Gray iron conference highlights metal's staying power.
The give and take of academics, researh and the hard knocks of practical application were very much in evidence during the American Founrymen's Society's Elemental Effects on Gray Iron conference held in September at Chicago's Hotel Sofitel. It was evident from the beginning of the two-day event that there is keen interest in the subtleties of the gray iron casters' art and the cold realities of its marketplace.
Solid subject presentations were folowed by lively exchanges and questions from the floor debating and testing theory with actual production experiences. For the 82 foundrymen and women who attended, the wealth and variety of information, much of it the product of extensive and recent research, provided new perspectives on old and some new technologies aimed at solving metallurgical problems in gray iron casting.
The roster of presenters assembled by Thomas D. Jennings, AFS technical director, included: Dr. Charles V. White, chairman of the AFS Gray Iron Research Committee; Dr. Charles Bates, head of the metals section, Southern Research Institute; John Wallace, professor, Case Western Reserve University; Dr. Carl R. Loper, Jr., professor, metallurgical engineering, University of Wisconsin; Bela Kovacs, vice presient, AFC Technical Center; William Henning, director, technical services, Miller & Co; John Greenhill, deputy manager, technical services group, BCIRA, Birmingham, England; and Richard Gundlach, metallurgical engineer, Climax Research Services.
In order to make the whole range of material covered by the presenters more accessible to the metalcasting industry, the proceedings of the conference were video-tapep. This is the first time such a conference has been video-taped, Jennings said, adding that an edited version of the tape should be available for purchase or rental within a few weeks.
Alvin W. Singleton, president and chief operating officer of the Lynchburg Foundry Co/Intermet Corp and AFS president, in his dinner address called attention to the staying power of gray iron castings despite the competition from highly-touted ductile and specialty irons like austempered, compacted graphite and silicon moly formuations.
He cited gray iron's cost-effective ration of machinability to strength, fast machinability compared to most other irons and its broad range of strength levels, ranging from Class 20 through Class 50, as significant factors in the relative stability of gray iron production. He also noted that gray iron has superior castability with fewer shrinkage and soundness problems than other materials, calling attention to its heat conductivity and wear resistance. Though admitting it has lost some market share in recent years, Singleton reported that gray iron production this year still will be close to seven million tons, almost twice that of ductile iron.
Conferees again were told that gray iron is not a simple material, notwithstanding its centuries of use. Many elements act and interact to influence its mechanical properties and casting qualities. While much is known, there is still more to learn about this old material.
Singleton urged that efforts to improve quality and cost-effectiveness remain priorities, its structure be made more consistent to avoid variations that impair its mechanical to meet the need to control metallurgists to meet the need to control the material's mechanical properties. He concluded by saying that he doesn't see the day when gray iron will cease to be a vital part of the foundry industry.
The thrust of the conference was summed up in opening remarks by Gundlach when he said that gray iron technology has advanced greatly in the last half century, allowing it to remain competitive for a broad range of structural components. Gray cast iron, he said, provides the technical advantages of high strength, soundness, good work-ability, dimensional stability and uniformity of properties usually required of cast products. Achieving these characteristics is dependent on metallurgical engineers working to achieve proper and uniform structures, producing high-strength gray irons that are easy to make and machine.
Conference subjects dealth with indepth included:
* elemental effects on mechanical properties in gray iron;
* effects of various gas levels on gray iron;
* controlling carbon, silicon and carbon equivalency to gray iron product quality;
* the effect of pearlitic forming elements (copper, tin, nickel);
* limits of sulfur, manganese and phosphorus in gray iron;
* minor elements of major importance;
* molybdenum, vanadium and chromium limitations for iron.
The amount and variety of technical information presented at the conference is a clear testament to the continued importance to gray iron in the castings marketplace.
