The ABCs of

Inspectors can perform their duties better if they possess an understanding of the qualities of , the most common base material they work with, and its inherent defects

BY RAYMOND R. SHEPARD

This article is the first in a four-part series dealing with steelmaking and how the quality of the base material affects the manufactured products inspectors must examine. It gives a his - torical perspective on and steel - making and an overview of the steel process. The next issue will highlight modern steel making with a basic furnace, and the final two articles will deal, respective - ly, with steelmaking using an furnace and the and form - ing of steel.

nspectors and nondestructive examination (NDE) technicians Iwork with a wide range of mate - rials — plastics, composites, and wood, as well as . Of all the materials we inspect, however, one is so ubiquitous, with an impact so far-reaching, that the majority of Fig. 1 — The Saugus Iron Works finery is shown center image. Notice the waterwheel that our society takes it for granted. powered the bellows for the hearth. The casting shed of the appears at the bottom That material is steel. left. (Photo by Raymond R. Shepard.) Inspectors and NDE techni - cians perform many types of tests. We perform tasks ranging from quality control of base materials other . The metalworking ’s impact on the econo - to verifying the success of the processes used for joining mate - my is significant, accounting for a large portion of the gross rials. In essence, our job is to investigate one or more properties national product of the United States. Today, we cannot imagine of a material and determine if that material meets or exceeds what our society would be like without the far-reaching effects design requirements. The inspection method utilized for a par - of steel. ticular application may depend on the material being investigat - But what exactly is steel? Steel is an . An alloy is a sub - ed, where in the manufacturing process the inspection is being stance consisting of a metal and at least one other element. performed, and the severity of the discontinuities that apply to Steel, therefore, consists of iron (Fe), minute amounts of the component in question and how they affect the safe use of (C), and possibly other alloying elements, depending on the that component. required properties of the steel. In any alloy system, the metal - To do our jobs properly, we need to understand not only the lic element of the greatest percentage is called the solvent. The types of defects associated with the joining process but also the alloying element is called the solute. inherent defects in the base material. To address this, we must There are several steps involved in making steel. The first have a firm understanding of how the material we are inspecting step is the production of from . The carbon con - is made. This article examines the history and manufacture of tent of pig iron is too high for it to be used as an end product; it iron and steel. is too brittle. Carbon content must be reduced to levels that are appropriate for the type of steel being produced. This task is per - formed in furnaces that make steel in batches of hundreds of The Steelmaking Process tons per heat. It is then cast into ingots or sent to a continuous In 1854, Henry Bessemer devised an improved method for casting mill. Once steel is in ingot form, it may be further making steel. Production soared and the face of the earth processed by rolling mills and formed into a variety of shapes changed forever. Worldwide, more steel is produced than any and products.

RAYMOND R. SHEPARD ([email protected]) is Quality Specialist/Training Coordinator, Kakivik Asset Management and Adjunct Professor, University of Alaska, Fairbanks College of Rural Alaska.

FALL 2003 • 19 Fig. 3 — A blast furnace in Fairfield, Ala. Notice its size. Historically, as blast furnaces grew larger in capacity, technology had to be developed that was more efficient in moving large amounts of raw materials faster. (Photo courtesy of United States Steel Corp.)

