EP0639651A1 - Intercritical heat treatment of cast iron workpieces - Google Patents

Abstract

Process for intercritical treatment at an austenisation temperature T of a charge of pig iron. It includes the following stages: - the said charge is heated in a furnace, according to a predetermined temperature curve, so that the curves of temperature rise of the different parts of the charge are substantially similar; - a temperature T' </= T is determined beforehand so that when the furnace is maintained at the temperature T', each part of the charge reaches but does not exceed the temperature T substantially at each moment of the period of maintaining the furnace at the temperature T'; - the temperature rise is stopped when the temperature of the furnace reaches the value T'; and - the furnace is maintained at the temperature T' for a predetermined period t; - the charge is then cooled. <IMAGE>

Description

The present invention relates to a method for intercritical processing of cast iron charges.

More specifically, the invention relates to a process for the heat treatment of pig iron charges, in particular but not limited to spheroidal iron with silicon content of the order of 1.5 to 4%, in which the pig iron charge is brought to a temperature corresponding to the diagram area between the critical temperatures A1 and A3 associated with the composition of the cast iron and maintained at this temperature for a controlled period, in order to obtain a predetermined proportion of austenite.

The evolution of certain techniques requires the availability of cast irons or more generally of materials with high characteristics capable of withstanding increasingly high mechanical stresses.

This is in particular the case in the automotive industry with the increase in the performance of motor vehicle engines which itself raises the level of stress on the various components of the mechanical components.

Currently, many grades of cast iron are available for these mechanical applications. They represent very diversified ranges of characteristics adapted to various types of stresses and they often constitute materials competing with other materials and, in particular, with high performance steels for the manufacture of extremely stressed members.

However, for certain applications, the economic constraints during competition are more and more severe and it is therefore particularly advantageous to be able to widen the range of building materials which can satisfy the very severe conditions of use and which can offer the development opportunity requested by the manufacturers.

It turns out that the production of high tenacity cast iron parts meeting these needs, that is to say having a tensile strength of the order of 700 to 800 MPa and an elongation at break of the of the order of 10 to 20%, requires the implementation of an intercritical treatment of the type defined above.

It goes without saying that the production of other parts from other types of cast iron may call, if necessary, for the implementation of an intercritical processing of these parts.

As already indicated, these intercritical treatment methods consist, in a known manner, of bringing the charge to be treated to a treatment temperature between the critical temperatures A1 and A3 corresponding to the composition of the cast iron for which the charge is made, at maintain this load at the processing temperature for a predetermined period, then cool the load also under predetermined conditions.

However, it has been observed that, for a given cast iron composition, due to the conditions of temperature rise of the charge, due to the thermal oscillations of the furnace around its set temperature and due to the particular shapes of the parts to be treated, they -same, the rate of austenite in the treated cast iron and the configuration of the austenite in the cast iron, actually obtained were not repetitively consistent with the results which should have been obtained at the intercritical temperature chosen, the fluctuations compared with theoretical results which may be important.

To overcome these drawbacks, an object of the present invention is to provide a method of intercritical treatment of cast iron charges, which guarantees good reproducibility of the austenite levels obtained in the parts of the charge, as well as an optimal distribution of the constituents of cast iron after treatment.

• on chauffe dans un four selon une courbe de température prédéterminée ladite charge de telle manière que les courbes d'élévation de température des différentes parties de la charge soient sensiblement similaires ;
• on détermine préalablement une température T' ≦ T de telle manière que lors du maintien du four à la température T', chaque partie de la charge atteigne mais ne dépasse pas la température T sensiblement à chaque instant de la période de maintien du four à la température T' ;
• on arrête la montée en température lorsque la température du four atteint la valeur T' ;
• on maintient le four à la température T' pendant une durée prédéterminée t ;
• puis on refroidit la charge.

