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Metal Casting Technologies : March 2006
46 Back to the INTRODUCTION teel is a wonderful, very versatile material. Its properties can be set by proper alloying, and then these properties can be manipulated afterwards by proper heat treatment. It is no wonder that a period in the world's development has been named after it -- the steel age. This paper is written to give understanding and guidance to the proper heat treatment procedures without dealing too much in the deeply technical jargon. This is addressed to the operator on the factory floor who has enough experience with steels to understand what I am talking about. This paper is intended to make him understand the principles behind the heat treatment process and, hopefully, improve his operations due to a better comprehension of these principles. The heat treatment of steels may be divided into five processes, namely: (a) Normalizing -- The process consists of heating the steel to a temperature (about 30-80˚ C or 55-150˚ F, with the lower the temperature giving a finer grain-size) above the critical temperature range, soaking it there to ensure uniformity of temperature throughout the section, and cooling it in still air afterwards. The critical temperature in steels is altered by the presence of carbon (C) in the iron (Fe), that is, the higher carbon contents give lower critical temperatures (Ac3 = temperature at which ferrite completes transformation to austenite upon heating), in inverse proportion. (b) Annealing -- Heating the steel, as in normalizing, to above the critical temperature, soaking it, and allowing it to cool slowly in the furnace. (c) Hardening -- Heating the steel as above and cooling it in a quenching medium like oil, water, or brine. Then it is reheated to a selected temperature depending on the hardness desired, soaking it here, and cooling it in any convenient manner. This process is also called "quenching and tempering." (d) Austempering -- This is like "hardening" with the difference that the quenching medium is held at a temperature in the range of bainite formation of approximately 250-370˚ C (400-700˚ F) and holding it there for the time required for transformation. (e) Martempering -- This is like "hardening," but this time it is quenched in a medium that is held at a temperature range just above the martensite transformation temperature, held there long enough to equalize the temperature throughout the section, and then cooling slowly in air through the martensitic transformation zone. Note: The last two named processes were developed by United States Steel Corporation of Pittsburgh, PA. The heat treatment process is better understood with the use of diagrams -- variously called I-T Diagrams (Isothermal Transformation Diagrams), TTT Diagrams (Triple T or Time- Temperature-Transformation Diagrams), or simply S-Curves (because they look like the letter "S"). These diagrams are used in this paper to explain the different heat treatment procedures. HEAT TREATMENT PRINCIPLES AND PROCEDURES Presented here are brief explanations of the various heat treatment procedures for a working understanding of each. NORMALIZING The properties of steel in the as-cast condition depend upon the presence of alloying elements in the steel; while in the as-rolled or as-forged condition, these properties depend upon the amount and type of reduction during processing, finishing temperature, rate of cooling, etc. Normalizing is employed to assure greater uniformity of microstructure. This process consists of heating the steel to a temperature above the critical range and cooling in still air. The purpose of normalizing is to refine the coarse as-processed grain size and to improve the uniformity of the microstructure that may vary in a piece because of temperature differences while processing. It is used especially on large castings or forgings that cannot be quenched and tempered. It is also used as a treatment preliminary to quenching and tempering to obtain a more uniform microstructure, reduce distortion to the piece, and to facilitate the solution of carbides and alloying elements. This treatment is illustrated by the schematic diagram of Figure 1. S www.metals.rala.com.au Prof. John Hermes D. Bautista, PMAI Technical Consultant Practical Heat Treatment of Steels