Residual stress measurements by x-ray diffraction in the ferritic-austenitic interface weldment
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I.
INTRODUCTION
A D V A N C E S in joining technology of engineering materials and improvements of the material properties through the better control of the microstructure have led to the expanding applications of the welding techniques in the fabricating processes. In particular, fusion welding of steel has been established as one of the main manufacturing processes of high performance, ensuring the integrity of the construction. Since World War II, welding dissimilar metals, e.g., ferrous alloys to stainless steel, has been increasingly utilized in transition joint applications such as f o s s i l - fire boilers, heat exchangers, power generation industry, and nuclear installation. The main reason for developing transition structures was the increasing demand for energy saving in the future as well as the interest in alternative metallurgical processes for improving the structure properties and subsequently the materials performance. In either case, a prominent position is shared, on tonage basis, by transition joints of ferritic-austenitic steels. The complex metallurgical processes which occur in fusion welding, such as shrinkage during solidification, phase transformation, or the different thermal expansion coefficients across the weld interface produce both tensile and compressive residual stresses in the different zones of the fabricated transition structure. These stresses are not immediately obvious. Since they are accommodated in the crystal structure as elastic energy, they are associated with the mismatching of the lattice parameters ab~ and af~ of the transition structure. Under favorable orientation and/or by incorporating external loading, the strain energy released causes cracking of the materials.
N. SARAFIANOS, Assistant Professor in Physical Metallurgy, is with the Laboratory of Physical MetMlurgy, Department of Mechanical Engineering, Aristotle's University, 540 06 Thessaloniki, Macedonia, Greece. Manuscript submitted May 27, 1992. METALLURGICAL TRANSACTIONS A
A large body of literature describing the phenomenon of residual stresses has been published, 11-51 with major contributions to the transition joint microstructure analysis,[6 lJl but more work needs to be done on the materials' interaction which gives birth to internal stresses. Residual stresses developed at the ferritic-austenitic interface welds may be one of the factors reducing the reliability of the joint. By using the X-ray method, the d spacing can be calculated by measuring the shift of the reflected angle O, which satisfies the Bragg law A= 2
9 dhk
I "
sin 0
The state of the residual stresses can then be evaluated across the ferritic-austenitic interface on manual metal arc (MMA) welding of steel. This work aims to calculate the residual stresses being developed at the weld interface of the ferritic-austenitic steels and was undertaken as a part of the project assessing the joining performance of dissimilar metals. Based on the bcc-fcc lattice structure continuity, an analytical model has been developed describing the in
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