GB2062014A - Method of hardening steels and metal alloys and apparatus therefor - Google Patents

Abstract

A method and apparatus are provided for hardening steels or other metal alloys by quenching with an aqueous solution of sodium borate. To enable the solution to absorb a rapid transfer of heat without serious fluctuations in temperature, the sodium borate is added in an amount of 20 to 50 wt. parts per 100 wt. parts of water, and hardening is carried out a the boiling point of the solution. The latter may additionnally contain 1 to 30 wt. parts of sodium sulphate, and/or 0.1 to 20 wt. parts of glycerine, per 100 wt. parts of water. The quenching bath (101) is provided with an oil jacket (102), and a heating coil (103), as well as cooled condensation surfaces above the coolant and an extractor funnel (201) connected to a cooler (202) for collecting and condensing vapour from the bath surface. <IMAGE>

Description

SPECIFICATION Method of hardening steels and metal alloys and apparatus therefor This invention relates to a method of hardening steels and other metal alloys by quenching with the aid of an aqueous solution to which sodium borate has been added.

The hardening of large-volume units in rapid succession, for example large wire coils in the working cycle of a rolling mill, makes special demands on quenchants and quenching apparatus. The effect of the quenchant must remain constant even though the high heat capacity of heavy wire coils results in a high rate of heat transfer to the quenchant and therefore an increase in temperature or fluctuations in temperature may easily occur. However, a change in the temperature of the bath would result in a change in hardening. The successive wire coils would not be hardened equally under these conditions. It is therefore necessary to use a quenchant which can be kept at a constant temperature without great expense.

Moreover, a change in the programme with regard to wire diameter or quality should not necessitate a change of quenchant and quenching process since such a change involves delay and therefore reduced production. The nature of the quenchant should therefore be such that the heat transmission from high to low temperatures is, on the one hand, sufficiently high and, on the other hand, does not show any extreme variations or rapid rises and therefore preserves the quenchant so that even critical grades of material which are sensitive to hardening cracks can be successfully hardened.

Moreover, in order that the object may be properly achieved, the great quantity of heat introduced into the quenchant should be capable of being dissipated economically.

In addition, the quenchant must meet other requirements. It must have the same effect over all the surface elements, even when the position varies, it must be thermally stable, it should not settle or be deposited on the material being treated and should be inexpensive so that the economy of the hardening process is not jeopardized. Likewise, it must be ensured that the quenching effect does not react sensitively to the normal impurities in the quenchant used.

More important than these requirements are the first three mentioned conditions, namely constancy of bath temperature, high heat transmission without extreme variation and easy coolability of the bath.

The known quenchants which have been introduced only partially achieve the complex object as outlined in each case, and the known quenching oils require considerable expense to maintain a constant oil bath temperature, very large quantities of oil being pumped by correspondingly large cooling systems. Moreover, the known quenching oils do not exhibit the required heat transmission which remains approximately constant. Also, the quenching oil is not thermally stable, being cracked at high temperatures and the efficacy of the oil being altered as a result. Fire hazard and the formation of dense smoke are further disadvantages. It is known practice to add to the conventional hydrocarbonbased quenching oils additives which are intended, for example, to prevent the proneness of certain steels to cracking from coming into play.Thus, additives comprising 2 to 20% of a sulphurous wetting agent with a phenol base are recommended in German Auslegeschrift No. 1 5 33 966, but additives of this type do not have sufficient thermal stability.

The cheap quenchant, water, likewise does not meet the requirements of approximately constant heat transmission from high to low temperatures. If additives are used to prevent the vapour film in the higher temperature range (Leidenfrost effect), a steep rise in heat transmission is maintained when the surface temperature drops, and a subsequent fall in heat transmission is maintained at even lower temperatures. Therefore, the problem as outlined is not solved by using conventional additives of 10% common salt or caustic soda solution or cyanogen-containing salts.

