Pinhole Porosity in High-Alloy Steel Castings Made in Green-Sand Molds

Transcription

1 Pinhole Porosity in High-Alloy Steel Castings Made in Green-Sand Molds Mucr~ is known and much has been written about pinhole porosity, especially in plain-carbon and low-alloy steels. The mechanism of pinhole formation in these steels has been explained, and the factors controlling this phenomenon have been identified and described in detail. Less attention has been directed toward the occurrence, of pinhole porosity in highalloy steels of the stainless type. Perhaps it has been thought that the stainless steels are less prone to this type of defect. Nevertheless, the development of pinhole porosity in castings made in green-sand molds is one of the hazards that has confronted the high-alloy-castings industry for many years. A measure of relief from this porosity problem has been brought about by the use of small additions of selenium to the melt.' The mechanism whereby selenium reduces or eliminates porosity is not known. However, there is ample evidence to demonstrate that, under the proper conditions, the addition is effective. On the other hand, the large number of factors in high-alloy foundry practice that may influence the formation of pinhole porosity has, in many cases, tended to obscure their relationship to the phenomenon. The cause-and-effect relationships between many of these factors and the incidence of pinholes has not been thoroughly understood. Moreover, while a selenium addition sometimes eliminates the defect, at other times it does,not. An References are on page 149.

2 improved understanding of the behavior of hind the frozen layer becomes high enough, selenium would be useful. For these rea- the hydrogen will react withoxygen (oxides) sons, it appeared appropriate to investigate in the melt to form water vapor, according Frozen skin,frozen skin Melt....,..;..,.,,., -..., FIG I-REACTION OF WATER VAPOR WITH METAL AT MOLD-METAL INTERFACE TO PRODUCE FILM OF OXIDE AND HYDROGEN, WHICH DIFFUSES INTO MELT BEHIND FROZEN SKIN. pinhole porosity in castings made in greensand molds from stainless-type steels. It is of interest to consider a mechanism for the formation of pinhole porosity based on some earlier resear~h.~~~ According to a theory proposed by Sims, when molten steel is poured into a green-sand mold, a thin skin of metal quickly freezes adjacent to the mold wall. At the same time, the nearby moisture in the mold is converted to steam by the heat from the metal, with a large increase in volume. Much of the steam diffuses back into the mold. However, some of it comes into intimate contact with the outer surface of the skin of frozen metal. Here, it reacts with the metal to form a film of metal oxide on the skin and atomic hydrogen, in accordance with the following equation: The atomic hydrogen penetrates the skin of frozen metal and concentrates in the still molten metal immediately behind the solid skin. The situation is shown schematically in Fig I. When the concentration of hydrogen in the molten metal immediately be- FIG 2-FORMATION OF MINUTE WATER VAPOR BUBBLES IMMEDIATELY BEHIND FROZEN SKIN BY COMBINATION OF HYDROGEN WITH OXYGEN 1N MELT. to the following equation: 2H + FeO Fe + H20 Because it is insoluble in molten steel, the water vapor thus created takes the form of minute bubbles. These bubbles usually appear in the hydrogen-rich region at the solid-liquid interface, as illustrated in Fig 2. The concentration of hydrogen in these bubbles initially is zero, while the concentration of hydrogen in the surrounding molten metal is a finite number and has an escaping pressure considerably above one atmosphere. Therefore, it moves toward equilibrium by diffusing into the steam bubbles, where it precipitates as molecular hydrogen (Fig 3). Thus, the water vapor bubbles act as nuclei for the precipitation of dissolved hydrogen from the molten metal, as well as for the additional hydrogen diffusing through the solidified skin of metal. As the hydrogen continues to precipitate into the bubbles, the bubbles continuously grow bigger. Because the metal is freezing during the process of bubble nucleation and growth, the bubbles elongate in the direction of freezing. As illustrated in Fig 4, frozen metal constitutes their side walls,

