EP0321370A1 - Process for modifying the cooling power of aqueous liquids for quenching metal alloys - Google Patents

Abstract

Process for modifying the cooling power of an aqueous solution of at least one water-soluble organic polymer which has a lower critical solubility temperature (LCST) with a view to its use for quenching metal alloys, characterised in that a water-soluble additive which modifies the said critical temperature is introduced into the said solution. To lower the LCST of the solution, an inorganic salt of an inorganic or organic acid is preferably employed as an additive in a concentration of between 0.1 and 200 grams per litre, this salt being chosen from the salts of alkali and alkaline-earth metals, including magnesium and zinc. To raise the LCST of the solution, a water-soluble organic derivative is added, in a concentration of between 0.1 and 200 grams per litre, chosen from aliphatic compounds containing at least one functional group chosen from alcohols, acids, aldehydes, amides and amines. <IMAGE>

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for modifying the cooling power of aqueous media intended for the quenching of metal alloys, these media being made up of so-called reverse-soluble water-soluble organic polymers, which precipitate by increasing the temperature. It also relates to aqueous quenching fluids, the cooling power of which can be modified by adding water-soluble additives.

STATE OF THE ART

It is known that attempts have been made for many years to substitute aqueous fluids for quenching oils, the drawbacks of which are numerous. Thus, in patent FR 1384244, solutions for the hardening of steels have been proposed, polyalkylene glycols having a molecular weight of up to 60,000, the aqueous solution possibly containing from 0.1 to 30% of polymer.
Many other publications have followed; there may be mentioned, among others, patents US 3,865,642 (which recommends the use of polyvinyl alcohol), US 3,902,929 (polyvinylpyrrolidone), US 4,087,290 (polyacrylic acid salts). Mention may also be made of patents FR 2507209 and 2538002 (polyoxyalkylenes glycols added with anticorrosive agents which act simultaneously on drasticity), FR 2537997 and FR 2537998 (polyvinylpyrrolidone with different additives which also act on drasticity).

In European patent application EP 206347, a quenching method is claimed in which the main constituent of the quenching medium is a water-soluble polymer which has been given, by a suitable choice of substituents, a hydrophobicity such that the cloud point is lowered by 'a predetermined value.
It is recalled that the aqueous solutions of this type of organic polymer precipitate at high temperature (in particular in contact with the parts being quenched) and redissolve at lower temperature. In the "Journal of Applied Polymer Science, 1959, vol: 1; n ° 1, p.56-62, FE BAILEY and RW CALLARD (Some properties of polyethylene oxides in aqueous solutions) studied the variation of the cloud point of these polymers as a function of the addition of variable amounts of mineral salts. However, this article does not refer to the application to quenching. The same is true for the article by EA BOUCHER & PM HINES, Journal of Polymer Science, Vol. 14, 1976, p. 2241-2251. It should also be specified that the "cloud point" of a polymer solution can be considered as a physical constant of this solution and that it can be determined by measuring the absorption coefficient of light as a function of temperature, through a cell containing this solution. At the point of trouble, there is an abrupt discontinuity in the transmission which spans a few tenths of kelvins.
It must also be remembered, for a good understanding of the invention, that the quenching of a metal part in an aqueous cooling fluid generally comprises three phases. If we take the case of a steel part previously brought to a temperature of about 850 ° C:
- the first stage is that of calefaction. The part is surrounded by a vapor sheath that isolates it from the liquid and slows down cooling. This stage may however, depending on the conditions of the operation, not exist or be eliminated by various means well known to specialists.
- the second stage is that of nucleated boiling, that is to say the formation of vapor bubbles on a large number of points of the part.
- the third stage corresponds to cooling by conduction and convection, by direct contact between the part and the quenching fluid.

It is possible, for each quenching medium, and at equal initial temperature of the quenched parts, to draw temperature curves as a function of time and of the cooling rate as a function of time or of temperature (derivative curves) which are called often: "drasticity curves". Taking into account the characteristics of the alloys to be quenched, the quenching medium is selected so as to slow or accelerate the cooling in the zones of high temperature or in the zone below approximately 300 ° C. (critical zone of the martensitic transformation) in which deformation or even "quenching nipples" may occur. However, this requires a whole range of quenching fluids, each corresponding to a type of alloy.

