Silicon: One key to thin-wall steel castings; Based on a paper presented at the 55th annual SFSA technical & operating conference, silicon is examined as an additive to increase the fluidity of molten steel.

Elimination is the name of the game on television's popular reality and game shows. While elimination on these shows is synonymous with losing, the word also can convey constructive and progressive developments. Elimination (in its positive sense) was a common theme at the 55th annual Steel Founders' Society of America's Technical & Operating Conference, held October 31-November 3 in Chicago.

The conference, which offered 34 presentations, featured research on the elimination of burn-in/burn-on defects, obstacles to competitive component manufacturing, and leaks and tears.

This article focuses on one such presentation, detailing the use of silicon in low carbon steel to eliminate pouring obstacles and problems associated with thin-walled steel castings.

Focusing on Fluidity

The limitations imposed by the fluidity of molten steel have restricted the use of steel castings in some markets, especially in the automobile industry. Steel castings have practical thickness limitations of 0.3125 in. in green sand and 0.25 in. in shell and no bake molding operations in part because of fluidity. Pouring temperature and metal composition also are important factors in determining the fluidity of steel.

Bipin Shah and Kandi Parsons, Grede Foundries Milwaukee Steel Div., Milwaukee, co-authored a paper, "Effects of Silicon on Low Carbon Steel," and provided conference attendees with an overview of a process to improve steel fluidity without adversely affecting mechanical properties and casting quality.

Modifying the chemistry of the steel seemed to be a potential solution. The goal was to elevate the silicon content in the molten steel to produce castings with a lower liquidus temperature, resulting in increased fluidity.

The ultimate application would be to produce thin-walled steel castings or lower the required pouring temperature in existing castings. A reduction in the temperature would alleviate some of the burden steel foundries bear due to high temperature-associated defects. In addition, lower pouring temperatures require lower tapping temperatures, translating into refractory and energy savings.

The foundry chose a plain carbon steel similar to AISI 1322 for the experiments. It chose to evaluate fluidity, liquidus temperature, chemistry, mechanical properties in quench and tempered state and normalized and tempered state, cleanliness, weldability and microstructure. The foundry then designed a pattern with varying thick-nesses that could provide a visual and measurable distance to which metal flows. This pattern featured thick-nesses ranging from 3-9 mm-the cope is shown in Fig. 1.

First Trial

The first trial was performed in an 800-lb induction furnace. Seventy-five percent ferro-silicon was added to raise the silicon levels--the silicon content became almost twice the manganese content. Samples were poured with 1.86% silicon content and with 0.44% silicon content. The pouring was designed to test the fluidity at various intervals during the pour. As anticipated, the higher silicon samples showed a decrease in the measured liquidus.

The chemical ideal diameters (ID) were not similar, dramatically impacting mechanical properties (Tables 1-3). This was anticipated because silicon is a solid solution alloying element that directly influences most of the properties of steel, primarily hardness and strength. As strength increases, ductility decreases. This was evident by the lower ductility and impact properties in the higher silicon sample.

Microstructural evaluation indicated no difference in microstructure, but grain size increased in the higher silicon samples. In addition, large voids were observed in the plates in the high silicon samples, especially those viewed in the quench and tempered state. This may be due to some visual 'frothing' while pouring these samples, which may account for the voids.

Figures 2a-d (letters correlate to the order poured) show that an increased fluidity resulting from the higher silicon steel improves casting distance.

Second Trial

Due to voids found in the elevated silicon plates, failed weldplates and low impact properties, a second trial was run with a lower silicon content of 1.2%.

The second trial also was melted in an induction furnace. The lower silicon content produced a liquidus temperature that fell between the initial high and low liquidus. The castings visually demonstrated that the achievable fluidity was in between the high and the low (Fig. 3a-c). A notable improvement in the impact strength over the initial 1.86% silicon was seen, even though the overall impact strength was still significantly lower than the low silicon.

Based on the results of this trial, a final trial was conducted to substantiate the findings.

Final Trial

This last trial also prevented the other alloying elements from interfering with the mechanical properties because the other elements were the same in both the high silicon and lower silicon samples. This trial was melted in a 6-ton electric arc furnace. The modification was done in a 500-lb shank pot using ferro-silicon to elevate the silicon.

Again, the higher silicon content produced a lower liquidus temperature, increasing fluidity (Fig. 4a-d). The results obtained confirmed the original findings. While there were some discrepancies in mechanical properties, this appeared to be a result of the variation in tempering temperature.

The chemical and mechanical properties obtained in the final trial confirmed results from the first trial (Tables 4-6).

Silicon as a Solution

The experiment clearly demonstrated that adding silicon content lowers the liquidus and increases fluidity, which allows thinner sections to be filled without increasing the refractory or energy requirements.

As silicon content increases, the liquidus temperature decreases. Although silicon has a detrimental effect on impact properties and a negative influence on weld properties, it produces positive benefits. Fluidity is increased with elevated silicon content and this increased content has a positive effect on casting strength.

In applications where impact strength is not a significant factor, a high silicon plain carbon steel would satisfy the requirements. This is particularly beneficial in applications where thin sections are needed, eliminating an obstacle of increased casting use.

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About the Authors

Bipin Shah, technical director, and Kandi Parsons, metallurgist, work at Grede Foundries Milwaukee Steel Div. Shah has 36 years of foundry experience specializing in metallurgy.

Table 1

First Trial Chemistries

Element                 High Silicon  Medium Silicon  Low Silicon

Carbon %                    0.21           0.24          0.22
Manganese %                 1.41           1.28          1.13
Silicon %                   1.86           1.24          0.44
Critical diameter (DI)      1.98           1.84          0.93
Table 2

First Trial Normalized and Tempered Properties

Property                High Silicon  Medium Silicon  Low Silicon

Tensile (psi)             100,601        111,385        88,789
Yield (psi)                69,138         83,551        48,678
Elongation %                 22            18.5           27
Reduction in Area %        41.27          42.18          60.1
Charpy (-40 ft-lb)           2              7              8
Temper temperature (F)      1120           1200          1120
Table 3

First Trial Quenched and Tempered Properties

Property                High Silicon  Medium Silicon  Low Silicon

Tensile (psi)             116,289        104,146        107,517
Yield (psi)                85,001         61,289        79,728
Elongation %                 21            20.5          19.5
Reduction in Area %        44.93          43.08          46.69
Charpy (-40 ft-lb)           3              3             18
Temper temperature (F)      1180           1160          1000
Table 4

Final Trial Chemistries

Element                 High Silicon  Low Silicon

Carbon %                    0.24         0.24
Manganese %                  1.2          1.2
Silicon %                   1.26          0.5
Critical diameter (DI)       2.1         1.45
Table 5

First Trial Normalized and Tempered Properties

Property                High Silicon  Low Silicon

Tensile (psi)              92,962       86,194
Yield (psi)                56,456       50,059
Elongation %                 25          25.5
Reduction in Area %        55.47         53.68
Charpy (-40 ft-lb)           7            10
Temper temperature (F)      1280         1060
Table 6

Final Trial Quenched and Tempered Properties

Property                High Silicon  Low Silicon

Tensile (psi)             114,286       105,798
Yield (psi)                90,196       80,774
Elongation %                17.5          18
Reduction in Area %        42.85         43.3
Charpy (-40 ft-lb)           21           18
Temper temperature (F)      1160         1060