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Guide for Heat-Straightening of Damaged Steel Bridge Members
Chapter 2: Heat Straightening Basics
Heat straightening is a repair procedure in which controlled heat is applied in specific patterns to the plastically deformed regions of damaged steel in repetitive heating and cooling cycles to gradually straighten the material. The process relies on internal and external restraints that produce thickening (or upsetting) during the heating phase and in–plane contraction during the cooling phase. Heat straightening is distinguished from other methods in that force is not used as the primary instrument of straightening. Rather, the thermal expansion/contraction is an unsymmetrical process in which each cycle leads to a gradual straightening trend. The process is characterized by the following conditions which must be maintained:
- The temperature of the steel does not exceed either (a) the lower critical temperature (the lowest temperature at which molecular changes occur), or (b) the temper limit for quenched and tempered steels.
- The stresses produced by applied external forces do not exceed the yield stress of the steel in its heated condition.
- Only the regions in the vicinity of the plastically deformed zones are heated.
When these conditions are met, the material properties undergo relatively small changes and the performance of the steel remains essentially unchanged after heat straightening. Properly conducted, heat straightening is a safe and economical procedure for repairing damaged steel.
A clear distinction should be made for two other methods often confused with heat straightening: hot mechanical straightening and hot working. Hot mechanical straightening differs from heat straightening in that external force is applied after heating to straighten the damage. These applied forces produce stresses well above yield, resulting in large movements during a single heat cycle. Often the member is completely straightened by the continued application of a large force during a single cycle. The results of this type of straightening are unpredictable and little research has been conducted on this procedure. Specific concerns about hot mechanical straightening include:
- Fracture may occur during straightening
- Material properties may be adversely affected
- Buckles, wrinkles or crimps may result
The Engineer should recognize that hot mechanical straightening is an unproven method which may lead to damaged or degraded steel. As such, its use should be considered only for non–load carrying elements when replacement or other methods are not viable.
Hot working is distinguished from heat straightening in that both large external forces and high heat are used. This method is similar to hot mechanical straightening in that external forces are used. In addition, the steel is heated well above the lower critical temperature and often glows cherry red indicating a temperature above the upper critical temperature. The results of this process are highly unpredictable and may result in:
- Fracture during straightening
- Severe changes in molecular structure which may not be reversible
- Severe changes in mechanical properties including a high degree of brittleness
- Buckles, wrinkles, crimps, and other distortions
Hot working should not be used to repair damaged structural steel.
Some practitioners will tend to over–jack and over–heat yet claim to be heat straightening. The reader is cautioned to be aware of these distinctions when specifying heat straightening as opposed to either hot mechanical straightening or hot working.
The basic concept of heat straightening is relatively simple and relies on two distinct properties of steel:
- If steel is stretched or compressed past a certain limit (usually referred to as yield), it does not assume its original shape when released. Rather, it remains partially elongated or shortened, depending on the direction of the originally applied force.
- If steel is heated to relatively modest temperatures (370–700°C or 700–1300°F), it expands at a predictable rate and its yield value becomes significantly lower while at the elevated temperature.
To illustrate how steel can be permanently deformed using these two properties; consider the short steel bar in Figure 5a. First, the bar is placed in a fixture, much stronger than the bar itself, and clamped snug–tight (Figure 5b). Then the bar is heated in the shaded portion. As the bar is heated it tries to expand. However, the fixture prevents expansion in the longitudinal direction. Thus, the fixture exerts restraining forces on the bar as shown in Figure 5c. Since the bar is prevented from longitudinal expansion, it is forced to expand a greater amount laterally and transversely through it’s thickness than in an identical unrestrained bar. Consequently, a bulge will occur in the heated zone. Because the bulge has been heated, its yield value has been lowered, resulting in some yielding which does not occur in the unheated portions. When the heating source is removed, the material will cool and contract three–dimensionally. The clamp cannot prevent the bar from contracting longitudinally. As cooling progresses the bar shortens and the bulge shrinks. However, a portion of the bulge remains even after the bar has completely cooled and the bar has shortened from its original length, Figure 5d. In essence a permanent redistribution of material has occurred in the heated zone leaving the bar slightly shorter with a small bulge. This permanent bulge, or thickening, in the heated zone is called “upsetting”. The redistribution of material is referred to as “plastic deformation” or “plastic flow”. The clamping force is often referred to as a restraining force. Through cycles of clamping, heating, and cooling, the bar could be shortened significantly.