3 FOUNDRY PRACTICES I39 while the interface between the bubble and the still molten metal advances into the melt. In this way the bubbles develop the, Frozen skin Melt --.,. \. b FIG 3-PRECIPITATION OF HYDROGEN IN WATER VAPOR BUBBLES. characteristic shapes observed in castings containing pinhole porosity. It should be noted that, in the description of pinhole porosity given here, the porosity does not start at the surface of the casting. Instead, it begins behind the skin that freezes when the metal is first poured into the mold. Thus, it is not visible in castings as shaken out of the molds. However, subsequent heat treating and cleaning operations may break the thin wall of metal between the pore and the surface, thus making the porosity visible. It is then observed as a small round hole at the surface of the casting. The appearance of such holes has given rise to the term "pinhole" porosity. Occasionally, porosity in the form of small holes is visible in castings as shaken out. In these cases, it is possible that the frozen metal skin was ruptured at a point of imperfection by the pressure of the steam in the mold. Then the steam may have entered the metal in considerable quantities through these ruptures and been responsible for the porosity. This description of the mechanism of pinhole formation in steel castings poured in green-sand molds suggests that water vapor, hydrogen, and oxygen play critical roles. The formation of water vapor bubbles in the molten metal by the reaction of hydrogen with oxides in the melt seems necessary in most cases. If minute bubbles FIG 4-GROWTH OF GAS PORE. Mold ;. of water vapor did not form to nucleate hydrogen precipitation, the development of pinhole porosity would be very unlikely. However, the formation of the necessary water vapor in the melt depends on the concentration of hydrogen and the availability of oxygen in the molten metal. If insuficient hydrogen is present, or if the oxygen in the melt is unavailable (i.e.7, combined as an oxide not readily reduced by hydrogen), these elements will not react to form the water vapor necessary to initiate porosity. Two general factors seem to contribute toward the hydrogen content of the melt immediately behind the skin of frozen metal where the water vapor bubbles that nucleate porosity are usually formed. One of these factors is the concentration of hydrogen in the metal being poured into the mold. This is the hydrogen introduced in the melt during the melting campaign, and subsequent teeming and pouring. This concentrates on the freezing face by dendritic segregation. The other factor is the contribution made by hydrogen diffusing into the melt through the freezing skin, a product of the reaction between the metal and the steam from the mold. These two factors are probably additive. Thus, if the initial hydrogen concentration in the metal entering the mold is low, more hydrogen must be supplied by the steam-metal reac-

4 tion at the mold-metal interface to build up the hydrogen concentration to the point where hydrogen will react with the oxides in the melt and form water vapor. Conversely, the higher the initial hydrogen concentration in the metal, the less hydrogen is required from the steam-metal ~eaction. Therefore, with a melt of sufficikntiy low hydrogen content, a certain amount of mold moisture can be tolerated without the development of pinhole porosity. By the same token, with molds that are dry enough it is possible to tolerate a considerable concentration of hydrogen in the metal in the ladle. In these ways, then, mold moisture and the initial hydrogen content of the melt are interrelated in the development of pinhole porosity. It is also seen that oxygen in the melt is necessary to the formation of water vapor bubbles required to nucleate porosity. If this oxygen is unavailable, the water vapor bubbles cannot form and porosity will not 'develop. Some oxygen is inevitably present in melted stainless steel. Exclusion of it from the melt is impractical. However, it would appear that its availability for the formation of water vapor could be altered. Speaking very generally, the availability of oxygen in a melt consisting essentially of iron is relatively high. When silicon is added to the system, the availability is substantially reduced. When sufficient aluminum, titanium, or zirconium is added, the availability of oxygen may become extremely low. Both experiment and practice demonstrate that proper and thorough deoxidation of the melt offers an effective means of controlling the incidence of gas porosity in plain-carbon and low-alloy steels. By the same token, it would seem logical to speculate that pinhole porosity in stainless steels could be brought under control by the use of strong deoxidizers. In the course of the investigation reported here, this proposition was tested. Aluminum additions of up to 0.6 pct were made to zoo-lb experimental melts of the 21 chro- mium, 9 nickel casting alloy, grade CF-8. No beneficial effects were observed relative to the incidence of pinhole porosity. I t was evident that this strong deoxidizer behaved differently when added to the stainless steel than when added to steels of relatively low alloy content. Some results, obtained in an investigation of the AISI 310 type of stainless steel by Perkins and Binder,4 appear to cast light on this difference in performance. Their data suggest that, while additions of aluminum and other strong deoxidizers lower the oxygen content of stainless steels considerably, such additions do not reduce the oxygen level to the low values attained in the less highly alloyed steels. Therefore, it might be postulated that even "strong" deoxidation does not make oxygen sufficiently unavailable in stainless steels to prevent combination with hydrogen to form water vapor bubbles. Furthermore, when sufficient deoxidizer is added to cause appreciable lowering of the oxygen content, it tends to form nitrides. Thus, it adds undesirable inclusions and depletes the steel of nitrogen needed to stabilize the austenite. This is especially pronounced with titanium and zirconium. It seems advisable, then, to consider other approaches to the problem of pinhole porosity in stainless steels. Nitrogen dissolved in molten steel may also play a part in the development of pinhole porosity. However, the " tolerance" of stainless steel for nitrogen is comparatively high and it is doubtful that this element takes a major part in pinhole formation in such steels except under unusual circumstances. It seems likely that dissolved nitrogen will precipitate into water vapor bubbles, just as hydrogen does, if it is available. In this way, nitrogen may contribute to gas porosity. However, dissolved nitrogen seems much less "available" than is dissolved hydrogen. It would appear that the presence of chromium, for example, in the alloy influences availability of nitrogen to precipitate into gas bubbles. It would