Some Conference Highlights
John Greenhill--(On dealing with the adverse effects of certain elements) The presence of aluminum in gray iron promotes moisture pickup of hydrogen from the mold environment, or in the melting or handling of refractories. On solidification ... hydrogen will either be thrown out forming graphite-lined pinholes adjacent to the casting surface or as larger fissures.
The degree of pinholing is generally related to the amount of aluminum in the iron ... aluminum above 0.003% is potentially hazardous. Of greater concern are concentrations of 0.002-0.005% ... resulting in isolated pinholes ... revealed at machining and certainly resulting in casting rejection. Achieving lower aluminum levels calls for extreme care over the quality of steel and cast iron scrap, ferrosilicon and inoculants.
The level of nitrogen varies between 0.004-0.014% ... in cupola melting, the higher the steel content, the higher the final nitrogen level. Nitrogen can have beneficial effects on mechanical properties, since increases shorten and compact the graphite flake form, increasing tensile properties and promoting the formation of pearlite.
Titanium and aluminum affect nitrogen which combines with them to form nitrides. Lower strength irons almost invariably will have higher titanium and aluminum contents which neutralize the nitrogen, thus, reducing its effectiveness. Nitrogen is an important alloying element, and any changes which affect it due to melting practice or charge material quality, can have significant effect on mechanical and machinability properties.
The presence of lead in gray cast irons ... seriously reduces its mechanical properties, often causing catastrophic casting failure. The most striking effect of lead in concentrations of 0.0004% and above on gray iron is on the graphite formation, but lead alone does not cause abnormal structures. Hydrogen must also be present. Molding material has a critical role, green sand molds tending to give rise to more hydrogen pickup than chemically-bonded molds.
Aluminum will also aggravate hydrogen pickup, causing irons with only marginal amounts of lead contamination to be more prone to form abnormal graphite. Lead also affects matrix structure, suppressing ferrite and promoting formation of pearlite. Increased lead content often has significant effect on hardness and machinability, and increasing failure rates in gray iron castings.
William A. Henning--(On the limits of sulfur, manganese and phosphorus in gray iron) Once regarded as impurities in gray iron, manganese and sulfur are now considered absolutely essential elements, as is phosphorus in certain applications. For a long time, founders were told to avoid sulfur; now it is known that it should be used with manganese in order to form manganese sulfide rather than iron sulfide which with its low melting point migrates to the eutectic cell boundaries. Manganese within normal ranges in cast iron will exist as manganese sulfide and prevent the embrittlement that accompanies the presence of iron sulfide in the grain boundaries. Manganese increases hardenability and minimizes growth when present at levels between 0.5-1.6%.
Commercial gray irons are always produced with a combination of manganese and sulfur, but the optimum manganese excess in relation to sulfur will vary somewhat according to residual alloy element levels. Base manganese in excess of sulfur, falling somewhere between 0.2-0.5%, enhances both tensile strength and hardness. Sulfur and manganese are more than just impurities in gray cast iron. The presence and control of each is absolutely essential in the production of desirable microstructures, optimum strengths and freedom from defects.
Phosphorus, in excess of the 0.02% that is soluble in austenite, exists in gray iron as a binary eutectic of iron and iron phosphide called steadite. The chemical composition of this eutectic is 10.2% phosphorus and 89.8% iron. Steadite can be observed at phosphorus level as low as 0.06%, and is a white, nonetching phase with a Brinell hardness of about 400.
In the presence of chrome, molybdenum or vanadium, phosphide eutectic may occur in ternary form as a eutectic of ferrite, iron carbide or iron phosphide, but the presence of the free carbide makes this form of iron much harder, causing markedly increased tool wear in machining operations.
For many years, phosphorus was viewed as essential in cast iron because of its positive effect on fluidity, but since reported as only a half to a third as effective as carbon in this respect. More recently, researchers have concluded that only the best foundry irons contained about 1% phosphorus with high-duty irons kept below 0.20%.
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|Title Annotation:||Elemental Effects on Gray Iron conference|
|Date:||Nov 1, 1989|
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