobe beam sconning images of elements in complex oxide inclusion form the matrix of the particle and in 52100 air melt steel.


(Mn, Ca 1 S -

Fig. %Major constituents of complex

difference in the composition of the sulfide particles as a function of melting practice. They were pre- dominantly MnS in both grades with most particles showing about 2-4% Ca, 1-2% Cr and 3-5% Fe substituted for the manganese.

The average index for B-type inclusions (Table 11) was shown to be very similar for VP and AM heats of 52100 steel. Microstructural and microprobe analysis was completed on a typical series of these inclusions from both AM and VP steel to establish the actual chemistry of the stringers and to determine whether significant differences resulted from vacuum processing. These results indicated that the composition of Type B stringers is largely the same for either melting practice. A typical series of X-ray scans is shown in Fig. 11 for a portion of a stringer in an air-melted heat. These scans, together with quantitative data taken from selected points in the particles, show that the stringers ar.e 70-90% Also, with the balance being primarily either CaO or MgO. Silica is frequently present in amounts up to about 5%. Sulfides are found randomly along the stringers and are either contained in the oxide, as they are in the particle in Fig. 11, or are interspersed between the ox-

oxide inclusion in 52100 air melt steel.

eralities are based on a relatively large number of observations and many individual quantitative analyses. A surprising degree of uniform- ity and reproducibility among inclusions was found in this work. Thus, the implications suggested by these data are believed reliable.

The major effect of vacuum processing on the microcleanliness of 52100 steel has been shown to be a decrease in globular oxide count and size and a slight increase in sulfide rating. The decrease in number of oxides is readily explained since the final additions are deferred in vacuum practice until after fairly complete carbon deoxidation has taken place. Thus, there is simply less oxygen available for the formation of oxides.

OPT l C A L MI1

The reason for the sulfide varia- tion with melting practice is less clear. Thermodynamically, based on existing data from simple systems, no sulfides should form in the steel until the temperature falls below the liquidus. This obviously will not oc- cur until teeming into the ingots. Thus the effects noted, in which sulfides nucleate on the globular oxides in AM heats but not in VP heats, may relate to the differences in deoxidation practice which introduce calcium to AM heats. Calcium apparently acts to scavenge the sulfides as they precipitate, causing them to nucleate on the globular oxide particles. In VP heats, to which calcium has not been added for deoxidation, the sulfides nucleate freely.

The lack of anv detectable effect of vacuum on Type B inclusions is not readily explained by the data obtained in the present study. This presents an important area for further research. To with- stand vacuum operations and the higher tap temperatures encount- ered with ladle degassing, high alumina refractories are needed for ladle linings. The latter must be rec- ognized as a potential source of alumina-type iwlusions. Additional studies dealing with the origin of Type B alumina particles in vacuum degassed steel should be conducted. Further modifications and refine- ments to melting practices and refractories are needed for additional improvements in microcleanliness.

CHEMICAL COMPOSITION OF BEARING STEELS

The present chemical compositions of bearing steels, both through- hardening and carburizing grades, should be revised for vacuum proc-

CROGRAPH ALUMINUM MAGNESIUM ides in the stringer. The Type B inclusions tended to show a denser spacing of the individual particles in AM heats than in V P heats. Further, the sulfides assoc~ated wlth B-type inclus~ons were generally CaS with 3-6% Mn in the air melts and MnS with 3-7% Ca in the VP heats.

Discussion of Microcleanliness -- and Microprobe Results SILICON CALCIUM S U L F U R

The inclusion studies presented in CI

this paper have dealt entirely with 52100 steel. They have been pre- lop sented in summary form and as Fig. l&Electron microprobe beam scanning images of elements in complex oxide inclusion fairly broad generalities. These gen- in 52100 vacuum process steel.


{OGRAPH CALCIUM ALUMINUM

MAGNESIUM SULFUR H

10~ Fig. 11-Electron microprobe scans in type B inclusion.

essing. The current compositions are casryovers from conventional electric furnace air melt practice. In fact, the EF compositions stem from open hearth practice, except for lower phosphorus and sulfur contents. For vacuum processing, certain compositional specifications, particularly those for silicon content, appear to be obsolete and serve no useful purpose in V P steels. With carbon vacuum deoxidation, silicon is not needed as a deoxidizer. In the amount present in most bearing steels, i.e., 0.20-0.35%, silicon does not contribute significantly to hardenability or other metallurgical characteristics. Thus, the addition of silicon to vacuum processed bearing steels seems to be a technically obsolete practice being continued to satisfy a n outmoded requirement I t would seem reasonable to revise the silicon specification to show only 0.35% maximum.

