US20150144234A1 - Microtreatment of carbide containing iron-based alloy and articles resulting therefrom - Google Patents

Microtreatment of carbide containing iron-based alloy and articles resulting therefrom Download PDF

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US20150144234A1
US20150144234A1 US14/404,007 US201314404007A US2015144234A1 US 20150144234 A1 US20150144234 A1 US 20150144234A1 US 201314404007 A US201314404007 A US 201314404007A US 2015144234 A1 US2015144234 A1 US 2015144234A1
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bainite
temperature
based alloy
steel
austenite
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Gary M. Cola, JR.
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/607—Molten salts
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite

Definitions

  • This invention relates to treated iron-based alloys, and more particularly relate to processes for transforming and/or shaping the same and the various microstructure materials resulting therefrom obtained by treating low, medium, and high carbon steel with other metallic components and other iron-based alloys to at least a mixed microstructure that contains martensite, bainite and un-dissolved carbides, and may also contain complex steel microstructures including portions of bainite, coalesced bainite, pearlite, ferrite, acicular ferrite, retained austenite and/or martensite along with combinations thereof by micro-treating said iron based alloy.
  • Processing of high strength steel generally takes intense capital equipment, high expenditures, expensive and dangerous heated fluids, such as quenching oils and quenching salts, and tempering/annealing processes which include the use of furnaces, heated equipment, and residual heat from pouring molten steel. These quenching procedures are intended to raise the hardness of the steel to a desirable value. Bainite and martensite can be made by these processes and are very desirable materials for certain high strength applications as they generally have Rockwell C hardness of from about 20 and up. The increased hardness correlates to a comparable increase in tensile strength.
  • Typical advanced high strength steels have generally included bainitic and/or martensitic phases.
  • Bainite is generally acicular steel structured of a combination of ferrite and carbide that exhibits considerable toughness while combining high strength with high ductility.
  • bainite has been commercially produced by austempering through lengthy austemper thermal cycling, bainite is a very desirable product.
  • One practical advantage of bainitic steels is that relatively high strength levels can be obtained together with adequate ductility without further heat treatment, after the bainite transformation has taken place. Such steels, when made as a low carbon alloy, are readily weldable.
  • Bainite has been found to be temper resistant and can form in the heat-affected zone adjacent to the weld metal, thereby reducing the incidence of cracking and providing for a less brittle weld seam. Furthermore, these steels having a lower carbon content tend to improve the weldability and reduce stresses arising from transformation. When austempered bainite is formed in medium and high carbon steels, weldability is reduced due to the higher carbon equivalence content.
  • the other conventional high strength steel constituent, martensite is another acicular microstructure made of a hard, supersaturated solid solution of carbon in a body-centered tetragonal lattice of iron. It is generally a metastable transitional structure formed during a phase transformation called a martensitic transformation or shear transformation in which larger workpieces of austenized steel may be quenched to a temperature within the martensite transformation range and held at that temperature to attain equalized temperature throughout before cooling to room temperature. Martensite in thinner sections is often quenched in water. Since chemical processes accelerate at higher temperatures, martensite is easily tempered to a much lower strength by the application of heat.
  • this effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead. Since quenching can be difficult to control, most steels are quenched to produce an overabundance of martensite, and then tempered to gradually reduce its strength until the right hardness/ductility structure for the intended application is achieved.
  • low grade ferrous alloys in strips, sheets, bars, plates, wires, tubes, profiles, workpieces and the like are converted into multi-phase high strength steels with a minimum of cost, time and effort.
  • Dual and more complex phase materials are achievable by practicing the present invention. Due to the short duration of the heating of the iron based alloy from the upper austenization temperature to the peak selected temperature, and the ability to rapidly achieve a partially bainitic microstructure, this method has become known as “Flash Bainite Processing”.
  • the iron-based, or ferrous, alloys/articles start out having a first microstructure, containing carbides, prior to the micro-treating, and are converted into a second complex microstructure by rapid heating and rapid cooling into high strength steels on at least a portion of the alloy/article.
  • a method for rapidly micro-treating an iron-based alloy for forming at least one phase of a high strength alloy, where the method comprises the steps of providing an iron-based alloy having a first micro-structure with an austenite conversion temperature.
  • This first microstructure is capable of being transformed to an iron-based alloy having a second micro-structure including the above mentioned phases by rapidly heating at an extremely high rate, such as 300° F./sec to 5000° F./sec from below the austenitic conversion temperature to a selected temperature above the austenitic conversion temperature.