5 FOUNDRY PRACTICES 141 seem that the higher the chromium content, the higher must the concentration of dissolved nitrogen be before it contributes to gas porosity. Austenite has a definitely higher solubility for nitrogen than does ferrite. When such elements as titanium are added to the melt in the proper amounts, the nitrogen becomes virtually unavailable because it forms stable titanium nitrides. It has been demonstrated that a small addition of selenium to stainless-steel melts can alleviate or eliminate pinhole porosity in castings made in green-sand molds. The mechanism by which selenium does this is not known, but it is possible to speculate with either a chemical or a physical viewpoint. From the chemical viewpoint, three alternative mechanisms are conceived: selenium might eliminate hydrogen from the metal, possibly in the form of a hydrogen selenide gas; selenium might reduce water vapor formed in the melt and thus prevent it from nucleating porosity; or, it might hold hydrogen in the metal, possibly dissolved in selenide inclusions, thus making it unavailable to react with the oxides in the melt to form the water vapor bubbles that trigger porosity. Several observations suggest that the first alternative is not the correct explanation. First, thermodynamic calculations indicate that the direct combination of selenium and hydrogen to form hydrogen selenide at steel melting temperatures is unlikely; second, expulsion of hydrogen from the melt in this manner would entail the loss of selenium. However, the evidence indicates that the selenium recovery in the metal is usually IOO pct when added in the small amounts used for porosity control. In addition, if hydrogen is removed by the addition of selenium, a boil should occur following the addition. Such a boil has not been observed. In addition, there is experimental evidence that hydrogen is not lost when selenium is added to the melt. The second alternative is also unlikely. It requires that selenium be a powerful deoxidizer. The evidence is to the contrary. Thus, if the chemical viewpoint is to prevail, it would seem that it must depend on the third alternative or some similar mechanism. The physical hypothesis postulates that the presence of selenium in the molten metal prevents bubbles from forming in some way. It is possible, for example, that selenium influences the surface tension characteristics of the liquid metal in such a manner as to discourage steam-bubble formation. It is extremely difficult for gas bubbles to form in molten steel. They must usually nucleate on a solid surface of very irregular contour. The melt side of the skin of metal that freezes against the mold wall when the casting is first poured would be a surface with the required irregularity of contour. It is conceivable, however, that even a moderate decrease in the surface tension of the steel-i.e., increase in its ability to "wet" itself-would cause a considerable reduction in the probability of bubble formation. While observations made in the course of the experimental work do not definitely prove or disprove any of these suggested mechanisms, they strongly discourage a "chemical" explanation of the effect of selenium. Preliminary Ezploralio~z As a prerequisite to the investigation, it was necessary to know whether pinhole porosity such as that observed in the foundry could be regularly produced and controlled in the laboratory.. Therefore, a series of preliminary experiments was performed to observe the effect of several factors on the formation of pinhole porosity with the objective of reproducing the phenomenon in the laboratory under controlled conditions. Since hydrogen and water vapor appeared to be important variables, the experiments were designed to

