Metallurgy; an introduction to the study of physical metallurgy . n to the ordinary temperature—either as the resultof quenching or by the presence of a third alloying element,as described below. The first stage in the decomposition of the 7-iron solid solu-tion consists in the formation of the acicular constituent alreadydescribed and illustrated (see Figs. 70 and 71, Plate XIV.). Thisconstituent has received the name of Martensite, and muchspeculation has been offered as to its true nature. It is un-doubtedly the hardest constituent of hardened steels, but it THE IRON-CARBON SYSTEM 179 varie
Metallurgy; an introduction to the study of physical metallurgy . n to the ordinary temperature—either as the resultof quenching or by the presence of a third alloying element,as described below. The first stage in the decomposition of the 7-iron solid solu-tion consists in the formation of the acicular constituent alreadydescribed and illustrated (see Figs. 70 and 71, Plate XIV.). Thisconstituent has received the name of Martensite, and muchspeculation has been offered as to its true nature. It is un-doubtedly the hardest constituent of hardened steels, but it THE IRON-CARBON SYSTEM 179 varies very widely in the details of its structure. When steelhas been quenched from very high temperatures, the Martensitela found to occur in large grains showing a rather coarse aciculaistructure. When, on the other hand, a steel of eutectoid com-position is quenched at a temperature just above the criticalpoint Ar^, then the structure of the resulting Martensite isexceedingly fine—so fine, indeed, that some workers, not perhaps 20000 n;j 5,000. 600 woo ?two Fig. 74. 100 eoo 900 Tempefalure in Decrees Centigrade Temperature-Tenaoity Curve for very Soft Steel at highTemperatures. provided with the best of microscopic appliances or employingunsatisfactory etching methods, have failed to detect theacicular structure, and have claimed that the best hardened steelcontains a structureless Martensite, which they have some-times distinguished by the term hardonita —a purely localterm whose general use is not to be recommended. The carefulexamination of samples of the best and most carefully hardenedsteel, however, has convinced both the author and the majorityof impartial observers that Martensite can always be shown to n2 180 STUDY OF PHYSICAL METALLUEGY have an aoicular structure, although in properly hardened steel that structure ia exceedingly minute. The question, What is Martensite and to what is its greathardness really due ? cannot as yet be answered quite co
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