  • This second microstructure is known to be heterogeneous due to the lack of time allowed above the upper austenization temperature for homogenization of alloying elements in the carbide containing iron based alloy.
  • the traditional austenitic conversion temperatures are elevated for given alloys due to the short duration of the thermal cycle initiated by the rapid heating.
  • This elevated austenization temperature is in part due to an averaging of austenization temperatures of the multiple alloy concentrations present within the steel in the individual austenite grains. Because different carbon concentrations have different upper austenization temperatures, the carbon concentration present in the majority of the prior austenite grains will have the greatest influence on the austenization temperature.
  • This heating step involves nearly immediate heating of the iron-based alloy to a selected temperature above its austenite conversion temperature starting from a temperature below the upper austenization temperature. Then, the alloy is substantially immediately quenched once the peak selected temperature is reached, without any substantial holding period at the elevated temperature, most often at an extremely fast rate, i.e. 300° F./sec to 5,000° F./sec on at least a portion of the iron-based alloy in a quenching unit adjacent the heating unit. In some instances, slower or interrupted quenching is desired to enact continuous cooling transformation or time temperature transformation of the carbide containing iron based alloy. This procedure forms at least one phase of a high strength alloy in a desired area, depending upon where the treatment was performed. Extremely rapid quenching will form at least one phase of a high strength alloy, as described more fully herein below.
  • Quenching may be accomplished nearly instantaneously by various methods and apparatuses, including water baths, water sprays, chilled forming dies, air knives, open air convection, final operation chilled progressive dies, final stage chilled line dies, chilled roll forming dies, and quenching hydroforms among others. Slower or interrupted cooling is possible through the use of molten salts, oils, steam, heated gaseous solutions, chilled quenching rollers, and many other means known to those skilled in the art. Regardless of quenching method, initiation of quenching occurring substantially immediately after reaching the peak selected heating temperature is required to limit carbon migration, carbide dissolution, and alloy homogenization.
  • FIG. 1A is a FEGSEM micrograph of bainite processed in accordance with the present invention.
  • FIG. 1B is a FEGSEM micrograph of bainite processed in accordance with the present invention.
  • FIG. 2A is a graph of typical temperature measurements at the inside wall of the processed tube
  • FIG. 2B is a cooling cycle time/temperature graph of the process in accordance with the present invention.
  • FIG. 2C is the heating and cooling cycle simulating austenite grain development over time
  • FIG. 3 is a transform analysis graph of temperature versus differential of temperature
  • FIG. 4A is a mechanical heterogeneity analysis of a raw material
  • FIG. 4B is a similar analysis of the flash processed material
  • FIG. 5 is a graph of elongation versus peak heating temperature
  • FIG. 6 is a stress versus strain graph of various examples of material
  • FIG. 7A is optical and SEM images of steel before and after Flash Bainite Processing
  • FIG. 7B is a transmission electron microscopy image of Flash Bainite Processed steel
  • FIG. 8 shows initial boundary conditions for simulation
  • FIGS. 9A , 9 B, 9 C, and 9 D are of cementite dissolution modeling at various heating rates
  • FIG. 10 is of the concentration gradients [C and Cr] at room temperature and at 1000° C.
  • FIG. 11 is a continuous cooling diagram of Fe-0.03C-0.3Cr wt % iron based alloy.
  • a new method of metal treatment which results in transforming a low grade iron based alloy into a high strength steel with extremely rapidly heating of the metal followed immediately by rapidly quenching the material.
  • the resulting steel is a heterogeneous composition of at least martensite, bainite, ferrite, and the other structures discussed in more detail hereinbelow.
  • the upper transformation temperature occurs when austenite transforms to nano-scale platelets.
  • the second transformation occurrence during cooling is the coalescing of the platelets into larger plates.
  • This double transformation notion leads us to another aspect of this invention. Since a double transformation is occurring, one could allow the first transformation to occur but halt the second. For example, rapidly heat the iron based alloy to a selected peak temperature, a few seconds later, quench the iron alloy in a quenching medium that is below the first transformation finish temperature but above the second transformation start temperature. The material would complete the first transformation but delay the second transformation.
  • first stage of Flash Bainite may cause for example, the first stage of Flash Bainite to form, for example, just the nano non-coalesced bainite phase, but then leave a significant amount of another phase, possibly retained austenite, or some other austenite daughter product.
