------------------------------------------------------------ ------------------------------------------------ INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY Department of Metallurgical Engineering and Materials Science ------------------------------------------------------------ ----------------------- S Y N O P S I S of the Ph.D. thesis titled MICROSTRUCTURAL AND THE CONSEQUENT LOCALIZED CORROSION RESISTANCE BEHAVIOUR OF HTHAZ OF UNS S32760 SDSS TIG WELDS PREPARED USING NITROGEN ADDITIONS TO ARGON SHIELDING GAS Proposed to be submitted in partial fulfillment of the degree of DOCTOR OF PHILOSOPHY of the INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY By Rahul Khandagale (Roll No. 99411401) Supervisors:
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Figure 1: A schematic indicating the involvement of HTHAZ via FZ in the process of exchange of nitrogen species due to the dynamic interaction between arc plasma/shielding atmosphere and highly convecting molten weld pool (FZ) during arc welding
Second generation duplex stainless steel contains nitrogen as a crucial alloying
element specifically added to enhance reaustenitization of the HTHAZ in the limited time
available as the weld cools. In the absence of nitrogen a highly ferritized [greater than
80% ferrite] can be obtained in the consequent loss in the expected physico – chemical
properties1. During arc welding there is an inevitable dynamic interaction between the
molten weld pool/arc atmosphere, molten weld pool/inert gas atmosphere and arc
atmosphere/inert gas interfaces leading to exchange of the metallurgically active nitrogen
species. The above leads to loss/gain of nitrogen from the metal depending upon the
partial pressure of nitrogen [diatomic] in the shielding gas and the monotonic nitrogen in
the arc atmosphere. The proposed model is shown in Figure 1.
The nitrogen plays two contradictory roles during the evolution of HTHAZ
microstructure i] Encouragement of reaustenitization which is beneficial ii] Formation of
1
FZ HTHAZ
LOSS GAIN(LOSS if PN2 in shielding gas mixture is below a critical value)
PN2PN
N
Cr2N which is harmful. Based on the CCT curve for the above two reaction2 it is seen that
beyond a particular level of nitrogen in the HTHAZ reaustenitization will dominate
leading to efficient trapping of nitrogen as it gets rejected from the ferrite phase, in which
the solubility of nitrogen is very low as compared to that in austenite. This leads to non
linearity of the effect of nitrogen contain in the HTHAZ on the formation of Cr 2N. This
has been confirmed by several weld simulation study on super-duplex stainless steels3, 4, 5
in the absence of adequate reaustenitization there is also a good possibility of forming
sigma phase in the HTHAZ in SDSS which contain higher levels of Cr and Mo as
alloying elements. Formation of both the above deleterious phases can lead to loss of
localized corrosion resistance.
The success of duplex stainless steel as a material with excellent pitting and SCC
noise analysis [ECNA] and the novel micro-electrochemical Ecorr corrosion potential
noise probing technique [a new technique under development in this laboratory]11
have been describe in chapter 5. a discussion on the rescaled range analysis7 [R/S][Hurst
analysis], a mathematical tool for appropriately analyzing the essentially non linear Ecorr
vs time noise data has also been included in this chapter.
Various experimental procedures adopted related to preparation of welds,
metallurgical characterization of the HTHAZ of the welds produced by optical and
electron microscopy, preparation of appropriate test specimens for different
electrochemical corrosion tests to evaluate the localized corrosion resistance of the
HTHAZ of the welds have been presented in chapter 6. The chapter also includes brief
description of various experimental set ups used for corrosion studies. The results
obtained from each of the above experimental procedure are also included in this chapter.
For the purpose of convenience results obtained have been presented in the form of
tables, figures, micrographs etc. immediately after each type of experiment. Some
representative results are shown in Figures 1 to 5 and Tables 1 to 3.
4
Figure 2: Low (25X) and high (250X) magnification optical micrographs respectively showing the composite zone [(a),(b)] and HTHAZ [(a’),(b’)] of welds prepared with 0 vol.% nitrogen additions to the argon shielding gas and t12/8 of 0.41 s [(a),(a’)] and 3.0 vol.% nitrogen additions to the argon shielding gas and t12/8 of 0.37 s [(b),(b’)]{Figure 2 (a): weld 30(W30),(b): weld 48(W48)}.[Note the significant amount of phase formation in the ferrite phase in the case of weld prepared with 0 vol.% nitrogen(a’).Cr2N although present can not be revealed by optical microscopy.]
5
500m
WZ
HAZ
(a)
(b)
50m
(a’)
(b’)WZ
HAZ
Base metal
HTHAZ
HTHAZ
Figure 3: Ecorr vs time noise data obtained with the novel micro-electrochemical
8. Bjorn E. S. Lindblom, Berthold Lundqvist and Nils Erick Hannerz, Scandinavian Journal of
Metallurgy, 20, pp.305-315 (1991).
9. M.A. Domninguez-Aguilar, R.C.Newman, Corrosion Science (2005) (article in press).
10. M.A. Domninguez-Aguilar, R.C.Newman, Corrosion Science (2005) (article in press).
11. R.Raman, DRDO sponsored project on “The development of micro-electrochemical corrosion
potential noise probing technique for evaluating local localized corrosion resistance of passive
materials in aqueous solution” (June2003 to August 2006). Being carried out in the Department
of Metallurgical Engg. and Material Science, I.I.T. Bombay (India).
14
Publication resulting from the Ph.D work:
1) Ramesh P , R.Khandagale, R.Dalvi, Depashree Nage, R.Raman “Repassivation Studies by Rapid Scratch Technique on Super Duplex Stainless Steel, Steel Welds in non-deaerated 3.5% NaCl Solution”, published in NACE India, conference held in Goa(India), from 28 th – 30th November 2002.
2) R.Raman, R.Khandagale, “Development of micro electrochemical Ecorr noise probing system to study degradation or enhancement of localized corrosion resistance of passive materials.”, published in NACE India, conference held in Goa (India), from 28th – 30th November 2002.
3) R.Raman, R.Khandagale, “GTAW of UNS S32760 Super Duplex Stainless Steel using Nitrogen additions in the shielding gas – microsturctural and localized corrosion resistance aspects of fusion zone / heat affected interface”, one day seminar on ‘Advance in Welded Fabrication’, held in BARC, Mumbai (India)on 18” January 2003.
4) R.Raman, P.Raju, P.J.Antony, R.Khandagale , “Analysis of corrosion potential noise by R/S analysis and wavelet transform methods” International conference on corrosion (CORCON2003) in Mumbai(India), Dec. 1 to 4, 2003.