It would therefore be extremely advantageous, for the convenience of use in an industrial environment, to have a single fluid whose drasticity would be easily adaptable to each particular case thanks to the addition of specific additives suitably chosen and dosed. This problem had not yet received a completely satisfactory solution.

OBJECT OF THE INVENTION

The present invention proposes to solve this problem. It relates to a process for modifying the cooling power of at least one organic polymer with reverse solubility, having a lower critical solubility temperature (usually abbreviated as LCST, Low critical solution temperature) this process being characterized in that one adds to the aqueous solution of said polymer a controlled amount of a water-soluble additive which modifies said critical temperature as the case may be, increasing or decreasing.
These additives are chosen:
- among certain water-soluble mineral salts if the LCST is to be reduced; however, some organic compounds also have this property
- among certain water-soluble organic compounds, but at the same time relatively hydrophobic, if one wants to increase (or in some cases, decrease) the LCST.
These additions are made at doses of between 0 and 200 grams per liter of the quenching medium.

Another object of the invention is an aqueous medium for the quenching of metal alloys, comprising at least one water-soluble polymer with reverse solubility, with an LCST point, characterized in that it comprises, in addition, at least one additive intended to modify, in a controlled manner, its LCST point, increasing or decreasing, and thus to model the cooling curve.

DESCRIPTION OF THE INVENTION

As previously reported, it was known from the article by BAILEY and CALLARD that the addition of salts in aqueous solutions of reverse solubility polymers changed their cloud point or "lower critical solution temperature" (abbreviated: LCST). However, no relationship had been made between this cloud point and the drasticity of the solutions. Several subsequent publications had also reported the harmful effect of the introduction of salts into these quenching fluids; in METALS HANDBOOKS, volume 2, 8 ° Edition, we note, in the chapter "Quenching of steels, page 19, the effect of contamination of quenching fluids when we introduce parts which have been brought to quenching temperature by immersion in a bath of molten salts. This is the case, for example, of aluminum alloy parts preheated in nitrite-nitrate baths to around 500 ° C. In the patents we have cited under in the prior art, several mention additions of small amounts of mineral salts, but these are generally small amounts intended to provide a side effect and mainly an anticorrosion effect, for example: 0.8 g / l of borax and 0 , 2 g / l of sodium nitrite as corrosion inhibitor in US 3902929 or in FR 1384244.

It is therefore quite unexpectedly that the Applicant has found that it is possible, by the controlled use of certain additives acting on the cloud point (LCST) of aqueous solutions of polymers with reverse solubility, to model the drasticity curve so as to adjust it as precisely as possible to the characteristics and structure of the alloy to be quenched.

The invention applies in particular, but not limited to, quenching media based on oxyethylene polymers, oxyethylene and oxypropylene copolymers known under the trade name "EMKAROX" (registered trademark of Imperial Chemical Industries ), in which the propylene oxide (OP) / ethylene oxide (OE) ratio can be between 0 and 1, as well as PVME (polyvinyl methyl ethers), PDME (polydimethoxyethylene) polymers, to name only the main ones. The product used in the examples of implementation of the invention is an EMKAROX which bears the reference FC31-165000 (OE / OP ratio = 3). This copolymer has the following characteristics:
- average molecular mass: M = 32,500
- intrinsic viscosity: n = 38.65 cm.g
- LCST (cloud point): 74.2 ° C.

• . La figure 1 montre la variation du point de trouble de l'EMKAROX FC31-­16500 en solution aqueuse à 4% en poids, en fonction de la concentration en divers additifs.
• . Les figures 2,3,4 et 5 montrent des courbes de refroidissement obtenues avec des additions diverses, selon l'invention, à une solution de base à 4% d'EMKAROX FC31-1650000 dans de l'eau. Les courbes des figures 2 et 4 montrent la variation de température en fonction du temps, et les figures 3 et 5, la variation de la vitesse de refroidissement en fonction de la température (courbes dérivées).
• . Les figures 6 à 9 montrent que l'effet des additions de sels minéraux sur le LCST ne dépend pas de la nature du polymère.
• . La figure 10 montre les variations de la vitesse de refroidissement en fonction du LCST pour différents additifs à plusieurs concentrations et pour différentes températures.