  • the material could then be brought down from the temperature between the first transformation finish temperature (i.e. 1022 F) and the second transformation start temperature (i.e. 860° F.).
  • first transformation finish temperature i.e. 1022 F
  • second transformation start temperature i.e. 860° F.
  • alloying elements known to be strong carbide formers has been found to be beneficial to the final microstructure's mechanical properties.
  • Such alloying elements include, but are not limited to, chromium, molybdenum, vanadium, silicon, aluminum, boron, tungsten, and titanium.
  • Two plates of 1020 based plain carbon were Flash Bainite processed. The only significant difference in the steel chemistry was that one had 0.33% weight chromium added. The steels were rapidly heated to 2050° F. in a few seconds. Upon reaching peak 2050 F, the steels were quenched in water spray to room temperature. The modified 1020 with 0.33% wt chromium tested at 1500-1600 MPa at 9 to 10% A50 elongation.
  • Flash Bainite Processed 4140 steel resulted in 1600 MPa yield strength, 2100 MPa tensile strength and 10% total A50 elongation. Testing of 4130, when using a full 1 ⁇ 2′′ width A50 specimen at 1 ⁇ 4′′ thick has resulted in 11-12% total A50 elongation at 1500 MPa yield and 1900 MPa ultimate tensile strength.
  • spheroidization of the prior microstructure was beneficial to enhancing the mechanical properties of the Flash Bainite processed steel. It is believed that spheroidization further limits carbon migration by organizing the carbon into spheres which are known to be the geometrically most efficient manner of volumetric storage with respect to surface area. By limiting carbon migration through speroidization, the percentage of bainite can be controlled. Other methods of developing texture in the steel to control prior austenite grain size and interaction with elemental migration has shown to be beneficial. For instance, in cases of 4130 steel, hot banded 4130 was found to have 10-15% lower ballistic resistance than 75% spheroidized 4130 against armor piercing and other small arms projectiles.
  • thermo-mechanical processing techniques require lengthy thermal processing to obtain a complex bainitic-martensitic microstructure
  • Flash Bainite Processing can do so with a single, rapid quenching operation which can take less than 10 seconds from above the lower austenitic temperature to below the lower martensitic temperature.
  • Other longer duration methods explained herein can provide other desirable metallurgical results provided that the first quenching step to below the bainitic finish temperature occurs substantially immediately after reaching peak heating temperature.
  • the micro-treated low, medium, or high carbon iron-based alloy may have varying thicknesses for different applications and may be readily weldable while having high tensile strength, along with the ability to minimize material and reduce weight.
  • One aspect of the present invention for the elevated quench interruption temperature is to use a quench medium at this temperature that could be molten salts, super heated gases, or heated oils, among others.
  • This aspect causes the first iron based alloy transformation that is caused by molten salt yet the temperature is above the second transformation start temperature so that the second transformation is slowed by molten salt's temperature.
  • all other iron alloy transformations are intentionally occurring in molten salt through either continuous cooling transformation or time temperature transformation. From this temp, below 1022° F. in the case of 4130 steel, the steel could be cooled in a manner in which the remaining austenite is brought down to ambient temperature with either minimal further transformation occurring or transformation to some other desirable austenite daughter phases.
  • Another aspect of this invention has to do with the heating and quenching apparatus.
  • Other previously filed patent applications for apparatus employ single or multiple heating and quenching heads to cool the material.
  • the present method employs a single heating unit to heat multiple pieces of material.
  • a rectangular induction coil could have material passing by it and heated both inside and outside of the coil. If the coil were appropriately sized, a rectangular tube could be heated inside the coil while other pieces, such as pieces of bar stock could be heated on the outside of the induction heating coil simultaneously.
  • Another aspect of this invention has to do with heating interrupted pieces of material.
  • a strip could have multiple cutouts removed from its shape. These pieces could be manufactured in the soft state and then Flash Bainite Processed to their final hardness state.
  • the edges near the cutouts will concentrate heat and melt the corners.
  • the present aspect of this invention will allow plugs of similar material to be held in place of the interruptions to absorb the heat. This will thus prevent the melted corners. Even when plugs are not employed, since the heating and quenching is so rapid, the corners of the shaped iron based alloy are not melted.
  • quench and partitioning technology austenizes steel over many minutes to level carbon, dissolve carbides, and homogenize alloy distribution in the steel. quench and partitioning then quenches to below the martensite start temperature, either holds at that sub-Ms temperature or reheats just above the martensitic start temperature and then quenches to ambient. The reason for such a hold is the partitioning of carbon to attain a desirable quantity of retained austenite in the final product.
