FR2697766A1 - Process for controlling the hardening of a lamellar iron - for components such as camshafts and rolling mill rolls. - Google Patents

Abstract

A process controls, in a foundry mould against at least one metal cooler, the hardening of a lamellar cast iron component in which additive elements are added to the molten iron before casting in order to promote the concomitant formation of carbides and graphite. The additives used to promote hardening are made up of bismuth and/or tellurium and/or boron whilst the additives used to promote the formation of graphite are made up of graphite based innoculants and/or silicon based innoculants. The graphite based innoculants and/or the silicon based innoculants may also include one or more of the following elements: strontium, zirconium, barium, calcium, aluminium or rare earth metals. The cast iron used in the process has the following compsn: 2.6 to 3.6% carbon; 1.2 to 2.6% silicon; 0.3 to 1.0% manganese; and 0.003 to 0.15% sulphur. USE/ADVANTAGE - Used to control the hardening of lamellar cast iron components such as camshafts, rolling mill rolls and other similar components. Allows greater control of the factors that affect the formation of graphite in a cast iron and consequently reduces to a max and stabilises the transition zone situated between a surface hardened zone in which the carbon is precipitated in the form of carbides and a softer central zone in which the carbon is precipitated in the form of graphite. The transition zone incorporates both carbides and graphite.

Description

Method to master. in a foundry mold confers at least one metallic cooler. quenching a piece of lamellar cast iron. such as a camshaft. a rolling mill or other.

 The present invention relates to a method for controlling, in a foundry mold against at least one metal cooler, the quenching of a piece of lamellar cast iron, such as a camshaft, a rolling mill cylinder or the like.

 The lamellar graphite cast iron camshafts are hardened at the location of the cams against metal coolers placed in the foundry mold.

 In this particular quenching technique, the founders encounter great difficulties in controlling the quenching depth which must extend over at least 3 mm. If the "quenching power" of the cast iron is increased too much, the quenching depth on the cams increases beyond 3 mm, and carbides are also found at the core, in the bearings or at the end of the latter so that the subsequent machining of the lubrication holes in particular is made very difficult.

Among the factors that influence the primary tempering of cast irons, the main ones are:
- the chemical composition of the basic pig iron,
- the thermal history of the metal,
- bath temperature,
- the presence of carburogenic elements which generate carbides by allying or quenching elements which favor the formation of carbides without allying,
- the possible presence of gas and in particular hydrogen in the metal bath.

 Any variation of at least one of these factors disturbs the graphitization potential of the cast iron and, consequently, modifies the quenching. Therefore, to stabilize the quenching depth of all the parts produced, it would suffice to keep all these factors constant. But this turns out to be practically impossible and, moreover, the measurement of these is delicate and random.

 The present invention aims to overcome as much as possible these factors which it is difficult to master in practice, by more or less masking their influence. It therefore aims to minimize and stabilize the transition zone between a surface hardened zone in which the carbon precipitates in the form of carbides and a soft core zone in which the carbon precipitates in the form of graphite, carbides and coexisting graphite in the transition zone.

 For this purpose and in accordance with the invention, the method consists in that addition elements are added to the molten iron before it is cast to concomitantly promote the formation of carbides and the formation of graphite.

 The addition elements favoring quenching consist of bismuth and / or tellurium and / or boron, while the addition elements favoring graphitization consist of a graphite-based inoculant and / or a base-based inoculant of silicon.

 Inoculants based on graphite and / or silicon may also contain at least one of the following elements: strontium, zirconium, barium, calcium, aluminum, rare earths.

Advantageously, for a cast iron with a weight content:
- carbon between 2.6 and 3.6%,
- silicon between 1.2 and 2.6%,
- optionally manganese between 0.3 and 1%, sulfur between 0.003 and 0.15% or others, the process consists in that, to promote quenching, the weight content of bismuth is between 0.003% and 0 , 03% and / or tellurium is between 0.003% and 0.1 QJo, and / or boron is between 0.003% and 0.2% while to promote graphitization the weight content of inoculant based on graphite is between 0.01% and 0.2% and / or inoculating with silicon is between 0.03% and 0.5%.

 Various other characteristics and advantages of the invention will also emerge from the detailed description which follows.

To more or less mask the influence of the aforementioned factors on the primary quenching, one proceeds to the concomitant addition of one or more quenching elements such as bismuth, tellurium, boron ... with an inoculant based on graphite and / or of ferro-silicon. The quenching element or elements favor quenching in contact with the metal coolers of the mold, that is to say the carburetion of the cast iron in the cooled zones of the mold. The inoculant (s) favor the graphitization of the cast iron in the mold zones where the temperature drop is less noticeable
In other words, these addition elements mixed with the primary cast iron in the mold intervene to favor the precipitation of carbon in the form of carbides in the zones where the temperature drops suddenly and in the form of graphite in the zone where the temperature lowers slowly .