In German Offenlegungsschrift No. 28 1 5 090 it is recommended that water-glass, borax, sodium nitrate or potassium nitrate, organic substances, wetting agents or agents having a similar effect be added to the water. The expert can scarcely learn a regular procedure from this very general recommendation because the choice is too wide and there is no precise indication of choice, use or concentration of the agent. According to this prior art aqueous solutions of 6 to 10% boiling water glass or 8 to 12% boiling borax or 4 to 6% boiling sodium nitrate or potassium nitrate are also suitable cooling agents for the patenting operation. However, the problem posed in this connection differs considerably from the problem underlying the present application.The known coolant is selected and set with regard to additives and temperature so that there is no hardening of the wire to be treated, but so that only a lamellar pearlite is formed. Moreover, the wire which is laid in open horizontal coils was cooled while passing over belt conveyors. Also, the method of cooling differs from the invention. Therefore, the solution to the problem outlined above cannot be inferred from German Offenlegungsschrift No.2815090.

Finally, water/salt solutions have also become known (C.R.M. No. 32, Sept. 1972, Page 14).

According to this publication a salt content of 1 to 10 times that of the water, or possibly even higher, should be selected in order that the boiling point of these solutions may be moved to higher temperatures of up to 3400C. The recommendation that 222 grams of CaCI2 be dissolved in 100 grams of water can for example be inferred from this publication. This solution has a boiling point of 1 600C. As a result of the low water content of this solution, a water loss has a particular effect on the cooling action of this agent.

The considerable quantity of salt discharged and the costly washing process resulting therefrom entail technical and economical disadvantages. Moreover, the heat transmission to be achieved is not sufficient for hardening coils of wire in the known solutions. The level of the mean heat transmission coefficient is 600 W/m2K in the case of these solutions, and 200 W/m2K in the case of the quenchant according to the invention. The high bath temperature of about 1 600C and the mentioned relatively low mean heat transmission coefficient of the quenchant reveal that this agent is not intended as a quenchant for solving the problem of the present application; it is proposed for the patenting of wires.

It is therefore evident that the problem cannot be solved by the known coolants. It has been found after extensive tests that it is possible to select a coolant which unexpectedly solves the problem posed with excellent results by considering the following facts: a) An aqueous solution in order that the evaporation heat of the water may be used for cooling the bath.

b) The quenching bath at boiling point in order to maintain the required constant temperature.

c) Adding 200 to 500 grams of sodium borate to 1000 grams of water in order to increase the heat transmission at a high temperature relative to water and to reduce the boiling phase and at the same time transfer the boiling point to a higher temperature.

d) Adding 10 to 300 grams of sodium sulphate in order that the density of the quenchant may be further increased. The sodium sulphate increases the solubility of sodium borate in water.

e) Adding 1 to 200 grams of glycerine in order to obtain a uniform heat transmission within a wide temperature range.

The optimum concentration of sodium borate according to the invention far exceeds the concentration which was hitherto considered advantageous. The advantageous additional provision of sodium sulphate and glycerine forms an essential part of the invention although it is not imperative to use these substances in every case.

The heat transmission coefficient of various quenchants, as a function of the surface temperature of the material to be hardened, is illustrated in Figure 1. Curve 1 applies to commercially available quenching oil at 60 to 800 C, and curve 2 applies to distilled water in a boiled state.Curves 3 to 7 refer to quenchants having the following compositions according to the invention: Sodium Sodium Water borate sulphate Glycerine (H20) (Na2B407) (Na2SO4) (C3H803) (3) 1.0 0.320 - - (4) 1.0 0.340 - - (5) 1.0 0.360 - - (6) 1.0 0.450 - 0.070 (7) 1.0 0.490 0.260 - It follows from Figure 1 that the new quenchant has a high heat transmission coefficient within a wide temperature range and having no troublesome extreme variations.

A special hardening plant is to be provided for carrying out the method according to the invention.