  • Another aspect of Quench and Partitioning technology austenizes steel over many minutes, quenches to below the martensite start temperature, reheats above martensitic start temperature and bainite finish temperature and then quenches to ambient after a desired amount of transformation has occurred.
  • the present innovation is a new technique of Quench/Partition technology.
  • For the Flash quench and partitioning all aspects are the same as conventional quench and partitioning with the exception of employing the rapidity of the Flash heating cycle. As soon as the peak austenization temperature is reached, rapidly heating at an extremely high rate, such as 300° F./sec to 5000° F./sec, quenching is initiated.
  • the novelty over traditional quench and partitioning is that a heterogeneous, non-homogenized, austenized steel is being quenched.
  • the heater will rapidly austenize the steel, quench the material to enact a transformation above the martensitic finish temperature, hold or reheat with the second heater, or heated quenching medium, to a subaustenitic temperature to stabilize or transform the existing microstructure, and then quench to room temp with the second quench.
  • Such methods are known to provide desirable quantities of retained austenite.
  • the resulting high strength steel may include at least one portion of the resulting high strength material made of coalesced bainite, upper bainite, lower bainite, martensite, ferrite, retained austenite, pearlite, ferrite, acicular ferrite, and/or dual or complex phase combinations thereof, depending on the placement of the treatments described and claimed herein below.
  • Complex phase materials can be made, such as martensite and bainite located next to ferrite and pearlite. These highly desired complex phase materials are achievable in the same workpiece by quenching only in various patterns so that a pattern of high strength steel can be manufactured in desired areas across the surface and/or cross section of an article after it has been heated. By only quenching certain areas, various material phases are possible in various locations where desired.
  • the flash bainite includes a bi-modal distribution of bainitic platelets or plates which exhibit highly desirable combinations of strength, ductility and toughness.
  • the flash processing of the present invention can create almost distortion free flat sheets, bars, plates and straight tubing.
  • the microstructure produces a fine grain structure within the bi-modal distribution of microstructures which yields the surprising strength and ductility.
  • a graph is shown charting temperature in degrees C. versus time in seconds to illustrate the cooling cycle as processed at the inside wall of one of the test tubes. Typical temperature measurements of this inside wall are showing that there is a very low temperature-time history ratio. In this particular example, utilizing AISI 4130 sheet metal tubing has a lower temperature to time history ratio.
  • FIG. 2B there is shown a graph of temperature versus time showing the flash processing temperature to time history ratio in addition to the conventional continuous annealing temperature to time history.
  • the temperature to time history ratio for the continuous annealing is much greater than that ratio for the flash processing.
  • FIG. 2C illustrates the austenite growth during the Flash Bainite Processing thermal cycle.
  • Region I shows prior austenite grains.
  • Region II shows austenite growth.
  • Region III shows heterogeneous austenite grains.
  • Region IV shows a complex mixture of bainite and martensite within the same prior austenite grains.
  • FIG. 3 illustrates an analysis of temperature in degrees centigrade versus the change in temperature also in degrees centigrade. This analysis shows transformations at between 1022 and 1202 degrees F. and 680 to 860 degrees F. during cooling. This analysis suggests that we have two different transformation conditions leading to very localized microstructural heterogeneity, although experiencing homogeneity on a macro scale.
  • FIGS. 4A and 4B there can be seen two mechanical heterogeneity analyses showing that there are two distinct regions of microstructure between the raw material and the flash processed material in accordance with the present invention. These findings are consistent with the previous analyses showing two separate transformations during the flash processing procedure.
  • FIG A and FIG B are graphs of normalized frequencies versus hardness, which illustrate the distribution of hardness.
  • FIG. 4A shows the base metal hardness distribution as very slight, while the material that has proceeded through flash processing illustrates both a high hardness zone as well as a low hardness zone over a broader distribution of hardness.
  • FIG. 5 yet another aspect of the invention is illustrated with fully strengthened with AISI 1010 material that has been flash processed.
  • This graph shows elongation versus peak flash temperatures, to show that the highest elongation occurs at the peak flash temperature, 1180 degrees C. having an A50 elongation of 7.9%.
  • the elongation percentage is 5.6.
  • Temperatures higher than 1180 C. have lower elongation than 7.9%.
  • Optimum elongation is found at medium to larger grain sizes which is counter-intuitive to the thought of grain refinement.
