Abstract
The invention relates to a method of heat treating a cast iron having graphite particles, in particular a cast iron having graphite nodules with a substantially spherical geometry. The method comprises the step of subjecting the cast iron to a first austenitizing temperature, in order to obtain a cast iron having an austenite matrix with a substantially homogeneous carbon content. Subsequently, at least part of the cast iron is subjected to at least a second, different austenitizing temperature in order to change, in at least part of the cast iron, the carbon concentration in a part of the matrix surrounding the (spherical) geometry of the graphite particles. The method yields improved controllability on strength properties characteristics for cast irons including malleable irons, in particular for ductile iron.
Claims
1. Method of heat treating a cast iron having graphite particles with a spherical geometry, the method comprising: A. subjecting the cast iron to a first austenitizing temperature (T1), in order to obtain a cast iron having an austenite matrix with a homogeneous carbon content; characterised in that: B. directly after step A, at least part of the cast iron is subjected to a second austenitizing temperature (T2), the first austenitizing temperature and the second austenitizing temperature being different relative to each other, wherein the second austenitizing temperature (T2) is higher than the first austenitizing temperature (T1), wherein in at least part of the cast iron, the carbon concentration in a part of the matrix surrounding the graphite particles, are changed.
2. Method according to claim 1, further comprising step D of, prior to step A, casting and solidifying a cast iron from a melt, for forming cast iron having graphite particles, wherein the method further comprises, in between step D and step A, preventing an Eutectoid transformation to take place in the cast iron by holding the temperature of the cast iron substantially above a Eutectoid temperature.
3. Method according to claim 2, wherein during the step of casting of the cast iron from the melt, a moulding method that allows a quick transition from the casting of the cast iron, breaking out of the mould, and exposing to the first austenitizing temperature is performed.
4. Method according to claim 1, wherein the cast iron is held at the second austenitizing temperature (T2) for a period of time.
5. Method according to claim 1, wherein a fluidized bed is used to perform said method.
6. Method according to claim 1, comprising step C of, subsequently to step B, cooling and holding the cast iron at a third temperature (T3) for a period of time, wherein said third temperature is below a Eutectoid temperature of the cast iron, wherein the third temperature (T3) is above a Martensite temperature for the cast iron.
7. Method according to claim 6, wherein the cast iron is cooled towards the third temperature (T3) with a rate of cooling that prevents formation of ferrite, perlite or ausferrite during cooling before reaching the third temperature (T3).
8. Method according to claim 6, wherein subsequently to step C, the cast iron is subjected to an austempering heat treatment, wherein the austempering heat treatment includes subjecting the cast iron to a first austempering temperature (T4).
9. Method according to claim 8, wherein subsequently to the step of subjecting the cast iron to the first austempering temperature (T4), the cast iron is subjected to a second austempering temperature (T5), the first austempering temperature and the second austempering temperature being different relative to each other, wherein the second austempering temperature (T5) is higher than the first austempering temperature (T4).
10. Method according to claim 8, wherein the austempering temperature (T4) is equal to, or higher than, the third temperature (T3) in step C.
Description
(1) In the following description, embodiments of the heat treating method according to the invention will be explained, based on the accompanying figures, in which:
(2) FIG. 1a to FIG. 1d show schematic time-temperature diagrams of embodiments of an austenitizing heat treating method;
(3) FIG. 2 shows a schematic time-temperature diagram of an austempering heat treating method;
(4) FIG. 3a to FIG. 3c show schematic time-temperature diagrams of alternative embodiments of austempering heat treating methods.
(5) FIG. 4 shows a schematic time-temperature diagram of an austempering heat treating method in which different carbon content resulting from the austenitizing heat treatment is accounted for; and
(6) FIG. 5a to FIG. 5c show schematic time-temperature diagrams of alternative embodiments of austempering heat treating methods, in which different carbon content resulting from the austenitizing heat treatment is accounted for.
