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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Describing the 'Macrome' method of magnetically tempering steel.

Identifier  WestWitteringFiles\W\May1931-August1931\  Scan084
Date  6th August 1931
  
To R.{Sir Henry Royce} from He/CHP.
c. Sg.{Arthur F. Sidgreaves - MD} Wor.{Arthur Wormald - General Works Manager}
c. Rg.{Mr Rowledge} E.{Mr Elliott - Chief Engineer}
c. Ev.{Ivan Evernden - coachwork}

HS{Lord Ernest Hives - Chair}/CHP1/AD6.8.31.

84587

"MACROME" TEMPERING OF STEEL.

During a recent visit to the Works of Messrs. A.C. Wickman Ltd. at Coventry, I had the opportunity of witnessing a demonstration of the "Macrome" method of magnetically tempering steel.

It should be explained that this process is a final operation, performed on the finished article after the usual hardening and heat treatment has been carried out. It is claimed that the treatment increases the fatigue resisting properties of the steel by a very considerable amount. The treatment is used very largely for cutting tools of all descriptions; greatly improved service and longer life are claimed for tools so treated. Springs are also said to have a very much longer life after undergoing this treatment.

The actual treatment is carried out as follows :-
The pieces to be tempered are fastened to a rod (or other convenient method) so that they may be suspended in a bath of mercury. After submerging the pieces, the lid{A. J. Lidsey} of the mercury bath is clamped tightly down and the bath is then heated by means of a large bunsen flame to a certain temperature, (too low to upset previous heat treatment). The bath and its contents are now allowed to cool, and when a certain critical temperature is reached an electric current is passed several times through the pieces undergoing treatment. The mercury bath and contents are now cooled off as quickly as possible by means of spraying with water and the pieces removed. There is no visible difference to the pieces after undergoing treatment.

The inventor of this tempering system, Mr. Priest, explained to me that the secret of the treatment lies in the critical temperature at which the current is applied to the articles. This temperature depends on the specification of the steel of which the articles are made, and a considerable amount of experiment is sometimes necessary before the correct temperature can be found for any particular specimen, although a reasonable estimate can generally be made from previous experience. The function of the mercury is simply to act as a medium for keeping the heat at a constant temperature around the pieces undergoing the treatment.
  
  


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