CP&A Tells Crane Industry to Adopt Heat Straightening

Casper, Phillips & Associates Inc. (CP&A) has advised the crane industry not to overlook heat straightening as an efficient method of repair when structural beams or columns are damaged.

Heat straightening, when considered, is often found to be the ideal solution when a beam or column has plastically deformed — but has not cracked. It has been used to straighten bent ship-to-shore container crane booms, where a failure of the boom up limit switch caused the boom to bend. Heat straightening is not for situations where cracks form because of high-cycle fatigue.

CP&A — the company offers a wide variety of services, including specification, design, manufacturing review, modification, and accident investigation — is filling a gap in typical college curriculum by passing knowledge about this repair procedure to its own engineers, and wider industry.

If an engineer is not familiar with the art of heat straightening, they might have heard of flame straightening or heat curving, which are just different names for using the process of thermal upsetting to change the shape of steel. Heat straightening and heat curving are recognized by the American Institute of Steel Construction (AISC), the American Association of State Highway and Transportation Officials (AASHTO), and the American Welding Society (AWS).

Regardless, industry trends indicate a more common repair procedure is to cut out and replace the deformed steel, even if this may not be the most economical or quickest solution depending on the location or extent of the damage. Patch or lap plates are also commonly used to reinforce damaged areas.

Notably, many engineers are only a few steps away from more wholesale utilization of heat straightening, in that most will understand the science behind deformation and yield stress, which are the building blocks to the concept. Elastic deformation occurs when a load is applied to a piece of steel and the steel returns to its original shape when the load is removed. Plastic deformation is permanent deformation of the steel. Permanent deformation is when the steel is loaded and does not return to its original shape when the load is removed. Yield stress is the amount of stress a piece of steel can withstand before it permanently (plastically) deforms.

Richard Phillips, mechanical engineer at CP&A, said: “We’re not pushing heat straightening lightly. In practice, it requires a qualified craftsman with extensive experience to apply the technique correctly in field conditions. It is not a very common process outside of the ship building and steel shop fabrication industries. Further, engineering is often needed to design the restraint systems and supports that may be needed to facilitate the heat straightening process.”

Phillips explained that, sometimes, curved surfaces and beams are intentional. These curved surfaces can be created using a process of thermal upsetting to put a curve into a flat plate or straight beam. For example, thermal upsetting was utilized to create the double curvature legs for the Seattle Space Needle. It can also be used to control weld distortions for higher precision assemblies or to create camber in beams.

“Typically, in cranes,” he continued, “We use the term heat straightening to mean repair of bent areas by using the process of thermal upsetting to straighten them out. Heat curving, on the other hand, is when the process of thermal upsetting is used to intentionally put a curve in a beam or plate. While all heat straightening uses thermal upsetting, not all thermal upsetting is heat straightening.”

Heating up a small area

Some engineers think thermal upsetting is hot bending or hot rolling, but these are completely different processes. For hot bending and hot rolling, the entire area of steel to be bent is heated up to a fairly high temperature. Thermal upsetting only heats up a small area at any given time and uses lower temperatures than what would be used for hot bending.

Typically, hot bending and hot rolling of steel is processed at over 1,700°F (925°C). Depending on the grade of steel, thermal upsetting is performed in the 700°F to 1,200°F (370°C to 650°C) range. Common structural steel for cranes is ASTM A572; the maximum thermal upsetting temperature for A572 is 1,100°F.

“At these temperatures the steel should be a very dull and dark red. If the steel turns orange, too much heat has been applied. For hot rolling and bending it is common for the steel to be orange,” said Phillips.

Prior to heat straightening, a structural engineer will design a temporary support and / or restraint system. CP&A designs these supports and restraints.  Depending on the importance of the structure, a civil or structural engineer will be assigned to supervise the design and repair.

A support system is needed when the structure must be repaired while under load and the load carried by the damaged is detrimental to the efficient use of the thermal upsetting method. In this case, jacks are used in conjunction with support steel to unload the damaged section, which make for a much more efficient use of the thermal upset method.

At the same time, the support structure is designed to act as a restraint system, which also maximizes the repair efficiency. In cases where there is not much load carried by the damaged area, a restraint system that uses jacks to apply a nominal restraint load is designed to help speed up the repair process. Hydraulic jacks attached to the support or restraint struts provide an adjustable length to continue diverting the load around the area or continue providing restraint as the repair process progresses.

