Nitrogen in cast iron

How much nitrogen

Source of nitrogen

compounds.Nitrogen can enter the ironnitrogen content resins.

Metallurgical effects

Compacted graphite in 12 in dia. bar containing 0.015%nitrogen. Etched 4% picrai. x 60

I when the moulds and/or cores contain high

These are seldom obvious except in very heavy sections.ns by nitrogen contents

increase in tensile strengthingot moulds as it lowers

can result from the increase in nitrogenSome of the advantages

cast irons arise from this effect.

and may lead to white ironcarbide and pearlite

~

high nitrogen content

Nitrogen pinholes «used by high-nitrogen resin-bonded cores. The defects occur wherethe core forming the hub of the casting meets the greensand mould

Excessive nitrogen contents appear as blowholes and fissures In the castings. It isu~ommon for nitrogen blowholes to occur below about 0.013 per cent nitrogen in lightsection castings and 0.008 per cent in heavy castings. These defects most frequentlyarise In thick section castings made from cupola iron of low carbon equivalent preparedfrom high steel charges.Nitrogen fissure defects occur adjacent to cores bonded with high nitrogen contentresins. These often appear as bubbles or fissures close to a re-entrant angle in acasting. Blowholes or fissures due to nitrogen may appear to have bright surfaces orcontain a continuous layer of graphite. However, If the defects have been exposed to theair, or the casting has been heat treated, they may have dull oxidlsed surfaces.

Effect on soundness

How to avoid the effects of high nitrogen contentThe effect of nitrogen can be neutralised by the addition of titanium to give a contentof 0.02-0.03 per cent in the iron before casting. Although additions of aluminium (togive a content in the iron of 0.02-0.04 per cent) are used to neutrallse nitrogen in theproduction of large ingot moulds and heavy castings of similar application, aluminiumshould never be added to iron for light section and general engineering castingsbecause it promotes hydrogen pinholes in such castings (see BCIRA Broadsheet 7).Carburiser materials having a high nitrogen content must be avoided.When defects are associated with moulds and cores bonded with high nitrogen contentresins, a lower nitrogen content resin should be substituted. Resins having below 3per cent nitrogen are unlikely to give trouble but sometimes it may be necessary tochange to a nitrogen-free resin.

Recommended further readingDAWSON (J.V.), SMITH (LW.L.) and BACH (B.B.)Some effects of nitrogen in cast Iron. BCIRA Journal of Research and Development. 1953, v. 4, June,pp. 540-552.MOUNTFORD (F.A.)The Influence of nitrogen on the strength, soundness and structure of grey cast iron. British Foundryman,1966. v. 59, April. pp. 141-151. External report 504.

Copies of any BCIRA Broadsheet can be obtained free of charge by BCIRA membersfrom British Cast Iron Research Association, Alvechurch, Birmingham B48 70B.

@ 1976 British Cast Iron Research Association, A/vechurch, Birmingham, B48 70B.

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Nitrogen in cast iron

How much nitrogen dissolves in castiron?Up to about 0-04 per cent nitrogen dissolves in cast iron,depending on the composition of the iron- A sharp drop insolubility occurs when the iron solidifies, accounting for thetendency of this element to cause blow-holes or fissures. Inpractice, the nitrogen content of castings cast in sandmoulds can vary between about 0 - 001 per cent and O. 015per cent, but castings are likely to be unsound with nitrogencontents above 0-010 per cent.

Sources of nitro~enSome nitrogen in cast iron is introduced by the chargematerials, but more commonly it is absorbed into the liquidiron together with carbon from the coke in the cupolafurnace. High nitrogen contents are most likely to occur incupola-melted irons using high-steel charges. When steelscrap charges are carburized in electric furnaces, highnitrogen contents can arise from the use of carburizingmaterials such as petroleum coke .that contain nitrogenouscompounds.Nitrogen can enter the iron during casting, when themoulds, cores or both are bonded with high-nitrogen-content resins.

Metallurgical effectsThese are seldom obvious except in very heavy sections.

A compacted form of graphite is produced in heavy sectionsby nitrogen contents above about 0.008 per cent. Compacted

Fig. 1 Compacted graphite in 30 cm bar0.015% nitrogen. Etched in 4% picral.