To achieve this object, according to the invention, the intercritical treatment process at a temperature T for austenization of a cast iron charge is characterized in that it comprises the following steps:

• the charge is heated in an oven according to a predetermined temperature curve so that the curves temperature rise of the different parts of the load are substantially similar;
• a temperature T '≦ T is determined beforehand so that when the oven is maintained at the temperature T', each part of the charge reaches but does not exceed the temperature T substantially at each instant of the period of keeping the oven at the temperature T ';
• the temperature rise is stopped when the oven temperature reaches the value T ';
• the oven is maintained at the temperature T 'for a predetermined duration t ;
• then the charge is cooled.

It is understood that by controlling the temperature rise curve of the different parts of the load, and by controlling the maximum effective temperature of the different parts of the load during the temperature maintenance phase, this temperature remaining at most equal at the chosen austenization temperature, an austenization rate and a suitable austenite configuration are effectively obtained.

It is also understood that T is the maximum temperature actually reached taking into account the end of the temperature rise and the temperature fluctuations due to the regulation of the oven.

• on amène ladite charge à une température sensiblement égale à la température critique A1 selon une courbe de température prédéterminée ;
• puis on amène la charge à la température T' par paliers de températures successifs, le pas des paliers P et la durée des paliers d étant prédéterminés.

According to a preferred embodiment of the process, the temperature rise of the load comprises the following steps:

• said charge is brought to a temperature substantially equal to the critical temperature A1 according to a predetermined temperature curve;
• then the charge is brought to temperature T 'in successive temperature steps, the pitch of the steps P and the duration of the steps d being predetermined.

Preferably, the charge is initially at a temperature close to but lower than the critical temperature A1.

Thanks to this particular mode of temperature rise of the load, it is possible to better respect the predetermined temperature curve in the intercritical zone and to better avoid the risks of fluctuations linked to the inertia of the furnace.

• la figure 1 illustre une courbe continue de montée en température de la charge ; et
• la figure 2 illustre une courbe par paliers de montée en température de la charge.

Other characteristics and advantages of the invention will appear better on reading the following description of several embodiments of the invention given by way of nonlimiting examples. The description refers to the appended figures in which:

• FIG. 1 illustrates a continuous curve of temperature rise of the load; and
• FIG. 2 illustrates a curve in stages of temperature rise of the load.

As already indicated, in order to obtain a cast iron part having homogeneously properties, in particular mechanical toughness properties, it is necessary that the treated cast iron part has not only a predetermined austenite level, but also a optimal distribution of the constituents, in particular austenite and ferrite.

For this, the method according to the invention essentially consists in controlling, for each part of the part, its temperature during the temperature rise phase according to a predetermined curve which is a function of the target austenite level, then in maintaining for a time. t , also a function of the target austenite rate, the charge treated at a temperature which reaches but does not at any time exceed the predefined austenization temperature T in the intercritical range between temperatures A1 and A3.

Figure 1 shows a first form of temperature rise curve.

This curve includes a first part A for temperature rise, a second part for maintaining the predetermined intercritical temperature T and a third part for cooling.

As already indicated, the portion of curve A is predetermined and controlled as a function of the desired austenite level. During phase B which lasts t (depending on the austenite rate), the melt charge must be maintained at a temperature strictly lower than or equal to T, but it must reach the temperature T. This condition must in particular be observed at the end the temperature rise of the oven. To meet this condition, despite temperature oscillations linked to the regulation of the oven, an experimental regulation temperature T ′ which is of course less than T. is determined experimentally by prior tests.

When we speak of a temperature of the charge, it must be understood that the different parts of the oven are at different temperatures T'i defined in such a way that the critical parts of the parts forming the charge placed in the oven are effectively at the temperature T '. The difference between the temperatures T and T ′ can vary appreciably as a function of the nominal capacity of the oven, its filling rate and the regulation system used.

• Température d'austénisation dans l'intervalle critique T = 820°C
• Température de consigne T' =818°C
• Montée en température continue entre la température ambiante et 818 °C : 10 heures
• Maintien du four à 818 °C : 2 heures
• Refroidissement en fonction des caractéristiques recherchées.

