Microstructural changes in HSLA-100 steel thermally cycled to simulate the heat-affected zone during welding
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I.
INTRODUCTION
MODERN high strength low alloy (HSLA) steels, which are used in the construction of buildings, bridges, pipelines,tq and ships, t2-51 achieve their excellent combination of strength and toughness by having low carbon levels (less than about 0.08 pct) in combination with alloying elements such as niobium, vanadium, titanium, copper, and nickel. Many of these alloying elements help to produce fine precipitates (e.g., niobium-, vanadium-, and titaniumcarbonitrides and copper) that increase the yield strength to levels of 80 to 100 KSI, despite the low carbon concentrations. The major reason for the current interest in these steels is that unlike the conventional, higher carbon HY grades, HSLA steels "have the potential for being welded with little or no preheat, and with less stringent process controls," thus significantly reducing the fabrication costs.[ 3]
Although HSLA steels can be processed to have excellent properties, major problems may still be encountered during welding, in the heat affected zone (HAZ) as well as in the weld metal. In the HAZ, the matrix phases may transform to austenite and the precipitates will coarsen and/or dissolve during heating; then the matrix will retransform to martensite, ferrite, and/or bainite with new precipitate distributions upon cooling (e.g., References 6 through 8).
G. SPANOS, Materials Scientist, R.W. FONDA, Postdoctoral Fellow, and R.A. VANDERMEER, Branch Consultant, are with the Physical Metallurgy Branch, Naval Research Laboratory, Washington, DC 203755320. A. MATUSZESKI, formerly Summer Student, Physical Metallurgy Branch, Naval Research Laboratory, is Undergraduate Student, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted May 13, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
These alterations may degrade the mechanical properties (e.g., strength, hardness, and toughness) of the welded structure. Consequently, there is a need to expand our knowledge base of the phase transformations and microstructural evolution which occur during welding of these steels, in order to enhance our understanding of the processing of the material and, in rum, to improve our ability to control the microstructure and mechanical properties. Although there have been a number of studies of HSLA steels within the last 10 years, many of these investigations have been centered about studying mechanical properties and have relied mostly upon optical microscopy and/or scanning electron microscopy to correlate the microstructure with these properties (e.g., References 5 and 8 through 13). Those studies which have used transmission electron microscopy (TEM) have usually focused on the effects of alloying elements and/or isothermal aging treatments on the microstructure (e.g., References 7, 14, and 15) or on the development of continuous cooling transformation (CCT) curves. [6,16]However, there is still no clear understanding of the detailed microstructures which give rise to the property variations across the HAZ as a function of position and weld
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