STEEL FOUNDERS' SOCIETY OF AMERICA TECHNICAL SERVICE REPORT #124 ANALYSIS OF DEFECTS OF THE BORED SURFACE OF SIX 1025 CARBON STEEL CASTINGS Published by the STEEL FOUNDERS' SOCIETY OF AMERICA Malcolm Blair Technical and Research Director August 1995
ANALYSIS OF DEFECTS ON THE BORED SURFACE OF SIX 1025 CARBON STEEL CASTINGS ABSTRACT Three 1025 carbon steel flange castings were analyzed to determine the cause of pores and oxide defects uncovered during machining. All of the defects were found near the cope surface of the casting. The defects were cut from the castings and analyzed using an optical microscope and a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectrometer (EDXRS). The defects consisted of large oxide inclusions and some associated porosity caused by reoxidation of the metal during pouring. The inclusions contained eroded silica sand grains embedded in a matrix of multiphase oxides, which is also consistent with metal reoxidation during pouring. Eliminating the defects will require reduced metal turbulence and opportunity to react with air. This might be achieved by shrouding the metal stream, inerting the mold with a gas, or by using a filter to control metal flow into the mold. 1
EXPERIMENTAL PROCEDURES Foundry Processing The metal was melted in a five-ton electric arc furnace and tapped into a five-ton ladle. Prior to tapping, the heat was blocked with 75% FeSi and then deoxidized with FeSi, FeMn, Al, FeTi, and CaSi during tapping. The tap temperature was about 3200 F and the pouring temperature was 2900-2950 F. The castings were poured from a 500 lb teapot ladle lined with silica sand bonded with sodium silicate resin. The breastplate and spout are lined with 70% alumina. The molds were green sand with bonded sand cores and shell downsprues. The poured weight was about 41 lbs using a standard gating system, and the casting weight was 11 lbs. There was one casting per mold. These castings have had a history of inclusions on or near the machined surface. Laboratory Procedure Three 1025 carbon steel castings were analyzed to determine the cause of defects uncovered during machining. The castings were designated CLS-1-3, CLS-1-4, and CLS-1-5. The defects in the castings were photographed, saw cut from the casting, and vacuum mounted in epoxy. These samples were ground, polished, etched in nital and examined with a light microscope and a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectrometer (EDXRS). RESULTS AND DISCUSSION A photograph of a casting is illustrated in Figure 1. A schematic of the casting showing typical locations of the defects is shown in Figure 2. Most of the anomalies were found within 1/4 inch of the cope surface. The Brinell hardnesses of all three castings are given in Table I. The hardness values were fairly consistent and ranged from 160 to 170 BHN. Photographs of some of the larger defects found are shown in Figures 3 and 4. Grey-green foreign material was visible in all defects. A 30x optical microscope montage of a defect from casting CLS-1-3 is shown in Figure 5. Higher magnification optical microscope images of this inclusion are shown in Figures 6 and 7. The defect consists principally of a multiphase matrix containing silica sand grains with some metal shot. An SEM micrograph of a defect from casting CLS-1-3a is shown in Figure 8. Higher magnification secondary and backscatter electron images of the defect are shown in Figures 9 and 10. A representative 2
EDXR spectrum from the points labeled 10 and 11 in Figure 10 is shown in Figure 11(A). This spectrum is that of silica sand. An EDXR spectrum representative of the needles in the inclusion (points 12 and 13 in Figure 10) is shown in Figure 11(B). The needles are an oxide rich in titanium and manganese. A representative EDXR spectrum from the matrix of the defect (points labeled 14, 15, and 16 in Figure 10) is shown in Figure 11(C). The matrix of the defect is an oxide rich in silicon, manganese, and aluminum. Oxides rich in silicon and manganese are typical of reoxidation products in carbon steel castings. An inclusion found on the bored surface of casting CLS-1-3 is illustrated in Figure 12. Secondary and backscatter electron images of the inclusion are shown in Figures 13. A representative EDXR spectra from the points labeled in Figure 13 are shown in Figure 14. Representative EDXR spectrum from point 1 in Figure 13 is shown in Figure 14(A). This phase consisted principally of manganese, titanium, and silicon oxides. A spectrum representative of point 2 is shown in Figure 14(B). This inclusion was composed principally of manganese and silicon oxides but did not contain silica sand grains. An optical microscope montage