This simple example illustrates the fundamental principles of heat straightening. However, most damage in steel members is much more complex than stretching or shortening of a bar. Consequently, different damage conditions require their own unique heating and restraining patterns.
The purpose of this chapter is to explain the basic techniques used in heat–straightening. There are three key elements to the heat–straightening process. The first is to select proper heating patterns and sequencing to fit the damage. The second is to properly control the heating temperature, and rate of heating and cooling. The third is to provide appropriate restraints during the heating cycle which can be relaxed or modified during the cooling cycle. The place to begin a discussion of heat straightening basics is with the first key: proper heating patterns and sequencing.
Several types of simple heating patterns exist. Effective heat straightening results when these patterns are combined into specific combinations. As a starting point in understanding heat straightening, first consider a flat plate. Most steel bridge members are an assemblage of plate elements arranged to maximize strength and stiffness while minimizing material. Once an understanding of the heating patterns for a single plate is developed, these concepts can be extended to other shapes. There are several basic heating patterns used for flat plates.
The vee heat is the most fundamental pattern used to straighten strong axis (category S) bends in steel plate elements. As seen in Figure 6, a typical vee heat starts with a very small spot heat applied at the apex of the vee–shaped area using an oxy–fuel torch. When the desired temperature is reached (usually around 650°C or 1200°F for mild carbon steel), the torch is advanced progressively in a serpentine motion toward the base of the vee. This motion is efficient for progressively heating the vee from top to bottom. The plate will initially move upward (Figure 6a) as a result of longitudinal expansion of material above the neutral axis producing negative bending. The cool material adjacent to the heated area resists the normal thermal expansion of the steel in the longitudinal direction. As a result, the heated material will tend to expand, or upset, to a greater extent through the thickness of the plate, resulting in plastic flow.
At the completion of the heat, the entire heated area is at a high and relatively uniform temperature. At this point the plate has moved downward (Figure 6b) due to longitudinal expansion of material below the neutral axis producing positive bending. As the steel cools, the material contracts longitudinally to a greater degree than the expansion during heating. Thus, a net contraction occurs. The net upsetting is proportional to the width across the vee, so the amount of upsetting increases from top to bottom of the vee.
This variation produces a closure of the vee. Bending is produced in an initially straight member, or straightening occurs (if the plate is bent in the opposite direction to that of the straightening movement, Figure 6c). For many applications, it is most efficient to utilize a vee that extends over the full depth of the plate element but, partial depth vees may be applicable in certain situations. When using partial depth vees, the open end should extend to the edge of the element. The vee depth is varied by placing the apex at a partial depth location. The most typical partial depth vees are the three–quarter and half depth. Applications for partial depth vees will be discussed in later sections.
Figure 7. Schematic diagram of edge heats used to heat–curve a beam. (note that line heats are applied about 2 in. from edge for inelastically stretched edges and thermal cut flanges with small notches)
If a smooth gentle bend is desired, a line near the edge of the member is heated. The line may be continuous or intermittent, depending on the degree of curvature desired. This pattern is often used to heat–curve rolled shapes in the fabricating shop. A schematic is shown in Figure 7.
Line heats are employed to repair a bend in a plate about its weak axis. Such bends, severe enough to produce yielding of the material, often result in long narrow zones of yielding referred to as yield lines. A line heat consists of a single straight pass of the torch, Figure 8. The restraint in this case is often provided by an external force although some movement will occur without external constraints. This behavior is illustrated in Figure 9. A line heat is applied to the underside of a plate element subjected to bending moments produced by external forces (Figure 9a). As the torch is applied and moved across the plate, the temperature distribution decreases through the thickness (Figure 9b). The cool material ahead of the torch constrains thermal expansion, even if external constraints are not present.
