Nitrogen in Ductile Iron 2002

presented at The 2002 Ductile Iron Society Annual Meeting
Pioneer Resort & Marina; Oshkosh, Wisconsin
June 13, 2002

ABSTRACT
This is an update of the information provided in the AFS Transactions 79-58 "Nitrogen Levels in Ductile Iron: AFS Committee 12-H Report". 

New information is provided on the effect of magnesium concentrations in treatment alloys, desulfurization, electric induction furnace frequency and cupola melting of base metal. Comments are offered on possible reactions between nitrogen and titanium, aluminum, magnesium and boron.

INTRODUCTION
Information on the nitrogen content of ductile iron has been accumulated over the past few years while working with various foundries. This paper will summarize some of this data.

The LECO TC 436 inert gas fusion analyzer was used to perform all the analyses reported. This inert gas fusion analyzer is both fast and accurate. All the nitrogen in each sample is released as molecular nitrogen and its thermal conductivity measured. The instrument is programmed to fuse a weighed sample under a helium atmosphere at a temperature of up to 3000°C. This analytical method measures total nitrogen in the sample. Previous nitrogen contents were done using the Kjeldahl method for soluble nitrogen and the Beeghly method for insoluble nitrogen. Wet chemical analysis is almost a lost art anymore and these methods have given way to procedures dominated by faster equipment. Machines and technology have replaced experience, technique and operator skill.

The samples utilized for analysis were spectrographic chilled discs or pins. They were broken up to produce the required sample weight. In some cases three samples were run and the results were almost identical. In other cases, one number would occur that was much higher or lower. This was probably due to the presence of segregated nitrides. Average results reported are probably plus or minus 5 ppm of the amount present.

Two foundries sampled were melting ductile base in cupolas. Both cupolas were acid lined and the charge contained ductile returns. Foundry "A" desulfurized the iron from 0.08-0.09% sulfur to 0.010% sulfur or less using a porous plug vessel patterned after the GM process developed at Defiance, OH. In this process nitrogen gas was injected through four porous plugs in the bottom of the ladle. The dwell time in the continuous treatment ladle was 10 minutes on average. The metal temperature was over 2700°F. Fine calcined lime and spar were fed into the ladle continuously. The nitrogen content of the cupola metal fell from 100 ppm to 82 ppm after treatment. The iron was then put in a holding furnace where no further drop in nitrogen was observed. The magnesium treatment in an open ladle using a 5% alloy resulted in a final nitrogen of 72 ppm.

Cupola "B" produced a base metal of 82 ppm nitrogen with a sulfur of 0.07-0.08%. The nitrogen content in the holder further dropped to 65 ppm. The metal was treated with 5% magnesium wire to desulfurize and nodulize. This was an excessive use of wire. The final nitrogen level was 52 ppm. When electric induction melted base iron with a 0.015% sulfur and a nitrogen of 56 ppm was treated with 5% wire the nitrogen content of the treated metal was 58 ppm. The amount of wire consumed was about half that consumed by the cupola foundry.

Five foundries that melted base iron in medium frequency induction furnaces reported nitrogen contents in the 41-56 ppm range with the average in the mid to high 40's. After treatment with 5% MgFeSi the nitrogen contents were about the same or slightly higher, averaging in the low 50's. The treatment was done in either open or tundish ladles. One foundry treated in an open ladle with 3% alloy and the nitrogen dropped from 48 ppm to 44 ppm. This was a very quiet reaction.

Another foundry melted a base iron with a 41 ppm nitrogen and reported a 74 ppm after treatment using pure magnesium and the Fisher process.

Several foundries using line frequency induction furnaces reported nitrogen levels of 57, 56 and 71 ppm. The nitrogen levels held about the same after treatment.

The nitrogen content of Brazilian nodular pig iron for three different lots was reported as 14, 20 and 30 ppm.

DISCUSSION
W.J. Evans, J.C. Harkness and J.F. Wallace reported in their paper "Factors Influencing the Formation of Pin Holes in Gray and Ductile Iron", DIS Project No. 6 (April, 1974) an estimated solubility of nitrogen, in a ductile iron of 3.80% carbon and 2.50% silicon, to be 72 ppm. The data reported in this study showed that when the nitrogen content was below the equilibrium level of 72 ppm in the base metal the content tended to increase. When the levels were higher they fell toward the equilibrium point. The exceptions were 1) the excessive wire treatment that may have created a high nitride level that floated out and, 2) the magnesium treatment using 3% MgFeSi.

The use of the equilibrium value of 72 ppm for total nitrogen makes a nice package of all the data. Most of the gas content levels, changes and results can be related to this value. It has always been taken as faith in ductile iron production that nitrogen content will approach this value due to the flushing effect of the magnesium.

The increases reported for the low nitrogen base metals are thought to be due to pick-up from the air during treatment as the gas content moved towards equilibrium. The more violent reactions picked up more gas. Whatever the mechanism, the total nitrogen, soluble and insoluble, moves toward the equilibrium value.

S. Morita and N. Inoyama reported that ductile irons of identical composition were carbide free and highly ferritic at 25 ppm soluble nitrogen and at 37 ppm the iron was carbidic. Further, they reported that graphite nodules could not be formed and that flake graphite occurred at 60 ppm soluble or mobile nitrogen.

Titanium and vanadium, present in some nodular pig iron, can practically eliminate soluble nitrogen. Morita and Inoyama report that, regardless of the nitrogen content of the base iron, both soluble and insoluble nitrogen are nearly constant at 50 to 60 ppm, somewhat lower than the Evans number. This is verified in the data presented for the medium frequency electric furnace induction melted iron.

