INTRODUCTION
This paper is an update of a previous paper authored by Mr. Humphry Udall, former
Director of Research and Development for Thermatool.
The welding of stainless steel tubes differs substantially from the welding of high quality
carbon steel tubes in a number of ways. The most significant difference is in the melting
point of the oxides that are formed on the faying edges during the welding process. The
oxides formed on carbon steel generally melt at a temperature lower than the melting
point of the steel. Since the oxides melt first and are fairly liquid even before the steel
melts, it is easier to squeeze them out in the weld rolls. Actually, it is possible to make
a fairly clean weld on carbon steel without actually melting the steel, though this is
rarely done.
Stainless steel, either austenitic or ferritic, has substantial quantities of chromium as an
alloying element that can oxidize during the welding process. These chromium oxides
are “refractory”, that is, they have very high melting points. In fact the melting point is
higher than the stainless steel itself. Because of this, the stainless steel becomes soft and
even liquid before the oxides melt, making the oxides difficult to squeeze out. If left on
the bond plane, these oxides constitute discontinuities that can cause brittle fracture of
the weld, low formability, and possibly weld line corrosion.
It is, therefore, essential that enough metal be melted at the edges to flush the oxides
out during the weld squeeze. If properly done, all of the oxides will be extruded during
the squeeze out and will be contained in the ID and OD flash (Fig 1). Conversely, if too
much metal is melted, the HF vee may become unstable leading to possible weld defects.
It is a well established fact that the “window of success” for stainless steel is smaller
than for carbon steel and greater attention must be paid to the forming and welding
parameters.
Fig 1 The HF welding process heats only the edges of the strip. The edges are
Forced together by the weld rolls causing the molten metal and metal oxides
to be ejected. The hot metal below the surface then fuses into a weld.
MATERIAL COMPOSITION
Because carbon can combine with the chromium to form very hard chromium carbides
in the weld area, keeping the carbon content low will reduce the incidence of hard
welds and cracks. Additionally, when the chromium forms carbides, it is no longer in a
form that will help protect the surface of the metal and weld area corrosion is possible.
By specifying the L versions such as 304L, the tube producer will reduce the potential
for problems without compromising performance of the tube material.
It is possible that excessive unreacted aluminum and titanium can form aluminum and
titanium oxides that are also refractory and just as difficult to remove as chromium
oxides. The maximum limits for soluble aluminum and titanium may have to be lower
than commercial limits. This is an issue to take up with your strip supplier. In addition,
large non-metallic inclusions in stainless steel can result in defects known as hook cracks
when they are upset in the weld area. Low levels of sulfur and phosphorus should be
specified when purchasing strip.
COIL CAMBER AND TEMPER
Flat and straight coil will run better in any mill and this is especially true of stainless. In a
properly adjusted mill, soft strip will usually run more smoothly than hard material
because of the ability of the mill to correct for slight strip camber. Care should be
taken to avoid introducing camber when recoiling strip after slitting.
STRIP EDGE CONDITION
Good strip edges are absolutely essential to achieving a good weld with stainless. The
edges should be smooth with a minimum of burr (Fig 2). This requires careful attention
to both edge condition and the set up of the slitting knives. Great care must be given to
handling and transporting of the slit strip to avoid scraping or bumping of the edges as
edge damage is a likely cause for weld defects. Good practice may also include the use
of edge trimming on the mill prior to the first breakdown roll. This generates a small
amount of trim scrap but this may be paid for by consistently higher quality welds and
reduced tubular scrap.


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