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THE QUALITY ADVANTAGESOF OLIN FINEWELDTUBE
WELDED TUBE
The manufacturing process utilized by Olin Fineweld Tube
for the production of tubing is technically described as ahigh frequency, induced current forge-weld process. Thisprocess is significantly different from the process used bymanufacturers of seamless tube in a number of importantproduct-quality areas. A critical review of these differenceshighlights the quality advantages of Olin Fineweld Tubeswelded tube versus competitive seamless tube.
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The ID of the extrusion die determines the OD ofthe extruded tube.
The OD of the piercing ram determines the ID ofthe tube.
The gap between the piercing ram and theextrusion die defines the tube wall thickness.
CONVENTIONAL EXTRUSION PROCESS
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Wall thickness uniformity (concentricity) isdifficult to control.
Good surface quality is difficult to achieve.
CONVENTIONAL EXTRUSION PROCESS
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HIGH FREQUENCY WELDEDTUBE PROCESS
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The Olin Fineweld Tube process starts with Direct Chill semi-continuous casting of three to four cast bars, or ingots. This
method results in exceptionally good homogeneity of the castbars such that laminations and inclusions are essentially non-existentin the strip, and therefore in the welded tube that is
made from the strip. Casting lots consist of 60,000 pounds or
greater. By comparison, seamless billets can weigh as little as300 pounds. The larger cast bars associated with the welded
tube process provide for greater chemical uniformity from one
tube to another, and traceability from a single tube back to itsoriginal casting is obviously much easier.
After casting, the bars are hot-rolled to a thickness of about
15 mm, and take on the form ofa coil, versus an ingot.
CASTING & HOT ROLLING
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COIL MILLINGA general characteristic of all castings, whether it is
a billet for a seamless tube or an ingot for a weldedtube, is that the surface of the casting will exhibit a
variety of defects including laps and seams, and that
it will be contaminated with carbonaceous mold
lubricant residues. The next step in the welded tube
process is therefore to mechanically mill the top andbottom surfaces of the hot-rolled coil to provide forthe positive removal of surface discontinuities,
surface oxides, and other contaminants inherent
with a casting operation.
Since either the top or bottom surface of the milledcoil will ultimately become the ID of a tube, the
possibility of a casting defect being present on theID of a welded tube is essentially non-existent. The
seamless process does incorporate a provision for
the removal of deleterious surface defects andresidues, but it does not involve a milling operation,
and the potential for the presence of surface defects
is therefore much greater with seamless tube than itis with welded tube. This point should not be
overlooked as surface defects of any magnituderepresent potential sites for subsequent localized
corrosion or pitting.
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COLD ROLLING
The next steps in Olins process are toalternately cold-roll and anneal the coils of
strip until the desired final thickness is
achieved. Both surfaces of the strip are
visible during these operations so anysurface defects that might be present can be
detected and corrected; thus further assuringthe superior surface qualityof welded tube
versus seamless tube. The use of cold-rolled
strip for the production of welded tube alsoresults in superior concentricityand a much
greater uniformity of wall thicknessas
compared to the seamless tube process.Uniformity of wall thickness and superior
concentricity are both highly desirable for therolling of the tubes into tubesheets. Benefits
are also realized if the tubing is subsequently
fabricated, expanded, or bent.
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ROLL-FORMING &WELDING
In Olins tube mill, a high-frequency inducedcurrent forge welding process is utilized.
With this process, high frequency current isconcentrated at the edges of the strip metal
which is being welded. Very rapid heating of
the strip edges occurs and they rapidly reach aplastic (almost molten) state. Concurrently, aheavy forging or squeezing pressure is applied
to the tube which forces the two edges together.
This forging pressure expels the plastic metal,
metal oxides, and impurities outward andinward in the form of a weld flash. The strip
edges are at temperature for only a fraction of asecond with the result being a narrow, hot
wrought forge-weld structure of exceptional
quality. Scarfing tools remove the ID and ODweld flash just downstream from the welding
operation.
