Effect of Heat Treatment Method in Minimizing the Residual Stress Level in Cold Drawn Welded Tubes (Cdw/Dom)

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Abstract:

Conventionally cold drawn welded tubes are stress relieved in a roller hearth furnace. In this study, induction stress relieving is compared with the conventional method for plain carbon steels. Technical comparison between the two methods is conducted by mechanical properties measurement and residual stress analysis by X-ray diffraction method.

Introduction:

Residual stress is a macroscopic stress that is set up within a metal during non-uniform plastic deformation, as in cold drawing or thermal gradients, as in quenching or welding. Up to 45% area reduction is employed in the manufacturing process of CDW tubes resulting in residual stresses in the tube. These can be tensile or compressive based on the type of cold work. Residual stress can affect the fatigue strength and fracture toughness of the material and can cause stress corrosion cracking and dimensional instability during further machining, welding and forming operations. Hence minimizing the unfavorable residual stress is an important process requirement for tubes involving critical applications. Residual stress is usually relieved by Stress Relieve Annealing (SRA), a low temperature annealing (350oC-600oC) in a roller hearth furnace.

Induction Heating System and Continuous Roller Hearth Furnace

Induction heating is a non-contact heating process, in which intense electromagnetic field generated by an AC current is used to heat the job. The shorter and higher temperature cycle in Induction heating yields the same mechanical properties as obtained in a conventional roller furnace where the tempering cycle time is higher. The high frequency induced generates skin effect, which ensures that the current will flow in a thin layer in the surface of the work-piece. This increases the effective resistance of the metal and enhances the heating effect.

In a continuous roller hearth furnace, CDW tubes are annealed, normalized, stress relieved or tempered with a protective atmosphere. Continuous process lines pass through the roller furnace, at temperatures specified for the process.

Definition of Residual Stress

When the stress applied exceeds the yield strength, the material deforms. The internal structure adapts itself to the load applied. But there exists a stress that can be tensile or compressive, which continue to exist in the material even after the load is removed. Metallurgically, three kinds of residual stresses are defined - the macro stresses (or stresses of first kind) over a few grains, the stresses of second kind over one particular grain and the stresses of third kind across sub-microscopic areas, say several atomic distances within a grain. The stresses of second and third kind are also called micro stresses. (Fig.1)

Generally, compressive residual stress has a beneficial effect on the fatigue life and stress corrosion because it delays crack initiation and propagation, as they tend to close the cracks. Tensile stress on the contrary reduces the mechanical performance of materials. In the elastic range, the residual stress can just be added to the applied stress as a static load. For this reason, compression can reduce the stress level of the layers where the applied load is high. This leads to an apparent increase of the fatigue limit.

Figure.1

This gains more significance in the case of cyclic loading where cracks initiate and propagate at a stress level much lower than the yield strength. This leads to a redistribution of the residual stress. It should also to be noticed that residual stress relaxation, due to fatigue or annealing, and surface layer removal (machining) produce dimensional changes of industrial components.

Stress Relieving

Stress Relieving rearranges atoms or molecules from their metastable position to a stable equilibrium position, associated with lower potential energy or stress state. The cold drawn tube is heated to 3500 C- 6000 C to held at the elevated...
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