Defect Characterisation on Zirconium Alloys Employing Multi-nde

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DEFECT CHARACTERISATION ON ZIRCONIUM ALLOYS EMPLOYING MULTI-NDE

 

ABSTRACT:

 Non-destructive Evaluation(NDE) is a tool widely used in research purpose and industry for evaluating the properties of material, component or system’s fitness for use, material soundness, and in-service inspection. The primary advantage of NDE is material evaluation &testing without causing damage. Non- destructive evaluation is one part of the function of quality control and is complementary to other long established methods. By definition NDT is the testing of materials, for surface or internal flaws or metallurgical conditions, without interfering in any way with the integrity of the material or its suitability for service. This project was aimed to characterize the defect present in Zirconium alloy components employing multi-NDE tools for ascertaining their root cause.

 

Keywords: .Zirconium Alloy. NDE

1.  INTRODUCTION:

1.1. Zirconium Alloy:

 Zirconium is a commercially available refractory metal with excellent corrosion resistance, good mechanical properties, very low thermal neutron cross section, and can be manufactured using standard fabrication techniques. Zirconium alloys have superior thermal properties compared to other traditional materials in consideration for spent nuclear fuel containers. Zirconium alloys have a thermal conductivity more than 30% higher than stainless steel alloys. The linear coefficient of thermal expansion for Zirconium alloys is nearly one-third the value for stainless steel giving zirconium alloys superior dimensional stability at elevated temperatures. This is an advantage in nuclear waste containers where temperatures could exceed 200ºC for hundreds of years.

 1.2. Zirconium Alloy Properties:

  Zirconium resists corrosive attack in most organic and mineral acids, strong alkalis, and some molten salts. Solutions of nitric acid (HNO3 ), sulfuric acid (H2 SO4 ), and hydrochloric acid (HCl) with impurities of ferric, cupric and nitrate ions generally result in corrosion rates of less than 0.13 mm/a (5 mpy) even at temperatures well above the boiling point curve. A tightly adherent and protective oxide film protects the metal-oxide interface to provide corrosion resistance. An additional benefit for zirconium alloys in long-term geological disposal options is the inert nature of zirconium oxide[1]. Application of zirconium alloys alleviates the concern of nickel and chromium contamination in the ground water in severely corroded spent fuel containers[2]. Zirconium alloys produced by ATI are available in a wide variety of sizes and shapes including plate, strip, sheet, foil, tubular products, rod, and wire. Wrought products are typically supplied in an annealed and conditioned form.

Table no: 1: THERMAL NEUTRON CROSS SECTIONS (BARNS)

Magnesium

0.059

Lead

0.17

Zirconium

0.18

Zircaloy

4 0.22

Aluminium

0.23

Iron

2.56

Austenitic Stainless Steel

3.1

Nickel

4.5

Non destructive testing:

 NDE stands for non-destructive evaluation. In other words it is a way of testing without destroying. NDE can be used to ensure the quality right from raw material stage through fabrication and processing to pre-service and in-service inspection.

 In today’s world where new materials are being developed, older materials and bonding methods are being subjected to higher pressures and loads[4], NDE ensures that materials can continue to operate to their highest capacity with the assurance that they will not fail within predetermined time limits.

 Apart from ensuring the structural integrity, quality and reliability of components and plants, today NDE finds extensive applications for condition monitoring, residual life assessment, energy audit, etc.

We have conducted NDE techniques mainly referring to these characteristics

  • Material Types
  • Defect Type
  • Defect Size
  • Defect Location (Refer to TABLE NO:2)

 

TABLE NO:2 COMMON NDE METHODS:

 Techniques

Capabilities

Limitations

Visual Inspection

Macroscopic surface flaws

Small flaws are difficult to detect, no subsurface flaws.

Microscopy

Small surface flaws

Not applicable to larger structures; no subsurface flaws.

Dye Penetrant

Surface flaws

No subsurface flaws; not for porous materials.

Magnetic Particle

Surface/near surface and layer flaws

Limited subsurface capability; only for ferromagnetic materials.

Eddy Current

Surface and near surface flaws

Difficult to interpret in some applications; only for metals.

Ultrasonic

Subsurface flaws

Material must be good conductor of sound.

Radiography

Subsurface flaws

Smallest defect detectable is 2% of the thickness; radiation protection. No subsurface flaws and not for porous materials.

2.Characterization of  zirconium alloy:

In our work we carried out tests on 4 types of specimen  INGOT, BILLET, BAR, WELD PART

2.1. Characterization Of Zirconium Alloy  Ingot:

 Triple melted ingots having a diameter of 350mm are machined to around 337mm. Visual examination is carried out on the machined ingot surface for defects such as porosities, blowholes, cavities, etc and the ingots, which are free from all these defects, are accepted for further processing. Before further processing these materials are examined by Ultrasonic Testing for ensuring soundness. During regular processing, one ingot shown significant indication in Ultrasonic Testing causing rejection. This ingot was sectioned and analyzed by NDE methods

Ultrasonic testing parameters:

Material: Zirconium alloy

Unit: Olympus

Probe frequency: 2 MHz, + 0.5″Ø

       Result:-

On conducting ultrasonic testing for an ingot, we have observed a surface echo, defect echo and a back wall echo.