Because of the thermal gradient, more upsetting occurs on the torch (or hotter) side of the plate. During cooling this side consequently contracts more, creating a concave movement on the torch side of the plate similar to that shown in Figure 9d. Thus, to straighten a plate bent about its weak axis, the heat should be applied to the convex side of the damaged plate. The movement can be magnified by the use of applied forces which produce bending moments about the yield line (Figure 9c). Referring to a section through the plate transverse to the line heat (Figure 9c), the restraining moments tend to prevent transverse expansion below the plate centerline. In a manner similar to the vee heat mechanism, the material thus tends to expand through the thickness, or “upset”. Upon cooling, the restraining moments tend to magnify transverse contraction (Figure 9d). The speed of the travel of the torch is critical as it determines the temperature attained. With proper restraints and a uniform speed of the torch, a rotation will occur about the heated line.
For a spot heat, a small round area of the metal is heated by moving the torch in a slow circular motion increasing the diameter until the entire area of the metal is heated. A spot heat causes upsetting of the metal through the thickness due to the restraint provided by the cool surrounding material. On cooling, a spot heat leaves tensile stresses in all the radial directions across the heated area. During a spot heat, the torch should not be held at a particular point for too long, as the spot may get too hot and buckling may occur due to excessive thermal expansion on the heated side of the member. Spot heats are used to repair localized damage such as bulges, dents, bellies, or dishes in a plate element.
Strip heats, also called rectangular heats, are used to remove a bulge in a plate element or to complement a vee heat. Strip heats are similar to vee heats and are accomplished in a like manner. Beginning at the initiation point, the torch is moved back and forth in a serpentine fashion across a strip for a desired length, Figures 10 and 11. This pattern sequentially brings the entire strip to the desired temperature. The orientation can be an important consideration. The strip heat may be initiated at the midpoint and moved toward both edges simultaneously using two torches. This approach would minimize weak axis bending of the beam shown in Figure 11a. A second alternative with similar effect is shown in Figure 11b using a single torch and starting from one side. Depending on the structural configuration, the strip may also be started at a free edge as shown in Figure 11c. However, without restraints, this orientation may produce some weak axis bending. By alternating the initiation point to opposite edges in successive heating cycles, the weak axis bending can be minimized.
The fundamental damage categories have previously been defined. A yield pattern is associated with each damage category. The yield zone of steel is that area in which inelastic deformation has occurred. It is important to recognize the region of yielding because heat should only be applied in the vicinity of the yield zones. Typical yield zones are shown in Figure 12.
These sketches are schematic to depict the basic patterns. The yield zones may vary in length depending on the type of loading and degree of damage. Often, these zones can be determined by visual inspection and are identified by paint peeling or loosened rust and mill scale. Analytical methods are also available when necessary to accurately determine yield zones.
Figure 12. Continued.
The repair of damaged steel members often requires a combination of vee, strip, line, or spot heats. A series of such heats, applied consecutively as a group, is referred to as a heating pattern. The order in which these individual heats are conducted is referred to as the heating sequence. The process of conducting a complete heating pattern and allowing it to cool is referred to as a heating cycle. Structural steel shapes for bridges can be considered as an assemblage of flat plates. Almost invariably, damage to these shapes involves the bending of some of these plate elements about their own major axes. Consequently, the heat straightening of steel begins with the application of vee heats to such plate elements.
The application of a single vee heat to a flat plate has already been described. This basic vee heat is the building block upon which heat straightening of bridge members rest. The heating patterns used for the four fundamental damage categories are outlined in this section for typical rolled shapes.
The yield zone for category S damage to a wide flange beam is shown in Figure 13 along with the appropriate heating pattern.