 Thus, the surface area of the molded part which is in contact with the metal coolers of the mold is quenched, while the core area containing only lamellar graphite is softened. Between the surface hardened zone of white cast iron containing practically only carbides and the soft zone with gray cast iron core containing practically only graphite, there is a relatively thin transition zone in which the proportion of carbides decreases from the periphery towards the core and in which the proportion of graphite decreases from the core towards the periphery.

The primary cast iron or base cast iron to which the treatment is applied contains in percentage by weight:
-2.6 to 3.6% carbon
- 1.2 to 2.6% silicon.

It can also contain:
- 0.3 to 1% manganese
- 0.003 to 0.15% sulfur.

For a cast iron of this quality, the treatment consists in adding, as a percentage by weight of addition to the base cast iron, concomitantly:
- to promote hardening
bismuth 0.003% to 0.03%
and / or tellurium at a rate of 0.003% to 0.1%
and / or boron at a rate of 0.003% to 0.2%
- to favor graphitization
a graphite-based inoculant at 0.01% to 0.2%
and / or a silicon-based inoculant in an amount of 0.03% to 0.5%.

 If a single element is added, its dosage is close to the maximum of the range indicated above and, if several elements having the same function are added, their dosages are lower.

 The refinement of the dosage is carried out during preliminary tests depending on the desired result.

 It is interesting to note that the inoculants based on graphite and / or silicon can contain at least one of the following elements strontium, zirconium, barium, calcium, aluminum, rare earths ... in more or less quantity. The tests make it possible to choose them and to determine their dosage.

 The quenching elements accentuate the quenching effect against the or each metal cooler and make it possible to guarantee a quenching thickness greater than 3 mm at the level of the cams of a camshaft or greater than the value which is appropriate at the level of the peripheral surface. of a rolling mill cylinder.

 The inoculating element (s) in turn increase the potential for core graphitization; they eliminate any carbide in the bearings and allow a transition zone to be obtained between the hardened part and the "soft" part of a very small thickness.

 The additions are advantageously carried out in a ladle before pouring or in the mold at the time of pouring or during the pouring of the cast iron into the ladle or in the mold.

 An example of treatment is given in the following.

 It involves casting in lamellar cast iron against metal coolers a piece in the shape of a rounded cam having a width of 45 mm, a length of 60 mm and a thickness of 15 mm.

The composition of the basic font is as follows:
carbon 3.40%
silicon 2.05%
manganese 0.96%
sulfur 0.12%
0.10% tin.

 It is made in an electric induction oven.

In this font, were added at the time of casting (in percentage by weight)
0.008% bismuth
and 0.04% of a graphite inoculant.

On a section of the piece passing through the middle, we observe
- a surface hardened area 3.5 to 4 mm thick
- a carbide-free and fully graphitized core zone
- And, between these zones, a transition zone of 1 to 1.5 mm thick in which carbides and graphite coexist.

Claims (7)

 1. A method for controlling, in a foundry mold against at least one metal cooler, the quenching of a piece of lamellar cast iron, such as a camshaft, a rolling mill cylinder or the like, characterized in that elements of Additions are added to the molten iron before it is poured to promote concomitantly the formation of carbides and the formation of graphite.  2. Method according to claim 1, characterized in that the addition elements favoring quenching consist of bismuth and / or tellurium, and / or boron, while the addition elements favoring graphitization consist of a graphite-based inoculant and / or a silicon-based inoculant.  3. Method according to claim 2, characterized in that the inoculants based on graphite and / or silicon also contain at least one of the following elements: strontium, zirconium, barium, calcium, aluminum, rare earths.  4. Method according to claim 2 or 3, applicable to a pig iron having a weight content:  - carbon between 2.6 and 3.6%,  - silicon between 1.2 and 2.6%,  - possibly manganese between 0.3 and 1%, sulfur between 0.003% and 0.15% or others, characterized in that, to promote quenching, the weight content of bismuth is between 0.003% and 0, 03% and / or tellurium is between 0.003% and 0.1%, and / or boron is between 0.003% and 0.2% while to promote graphitization the weight content of inoculant based on graphite is included between 0.01% and 0.2%, and / or inoculating with silicon is between 0.03% and 0.5%.  5. Method according to any one of claims 1 to 4, characterized in that the addition elements are added to the molten iron in the pocket before the casting of the iron.  6. Method according to any one of claims 1 to 4, characterized in that the addition elements are deposited in the foundry mold before the casting of the cast iron.  7.- Method according to any one of claims 1 to 4, characterized in that the addition elements are incorporated during the transfer of the cast iron into the ladle or into the mold.