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Journal of Failure Analysis and Prevention

, Volume 13, Issue 2, pp 140–143 | Cite as

Failure and Stress Analysis of Deformed Steel Tube

  • A. Nusair Khan
  • M. Mudassar Rauf
  • I. Salam
  • S. H. Khan
Case History---Peer-Reviewed
  • 176 Downloads

Abstract

A deformed steel tube was received for failure analysis. The buckling of the tube had occurred during the heat-treatment operation. The received tube was subjected to different metallurgical tests, and a nondestructive test was also employed to confirm the presence of residual stresses in the tube. It was observed that the microstructure of the tube was homogenous and had no banded structure. However, X-ray diffraction analysis confirmed the presence of up to 6% retained austenite which may have caused the distortion of the tube.

Keywords

Retained austenite Deformation of the tubes 28Cr3SiNiWMoV steel Bainitic steel Eddy current testing 

Introduction

The increase in temperature increases the thermal vibration pushing the atoms apart, increasing their mean spacing, and causing thermal expansion (α). Perhaps the most important physical property of steel to be considered in design is its coefficient of thermal expansion. Most heat-treating problems could be solved if this coefficient could be controlled [1]. Almost all solids expand on heating and contract upon cooling. The relationship between thermal conductivity and thermal expansion is important in designing to prevent thermal distortion. Thermal gradients can cause a change of shape, which is a distortion of the component. The distortion is proportional to the gradient of the strain, and so it is proportional to the thermal gradient. By Fourier’s first law, the heat flow is proportional to the thermal gradient through the thermal conductivity (λ). For a given geometry and heat flow, the distortion is minimized by selecting materials with large values of λ/α [2]. For example, austenitic stainless steels have low thermal conductivity and high thermal expansion. Thermal expansion has a strong influence on the development of residual stress. Whenever the thermal expansion or contraction of a body is prevented, thermal stresses appear. The magnitude of stresses is related to the yield stress of the material at the temperature at which the deformation occurred; if these stresses are large enough, they cause yielding, fracture, or elastic collapse (buckling).

The evaluation and monitoring of these stresses by nondestructive method may play an important role in assessing the life of the components. In this study, the presence of residual stresses is analyzed by eddy current method. Further, a complete investigation was made to analyze the causes of deformation of the steel tube, during the heat-treatment cycle.

History

Steel tubes having diameter 100 mm and length 300 mm were severely cold deformed. These tubes were heat treated at 910 °C for 30 min and cooled down to room temperature in the same furnace. During heating and cooling, nitrogen atmosphere was provided to minimize the scale on the surface of the tubes. After completing the heat-treatment cycle, it was observed that some tubes had a problem of distortion at one end, while the other end had no such deformation.

Visual Observations

Visual observation revealed that the received tube (deformed tube) was highly deformed at one end, Fig. 1, while the other end had no such deformation. In order to obtain samples, the tube was cut (1.5 mm) from the deformed side. It was observed that the tube regained its original shape after cutting.
Fig. 1

Schematic showing the dimensions of the tube before and after annealing cycle

Chemical Composition

Chemical composition was determined using energy dispersive scanning spectrometer (EDS) and combustion type carbon/sulfur analyzer. The results showed that the material is equivalent to 28Cr3SiNiWMoV steel, Table 1.
Table 1

Chemical composition of the pipe material

Elements

Chemical composition, wt.%

Sample

Standard material 28Cr3SiNiWMoV

Fe

Balance

Balance

Cr

2.9

2.8–3.2

Ni

1.0

0.8–1.2

W

0.7

0.8–1.2

Mo

0.8

0.35–0.55

V

0.2

0.05–0.15

Si

1.0

0.9–1.2

Mn

0.7

0.5–0.8

C

0.3

0.25–0.32

S

0.0054

···

Hardness Testing

Vicker’s hardness testing was performed on the circumference of the tube in the deformed region. It was observed that the hardness remains same throughout the scan. The obtained hardness was in the range of 430–450 Hv.

Phase Analysis

In order to investigate the presence of retained austenite, the samples were subjected to X-ray diffraction (XRD) analysis. For this purpose, the samples were analyzed from 12 different locations along the periphery of the tube, Fig. 2. The results show retained austenite at four different locations, Fig. 3. The retained austenite was up to 6% in these locations, whereas, other locations were observed to be having 100% bainite/martensite. The uneven distribution of retained austenite demonstrates that the material is inhomogeneous in terms of chemical composition. Thus, during cooling cycle in the annealing treatment, stresses were developed because of nonhomogeneous transformation of the material.
Fig. 2

Schematic after opening of the tube showing the positions at which the XRD analysis was performed