The deformed shape of the typical bent plate is shown in Figure 14. The heating pattern is the full–depth vee as shown. Because the net change in curvature after one pattern of heats is small, cycles of heating and cooling are required to completely straighten a damaged plate. For each cycle, the vee (or vees) should be moved to a different location in the vicinity of the yield zone region as suggested by the dashed lines in Figure 14 so that the exact same spot is not continually reheated. More heats should be placed in the central part of the yield region and fewer near the extremities to reflect the difference in damage curvature. This principle applies for all heating patterns in the following sections.
As shown in Figure 15, the heating patterns for these cases consist of a vee and strip heat combination. For purposes of defining heating patterns, it is convenient to refer to the elements of a cross section as either primary or stiffening elements. The primary elements are those damaged by bending about their major axes, such as the webs in Figure 15. The stiffening elements are those bent about their minor axes, such as the flanges in Figure 15. Typically, vee heats are applied to primary elements while strip, line or no heat at all may be applied to stiffening elements. For the case under consideration here, a vee heat is first applied to the web. Upon completion, a strip heat is applied to the flange at the open end of the vee.
The width of the strip equals the width of the vee at the point of intersection. This procedure allows the vee to close during cooling without restraint from the stiffening element. No heat is applied to the flange at the apex of the vee. This vee/strip combination is repeated by shifting over the vicinity of the yield zone until the member is straight.
The heating pattern for these cases is similar to the previous case but note the primary and stiffening elements are reversed. The vee heat is first applied to both flanges (either simultaneously or one at a time) as shown in Figure 16. After heating these primary elements, a strip heat is applied to the web. The only exception is that no strip heat is applied to stiffening elements located adjacent to the apex of a vee heated element since this element offers little restraint to the closing of the vee during cooling. Note that the width of the strip heat is equal to the width of the vee heat at the point of intersection. For all cases the pattern is repeated by shifting within the vicinity of the yield zone until the member is straight.
The heating pattern for this damage case is shown in Figure 17. The vees on the top and bottom flange are reversed to reflect the different directions of curvature of the opposite flanges. The vee heats are applied first and then the strip heat is applied. Note that for the channel, the strip heat need only be applied to half depth. This half depth strip allows the lower flange vee to close with minimal restraint from the web.
A local buckle or bulge reflects an elongation of material. Restoration requires the bulging area to be shortened. A series of vee or line heats can be used for this purpose as shown in fig. 18. These vees are heated sequentially across the buckle or around the bulge. For web bulges either lines or vees may be used. If vees are used, they are spaced so that the open ends of the vees touch. There is a tendency for practitioners to over–heat web bulges. For most cases, too much heat is counter–productive. The preferred pattern is the line heats in the spoke/wagon wheel pattern. For the flange buckle pattern (Figure 18b) either lines or a combination of lines and vees may be used. For most cases, the line pattern with few or no vees tends to be most effective. Since the flange damage tends to be unsymmetrical, more heating cycles are required on the side with the most damage.
Since angles usually do not have an axis of symmetry, the heating pattern requires special consideration. Typically, the heating pattern is similar to that of a channel. However, the vee heat on one leg of an angle will produce components of movement both parallel and perpendicular to the heated leg. Thus, the heating pattern shown in Figure 19 may need to be alternated on the adjacent leg. Another method to minimize out–of–plane movement is to use the strip heat patterns suggested in Figure 11.
Most damage situations do not fit neatly into one of the fundamental damage categories. Rather, the damage is a combination of several of these categories. To repair these more complex cases, the damage should be viewed as a combination of the fundamental cases. The approach is to preplan the entire set of sequences, starting with the component of damage that is most severe. As straightening progresses, the process is redirected to other components, minimizing overlaps that counteract or unnecessarily reheat areas. By focusing on the fundamental damage categories in sequence, complex damage can be repaired by using the basic heating patterns described in the previous sections.
