Evaluation of steel properties and cracking in “k”-area of W shapes

Historically, the chemical composition and physical properties specified by the appropriate ASTM Standards were an indication that the structural steel material was adequate for most common applications. Typically, only the chemistry, yield point, tensile strength and elongation were provided on the mill test report. Changes in steel making processes came about slowly so that there was no need for major changes in the ASTM Standards. During the post World War II period, riveted construction was replaced by bolted and, more importantly, welded connections and structural shapes. At the same time, larger and longer span structures became common. Within years, major problems were encountered. Fracture of bridge girders using heavy plates and frames using heavy jumbo shapes alerted designers to the phenomena of Charpy V-notch (CVN) toughness and brittle fracture. Appropriately, the governing codes such as AASHTO, AREA and AISC introduced material requirements and testing to prevent steel with unacceptable properties from being used. Recently, another material property deficiency has been identified, namely cracking in the `kʹ area of hot rolled wide flange shapes. The shapes are produced from scrap iron in an electric furnace and continuously cast into near shape configuration. It is not the scrap iron or the continuous casting into near shape that is the problem, but the final cold straightening that alters the steel properties in the `kʹ area. Extremely high yield stress almost equal to the elevated tensile strength occurs with corresponding lowered elongation and single digit CVN toughness. This paper will show examples, discuss this phenomenon and explain its ramifications.