Fig. 3

XRD scan showing the peak of retained austenite

Metallography

The sample from the tube was mounted in two directions, i.e., along the tube and across the tube. The samples revealed that the grain size is in the range of 5–10 μm and are homogenously distributed throughout the tube, Fig. 4. No banded structure was observed in the samples. Further, some nonmetallic inclusions were also identified within the tube which was revealed to be rich in Molybdenum, Fig. 5 (confirmed by EDS at 30 KV).
Fig. 4

Surface of the tube showing fine grain size

Fig. 5

Molybdenum-rich inclusions observed in the material

Characterization by Eddy Current Method

Eddy current testing is based on the principles of electromagnetic induction and is sensitive to the electrical conductivity and magnetic permeability of conductive materials. Eddy current evaluation technique can be applied on both conductive and nonconductive materials depending upon their permeability: however; the permeability must not be equal to one. The steel specimens are good conductors and have the magnetic permeability greater than 1. The presence of residual stresses may affect the change in eddy current response since the stresses may distort the structure of the material [3]. This localized distortion of the structure disturbs the magnetic behavior of the material, and consequently, the eddy current response varied from point to point in the presence of residual stresses. The eddy current response of the specimens was studied at 4 KHz. Figure 6 demonstrates the change in eddy current response at different locations of the tube. The impedance values are plotted around the circumference of the tube, whereas, each curve has been taken along the tube axis.
Fig. 6

Changes in impedance measured at the circumference of the tube with schematic of the tube showing the positions at which the values are measured

It can be observed that the stresses at both ends of the tube are either in compressive or in tensile state; this is because there is no variation in the curves, as the stresses in a body usually balance each other. Whenever the stresses are in unbalanced state, they may induce fracture, cause to yield, or collapse (buckling) the body elastically, depending on the magnitude and the strength of the material. The buckling at one end of the tube demonstrates that the magnitude of the stresses is high enough to distort the shape elastically.

Discussion

28Cr3SiNiWMoV steel is a bainitic steel which during furnace cooling revealed 100% bainite. The steel showed a homogenous structure with no presence of banded structure ascribing to the fact that the material is apparently homogenous. Further, the uniformity in hardness along the circumference of the tube also did not indicate the inhomogenity in material. However, XRD revealed the presence of 1–6% retained austenite at various locations. Since the austenite is the softer phase compared with bainite, it will be expected that the transformation of rest of the bainite induced the mechanical stresses, upon cooling, which will be distributed equally in all directions. However, the regions having the retained austenite did not sustain the stresses thus forcing the tube to deform. The produced stresses are well within the range of elastic limit, as when one end of the tube is cut, it regains its original shape.

Eddy currents are sensitive to any changes in material which may be due to microstructure, metallurgical phases, or residual stresses [4]. In the present study, the grain size is similar throughout the tube because of severe plastic deformation operations. However, retained austenite, which is present at some locations, may interfere with the impedance of the material, as observed while studying the data of residual stresses induced through eddy current in deformed tubes. The eddy current data revealed that trough and crest are formed after constant intervals, Fig. 6. This perhaps indicates that the residual stresses are induced in the tube and not in the retained austenite, since the retained austenite is distributed randomly and is present only in very small percentage. The rise and fall in peaks indicate the compressive and tensile nature of stresses in the tube. Usually, when the system is in equilibrium, the stresses are always in balance.

Conclusion

Bainitic steel tube was received for failure analysis. The failure was due to the deformation of the tubes during heat-treatment cycle. The samples were metallurgically analyzed and were found to contain up to 6% retained austenite which probably caused the distortion in tubes. Further, the residual stresses were also studied effectively by a nondestructive eddy current method.

References

  1. 1.
    Kernand, R.F., Suess, M.E.: Steel Selection, p. 35. Wiley, New York (1979)Google Scholar
  2. 2.
    Ashby, M.F.: Material Selection in Mechanical Design. Butterworth Heinemann, Oxford (1999)Google Scholar
  3. 3.
    Zergoug, M., Kamel, G., Boucherou, N.: Mechanical stress analysis by eddy current method. J. Am. Sci. 4(4), 1–6 (2008)Google Scholar
  4. 4.
    Barac, D., Katcher, W., Soules, J.: Advances in eddy current measurement of residual stress. In: 7th International Conference on Shot Peening, pp. 326–335. The Institute of Precision Mechanics, Warsaw (1999)Google Scholar

Copyright information

© ASM International 2013

Authors and Affiliations

  • A. Nusair Khan
    • 1
  • M. Mudassar Rauf
    • 1
  • I. Salam
    • 1
  • S. H. Khan
    • 1
  1. 1.Institute of Industrial and Control SystemsRawalpindiPakistan

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