CH655739A5 - PROCESS FOR HARDENING FERROUS ALLOYS IN AQUEOUS MEDIA. - Google Patents

Description

The present invention relates to a quenching process in an aqueous medium for ferrous alloys and, more particularly, for carbon steels and alloyed and low-alloyed steels.

It is known that the optimal mechanical characteristics of steels are only obtained after heating at high temperature followed by quenching. The speed and conditions of cooling of the steel during quenching have a decisive influence on the mechanical properties. If these conditions are not met, it may also result in deformations and even sparking of the hardened parts.

The quenching is generally carried out in a liquid or fluid medium. The liquid medium, depending on the desired cooling rate, can be of the aqueous, oily or igneous type (molten salt).

The theory and practice of quenching steels are discussed, for example, in the chapter "Quenching of Steel", pp. 15 to 36, t. 2, of the "Metals Handbook", 8th edition, published by American Society of Metals.

When a steel part, previously brought to a high temperature, for example 850 "C, is soaked in a liquid at a substantially lower temperature, the cooling takes place in three very distinct stages:

- The first stage, which corresponds to the temperature range from 850 to about 500 "C, corresponds to the heat build-up. The part is surrounded by a vapor sheath which isolates it from the liquid and slows down the cooling.

- The second stage, which corresponds approximately to the temperature range of approximately 500 to approximately 350 ° C., in the case of a quenching oil, corresponds to the nucleated boiling, that is to say

the appearance of steam bubbles at a large number of points in the room.

- Finally, the third stage corresponds to cooling by conduction and convection, thanks to direct contact with the quenching liquid. This stage can start from 350: C in the case of an aqueous medium.

The use of quenching oils generally leads to satisfactory results as regards the characteristics of the quenched parts. But it is well known that, in industrial practice, the use of quenching oils involves drawbacks and servitudes such as: soiled workshops, environmental pollution, sometimes unpleasant odors, risks of ignition , the need to preheat the oil tanks, degrease the hardened parts, etc.

For these various reasons, attempts have been made for many years to develop aqueous quenching media which would not suffer from these drawbacks and would provide quenched parts with mechanical characteristics substantially identical to those obtained by quenching. oil. The increase in the price of petroleum products has amplified these research efforts.

As early as 1960, in a commercial notice, the company Wyandotte Chemical Co. recommended the use of polyoxyalkylene glycols as additives to water quenching media. The product, designated by the registered trademark Pluracol V 10, had a molecular weight of 25,000 to 35,000.

In the previously mentioned “Metal Handbook”, it is pointed out that the addition of 0.01% of polyvinyl alcohol to the quenching water appreciably increases the cooling rate during the heating phase.

In French patent FR-A No. 1384244 (= US No. 3220893), from Union Carbide, aqueous media based on polyalkylene glycols added with anticorrosive agents such as nitrites or borates have been described.

In French patent R No. 1525603 in the name of BASF, AG, the addition of a water-soluble polymer containing groups (—CO — NH—) is recommended at a content of between 0.1 and 1% by weight.

In German patent application DE No. 2349225, 0.4 to 10% by weight of a salt of poly-acrylic acid in water is added.

In French patent FR No. 2316336 (= US No. 4087290) in the name of Houghton & Co., the additive is also a water-soluble salt of polyacrylic acid.

Finally, in US Pat. No. 3902929, assigned to Park Chemical Company, the use of polyvinylpyrrolidone, of average molecular weight between 5,000 and 400,000, with the addition of nitrite and / or borax (Na2B407) is recommended. .

However, it does not appear that the various formulations described in the prior art have led, in industrial practice, to identical results, or at least comparable, to those obtained by oil quenching.

In fact, in aqueous media, we encounter three major difficulties:

- the instability and the non-reproducibility of the caléfaction regime and of the transition to nucleated boiling,

- the position, at around 100 "C (boiling water) of the transition between the nucleated boiling regime and the convection regime,

- the relatively low convection speed below 100 C.

The ideal aqueous quenching medium for ferrous alloys should make it possible to stabilize - and possibly completely eliminate - the heat-up regime, and to bring the transition point known as 62 between the nucleated boiling regime to around 330 to 350 C. and the convection regime, the temperature of 350 "C corresponding, on average, to the point known as Ms which marks the beginning of the martensitic transformation. As to the point known as 0ls which corresponds

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655739

at the transition temperature between the caléfaction regime -

when it exists - and the nucleated boiling regime, it can be between 450 and 700 C, depending on the type of oil considered.