The primary equipment utilized for heat straightening is a heating torch. The heat source is typically an oxygen–fuel mixture. Typical fuels include acetylene, propane, and natural gas. The appropriate fuel is mixed with oxygen under pressure at the nozzle to produce a proper heating flame. A regulator is used to reduce pressures to working levels of 100–140 kPa (15–20 psi). Either a single or a multiple orifice tip may be used. The size and type is dictated by the fuel selected and thickness of material to be heated. A No. 8 single orifice tip is generally satisfactory for thicknesses up to 20–25 mm (3/4 or 1 in) with acetylene. For thinner material a smaller tip is recommended. If heavy sections are being heated, a single orifice tip may not be adequate. For such cases a rosebud or multiple orifice tip is recommended. The size may vary depending on the material thickness. The determining factor is the ability to raise the through–the–thickness steel temperature to the specified level. Note that whether single or multiple orifice, the torch should be a heating torch and not a cutting torch. The oxyacetylene fuel is preferred by many because it is a "hot" fuel. However, this fuel is also highly volatile. Some prefer a propane fuel, which is safer to handle. Since it does not burn as hot, a larger tip or rosebud orifice may be required. In either case the key is to be able to quickly heat a small area. Torch size and fuel must be adjusted to meet these criteria.
The fuel used in heat straightening is volatile and dangerous. Fuel tanks should always be handled with extreme care. Safety precautions include:
- Always place a protective cap on head of each tank before transporting. Always secure tanks prior to heat straightening.
- Examine tanks for damage prior to each use.
- Check lines and fixtures for leaks or damage prior to each use and that proper check valves are installed.
In addition, the technician using the torch must be safety conscious at all times. Precautions include:
- Wear protective goggles while heating (a no. 3 lens is recommended).
- Be careful of where the lighted torch is pointed at all times.
- Wear protective gloves and clothing.
- Always be in a stable, secure position prior to opening valves and lighting the torch.
- Follow proper procedures when using scaffolding and use safety harnesses when working above the ground. Secure tanks and hoses in safe positions prior to heat straightening.
One of the most important and yet difficult–to–control parameters of heat straightening is the temperature of the heated metal. Factors affecting the temperature include size and type of the torch orifice, intensity of the flame, speed of torch movement, and thickness and configuration of the member. Assuming that adequate control of the applied temperature is maintained, the question arises as to what temperature produces the best results in heat straightening without altering the material properties. Early investigators had different opinions on temperature control. However, more recent comprehensive testing programs have shown that the plastic rotation produced is directly proportional to the heating temperature, up to at least 870°C (1600°F).
The maximum temperature recommended by most researchers is 650°C (1200°F) for all but quenched and tempered high–strength steels. Higher temperatures may result in greater rotation but out–of–plane distortion becomes likely and surface damage such as pitting will occur at 760°–870°C (1400° to 1600° F). Also, temperatures in excess of approximately 700°C (1300°F) (metallurgically referred to as the lower phase transition temperature) may change the molecular composition, altering material properties after cooling. (See section 4.1 for a more detailed discussion justifying these temperature limits.) The limiting temperature of 650°C (1200°F) allows for about one hundred degrees of temperature variation, which was found to be a common range among experienced practitioners. AASHTO/AWS D1.5 (1996) specifies maximum heating temperatures of 590°C (1100°F) for quenched and tempered (Q&T) steels and 650°C (1200°F) for all others.
For A514 and non–HPS A709 (grades 100 and 100W), a minimum tempering temperature of 620°C (1150°F) is required. Thus, the 590°C (1100°F) limit provides a 30°C (50°F) safety factor. However, for Q & T A709 Grade 70W the specified minimum tempering temperature is 590°C (1100°F). A maximum heating temperature of 565°C (1050°F) is recommended for this grade to provide a 30°C (50°F) safety factor and to avoid property changes. HPS Grade 70W produced by thermo–mechanically controlled processing (TMCP) is not Q % T, so 650°C (1200°F) applies.
To control the temperature, the speed of the torch movement and the size of the orifice must be adjusted for different thicknesses of material. However, as long as the temperature is rapidly achieved at the appropriate level, the contraction effect will be similar. Various methods can be used to monitor temperature during heating. Principal among these include: visual observation of color of the steel (see 2.11.3); use of special temperature sensing crayons or pyrometers; and infrared electronic temperature sensing devices.
