Method for the heat treatment of a part made from maraging steel

Abstract

A method for the heat treatment of a part made of maraging steel, which part is obtained by selective laser melting, it comprises the steps of: heating the said part made of maraging steel from ambient temperature T0 to a maximum temperature Tmax of between 600° C. and 640° C., maintaining the said maximum temperature Tmax for a duration of between 5 hours and 7 hours, and rapidly cooling the said part.

Claims

1. A method of manufacturing a tire curing and vulcanizing mold or mold segment made of aluminium, the method comprising: fabricating, by selective laser melting, at least one thin blade for forming grooves in a tread of the tire, the thin blade being made of a maraging steel comprising a carbon content of less than or equal to 0.03%, a nickel content of between 17% and 19%, a cobalt content of between 8.5% and 9.5%, a molybdenum content of between 4.5% and 5.2%, a titanium content of between 0% and 0.8%, an aluminium content of between 0% and 0.15%, a chromium content of between 0% and 0.5%, a copper content of between 0% and 0.5%, a silicon content of between 0% and 0.1%, a manganese content of between 0% and 0.1%, a sulfur content of between 0% and 0.01%, a phosphorus content of between 0% and 0.01%, the remainder being iron and all percentages being expressed by weight with respect to the total weight of the thin blade; heating the at least one thin blade from ambient temperature To to a maximum temperature T.sub.max of between 600° C. and 640° C.; maintaining the maximum temperature T.sub.max for a duration of between 5 hours and 7 hours; cooling the at least one thin blade; placing the thus-obtained at least one thin blade in a die cavity for forming the tire curing and vulcanizing mold or mold segment; and pouring or injecting aluminium into the die cavity around the at least one thin blade so as to obtain the aluminium tire curing and vulcanizing mold or mold segment.

2. The method according to claim 1, wherein the maximum temperature T.sub.max is between 610° C. and 630° C.

3. The method according to claim 1, wherein the maximum temperature T.sub.max is maintained for a duration of 6 hours or of around 6 hours.

4. The method according to claim 1, wherein a rate of cooling V.sub.c is between 420° C./min and 480° C./min.

5. The method according to claim 4, wherein the rate of cooling V.sub.c is between 440° C./min and 460° C./min.

6. The method according to claim 1, wherein a rate of heating V.sub.h is between 5° C./min and 10° C./min.

7. The method according to claim 1, wherein the heating of the at least one thin blade is carried out in a vacuum furnace.

8. The method according to claim 1, wherein the cooling is carried out under a rotating flow of cooled inert gases or under air.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Further features and advantages of the invention will become apparent from the description which will now be given thereof, with reference to the appended drawings which depict, by way of non-limiting example, one possible embodiment thereof.

(2) In these drawings:

(3) FIG. 1 is a perspective view of one exemplary embodiment of a thin blade made of steel for forming grooves in the treads of tyres,

(4) FIG. 2 is one exemplary embodiment of a tyre curing and vulcanizing mould equipped with at least one of the aforementioned thin blades, and

(5) FIG. 3 is a depiction, in graph form, of how the temperature changes as a function of time, over the course of the various steps of the heat treatment method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) The heat treatment method according to the invention seeks to treat a part made of maraging steel, which part is obtained by selective laser melting.

(7) The maraging steel employed is a steel containing, in the conventional way, a percentage of carbon less than or equal to 0.03%, a nickel content of between 17% and 19%, a cobalt content of between 8.5% and 9.5%, a molybdenum content of between 4.5% and 5.2%, a titanium content of between 0% and 0.8%, an aluminium content of between 0% and 0.15%, a chromium content of between 0% and 0.5%, a copper content of between 0% and 0.5%, a silicon content of between 0% and 0.1%, a manganese content of between 0% and 0.1%, a sulfur content of between 0% and 0.01%, a phosphorus content of between 0% and 0.01%, the remainder being iron, (these percentages being expressed by weight with respect to the total weight of the product). This steel has a martensitic structure.

(8) This steel exhibits the following properties, after solution heat treatment followed by ageing: high hardness (>550 Hv), good properties under tension (Re>1500 MPa, Rm>1600 MPa) and in terms of fatigue, low thermal expansion coefficient less than or equal to 10.2×10.sup.−6 m/m. ° C.

(9) When this maraging steel has been obtained by a method of selective laser melting (SLM), namely by an additive manufacturing method, it also has specific properties inherent to additive manufacturing without a post-heat treatment, such as a very fine microstructure (brought about by the significant thermal gradients associated with this method), which is beneficial to certain properties such as the elastic limit (>800 MPa), the load at break (>900 MPa) and the hardness (>380 Hv).

(10) As set out hereinabove, one particular example of an application of the invention is the manufacture of a tyre curing and vulcanizing mould.

(11) FIG. 2 shows one exemplary embodiment of such a mould 1, of which only part is depicted. This mould 1, of annular overall shape, is made up of a plurality of curved segments 10, shaped as portions of an annulus in the shape of arcs of a circle, which are intended to be assembled in order to form the entire mould 1. Each mould segment 10 has, on its concave interior surface, several thin blades 2, shaped to exhibit the shape of the grooves in the tread.

(12) An exemplary embodiment of such a thin blade 2 can be seen by referring to FIG. 1.

