The Use of Heheat Straightening to Repair Damaged Steel Structures
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The use of heat straightening to repair damaged steel structures has gained popularity in recent years. However, applications have been limited due to concerns related to degradation of material properties after repair. Most research has been limited to small undamaged plate specimens where only one to three heats were applied. The purpose of this paper is to report on a study in which structural members were damaged and completely repaired by heat straightening, after which material properties were investigated. The research data indicate that heat straightening does affect mechanical properties of steel. Yield stress may increase by as much as 20%, especially in the vicinity of the apex of vee heats. Tensile strength also increases but at only half the rate of yield stress. The ductility as measured by percent elongation may decrease by one-third, and the modulus of elasticity may decrease by over 25% in some heated regions. A portion of this study included the evaluation of the effect of degree of damage on material properties after heat straightening. Damage with maximum strains up to 100 times the yield strain were repaired, and material properties were compared to members damaged with much smaller strains. The degree of damage had a minimal effect on the material properties of heat-straightened steel. Some members were damaged and completely heat straightened more than once to evaluate the effect of repetitive damage on the material properties of heat-straightened steel. Changes in material properties are small after two cycles of damage and repair. However, additional cycles produced a more brittle material and, in some cases, resulted in fracture of the material. In summary, heat straightening is a viable alternative for the repair of damaged structural steel. However, the use should be aware that some material properties will be changed during the process. Guide for Heat-Straightening of Damaged Steel Bridge Members Chapter 4: Effects of Heat Straightening on the Material Properties of Steel 4.1 Introduction The potential for detrimental effects from heating damaged steel has limited the implementation of heat straightening. However, with an understanding of the properties of steel, heat straightening can be safely conducted. Heating steel reduces the yield stress as well as the elastic modulus but the coefficient of thermal expansion increases with temperature. The behavior of these parameters complicates attempts to understand the response of steel to heat straightening. In addition to these short–term effects, heat can result in long–term consequences which may be detrimental. Most structural steel used for bridge construction in the United States is classified as low carbon, high strength low alloy (HSLA) or quenched and tempered (Q & T) steel. At ambient temperature, these steels have three major constituents: ferrite, cementite and pearlite. The iron–carbon equilibrium diagram shown in Figure 28 illustrates the relationship of these components. Ferrite consists of iron molecules with no carbon attached, cementite is an iron–carbon molecule, (Fe3C); and pearlite is a mixture of cementite (12 percent) and ferrite (88 percent). A low carbon steel has less than 0.8 percent carbon, too little to develop 100 percent pearlite, resulting in pearlite plus free ferrite molecules. High carbon steels (carbon content between 0.8 and 2.0 percent) have more carbon than required to form pearlite, resulting in steel with partial cementite. Low carbon steels tend to be softer and more ductile, characteristics of ferrite, but cementite is hard and brittle so high carbon steels are harder and less ductile, poor properties for bridges. Temperatures greater than about 700°C (1300°F) begin to produce a phase change in steel. This temperature is often called the lower critical (or lower phase transition) temperature. The body centered cubic molecular structure begins to assume a face centered cubic form. With this structure, a larger percentage of carbon will be carried in solution. When steel cools below the lower critical
