1 Foundry: 4 Paths to melt quality.

Deoxidation and gas removal is of vital importance in the production of defect-free steel castings. AG Anderson Ltd., a 100-employee jobbing foundry in London, Ontario, Canada, has focused on determining the cause of gas porosity in its steel castings and then reducing it.

The foundry, which produces corrosion-resistant, heat-resistant, carbon- and low-alloy steel and corrosion-, wear-and abrasion-resistant iron castings ranging between 2-4500 lb, was experiencing steel casting inclusions in excess of the industry average. Being new to high volume steel casting production, the foundry knew it had to solve its gas problems to remain competitive.

Melting

At AG Anderson, melting is performed in three medium frequency induction furnaces with capacities of 600, 2500 and 4000 lb. A spinel bond fused alumina-based refractory is used for lining the three furnaces. The refractory is a combination of 88.2% alumina ([Al.sub.2][O.sub.3]) and 10% magnesium oxide. Because of the wear and tear, each furnace is completely relined every 65-75 heats.

Pouring is performed using lip pour ladles ranging from 600-4000 lb. Ladles are lined with fused alumina-based castable refractory materials. Teapot and pre-cast disposable ladles also are employed.

Charges consist of 40-60% foundry returns with the balance made up of low carbon iron or 1010 steel, ferroalloys and deoxidizers. All foundry-generated returns are color coded and segregated, while purchased scrap is not used in recipes. To avoid contamination from unwanted elements, a wash heat of medium carbon steel always is employed when switching from melting iron to stainless steel or when switching from melting stainless steel to ductile iron. Ladles also are washed following the same principle.

As it cast steel in the early 90s, AG Anderson experienced defect problems in its carbon and low alloy components that required extensive rework time. The high number of upgrade hours combined with the number of castings unable to successfully pass nondestructive testing prompted a study. Beginning in 1997, the foundry observed and addressed potential gas sources, including:

* inefficient deoxidation;

* hydrogen absorption during processing of steel;

* nitrogen absorption from the furnace atmosphere during melting and superheating;

* entrapped air.

As it examined these gas sources, the foundry developed methods to reduce their prevalence, thereby improving the steel melt quality. The following discusses these sources and the methods used to decrease them.

Inefficient Deoxidation

Oxygen dissolved in the steel reacts with carbon and produces carbon monoxide (CO). The reaction is usually responsible for gas porosity defects (pinholes or blowholes) in steels insufficiently or inefficiently deoxidized. The usual method to reduce the oxygen content is the addition of a final deoxidizer that has a greater affinity for oxygen than carbon, and that forms a solid or liquid oxide product.

In the steel casting industry, aluminum is universally used as a final deoxidizer. The deoxidation products are [Al.sub.2][O.sub.3] and hercynite ([FeO-[Al.sub.2][O.sub.3]).

AG Anderson experimented with adding different quantities of aluminum to its carbon and low alloy steel melt and determined that the optimum aluminum addition to eliminate gas problems would be 0.07-0.09%. However, this range caused undesired outcomes such as filter blocking, ductility loss and aluminum nitride embrittlement.

Aluminum additions of 0.04-0.06% eliminated filter clogging problems but were inefficient because inclusions were still present in the castings. The foundry focused on eliminating the problems caused by higher aluminum additions so that a higher concentrated final deoxidizer could be used.

Filter blocking--As a common practice at AG Anderson, all steels are filtered using 10 pore/in. ceramic foam filters. Each filter is characterized by a specific filter blockage factor (FBF) measured in lb of metal (capable of being filtered before filter seizing) per sq in. of filter. In the case of aluminum-final-deoxidized steels, the filter blockage factor is dramatically reduced because aluminum and oxygen form solid [Al.sub.2][O.sub.3]. Solid [Al.sub.2][O.sub.3] agglomerates the filter pores and significantly reduces the FBF.

Filter blockage causes two problems: it may prevent the metal from entering the mold, thus generating an incomplete casting, and it also can cause filter breakage. As the filter starts clogging, the metallostatic pressure on one side of the filter increases and, as a result, the filter collapses and parts of it enter the mold cavity.