6 FIG 6-SOUND CASTING. FIG 7-MILD DEGREE OF POROSITY. FIG 8-MEDIUM DEGREE OF POROSITY. FIG 9-SEVERE POROSITY. determine their effect over a wide concentration range. The procedure used was to melt virgin stock in a magnesia-lined induction furnace to provide melts in the CF-8 alloy composition range. The hydrogen content of the melts was varied over a range of about 4 to 18 parts per million by bubbling tank hydrogen through the molten metal. Castings were poured into calcined clay and core-sand molds, and into green-sand molds of varying moisture content to observe the effect of mold moisture. All of the castings in this preliminary work were poured at a temperature of zgoo F as measured by an optical pyrometer. A gland casting (Fig 5) was used to observe the extent of porosity under the various conditions. This particular shape was reported from practice to be very susceptible to the formation of pinhole porosity in the four thin ear sections. Radiographs were made to show the extent of pinhole porosity. Typical radiographs are shown in Figs 60through 9. The results of these preliminary experiments showed that pinhole porosity could

7 FOUNDRY PRACTICES 143 be produced readily in the laboratory, and indicated the importance of the hydrogen content of the melt and the moisture in the mold. The severity of the porosity in the melt hydrogen content and pouring temperature. Such information would be useful in evaluating the effects of other variables. The procedure used was to induction Hydrogen.Content, ports per million -INFLUENCE OF POURING TEMPERATURE AND MELT HYDROGEN CONTENT ON OCCURRENCE OF PINHOLE POROSITY. Castings containing no selenium and made in molds of 120 to 130 permeability containing 3 pct moisture. castings increased with the moisture con- melt a series of 200-lb to 240-lb heats of tent of the core-sand and green-sand molds. CF-8 alloy. After meltdown, the hydrogen No porosity was found in the calcined clay levels were adjusted by bubbling tank molds (zero moisture content), even though hydrogen into the melts at approximately the hydrogen content of the metal ranged 31oooF for various periods of time dependup to 18 ppm. However, in castings made in ing on the hydrogen content desired. To green-sand molds, the degree of porosity determine the effect of pouring temperawas more severe as the hydrogen content ture on the incidence of porosity, the of the melt was increased. melt was allowed to cool and metal was The effectiveness of ferroselenium addi- poured directly from the furnace into tions in controlling porosity was also con- the molds as the desired temperatures firmed in the laboratory. It was interesting were reached. Pouring temperatures were to note that the hydrogen contents of the measured in the furnace with immersion melts were not altered by the selenium thermocouples. Samples for hydrogen additions. analysis were taken at various intervals during the pouring of a series of castings. Relationship of Melt H~drogel~ Cmieflt, 1, some cases, metal remaining in the Poz*"ing Temperature, Sezewiunt furnace, after one series of castings was Content to Occurrence of Pinhole made, was reheated for subsequent tests. Porosily Additions of selenium (when used) were The preliminary results suggested the made just prior to the beginning of the advisability of attempting to establish in a cool-down. Castings were poured into quantitative way the interrelated effects of molds containing 3 and 4.5 pet-water.

8 Hydrogen Content.paHs permillion FIG I I-INFLUENCE OF POURING TEMPERATURE AND MELT HYDROGEN CONTENT ON OCCURRENCE OF PINHOLE POROSITY. Castings containing pct selenium and made in molds of 120 to 130 permeability containing 3 pct moisture. FIG I 2-INFLUENCE Hydrogen Content, ports per rnillicn s OF POURING TEMPERATURE AND MELT IIYDROGEN CONTENT ON OCCURRENCE OF PINHOLE POROSITY. Castings containing pct selenium and made in molds of 120 to 130 permeability containing 3 pct moisture. 0 The molds were squeezed to a hardness that would give them a permeability in the 120 to 130 range. This was determined from a curve of hardness versus per- meability. The castings were radiographed to determine the extent of porosity. The results obtained are plotted in Figs 10 through 15 in such a way as to

9 FOUNDRY PRACTICES o\ Hydrogen Content. ports per million A-~1086 PIG I~-~NFLUENCE OF POURING TEMPERATURE AND MELT HYDROGEN CONTENT ON OCCURRENCE OF PINHOLE POROSITY. Castings containing no selenium and made in molds of 120 to 130 permeability containing 4.5 pct moisture. Hydrogen Content, parts, per million FIG 14-INFLUENCE OF POURING TEMTERATURE AND MELT HYDROGEN CONTENT ON OCCURRENCE OF PINHOLE POROSITY. Castings containing pct selenium and made in molds of 120 to 130 permeability containing 4.5 pct moisture. show whether the castings were porous the conditions under which sound or or sound as the hydrogen in the melt and porous castings would be expected. Figs 10, the pouring temperature were varied. 11, and 12 show the effects of pouring In each case, it was possible to draw a temperature and hydrogen content on the line separating the areas representing occurrence of porosity in castings poured

