Research method

Suppression of Solidification Crack upon Steel Welding

X-ray diffraction/scattering

The world’s first successful Two-dimensional Time-resolved X-ray diffraction of metallographic changes in weld metals
during real welding at a resolution of 0.01 sec.

Role of SPring-8

Achievements

Paving the way to suppression of welding solidification crack* in weld metal bydetailed measurement of metallographic changes of high-alloy steel** weld
metal in the melting and solidification processes during welding
R&D facility: Sumitomo Metal Industries, Ltd., Osaka University,and Japan Synchrotron Radiation Research Institute

BackgroundResults

The prevention of welding solidification crack
is essential for improving the reliability ofmaterials at the welded part. For this purpose, areduction of metals causing a decrease in thesolidification temperature at the welded part has
been tried; however, a number of difficulties
remain for practical application.
To prevent solidification crack, it is necessary
to examine the welding solidification process indetail, clarify the mechanism behind theformation of solidification crack, such as when
and what substances are produced, and predictthe formation of solidification crack.

For the in situ observation of metallographic
changes at the welded part, we used our actual welding
machine and applied PILATUS,* a two-dimensional
pixel detector to catch week signals. As a result, we
succeeded in two-dimensional capturing themetallographic changes during rapid coolingover 100 K/s, and in establishing guidelines formaterial design to prevent solidification crack.
For example, niobium forms a carbide earlier as
the amounts of niobium and carbon in an alloy areincreased. The importance of the timely and stableformation of niobium carbide was clarified toprevent solidification crack.

*Welding solidification crack: Welding involves the rapid melting and solidification of weld metal. A membrane (liquid phase) with a low melting
point is formed during welding, and stress due to its contraction acts on the membrane while solidifying, causing a network of cracks. This is known
as welding solidification crack.
**High-alloy steel: The percentage of alloy elements added to steel is at least 10%.

*PILATUS: High-sensitivity high-speed two-dimensional X-ray detector developedin a collaboration between Japan Synchrotron Radiation Research Institute andPaul Scherrer Institut (PSI) in Switzerland.

Publication: M. Yonemura et al.; Materials Transactions 47, 2292-2298 (2006) Y. Komizo et al.; Quarterly journal of the Japan welding society 24, 57-64 (2006)
M. Yonemura et al.; Tetsu-to-Hagané 93, 68-74 (2007)

Output (10 V, 150 A)

Equipment developed forobservation of welding process

Halo pattern (liquid phase)2D pixel detector,PILATUS

The melting and solidification phenomena can beobserved in situ at the specific welded part with ahigh time resolution of 0.01 s.

Gas tungsten
arc welding machine

MoveArc
SPring-8
high-brilliance X-rays

Specimen

X-ray diffraction patternfor high-alloy steel
Niobium carbide (NbC) appears from a liquid
phase after a few seconds of rapid cooling, andgrows stably. As a result, its microstructure isstabilized and solidification crack is suppressed.
(1) First, the metal melts and the temperature
decreases. At the moment the metal starts
to solidify, a δ phase first appears from aliquid phase (halo pattern).
(2) A γ phase appears with further decreasingtemperature.
(3) When the temperature further decreases,
NbC appears (is crystallized) and the liquid
phase disappears. At this time, the γ-phaseand NbC spots are aligned, indicating a
structurally stable crystal orientation.
(4) Finally, a γ+δ+NbC triple phase forms.

(1)1420 Co(2)1370oC

Halo pattern: Ring-shaped wide
pattern observed in the presence ofa liquid phase
Nbc: Niobium carbideδ phase: Primary phase of Fe-Ni-Cralloyγ phase: Secondary phase ofFe-Ni-Cr alloy
Figures in parentheses represent
the indices of crystal orientation.

(3)1150oC

Example of welding solidification crack(4)300oC

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