,

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graphite causes an increase in tensile strength but thisgraphite structure is undesirable in some designs of ingotmoulds as it lowers the thermal shock resistance.An increase in tensile strength of 30 - 45 N/mm2 can resultfrom the increase in nitrogen content arising from the use ofhigh-steel charges in the cupola. Some of the advantagesclaimed for the use of high-steel charges for high-duty castirons arise from this effect.

High nitrogen contents promote pearlite formation and maylead to white-iron structures.

In malleable irons, high nitrogen contents stabilize carbideand pearlite and hinder annealing, but problems of this typeare uncommon.

Nitrogen has no significant effect in ductile irons.

Effect on soundnessExcessive nitrogen contents result in blow-holes andfissures in the castings. It is uncommon for nitrogen blow-holes to occur below about 0.013 per cent nitrogen in light-section castings and O. 008 per cent in heavy castings.These defects most frequently arise in thick-section castingsmade from cupola iron of low carbon equivalent preparedfrom high steel charges, or electric-melted irons carburizedwith materials such as petroleum coke.

containingx60

Fig. 3 Nitrogen pinholes caused byhigh-nitrogen resin-bondedcores. The defects occur wherethe core forming the hub of thecasting meets the greensandmould.

Nitrogen fissure defects occur next to cores bonded withhigh-nitrogen-content resins. These often appear as bubblesor fissures close to a re-entrant angle in a casting.

Blow-holes or fissures due to nitrogen may have brightsurfaces or contain a discontinuous layer of graphite.However, if the defects have been exposed to the air, or thecasting has been heat treated, they may have dull oxidizedsurfaces.

How to avoid the effects of high

nitrogen contentThe effect of nitrogen can be neutralized by the addition ofenough titanium to give a content of 0.02-0.03 per cent inthe iron before casting. Although additions of aluminium(to give a content in the iron of 0.02-0.04 per cent) areused to neutralize nitrogen in the production of large ingotmoulds and heavy castings for similar applications,aluminium should never be added to iron for light-section andgeneral engineering castings because it promotes hydrogenpinholes in such castings (see BCIRA Broadsheet 7*).

Large amounts of carburizer materials having a highnitrogen content must be avoided.

When defects are associated with moulds and cores bondedwith high-nitrogen-content resins, a resin with less nitrogenshould be substituted. Resins with less than 3 per centnitrogen are unlikely to give trouble, but sometimes it maybe necessary to change to a nitrogen-free resin.

free copies of any BCIRA Broadsheet from BCIRA, Alvechurch, Birmingham B48 7aB@ 1989 BCIRA

Members can obtain

RECOMMENDED FURTHER READINGBClRA Broadsheet 7*: Pinholes formed by hydrogen gas duringsolidification of iron castings.BClRA Broadsheet 49*: Holes in castings caused by high-nitro-gen-content resin-binder systems.BClRA Broadsheet 165: Changes in the nitrogen content of castiron during melting and holding in a coreless induction furnace.BClRA Broadsheet 218: The determination of nitrogen inchemically bonded sands.BClRA Broadsheet 294:DAWSON, J. V., SMITH, L. Wnitrogen in cast iron. BCIRA

in ctst irons.&; BACH, B. B. Some effects of

355.strength,

Foundrymsn,

the complementary effects of

MOUNTFORD, F. A. The influence of nitrogen on thesoundness and structure of grey cast iron. BritishApril 1966, Vol. 59, 141-151.SIMMONS, W. Experiments intohydrogen and nitrogen in the fonnation of gas holes in ironcastings. BCIRA Repon 1122. t

SMITH, L. W. L. Detennination of the nitrogen content of carbonmaterials. BCIRA Repon 1177. t

GREENHILL, J. M. & REYNOLDS, N. M. Nitrogen defects in ironcastings. BCIRA Repon 1189.tEVANs, E. R. Nitrogen in cast irons-and its neutralization byaluminium. BCIRA Repon 1260.tThe effects of alloying elements in cast iron-5: nitrogen. BCIRARepon 1299.tFALLON, M. J. Nitrogen pinhole defects in manensitic white ironcastings. BCIRA Repon 1635. t

DAWSON, J. V. The influence of nitrogen on the tensile propertiesof blackhean malleable cast irons. BCIRA Repon 1674. t

DAWSON, J. V. The litrogen on the structure andproperties ofwhitehc iron. BCIRA Repon 1709.t

tAvailable only to BCIRA Members.