LETTERS To the Editor: It is surprising to note the number of chemistry textbooks that contain erroneous information in regard to the iron-carbon system in steel. I agree that this is not a very important aspect in the study of general chemistry and that many students and teachers never give this matter second thought. But in respect to .the informants it would be preferable for the material in the textbooks to be in accord consistently with the right idea than to agree cousisteutly with the wrong idea. The subject which prompts me to write this letter concerns the property of quenched steel relative to composition and hardness. Eighty per cent of the books I have examined express the opinion, in some form or another, that martensite (in austenite), the constituent of quenched steel, is really a supersaturated solution of cementite (FeaC) in ferrite, the hardness of the product being due to the rigidness of the ferrite crystal by the intrusion of cementite into the ferrite lattice. These ideas have no foundation. The fact is that when austenite (solid solution of carbon in gamma-iron) is continuously cooled a t a rate exceeding the critical cooling rate i t is transformed into martensite which is a solid solution of carbon in a structure not unlike femte (alpha-iron). At this rapid rate of cooling and a t the final temperature reached, which is about normal room temperature, the carbide cannot form. I t is only when steel is tempered for a period of time above 100°C. that the carbide begins to precipitate, and even then the product is not a solid solution. Martensite is only one phase and not a system containing two components as would be suggested by the above opposing statement. From the definition of a phase it may be seen that even if ferrite and cementite existed together in this product they would not be in solution. A phase is a distinct physical body. Therefore two or more phases can be separated mechanically, hence-
forth being in heterogeneous equilibrium. Since ferrite and cementite are phases they could be separated mechanically-but not so if the carbide was considered as a solid solution in ferrite, because a solution is homogeneous, i. e., a system alike through and through from which distinct physical bodies cannot be separated. Martensite is a solid solution, but only with respect to carbon; a phase when i t is regarded in relation to cementite. Consequently the two may be separated if one is present with the other, as is the case when austenite is air cooled. This is not the case in quenched ,steel; cementite cannot be separated from martensite because it is not present. Much is not known on why martensite is harder than the other phases of steel. Theories have been advanced which for the most part state: (1) that martensite is intrinsically hard because of grain structure (the carbon atoms are interstitially dissolved in the austenite, which dissolves approximately 8.5 atomic per cent of carbon, and are retained in supersaturated solution in the ferrite-like structure which normally would dissolve only a few tenths per cent): (2) that residual stresses (distortion) are-introduced into the entire matrix of austenite plus martensite by the precipitation of the plates of martensite. These facts are based on information taken from the following works: (1) BAW, EDGARC., "The Alloying Elements in Steel," U. S. Steel Corporation, .New York, 1939.
"The Physics of Metals," (2) SEITZ,FREDERICK, McGraw-Hill Book Company, Inc., New York, 1943.
(3)
BARRETT, CHARLES S., "Structure of Metals," McGraw-Hill Book Company, Inc., New York, 1943.
WILLIAM P. CLANCY WATEKTO ARSENAL ~
WATERTOWN, MASSACHUSETTS
