Heat-Treating Steels
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340 Technology Fundamental Principles of Heat-Treating Steels by Richard D. Sisson, Jr. and Daniel H. Herring photo provided by Chen Nan Iron Wire The critical heat-treating process iron can only contain 0.022 wt% C. This eat-treating is a process that parameters are time, temperature, 100-fold difference in carbon solubility is critical to the fabrication atmosphere, and cooling rates. In in the two iron crystal structures is also of steel fasteners. It is this article the fundamental physical necessary for the heat treatment of steel. H a necessary process in metallurgy of steels will be presented order to achieve the mechanical for each step in this important process. The Iron - Carbon phase diagram properties. Heat-treating is typically Future articles will talk about each aspect is presented in Figure 1. Some of conducted after the forming of the heat treatment operation. the key features of the phase diagram processes and before any coating or were described above. A phase diagram finishing processes. Heat-treating is a map of the phase or phases that may be done by the manufacturer The Iron - Carbon Phase exist in equilibrium as a function of or outsourced to a commercial Diagram temperature and composition. For steel shop. The equipment necessary the composition is given in wt% carbon to effectively heat treat steel Pure iron can exist with two crystal and the temperature is in either Celsius fasteners includes well controlled structures. At temperatures below 912℃ or Fahrenheit. Typical steels contain furnaces with temperature control, (1,675℉), the crystal structure of iron is between .10 and .80 wt% carbon. atmosphere control, quenching body centered cubic (bcc) and is referred tanks, and cleaning equipment. to as Ferrite (α). At temperatures between 912℃(1,675℉) and 1,394℃ Pearlite Formation T h e s t e e l h e a t t r e a t m e n t (2,540℉) the crystal structure of iron is process consists of heating the face centered cubic (fcc) and is known as Let's consider steel that contains steel fasteners into the Austenite Austenite (γ). Above 1,394℃ (2,540℉) 0.76 wt% carbon. As seen on the phase range, that is to a high temperature and below the melting point of pure iron diagram, if this steel is heated to 800℃ 840℃~980℃ (1,550℉~1,800℉), 1,538℃ (2,800℉), iron returns to bcc (1,470℉), the fcc Austenite (γ) solid in which the steel becomes "red and is designated as delta (δ) Ferrite. solution exists. However, if the Austenite hot" for some time. Following the Steel can be heat treated as a result of this is cooled to below 723℃ (1,333℉) a heating process, the parts must be Austenite to Ferrite phase transformation. phase transformation will take place to cooled (quenched) rapidly usually in form the iron carbide (Fe3C) also known a liquid media such as oil or water. Iron can form a solid solution with as cementite and the bcc Ferrite (δ) solid When quenching has finished, the carbon to produce steel. The carbon solution containing less than 0.02 wt% next step is to re-heat (temper) the atoms dissolve into the solid by filling carbon. At 723℃ (1,333℉) a three-phase quenched parts at a temperature the interstitial sites of the fcc and bcc equilibrium exists as seen on the phase lower than that which was used crystals. Due to the size and shape of diagram. Austenite is in equilibrium with to harden the parts 200℃~480℃ these interstitial site the fcc iron can Ferrite and Cementite at 723℃ (1,333℉). (400~900℉). dissolve up to 2.14 wt% C while the bcc 342 Technology [1] Fig. 1 Simplified Iron-Iron Carbide Phase Diagram Fig. 3 Hypoeutectoid steel with approximately 50% Ferrite and 50% Pearlite Cooling Fig. 2 Optical photomicrograph of Steels containing more than 0.76 Austenite (0.76 wt%C) --> Ferrite (a) + Pearlite wt.%C are called hypereutectoid steels. Fe3C (0.02 wt%C) As seen in the phase diagram, when a hypereutectoid steel containing 0.90 When this phase transformation wt.%C is heated to 900℃ (1,650℉), the takes place on relatively slow cooling Austenite solid solution forms. When a microstructural component called this steel is slowly cooled the primary Pearlite is formed. Pearlite consists of phase is iron carbide (Fe3C). Upon plates of Fe3C in a matrix of Ferrite (α) cooling below 723℃ the Austenite, as seen in Figure 2. Pearlite is formed which now contains 0.76 wt.%C from Austenite containing the eutectoid transforms to Pearlite. The resulting composition of 0.76 wt.%C. This microstructure of this alloy will consist microstructure develops as a result of the of approximately 2% primary carbide decomposition of Austenite by nucleating and 98% Pearlite as seen in Figure 4. carbide plates first and then the Ferrite on the sides of the carbide plates. Time Temperature Transformation (TTT) Steels that contain less than 0.76 Diagrams wt.