Figures 1 to 10 illustrate the invention.

• . Figure 1 shows the variation of the cloud point of EMKAROX FC31-16500 in aqueous solution at 4% by weight, as a function of the concentration of various additives.
• . Figures 2,3,4 and 5 show cooling curves obtained with various additions, according to the invention, to a 4% base solution of EMKAROX FC31-1650000 in water. The curves of FIGS. 2 and 4 show the variation of temperature as a function of time, and FIGS. 3 and 5, the variation of the cooling rate as a function of temperature (derived curves).
• . Figures 6 to 9 show that the effect of mineral salt additions on the LCST does not depend on the nature of the polymer.
• . FIG. 10 shows the variations in the cooling rate as a function of the LCST for different additives at several concentrations and for different temperatures.

• a- ceux qui abaissent le LCST et qui appartiennent au groupe des sels alcalins et alcalino-terreux (y compris le magnésium) et des sels de zinc d'acides minéraux ou organiques et qui comportent également quelques dérivés organiques.
• b- ceux qui élèvent (ou qui, pour certains, abaissent) le LCST, et qui appartiennent au groupe des composés organiques hydrosolubles et notamment au groupe comprenant au moins une fonction choisie parmi les alcools, les acides, les aldéhydes, les amides, les amines.

The Applicant has found that the additives capable of modifying the cloud point (LCST) of a water-soluble polymer with reverse solubility, fall into two categories:

• a- those which lower the LCST and which belong to the group of alkaline and alkaline-earth salts (including magnesium) and zinc salts of mineral or organic acids and which also contain some organic derivatives.
• b- those which raise (or which, for some, lower) the LCST, and which belong to the group of water-soluble organic compounds and in particular to the group comprising at least one function chosen from alcohols, acids, aldehydes, amides, amines.

In the first group, that of mineral salts, it is found that the effect of decreasing the LCST as a function of the concentration in the polymer solution, varies significantly with the nature of the cation and of the anion.

The orders of efficiency of cations, anions and salts are as follows (indicated in the order of decreasing efficiency):
- At equal molar concentration:
K⁺, Na⁺, Ca⁺, Mg⁺⁺, Zn⁺⁺, Li⁺
B₄O₇⁻⁻, PO₄⁻⁻⁻, CO₃⁻⁻, SiO₃⁻⁻, SO₄⁻⁻, F⁻, Cl⁻,
CH₃COO⁻, Br⁻, ClO₃⁻, I⁻,
Na₃PO₄, Na₂B₄O₇, Na₂CO₃, Na₂SiO₃, K₂SO₄, MgSO₄, ZnSO₄, Li₂SO₄, KF, NaCH₃CO₂, KCl, NaCl, CaCL₂, MgCl₂, KBr, KClO₃, LiCl, KI.
- At equal weight concentration of anhydrous salts:
KOH, Na₂CO₃, Na₂B₄O₇, Na₃PO₄, K₂CO₃, MgSO₄, Na₂SiO₃, KF, K₂SO₄, Li₂SO₄, NaCl, ZnSO₄, KCl, NaCH₃CO₂, LiCl, MgCl₂, CaCL₂, KClO₃, KBr, KI.

In the second group, that of water-soluble organic compounds, the effect on the cloud point depends on the nature and number of polar groups as well as the length of the paraffin chain.

We observed the following order of effects for the products we studied (cited in order of decreasing effectiveness):
- At equal molar concentration:
- positive effect (increased LCST):
1,3-butane diol, acetamide, propylene glycol, ethanol, methanol, formamide, ethylene glycol
- negative effect (lowering of the LCST):
butanol, propanol.
- At equal weight concentration:
- positive effect:
acetamide, butane diol 1-3, methanol, propylene glycol, ethanol, formamide, ethylene glycol
- negative effect :
butanol, propanol.

