GB2133048A - Method of quenching ferrous alloys in an aqueous medium - Google Patents

Abstract

Articles made of ferrous alloys, particularly articles made of carbon steels and alloyed steels, that have previously been brought to an elevated temperature of over 750 DEG C and generally from 800 to 950 DEG C, are quenched in an aqueous solution of polyvinylpyrrolidone containing an additive that causes reversible precipitation of polyvinylpyrrolidone at the surface of the articles at the time when they are placed in the quenching medium, the quenching medium being subjected to agitation. The most appropriate precipitating additive is sodium sulphate at a concentration of 5 to 10 g/l.

Description

SPECIFICATION Method of quenching ferrous alloys in an aqueous medium The invention concerns the aqueous quenching of ferrous alloys, more particularly, but not exclusively, carbon steels and alloyed steels, including steels with only small amounts of alloying elements.

It is known that optimum mechanical properties of steels are obtained only after heating to an elevated temperature followed by quenching. The speed and conditions of the cooling involved in quenching determine the mechanical properties. if the desired conditions are not obtained, deformation and cooling cracks may occur in the quenched articles.

Quenching is usually carried out in a fluid, e.g.

liquid, medium. Depending on the cooling speed desired, the liquid medium may be ofthe aqueous, oily or molten salttype.Thetheoryand practice of quenching steels are set out, for example, in the chapter "Quenching of Steel", pages 15to 36 of volume 2 of the "Metals Handbook", 8th edition, edited by the American Society of Metals.

When a steel article that has previously been brought to an elevated temperature of e.g. 850"C is quenched in a liquid at a substantially lower tempera- ture, cooling takes place in three very distinct stages.

The first stage, viz calefaction, corresponds to the temperature range from 850into approximately 500"C. In th is stage the a rticle is su rrou nded by a sheath of steam which isolates itfrom the liquid and slows down cooling.

The second stage, viz nucleated boiling, corresponds approximately to the temperature range from 500into 350"C in the case of a quenching oil. In this stage, bubbles of steam appear at a large number of points on the article.

The third stage involves cooling through conduction and convection as a result of direct contactwith the quenching liquid. This stage may startfrom 350"C in the case of an oil, orfrom somewhere in the region of 1 OO"C in the case of an aqueous medium.

Generally speaking, the use of quenching oils gives satisfactory properties to the articles quenched. In industrial practice, however, the use of quenching oils brings disadvantages and obligations such as dirty workshops, environmental pollution, unpleasant odours, dangers of ignition, and the need to preheat the oiltanks and degreasethe articles quenched.

Forthese reasons attempts have been made for manyyearstoperfectaqueousquenching media that would notsufferfrom these disadvantages and that would give the quenched articles mechanical properties substantially identical with those obtained by oil quenching. The increase in the price of petroleum products has increased such research efforts.

From 1960, in a commercial report, the Wyandotte Chemical Co recommended using polyoxyalkylene glycols asadditivesto aqueous quenching media.

One such product, which issoldunderthe registered trade mark Pluracol V 10, has a molecular weight of from 25,000 to 35,000.

The above-mentioned "Metals Handbook" indicates that the addition of 0.01% of polyvinyl alcohol to the quenching water substantially increasesthecool- ing speed during the calefaction phase. Union Carbide French Patent No 1 384 244, which is equivalent to US Patent No 3 220 893, describes aqueous media based on polyalkylene g lycols with anticorrosive agents such as nitrites or borates added to them. BASFAG French Patent No 1 525 603 recommends adding a water-soluble polymer containing iminocarbonyl ( CO-NH-) groups in a proportion of 0.1 % to 1 % by weight. German PatentApplication DE2 349 225 adds from 0.4to 10% byweight of a polyacrylic acid salt in water.In Houghton & Co French Patent No 2 316 336, which is equivalentto US Patent No 4 087 290,the additive is again a water-soluble salt of polyacrylic acid. US Patent No 3 902 929, which is assigned to Park Chemical Company, recommends using polyvinyl pyrrolidone of an average molecularweightfrom 5,000 to 400,000, with a nitrite and/or borax (Na2B4O7) being added as a anticorrosive agent.

In industrial practice, however, the various formula- tions described in the prior art do not appearto have produced results identical or even comparable with those obtained by quenching with oil. In aqueous media three major difficulties are in fact encountered, viz (a) the instability and non-reproducibility ofthe calefaction stage and the transition to nucleated boiling; (b)the position, (about 100 C,the boiling point of water) of the transition between the nucieated boiling stage and the convection stage, and (c) the relatively low speed of convection below 1 OO"C.

The ideal aqueous quenching mediumforferrous alloys should make it possible to stabilise or possibly eliminate the calefaction stage and to bring the transition temperature 62 between the nucleated boiling stage and the convection stagetothe region of 330to 350"C. The temperature 350"C corresponds on an averagetothe point known as Moss which marksthe beginning of martensite transformation. Thetransi- tion temperature 61 between the calefaction stage (if any) and the nucleated boiling stage may be between 450 and 700"C according to the type of oil considered.