Phillips said: “The basic mechanism of thermal upsetting is to create localized plastic flow at the area of heat application. For repairs, heat is applied to the area of steel that has yielded in tension, which is generally the convex side of bent steel. The application of heat causes the steel to locally expand. But the steel around the heated area resists the expansion, as it is cooler and provides local restraint against expansion in the plane of the plate. But because the steel is not restrained in the through-thickness direction, and at relatively low temperatures, plastic flow will occur at the point of heat application and make the steel nominally thicker at that location. The increase in thickness occurs locally, and only on the heated side of the plate. When the heat is removed, the plastic flow stops, and the ‘now thicker’ steel cools down. As the steel shrinks while cooling, it ‘pulls inward’ towards the center of the heated location, such that there is a net tension on the thickened/heated side of the plate. For an initially flat plate, the heated side will thus produce a concave curvature in the plate around the heated area. Conversely, if heat is applied to the convex side of a bent plate, the tension will add concavity to the convex surface, making it less convex — in other words, it will start to straighten the bent plate. This process can be repeated multiple times, or ‘cycles’, but with each cycle the effectiveness tends to reduce slightly due to the geometry of the plastic flow.”

It is noteworthy that, as heat is applied to one side of a plate or piece of steel, the effect of the heat application prior to plastic flow will cause the heated surface of the plate to expand, causing a convex curvature to occur while heating prior to the initiation of plastic flow. This convex curvature is elastic and will rebound upon cooling.

Since the intent of the thermal upset process is to create a concave surface, any amount of elastic convex curvature created by the heating process is detrimental to the efficiency of the thermal upset process, as it reduces the net amount of concavity that is produced on a single heat cycle. To help the efficiency of the thermal upset process, restraints are used to reduce or eliminate the initial elastic convex curvature, thus forcing the plastic flow to occur more quickly and resulting in more concave curvature upon removal of heat. It is important to understand that application of restraint is not cold bending of the steel but is rather an application of elastic preload that results in a more efficient thermal upset process.

If you slowly move the point of heat application in a line, the result will be a concave linear bend in the plate along the line of heat application. If you apply heat to both sides of the plate, a local tension zone is created. If you make a filled-in ‘V’ pattern on both sides of a plate, the plate will shrink more at the top of the ‘V’ pattern, and only a small amount at the point of the ‘V’ pattern. This will result in a gradual ‘bend’ in the plate, while at the same time the plate will remain flat. Various locations and patterns of heat application are used to gradually straighten the steel.

Hydraulic jacks are used in restraint and support systems to unload the damaged area and / or to apply a limited restraint load to the damaged area. As noted earlier, this external force is not the primary mechanism of straightening. The low temperatures used in the thermal upset process helps maintain the material properties.

“The heat is typically applied by an oxyacetylene torch,” said Phillips, “With special tips to help the worker control the heat. Controlling the temperature is one of the most important parameters — it is also one of the most difficult, which is why the skill of the tradesperson is extremely important.”

Another equally important skill is understanding the location, pattern and sequence of heat and restraint application. The experienced tradesperson will attempt to ‘reverse’ the damage sequence to restore the bent plates. This sequence can be quite complex for compound bends and for structures with bends at multiple locations and / or faces. Application to closed sections that do not allow heat application to both sides of a bent plate are even more challenging and require a thorough understanding of the process and experience with similar repairs.

Heat straightening can be significantly more economical to repair a bent structural beam or column as compared to removal and replacement, provided the bent areas do not have significant cracking. Heat straightening may not be applicable in cases where deformations and damage is a result of brittle fracture. However, if done correctly, heat straightening can be used to repair damaged areas that are subject to high cyclic stresses, but extra care should be taken to ensure proper post repair alignment and the appropriate NDT used to assure the repaired area is free of initial cracks. Additional situations that may complicate the applicability of heat straightening include material degradation such as significant corrosion, or exposure to a high temperature such as a fire when the damage occurred. An experienced engineer can determine the applicability in cases where these items are present.

• The American Association of State Highway and Transportation Officials (AASHTO) has published a technical report (FHQA-IF-08-999) that goes deeper into heat straightening science.