10 146 PROCEEDINGS OF ELECTRIC FURNACE CONFERENCE, 1958 in molds containing 3 pct moisture with additions to the melt of zero, 0.005, and o.ozo pct selenium, respectively. Figs 13 and 14 show similar effects for castings are used. Also, castings produced in molds containing 4.5 pct moisture were more sensitive to small variations in melt hydrogen content than those produced FIG 15-CO~ARISON Hydrogen Content,ports per million OF CURVES OF FIGURES 10 THROUGH 14. made in molds having 4.5 pct moisture. In Fig 15, the curves in the foregoing figures are superimposed. These figures show that porosity could be developed by increasing either the pouring temperature or the melt hydrogen content. The slopes and shapes of the lines separating the conditions under which sound castings result from those for porous castings indicate the relative importance of temperature and hydrogen content. The effect of other variables (for example, mold moisture or selenium additions, shown in Fig 15) are indicated by the displacement of the lines from those used for comparison. The effect of increasing the mold moisture from 3 to 4.5 pct in the absence of selenium, for example, was to narrow the area that is termed "sound castings." This means that the melt hydrogen content or the pouring temperature must be lower when higher mold moisture contents in molds containing 3 pct moisture. This is shown by the difference in the slope of the lines. This interrelated effect of mold moisture content, melt hydrogen content, and pouring temperature is especially significant, because the two curves surround an area that encompasses a range of hydrogen content and pouring' temperature that is expected in commercial practice. Therefore, it is evident that close control of these variables within the proper limits may be required for the commercial production of sound castings. The beneficial effect of selenium at both the 3 and 4.5 pct mold moisture conditions is shown in the plot. The effect of selenium was to shift the position of the dividing lines to the right so that more melt hydrogen can be tolerated and higher pouring temperatures may be used. The amolint of shift produced by anaddition of 0.02 pct seleniuni was much greater at the 4.5 pct mold moisture level than

11 I FOUNDRY PRACTICES 147 at the 3 pct level. This can be illustrated the detrimental effect of the increased by comparing the curves at 2g500F. For mold moisture. the 3 pct mold moisture, zero selenium condition, the boundary between porous Efect of Mold Permeability on the Occzdrrence and sound conditions occurs at about of Pinhole Porosity 8 ppm hydrogen. When 0.02 pct selenium The results obtained during the experiis added, the division line is shifted to ments concerned with the plotting of the TABLE I-Hydrogen Content of Samples Taken From Arc-Melted Heatsa I CP-8 Alloy 1 CP-8M Alloy HP Alloy ( HT Alloy Sample 1 ~ornpany I 1 Heat Heat Heat Heat Heat I Meltdown Before oxygen blow... After oxygen blow b After chromium addition. After refining before tap Bull ladle... Bull reladle... First shank ladle: Last shank ladle Mold riser company 2 company 3 1 company 4 company 5 ~ Heat Heat b a Hydrogen content reported in parts per million. a Also carbon boil Heat Heat I Heat I about 10 to 11 ppm hydrogen. On the other hand, for the 4.5 pct mold moisture, zero selenium condition, the boundary line at 2850 F is at 6 ppm hydrogen. The addition of 0.02 pct selenium under these conditions causes a shift of the boundary line to the 10 to 11 ppm hydrogen range. The shift for the 4.5 pct mold moisture condition is nearly twice that for the 3 pct mold moisture condition when compared at 2850 F. The spread over the entire temperature range is indicated by the area bounded by the two curves. The effect of changing the mold moisture from 3 to 4.5 pct was not apparent when a 0.02 pct selenium addition was used. That is to say, the curves under these conditions were nearly identical. This indicates that the detrimental effect expected from the higher mold moisture content was not observed. Evidently, the beneficial effect of the selenium addition overshadowed curves for melt hydrogen versus pouring temperature indicated that mold permeability might be a variable to consider in the porosity studies. Accordingly, experiments were performed in which mold permeability was varied. Castings were made in molds of which the permeabilities were in the 80 to go, 120 to 130, and 160 to 170 ranges. Permeability was controlled by first determining a hardness versus permeability curve for the sand mixture and then squeezing the mold to the hardness necessary to give the desired permeability, as indicated by the hardness. Pouring temperatures were selected on the basis of the previous work so that the dividing line between porous and nonporous conditions would be traversed. No clear-cut effect of permeability was observed either when the mold moisture content was 3 pct or when it was 4.5 pct. It is surmised that considerably lower

