Steel and aluminium: Best of both worlds
It’s a given that most energy usage for steel automotive components occurs during application, as opposed to manufacture. The obvious way to limit this energy expenditure is by reducing the weight of the individual components and the overall weight of the vehicle, hence the ongoing development of hollow transmission shafts and substitution of steel for lighter materials.
Another approach is to substitute a large proportion of the volume of a given component with a lower density filler material. In the case of a transmission shaft normally composed of solid steel, it might be advantageous to manufacture the shaft instead from a steel case filled with an aluminium core. The challenge here is to securely bond the materials together. Aluminium rapidly is passivated and the resultant Al2O3 film inhibits bonding to the steel surface. Furthermore, the difference thermal expansion coefficients of the two materials makes it unlikely that any bond formed will be stable. That’s not to say it’s impossible though!
(c) Wiley-VCH Verlag GmbH & Co. KGaA
The main point here is that the casing and core are heated prior to forming, but that the forming process itself results in further heating (the amount of heat generated being proportional to the strain rate) and, as the aluminium is strained, its passivation layer breaks up. Where the aluminium and steel surfaces are in contact, an intermetallic phase forms during forging to provide a strong, stable bond, (if the temperature is right, of course!) as shown here:
(c) Wiley-VCH GmbH & Co. KGaA
So, given appropriate preparation of the surfaces, heat treatment and a high strain rate, they’ve shown that steel-cased aluminium shaft can be readily forged. What other methods are out there to reduce component weight?
Don’t let it boil over
(c) Wiley-VCH GmbH & Co. KGaA, Weinheim
One thing to look out for in Basic Oxgen Steelmaking (BOS) is excess slag foam formation. Slag foam is itself crucial to the BOS process, providing the reaction vessel with a protective lining and improving the phosphorous refining conditions. If the foam formation isn’t carefully monitored and controlled, however, the foam can force itself out of the vessel, causing significant damage and loss of metal yield. Steel-makers call this slopping, and they hate it.
The method most commonly used to estimate height level and to control slopping is audiometry. Noise from the oxygen jet is attenuated by slag foam and so the noise is reduced as the foam height increases. Installing a microphone as illustrated here and analyzing the audio spectra is an effective method to monitor the foam height.
Another method described by Brämming et al is vessel vibration monitoring. The method involves recording the BOS vessel’s vibrations and can be used to monitor the foam by exploiting the characteristic transfer of kinetic energy from the slag to the vessel. The team from Luleå University of Technology, SSAB and Tata Steel ran trails of the new system with BOS vessels in Luleå and Scunthorpe in parallel with the audiometry method. They found that audiometry and vessel vibration methods result in comparible estimates of foam height and could be used in parallel to more accurately predict slopping.
Nice flat
The rail industry is reliant upon understanding and controlling the behavior of steel. Regular inspection and maintenance are required to keep both rails and wheels in good operating condition.
Here, then, is an example of both rail and wheel getting seriously abused. A train in southern England was rounding a bend at ~ 160 kph when a tree fell in front of it. The driver applied the emergency break but a collision was unavoidable and the front of the train smashed into the tree. Luckily the train did not derail, although the driver received cuts and bruises.
During the collision a switchboard was damaged which disabled, among other things, the wheel slide prevention system (a kind of ABS for trains). This meant that the train took an agonizing 1 km to stop, much longer than normal, and traveled a significant portion of that distance with the leading power car’s wheels locked up, and this is what they looked like afterwards:
Reproduced from RAIB Report 08/2011 with permission.
Ouch! This is a wheel flat – and what a beauty! Have you studied wheel flats or sliding contact in general? What articles are out there, and do you have any micrographs you want to share?
Interrogating martensite
Steel Research International is published monthly and in every issue we pick what we think is a particularly outstanding article and highlight it as The Editor’s Choice.
This month’s winners are a team from KTH (Royal Institute of Technology), Sweden. Their article “Spontaneous and Deformation-Induced Martensite in Austenitic Stainless Steels with Different Stability” is a comprehensive study that addresses some key issues regarding the role of martensite formation in these steels and how this affects the strain hardening behaviour.
So what next for austenitic stainless steels? Do we have a comprehensive understanding? What are the burning questions that have yet to be answered?
