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 temperature for some time and cooled to room temperature. When stress relieving is performed at relatively low temperatures, hardness, tensile strength and elastic properties of most cold-drawn steel are increased. The choice of a specific time and temperature is dependent on chemical composition, cold drawing process and the mechanical properties required in the finished tube. 

Evaluation of Residual Stress

Various measurement techniques are developed to measure the Residual Stress, which include ring core technique and bending deflection method. Advanced technologies include X-ray diffraction, neutron diffraction, ultrasonic technique and magnetic methods.

X - ray diffraction method uses Bragg’s law to evaluate the residual stress. The diffraction angle calculated with the lattice spacing –measured by strain gauges – is used for stress tensor computations.
 
Experiments & Results
 

Residual stress testing procedure & results

A spot is identified for testing and is electrolytically polished. The variables for polishing are selected based on the material and conducted as per the guidelines from ASME handbook.
The scattered X-rays from a chromium target from the irradiated atoms/crystals are reinforced in a specific direction of an extremely narrow range and propagated. The schematic of the XRD set up used is indicated in Fig. 2. The condition of diffraction is met using Bragg’s law, nl = 2d sinq, where d is inter atomic spacing, q is the Bragg’s angle and l is the wavelength characteristics of the X ray radiation. The diffraction patters so obtained is measured for various Y angles i.e., 0,5,10,15,20,25 and 40. (Y = Yo+ q), in terms of a shift in 2q, which accounts for a change in the inter planar spacing. If tension is applied in parallel to the sample surface, the applied tension will act on the different groups of crystals in different angles, as force is a vector. Accordingly the inter planar spacing will vary with respect to the angle between the sample plane and the lattice plane. The stress value is determined from the slope of the plot of 2q Vs sin2Y.
 
 

Figure 2.

Stress s = K x D (2q/ sin2Y)

K = -1/2 x [E / (1+m) ] cot q (p/180)

The variables in the X-ray measurements like the type of target material used, (Chromium, cobalt or copper.) measurements method (Fixed Y method or Fixed Y0 method), tube current stepwidth, low / high 2q angle, oscillation number, fixed time etc. are standardized on case-to-case depending up on the material / component, texture, grain size (coarse or fine grained) etc.

Residual stress measuring was carried out on three types of steels listed in Table 4. Negative values indicate compressive stress and positive values indicate tensile stress.
 

AD indicates As Drawn

Results

Stress relief annealing using a continuous roller hearth furnace and induction heating system are compared. The following are the observations established from this study.

The tensile properties of medium carbon steels was shown to be higher for samples SR annealed in an induction heating system, without compromising the elongation properties of the material. For low carbon steels, both the processes yield almost the same result.

Surface residual stresses in the material decrease using both methods. However, the values close to the no-stress range are observed after SR annealing in the induction heating system.
Induction heating system can be used for small batch production and development and research activities. This is the result of conformance to the desired properties – as conventionally obtained using a roller hearth furnace – along with the inherent advantages of higher speed, reduced dwell periods, selective heating capability to produce quality parts (at lower cost), lower space requirement and better operating environment.

The residual stress analysis in this study is limited to the surface of the material. Therefore further studies are required to compare the core residual stress of the material after the two processes, the extent of residual stress relief required to achieve the desired properties and to conclude the better between the two SRA processes.

Contact Person:
 
Sandeep Mathew Olickal/Sekar C K,
Process Development,
Tube Products Of India – Exports Division,
M.T.H Road, Avadi, Chennai,
INDIA – 600 054
Ph: 91-44-2638 3338


www.tubeproductsofindia.com