Simple Solutions That Work! Issue 11

Example of FeSe additions at Foundry X: To illustrate how effective the addition of FeSe to a high chrome-nickel alloy valve casting (nominally 1.6% C, 21% Cr, 6% Mn, 4% Ni, 0.2% N) is shown in this example from Foundry X. Nitrogen (2000 ppm) is intentionally added to stabilize austenite and added as a nitrogen bearing FeCr. Foundry X is a shell sand foundry pouring a variety of high chrome-nickel stainless steel castings. In the spring during the rainy (high humidity) season, outbreaks of subsurface porosity would inevitably occur. Due to the seasonal nature of this problem, finished scrap has occasionally risen to as high as 80% for a given month. During the investigation it was thought that complex hydrogen/nitrogen pinholing arising from breakdown of the nitrogen-bearing synthetic resin binders used for shell molds was partly responsible for the porosity. Examples of the type of subsurface porosity are shown in sectioned valve castings (see Figure 4). In an effort to reduce overall gas levels, Foundry X embarked on an extensive study of variables that might be the cause of the sub- surface porosity. This included: • Varying metal pouring temperatures from 2800 o F to 3050 o F • Melting under a protective argon cover • Rapid melting and minimizing metal holding times • Adding iron ore and nickel oxide to initiate a moderate carbon boil • Deoxidation practice using various CaSi, CaSiBa, Aluminum and Misch Metal additions • Additions of ferrozirconium, ferrotitanium and a complex alloy consisting of (Fe-Si-Mn-Al) • Reducing the charged nitrogen levels To determine the effect of charged nitrogen, heats made without nitrogen additions contained 0.05% to 0.07% (500 to 700 ppm) nitrogen. Three factors were responsible for the increase in nitrogen levels: 1) melting in air (3) , 2) nitrogen pickup from the shell molds and 3) increased nitrogen solubility from chrome and manganese in the alloy. Even at these lower nitrogen levels, intermittent porosity still occurred. Extensive analytical work was conducted along the way to determine if nitrogen and/or hydrogen was responsible for subsurface defects, extreme care was taken in analyzing for hydrogen. Test castings were immediately quenched and stored in liquid nitrogen to prevent diffusion of hydrogen from the samples. Almost without exception, no definitive variable was identified as the culprit for the sub-surface pinholes. Only when adding 0.02% FeSe to the ladle after deoxidation with 0.10% aluminum Contact: ROD NARO rod@asi-alloys.com and 0.06% ferrotitanium did the porosity problem disappear. It appeared likely that the subsurface porosity was the result of complex hydrogen/nitrogen interactions. Conclusion: The addition of small controlled amounts of ferroselenium (up to 0.02%) and keeping the charged nitrogen on the lower end of the specification was the only foolproof method that was effective in preventing subsurface porosity at Foundry X. References: 1. W. O, Philbrook and M. B. Bever (Eds.). Basic Open Hearth Steelmaking, Physical Chemistry of Liquid Steel. Chap. 16. 621- 690. AIME New York 1951 2. Courtesy of Doug Trinowski, HA International. 3. D. B. Roach and W. F. Simmons, "Effects of Nitrogen Additions to Stainless Steels", DMIC Technical Note, Battelle Memorial Institute, Columbus, Ohio, 1966. 4. Naro, R.L, "Formation and Control of Lustrous Carbon Surface Defects in Iron and Steel Castings," Silver Anniversary Paper, ASI International, Ltd, AFS Transactions, paper 02-154, (2002). 5. H. P. Rassbach, E. R. Saunders and W. L. Harbrecht, "Nitrogen in Stainless Steel, Electric Furnace Steel Proceedings of the AIME", Vol. 11, 1953. 6. A. M. Hall, C. E. Sims, "Reducing Pinhole Porosity in High Alloy Steel Castings by Additions of Selenium", Battelle Memorial Institute, American Society for Metals, Technical Report P9-41.2, Presented at the 1969 Materials Engineering Exposition, 1969 53 Figure 4: Macrophotographs of sub- surface pinholes found in high chrome nickel valves. Top of valve head and longitudinal section. 7 MAKING YOUR INSTALLATION ASUCCESS