Forging Rolls
High Frequency Source
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WELD STRUCTUREThe photomicrograph shown to the left is that of a typical weld produced by Olins forge-
welding process. Note the narrow homogeneous structure of the weld. The width of the weld
is uniform from top to bottom indicating that the heat input was uniform. Flow lines pass
parallel to and away from the weld indicating the high forging pressure which formed themetallurgical bond. Since a high frequency induction weld is a wrought structure similar to
the parent material, its strength, ductility, and corrosion resisting characteristics are similar
to the parent material. This permits the use of as-welded tube for most alloys in corrosiveenvironments.
The photomicrograph shown to the right is that of a weld produced by the competitive TIGwelding process. Note the cast structure associated with this process. This weld would need
to be cold worked and annealed for use in a corrosive environment.
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WELD STRUCTUREThe photograph shown on the left is another example of the weld produced by Olins
forge-welding process. The photograph on the right is a photo of the same weld shownon the left, but after annealing. Note that the forge-weld area is indiscernible from the
rest of the tube after annealing.
Weld Zone(50X Mag
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RE-CUT LINE & PACKINGThe final production operations in the production of Olin Fineweld Tubes welded tube areto precision re-cut the tube to the customers desired length, to deburr the tube ends, and
to pack the tube.
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QUALITY ASSURANCE & TESTINGThe Quality Assurance System at Fineweld Tube is ISO 9001: 2000 approved. Test methodsand frequencies are in accordance with ASTM B-543 and/ or ASME SB-543 (for condenser,
heat exchanger, and desalination tube). The primary nondestructive test utilized is 100% eddycurrent testing. The eddy current test unit is located in-line just after the tube is welded and
just prior to the initial cut-to-length operation. In-line eddy current testing avoids endeffects, which are associated with off-line eddy current testing utilized for seamless tube, and
assures that every inch or millimeter of tube is tested. Destructive testing includes expansiontesting, flattening, and reverse flatten testing, and dimensional checks are conducted for
diameter, wall thickness, length, straightness, and RIF. Hydrostatic and pneumatic testing isalso conducted when specified by the purchaser.
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Eddy current testing 100% testing required in both specs
Expansion testing Requirements identical; Olin testing exceeds ASTM by 600%
Flattening testing Required in both specifications; Olin testing exceeds ASTM by600%
Reverse flattening testing Not required in B-111; Olin testing exceeds ASTM by 100%
Diameter tolerance Requirements identical; Olin testing exceeds ASTM by 250%
Wall thickness tolerance Requirements identical; Olin testing exceeds ASTM by 250%
Length tolerance Requirements identical; Olin testing exceeds ASTM by 200%
Squareness of cut Requirements identical; Olin testing exceeds ASTM by 200%
Straightness Not required in either specification Olin applies the
requirements of ASTM B-587
RIF (Residual Internal Flash) Applies only to ASTM B-543 . +0.006/ -0.000 Olin testingexceeds ASTM by 250%
COMPARISON OF ASTM B-111 & ASTM B-543
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Eddy current testing 100% testing required in both specs
Expansion testing Requirements identical; Olin testing exceeds ASTM by 600%
Flattening testing Required in both specifications; Olin testing exceeds ASTM by
600%
Reverse flattening testing Not required in B-135; Olin testing exceeds ASTM by 100%
Diameter tolerance Requirements identical; Olin testing exceeds ASTM by 250%
Wall thickness tolerance Requirements identical; Olin testing exceeds ASTM by 250%Length tolerance Requirements identical; Olin testing exceeds ASTM by 200%
Squareness of cut Requirements identical; Olin testing exceeds ASTM by 200%
Straightness Requirements identical
RIF (Residual Internal Flash) Applies only to ASTM B-587 - +0.006/ -0.000 Olin testing
exceeds ASTM by 250%
COMPARISON OF ASTM B-135 & ASTM B-587
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WELD INTEGRITY FOR DEMANDINGAPPLICATIONS
The true test of a product is how it performs in a demanding application.Shown below are several applications in which Olins welded tubing isused that demonstrate the superior weld integrity of the product.
Integral Fin Tube
Twisted Tube
U-Bent Hairpin Tube
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SUMMARY
Specification requirements for welded tube are equal toor greater than seamless tube.
Performance in demanding applications is equal to orgreater than seamless tube.
More uniform temper and chemistry from one tube toanother.
Superior concentricity and wall thickness uniformity.
Eddy current end effects are avoided.
Superior surface quality and cleanliness on both the ODand ID.