Oxidation behavior of a V–4Cr–4Ti alloy during the commercial processing of thin-wall tubing

doi:10.1016/j.jnucmat.2007.03.073

Journal of Nuclear Materials

Volumes 367–370, Part A, 1 August 2007, Pages 839-843

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Abstract

Because of the high solubility and mobility of oxygen in vanadium, composition control during the fabrication of thin (0.25 mm) wall tubing from vanadium alloys by cold drawing and annealing, presents a technological challenge. During intermediate annealing at 1000 °C in the 10⁻⁴ Torr vacuum regime, oxygen penetration into the tube wall is controlled by the development of a semi-protective surface oxide (linear-parabolic oxidation conditions); oxygen-hardened surface layers lead to a high incidence of surface cracking during the final stages of cold drawing. In the 10⁻⁵ Torr regime, under linear kinetic oxidation conditions, rapid oxygen penetration results in unacceptably high levels of oxygen pick-up (∼1500 wppm). In the 10⁻⁷ Torr vacuum regime, molecular impingement rates are reduced to the point where overall oxygen pick-up is reduced to <100 wppm. Improved cleaning/gettering procedures also restrict carbon and nitrogen pick-up to very low levels.

Introduction

The determination of the thermal and irradiation creep properties of V–4Cr–4Ti using the pressurized tube technique requires a supply of 4.57 mm outside diameter tubing with a wall thickness of 0.250 mm. Because of the high solubility and rapid mobility of oxygen in this material, maintaining the desired chemical and microstructural characteristics of the alloy during the fabrication of thin wall tubing presents a difficult technological challenge. In 1995, the US fusion program procured some 6 m of tubing, using the US heat No. 832665, utilizing commercial vendors [1]. This effort, (Batch A), met with mixed success since a large fraction of the tubing developed cracks at both surfaces and during processing, the carbon concentration increased from 80 to 300 wppm, oxygen increased from 310 to 700 wppm, while nitrogen remained fairly constant. During the intermediate annealing cycles at 1000 °C for this batch the furnace vacuum was maintained in the 10⁻⁴ Torr range. In an effort to control oxygen pick-up during the intermediate annealing heat treatments, further processing of thin-wall tubing has been undertaken utilizing vacuum conditions in the 10⁻⁵ Torr regime and in the 10⁻⁷ Torr regime.

Section snippets

Processing conditions for batches A and B

The intermediate heat treatments of 1 h at 1000 °C for Batch A were carried out in a ∼170 l tube furnace operating with a vacuum in the 10⁻⁴ Torr range. During the nine drawing and annealing cycles, the average oxygen content increased from an initial ∼300 wppm up to ∼700 wppm [1]. Cracks developed at both surfaces which were frequently linked across the tube wall via a band of severe macroscopic deformation; this cracking was probably related to the high surface oxygen levels picked up during the

Interstitial pick-up

Archive samples for chemical analysis and metallography were obtained after each draw cycle and after each anneal. (For Batch A only the initial and final interstitial analyses were obtained). The interstitial analyses for Batch B are summarized for both heats in Table 1.

Because of the order-of-magnitude improvement in the vacuum conditions it was expected that the level of interstitial pick-up would be lower for Batch B than for Batch A. Contrary to these expectations, the pick-up of oxygen

Oxidation behavior of batches A and B

The increase in oxygen for each annealing stage for Batch B tubing, obtained from Fig. 1, is plotted against the SAV ratio of the tubing at each stage in Fig. 2. The observed linear dependence of gas absorption on specimen SAV ratio is in agreement with the following relationship described by Inouye [7] for conditions where absorption of the oxygen is not limited by the formation of a protective oxide film; $Δ C_{m} = Q_{a} \cdot Δ t \cdot A / ρ V,$ where ΔC_m is the change in concentration, (g of gas/g of metal); Q_a is the

Oxidation behavior of Batch D tubing

The above results indicated that at least an order-of-magnitude improvement in vacuum conditions would be required to maintain satisfactorily low oxidation rates during intermediate annealing at 1000 °C. Following an experimental Batch C, a further batch of tubing from the NIFS-2-HEAT (Batch D) was processed utilizing a 10 cm dia. tube furnace capable of maintaining a vacuum of ∼3 × 10⁻⁷ Torr throughout the annealing cycle. Residual gas analyzer measurements indicated oxygen partial pressures of <2 ×

Conclusions

1.
During intermediate annealing at 1000 °C in the 10⁻⁴ Torr vacuum regime, oxygen pick-up during fabrication of 0.250 mm wall tubing was limited by the formation of a semi-protective visible oxide film which was removed after each draw cycle by acid cleaning. Under these conditions, (Batch A), oxygen and carbon pick-up were ∼400 wppm and ∼220 wppm, respectively; however the frequency of surface cracking was unacceptably high.
2.
With vacuum annealing conditions in the 10⁻⁵ Torr regime (Batch B tubing),

Acknowledgements

This work was sponsored by the Office of Fusion Energy Sciences, US Department of Energy under Contract DE-AC05-00OR22725 with UT-Battelle and DE-AC06-76RLO1830 with Battelle Memorial Institute. The authors wish to acknowledge the technical support provided by L.T. Gibson and M.J. Gardner.

References (8)

B.A. Pint et al.
J. Nucl. Mater
(2002)
H. Tsai, M.C. Billone, R.V. Strain, D.L. Smith, Fusion Materials Semi-Annual Report DOE/ER-0310/23, December 1997, p....
A.F. Rowcliffe, W.R. Johnson, D.T. Hoelzer, Fusion Materials Semi-Annual Progress Report DOE/ER-0310/33, December 2002,...
M.L. Grossbeck, R.J. Kurtz, L.T. Gibson, M.J. Gardner, Fusion Materials Semi-Annual Progress Report DOE/ER-0310/32,...

There are more references available in the full text version of this article.

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Citation Excerpt :
The manufacturing technology for low activation V–4Cr–4Ti has made significant progress in recent years [1–3].
Neutron irradiation in JMTR at 673 K to ∼0.11 dpa and post-irradiation examinations were carried out on V–4Cr–4Ti (NIFS-HEAT-2) heat-treated at 1073–1373 K for 1 h, for the purpose of optimizing the heat treatment conditions considering the effect of neutron irradiation.
After irradiation, hardness and Charpy impact properties were similar with each other at 1073–1273 K, which is in strong contrast with the unirradiated V–4Cr–4Ti, which showed clearly minimum hardness and optimum impact property at 1173 K. Significant hardening and an increase in DBTT occurred after irradiation, however, at 1373 K. TEM observations showed similar microstructure with high density of precipitates and very low dislocation density at 1173–1273 K following irradiation.
The mechanism of the weak dependence of properties after irradiation for the heat treatment temperatures between 1073 and 1273 and implication of the present study to determining annealing temperature was discussed.
Research progress on high-temperature oxidation resistance of vanadium alloys
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