The chemical specification for manganese in 52100 should also be reviewed. At the level normally present in this steel, manganese does not contribute greatly to hardenability, nor is this amount necessary to tie up the relatively low content of sulfur present." On the other hand, the manganese content of scrap pre- vents attainment of ultra low manganese in the finished steel except by use of extraordinary means. Here again, a specification on the maximum alloy content should suf- fice.

S U M M A R Y The transition from air melted

steel to , carbon-deoxidized, ladle vacuum degassed steel for bearings has been accomplished smoothly. An improvement of bearing life ratings has developed simultaneously with the use of vacuum degassed

steel. Through-hardening 52100 bearing steel produced by ladle vacuum degassing shows improved microcleanliness. The most notice- able effect of this degassing practice on the microcleanliness of 52100 is the reduction in size and frequency of D-type globular oxides and the increase in sulfide ratings. Micro- probe and metallographic investigations have shown that sulfides are associated with D-type oxide inclusions in air melt 52100; however, this is not the case for 52100 VP. This results in the presence of greater numbers of sulfides in the degassed steel. Degassing practice has now developed to the stage where steel compositions tailored to this process are desirable.

REFERENCES 1 J. D. Murray and R. F. Johnson: Special

Report 77 on Clean Steel, Iron and Steel Inst., London. 1963, pp. 110-118. CL. 0. Uhrus: Special Report 77 on Clean

Steel. Iron and Steel Inst.. London. 1963. pD. io4-109.

XR. F. Johnson and J. F. Sewell: J . Iron and Steel Inst., December 1960, pp. 414-443.

I E. Harness: Private communication. fiT. E. Perry: High Vacuum Deocissing-

Tile Pvocess rind The Results , oaoer ore- sented at General Meetlng of ~rnerccan iron and Steel Inst., New York, May 26, 1965.

APPENDIX

Inclusion Identification by Electron-Microprobe Analysis

The Electron Microprobe X-ray Analyzer provides the metallurgist with the ideal technique for the identification of inclusion particles: he can establish the composition of observed particles in situ while ob- serving the particle in question. This is accomplished by coupling the long-established principles of X-ray spectrography with recent advances in electron optics which make possible the production of a micron-size

electron beam. By centering the beam on a specific micro-constitu- ent, an X-ray spectrum is generated which contains the wave lengths characteristic of the elements present in the irradiated area. The oper- ator can collect these characteristic X-rays in suitable spectrometers and electronically record the chemical composition of a microvolume of material, in situ. At the time of these studies, analysis could be made for all elements in the periodic table above sodium. Present capabil- ities of our equipment also permit analysis for the interstitial elements. A standard metallographic polish is applied to all samples prior to analysis.

The microprobe was used to study the composition of the inclusions in 52100 steel with emphasis on the effects of vacuum processing on inclusions. The inclusions were first identified qualitatively using an Electron Beam Scanning (EBS) system. This system electrically couples the sweep drives on an oscilloscope tube to a set of deflection plates around the electron beam. In this way the electron beam can be caused to scan a square area on the sample whose size can be varied in steps of 2X from 45 to 360 microns square. This results in an image magnification of from 250 to 2000X. As the beam sweeps across the sample, the instantaneous signal from any of the detectors in the in- strument can be amplified to modu- late the intensity of the oscilloscope beam. Thus, by setting a n X-ray detector for the wave length of an element being studied, the distribution of that element in the area being scanned is presented on the oscilloscope tube. By taking successive photographs of the same area with the X-ray detector set for the wave lengths of the various elements being studied, a comparative distribution of these elements can be made. The EBS photographs can be compared directly with optical micro- graphs of the areas being analyzed.

Following the qualitative deter- mination of the elements contained in the inclusions, a quantitative analysis was made of each of the inclusions. Standards used for the elements were pure metals with the following exceptions: FeS for S, CaCo, for Ca, and KBr for K. Back- ground corrections, wave length shift corrections, and deadtime corrections were made to all data. Ab- sorption corrections were based on the measured relative X-ray intensi- ties, using published mass absorption coefficients. All inclusions, except the sulfides, were assumed to be oxides during the absorption corrections and, hence, all inclusions, except sulfides, are reported as oxides, although they are not neces- sarily present as oxides. Their ap- pearance in the optical microscope indicates that they are oxide types, however.


.

Discussion B. A. STRATHDEE [Dominion Foun-

dries and Steel, Hamilton , Ont.] : What techniques do you use for sampling both liquid and solid steel, and how do you analyze them for oxygen, hydrogen and nitrogen?

C. P. CHURCH: We have a Leco oxygen analyzer. Because of our product we are not overly concerned with hydrogen in our finished product, and we have very few results on hydrogen or nitrogen. At present, we are primarily concerned with oxygen. We take these from billet samples. We have taken tests at each stage of processing, and the final tests in the 4 x 4 reforged bloom. We have taken samples before going into the . degasser and at various stages along the line, too.

