12 Industrial Pipeline Welding
409
The requirement for pipes of higher strength and ductility has led to an increase
in manganese contents in the hot rolled sheet steels. However, a higher manganese
content (>1.2 wt%) and a greater wall thickness are likely to induce “penetrator” or
cold weld defects in the welded zone. These defects are generally classified as:
1. Residual FeO–MnO–SiO 2 –(Al 2 O 3 ) oxides left without being squeezed out from
the joint.
2. Exposed cracks due to cavity formation and hot cracking.
3. Blow-holes including oxides.
In cold weld, oxide inclusions, e.g., in form of continuous thin films within the
weld zone, are frequently observed when the heat input is low to cause adequately
melting of the metal. The heat input depends on various process parameters such as
strip thickness, speed, V angle, length, and edge alignment. Insufficient heat input
may result to melting only a thin metal layer on the edge surface and, consequently,
the oxides formed on the surfaces during heating or storage of the strip may not be
squeezed out leading to cold weld defects.
Penetrator like defects become more frequent as the heat input is increased, and
the mill speed is reduced. Another parameter is the manganese-to-silicon (Mn/Si)
ratio, which influences the oxidation sequence. Penetrators usually consists of Fe,
Mn, and Si oxides. Their sizes range from a few millimeters to a centimeter and affect
the mechanical properties [45]. The Mn and Si content at the welded joint noticeably
decreases with the heat coefficient Q defined as:
Q =
E P I P
vt
(12.6)
where E P is the voltage on the plate (kV), I P is the current on the plate (A), v is
pipe welding speed (m/min), and t is the pipe’s wall thickness. This heat coefficient
is generally employed as an index for pipe welding conditions. This value is higher
when the mill speed decreases and the welding heat input increases [46].
With regard to the penetrator formation, the following reactions occur:
(FeO) + [Mn] ↔ [Fe] + (MnQ)
2(FeO) + [Si] ↔ 2[Fe] + (SiO 2 )
where [] denotes the element’s concentration in the molten iron and () the slag’s free
oxide content.
The MnO–SiO 2 phase diagram in Fig. 12.22 shows that the melting points of
penetrators. The molten bead temperature has to remain close to the melting point
of steel, because due to the proximity effect the current flows mainly along the edge
surfaces. A part of the molten slag changes to penetrator although the density of the
slag is significantly lower (about half) comparing to the density of the molten metal
[47]. Figure 12.23 shows a typical morphology of penetrator found in weld metal of
pipeline steel.
409
The requirement for pipes of higher strength and ductility has led to an increase
in manganese contents in the hot rolled sheet steels. However, a higher manganese
content (>1.2 wt%) and a greater wall thickness are likely to induce “penetrator” or
cold weld defects in the welded zone. These defects are generally classified as:
1. Residual FeO–MnO–SiO 2 –(Al 2 O 3 ) oxides left without being squeezed out from
the joint.
2. Exposed cracks due to cavity formation and hot cracking.
3. Blow-holes including oxides.
In cold weld, oxide inclusions, e.g., in form of continuous thin films within the
weld zone, are frequently observed when the heat input is low to cause adequately
melting of the metal. The heat input depends on various process parameters such as
strip thickness, speed, V angle, length, and edge alignment. Insufficient heat input
may result to melting only a thin metal layer on the edge surface and, consequently,
the oxides formed on the surfaces during heating or storage of the strip may not be
squeezed out leading to cold weld defects.
Penetrator like defects become more frequent as the heat input is increased, and
the mill speed is reduced. Another parameter is the manganese-to-silicon (Mn/Si)
ratio, which influences the oxidation sequence. Penetrators usually consists of Fe,
Mn, and Si oxides. Their sizes range from a few millimeters to a centimeter and affect
the mechanical properties [45]. The Mn and Si content at the welded joint noticeably
decreases with the heat coefficient Q defined as:
Q =
E P I P
vt
(12.6)
where E P is the voltage on the plate (kV), I P is the current on the plate (A), v is
pipe welding speed (m/min), and t is the pipe’s wall thickness. This heat coefficient
is generally employed as an index for pipe welding conditions. This value is higher
when the mill speed decreases and the welding heat input increases [46].
With regard to the penetrator formation, the following reactions occur:
(FeO) + [Mn] ↔ [Fe] + (MnQ)
2(FeO) + [Si] ↔ 2[Fe] + (SiO 2 )
where [] denotes the element’s concentration in the molten iron and () the slag’s free
oxide content.
The MnO–SiO 2 phase diagram in Fig. 12.22 shows that the melting points of
penetrators. The molten bead temperature has to remain close to the melting point
of steel, because due to the proximity effect the current flows mainly along the edge
surfaces. A part of the molten slag changes to penetrator although the density of the
slag is significantly lower (about half) comparing to the density of the molten metal
[47]. Figure 12.23 shows a typical morphology of penetrator found in weld metal of
pipeline steel.