AG Anderson carried out extensive projects using trial and error methods to determine the FBF for aluminum deoxidized. The results were compared with the filter manufacturer ratings and recommendations. The foundry discovered that additions of aluminum in excess of 0.05% reduce the FBF of ceramic filters from 25 to as low as 5 lb/sq in.

The study determined that to increase the FBF, the amount of aluminum used for deoxidation must be drastically reduced. In this case, an additional final deoxidizer was needed. A combination of 0.05% titanium and 0.02% aluminum increased the FBF to 45 lb/sq in.

At the same time, the foundry noticed that the amount of gas porosity-related defects was reduced significantly. The 0.05% titanium and 0.02% aluminum combination was used as a final deoxidizer, and then gradually aluminum was excluded completely. From this study, AG Anderson developed filter sizing guidelines for aluminum and titanium deoxidized carbon and low alloy steels (Table 1).

Loss of ductility--The foundry noticed that a large number of test bars containing the higher aluminum content were failing elongation tests. Once the foundry switched to titanium as its final deoxidizer, the tensile and yield strength and elongation measurements improved, eliminating this problem.

Aluminum nitride embrittlement--Even though in most cases the problem is limited to quench-and-tempered, high strength steels, AG Anderson occasionally noticed "rock candy" fractures in its carbon steel castings. Increased aluminum additions created this rock candy cracking through aluminum nitride precipitation. Aluminum nitrides precipitate in proportion to the aluminum and nitrogen contents, and in inverse proportion to the cooling rate (Fig. 1).

The occurrence was more obvious when exothermic riser sleeves were used. The foundry believed that alumina-containing exothermic sleeves increased the aluminum level in the riser metal.

Aluminum levels in the riser metal in excess of 0.08% when combined with nitrogen levels in excess of 0.004% were believed to be the cause of hairline rock candy cracks that developed under the riser and were noticed after the final temper. AG Anderson adjusted nitrogen levels by adding titanium. The product of this reaction titanium nitride, has a less detrimental effect on steels than aluminum nitride because it doesn't facilitate the formation of rock candy cracks, which aluminum nitride does.

Hydrogen Absorption

AG Anderson also experienced gas stemming from moisture and hydrocarbons present in the furnace atmosphere and mold coatings and binders. Water vapor in molten steel dissolves to form hydrogen and oxygen.

A number of conditions are responsible for the absorption of hydrogen leading to pinhole formation. During melting and refining, hydrogen absorption can occur as a result of:

* hydrogen induced into the charge by wet, rusty or oily returns;

* water vapor formed by wet refractory materials;

* wet ferroalloy additions;

* holding of heats in the furnace after final deoxidizers have been added;

* humidity of the furnace atmosphere due to the weather.

The only method to drive hydrogen from the melt is carbon boil. The CO bubbles escaping from the bath act as a carrier of the hydrogen. In the case of induction steelmaking, however, carbon boil is not a practical option because the coil around the furnace does not allow for a porous plug to introduce gases. As a result, no hydrogen removal can be performed at this stage. Rather, an effort must be made to produce steel with low hydrogen content by using dry charge materials. Realizing this, AG Anderson adopted inside storage of its charge material.

The foundry also has concentrated on faster melting and avoiding extended exposure of bare liquid steel to humidity. In addition, hydrogen absorption is decreased by ensuring that the furnace spout and ladle refractories are properly dried.

Nitrogen Absorption

Nitrogen as gas in the atmosphere and charge materials are the only sources of nitrogen in the induction furnace melting of steels. The rate of nitrogen pickup is relatively slow as compared to oxygen and hydrogen, but high levels of dissolved nitrogen in steels can cause gas porosity and aluminum nitride embrittlement.

Carbon boil is the only method of nitrogen removal commonly employed by the electric arc furnace steelmaker. As is the case with hydrogen, when melting takes place in an induction furnace, nitrogen cannot be removed from the bath via carbon boil. To produce steel with low nitrogen content in an induction furnace, AG Anderson considers the following factors:

* charge materials. Purchased charge materials (including ferroalloys) must be of low nitrogen content;

* foundry returns. The use of foundry returns that already contain small amounts of nitrogen must be limited to 50-60%;

* charge chemistry. Adding silicon to the charge early in the steelmaking process forms a protective slag. At AG Anderson, 75% of the total ferrosilicon required by the recipe is added with the initial charge;

* melting and refining. Nitrogen is introduced when bare molten metal is exposed to the atmosphere. To reduce nitrogen absorption, exposure times must be reduced to a minimum. Whenever practical, bath protection via argon blanketing must be performed;

* tapping and pouring. The transfer stream must be as laminar as possible.