The term "restraining forces" can refer to either externally applied forces or internal redundancy and self–weight. These forces, when properly utilized, can expedite the straightening process. However, if improperly applied, restraining forces can hinder or even prevent straightening. In its simplest terms, the effect of restraining forces can be explained by considering the previous plate element as shown in Figure 6. The basic mechanism of heat straightening is to create plastic flow, causing expansion through the thickness (upsetting) during the heating phase, followed by elastic longitudinal contraction during the cooling phase. This upsetting can be accomplished in two ways. First, as the heat progresses toward the base of the vee, the cool material ahead of the torch prevents complete longitudinal expansion of the heated material, thus forcing upsetting through the thickness. However, as shown in Figure 6, some local longitudinal expansion occurs because the surrounding cool material does not offer perfect confinement. After cooling, the damage induced distortion is reduced in proportion to the confinement level from the internal restraints.
A second method of producing the desired upsetting (usually used in conjunction with the vee heat) is to provide a restraining force. The role of the restraining force is to reduce or prevent longitudinal plate movements associated with expansion during the heating phase. For example, if a restraining force is applied as shown in Figure 6, the upsetting effect will be increased by constricting the free longitudinal expansion at the open end of the vee. A restraining force is usually applied externally, producing a bending moment tending to close the vee. Caution must be used in applying external forces, since over–jacking may result in fracture of the member. To minimize the cracking potential, it is recommended that an external force be calculated and set prior to actual heating and not be increased until the cooling phase of the cycle is complete.
In essence, a restraining force acts in a similar manner to the cool material ahead of the vee heat torch movement. The material behavior can be viewed as shown in Figure 20. A small element from a plate, when constrained in the x–direction and heated, will expand and flow plastically primarily through the thickness (Figure 20c).
Secondary plastic flow will occur in the y–direction. However, this movement will be small in comparison with that of the z–direction, because the plate is much thinner than its y–dimension and offers less restraint to plastic flow. Upon cooling with unrestrained contraction, the final configuration of the element will be smaller in the x–direction and thicker in the z–direction (Figure 20d) than its original size. Regardless of the cause of the constraint, either cooler adjacent material, self weight, or an external restraining force, the plastic flow occurs in an identical manner.
Sometimes the structure itself provides additional restraint through redundancy. For example, if the simply supported beam depicted in Figure 6 were fixed at the supports, the member stiffness increases by 33 percent. This increased stiffness would provide additional restraint over the simply supported case.
In order to stay within the criteria for heat straightening, the restraint forces must not produce stresses greater than yield in the heated zone. At a heating temperature of 650°C (1200oF), the yield stress is reduced by approximately 50%. Therefore, a restraining force producing stresses of 50% yield (at ambient temperature) in the heated section would result in stresses at near initial yield when heated. Anything higher pushes the procedure into the hot mechanical straightening range. Therefore limit forces due to self–weight and applied restraint to those producing a maximum moment of 50% of the member capacity (in the heated area) at ambient temperature. This recommendation is somewhat conservative since the entire cross section is never at 650°C (1200oF). Rather, just the immediate area around the torch is at that temperature and the remainder of the cross section has already begun to cool (behind the torch) or is not yet heated (ahead of the torch). Thus, limiting the moment to 50% of member capacity keeps the procedure within the heat straightening zone. Another reason for limiting the force is that higher jacking forces increase the risk of fracture. This aspect is discussed in section 4.4.
In light of this, a set of criteria for restrain forces can be developed. These criteria apply for internal as well as external constraints.
- Constraints should be passive during the heating phase; that is, they should be applied before heating and not increased by external means during heating or cooling.
- Constraints should not impede contraction during the cooling phase.
- Constraints should not cause local bucking of the compression element during the heating phase.
- Constraints should not produce an unstable structure by either the formation of plastic hinges or member instability during the procedure.
- Constraints should be limited such that the maximum moment in the heated zone does not cause stresses that exceed 50% of yield at ambient temperature.