Fig. 2 — This bellows blows air on one of the finery hearths. The late 1800s that came into use as the primary in blast same type of bellows system also supplied air to the blast furnace. furnaces. Today, -tar chemicals released during the coking The shaft is connected to a waterwheel outside the building. (Photo process, such as asphalt, are vital industrial products. Due to a by Raymond R. Shepard.) better understanding of chemistry, modern blast furnaces use a combination of coke in the furnace and powdered coal added to Pig Iron: Ingredients and Process gases for their fuel. Iron production in a blast furnace is a chemical reduction Pig iron production starts with three main ingredients — iron process. At high temperatures, carbon has great affinity for oxy - ore, , and a source of fuel — that are charged in a blast gen. When oxygen comes in contact with melted iron ore, car - furnace. Additionally, oxygen is required to convert the raw ore bon and oxygen combine to form carbon monoxide (CO) gas. into usable iron. While iron can be found in a natural state, it is The CO then combines with another molecule of CO to form very rare. For instance, meteorites are sources of natural iron, but carbon dioxide (CO ). Solid carbon is removed from the molten due to their scarcity, they could never be used for iron production 2 iron by conversion into a gas. In the furnace, gases work their on an industrial scale. Iron ore, however accounts for approxi - way to the top, while the heavier liquids fall to the bottom. The mately 5% of the earth’s crust. The waste material (noniron-bear - heated gas is vented off and recycled as hot fuel gas is delivered ing rock) in lower-grade iron ore must be removed. This waste to the blast furnace. material is called gangue from the word gangen meaning “to go.” Iron ore with concentrations of iron exceeding 50% can be directly charged into a blast furnace. Ores of lower concentration must be An Early American Blast Furnace refined through the process of beneficiation (concentration). The During colonial times, iron and steel were very scarce in the main ores utilized for iron making are hematite (Fe 2O3), mag - New World. produced iron and steel, but in limited netite (Fe 3O4), and tacomite (Fe 3O3). quantities and not of the highest quality. Sweden and Limestone acts as a fluxing agent within a blast furnace. It produced excellent steel; however, they released only limited performs an important function by removing impurities such as amounts for export to countries such as England. Additionally, . The limestone melts to form a liquid within the fur - England did not have the prodigious forests that the nace. Being less dense than liquid iron, the slag floats above it. As the slag builds up, it is periodically tapped and drained from Scandinavian countries possessed. Demand for steel outpaced the furnace. Historically, slag was disposed of as a waste prod - supply. Colonists in the New World realized they needed to pro - uct, but these days it may be used as aggregate in or duce iron and steel themselves instead of relying on imports. In asphalt roadways. 1620, an of limited capability was started in In the past, was used to provide the fuel necessary Jamestown, Va., and met with minimal success. In 1646, the first for the reduction process; coke is used today. Blast furnaces blast furnace and comprehensive, integrated ironworks was required large volumes of charcoal, thus many furnaces were built along the banks of the Saugus River in Saugus, Mass. located in areas that had healthy forests. Coal could not be used Today, restored to its original state and now a U.S. National directly for the production of iron due to the fact coal contains Historic Site, the ironworks allows a glimpse of what iron pro - unacceptably high levels of sulfur and other impurities. When duction was like during the colonial period in America. experimenting with coal as a fuel, early ironmasters realized the Saugus Iron Works was technically advanced for its time and end product was too brittle to be of any use. on par with the leading technology found in Europe. As it pro - In England, Abraham Darby discovered a way to convert coal gressed through the site, iron production went from the blast into coke in the early 1700s. By placing coal in an oxygen- furnace to a finery, chafery, slitting mill, and then to a black - reduced atmosphere and burning it, the majority of the sulfur smith shop — Fig. 1. The slitting mill was state of the art and and other coal tar chemicals are driven off as gas, leaving produced elongated stock sold to for production of behind the carbon. Upon cooling, the resulting product is a items such as wagon wheel rims, nails, etc. Products produced hard, porous material. Darby then used the coke to produce cast at Saugus included finished wares such as stove backs iron. Darby kept his method of converting coal into coke a and pots, blooms and bars for resale, bar for nail manufacturing, secret for many years. In the United States, it wasn’t until the and forged products such as tongs and hinges.

20 • INSPECTION TRENDS Fig. 4 — A Bessemer Converter during a blow at the Homestead Iron Works, which was located along the banks of the Monongahela River in Pennsylvania. Built in 1879, Homestead shut down in 1986. (Photo courtesy of United States Steel Corp.)

