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Guide for Heat-Straightening of Damaged Steel Bridge Members
Chapter 4: Effects of Heat Straightening on the Material Properties of Steel
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 temperature, it attempts to return to its body centered structure. Since this retransformation requires time, rapid cooling may not permit the complete change to occur and a hard, brittle phase called martensite occurs. This form has reduced ductility and is more sensitive to brittle fracture under repeated loads.
The upper critical (or upper phase transition) temperature is the level at which the molecular change in structure is complete. At this temperature (around 815–925°C or 1500–1700°F for most steels, depending on carbon content) the steel assumes the form of a uniform solid solution called austenite. It is at temperatures between the lower and upper critical that a wide range of mill hot rolling and working can occur. As long as the temperature is lowered slowly in a controlled manner from these levels, the steel assumes its original molecular configuration and properties.
This temperature control is more difficult to maintain at a fabrication shop or in the field when conducting heat straightening repairs. Consequently, if the temperature during heat straightening is not kept below the lower critical temperature, undesirable properties may be produced during cooling. It is this concern that has limited the application of heat straightening in many cases.
A related issue is the question of residual stresses. When heated steel cools, the surfaces having the most exposure to the cooling environment contract more rapidly. This unequal contraction produces residual stresses found in most steel shapes and it is important to understand how heat straightening affects these patterns. The purpose of this chapter is to provide a summary of how heat straightening affects material properties and residual stresses.
Significant residual stresses occur in most structural steel members. Such stresses usually result from differential shrinkage during cooling in the manufacture of both rolled and welded built–up shapes. However, the cutting and punching process during fabrication may also produce residual stresses. Residual stresses in fabricated steel can be quite high and may reach 50 percent of yield for some rolled shapes and approach yield for some welded members. With one exception, residual stresses have been neglected in code requirements governing steel design. The reasons for neglecting residual stresses relate to two characteristics: (1) The ductility of steel allows for a moderating redistribution of residual stresses when a member is subjected to large loads, and (2) since residual stresses are self–equilibrating, large compressive stresses at one location on a cross section are balanced by tensile stresses at another location. As a consequence, the stresses at a specific cross section produced by applied loads is additive to the residual stresses at some points and are subtractive at others so the ultimate strength of a member is usually not affected. The exception is compression members in which high residual stresses may reduce the buckling strength. American design codes account for residual stresses in compression members by assuming an average residual stress value of 50 percent of the yield stress. This assumption may lead to somewhat conservative designs for rolled shapes (which have smaller residual stresses) and slightly less conservative designs for welded built–up shapes (which have larger residual stresses). European codes have adopted the multiple column curve approach in which different formulas are used depending, on the magnitude of residual stresses. For these codes the level of residual stress affects the design capacity.
Avent, et. al. (2001) conducted research to assess whether heat straightening produces some negative effects due to residual stresses. The distribution of residual stresses for vee heated plates is shown in Figure 29 and those for various heat–straightened rolled shapes are shown in Figures 30–34.
Figure 29. Residual stress distribution for
plates damaged and then vee heated
Figure 30. Typical residual stress distribution for
a heat straightened angle
Figure 31. Typical residual stress distribution for
a heat straightened angle
Figure 32. Typical residual stress distribution for
a heat straightened channel
Figure 33. Typical residual stress distribution
for a Category S wide flange beam
Figure 34. Typical residual stress distribution for
a Category W heat straightened wide flange beam
In summary, the residual stresses in heat–straightened plates are fairly consistent having maximum compression stresses of about 150 MPa (20 ksi) at the edges and tension stresses of about one–half that value at the center of the plate. Residual stresses in heat–straightened angles and channels tend to have maximum values approaching yield in compression at the toes and heel. Relatively high tension stresses are found near the middle of each leg. Maximum residual stresses in wide flange beams approach the yield stress in compression at the flange edges.
The large residual stresses created during heat straightening have several implications. First, if the member is a compression element, the high residual stresses are similar to welded built–up members. Since U.S. codes use a singe column curve concept, these members are all treated the same and no capacity reduction should be assumed. Second, high tensile residual stresses reduce the effectiveness of jacking forces by effectively canceling out compressive stresses in areas where externally applied forces would cause them. Movement could be reduced or even reversed, if the jacking force moment does not compensate for the residual stresses.
Finally, large compression residual stresses may produce bulges in the compression elements of a cross section during heat straightening. Special heating patterns and sequences may be required to prevent this effect.
Research data (Avent, Mukai and Robinson, 2000) indicate that heat straightening affects the mechanical properties of steel. Early researchers used undamaged steel and a small number of heats to conclude that property changes after cooling were minimal. However, tests on damaged and subsequently heat–straightened plates and beams indicate that some property changes may be of significance. The modulus of elasticity may decrease by over 25% in some heated regions.
Yield stress may increase by as much as 20% in some cases, especially in the vicinity of the apex of vee heats. Specimens heated for various lengths of time, cooled both by air and by quenching with a mist, and subjected to various superimposed loads and residual stresses have been tested. None of theses variables had significant effect on the yield stress with the possible exception of the quenched and tempered steel. In the case of quenching, the yield stress was, on the average, unchanged from the original yield. Overall, the data indicates that the long term effects of the heat straightening process on yield stress are small but generally increase it.
Tensile strength also increases but at only half the rate of yield stress. The ductility as measured by percent elongation typically decreases by one–third.
In general, the fatigue–crack initiation threshold increases with tensile and yield strengths, but tensile strength increases in the heat–straightened plates were relatively small, when compared to ductility losses. Thus, improvement of the fatigue–crack initiation threshold, based solely on tensile strength could possibly be more than offset by the reduced stress redistribution permitted by the ductility loss. Some reduction in the fatigue limit might occur as a result.
