Elsevier

Scripta Materialia

Volume 43, Issue 8, 29 September 2000, Pages 751-757
Scripta Materialia

Original articles
In-situ observations of phase transformations during solidification and cooling of austenitic stainless steel welds using time-resolved x-ray diffraction

https://doi.org/10.1016/S1359-6462(00)00481-4Get rights and content

Introduction

The microstructure of many AISI 300-series stainless steels consists of a mixture of γ-austenite and δ-ferrite phases 1, 2, 3, 4. Austenite, which is paramagnetic, has a face centered cubic (fcc) crystal structure, and is the predominant phase in these alloys. The remainder is ferrite, which is ferromagnetic and has a body centered cubic (bcc) crystal structure. The effects of alloy composition and weld solidification rate on the microstructure of austenitic stainless steel alloys have been extensively studied because residual ferrite in the microstructure can have both beneficial and deleterious effects on weld integrity 5, 6, 7, 8, 9, 10, 11, 12.

The effect of composition on the phases that exist in arc welds was studied by Schaeffler more than 50 years ago [1] who developed a diagram to predict the amount of ferrite in stainless steel welds. This empirical diagram has been modified several times to enhance its accuracy 2, 3, 4. The weld solidification rate further affects both the microstructure and the relative fraction of austenite and ferrite in welds 13, 14, 15, 16, 17. Solidification rate studies such as these have been important in developing an understanding of the effects that different welding processes have on the resulting weld fusion zone microstructure through its influence on solidification and solid state transformations.

In this investigation, direct observations of the ferrite and austenite phases and their transformation dynamics were made with intense synchrotron radiation in-situ during arc welding of an AISI type 304 stainless steel using time-resolved x-ray diffraction (TRXRD). In-situ synchrotron x-ray diffraction has been used in a novel spatially resolved mode to investigate phase transformations in commercially pure titanium 18, 19, 20, 21, and has now been modified to a time-resolved mode to study microstructural evolution in stainless steel welds. The goals of these experiments were twofold: first, to verify experimentally whether δ-ferrite or γ-austenite is the primary phase to solidify from the melt, and second, to follow the dynamics of the γ→δ phase transformation during cooling in the arc welds.

Section snippets

Materials and welding

AISI type 304 stainless steel was acquired in a 100 mm diameter bar. Chemical analysis was performed on this material using combustion analysis for C, N, S and P, and inductively coupled plasma analysis for the remaining elements. The results yielded the following concentrations (in wt. %): 18.44% Cr, 10.71% Ni, 0.019% C, 0.053% N, 1.67% Mn, 0.04% Mo, <0.005%Nb, 0.46% Si, 0.04% V, 0.04% Cu, 0.02% Co, 0.016%S, 0.015% P, balance Fe (68.5%). The welded samples were polished using standard

Phase equilibria

The AISI 300-series stainless steels alloys are based on the Fe-Ni-Cr ternary system and have compositions that typically contain 65% to 70%Fe by weight [26]. The constitution of these alloys has been critically evaluated 27, 28 and, studies such as these have provided the basis for understanding the effects of alloy composition and cooling rate on the microstructure of stainless steel welds. One useful way to visualize the relative amounts of austenite and ferrite as a function of temperature

Conclusions

Time resolved diffraction measurements have been performed using a 730 μm synchrotron beam to identify the transformation dynamics of AISI type 304 stainless steel stationary ‘spot’ welds upon solidification and cooling to room temperature. Using a time resolution of 50 ms, these experiments showed directly, for the first time, that δ-ferrite is the first phase to solidify from the liquid weld pool. The δ-ferrite phase existed as the only solid phase for 500 ms before beginning to transform to

Acknowledgements

This work was performed under the auspices of the U. S. Department of Energy, Lawrence Livermore National Laboratory, under Contract No. W-7405-ENG-48. This work was supported by DOE, Office of Basic Energy Sciences, Division of Materials Science. Synchrotron experiments were conducted at SSRL supported by DOE, Division of Chemical Science. T. Ressler wishes to thank the Alexander von Humboldt Foundation for a Feodor Lynen research fellowship. The authors express gratitude to Dr. J. A. Brooks

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