12 ~ermeabilities would have been required for this variable to influence the incidence of pinhole porosity. FIELD EXPERIENCE To evaluate the laboratory investigation of pinhole porosity in terms of commercial Alloy Type TABLE 2-Hydrogen Sample Tap ladle Meltdown Ladle Meltdown Ladle Furnace before tap Meltdown Meltdown difficulty would be expected, while at the upper end, troubles from pinhole porosity would be likely. Where applicable data from the commercial heats were available they demonstrated the ability of the oxygen.blow to reduce melt hydrogen content. In most Content of Samples Taken From Induction Heatsa Company A I I I I I I I I I I foundry practice and experience, field experiments were carried out to gain information on the range of meltdown hydrogen contents to be expected and the influence of foundry operations on melt hydrogen content. Such information could very well aid in translating the laboratory findings to the commercial foundry and thus improve the facility with which the foundry can control porosity. Toward these objectives, several Alloy Casting Institute members provided samples from various heats for hydrogen analysis at Battelle. The samples were taken from arc-melted heats of CF-8, CF-8M, HE, and HT alloys and from induction melted HE, HH, HK, HU, HT, CF-8, and CF-8M alloys. Samples from the arc-melted heats were taken at various stages during the melting campaign. The results are given in Table I. The results show that the meltdown hydrogen contents varied from 3.2 to 9.9 ppm. This is a very considerable spread and spans most of the range of melt hydrogen contents within which the laboratory work was done. The laboratory results suggest that, at the lower end of this spread, little Company B a Hydrogen content reported in parts per million. Average of four samples. cases, the extent of reduction was dramatic. The oxygen blow certainly seems to be an effective means for obtaining a low hydrogen content during at least one period in the making of a heat of stainless-type alloy. In general, however, subsequent refining and ladling increased the hydrogen content over that prevailing after the blow. These observations emphasize strongly the liecessity of avoiding contact with moisture, especially after the oxygen blow. Chromium and other alloy additions should be thoroughly dry. Likewise, slag-making materials should be completely free from moisture. Ladles are of equal importance; they are likely to contribute significant amounts of hydrogen unless preheated above 2ooo F for several hours. An interesting point was brought out by analysis of mold riser samples taken before the metal froze. Whenever this was done the hydrogen content was considerably higher than that in the metal before pouring. This adds evidence to the theory that the mold moisture contributes hydrogen to the molten metal before solidification. Some samples for hydrogen analysis were submitted by several companies from in- - 1 I