The bath is preferably heated indirectly, having a double jacket filled with thermal oil. The heated jacket rises above the level of liquid in the bath and also encloses the bottom so that it is impossible for the additives to be deposited or for incrustations to form. The constant composition of the bath can be maintained more easily in this way. The tapered bottom of the bath has such an angle that deposition of scale particles is prevented. The bath is filtered and cleared of particles of scale in a bypass. Only a quantity of water equal to the evaporation loss is added in the bypass. Means are also advantageously provided for reguiating this process so that the constant composition of the bath is ensured.

In addition, condensation surfaces above the quenching bath are instrumental in maintaining the constant composition of the bath. The condensate deposited on these condensation surfaces is conveyed back to the bath. The vapour which does not condense on these surfaces is removed by suction, cooled and the condensate which is formed as a result is likewise returned to the bath. The additives which are removed with the material to be hardened are returned to the bath continuously or even in an intermittent operation, triggered by a concentration control system.

Figure 2 shows a hardening plant which has these features and is described below. The quenching bath 101, which has the double jacket 102 filled with thermal oil, is tapered at the bottom. A heating coil 103 encloses the said bath. Measuring instruments for monitoring and regulating concentration, density and temperature 104 and for monitoring and regulating the liquid level 105 are provided. The introduction of additives can be effected both manually and automatically and is not included in the diagram. The water is fed in via a pipe 106 regulated by a three-way valve 107 which allows some of the return water to be fed to the storage tank via pipe 108. The return water can be brought to the desired temperature by an additional heater 114 which is regulated by the temperature controller 109.

The jet pump 111 causes a bypass stream, driven by the stream of return water, to be extracted from the quenching bath 101 via valve 112 and forced through the filter 110 together with the return water which is driven by pump 11 3. The valve 11 5 and orifice gauge 11 6 permit regulation of the water inflow to the storage tank 501 or quenching bath.

Above the quenching bath there are provided condensation surfaces (not shown) which return the condensed water directly to the quenching bath. The vapour which has not condensed is collected by the extractor funnel 201 and condensed in the cooler 202. The condensate is used for inundating and therefore for washing the coils in the bath 301.

With the aid of the pump 304, the water is fed to the washing tank 401 via valve 302 and via the filter 305 and pipe 306, the first wash of the coils being effected by immersion in the said washing tank.

This water is fed via the valve 403 by the pump 404 and via the filter 405 to the quenching bath 101 or to the storage tank 501.

The diagrammatic view in Figure 2 can only give a general impression of the hardening plant and modifications are conceivable without departing from the basic concept of the invention. Thus, it is easily possible, for example, to combine or connect the individual parts of the plant in such a way that in no case is a second source of energy required during operation.

The method according to the invention and the apparatus for carrying out the said method can be used, for example, for hardening steel coils. However, other forms of material for hardening, such as rods or forgings, can also be advantageously hardened by using the new method in the plant described. Moreover, the invention can also be applied to non-ferrous metal alloys which have to be quenched by a high solution temperature.

Claims (7)

1. A method of hardening steels or other metal alloys which comprises quenching with an aqueous solution containing sodium borate in an amount of 20 to 50 wt. parts per 100 wt. parts of water, and wherein hardening takes place at the boiling point of the solution.

2. A method according to Claim 1, wherein 1 to 30 wt. parts of sodium sulphate per 100 wt. parts of water are added to the quenchant.

3. A method according to Claim 1 or Claim 2, wherein 0.1 to 20 wt. parts of glycerine per 100 wt.

parts of water are also added to the quenchant.

4. Apparatus for carrying out the method according to any one of the preceding Claims which comprises a quenching bath containing said aqueous solution, cooled condensation surfaces above the quenching bath and an extractor funnel above the bath connected to an adjacent cooler for collecting and condensing vapour from the bath.

5. Apparatus for carrying out the method according to any of the preceding Claims which comprises a quenching bath containing said aqueous solution, the quenching bath being provided with an outer jacket filled with thermal oil and with a heating oil, and being tapered at the bottom.

6. A method according to Claim 1 substantially as therein described with reference to Figure 2 of the accompanying drawings.

7. Apparatus according to Claim 4 or Claim 5 substantially as herein described with reference to Figure 2 of the accompanying drawings.