Evaluation and Repair of Wrought Iron and - Purdue e-Pubs ...

78strength from the existence of pearlite is negligible. Carbon in wrought iron typicallyexists as fine precipitates of iron carbide, or cementite intermixed with the ferrite(Gordon,1988). This is because the wrought iron was never fully heated to a liquid formduring the manufacturing process, and so the carbon is not heated fully enough to formpearlite.The dark, elongated areas in the micrograph in Figure 4.1 are inclusions whichconsist of a variety of impurities like phosphorous, sulfur or silicon. The majority ofthese impurities are iron silicate and other oxides which are commonly grouped togetherand known as slag. The slag was intertwined into the microstructure of the wrought ironduring the manufacturing process where the molten slag was used to help heat the ironore. Most of the molten slag was squeezed out of the material during rolling of thewrought iron into the eyebar shape.The slag inclusions that remained in the material were typically elongated andextended along only one direction in the material. These inclusions were larger thaninclusions that are typically found in other metals such as steel. Some of these inclusionsare large enough to be seen with the naked eye. Figure 4.2 shows a typical largeinclusion found throughout the material.Figure 4.3 is a photograph of a micrograph taken of the scrap piece of steel. Thismicrograph was used to compare the microstructure of the historic wrought iron to that ofa common structural steel. The micrograph of the steel indicated that steel consisted of asmaller grain structure than wrought iron. It also shows a mixture of ferrite, pearlite andimpurities that create the microstructure of steel. The impurities, or inclusions in the steelwere much smaller and more distributed, unlike the inclusions in wrought iron.The addition of carbon in the form of pearlite increases the strength and ductilityof pure iron to form steel. Since the composition of wrought iron consists mainly offerrite with widely dispersed areas of cementite and impurities, the mechanical properties