13 FOUNDRY PRACTICES I49 duction heats of various alloys. Table 2 presents the results of the hydrogen determinations made on these samples. No conclusions can be drawn from these results except that they probably indicate the levels of hydrogen to be expected in normal induction practice. The results of this investigation show that the important foundry variables influencing pinhole porosity in stainless-type castings are melt hydrogen content, pouring temperature, and mold moisture content. Careful control of these major variables will minimize the occurrence of pinhole porosity in castings under normal conditions. The curves that show the, relationship between pouring temperature and hydrogen content of the melt at several selenium and mold moisture contents can constitute a useful guide for improvicg foundry practice. These were obtained for the CF-8 alloy and may apply only in a very general way to other high-alloy grades. The position of the lines of demarcation between porous and sound casting coilditions were established as a result of numerous experiments made over a period of about 1% years. Thus, they can be considered to have some quantitative meaning. and might be used to select conditions for sound castings within the limits set by the experiments. For example, the interrelated effects of melt hydrogen and pouring temperature are shown for castings made in molds containing either 3 or 4.5 pct moisture. If the hydrogen content is known, the curves will indicate the maximum pouring temperature that should result in sound castings when poured in molds in the indicated range of water content. At other mold moisture levels, the curves would no longer apply. Within reasonable limits, perhaps some extrapolation could be made. Beyond such limits, new curves would have to be prepared from results of additional experiments. This type of plot indicates what control is necessary of both melt hydrogen content and mold moisture to achieve sound castings. Also, it points the direction for overcoming the harmful effect of one variable by compensating with a change in the other variable. Thus, if weather conditions (i.e., humidity) should prevent the control of melt hydrogen within a desired low range, it may be possible to minimize porosity by reducing the mold moisture, or by pouring at a lower temperature if practicable. Another useful application of the curves is the evaluation.of the effect of changes in practice. For example, the curves show that the addition of 0.02 pct selenium permits more hydrogen in the melt and higher pouring temperatures before conditions resulting in porous castings are reached. Similarly, the detrimental effect from high moisture content of the molds, within the range investigated, can be compensated by the selenium addition. In the absence of selenium, the moisture content of the mold becomes more important. This paper is based on results of an investigation that was carried out at Battelle Memorial Institute under the sponsorship of the Alloy Casting Institute. The authors wish to express their gratitude to the Alloy Casting Institute for permission to publish this material. Wilcox, R. J.: Induction Melting of Stainless Steels. Elec. Fur. Steel Proc., AIME (1950) 8, Sims, C. E., and C. A. Zapffe: Mechanism of Pin-Hole Formation. Trans.Am. Found- ' rymen's Assn. (1941) 49, Zapffe. C. A,, and C. E. Si~ms: Hydrogen and Nitrogen as Causes of Gassiness in Ferrous Castings. Trans. Am. Foundrymen's Assn. (1944) 51, 40. Perkins. R. A., and W. 0. Binder: Deoxidation and Hot Ductility of Type 310 Stainless Steel. Elec. Fur. Steel Proc., AIME (1956) 14, Carter, S. F.: Effect of Melting Practice on Hydrogen. Elec. Fur. Steel Proc., AIME (1949) 7,

14 DISCUSSION A. F. GROSS, CHAIRMAN-Thank YOU, Mr. Hall. This has been a very important work from the standpoint of the high-alloy producers. We have a few minutes here, are there any questions on the paper? M. J. BORIS-I should like to know whether ferroselenium works on carbon steels in the same way it does on stainless steels. A. M. HALL-I do not know. However, I would suppose that it would, especiallv if the heat were fairly well deoxidized before the addition was made. Selenium is added to low-alloy steels to improve machinability and one might inquire what the experience has been on the freemachining grades of steel in comparison with the non-free-machining grades that otherwise are made in the same way. M. J. BORIS-Have you tried al;y.other techniques that have been successful? A. M. HALL-Yes, a technique that has been used in the induction furnace is to melt down low-chromium scrap or virgin metal with iron oxide and use a carbon boil before adding chromium; that is, before bringing the chromium up to the desired level. This has merit in driving hydrogen out of the bath. A. F. GROSS, CHAIRMAN-111 Our own foundry, we had some success in using a new alloy developed by Electro Metallurgical Corporation, which is a magnesium-calcium-silicon alloy. It has prob-, ably worked about the same as selenium in limited trials. M. J. BORIS-Does high nitrogen content affect the effectiveness of the selenium addition? A. M. H~LL-when the nitrogen level in stainless steels is extremely high, nitrogen will, contribute to porosity. But the level has to be far beyond that encountered in normal practice. For an 18-8 type steel, it has been necessary to raise the nitrogen content to over 0.25 pct before a contribution to porosity was made. The reason probably is tied in with the chromium content of the steel, the chromium limiting the activity of the nitrogen almost as though it combined with the nitrogen. This is a left-handed answer to your question. In other words, the question generally does not come up. A. F. GROSS, CHAIRMAN-T~~~~ YOU, Mr. Hall. During the past few years we have stressed nitrogen and then, hydrogen, and now it looks as though oxygen were becoming the most important, so possibly more and more papers will pesent figures on oxygen, as some of the papers did this morning. The next paper is by Mr. J. T. Gow, Vice-President in Charge of Manufacturing and Engineering, and Mr. C. F. Hamilton, Production Control Metallurgist; Sandusky Foundry and Machine Co., :Sandusky, Ohio. The paper will be presented by Mr. Hamilton. Frank Hamilton obtained - his B.S. Metallurgical Engineering from Case Institute of Technology and has been associated with Sandusky Foundry and Machine Co. during the last four years. His present position is Manager of Production Control. Mr. Hamilton.