Steel casters examine melting practices and metal processing.
Raymond Monroe, Steel Founders' Society of America, presented a general outline of the challenges and tasks currently facing steel foundries. Noting that steel castings were sought after in the 1970s but shunned in the 1980s, Monroe said that to regain lost ground, foundries must produce "thin walled, high-quality components. We have to be twice as good at half the cost. We have to change our image."
To accomplish that, steel foundries must reinvest in process technologies, he said. "In the '80s, we were content to hang on. In the last few years, business has been much too good just to hang on."
The elimination of macroinclusions from steel castings is an area Monroe called critical. Steel casters have to look at the interaction of metal and oxygen at every point in the process.
"Melting offers about a 30% chance for improvement," he said, "but we also have to look at pouring. It isn't enough to have a good mold and good steel, we have to optimize pouring practices." Some overall steps toward improvement are adopting proven processes from steel minimills, obtaining critical measurements of slag chemistry, better using suppliers and designed experiments. "We should be learning from every trial.
"We tend to think everything important happens in the furnace or in the muller. We lack discipline in that respect. We have to see the total process."
Acid vs. Basic Melting
Robert Shepherd and John Carpenter, Harrison Steel Castings Co., discussed the results of testing done at that foundry to gauge the cost and quality effects of switching from acid refractories to basic. The 13-month test assessed more than 900 heats from an acid-lined electric arc furnace and one lined with basic refractories. Part of the experiment also included gauging the impact of calcium wire injection on the heats.
Using cracks and "dirt" as the casting defects to be measured "to determine if a reduction in cleaning room costs would offset increased refractory costs from basic melting," the authors said that basic melting reduced the incidence of these defects by 16.5%. In addition, the impact values for basic-melted quenched and tempered steels were significantly higher, while the basic steel "exhibited lower total oxygen and fewer microinclusions than did their acid counterparts," they said.
Despite these improvements, the refractory and other related costs in using basic melting were 13% higher than acid melting, and "the switch from acid to basic melting was not justified at the time," they concluded. Calcium wire injection, which reduced dirt in the acid steel 32%, as opposed to only 5.5% for basic, proved to be a far more cost-effective means of improving quality. As for the improved impact values, only 10% of Harrison Steel's castings have impact requirements, which can be met by purchasing low-sulfur, low-phosphorous scrap. But "as casting requirements become more stringent," the authors cautioned, "these conclusions will have to be reexamined."
Direct Carbon Probe
Maynard Steel's Brad Kokal detailed the foundry's efforts to shorten hold time on molten steel heats and thus lessen the chance of reoxidation. Kokal explained that waiting for the results of metallurgical tests can harm the quality of the melt and cost the foundry in productivity.
"If a furnace on one end of the plant has just tapped a heat while a furnace on the other side sends a carbon check to the lab, the carbon check will wait - sometimes up to 20 min," he explained. "Anyone who has stood next to a furnace waiting for test results knows how long this feels."
To alleviate the situation, Maynard began testing a direct-read carbon probe from Minco. The Metnet system consists of a carbon sampler used on the end of a lance in combination with a thermocouple. Thermal arrest data from the sampler is converted to a cooling curve calibrated for carbon determination. The carbon content is displayed on a computer screen.
"By having accurate carbon data available instantly at the furnace, we've saved between 10-30 min per heat by eliminating priority conflicts that arise at the lab," Kokal said. He also stated that on Maynard's 25-ton arc furnace, this saves up to $25 per min.
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|Title Annotation:||American Foundrymen's Society conference|
|Author:||Philbin, Matthew L.|
|Date:||Nov 1, 1996|
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