Evaluation of Friction Stir Processing of HY-80 Steel Under Wet and Dry Conditions
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Authors
Young, Garth William II
Subjects
Friction Stir Welding, Friction Stir Processing, Underwater Welding, High
Strength Steel, Microstructural Properties, Hardenable Alloy Steel, Weld Repair, HY-80, Charpy
Impact Test, Tensile Test, Microhardness Test, Martensite, Austenite
Advisors
McNelley, Terry R.
Date of Issue
2012-03
Date
Mar-12
Publisher
Monterey, California. Naval Postgraduate School
Language
Abstract
This thesis describes the microstructural and mechanical property changes associated with Friction Stir Processing
(FSP) of HY-80 steel under dry and underwater conditions. HY-80 is a low-carbon alloy steel that is used in a
quenched and tempered condition and is highly susceptible to hydrogen assisted cracking associated with
conventional fusion welding. FSW/P (400 RPM/ 2 IPM) was conducted using a polycrystalline cubic boron nitride
tool having a pin length of 6.35 mm. Two sets of processing runs were completed on HY-80 plates, 6.35 mm in
thickness, one dry and the other under seawater. Analysis of the residual hydrogen content revealed little change in
hydrogen content associated with FSW/P. Detailed microstructural characterization (optical and scanning electron
microscopy) with emphasis on the differences in the distribution of constituent phases in the rolling, transverse and
normal planes was carried out. Microstructural analysis of the “plan-view” section at the tool extraction sites of
FSW/P samples was executed. Mechanical properties of these samples were evaluated by tensile, microhardness, and
Charpy V-notch impact resistance tests. The results indicated that austenitization occurred within the stir zones and
subsequent cooling influenced the final microstructure and properties. The thermomechanically affected zone
displayed a gradual change in near-equiaxed austenitic grain size towards both the tool axis as well as the base plate
with little grain shape change throughout. These results have been rationalized on the basis of expected temperature
and deformation fields generated setup in these steel samples and the nature of phase transformations occurring in low
carbon steels such as HY-80.
Type
Thesis
Description
Series/Report No
Department
Mechanical Engineering