Introduction

 

Recently Horwath and Goodrich [1] developed a micro-inclusion classification in steel castings as part of an overall project to develop techniques for producing and delivering consistently clean steel to the casting cavity. As part of this project an initial inclusion atlas was developed and Horwath and Goodrich recommended that this work should be continued with a second project to classify macro-inclusions in a similar manner. Although an earlier program by Svoboda et al. [2], funded by the Department of Commerce, recognized that, at that time, 80% of the macro-inclusions that troubled the foundry industry were due to reoxidation, it was clear that in order to eliminate all macro-inclusions it would be necessary to not only develop techniques to reduce reoxidation but also to eliminate other sources of contamination that could result in macroinclusions.

As a result of Horwath's suggestion a project was initiated by the AFS and the DOE at Carnegie Mellon University to characterize the macro-inclusions found in foundry cast steels, to produce of a atlas of the micro and macro-inclusions that are found in foundry processed steels and to suggest procedures to eliminate identified macro-inclusions. In addition it was decided that a web based homepage would be developed to allow easy access to the atlas.

The presence of micro-inclusions in castings is unavoidable as they are the natural inclusions which are formed in liquid steels due to reaction between alloying elements and oxygen; however, it is necessary to minimize these inclusions.

The presence of macro-inclusions in steel castings is avoidable but their presence has plagued all forms of steel casting and is particularly problematic in both foundry processing and in the continuous casting of sheet steels and wire. In general, macro-inclusions are related to three major problems (1) deoxidation/reoxidation, (2) slag entrapment and (3) erosion of refractory materials during steel pouring. All three problems occur during foundry processing.

Macro-inclusions are always practice related and analysis of the size and chemical composition of a macro-inclusion can lead to the identification of potential sources of this problem. Once an inclusional source is developed a clear and effective process change can be made to eliminate such problems in the future. Therefore, the techniques already developed by integrated steel manufacturers can be readily applied to foundries by coupling inclusion identification with an in-depth study of steelmaking and casting practices in the foundry.

Macro-inclusions were previously identified in the study of Horwath and Goodrich and Svoboda et al. to be a significant problem in cast steels that can lead to problems in castings that result in excessive casting repairs or rejected castings. To avoid these problems truly clean steels must be cast and although there are many ways to quantify clean steel, one method is to ensure that there are no inclusions in cast materials above a size that leads to failure during ultra-sonic or visual inspection of the casting. In this definition all macro-inclusions must be eliminated. In general this means that macro-inclusions greater than 100 microns in diameter must be eliminated in most applications; however, in certain more severe applications macro-inclusions greater than 50 microns in diameter may become problematic. It is thus the purpose of this work to outline the operating conditions that can lead to the production of such inclusions and to suggest practices that can lead to the elimination of these potential casting problems.

This work will be fundamental in nature but will have great practical significance. First potential mechanisms of inclusion formation will be identified by review of the known engineering principles where the criteria for inclusion formation will be outlined. This will use the work already carried out by other parts of the steel industry. This will lead to our ability to predict potential processing problems during foundry casting. Secondly, the survey of macro-inclusional defects found by AFS members will be used to determine potential practices that can lead to macro-inclusion formation and cataloguing of this relationship will lead to a rational explanation of the types of macro-inclusional causes.

 

Inclusion Formation

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References

(1) J. A. Horwath and G. M. Goodrich: "Micro-inclusion Classification in Steel Casting", AFS Transactions, 1995, p 495- 510

(2) J. M. Svoboda, R. W. Monroe, C. E. Bates and J. Griffin: "Appearance and Composition of Macro-inclusions in Steel Castings", AFS Transactions, 1987, p 187- 202