The effect of the surface on grain boundary migration in austenitic stainless steel weld metal

Abstract

The direction of grain boundary (GB) migration that occurs after solidification in the original surface of the weld of an austenitic stainless steel was compared to that in the interior (the depth 100 m from the original surface). The results are as follows. In the interior, the direction of GB migration was decided by the moving direction of the triple junctions that bring about their equilibrium. This equilibrium results from the GB approaching a cross angle of 120 deg with another GB in the vicinity of the triple junction. Meanwhile, in the original surface, the direction of GB migration was decided by the direction that makes the GB plane, which is just at solidification, being perpendicular to the surface. That is, the GB plane migrates to be perpendicular to the original surface. This GB migration was often observed when the angle between the GB plane and the original surface had been less than 40 deg. Furthermore, the amount of the GB migration had rapidly increased with the decrease in the angle. The validity of these results is examined with a discussion of the reduction of the interfacial energy.

Migration of solidification grain boundaries and prediction

“…As pointed out earlier, since can be regarded as a constant (excluding nano-grains), we have where the subscript E refers to the equilibrium state and the subscript I refers to the initial state. Equation ( 2 ) is consistent with the current understanding of the main DF for SGBM being the reduction in the total GB energy 18 , 21 , 25 , 35 .…”

“… 16 ). Additionally, we have assessed the Fe-C system for SGBM in stainless steels 18 – 25 . The inhibiting factor for each of these alloy systems and the four Mg-X alloy systems discussed in Fig.…”

“…More SGBM phenomena were subsequently observed in as-cast, welded, wire-deposited (AM-fabricated) metals or alloys. Examples include Al-4.6 wt.% Mg 10 , Al-0.2 wt.%Cu 11 , Cu (Cu-0.006% wt.%O) 12 , 13 , Cu-3 wt.%Sn 13 , 14 , Ti-6 wt.%Cr 15 , Zr-2 wt.%Mo 16 , commercial magnesium alloy AM50 (Mg-4.9 wt.%Al-0.34 wt.%Mn) 17 , maraging steels (both as-cast and wire-deposited) 9 , stainless steels including both commercial (304, 309, 310, 347, 321, 316, 430) 18 – 25 and non-commercial grades 21 , 22 , 24 , 25 , Ni-based filler alloys for welding 26 , and high entropy alloys 27 . In general, SGBM is less often observed in commercial cast alloys that contain a noticeable presence of second-phase particles, due to the particle pinning effect (note that GB particles can migrate along with moving GBs, known as ‘mobile particles’ 28 , especially at elevated temperatures, depending on their properties, sizes, and interactions with GBs).…”

“…Information on SGBM scattered in the literature since 1961 provides the following basic understanding: (i) SGBM occurs near the solidus temperature (T solidus ) of the alloy 9 , 21 , 22 , 25 , 33 ; (ii) the strain generated during cooling after solidification has a minor or negligible influence based on experiments using specially designed samples 20 − this clarifies an important concern about the underlying mechanism; (iii) tortuous or irregular SGBs are apt to migrate 20 , 34 , but SGBM is not due to grain growth 34 ; and (iv) the main driving force for SGBM is the reduction in the total GB energy 18 , 21 , 25 , 35 , and the GB triple junctions approach equilibrium (straight GBs in 120°–120°–120°) after SGBM 18 . However, compared to the GBM in recrystallisation or grain growth, the fundamental factors affecting SGBM, including alloy composition (solute type and content), cooling rate, solidification characteristics and grain size, remain essentially unexplored.…”