From a practical viewpoint, these criteria mean that (a) the vee angle should be kept small enough to avoid local buckling, (b) the external restraining forces must be applied before heating and be self–relieving as contraction occurs, and (c) the maximum level of any externally applied forces must be based on a structural analysis of the complete structure that includes the reduced strength and stiffness of a member due to the heating effects.
This description of the heat straightening process provides the basic methodology. However, the proper application of heat is a skill requiring practice and experience: at this juncture, the art of heat straightening meets the technology. The practitioner needs to understand the variables involved in the process and how to control them. Some of the more important variables are discussed here.
The amount of heat applied to a steel surface is a function of the type of fuel, the number and size of the orifices, the fuel pressure and resulting heat output at the nozzle tip. Selecting the appropriate tip size is primarily a function of the thickness of the material. The goal is to rapidly bring the steel in the vicinity of the torch tip to the specified temperature, not just at the surface, but throughout the thickness. Once this condition is obtained at the initial heating location, the torch should be moved along the path at a rate that brings successive sections of steel to the specified temperature. A tip that is too small for the thickness will result in insufficient heat input at the surface that does not penetrate effectively through the thickness. If the tip is too large, there will be a tendency to input heat into the region so quickly that it is difficult to control the temperature and distortion. Table 1 is a general guide for selecting a tip size. Intensity of the torch, ambient temperature, steel configuration, access, and fabrication details influence the choice of tips. Adjustments can also be made in the torch intensity to improve the heating response. A hotter flame is helpful if the configuration of the steel tends to draw heat away from the spot of heating. A less intense flame allows for a slower pace as the torch is moved along the path. The intensity may be adjusted so as to compensate for variables encountered in the field.
The pace of moving the torch along the path will be a function of the configuration of the member, location of damage and pattern selected. At the initiation of heating, the torch typically remains on a single spot as the temperature rises. Once the heating temperature is reached, a steady movement along the path of heating can usually be maintained. Practice heats will allow technicians to develop a feel for how to vary the torch speed over various configurations.
Attachments such as stiffeners may serve as a heat sink requiring the slowing of the torch movement over certain zones. One typical example is the heating of the flange of a rolled beam where the web–flange juncture must be heated more slowly since the web draws heat away from the flange.
Sometimes the pace must be quickened to maintain a uniform heat. A common example is the conclusion of a vee heat at a free edge. By the last pass along that edge, the wave of heat moving down the vee almost overtakes the torch. As a result, the last pass is usually conducted very quickly.
In theory, control of temperature may seem easy: watch the color of the steel and use temperature crayons. In practice, temperature control is quite difficult. First, the satiny silver color of steel indicating 650°C (1200°F) is often obscured. The torch flame often reacts with surface impurities including paint, oil or previous temperature crayon marks themselves. When the flame hits these, it may burn bright yellow or orange
and hide the surface near the tip. Additionally, the surface temperature directly under the flame will briefly exceed specified limits in order to convey heat into the metal. Therefore, temperature should not be checked until the flame leaves the area for a 3 to 5 second “soak time”. The available light also influences observations. In daylight or bright indoor light, the silver color is easier to read and no dull red can be seen. However, in dark shadow zones or on overcast days or with limited artificial light, the steel will emit a dull red glow at the same temperature. No. 3 goggles may mask subtle colors so an observer without goggles may be needed. As a general rule, if red is visible in normal lighting, the steel is too hot. When heat straightening is done properly, the steel is not heated above its lower phase transition temperature and its properties will not change significantly. Overheating may create brittle, fracture sensitive zones, which could result in a sudden failure. Constant attention is required to maintain the heating temperature in the correct range. Practice is essential to recognize and control the temperature.
Earlier, a clear distinction was made between hot mechanical straightening and heat straightening. The technique of hot mechanical straightening consists of lowering the yield strength by heating and then applying sufficient jacking loads in a single application to straighten the damage by inelastically deforming the section. Heat straightening on the other hand, requires that the restraining forces result in stresses not exceeding yield at the elevated temperature. Movement occurs as a result of plastic deformations during contraction, not by mechanical overload. Therefore, initial restraining forces are an integral part of heat straightening.