As an example, the following thermal cycle was subjected to a spheroidal cast iron load of 2 tonnes:

• Austenization temperature in the critical range T = 820 ° C
• Setpoint temperature T '= 818 ° C
• Continuous rise in temperature between room temperature and 818 ° C: 10 hours
• Maintaining the oven at 818 ° C: 2 hours
• Cooling according to the desired characteristics.

Figure 2 shows a second controlled temperature rise curve. Phase A is broken down into a first half-phase A 'in which the curve is continuous and predetermined until the load reaches at all points the critical temperature A1, and into a second half-phase A ". During the second half-phase the temperature is increased in stages, the step p and the duration of each stage d are determined according to the desired austenite rate until the oven reaches the temperature T 'lower than the temperature T for the reasons indicated above.

• Température d'austénisation dans l'intervalle critique T = 820°C
• Température de consigne du four T' =818°C
• Montée en température entre la température ambiante et 778°C : 8 heures
• Maintien à 778°C pendant 1 heure
• Montée entre 778 et 798°C : durée 1 heure
• Maintien à 798°C pendant 1 heure
• Montée entre 798°C et 818°C : durée 1 heure
• Maintien à 818°C : durée 1 heure
• Refroidissement en fonction des caractéristiques recherchées.

As an example, the following thermal cycle was subjected to a spheroidal cast iron load of 2 tonnes:

• Austenization temperature in the critical range T = 820 ° C
• Oven set temperature T '= 818 ° C
• Temperature rise between room temperature and 778 ° C: 8 hours
• Maintained at 778 ° C for 1 hour
• Ascent between 778 and 798 ° C: duration 1 hour
• Maintained at 798 ° C for 1 hour
• Ascent between 798 ° C and 818 ° C: duration 1 hour
• Maintained at 818 ° C: duration 1 hour
• Cooling according to the desired characteristics.

The temperature rise in stages in zone A "allows better control of the temperature rise curve and in particular its speed when the temperature arrives in the intercritical zone. This control is particularly important for obtaining the austenite rate and the austenite composition - desired ferrite.

In a particular example of treatment carried out according to the invention, jets of different diameters (25, 39 and 58 mm) cast with three irons well suited to intercritical treatment were austenitized at two temperature levels in the critical interval (805 ° C and 820 ° C) by respecting the procedure described above and in particular by applying a temperature setting in stages. These jets were then cooled with supply air. The results show that, for each type of cast iron, the differences obtained between the results relating to the different diameters are very small.

Claims (5)

Intercritical treatment process at a temperature T for austenization of a cast iron charge, characterized in that it comprises the following stages: - Heating in an oven, according to a predetermined temperature curve, said load so that the temperature rise curves of the different parts of the load are substantially similar; - A temperature T '≦ T is determined beforehand so that when the oven is maintained at the temperature T', each part of the charge reaches but does not exceed the temperature T substantially at each instant of the period for maintaining the oven at the temperature T '; - the temperature rise is stopped when the oven temperature reaches the value T '; - The oven is maintained at the temperature T 'for a predetermined duration t ; - then the load is cooled. Method according to claim 1, characterized in that the temperature rise of the load comprises the following steps: - Said charge is brought to a temperature close to the critical temperature A1 according to a predetermined temperature curve; - Then the load is brought to temperature T 'in successive temperature steps, the pitch of the steps P and the duration d of the steps d being predetermined. Method according to claim 2, characterized in that the charge is initially brought to a temperature close to but less than the critical temperature A1. Method according to any one of Claims 1 to 3, characterized in that the duration t of maintaining at the temperature T 'is a function of the temperature rise curve of the said charge. Process according to any one of Claims 1 to 4, characterized in that several temperatures T'i are determined for different parts of the furnace, so that the different parts of the charge effectively remain at a temperature at most equal to T during the entire duration of the treatment.

Images