Entrapped Air

AG Anderson found that uncontrolled, turbulent metal flow causes air entrapment, appearing as a localized gas defect.

The foundry's engineers and technicians designed gating systems capable of reducing turbulence to cut down on gas defects. This involved adapting a significant change in gating philosophy.

Rigging was designed with smooth transitions. Bottom filling replaced filling at the parting line or top filling. In order to accommodate this type of mold filling, gating systems in a three-part mold (two-part drag) were developed. In the designs developed, the metal experienced turbulence in the sprue and runners, allowing it to fill quietly as it entered the mold cavity.

Figure 2 shows a molded AG Anderson casting--an open impeller molded according to its gating. This design required that the main runner bar be placed in the bottom part of the drag.

A change in sprue design also aided production. Properly-sized. square-shaped sprues avoid vortexing and remain full throughout pouring, which helps decrease air entrapment.

Another technique used was a pressurized gating system with the choke at the ingates. Main runners were designed to reduce velocity and allow nonmetallic inclusions to float. Both cope and drag runners and ingates were used. Ceramic filters were adopted for laminating turbulent flow in addition to their primary function of retaining nonmetallic inclusions.

Lessons Learned

The control of detrimental gas is a vital step in producing good quality steel castings. The success stemming from AG Anderson's efforts to eliminate gas in steel casting production has resulted in castings with fewer defects Since implementing these practices, rework time has decreased by 40%.

Even though gases are more difficult to control in induction melting furnaces than in arc furnaces, continuous efforts to reduce gas-related defects in steel castings must be made. However, the final deoxidizer, whether titanium or aluminum, is only a part of the solution. Many other factors contribute to the level of dissolved gases in steel castings.

Table 1.

AG Anderson's Filter Selection Guidelines Based on Maximum Recommended
Pour Rate.

Filter Size          (10 pore/in. filters) Maximum
                      recommended pour weight (lb)
                                Aluminum
                               Deoxidized
                                 Steels
                   Single                  Double
                   Filter                  Filter

4 x 4 x 1 in.       400                     850
5 x 5 x 1 in.       625                     1328
6 x 6 x 1 in.       900                     1912
3 in. dia x 1 in.   177
4 in. dia x 1 in.   314
5 in. dia x 1 in.   491

Filter Size         (10 pore/in. filters) Maximum recommended pour
                                     weight (lb)
                         Aluminum and Titanium            Titanium
                           Deoxidized Steels             Deoxidized
                          (0.02% Al, 0.05% Ti)             Steels
                   Single                 Double     Single
                   Filter                 Filter     Filter

4 x 4 x 1 in.       720                    1530       800
5 x 5 x 1 in.       1125                   2391       1250
6 x 6 x 1 in.       1620                   3442       1800
3 in. dia x 1 in.   318                               353
4 in. dia x 1 in.   565                               628
5 in. dia x 1 in.   883                               981

Filter Size           (10
                    pore/in.
                    filters)
                    Maximum
                   recommende
                     d pour
                     weight
                      (lb)
                    Titanium
                   Deoxidized
                     Steels
                   Double
                   Filter

4 x 4 x 1 in.       1700
5 x 5 x 1 in.       2656
6 x 6 x 1 in.       3825
3 in. dia x 1 in.
4 in. dia x 1 in.
5 in. dia x 1 in.

For More Information

Visit www.moderncasting.com for the 1992 AFS Transaction Paper, (92-08) "Variables Affecting the Nitrogen Content of Carbon and Low Alloy Acid Electric Arc Furnace Steels," C. F. Pilliod (1992).

"Efficient Steel Melting with Induction," P.D. Cerverello, AFS International Conference on Steel Melting & Refining (2000).

This article was adapted from a presentation at the 54th Annual Technical & Operating Conference of the Steel Founders' Society of America.

For a free copy of this article circle No. 345 on the Reader Action Card.

About the Author

Vasile lonescu has worked in foundries since 1981 and has been a metallurgist at AG Anderson, Ltd. for the last 5 yr.