(13) The thin blade 2 has a wavy shape. It comprises two parts, namely a part 21 referred to as “interior” because it is intended to be embedded in the aluminium that constitutes the rear 100 of a mould segment 10, and a part 22 referred to as “exterior” because it is intended to project from the rear of the mould segment towards the centre of the mould 1.

(14) It is therefore, for example, this thin blade 2 made of maraging steel and obtained by selective laser melting that will undergo the heat treatment according to the invention.

(15) This method will now be described with reference to FIG. 3.

(16) This heat treatment is preferably carried out by placing the part made of maraging steel that is to be treated in a vacuum furnace. It is also possible to use a furnace under an inert atmosphere, for example of argon or of nitrogen.

(17) This part is heated for a heating time d.sub.1, during which it transitions from ambient temperature T.sub.0 to a maximum temperature T.sub.max. It is then held at said maximum temperature T.sub.max for a time duration dz. Finally, it is cooled rapidly in a time d.sub.3, during which it makes the transition from the maximum temperature T.sub.max to the temperature T.sub.0 or substantially T.sub.0.

(18) The rate of heating V.sub.h is equal to (T.sub.max−T.sub.0)/d.sub.1. This rate of heating V.sub.h is preferably comprised between 5° C./min and 10° C./min, and more preferably still, it is 7.5° C./min.

(19) The initial temperature T.sub.0 corresponds to ambient temperature, namely around 20° C.

(20) The maximum temperature T.sub.max is comprised between 600° C. and 640° C., preferably between 610° C. and 630° C., and more preferably still, equal to 620° C.

(21) The duration d.sub.2 of the time spent held at this maximum temperature T.sub.max is comprised between 5 and 7 hours, more preferably still, a time of 6 hours or around 6 hours.

(22) The rate of cooling V.sub.c is equal to (T.sub.max−T.sub.0)/d.sub.3. This rate of cooling V.sub.c is preferably comprised between 420° C./min and 480° C./min, is preferably comprised between 440° C./min and 460° C./min, and more preferably still, it is equal to 450° C./min.

(23) The rapid cooling in the vacuum heat treatment furnace occurs with the vacuum eliminated, namely under air or by injecting cooled inert gases, such as nitrogen and hydrogen for example, for example as a rotating flow. These two inert gases specifically allow a rapid exchange of heat with the part treated in the furnace.

(24) When the method is performed in a furnace under an inert atmosphere, all of the steps (temperature increase and decrease) are performed under an atmosphere of inert gas(es).

(25) At the end of the heat treatment according to the invention, the part made of maraging steel obtained contains between 45% and 65% austenite and between 55% and 35% martensite.

(26) The residual presence of Fe.sub.2Mo.sub.6 and of M.sub.6C carbide precipitates is also noted.

(27) In this instance in which the aforementioned heat treatment is applied to thin blades 2 for the creation of the tread grooves, it may be carried out while the thin blades are still on their manufacturing platen, thereby making it possible to avoid deformation. They can also be detached and placed in baskets that can go into the furnace.

(28) The invention also relates to a method for manufacturing a metal product, which comprises the steps consisting in general in positioning at least one part, made of maraging steel that has undergone the aforementioned heat treatment method, in a die cavity and pouring or injecting aluminium around this part so as to obtain the said end product.

(29) In the case of the manufacture of the aforementioned mould 1, the thin blades 2 are placed in a cavity, for example made of plaster, the shape of which is the negative (the reverse image) of the mould 1 or of a segment 10 of this mould.

(30) The outer part 22 of each thin blade 2 is embedded in the plaster. The die cavity is closed and the aluminium is poured or injected into it. Once the aluminium has cooled, the plaster die is broken away, yielding the aluminium mould 1 or mould segment 10 in which the interior part 21 is embedded in the aluminium and from which the exterior part 22 of the thin blades 2 projects.

(31) Tests were conducted to measure the elongation at break A of a part made of maraging steel that has undergone the various methods. These results are summarized in Table 1 below.

(32) TABLE-US-00001 TABLE 1 Various method steps to which Elongation at the parts are subjected break A (%) SLM 6% max SLM + cast alu 0.2% to 1.5% SLM + HT 6% max SLM + HT + cast alu 5% max SLM = part made from maraging steel obtained at the end of a selective laser melting (SLM) method. HT = heat treatment according to the invention. Cast alu = cast with aluminium poured at a temperature of around 750° C.

(33) The results in Table 1 show the positive effect that the heat treatment has on the fatigue behaviour of the part

(34) Specifically it may be noted that if the treatment of pouring cast aluminium is performed directly after the part made of maraging steel has been obtained using the SLM treatment, without the heat treatment of the invention (Row 2 of the table), the elongation at break A is greatly reduced.

(35) By contrast, the heat treatment according to the invention makes it possible to keep the elongation at break A identical or substantially identical to what it was on the original part as obtained at the end of the selective laser melting treatment. Furthermore, when the pouring of the cast aluminium is performed after the heat treatment according to the invention, it is found that the elongation at break A is simply reduced slightly by comparison with its original value.

(36) Additional tests conducted on the tensile properties of elastic limit and load at break show that these values remain at least equivalent to those obtained on the part obtained by SLM, if the heat treatment according to the invention is performed between the aforementioned SLM step and the pouring of the aluminium.

(37) The heat treatment method according to the invention also has a beneficial effect on the shock resistance of the parts obtained.