G. H. OCKENHOUSE: In respect to fatigue testing, what percentage of fatigue failures could be traced directly to nonmetallic inclusions?

T. M. KREBS: I cannot answer that. This part of the test program-the search for the cause of the failure -proceeds very slowly. In most cases the evidence of what was at the point of failure is destroyed by formation of the failure itself, so I really cannot say that we have very good evidence in all cases relating the failure to some inclusion.

However, in doing some of our ultrasonic test work on samples, bearings which have been run in fatigue, we have located small indications which, when we have cut into the sample, have shown an inclusion with a small initiation point perhaps of a fatigue crack at that point. Once the failure has actually begun and has occurred, unless the inclusion was very gross, I doubt very much that evidence would be found asso- ciating the inclusion with the failure. We have not done a great deal of this type of investigation on the fatigue specimens that we have generated in our testers, however.

G. H. OCKENHOUSE: SKP has been doing a lot of work. They are a bearing company, and their recent work indicates that it is more of a problem in the making of the bearing and heat treatment. I believe new depar- tures and new heat treatment procedures bear this out-that some of the improvement in the B10 life has not only come from vacuum treatment but also the new heat treating procedures they are using by putting compressive stresses in the subsurface area of the parts.

T. M. KREBS: That is certainly true.

There is no intent here to give all the credit to the steel companies for the new ratings.

FREDERICK DELVE [Jcmes & Laugh- lin Steel Corp., Pittsburgh, Pa.] : Do you propose to decrease the silicon content from 50 and 30 down to what level? You say i t will have no effect on the response to heat treatment.

T. M. KREBS: That is correct. I said presently there is a 20 min and 35 max. I think we should take out the 20 min since it seems to be con- tributing nothing. You can leave in the 35 max, which will cover variations in the present practice.

G. C. DUDERSTADT [Jones & Laugh- lin Steel C w . , Pittsburgh, Pa.]: Did you get some additional infor- mation on the manganese oxide content of the globular inclusions? All the other microprobe tracings have shown certain variations between the two steelmaking practices, but we did not see the manganese, which may be very important.

J. P. ROWE: Only briefly I can say that in all cases where we have reported data on manganese, we find it associated in an area of the specimen which also contains sulfur, leading us rather naturally to believe that it is in the form of manganese sulfide. Our microprobe has only recently been equipped for light-element analysis, and for that reason we have not looked at the oxygen content in these areas. I am afraid we cannot make any further comment on that question.

Chairman PREUSCH: Your sug- gestion that manganese be lowered brings with it an implication that you feel some improvements might be realized from lower manganese in the steelmaking. Do you have indications of this?

Secondly, have you actually poured ingots of 52100 made without silicon, and have you found them to be entirely free from subsurface small gas bubble rejection?

T. M. KREBS: Regarding the first question, I really do not have any in- formation on the effect of manganese at lower levels. Clark, could you comment on the second question after Mr. Preusch repeats it?

Chairman PREUSCH: YOU have suggested that there should be 35 max on silicon but no minimum. The thought is that the steels might be poured or perhaps should be poured at substantially lower silicon levels than the 20 min. Have you actually

poured ingots at the 10 or less vicin- ity? If you have, have you done it in air, and have those ingots been entirely free of subsurface carbon monoxide injection?

C. P. CHURCH: We have not done it purposely. Once in a while we have an accident and end up with low silicon, and that is why at the present time we are looking for some bearing company with as much courage as we have to process a heat with low silicon. On some of our work, where we have ended up with low silicon, we have made magnaflux and microcleanliness tests, and it seems to us that on the subsurface indication we got quite an improvement. Therefore we are quite in- terested in trying to find out what happens down in the very low sili- . cons.

G. C. DUDERSTADT: If I understood you correctly, you said some of your improvements were obtained through the placement of calcium silicon through the carbon deoxidation practice. Does this mean you do not add any calcium silicon any more?

Secondly, we have heard quite of- ten that vacuum process results in an increased rating for sulfides. Do you have any evidence that this has a detrimental effect on the properties of the bearing steel, or does this mean, if you would not have this increase in the sulfide rating, you might have even better properties?

C. P. CHURCH: We do not use any calcium silicon at the present .time in any stage of melting. We find when we do introduce it, even though it does have benefits for ball- ing up the alumina inclusions, we come out with large ball type inclusions, which our customers do not like, either. You have to decide which one you want. We have de- cided we do not want the ball type inclusion. With our practice we have enjoyed very good success without the calcium silicon. It has not made an inferior product as far as we know.

G. C. DUDERSTADT: Does this' increased rating in sulfides have any detrimental effect on your properties, or do you get some evidence of this?

C. P. CHURCH: The majority of those using it say no; but when it comes to increasing the sulfide limit they are not so courageous. I believe most of the bearing people do not think the high sulfur causes them much trouble.

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