The reported nitrogen content of gray iron is probably mostly soluble nitrogen as it is known that if certain levels are exceeded in the absence of titanium and zirconium, porosity will be present in the castings. 

Ductile iron, however, is usually produced with about 50% returns. The magnesium in the returns can form MgSiN2 nitrides as reported by H. Wada and R.D. Pehlke. The tendency to react with nitrogen is higher for magnesium than it is for titanium, aluminum or boron. This may account for the softening of flake graphite irons when ductile iron returns are included in the charge. Nitrogen is an important alloying agent in gray iron. The increase in graphitic carbon could also be a factor.

In "The Physical Metallurgy of Cast Iron", I. Minkoff reports the following: "The sensitivity of gray, ductile and malleable cast iron to the melting process lies in the influence of small concentrations of impurities on graphite nucleation and subsequent growth. In spheroidal graphite cast iron, after reactive elements, in particular magnesium, have combined with some of the impurities including sulfur, the remaining impurity elements interact with graphite during the solidification process. The concentration of the impurities increases in the liquid due to segregation. Dissolved gases play a role in this process and composition control therefore relates to gases as well as all other elements."

Elements such as magnesium, aluminum, cerium, titanium, boron, bismuth, lanthanum, etc. can form a variety of compounds such as oxides, nitrides, sulfides and carbides. The presence or absence of these compounds and their composition can affect the graphite morphology. The effect of oxygen content on nucleation is beginning to be acknowledged. Nitrogen may have an important if dissimilar role in the process.

BORON
Where does it come from, what is its effect and how can its effect be controlled?

Boron, when added to a well deoxidized steel, will act as a hardenability intensifier in hypoeutectoid steels (less than 0.77% carbon). Boron oxide and boron nitride are totally ineffective, however, in producing any degree of hardenability. The boron present that is free of oxygen and nitrogen reduces ferrite nucleation, slows the diffusivity of carbon, reduces the number of nucleation sites and can precipitate as a complex carbide (this is why boron is considered a strong carbide former in gray iron). Boron steels are favored because they are inexpensive to produce.

Another grade of boron-containing steel used primarily by the auto industry is interstitial free (I-F) ultra low carbon sheet steel. This is a superior grade for hot dip galvanizing. This grade has recently seen greatly increased usage. The boron is added to this grade of steel to tie up the nitrogen.

Boron treated steels are also utilized for wire drawing. For these steels the formation of boron nitride is favored because of its small inclusion size and interstitial dispersion within the austenite matrix.

Note that controlling the formation of the boron compound produces very different types of steel. Adding boron as ferroboron results in the formation of boron nitride. Adding an alloy of titanium, aluminum, zirconium and boron produces boron carbide.

"Russian Castings Production", 1975, contains an article describing the addition of boron and nitrogen to high strength iron. Greatly decreased hardness and chill were observed. Either element added separately had the opposite effect. The boron nitride formed was thought to act as a graphite nucleus.

Boron is present in a wide variety of steels. Boron has also been reported in some pig irons. When both items are added to the charge and the recycling of the returns are considered, the boron content may increase to the point that the production of pearlitic grades of ductile iron is hindered.

In ductile iron, boron will combine with soluble nitrogen to form boron nitride. Boron nitride is considered to act as a nucleus for graphite formation. Titanium, as indicated in the example for steel, is a stronger nitride former than boron, consequently, if the magnesium and titanium are present in sufficient quantities to convert all the nitrogen to nitrides, the boron will not form boron nitride. The boron thus will not promote the formation of graphite but it could, however, promote carbides. If there is soluble nitrogen available, boron nitride will form, promoting graphite nucleation, excessive graphitic carbon and a ferritic matrix. To counter the presence of boron in the base metal, additional titanium should be added to remove any soluble nitrogen.

CONCLUSIONS
Ductile base iron melted in medium frequency electric induction furnaces with graphite recarburizers and approximately 50% ductile iron returns with the balance steel or steel and pig iron, will produce a nitrogen content in the 41-57 ppm range. The average was in the high 40's.

Line frequency electric furnace melting of ductile base appears to produce an iron with a nitrogen content of about 55 ppm.

Treatment with 5% MgFeSi tends to raise the nitrogen content of irons when initial concentrations are in the 40-50 ppm range. 3% MgFeSi treatment alloys do not raise the nitrogen levels even when the base iron is very low.

Cupola melted base iron, when large amounts of ductile iron returns are utilized, produce nitrogen levels of 80-100 ppm.

Treating the cupola iron in a desulfurizing vessel using lime spar results in a 20 ppm drop in nitrogen content. 

The nitrogen content of the base iron in the holding furnace was observed to drop when the cupola iron went directly to the holder. This was probably due to nitrides floating out. When the iron went through the desulfurization vessel the nitrides present were flushed out and no further loss was experienced in the holder.

No meaningful difference in nitrogen content could be detected between open and tundish ladles.

It is proposed that if the formation of boron nitrides can be prevented, boron should not hinder the ability to produce pearlitic ductile iron that meets the required mechanical properties. The addition of titanium to prevent the formation of boron nitrides is suggested.

It is concluded that most of the nitrogen present in ductile iron is in the form of insoluble nitrides. The total nitrogen reported is related to the equilibrium value of 72 ppm, as stated by Evans, Harkness and Wallace. The nitrogen content either increases or decreases depending on its relationship to the equilibrium value.

The 50-60 ppm nitrogen value reported by Morita and Inoyama was consistent for electric induction melted ductile iron.