Direct observations of the <Emphasis Type="Italic">α</Emphasis> → <Emphasis Type="Italic">γ</Emphasis> transformation at different input powers in the heat-affected zone of 1045 C-Mn steel arc welds observed by spatially resolved X-ray diffraction |
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Authors: | T A Palmer J W Elmer |
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Affiliation: | (1) the Lawrence Livermore National Laboratory, 94550 Livermore, CA |
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Abstract: | Spatially resolved X-ray diffraction (SRXRD) experiments have been performed during gas tungstenarc (GTA) welding of AISI
1045 C-Mn steel at input powers ranging from 1000 to 3750 W. In-situ diffraction patterns taken at discreet locations across the width of the heat-affected zone (HAZ) near the peak of the heating
cycle in each weld show regions containing austenite (γ), ferrite and austenite (α+γ), and ferrite (α). Changes in input power have a demonstrated effect on the resulting sizes of these regions. The largest effect is on the
γ phase region, which nearly triples in width with increasing input power, while the width of the surrounding two-phase α+γ region remains relatively constant. An analysis of the diffraction patterns obtained across this range of locations allows
the formation of austenite from the base-metal microstructure to be monitored. After the completion of the α → γ transformation, a splitting of the austenite peaks is observed at temperatures between approximately 860 °C and 1290 °C.
This splitting in the austenite peaks results from the dissolution of cementite laths originally present in the base-metal
pearlite, which remain after the completion of the α → γ transformation, and represents the formation of a second more highly alloyed austenite constituent. With increasing temperatures,
carbon, originally present in the cementite laths, diffuses from the second newly formed austenite constituent to the original
austenite constituent. Eventually, a homogeneous austenitic microstructure is produced at temperatures of approximately 1300
°C and above, depending on the weld input power. |
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