Wednesday, April 23, 2014
Monday, April 21, 2014
Hot rolling & Cold rolling steel
Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces and for smaller workpieces induction heating is used. As the material is worked the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature; this is usually 50 to 100 °C (90 to 180 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling.
Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than 20 mm (0.79 in) thick often have some directional properties. Also, non-uniformed cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams. While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high-temperatures. It is usually removed via pickling or the smooth clean surface process, which reveals a smooth surface. Dimensional tolerances are usually 2 to 5% of the overall dimension.
Hot rolled mild steel seems to have a wider tolerance for amount of included carbon than cold rolled, making it a bit more problematic to use as a blacksmith. Also for similar metals, hot rolled seems to typically be less costly.
Hot rolling is used mainly to produce sheet metal or simple cross sections, such as rail tracks. Other typical uses for hot rolled metal includes truck frames, automotive wheels, pipe and tubular, water heaters, agriculture equipment, strappings, stampings, compressor shells, truck frames, railcar components, wheel rims, metal buildings, railroad-hopper cars, doors, shelving, discs, guard rails, pipe and tubular, automotive clutch plates.
Cold rolling
Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used.[2] Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.
Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin-rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction.Skin-rolling, also known as a skin-pass, involves the least amount of reduction: 0.5-1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield point phenomenon (by preventing Lüders bands from forming in later processing). It locks dislocations at the surface and thereby reduces the possibility of formation of Lüders bands. To avoid the formation of Lüders bands it is necessary to create substantial density of unpinned dislocations in ferrite matrix. It is also used to breakup the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.
Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small. Cold rolling shapes requires a series of shaping operations, usually along the lines of sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.
If processed by a blacksmith, the smoother, more consistent, and lower levels of carbon encapsulated in the steel makes it easier to process, but at the cost of being more expensive.
Typical uses for cold rolled steel include metal furniture, desks, filing cabinets, shelves, tables, chairs, motorcycle exhaust pipes, computer cabinet and hardware, all home appliances and components, shelving, lighting fixtures, hinges, tubing, steel drums, lawn mowers, electronic cabinetry, lighting fixtures, water heaters, metal containers, and a variety of construction related products.
Wednesday, April 9, 2014
Tuesday, April 8, 2014
EMC Environment
Assemblies can emit and the must be immune to external
electromagnetic disturbances. IEC defines two categories
a) Environment A - relates to low-voltage non-public or industrial
networks / locations / installations including highly disturbing sources.
b) Environment B - relates to low-voltage public networks such as
domestic commercial and light industrial locations / installations.
This environment does not cover highly disturbing sources such as arc welders.
The specifier should detail a requirement for either Environment A or B.
In exceptional applications, for example, some rail applications, it is
necessary to specify a higher level of immunity.
Monday, April 7, 2014
Internal Arc inside Switchboard
Arc Proof metal-clad switchgear and controlgear is designed
and manufactured to prevent the occurrence of internal faults.
If the switchgear and controlgear is installed, operated and
maintained following the instructions of the manufacturer, there
should be little probability that an internal arc occurs during its
entire service life, but it cannot be completely disregarded for
hazard risk assessment.
An internal arc fault, which constitutes a hazard, if operators
are present, though extremely rare might occur due to reasons
such as failure of insulation, contacts due to ageing, overvoltages
in system because of switching or lightning surges, pollution
due to environmental conditions, mal-operation or insufficient
maintenance.
Engineers and site managers have a legislated “duty of care” to
make proper equipment selection, operating procedures and
service conditions. The effectiveness of the selection, at
providing the prescribed level of protection of operators in case
of an internal arc, can be verified by type testing.
Designs which have been successfully type tested qualify as
IAC classified. This classification is intended to offer a tested
level of protection to operators in the vicinity of the equipment
in normal operating conditions and with the switchgear and
controlgear in normal service position, in the event of internal
arc.
Other enhanced measures may be adopted to provide the
highest possible level of protection to operators in case of an
internal arc. These best practices measures are aimed to limit
the external consequences of internal arc;
a) Rapid fault-clearance times initiated by arc detect sensors or
by a busbar protection.
b) Application of fault current limiting fuses to limit the let-through
current and fault duration.
c) Fast elimination of arc by diverting it to metallic short circuit
by means of fast sensing and fast closing devices (arc terminator).
d) Remote control to allow operators to stay outside arc flash
boundary.
e) Pressure relief device.
f) All operations behind type tested arc proof doors.
Normal operating conditions means the conditions of metalclad
switchgear and controlgear required to carry out operations
such as opening or closing HV switching devices,
connecting and disconnecting withdrawable parts, reading of
measuring instruments and monitoring equipment, etc. Therefore,
if to perform any of such operations any cover has to be
removed and/or any door has to be opened, the test shall be
carried out with the cover and/or door removed.
Removing or replacing active components (for example, HV
fuses or any other removable component) are not considered
to be normal operations, neither those required to carry out
maintenance works.
The Internal Arc Classification IAC makes allowance for internal
overpressure acting on covers, doors, inspection windows,
ventilation openings, etc. It also takes into consideration the
thermal effects of the arc or its roots on the enclosure and of
ejected hot gases and glowing particles, but not damage to
internal partition and shutters not being accessible in normal
operating conditions.
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