(refer to fig 1 (II.A))

Fig.1  Defect echo in ingot

2.2.Characterization Of Zirconium Alloy Billet:

 Hot extruded and machined Zirconium alloy billet stocks are being processed further for fabrication of precision tube and components. Before further processing these materials are examined by Ultrasonic Testing for ensuring soundness. During regular processing, one billet shown significant indication in Ultrasonic Testing causing rejection. This billet was sectioned and analyzed further employing Ultrasonic Testing, Radiography and Metallography.

 Ultrasonic Testing:

       Parameters:-

Material: Zirconium alloy

Unit: Olympus 

Probe frequency: 4 MHz, + 0.5″Ø

Velocity: 4800 m/s

Gain: 67-68 dB

Standard defect size: 0.8mm flat bottom hole

       Result:-

 On conducting ultrasonic testing for a billet, we have observed a surface echo, defect echo and a back wall echo.( Refer to fig 2  (II.B))

Fig .2 Defect echo in a billet

  Radiography Testing:

       Parameters:-

Material: Zirconium alloy

Current: 5 mA

Voltage: 195 kV

Time: 9 mins.

Penetrameter:9 thou.

 

Fig.3 Porosity defect

       Result:-

 After radiography testing for billet, we have seen porosity in circumferential manner near the outer surface.(refer fig 3(II.B))

Metallography:

       Result:-

The following microstructural details and porosity defects have been observed under the optical microscope.(refer fig 4 & 5)

    

Fig.4 Zircaloy Longitudinal section               Fig.5  Zircaloy Transverse section

2.3. Characterization Of Zirconium Alloy Bar:

 Hot extruded and machined Zirconium alloy billets produce bars which are being machined to manufacture end plugs which are being used for encapsulation of PHWR cladding tubes. The challenge in manufacturing of end plug material was to avoid micro flaws and porosities, which affect the integrity of fuel element. The bar material undergoes NDE methods such as Ultrasonic Testing and Metallography. Observation and results are given below.

  Ultrasonic Testing:

       Parameters:-

Material: Zirconium alloy

Method: Pulse – echo immersion testing method

Rod size: 13.47 – 13.69 m

Probe 1 frequency: 15 MHz, normal beam

Probe 2 frequency: 10 MHz, normal beam

Probe 3 frequency: 10 MHz, offset – angle beam

Standard defect size: 0.6 mm and 0.9 mm radial hole

       Result:-

 On conducting ultrasonic testing for a bar, we have observed a surface echo, defect echo and a backwall echo. (refer to fig 6)

 

Fig. 6   Defect in 37element  standard rod

 Metallography:

       Result:-

 The following micro structural details and porosities have been observed under the optical microscope. (refer to fig 7 & 8)

  

  Fig.7  Longitudinalsection    Fig.8 Transverse section

2.4.Characterization Of Zirconium Alloy Weld Samples:

 The weld material undergoes NDE methods such as Liquid Penetrant test and Radiography in order to identify weld defects such as porosities, lack of penetration, lack of fusion, lamination, etc. These weld defects affect the integrity of fuel element and hence several NDE methods are used to check the soundness of the weld.  Liquid penetrant testing is a widely used NDE method for detecting surface defects on Zirconium alloy components. Observation and results are given below.

Liquid Penetrant Testing:

 This method consists of applying a liquid capable of penetrating surface discontinuities onto the surface of interest, wiping off any excess penetrant and then applying a developer capable of absorbing any penetrant entrapped in the defects, thus magnifying the indication and making it visible. The analysis describes the tests performed to characterize the products to be used in liquid penetrant testing on non-porous weld materials whose surface temperature is higher than 500C.

fig.9 Liquid Penetrant Test

       Result:-

 On conductingLiquid Penetrant testing for a weld melt , we have observed surface discontinuities in the welded zones . (refer to fig 10)

 Radiography Testing:

Parameters:-

Material: Zirconium alloy

Current: 8 mA

Voltage: 160 kV

Time: 3 mins.    

Penetrameter:6 thou.

Fig.10 Cluster porosity in weld sample III(II.D)

       Result:-

 After conducting radiography test, we have seen 0.2 mm cluster porosity in the welded sample.(refer to fig 10)

Conclusion:

 On a whole, we can say that NDE is a revolutionary advancement in field of mechanical testing.  on the basis of its scope and increasing areas of application we can say that many more new advancements in NDE are yet to come. So hence we conclude that

       In conducting ultrasonic testing for an ingot, Billet, bar  we have observed a surface echo, defect echo and a back wall echo.

       In  radiography testing for billet,bar, weld we have  observed a porosity in circumferential manner near the outer surface.

       In  Metalography Testing for ingot, Billet, bar    We have observed  porosity defects have been observed under the optical microscope.

References:

[1].www.ndt.net/article/apcndt2013/papers/098.pdf

[2].www.nde-ed.org/index_flash.html

[3].www.wahchang.com

[4].www.barc.gov.in

[5].web.itu.edu.tr

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