of an inclusion in casting CLS-1-4 is shown in Figure 15, and higher magnification images of localized areas are shown in Figure 16. Again, the inclusion consisted of silica grains and few metal shot in a multiphase metal oxide matrix. An SEM image of an inclusion in CLS-1-4 is shown in Figure 17, and an EDXR spectrum from the inclusion (point 4, Figure 17) is shown in Figure 18. The oxide was composed primarily of alumina, silica, and manganese oxide which are typical reoxidation products. An analysis of another inclusion from the casting (CLS-1-4a) gave similar results. The inclusion contained grains of silica sand in a complex matrix consisting of oxides containing silicon, aluminum, manganese, and titanium. Optical microscope images of inclusions found in casting CLS-1-5 are shown in Figures 19, 20, and 21. These inclusions consisted of sand particles and metal shot in a multiphase matrix metal oxide. An EDXR spectrum from points 1 and 2 in Figure 21(A) are shown in Figure 22(A) and (B) and indicate the presence of a large silica grain embedded in a complex oxide matrix rich in titanium, manganese, silicon, and aluminum. The results obtained from the inclusions from all three castings indicated that reoxidation of the metal during pouring led to the formation of the defects. Reoxidation is the reaction of elements in steel with oxygen after the steel has been deoxidized. Reoxidation products are usually composed of aluminum, silica, and manganese that have a strong tendency to react with oxygen at steel pouring temperatures. Aluminum-rich oxide particles usually lie in multiphase metal oxide matrix. The oxide matrix is usually rich in manganese and silicon but, in extreme cases, may contain some iron oxide. The presence of iron oxide may cause gas holes to be formed adjacent the inclusion as the iron oxide is reduced by carbon during solidification. 3
The presence of silica grains mixed with reoxidation products is common. Iron and manganese oxides wet silica sands, adhere to the sand and may pull sand grains from the mold surface as metal flows into the cavity. Conclusions The defects found on the cope side of the 1025 carbon steel castings examined on this study were principally the result of reoxidation of the metal during pouring. The associated gas porosity was a result of carbon monoxide formation during solidification as oxidized iron was reduced by carbon in the steel. The reoxidation inclusions also contained silica sand grains resulting from mold wetting and erosion. All of these phenomena are associated with reoxidation of the metal during pouring. Eliminating the defects will require reduced metal turbulence and opportunity to react with air. This might be achieved by shrouding the metal stream, inerting the mold with a gas, or by using a filter to control metal flow into the mold. Submitted by, Robin Griffin, Ph.D. Optics Laboratory Manager Approved by: Zoe B. Dwyer, Ph.D. Postdoctoral Fellow Charles Bates, Ph.D. Research Professor 4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
USE OF THIS REPORT AND INFORMATION CONTAINED THEREIN Publicity This report and the information contained therein is the property of the individual or organization named on the face hereof and may be freely distributed in its present form. However, the University of Alabama at Birmingham (UAB) hereby reminds Sponsor that no advertising or publicity matter, having or containing any reference to the University of Alabama at Birmingham, shall be made use of by anyone, unless and until such matter shall have first been submitted to and received the approval in writing of UAB. (UAB does not usually approve any type of endorsement advertising.) Limitation of Liability The faculty and staff of UAB associated with this project have used their professional experience and best professional efforts in performing this work. However, UAB does not represent, warrant or guarantee that its research results, or product produced therefrom, are merchantable or satisfactory for any particular purpose, and there are no warranties, express or implied, to such effect. Acceptance, reliance on, or use of such results shall be at the sole risk of Sponsor. In connection with this work, UAB shall in no event be responsible or liable in contract or in tort for any special, indirect, incidental or consequential damages, such as, but not limited to, loss of product, profits or revenues, damage or loss from operation or nonoperation of plant, or claims of customers of Sponsor. Report No.: WAB-MTG-TSR-124 To: Steel Founders' Society of America 455 State Street Des Plaines, IL 60016 Date: August 1, 1995 UAB Account No. 635890 and 634950 E-File: cls-1.eg7 28