The Saugus site was chosen because it had all the necessary ingredients for the production of iron nearby. Iron ore was avail - able in the form of bog iron (limonite). The virgin forests of New England supplied the tons of charcoal necessary to fire a great blast furnace. Gabro rock, a fluxing agent, was available on the nearby Nahant Peninsula. In addition, two other vital compo - nents were in place that made the Saugus site attractive. A near - Fig. 5 — An being tapped. Slag is flowing off by pond supplied water via a raceway to power the waterwheels the top of the ladle in this picture. The open hearth process has been replaced by the basic oxygen furnace (BOF) and elelctric arc that, in turn, operated the bellows and powered the slitting mill. furnaces (EAF). (Photo courtesy of United States Steel Corp.) The river also allowed receiving of raw materials and shipping of finished products. several cubic meters of air per cycle — Fig. 2. The air was fed Bog iron is a renewable resource that originates along the bot - into the bottom of the furnace through tuyeres. tom of a bog, a depression where water collects but cannot drain The blast furnace had to be tapped at regular intervals. Slag, because there isn’t an available outlet. Iron-rich underground which floated on top of the molten metal, was removed and dis - springs provide the water to fill the bog. Evaporation eliminates posed of at the site. Molten iron was removed from the bottom some water but, in the process, minerals and water-soluble metals of the furnace through a taphole. The ground at the base of the such as iron are left behind. As time goes by, iron may concentrate furnace was covered with fine sand. If cast-iron products were along the bog bottom. Workers with long-handled shovels scraped being made, a wooden model of the item being cast was pressed the ore off the bottom of bogs from small boats. This “bog ore” into the sand and a series of channels was dug to the pattern. was then transported to the ironworks in a larger boat. When the furnace was tapped, gravity caused the iron to flow Gabro rock was quarried on a nearby peninsula and trans - into the depressions. To make pig iron, a central channel was ported to the ironworks. The gabro acted as the needed to dug straight out from underneath the taphole. At approximately facilitate the removal of waste from the ore. Charcoal was man - 90-degree angles to this central “sow,” smaller channels were ufactured by colliers throughout the region and transported by dug. These side channels were called “pigs,” hence the term pig wagon or boat, depending on the distance to the ironworks. iron. Once solidified, the pigs would be broken off the main sow. Charcoal was in high demand and its manufacture employed The pigs were then further processed into in the many individuals as fellers, teamsters, and laborers. All materi - finery. Due to financial difficulties, the Saugus Iron Works fin - als were harvested by hand and moved by horse or boat. ished its last blast in 1668. Raw ingredients were loaded into the blast furnace from the top. The furnace was built into the hillside. Workers accessed the charging hole of the furnace via a flat wooden deck called a Modern Blast Furnace charging bridge. To start the blast, alternating layers of char - The modern blast furnace may look completely different coal, ore, and gabro were placed in the oven and the charcoal from its colonial predecessors, but in actuality, the theory of was ignited. Once fully up to heat, iron ore, gabro, and charcoal operation has not changed all that much. The technology, how - were shoveled by hand into the gaping maw of the furnace 24 ever, has advanced tenfold. Instead of stone walls, modern blast hours a day for weeks at a time. A typical blast would run con - furnaces are supported by steel framework — Fig. 3. The fur - tinuously for 30 to 40 weeks. nace shell is made of steel. Lining the furnace is a Air was supplied to the furnace through sets of bellows pow - brick material resistant to high temperatures. Changes to blast ered by a waterwheel. The central shaft of the waterwheel was furnaces came about as technology evolved. Oxygen, which pre - set with a series of adjustable cams that actuated levers to pump viously was not available in mass quantities, started to be added the bellows. To pump the bellows faster, more cams were added to the air. This provides significantly more oxygen to the blast as to the shaft, which would speed up the frequency of the pump - compared to straight air. Skip cars and elevators replaced work - ing. The bellows were made of wood and leather and displaced ers shoveling ingredients into the furnaces as the quantities