First, one should know how much external force is being applied to the system. Thus, all jacks should be gauged and calibrated. Second, the maximum jacking force should be calculated to insure that over–stress at elevated temperatures will not occur. Often, these computations require a structural engineering analysis, but for frequently encountered cases, some rules of thumb can be established. The practitioner must be aware that over–jacking may cause over–correction, buckling or a sudden fracture during the process. It might also result in difficult to detect micro–cracks which could severely reduce fatigue resistance.
One key to heat straightening is selecting appropriate heat patterns to fit the yield zones of the steel. Basic patterns were illustrated in Figures 14–19. Yield zones, where the steel has inelastically deformed, occur in regions of sharpest curvature. Some practitioners have a tendency to heat in a broader zone, but this again is a case of more being less. Stay with the recommended patterns and do not expand them. Heat straightening is a cyclic process and the movement occurs gradually by contraction during cooling. Sometimes 20 or more heating cycles may be required to straighten a damaged member. Since a heating pattern usually covers only a portion of the yield zone, the pattern should be shifted on a cycle–by–cycle basis. The significant portion of a heating pattern array should be in the yield zone with fewer heating cycles having patterns near the edges and more near the center where curvature is the sharpest. Also, do not duplicate continuous passes through a given zone during one heating cycle. Going back and re–heating before the material has cooled interrupts the contraction process. The heat straightening predictability and effectiveness is consequently reduced.
|Steel Thickness||Orifice Type||Size|
When a combination of vee, strip and/or line heats are used, the order of heating is referred to as the sequence. The sequencing of heats may be important in some straightening operations. However, little research has been conducted to verify its effects. Some practitioners feel that proper sequencing will accelerate the straightening and help keep residual stresses to a minimum. Consider the case of an I beam with Category S damage requiring a vee heat in the web and a strip heat in the flange as shown in Figure 15.
A common sequence is to heat the vee first, followed immediately by the strip. The available research data and difference sequences used in practice indicates that more than one sequence can be successful. At this time there is not adequate documentation to mandate one sequence for a particular heating pattern. The experience of the practitioner is the most reliable guide to proper sequencing. The sequencing patterns shown in this manual are based on those often successfully used in practice.
One of the more perplexing aspects of heat straightening is that sometimes there is no movement. Should this happen, perform several cycles, making sure to shift to new locations within the yield zone after each cycle. Sometimes there is an existing residual stress pattern or restraint imposed by the structure tending to oppose movement. Several heating cycles will tend to redistribute or dissipate these opposing stresses and may lead to the desired movement. Should the problem persist, the jacking forces may be too low. A re-analysis of the jacking layout is recommended, particularly in light of redundancies that may exist. Finally, check the heating patterns to insure they are consistent with the damage. For example, neglecting to heat all separate yield zones during one heat cycle could prevent movement. The key point is that if the steel doesn’t move, there is a reason. It is a matter of finding the reason. Difficult problems may require a consultant more experienced in heat straightening or replacement of the element. Over-heating or over-jacking is not a solution.
Ambient air cooling is the safest method. Rapid cooling is dangerous if the steel has been over-heated and may produce brittle “hot spots”. However, once the steel has cooled below the lower phase transition temperature, rapid cooling is not harmful. Many practitioners allow the surface of the steel to cool below 315°C (600°F) prior to accelerating cooling. Such a surface temperature reduction insures that the interior steel temperature has dropped. One approach to accelerated cooling is to use compressed air blown on the heated surfaces. Faster cooling can be obtained with water mist cooling. However, the steam generated could result in burns and the water runoff could lead to a clean-up problem especially if it covers areas which must be subsequently heated. The following cautionary measures should be taken when considering this option: (1) a mist applicator which allows the technician to remain at a safe distance; (2) protective clothing and goggles; and (3) a method for safely disposing of the waste water.
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