Question

In the heat treatment of steel, when is tempering not required? (1 mark)

You are an engineer and you are required to design a self-compacting concrete mix. List 2 important considerations in your mix design. (1 mark)

Answer 1

(1)
Depending on the holding-temperature, austempering can produce either upper or lower bainite. Upper bainite is a laminate structure formed at temperatures typically above 350 °C (662 °F) and is a much tougher microstructure. Lower bainite is a needle-like structure, produced at temperatures below 350 °C, and is stronger but much more brittle.
This produces steel with superior impact resistance.
Modern punches and chisels are often austempered.  
Because austempering does not produce martensite,
the steel does not require further tempering.

(2)
(I) In designing the mix it is most useful to consider the relative proportions of the key components by volume rather than by mass.
(ii) In the event that satisfactory performance cannot be obtained, then consideration should be given to fundamental redesign of the mix. Depending on the apparent problem, the following courses of action might be appropriate:
the use of additional or different types of filler, (if available);
modify the proportions of the sand or the coarse aggregate;
the use of a viscosity modifying agent, if not already included in the mix;
adjust the dosage of the superplasticizer and/or the viscosity modifying agent;
the use of alternative types of superplasticizer (and/or viscosity modifying agent) which may be more compatible with local materials;
different dosage rates of admixture to modify the water content, and hence the water / powder ratio.