  • the chemistry of this material in percent by weight is 0.10 C, 0.31 Mn, less than 0.01 Si, sulfur, phosphorus, and 99.41 iron.
  • FIG. 6 is a graph of tensile strength in KSI versus tensile strain in percentage.
  • Flash Bainite Processed AISI 1020 modified to include 0.32% wt chromium after tempering to various temperatures in a range from 400 to 700 degrees C.
  • 8 examples are shown with varying widths in inches. This experiment shows that even after 300 seconds of tempering at 932 degrees F., flash processed AISI 1020+Cr will retain 79% of its “as quenched” tensile strength. Furthermore, the elongation does not drop to + or ⁇ 5% with less than 5 seconds of tempering.
  • FIG. 7A microstructures of both initial (ferrite and carbides) and final flash processed (lath morphology and carbides) samples are provided. In order to validate the mechanism and observed microstructure from a theoretical standpoint, diffusion controlled growth modelling was performed.
  • FIG. 7A are microstructure images from optical and backscatter electron imaging before and after flash processing. The rectangles mark the location of the scanning electron microscopy images.
  • FIG. 7B transmission electron microscopy revealed bainitic sheaves growing from a prior austenite grain boundary that confirmed the hypothesis above.
  • DICTRA software commercially available from Thermo-Calc Software, Inc. of McMurray, Pa. was used to simulate re-austenization kinetics during rapid heating conditions. The software predicts the interface velocity during phase transformation using critically evaluated thermo-dynamic and kinetic data.
  • the dissolution of Cr-enriched cementite (M 3 C) is considered in a Fe—C—Cr system.
  • the initial compositions of the ferrite and cementite were calculated at 1292° F. (spheroidization temperature) based on bulk Cr and C concentration (0.3 wt. % C and 0.88 wt. % Cr) for a typical 4130 steel. Boundary conditions for the simulations are shown schematically in FIG. 8 . The simulations were performed for heating from 1292° F. to 1832° F. at rates of 1, 10, and 100° Ks ⁇ 1 . Austenite was allowed to form at the (a/M 3 C) interface and grow into both cementite and ferrite.
  • the first two simulations showed the full dissolution of cementite into a-ferrite, even before the nucleation and growth of austenite into the ferrite ( FIGS. 9A and 9B ).
  • the cementite does not have enough time to fully dissolve into ferrite ( FIG. 9C ).
  • the austenite forms at the cementite-austenite interface and grows into both the cementite and ferrite.
  • the cementite is fully dissolved well before the ferrite is consumed by the austenite.
  • FIG. 9D shows minimal dissolution of the cementite and nucleation of austenite at the ferrite cementite interface. However, even after reaching 1000° C., the dissolution of cementite is incomplete.
  • concentration gradients [C and Cr] at room temperature and at 1000° C. are compared in FIG. 10 . As the temperature is increased slowly, austenite consumes the ferrite slowly.
  • carbon migration and carbide dissolution yield many different co-existing compositions of steel. These co-existing compositions can exist due to localized heating gradients occurring at the atomic level throughout the bulk of the material being treated. These compositions are somewhat predictable by using alloys with from about 0.01 to about 0.05 wt. Percent of carbon, and from about 0.2 to about 0.5 wt. Percent chromium. For example, if using a steel such as 4140, which contains 0.40 wt percent carbon, and 0.90 wt. Percent chromium, it is controllable and likely that you can attain 0.02 wt.
  • Percent carbon and 0.3 wt percent chromium in the high multiplicity of heterogeneous chemistry on the localized austenite grain scale. Further, it is anticipated, for example, that utilization of a 1002 steel with 0.02 wt % carbon and 0.3 wt % chromium would result in nearly 100% bainitic microstructure after prolonged austenitization and homogenization followed by rapid quenching at a rate of between 200° C./sec. and 3000° C./sec.
  • the composition of the bulk austenite would contain between 0.01-0.03 wt. % C and 0.33 wt. % Cr after quenching with a cooling rate of 1000° C./sec.
  • the resultant CCT diagram ( FIG. 11 ) for this austenite is calculated using the model developed by Dr. Harry Bhadeshia at Cambridge University in the UK. When the calculations were overlaid with a cooling rate from flash processing (1000° C.s ⁇ 1 ), the formation of bainite was shown to be possible.
  • the present invention finds applicability in the metal treatment industry and finds particular utility in steel treatment applications for the processing and manufacture of high strength steels in high volume processing.
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