(7) FIG. 1a to FIG. 1d show time-temperature diagrams. On the horizontal axis is time, and on the vertical axis is temperature. The Eutectic solidification temperature is indicated by reference Tm. The Eutectic temperature may be approximately 1150 C. The Eutectoid temperature is indicated by reference Te. The Eutectoid temperature may be in the range of 750 C. to 950 C., and depends (amongst others) strongly on a cooling or a heating situation (kinetic effects) and on the Silicon content of the cast iron. In between the Eutectoid temperature Te and the Eutectic temperature Tm lies the so-called austenite region, in which formation of austenite, with in an embodiment a homogeneous interstitial distribution of Carbon, occurs. The line in the diagram schematically shows the (average or indicative) temperature of the cast iron as a function of time, during the heat treating method.
(8) In FIG. 1a, the cast iron starts at room temperature. The cast iron is heated to a first austenitizing temperature T1. This temperature T1 is above the Eutectoid temperature Te, and below the melting temperature Tm. The cast iron having graphite particles, is held at this first austenitizing temperature (T1) until the entire casting becomes essentially austenitic and the matrix becomes saturated with a carbon level belonging to the chosen austenitizing temperature. After a while, in a subsequent second step, the cast iron is heated (solid line) or cooled (dashed line) within the austenitizing temperature region, to a second temperature T2 or T2, respectively. The second temperature T2, T2 is different from the first temperature. As can be seen in FIG. 1a, the heating or cooling to the second temperature T2, T2 is performed very rapidly. The cast iron is held at this temperature for a relatively short period of time, to influence only a small range around the graphite particles, resulting in locally (i.e. around the particles) increased or decreased Carbon content. After this, the cast iron is relatively rapidly cooled to a temperature T3. This temperature lies below the Eutectoid temperature.
(9) Not shown in FIG. 1a is that preceding the exposure to the first austenitizing temperature T1, the casting may be subjected to a pre-austenitizing temperature, which is in an embodiment as high as possible, and more specifically just below the Eutectic melting temperature Tm, to allow the (pro-eutectic) ferritic parts present in the casting to transform to austenite.
(10) The effect of this time-temperature-curve on the cast iron, and more specifically the change in austenitizing temperature, is that graphite atoms will diffuse from the graphite particles into the matrix when the second temperature is higher (solid line), or from the matrix to the graphite particles when the second temperature is lower (dashed line). Due to this, a relatively small substantially spherical layer in the matrix surrounding the graphite nodule will form, in which the graphite concentration is higher (solid line) or lower (dashed line) relative to the rest of the matrix. As described before, this slightly higher or lower graphite concentration around the graphite particles results in a change in strength properties characteristics of the cast iron. When subjected to a higher second temperature T2, the cast iron will increase in strength properties. Also, a detrimental effect of cooling towards the third temperature T3 is countered, as described before. When subjected to a lower second temperature T2, the cast iron will decrease in strength and hardness but will show improved ductility. Thus, the strength properties of the cast iron may be controlled by using a second, different austenitizing temperature.
(11) FIG. 1b to FIG. 1d show embodiments of the heat treating method. In these embodiments, large energy savings may be obtained. Instead of heating the cast iron from room temperature to the first austenitizing temperature T1, the cast iron is subjected to the first austenitizing temperature T1 directly after the step of casting and solidifying the cast iron from a melt for forming cast iron having graphite particles, e.g. with a substantially spherical geometry. By doing so, a large amount of energy may be saved, since no heat is required to heat the cast iron to the first austenitizing temperature. A further important advantage is that the formation of (pro-eutectic) ferrite is prevented. Once formed, this pro-eutectic ferrite may in some cases be very stable. Only after a long exposure in the austenitizing temperature range, the pro-eutectic ferrite may transform to austenite, thus increasing the time necessary for the cast iron to become fully austenitic. By directly exposing the cast iron to the first austenitizing temperature after solidification, austenitizing times may be shorter, resulting in less coarsening of the austenite microstructure, and also further improvements in subsequent heat treatment steps are obtained.