In general, for the implementation of the invention, additives will be preferred whose efficiency is greatest so as to limit the salt concentration of the quenching medium to the strict minimum for obvious reasons.

It has also been found that the effects of the substances of the two groups (increasing or decreasing LCST) are algebraically additive, that is to say that, for example, the increase in LCST obtained by an organic derivative can be reduced, or even canceled, or even reversed by the addition of a mineral salt. This effect can be used to compensate for the undesirable effect on the LCST of an additive introduced with the aim of providing an anti-corrosion, biocidal, anti-foaming or other effect.

Finally, it was found that the results obtained on the copolymer of ethylene oxide and of propylene oxide of ratio OR / OP = 3/1 were strictly reproducible with copolymers having ratios OE / OP of 2/1 and 5/1, as clearly shown in Figures 6 to 9.

Figure 1 embodies all that has just been exposed on the effect of the two groups of additives; we note, moreover, that the cloud point is lowered in a substantially linear manner, by addition of increasing amounts of mineral salts: it appears that the disodium carbonate Na₂CO₃ has, at equal concentration, an efficiency greater than that of sodium metaborate ( Borax). Propylene glycol and acetamide raise the cloud point more effectively than their respective lower counterparts, ethylene glycol and formamide, while the effect of potassium iodide is insignificant as previously reported. .

Table I gives the characteristics of the solutions used for the drasticity measurements. In all cases, the basic solution is EMKAROX FC31-165000 at 4% by weight. ADDITIVE Na₂CO₃ KI Propylene glycol conc. % 0 0.76 1.52 3.04 4 5 10 20 viscosity (cp) 1 1.03 1.06 1.13 0.96 1.2 1.4 1.8 viscosity 3.5 3.4 3.27 3.01 3.46 4.14 4.72 5.9 η cm. g 38.7 29.7 38.0 43.8 LCST ° C 74.2 64.4 54.8 35.3 75.3 78.6 83.5 96 pH 7.9 11.5 - 6.9

FIG. 2 shows the influence of the additions of sodium carbonate according to Table 1 on the shape of the cooling curves, the initial temperature of the quenched steel part being 850 ° C. This influence, which is not very marked in the nucleated boiling zone is amplified in the conduction / convection cooling zone, below 300 ° C. where there is a noticeable reduction in the cooling rate.
FIG. 4, which corresponds to the same conditions as FIG. 2, the additive being propylene glycol, shows that the modeling effect of the drasticity curve begins at 850 ° C., with a significant decrease in the cooling rate, and reverses below about 300 ° C.
In addition, it is clear that the addition of Na₂CO₃ makes it possible to increase the transition temperature between the nucleated boiling and natural convection, while the addition of propylene glycol causes the opposite effect. By playing on these different tests, it is thus possible to model the cooling curve, for a given polymer or family of polymers, as a function of the quenching conditions required by the parts treated.
The variations are better perceived in Figures 3 and 5 where the derived curves have been drawn (cooling rate as a function of temperature).
In Figure 3, the derived curves are plotted for disodium carbonate concentrations of 0.76 1.52 and 3.04% by weight. Similarly, in Figure 5, the derived curves are plotted for propylene glycol concentrations of 5 to 20%.
We can distinguish two temperature ranges:
- between approximately 850 and 300 ° C, the cooling rate for a given temperature, increases when the Na₂CO₃ concentration increases and decreases when the propylene glycol concentration increases
- between about 300 and 25 ° C, the cooling rate, for a given temperature, decreases when adding disodium carbonate and increases when adding propylene glycol.
Figures 6 to 9 have already been commented on.
FIG. 10 shows the variations in the cooling rate in the range 25-300 ° C. (approximately) as a function of the cloud point for three temperatures: 100, 150 and 200 ° C. and for several concentrations of disodium carbonate, disodium phosphate, ethylene glycol, propylene glycol and potassium iodide.

It is known that in this type of aqueous fluids, the cooling process is slower the higher the viscosity. In the case of solutions containing propylene glycol, the overall viscosity of the solution is an increasing function of its concentration (see Table I) while, for solutions containing Na₂CO₃, the overall viscosity decreases. The variations in dT / dt in the range 850-300 ° C could therefore be explained by the changes in viscosity.