%C are called hypoeuctoid steels. a microstructure of primary Ferrite and As seen in the phase diagram, when Austenite is developed. Upon cooling T h e p h a s e t r a n s f o r m a t i o n s a hypoeuctoid steel containing 0.40 below 723℃ (1,333℉) the Austenite, described above are diffusion controlled wt.%C is heated to 900℃ (1,650℉) which now contains 0.76 wt%C is and occur by nucleation and growth. Austenite forms. When this steel is transformed to Pearlite. The resulting The rates of these transformations are slowly cooled the first new phase (also microstructure of this alloy will consist a function of temperature and steel called the primary phase) is Ferrite. of approximately 50% primary Ferrite composition and can be described with During this cooling to 723℃ (1,333℉) and 50% Pearlite, as seen in Figure 3. time - temperature - transformation 344 Technology diagram for the eutectoid steel as very rapidly to 600℃ (1,100℉) the For this eutectoid steel, when rapidly seen in Figure 4. This particular Austenite becomes unstable and will cooled to a temperature below the nose of diagram presents the rates for over time transform to Pearlite. As the TTT curve a slightly different type of isothermal (constant temperature) seen in Figure 4, after 2~3 seconds transformation occurs, it will result in the transformations of a eutectoid steel. the Austenite starts to transform formation of Bainite. Similar to Pearlite, In these diagrams the y-axis is to Pearlite, after 6~7 seconds the Bainite consists of a mixture of Ferrite and temperature and the x-axis is time. transformation is 50% complete (i.e. carbide. However, the morphology is finer In this instance the logarithm of time 50% Austenite and 50% Pearlite), and can have a more feathery or acicular is used. As seen on this diagram at and after about 15 seconds the appearance. As seen in Figure 4, after rapid temperature above 723℃ (1,333℉) transformation is complete, meaning cooling to 400℃ (750℉), the Austenite is the Austenite phase is always stable. 100% Pearlite is all that remains. unstable, after about 7 seconds the Austenite However, when this steel is cooled starts to transform to Bainite, after about 80 seconds the transformation is 50% [3] Fig. 4 Time - Temperature Transformation Diagram for the complete and after about 200 seconds the Eutectoid Steel transformation to Bainite is 100% complete. These TTT diagrams are available for a wide variety of steel compositions. As presented in these TTT diagrams the transformation from Austenite to Pearlite, Bainite, primary Ferrite or carbide takes time. These transformations are diffusion controlled and diffusion takes time. Martensite Formation and Tempering When Austenite quenched to room temperature a new phase transformation can occur. The fcc Austenite can transform to Martensite a body centered tetragonal (bct) phase. This reaction is not diffusion controlled and takes place by a shear-type transformation and is solely a function of temperature, not time. The as-quenched Martensite is extremely hard and brittle. The hardness of the Martensite is dependent on the carbon content of the Austenite in the steel. The Martensite and the Austenite it came from have the same chemical composition. 346 Technology As quenched Martensite is too To avoid carburizing or Conclusion hard and brittle for most practical uses, d e c a r b u r i z i n g , t h e c a r b o n however it can be heat treated to recover potential of the atmosphere in Heat-treating steel fasteners to some toughness and ductility. This the high temperature furnace achieve the desired microstructure and heat treatment is known as tempering. must be closely controlled. properties can be achieved by control Tempering consists of re-heating the Typically, the atmosphere in these of the important process parameters of Martensite to a temperature between furnaces is controlled with an temperature, time, and cooling rates. 200 ℃ (400℉) and 400℃ (750℉) for Endothermic gas generator. The details Knowledge of the fundamental physical several hours. During this heat treatment of atmosphere control will be a subject metallurgy of the phase transformations carbides precipitate in the Martensite of a future article. in steels is critical to understanding the matrix. The result of this transformation importance of these process parameters. is an increase in the toughness of the The next step in the process is steel by the carbide precipitates as well cooling. The cooling rate must be as the decrease in the carbon content of controlled to achieve the desired the Martensite.