FALL 2003 • 21 required increased nace and preheated gas flowed over the top of the melt. Furnace beyond human capac - operators worked the furnace with long steel tools — Fig. 6. ity to move them and Utilizing some of the same concepts of a reverberatory fur - the heights of fur - nace but in a different configuration, the process involves recy - naces reached more cling waste heat and gases by routing them alternately through than a hundred feet. fire brick. The brick picks up heat from the exhaust gases. The Modern computer direction of flow would then be reversed and air would flow modeling and strict through the brick. The air would pick up the latent heat in the metallurgical con - brick and the air was effectively preheated. The exhaust would trols have replaced then flow out through another set of fire bricks and the process guesswork and trial would continue. Open hearth furnaces were used in the United and error in the blast States up into the 1960s, when the process started to be phased furnace. out by the more economical basic oxygen furnace and eventual - ly the . Bessemer The production of pig iron is but the first step in the production of steel and steel products. A basic overview of how iron and steel Fig. 6 — A worker is seen taking a Process sample from an open hearth furnace. are produced will help us perform our inspection tasks better, and Notice the heat waves engulfing him To make steel, provide a better understanding and appreciation for what many while he performs his task. (Photo pig iron must be fur - take for granted, the complex process of iron and steelmaking. O courtesy of United States Steel Corp.) ther processed. In the early 1850s, Bibliography Henry Bessemer was looking for a source of inexpensive steel. He had invented a can - 1. Dell, K. A. 1969. Metallurgy Theory and Practice . non projectile that rotated during flight, thus giving the round Homewood, Ill.: American Technical Publishers, Inc. added stability. The projectile required cannon bore pressures 2. Gordon, R. B. 1996. American Iron, 1607–1900. Baltimore, so high that cannon failure was a common problem. He was not Md.: The Johns Hopkins University Press. able to find a reasonable source for the quantity and quality of 3. Linnert, G. 1994. Welding Metallurgy , Vol. 1. Hilton Head steel he needed, so he set out to solve the problem of steel pro - Island, S.C.: GML Publications. duction on his own. Bessemer was an experienced inventor, hav - 4. Neely, J. E. 2000. Practical Metallurgy and Materials of ing devised such improvements as a sugarcane press, metal leaf Industry . Upper Saddle River, N.J.: Prentice-Hall Inc. production machinery, and improvements to the glass manufac - turing process. Instead of starting with low-carbon wrought iron and diffus - ing carbon into the iron over a period of weeks to make a small batch of steel, Bessemer started experimenting with high-car - bon cast iron. He believed that through the introduction of air (oxygen) over the molten steel, carbon could be removed from the molten bath. He built an experimental converter and con - ducted a series of experiments with it. Instead of waterwheels, he used a steam engine to power the blowers that provided the air to the converter. Four tuyeres, long ceramic tubes, delivered the air to the bottom of the con - verter. To assist him with the process, Bessemer hired an iron - master. But when Bessemer told the ironmaster what he planned to do, the ironmaster balked. The ironmaster assumed blowing air over molten metal without the addition of fuel would make the metal cool and solidify. However, the chemical reac - tion between carbon and other impurities with oxygen is an exothermic reaction; heat is given off. The first reaction was so violent it melted the protective iron lid on the top of the con - verter. Bessemer continued to work on improvements to the process, which allowed large-scale steel production for the first time. The was an important step in steel pro - duction and was the precursor to the modern basic oxygen fur - nace that is the backbone of the steel industry today — Fig. 4.

Open Hearth Furnace Bessemer was not the only person credited with major improvements to the steelmaking process. Approximately ten years after the Bessemer process was introduced, the Siemens brothers in Germany developed another steelmaking process. In 1864, the open hearth process was introduced, which was simi - lar to its predecessor, the . Short and stout (Fig. 5), open hearth furnaces had a wide hearth and a low roof above the melt. Pig iron and were placed within the fur - Circle No. 7 on Reader Info-Card