The object of the present invention is a quenching process in an aqueous medium which makes it possible to reproduce the results obtained by quenching in oil, and even to go beyond it, since it eliminates the calefaction phase and raises the temperature 02 up to 350th C.

To this end, this process has the characteristics specified in claim 1.

Preferably, the polyvinylpyrrolidone must have an average molecular mass at least equal to 400,000 and, for best results, of between 500,000 and 1,000,000.

The optimum concentration of PVP is between 5 and 50 g per liter of water and, preferably, between 10 and 35 g. The precipitation aid can be chosen from a wide range of substances causing, upon contact with the quenched parts, at the time of their introduction into the quenching medium, a reversible precipitation of the PVP, the word reversible meaning that when the quenched part is in thermal equilibrium with the quenching medium, the PVP layer, which precipitated when hot, is completely redissolved. It should be noted that this phenomenon is specific to PVP and is not of the same nature as the inverted solubility which is encountered in the case of water-soluble polymers having, in their molecular structure, oxygen bridges on which a water molecule can be set reversibly depending on the temperature.

The adjuvants causing the precipitation of PVP have been studied on a theoretical level, in particular in the articles by B. Jir-gensons: "Solubility and fractionation of PVP", "Journal of Polymers Science", 1952, 8, N ° 5, pp. 519-527, and by J. Elissaf S. Ericks-son and F.R. Eirich, "Journal of Polymers Science", 1960,17, pp. 193-202 ("Interaction of PVP with cosolutes"). These reversible precipitation adjuvants can be either water-soluble organic solvents, such as ketones or alcohols, or mineral salts and, in particular, sodium salts such as chloride, sulfate, perchlorate, thiocyanate, borax, disphosphate, hydroxide, or ammonium salts such as sulfate. Among these adjuvants, sodium chloride and disodium sulphate Na2SO4 have been shown to be particularly suitable for implementing the invention, at a concentration of 50 to 150 g per liter in the case of NaCl, and at a concentration of between 5 and 50 and preferably between 5 and 10 g per liter of water, in the case of Na2SO4. The agitation which is necessary to obtain the optimum characteristics of the quenching medium can be constituted by a circulation device with, for example, withdrawal and reinjection of the liquid at two opposite points of the container. More vigorous stirring - for example by injecting the quenching fluid under a pressure of a few bars - is also suitable.

The invention was implemented under the following conditions: steel specimens of 20 and 35 mm in diameter and 60 and 105 mm in height (previously heated at 850 ° C. for 20 min) were soaked in a tank containing 15 l of aqueous quenching medium, according to the invention, stirred by recirculation.

The tests were carried out on steels having the following chemical composition:

Designation

VS%

Cr%

MB%

35 CD4

0.37

0.99

0.17

37 C4

0.37

1.0

-

42 CD4

0.40

1.04

0.185

For each test, the temperature variation as a function of time was recorded by means of a thermocouple placed in the sample, and the temperature of the transition points 9j between the calefaction and the nucleated boiling was noted, and ô2 transition between nucleated boiling and convection. When Qj is at 850 ° C, this shows that there is no calefaction.

Tables 1 and 2 give the results of these tests, Table 1 being comparative, Table 2 relating to the invention. Figs. 1 to 5 show the results of the Vickers hardness measurements (HV30) on the cross-section of the cut-off specimens without heating, at mid-height, along a plane perpendicular to their axis.

Fig. 1 relates to a 37C4 steel 35 mm in diameter.

Fig. 2 refers to a 37C4 steel 20 mm in diameter.

Fig. 3 relates to a 42CD4 steel 40 mm in diameter. 15 Fig. 4 refers to 35CD4 steel 20 mm in diameter.

Fig. 5 relates to 35CD4 steel 35 mm in diameter.

TABLE 1 - Comparative tests

20

30

35

Quenching medium and conditions e ^ c e2 ° c

Water between 20 and 80 ° C

random

100 ° c

Classic quenching oil at 50 ° C, not stirred

480

350

High performance quench oil, 50 ° C, unstirred

655

350

Additives for water quenching currently on the market, at 20 ° C

random

100 to

200 ° C

PVP + water at 20 ° C without stirring 5 g / 1

10 g / 1 15 g / 1 20 g / 1 35 g / 1 50 g / 1

325 325 300 310 300 300

100 125 125 140 140 140

PVP + water at 20 ° C stirring 5 g / 1 by recirculation 10 g / 1

15 g / 1 20 g / 1 35 g / 1 50 g / 1

625 600 340 340 320 320

100 130 125 160 160 160

The quenching medium was obtained from PVP-K 90 from the company Badische Anilin und Soda Fabrik (BASF); according to the supplier, this PVP has an average molecular weight of around 700,000.