(12) Although FIG. 1c and FIG. 1d both show a heat treating method in which a second austenitizing temperature T2, T2 is used, it should be noted that energy savings may be obtained for any austenitizing method that is directly started after the casting and solidification of the metal (i.e. that is started while maintaining the temperature of the cast substantially above the Eutectoid temperature Te). A single austenitizing temperature may also be used. The applicant reserves the right to apply for protection for this subject matter, in this application and/or in other applications.
(13) In FIGS. 2 and 3, the ferrite and perlite transformation region (I), and the ausferrite transformation region (II) are schematically indicated by roman numerals I and II, respectively. By selecting the appropriate temperature-time curve, a desired micro-structure in the cast iron may be obtained. In general, this is known to those skilled in the art.
(14) FIG. 2 shows an embodiment of a subsequent austempering method, to further increase the strength properties of the material. Such an austempering method is known per se. In the austempering method, the cast iron is held at a constant temperature to allow the formation of an ausferritic structure.
(15) FIGS. 3a to 3c show further improvements relating to austempering methods. It should be noted that these improvements have positive effects when combined with the austenitizing heat treatment according to the present invention. Nevertheless, these austempering methods may also be beneficial when the austenitizing heat treatment according to the present invention does not precede the austempering method. The applicant therefore reserves the right to apply for protection for these austempering methods in this application, and/or in other applications.
(16) It can be seen in FIGS. 3a and 3b that following cooling to the third temperature T3, the austempering is started at the a first austempering temperature T4. In this embodiment, the austempering temperature T4 is equal to the third temperature T3. In the matrix, ausferrite is formed. This temperature is held for a certain period of time. Then, subsequently, the temperature is raised (FIG. 3a) or lowered (FIG. 3b) to a second, different austempering temperature T5, T5. The higher temperature T5 causes coarser high-ausferrite, with a higher carbon content in the ausferrite. The lower temperature T5 causes a transformation with finer low-ausferrite, but also a lower carbon content in the ausferrite. The chosen temperature T5 and T5 influences the percentage retained austenite, the coarseness of the ausferrite, and the percentage of carbon in the retained austenite. Thus, the temperature may be used to influence and control the desired characteristics of the cast iron. Finally, the casting is cooled towards room temperature.
(17) A special variant (not shown) is where after starting the austempering at T3, the temperature is lowered after some time, e.g. somewhere halfway the austempering time. After this time, the increasing carbon content results in the Martensite starting temperature to be lower, enabling a lower austempering temperature whilst staying above the Martensite starting temperature.
(18) FIG. 3c shows an austempering method that is especially suitable for relatively thick castings. Here, the casting is cooled to the third temperature T3. However, compared to FIG. 3a, this third temperature T3 is lower, to allow the core (or at least deeper parts) of the casting to reach a level equal to, or lower than, the desired austempering temperature T4. After this cooling to the relatively low third temperature T3, the casting is subjected to the desired austempering method. Once again, this temperature is held for a certain period of time. Then, subsequently, the temperature may be raised or lowered to a second, different austempering temperature T5, T5. Finally, the casting is cooled towards room temperature.
(19) In FIGS. 2 and 3, the ferrite and perlite transformation region (I), and the ausferrite transformation region (II) are schematically indicated by roman numerals I and II, respectively. By selecting the appropriate temperature-time curve, a desired micro-structure in the cast iron may be obtained. In general, this is known to those skilled in the art.
(20) It should be noted that with the heat treatment method provided by the invention, it is possible that large gradients in carbon concentrations exist. In different parts of the cast iron, one may find different carbon concentrations. In general, the time and temperature at which a ferrite and pearlite transformation, and/or the ausferrite transformation takes place, depends on the (local) carbon concentration of the cast iron. In general, a higher carbon content results in the transformation taking place later. This effect may be used to influence the material properties, as will be discussed based on FIG. 4 and FIG. 5a to FIG. 5c.
