Friday, December 14, 2018

Understanding the Hot Rolling Process

In metalworking, rolling is a fabrication process that consists of passing the metal through a pair of
rollers. Flat rolling creates a sheet, while profile rolling creates a bar or a rod. In addition, rolling
processes are temperature-dependent. Hot rolling is performed when the material is above its
recrystallization temperature, while cold rolling is performed below the recrystallization temperature.
Advantages of Hot Rolling
As you might suspect, hot rolling and cold rolling create products with very different characteristics,
meaning that each process has its place. Hot rolling requires less processing than cold rolling,
meaning that hot rolled products are significantly less expensive. In addition, hot rolled metals cool at
room temperature, avoiding the internal stresses of quenching or work-hardening. Hot rolling allows
metals to be easily manipulated into a variety of shapes, and it creates a denser finished product.
It is important to note that dimensional tolerances are not as precise with hot rolling as with cold rolling,
but the material strength is higher. Therefore, it is best suited to applications in which strength matters
more than minute distortions, such as construction projects and railroad tracks.
Hot rolled metals do not have the same smooth surface as cold rolled metals, as scaling is a byproduct
of the cooling process. However, this scaling can be removed through acid-bath pickling, sandblasting,
or grinding, and a new finish can be applied. In addition, descaled steel provides an optimal surface for
coatings such as paint.
How Is Hot Rolling Done?
The first step is to heat a large, rectangular billet of metal to an extremely high temperature based on
the temperature at which the molten metal recrystallizes (for example, more than 1700 degrees F for
steel), and then pre-process it by flattening it into a single large roll. While maintaining the temperature,
the metal is then pushed through a series of rollers at high speeds to create its finished shape and size.
To create sheet metal, the rolled metal is spun into coils before cooling. For bars, plates, or other
profiles, the material is sectioned, cooled, and then packaged. Note that hot rolled metal is allowed to
cool naturally, which will result in a bit of shrinkage and perhaps minute warping.
Each metal and each fabrication process has its own unique pros and cons, and each is best for
specific applications. Because there is no one size fits all solution, it is vital to work with an expert to
ensure that you choose just the right metal and fabrication process for your specific project.
Ready to Start?
Atlas Bronze is a leading U.S. distributor of bronze, copper, brass, iron, and more. Contact us today at 1-800-478-0887 to place an order or learn about our custom products.

Understanding the Sintering Process

Powder metallurgy is the process of creating metal objects from metal powder rather than formed billets.
It is highly useful for smaller items in which casting or forging would create a great deal of loss, for
metals such as tungsten with extremely high melting points, and for creating alloys of mutually insoluble
metals. It is also used when a highly porous item is desired.
Sintering is the process of heating and compacting the metal powder to create the desired object. It is
done at temperatures higher than room temperature but lower than the melting point of the metal.
Solid State Sintering
If solid state sintering is desired, two things must happen after the powder is heated and compacted.
First, the pressing lubricant must be removed through evaporation and vapor burning. Then, the surface
oxides of the powder particles must be removed. At this point, the contact points of the object will grow
“sintering necks” that join them together, which can later be cold welded for strength. Most commonly,
the entire process occurs in a single, continuous furnace kept at an appropriate temperature.
Sinter Hardening
To enhance the strength imparted by sintering, it is possible to accelerate the cooling rate of the
sintering furnace. Special material grades are available that create tougher microstructures at specific
cooling rates.
Liquid Phase Sintering
There are two types of liquid phase sintering: transient and permanent.
Transient liquid phase sintering: When working solely with iron powder, solid state sintering would
lead to shrinkage as the sintering necks develop. Adding fine copper powder, though, guards against
shrinkage and adds extra strength to the finished product. In transient liquid phase sintering, the copper
melts at sintering temperature, diffusing throughout the iron powder.
Permanent liquid phase sintering: In some cases, such as cemented carbides, a permanent liquid
phase is desired throughout the sintering process. This involves adding a melting binder to the
powdered metal. There are three distinct stages to this process:

Rearrangement: During melting, the liquid will be pulled into pores and the grains will rearrange
through capillary action.
Solution-precipitation: Where capillary action is high, atoms will move into solution and then precipitate into areas of lower chemical potential. The result is grain boundary diffusion and
Final densification: At this point, grains are efficiently packed, allowing liquid to easily flow into pores, gradually densifying the full solid skeletal network.
Ready to Start?

Atlas Bronze is a leading U.S. distributor of bronze, copper, brass, iron, and more. Contact us today at
1-800-478-0887 to place an order or learn about our custom products.