The PVP concentration was varied between 5 and 50 g / l, the Na2SO4 concentration between 5 and 30 g / l and the NaCl concentration between 50 and 200 g / l.

TABLE 2 - Implementation of the invention

Quenching medium and conditions e ^ c e2 ° c

PVP + H20, 15 g / 1 Agitation by recirculation 20: C

NaClen g / 1 50 100 150 200

850 850 850 850

355 350 130 130

PVP + H2O, 15 g / 1 Agitation by recirculation 20th C

Na2S04 g / 1 5

10 20 30

850 850 850 850

330 330 325 325

Constant Na2SO4 concentration equal to 5 g / 1

Recirculation agitation 20 ° C

PVP content g / 1 15 20 30 35

850 850 850 850

330 340 350 380

Constant concentration of Na2SO4 equal to 10 g / 1

Recirculation agitation 20 'C

35 40 50

850 850 725

350 300 300

It is found that the quenching medium, which is the subject of the invention, gives thermal results equivalent to those of the best oils currently known and, in particular, eliminates the heat build-up (which

655,739

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is not the case for oils) and goes back to point 02 at 350 C (and even a little more in the best case).

Optimal concentrations are around 15 g. 1 for PVP, 50 to 100 g l for NaCl, 5 to 10 g / 1 for Na2SO4. s

Mechanical tests, the results of which are given in figs. 1 to 5, were obtained with a quenching medium at 20 ° C., stirred by recirculation and consisting of:

- pure water at 20 C (curve N ° 1),

- oil at 50 C (curve N ° 2); io for comparison,

- two aqueous media according to the invention with:

• 12.5 g / 1 PVP + 5 g / Na2SO4 at 20 "C (curve N ° 3)

■ 35 g / 1 PVP + 5 g / 1 Na2SO4 at 20 C (curve N ° 4).

It is clear that: is

I) The water and the oil give hardness profiles, on the section of the test piece, called “U”, because they show a depression in the heart, due to poor transmission of the heat flux between the heart of the test tube and the quenching medium.

2) The quenching medium according to the invention gives practically flat hardness profiles, results which no aqueous quenching medium currently known can provide or even approach. In addition, it is important to emphasize that this flat profile is obtained without any alteration of the overall hardness which remains, depending on the case, equivalent to that given by pure water (PVP at 12.5 g. L) or the oil (PVP at 35 gl).

Finally, the quenching media according to the invention have the same advantages as all the aqueous media already known, based on water-soluble polymers: absence of odor, risk of flammability, toxicity, ease of cleaning of the quenched parts , biodegradability of effluents.

Finally, like other known aqueous quenching media, they can be supplemented with various anticorrosive or biocidal additives.

R

3 sheets of drawings

Claims (8)

655,739 1. A method of quenching, in an aqueous medium, of ferrous alloy parts and in particular of carbon steels and alloyed and low alloyed steels, previously brought to a temperature above 750 C, characterized in that said parts are introduced into a quenching medium consisting of an aqueous po-lyvinylpyrrolidone solution, added with an adjuvant causing, on the surface of said parts, at the time of their introduction into the quenching medium, a reversible precipitation of polyvinylpyrroli-done, and in that the quenching medium is subjected to stirring. 2. Method according to claim 1, characterized in that the poly-vinylpyrrolidone has an average molecular weight greater than 400,000 and, preferably, between 500,000 and 1,000,000. 2 3. Method according to claim 1, characterized in that the quenching medium comprises from 5 to 50 g and, preferably, from 10 to 35 g of polyvinylpyrrolidone per liter of water. 4. Method according to one of claims 1, 2 or 3, characterized in that the adjuvant causing precipitation is chosen from acetone, alcohols, sodium salts and hydrosoluble ammonium salts. 5. Method according to claim 4, characterized in that the adjuvant causing precipitation is introduced into the quenching medium at a concentration between 5 and 150 g and, preferably, between 50 and 100 g per liter of water. 6. Method according to claim 5, characterized in that the adjuvant is sodium chloride at a concentration between 50 and 100 g per liter of water. 7. Method according to claim 5, characterized in that the adjuvant is disodium sulfate at a concentration between 5 and 50 g and, preferably, between 5 and 10 g per liter of water. 8. Method according to one of claims 1 to 7, characterized in that the temperature to which the parts are brought, prior to their introduction into the quenching medium, is between 800 and 950 C.