316 316L stainless steel round pipe hollow bar use for hydraulic
Metal tube and hollow bar are used in a wide variety of structural
applications. They offer a cost advantage to solid metal bar stock,
while weighing less than a solid bar of the same material and size,
yet still providing much strength. While tube and hollow bar can be
similar in shape, we will look at the difference between tube and
hollow bar.
The difference between tube and hollow bar include:
Difference:
Manufacturing Process
Quality
Materials
Manufacturing Process:
A big difference between welded tubing and hollow bar is how the
two are made. Tube is made by taking slit coils of a base metal and
running it through a tube mill, which is a series of dies that form
the slit coil into a tube shape; either round, square, or
rectangular for example. Once formed into the desired shape, the
edges are joined together with a weld. The welding process can vary
depending on the material type, material thickness, and preference
of the manufacturing team. Typically, high frequency welding or
electric resistance welding is used. The excess weld metal is then
removed from the outside. Frequently, the newly joined tubing is
then brought through an annealing process to create a homogenous
final product.
With hollow bar, welding is not used. There are two main ways to
create hollow bar, with the process depending on material type and
dimensional tolerance requirements. One method for creating hollow
bar is called drilling. This process involves sending a drill
through a solid bar stock of material. This process can produce
highly accurate part dimensions for critical applications. The
second method is centrifugal casting. This is when a round mold is
rotated at high speeds as molten metal is poured into the mold. The
force created by the spinning mold forces the molten material to
the inside mold wall where it cools and hardens. Centrifugal casted
hallow bar tends to be dense and uniform. There are really only two
metals that refer to hollow bar; stainless steel and bronze.
Quality:
Hollow bar can be made to be extremely homogeneous and can also
possess very tight dimensional tolerances. On the other hand,
achieving tight dimensional tolerances can be more challenging with
tube. Also, if care is not taken by the manufacturer, the welding
process used for manufacturing tube can potentially create a lower
quality product than desired. The excess weld metal should be
removed to avoid stress concentrators in the welded tubing. Also,
the tube should be annealed or heat-treated after welding to ensure
that a homogenous product is being created. Welding processes can
create different crystalline structures near the weld zone which
could affect the desired mechanical properties of the tubing.
Materials:
A wide variety of materials can be formed into welded tubing. If
the material can conduct electricity, electric resistance welding
can be used to weld the material onto itself to create tubing.
Hollow bar can be created out of most materials as well. However,
harder and stronger materials may prove to be too difficult to
extrude so the piercing method could be used instead.
In the end, the choice between using tube or hollow bar will
ultimately come down to the application, as one is not universally
better or cheaper than the other. Depending on the material type
and tolerances, several design considerations should be taken into
account before a choice between the two is made.
Chemical Composition Compared:
| Material | C | Si | Mn | P | S | Ni | Cr | Mo | N |
| 316 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.035 | ≤0.03 | 10.0~14.0 | 16.0~18.5 | 2.00~3.00 | ≤0.10 |
| 316L | ≤0.03 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.03 | 10.0~14.0 | 16.0~18.0 | 2.00~3.00 | ≤0.10 |
Mechanical Properties Compared:
| Grade | UNS | Tensile Strength (MPa) min | Yield Strength (MPa) min | Elongation (%) min | Brinell Hardness (HB) max | Rockwell Hardness (HRB)max |
|---|
| 316 | S31600 | 515 | 205 | 40 | 217 | 95 |
| 316L | S31603 | 485 | 170 | 40 | 217 | 95 |
Product Photos:
