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Engineering materials: Structure/property relationships in forming processes
Rolling
This unit of work addresses aspects of the following syllabus outcomes.
The student:
H2.1 differentiates between properties of materials and justifies the selection of materials, components and processes in engineering
Source: Stage 6 Engineering Studies syllabus, page 37. Board of Studies NSW, 1999.
Introduction
One of the most diverse areas of engineering would have to be the processes designed to produce and form raw materials for machine and structural components.
These forming processes have a major impact on the structure of the material and the final ‘working properties’ required by the material as it performs its ‘component function’. It is often necessary to change or modify a material’s structure after forming to allow it to meet the demands of its operating environment.
In this section students are required to ‘learn to’:
‘analyse the structure/property relationship developed through forming processes’
Rolling is a forming process, which causes permanent change of shape (set) by plastic deformation of the material as it passes between sets of steel rolls.
Sets of cylindrical rolls reduce the cross-sectional thickness of the metal whilst simultaneously causing it to become elongated. Other rolling processes are employed to change the cross-sectional shape of the metal using shaped rolls while other methods form the metal into a specific shape for a particular purpose.
In its simplest form, rolling produces flat sheet of a specific thickness related to its ‘end use’.
Many parts of a crane are made of cold rolled steel. These include body panels, which are pressed to shape, plate sections for platforms, and foot rests, jib structural frame members or stainless steel for exhaust cowlings. Cold rolling has the added effect of work hardening the material thus further improving the material’s mechanical properties.
The Rolling process is able to form steel and other metals into a wide range of shapes and sizes. Slip rolls turn flat plate into a cylinder while heavy-duty rolls are able to form the gentle curves in railway lines for the monorail.
The slip rolls of this machine are progressively adjusted until the cylinder closes.
Square tube, channel and ‘I’ beams pose no problem for these sets of powerful rolls.
Forming an annulus across a thick section is a simple task for these special rolls.
The rolling process is carried out as either a ‘Hot Working’ or a ‘Cold Working’ process.
Hot rolling is a plastic deformation process, which mechanically shapes a metal at a temperature above its recrystallisation temperature. Any induced stresses and grain deformation is removed due to the elevated working temperature. The resultant grain structure is equiaxed.
http://www.stahlseite.de/eko2.htm 
This link will take you to a steel production factory in Germany. Here you will see an image of a slab of hot steel in a rolling mill.
As the metal passes through the rolls its original equiaxed grains are elongated relative to the degree of deformation.
Different grain orientation has resulted in reflected light producing the colours shown.
http://www.stahlseite.de/eko4.htm
This link will take you to a steel production factory in Germany. Here you will see an image of a coil of hot steel in a rolling mill.
The high rolling temperature has allowed the grains to recrystallise thus becoming equiaxed once again. Note that the Hot Working / Recrystallisation temperature is different for different metals and metal alloys.
When metal is worked below its 'recrystallisation' temperature it is said to be 'cold worked'. However, the metal may be purposely heated or still quite hot from another forming process such as hot rolling or forging.
When a metal with an equiaxed grain structure is cold worked, as in rolling, the grains are 'squashed' slightly to form a type of oval shape. These elongated grains will provide directional strength to the metal whilst under a load.
The deformed grain structure will remain unless further heat treatment (such as annealing or normalising) is carried out.
Elongated grains provide directional bend-strength properties.
Activity 1
Go to the following web site and answer the questions below
http://www.bluescopesteel.com/navajo/display.cfm/objectID.5248A606-DDF6-4E80-B74FA4A95B3F5107#rolling
- Cast steel is a relatively weak mass of coarse, uneven metal crystals. Describe the benefits of rolling cast metals and list some of the properties, which are achieved by this process.
- Justify the reason for heating steel slabs, blooms and billets to 1200°C prior to rolling.
- Cold rolling has quite a different effect on the finished product when compared to hot rolled steel. Describe some of these differences.
Activity 2
Go to the following web site and answer the following questions
http://www.steelstrip.co.uk/coldrolled.htm
- List some reasons for cold rolling following a hot rolling process
- Explain the term ‘reversing mill’ when applied to cold rolling mills.
Activity 3
The cold rolling process plays an important role in determining the properties of metal, which affect further forming operations. Answer the following questions by researching the site below.
http://efunda.com/processes/metal_processing/cold_rolling.cfm
- What does the amount of strain, introduced during cold rolling, have on the rolled metal?
- Cold rolled steel can be produced in various 'conditions'. Compare some of the characteristics and differences of 'skin rolling' and 'quarter hard rolled' stock.
- Describe some of the 'bend' possibilites for 'quarter hard', 'half hard' and 'full hard' meterial.
Activity 4
The tower crane illustrated below has quite a long jib fabricated in a triangulated shape by welding. It is constructed from cold rolled steel for the long members, cold drawn steel tube for the short members and the cross braces at the bottom of the jib are hot rolled ‘angle iron’. A trolley, circled, travels along the jib to raise and relocate loads thus inducing a bending moment in the jib.
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Activity 5
http://www.activecranehire.com.au/hire.htm
You can see a schematic of this self-erecting crane and read more about it on the web site above.
The images below show portions of a small portable tower crane used on low-level construction sites. It is easily assembled and disassembled in difficult site locations. The crane is primarily fabricated, by welding rolled steel plate approx. 12mm to 16 mm thick, as a ‘box section’ member.
From your knowledge of forming processes and structure/property relationships, select and justify the best possible position, on the steel plate, (for the ‘oxy-cut’ profiles for the side panels) to make the Support Arm. Your selection must achieve the best possible bend strength for the panels.
Note the direction of cold rolling.
Vertically applied loads are resisted by the elongated grain structure.
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