The present invention provides a method of quenching in an aqueous medium that cuts outthe calefaction stage and raises the temperature 62 to about350 C and that comprises submerging a steel article at a temperature ofover7500C, usually from 800 to 950"C, in an aqueous solution of polyvinylpyrrolidone containing an additive that causes reversible precipitation of polyvinyl pyrrolidone at the surface of thearticlesatthetimewhentheyare put into the quenching medium, and agitating the quencing medium so as to ensure constant renewal by circulat ing the quenching fluid around the article to be quenched.The results obtained from oil quenching can thus be reproduced and even surpassed.

The polyvinylpyrrolidone preferably has an average molecularweight of at least 400,000, from 500,000 to 1,000,000 giving the best results.

Theoptimum concentration ofthe PVPinthewater is from Sto 50 g/l, preferably from 10 to 35 gill. The precipitation additive may be chosen from a wide range ofsubstancesthat, on coming into contact with the articles being quenchedwhentheyareplaced in the quenching medium, cause reversible precipitation of the PVP.The word "reversible" signifies that when the article being quenched has come into thermal equilibriumwiththequenching medium,thelayerof PVP, which has precipitated hot, is entirely redissolved. It would be noted thatthis phenomenon is peculiarto PVP and notto the same nature as the reversed solubility met with in the case of watersoluble polymers whose molecular structures have oxygen bridges on which a molecule of water can be fixed reversibly as a function of the temperature.

Additives leading to the precipitation of PVP have been studied theoretically, particularly in the articles by B. Jirgensons: Solubilityandfractionation of PVP, Journal of Polymers Science, 1952,8, no.5, pp.

519-527 and J. Elissaf, S. Ericksson and F. R. Eirich, Journal of Polymers Science, 1960, 17, pp. 193-202 (interaction of PVP with co-solutes). These reversible precipitation additives may be eitherwater-soluble organic solvents, such as ketones, e.g. acetone, or alcohols, inorganic salts and particularly sodium salts such as the chloride, sulphate, perchlorate, thiocyanate, borate and diphosphate, and ammonium salts such asthe sulphate, or sodium hydroxide. Ofthese additives sodium chloride NcCI and sodium sulphate Na2SO4 have proved to be particularly well adapted to use in the invention, at a concentration offrom 50 to 150 9/l in the case of NaCI, and at a concentration of from 5to 50 and preferablyfrom Sto 10 girl in the case of Na2SO4. The agitation necessary to obtain the optimum properties ofthe quenching medium maybe provided by a circulating arrangement, for example with the liquid being taken out and reinjected attwo opposed points in the vessel. More vigorous agitation, e.g. by injecting the quenching fluid at a pressure of a few bars, is also a suitable provision.

The invention is used underthefollowing conditions: Steel samples 20 and 35 mm in diameter and respectively 60 and 105 mm high that have previously been heated to 850"Cfor20 minutes are plunged into a tank containing 15 litres of aqueous quenching medium according to the invention, which is agitated by recirculation.

The tests are carried out on steels ofthe following chemical composition:

Name C% Cr% Mo% 35 CD4 0.37 0.99 0.17 37 C4 0.37 1.0 42 CD4 0.40 1.04 0.185 The quenching medium is obtained from PVP-K 90 from BASF: the suppliers that state thatthis PVP has an average molecularweightofapproximately 700,000.

The concentration of PVP is varied from Sto 50 g/l, the concentration of NA2SO4 from 5 to 30 g/l and the concentration of NaCI from 50 to 200 g/l.

For each test the change in temperature as a function of time is recorded by means of a termocou ple placed in the sample, and the temperatures 81 and 62 (as defined above} are noted. When 81 is 850"C there is no calefaction.

Tables 1 and 2 give the results of these tests, Table 1 being comparative and Table 2 relating to the invention. In the accompanying drawings, Figures 1 to 5 show the results of measuring the Vickers hardness (HV30) on the cross-section ofthe samples cut up without heating, halfway up their height, in a plane perpendicularto their axis.

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

Figure 2 relates to a 37C4 steel 20 mm in diameter.

Figure 3 relates to a 42CD4 steel 40 mm in diameter.

Figure 4 relates to a 35CD4 steel 20 mm in diameter.

Figure 5 relates to a 35CD4 steel 35 mm in diameter.