(21) FIG. 4 shows an embodiment of an austempering method, to further increase the strength properties of the material. It can be seen that a first transformation line CL is shown, which relates to the start of the transformation for the low carbon concentration. Another transformation line CH is visible, which relates to the start of the transformation for the high carbon concentration. The CL transformation line is further referred to as the low carbon transformation line CL. The CH transformation line is further referred to as the high carbon transformation line CH. It can be seen that the CH transformation line relates to a later time period, and generally also to a bit lower temperature. With the austempering method shown in FIG. 4, initially a volume having a low carbon concentration will be subjected to a transformation according to the CL transformation line. A bit later in time, also the volume having a relatively higher carbon concentration will be subjected to a likewise transformation, according to the CH transformation line.
(22) FIGS. 5a to 5c show further improvements relating to austempering methods using a cast iron having regions with different carbon contents, i.e. a low carbon concentration region and a high carbon concentration region. It should be noted that these improvements are feasible when combined with the multi-stage austenitizing heat treatment according to the present invention.
(23) It can be seen in FIGS. 5a and 5b that following cooling to the third temperature T3, the austempering for the low carbon concentration is started at the a first austempering temperature T4. In this embodiment, the austempering temperature T4 is equal to the third temperature T3. In the matrix, ausferrite formation is started in the regions with lower carbon content. This temperature is held for a certain period of time. Then, subsequently, the temperature is raised (FIG. 5a) or lowered (FIG. 5b) to a second, different austempering temperature T5, T5. The change in temperature is effected just before the CH transformation line is reached. Transformation in the regions having a relatively high carbon concentration has therefore not started yet. The transformation for the higher carbon regions thus starts later, and at a higher or lower temperature, respectively.
(24) The higher transformation temperature T5 (FIG. 5a) causes a faster transformation resulting in a coarser ausferrite containing less acicular ferrite and more retained austenite. The lower transformation temperature T5 (FIG. 5b) causes a slower transformation resulting in a finer ausferrite phase containing more acicular ferrite and less retained austenite. The higher transformation temperature T5 (FIG. 5a) causes a faster transformation resulting in less and coarser high-ausferrite phase and lower carbon content in the retained austenite. The lower transformation temperature T5 (FIG. 5b) causes a slower transformation resulting in more and finer low-ausferrite phase and higher carbon content in the retained austenite. The chosen temperature T5 and T5 influences the percentage and coarseness of the retained austenite, the amount and coarseness of the acicular ferrite, the amount and coarseness of the retained austenite and the percentage of carbon in the retained austenite. Thus, the temperature may be used to influence and control the desired characteristics of the cast iron. Finally, the casting is cooled towards room temperature.
(25) A special variant (not shown) is where after starting the austempering at T3, the temperature is lowered after some time, e.g. somewhere halfway the austempering time, in an embodiment before the CH transformation line is reached. After this time, the increasing carbon content in the retained austenite results in the Martensite starting temperature to be lower, enabling lower austempering temperatures whilst staying above the Martensite starting temperature.
(26) Another special variant (not shown) is starting a partial Martensite transformation at a temperature T3 just below the Martensite starting temperature directly followed by the austempering transformation at T4 and subsequently T5 or T5.
(27) FIG. 5c shows an austempering method that is especially suitable for relatively thick castings. Here, the casting is cooled to the third temperature T3. However, compared to FIG. 5a, this third temperature T3 is lower, to allow the core (or at least deeper parts) of the casting to reach a level equal to, or lower than, the desired austempering temperature T4. After this cooling to the relatively low third temperature T3, the casting is subjected to the desired austempering method. Once again, this temperature is held for a certain period of time. Then, subsequently, the temperature may be raised or lowered to a second, different austempering temperature T5, T5, just before the HC transformation line is reached. Finally, the casting is cooled towards room temperature.
(28) The method according to the invention is especially suitable for large scale production of iron castings with graphite particles, in particular ductile iron castings, having improved strength characteristics. With the method according to the invention, and improvements thereon, large energy savings and (resulting) environmental benefits are obtainable.
(29) It may be clear to a person skilled in the art, that the invention has been described based on several embodiments thereof. Alternatives and modifications may be made, all of which may be within the scope of the requested protection according to the attached claims.
