Metal Rolling and Shaping

Metal Rolling Process

 

Metal rolling is the metal forming process used to reduce the thickness of metal and make the thickness of the reduced metal uniform.

The process is very similar to the rolling of dough with rolling pins.

There are two types of this process depending on the recrystallization temperature of the metal used, hot rolling and cold rolling.

When the temperature is above the recrystallization temperature of the metal it is hot rolling, and when below it is cold rolling.

Hot Rolling

 

Hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all cold working processes.

Roll stands holding pairs of rolls are grouped into rolling mills that quickly process metal into products like I-beams, angle stock, channel stock and other structural steel, bar stock and rails.

Steel mills often have rolling mill divisions that convert the semi-finished casting products into finished products.

Hot rolling allows the devormed grains of the metal to recrystalize equally to keep the microstructure symmetrical and prevent the material from work hardening.

It uses semi-finished casting products like slabs, blooms, and billets.

In continuous casting the products are usually fed directly into the rolling mills at the proper temperature.

Smaller operations heat the material with induction heating, or in the case of larger workpieces a gas or oil fired soaking pit is used.

The temperature of the workpiece must remain above the recrystallization temperature of the metal throughout the process.

Non-uniform cooling adds significant residual stress, usually in non-uniform cross section shapes like I-beams.

Good quality products have their surfaces covered in mill scale, an oxide that forms at high temperatures.

Mill scale is usually removed via pickling or smooth clean surface (SCS) process, revealing a smooth surface.

Hot rolling has a higher carbon tolerance for included carbon, and is therefore more difficult for a blacksmith to use.

Cold Rolling

 

Cold rolling occurs usually at room temperature, which increases the strength of the metals via strain hardening.

This strengthens the metal up to 20% while improving the surface finish and holds tighter tolerances.

Sheets, strips, bars, and rods in smaller sizes than the ones formed by hot rolling are often produced by cold rolling.

Cold rolling cannot reduce the thickness of a workpiece as much in a single pass as hot rolling can.

Sheets and strips produced by cold rolling come in various conditions, full-hard, half-hard, quarter-hard, and skin-rolled.

Full-hard rolling reduces the thickness of the metal by 50%, half-hard and quarter-hard create smaller reductions in thickness.

Skin-rolling involves the smallest reduction, at 0.5-1%.

It produces a smooth surface, uniform thickness, and reduces the yield point phenomenon. (Where the metal can deform under stress and not return to its original shape.)

Locking dislocations at the surface, and breaking up spangles in galvanized steel are also produced by skin-rolling.

Blacksmiths can more easily process cold rolling, because of its results are smoother, more consistent, and encapsulate low amounts of carbon in the metal.

The process as a whole is more expensive than hot rolling.

Products of cold rolling

• Metal furniture
• Desks
• Filing cabinets
• Tables
• Chairs
• Motorcycle exhaust pipes
• Computer cabinets and hardware
• Home appliances and components
• Shelving
• Lighting fixtures
• Hinges
• Tubing
• Steel drums
• Lawn mowers
• Electronic cabinetry
• Water heaters
• Metal containers
• Fan blades
• Frying pans
• Wall and ceiling mount kits
• Construction related products

Rolling Mill Configurations

 

The most basic rolling mill configuration is a two-high non-reversing with two rolls that turn in only one direction.

Two-high reversing mills can rotate in both directions, but are at a disadvantage because the rolls must be stopped, reversed, and then brought back up to rolling speed between each pass of the metal.

The three-high mill solves this problem with three rolls that move in one direction, so that the metal can be fed through two of the rolls and then through the other pair.

An elevator is needed to lift and lower the workpiece for a three-high mill.

Small roll diameters result in less power and force required, and four-high or cluster mills use this to their advantage.

Four-high mills use four rolls, two small and two large, the two smaller rolls use contact with the larger rolls to overcome their main problem which is reduction of stiffness.

All four rolls are placed in a vertical line, with the smaller ones placed on the outside.

Cluster mills use the same principal, but place their gradually larger rolls at angles to the small center rolls to maximise contact and use very small rolls to do the work.

Ring rolling increases the diameter of a ring, it is a specialized type of hot rolling.

Two rolls, an inner idler roll and a driven roll which presses the ring from the outside, during rolling the wall thickness decreases as the diameter increases.

This process results in circumferential grain structure, giving better mechanical properties to the finished rings.

Diameters as large as 26 ft and face heights as tall as 79 in can be made.

Ring rolling is used in railway tyers, bearings, gears, rockets, turbines, airplanes, pipes, and pressure vessels.

Roll bending produces a cylindrical product from sheet metals.

Roll forming is a continuous bending operation which uses a long strip of metal (Usually coiled steel) passing it through consecutive sets of rolls, each performing only an incremental part of the bend, until the desired cross-section profile is obtained.

It is ideal for parts with long lengths or large quantities.

Forge rolling is a longitudinal rolling process reduces the cross-sectional area of heated bars or billets, by moving them between two roll segments rotating in opposite directions.

Producing optimized material for die forging.

Forge rolling produces crankshafts, connection rods, steering knuckles and vehicle axles.

Controlled rolling uses controlled deformation and heat treating to control many facets of the worked metals.

Heat treatments can produce fine grain structure, control the nature, size and distribution of transformation products (Like ferrite, austenite, pearlite, bainite, and martensite in steel.) ; including precipitation hardening, and controlling the toughness.

This entire process must be closely monitored and controlled, but result in better mechanical properties and energy savings.

Metal Shaping

 

Deforming a workpiece through mechanical deformation, and changing the shape of the piece without altering its mass is the process of metal shaping.

It can be done by hand with a form, or an object that has the shape of the finished piece, and a hammer.

The most common form is a tree stump or log that have been carved with bowl shaped tops, with a variety of bowl diameters and depths allowing for more varied final shapes.

Other forms include a leather bag filled with sand or lead shot, or a metal piece already in the shape of the finished piece.

Repeatedly striking the workpiece with the hammer gradually deforms the metal into the desired shape.

This process can also use shop hammers to deform the metal far more quickly.

Planishing is then used to smooth out small irregularities, it uses a smooth, flat, and relatively hard hammerhead and a hard dolly that matches the contour of the part.

Mechanical hammers are especially useful for this process as it requires thousands of small hammer blows.

When a workpiece is finished it is clean and matches the dimensions intended.

Article refrences:

Metal Rolling:

https://en.wikipedia.org/wiki/Rolling_(metalworking)

https://thelibraryofmanufacturing.com/metal_rolling.html

Metal Shaping:

http://www.britishv8.org/Articles/Hammerforming-and-Planishing.htm

https://en.wikipedia.org/wiki/Forming_(metalworking)