Table 1 - comparative tests

Quenching medium and conditions 01 C 02 C variable 100 C Water between 20 and 80 C variable 100 C (aleatoire) Conventional quenching oil at 480 350 50 C, not agitated High performance quenching oil 655 350 at 50 C, not agitated Additives for water quenching variable 100 to 200 C now on the market, at 20 C PVP + water at 20"C 5 g/l 325 100 without agitation 10 gA 325 125 15 gA 300 125 20 g/l 310 140 35g/l 300 140 50 gA 300 140 PVP + water at 200C 5 g/l 625 100 agitation by 10 gA 600 130 recirculation 15 gA 340 125 20g/l 340 160 35g/l 320 160 50g/l 320 160 Table 2 - Use of the invention

Quenching medium and conditions 01 C 32 C NaCI in g/l PVP + H20, 15 gIl 50 850 355 Agitation by recirculation 100 850 350 20 C 150 850 130 200 850 130 Na2SO4 g/l PVP + H20, 15 g/l 5 850 330 Agitated by recirculation 10 850 330 20+C 20 850 325 30 850 325 Content of PVP g/l Concentration of Na2SO4 15 850 330 constant at 20 850 340 5 g/l 30 850 350 Agitation by recirculation 35 850 380 20 C Concentration of Na2SO4 35 850 350 constant at 40 850 300 10 gIl 50 725 300 Agitation by recirculation 20 C The quenching medium according to the invention can be seen to give thermal results equivalent to those of the best oils now known, and in particular to eliminate calefaction, which oils do not, and to raise 62 to 3500C and even slightly higher in the best case.

The optimum concentrations are in the region of 15 g/l for PVP, 50 to 100 g/i for NaCI and 5 to 10 g/l for Na2SO4.

The mechanical tests, the results of which are given in Figures 1 to 5, are obtained with a quenching medium at 20"C, which is agitated by recirculation and comprises pure water at 20"C in curve no 1 and oil at 50 in curve no 2.

Bywayofcomparison, two aqueous media according to the invention with: 12.5 g/l of PVP + 5 g/l NA2SO4 at 20"C in curve no 3 and 35 gIl of PVP + 5 g/l Na2SO4 at 20"C in curve no 4, are tested.

Itwill be seen that: 1. Water and oil give so-called "U-shaped" hard ness curves overthe section of the sample, since they show a depression at the core, due to poor transmission of heat between the core of the sampleandthe quenching medium.

2. The quenching medium according to the inven tion gives virtually flat hardness curves, a result which no aqueous quenching medium previously known can provide or even approach.

Furthermore, it is important to emphasise that the flat profile is obtained without any detrimental effect on the overall hardness, which remains equivalent to that given by pure water (PVP at 12.5 gill) or oil (PVP at 35 g/l) as the case may be.

Finally, the quenching media according to the invention have the same advantages as all known aqueous media based on water-soluble polymers, viz, the absence of odour, of danger of ignition and of toxicity, the ease in cleaning the quenched articles and the fact that the effluent is biodegradable.

Like other known aqueous quenching media, they may have various anticorrosion additives or biocides included in them.

Claims (16)

1. A method of quenching, in an aqueous medium, articles made of ferrous alloys that have previously been broug htto an elevated temperature of over 750"C, comprising placing the articles in a quenching medium comprising an aqueous solution of polyvinylpyrrolidone containing an additive that causes reversible precipitation of polyvinyl pyrrolidone atthe surface of the articles at the time when they are put into the quenching medium, and subjecting the quenching medium to agitation.

2. A method as claimed in claim 1, in which the ferrous alloy is a carbon steel.

3. A method as claimed in claim 1, in which the ferrous alloy is an alloyed steel.

4. A method as claimed in any preceding claim, in which the temperature is 800 to 950"C.

5. A method as claimed in any preceding claim, in which the polyvinylpyrrolidone has an average molecularweight of over 400,000.

6. A method as claimed in claim 5, in which the average molecular weight is in the range 500,000 to 1,000,000.

7. A method as claimed in any preceding claim, in which the quenching medium contains from 5 to 50 of polyvinyl pyrrolidone per litre of water.

8. A method as claimed in claim 7, in which the quenching medium contains from 1 Oto 35 g of polyvinylpyrrolidone per litre of water.

9. A method as claimed in any preceding claim, in which the additive that causes the precipitation is acetone, an alcohol, a sodium salt or water-soluble ammonium salt.

10. A method as claimed in claim 9, in which the additive is put into the quenching medium at a concentration of from 5 to 150 g/l.

11. A method as claimed in claim 10, in which the concentration is from 50 to 100 g/l of water.

12. A method as claimed in claim 11, in which the additive is sodium chloride at a concentration offrom 50to 100 g/l.

13. A method as claimed in claim 11, in which the additive is disodium sulphate as a concentration of from 5 to 50 g/l.

14. A method as claimed in claim 13, in which the concentration is from 5 to 10 g/I.

15. A method as claimed in claim 1, carried out substantially as hereinbefore described in any one of the tests in Table 2.

16. Articles made of ferrous alloys that have been quenched bya method as claimed in anyone ofthe preceding claims.