BENEFITS PROVIDED BY THE INVENTION

The present invention provides a solution to the problem of the multiplicity of aqueous quenching fluids required by the treatment of parts made of different alloys or requiring different quenching speeds in particular temperature ranges. It is possible to model the cooling curve at the option of the users, from a single solution of water-soluble polymer, to which a cloud point of a predetermined value can be given.
It also allows, on a secondary basis, to correct the effect, on the point of cloudiness of auxiliary additives, such as biocides, defoamers, anticorrosives, etc.
Finally, it can temporarily restore the LCST point on solutions which begin to be affected by aging due to prolonged use, for example to complete a series of quenches before the complete renewal of the solution.

Claims (15)

1. Process for modifying the cooling power of an aqueous solution of at least one water-soluble organic polymer having a lower critical solubility temperature (called "LCST"), with a view to its use for the quenching of metal alloys, characterized in that that a predetermined amount of a water-soluble additive is introduced into said solution which modifies said critical temperature, in a controlled manner, decreasing or increasing. 2. Method according to claim 1, characterized in that, to decrease the LCST of the solution, a mineral salt of a mineral or organic acid is used as an additive at a concentration of between 0.1 and 200 grams per liter. 3. Method according to claim 2, characterized in that the mineral salt is chosen from the alkali and alkaline earth metal salts, including magnesium, and zinc. 4. Method according to claim 2, characterized in that the anion of the mineral salt is chosen from the anions B₄O₇⁻⁻, PO₄⁻⁻⁻, CO₃⁻⁻, SO₄⁻⁻, SiO₃ F⁻, Cl⁻, CH₃COO⁻, Br ⁻, ClO₃⁻, I⁻. 5. Method according to any one of claims 1 to 4 characterized in that the mineral salt is preferably chosen from disodium carbonate, trisodium phosphate, borax, sodium silicate, disodium sulfate. 6. Method according to claim 1, characterized in that, to increase, or optionally, to decrease the LCST of the solution, an organic water-soluble derivative is added, at a concentration of between 0.1 and 200 grams per liter. 7. Method according to claim 6, characterized in that the organic derivative is chosen from aliphatic compounds having at least one function chosen from alcohols, acids, aldehydes, amides and amines. 8. Method according to claim 7, characterized in that the organic derivative increasing the LCST is chosen, preferably from butane diol 1-3, acetamide, propylene glycol, ethanol, methanol, formamide, l ethylene glycol. 9. Method according to claim 7, characterized in that the organic derivative reducing the LCST is propanol or butanol. 10. Method according to any one of claims 1 to 9, characterized in that the water-soluble polymer having an LCST point is chosen from oxyethylene polymers, the copolymers of ethylene oxide and of propylene oxide having a propylene oxide / ethylene oxide ratio of between 0 and 1, polyvinyl methyl ethers, polydimethoxyethylenes. 11. Method according to any one of claims 1 to 10, characterized in that one compensates for the variation of the LCST due to the presence of at least one compound causing this LCST to vary in a given direction, by an addition of '' at least one compound acting in the opposite direction on the LCST. 12. Aqueous medium, for the quenching of metal alloys, comprising at least one water-soluble organic polymer having an LCST point, characterized in that it comprises, in addition, at least one additive for modifying (increasing or decreasing) LCST . 13. Aqueous media, according to claim 10, characterized in that the additive making it possible to decrease the LCST is chosen from water-soluble, alkaline and alkaline-earth salts (including magnesium), and of zinc, of mineral or organic acids , at a concentration between 0.1 and 200 grams per liter. 14. Aqueous medium, according to claim 10, characterized in that the additive making it possible to increase the LCST is chosen from aliphatic organic derivatives, water-soluble, at a concentration of between 0.1 and 200 grams per liter, having at least one function chosen from alcohols, acids, aldehydes, amides and amines. 15. An aqueous medium according to any one of claims 10 to 12, characterized in that it simultaneously comprises at least one compound acting on its LCST in a given direction and at least one compound acting on its LCST in the opposite direction.

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