Similar to ductility, fracture toughness (a value proportional to the energy consumed during plastic deformation) may decrease as a material’s yield strength changes during heat straightening. Research data indicates that considerable variation may occur in Charpy vee notch tests before and after heat straightening. However, no clear relationships have been established for first time heat straightening repairs.
Most research on material properties effects has been limited to strain ratios of 100 or less. This range of strain ratios includes a majority of typical bridge damage. Of particular significance is that, within this range, changes in mechanical properties after heat straightening are not a function of degree of damage as measured by angle of damage or strain ratio. However, for strain ratios over 100, yield stress is directly proportional and elongation is inversely proportional to strain ratio. Thus, except for high degree of damage areas, material properties should not be the primary determining factor when contemplating the use of heat straightening.
An important issue is how many times a girder can be damaged and heat straightened. Changes in all the material properties become more evident with the increasing number of damage/repair cycles. These changes are particularly significant at the region associated with the apex of the vee. After two damage/repair cycles, the property changes remain relatively modest. But after four damage cycles, the increase in yield and tensile strengths and the loss in ductility were much more pronounced (Figures 35–37). Because the variation in yield is larger, the gap between yield and tensile strengths decreases as the number of damage/repair cycles increase. The ratio of yield–to–tensile strength is around 68% for undamaged steel. That ratio typically increases to 78% after one damage/repair cycle and to 88% after eight cycles. The elongation after one or two damage/repair cycles (31–32 percent) followed the trend of results for a single repair with about a one–third reduction. However, for four or eight cycles the elongation and ductility are proportionally reduced as shown in Figure 37. This behavior with each damage/repair cycle results in an increasingly brittle material. These data illustrate why over–jacking may result in brittle fracture after a number of damage/repair cycles in the same zone.
The point at which loss in ductility becomes dangerous is case–specific. However, the extreme losses encountered in the repetitively damaged beams show that there is probably a limit to the number of times that any given member should be repaired. Material property changes were usually acceptable after two cycles. Thus, a condition that is safe to straighten once could usually be safely straightened twice. The changes become significantly greater after four and eight damage/repair cycles, respectively. These findings are further substantiated by the fact that during one full–scale study (Avent and Fadous, 1989), one girder exhibited brittle behavior by cracking during a heat in its fourth damage/repair cycle. Based on this research, re–damaged members at the same location should not be subjected to heat straightening more than twice. Connor, Kaufmann and Urban (2008) reached the same conclusion in their full–scale testing to evaluate fatigue and fracture performance.
The recommended maximum jacking force is 50% of the member capacity as discussed in Section 2.9. The basic concept is to keep the stresses due to jacking below the yield stress at the elevated temperature. For bending members the computation of capacity is straight–forward and computed as the plastic moment capacity, Mp. While some small zones of yielding may occur under the conditions of jacking equal to 50% of Mp, the majority of the cross section remains below yield. However, when considering local damage (Category L) or composite girders, the computation of capacity is not well–defined. For local damage the best way to determine the capacity is experimentally such as applying a jacking force in an undamaged low stress area until initial yielding is reached. For composite girders, refer to Avent and Mukai (1998) for computation methods.
Little research has been conducted on the effects of higher jacking forces. Avent and Mukai (1998) conducted some large scale repairs on damaged girders. One case included using jacking forces producing moments greater than 50% Mp. In this case, the movements observed during heat straightening were excessive and indicated that some hot mechanical straightening had taken place. During the 7th heating cycle, the lower flange of the composite beam fractured as shown in Figure 21. The fracture occurred on edge of the lower flange compressed by the force which induced the damage. Similar fractures have also been observed in actual field repairs. In each case the fracture occurred on the flange edge compressed when damaged. During the heat straightening repair, the jacking force induced tension in the area that fractured. This case indicates that excessive jacking forces increase the risk of sudden fractures.
Recent research by Sharma (2005) has provided insight as to why such fractures occur. A series of plates were bent about their weak axes and heat straightened using line heats. Jacking forces producing plastic moments of 50, 70 and 90 percent of capacity were used. As expected, the plate movement during heat straightening was directly proportional to the level of jacking force. Material properties tests showed that the level of jacking force had little effect on yield stress, tensile strength, modulus of elasticity, or ductility. However, there were significant differences in material properties on the side compressed by damage. Comparing material properties from the areas placed in tension and compression by the damage, the compression side had significantly: (1) higher yield stress, (2) lower ductility, and (3) less toughness based on Charpy tests. These results indicate that the compressed side is more brittle and thus more likely to fracture during repair with large jacking forces.
The body of research indicates that heat straightening can be used without significantly compromising the material for strain ratios less than 100. Sharma (2005) also conducted weak axis plate tests that included damaged plates with strain ratios of 65, 150 and 200. He found the following relationships: (1) plate movement during heat straightening was inversely proportional to the strain ratio, (2) the increase in yield stress after heat straightening was directly proportional to the strain ratio, and (3) ductility after repair was inversely proportional to the strain ratio. This behavior indicates that the likelihood of fracture during heat straightening is directly proportional to the strain ratio, particularly when the strain ratio is greater than 100. Thus, the risk of fracture increases with strain ratios greater than 100.
Connor, Kaufmann and Urban (2008) conducted the first major study on fatigue and fracture performance of heat–straightened steel. Their full–size tests led to the conclusion that damage and repair cycles did not have a significant effect on fatigue life of girders at stiffeners and cover plates. However, live load stresses may be magnified by residual local damage (even within normal tolerances) after heat straightening. They recommend stress adjustment factors be applied to ensure that the residual damage will not cause an unacceptable increase in live load stress that would result in a fatigue failure.