GB2075396A - Brazing and subsequent heat treatment of steel - Google Patents

Description

SPECIFICATION Brazing and subsequent heat treatment of steel This invention relates to brazing and subsequent heat treatment of steel such as soft steel.

It has been usual hitherto that in the manufacture of steel articles requiring brazing, the steel has been heated to a temperature of about 1 100--12000C for effecting a copper solder brazing treatment, using a converted gas, such as a decomposed ammonia gas, as an atmosphere in the furnace, and thereafter the steel has been gradually cooled by furnace cooling to a normal temperature. This whole cycle is illustrated by the full line curve as continued by the dotted line curve a in Figure 1 of the accompanying drawings, in which temperature in OC is plotted as ordinate against time in minutes as abscissa.This heat treatment results in a product which is lower in mechanical properties such as fatigue strength, bending strength and others than the untreated steel before the brazing treatment, and also the product is comparatively weak in tensile strength at its soldered portion. Therefore, to obtain a product having the required mechanical properties after the heat treatment, it has been necessary that a special shape or configuration of the material to be treated is designed, for example the thickness thereof is increased, or that a raw material higher in mechanical properties by one rank is used, and thus this has íed to such inconveniences as increase in weight of the product, rise in cost, and others.

According to the present invention there is provided apparatus for brazing and subsequent heat treatment of steel such as a soft steel, the apparatus comprising a heating furnace for heating for brazing a steel such as a soft steel having solder applied at joining portions to a temperature above the melting point of the solder in an oxidation-free atmosphere, a furnace cooling section for furnace cooling the steel after this heating for brazing to a temperature in the range 720-5700C which is below the A, transformation point of the steel in an oxidation-free atmosphere, a rapid cooling section for rapidly cooling the steel after the furnace cooling, and a conveying means for conveying the steel through the heating furnace, the furnace cooling section and the rapid cooling section.By utilising this apparatus in this way there can be obtained a steel material which after being subjected to the brazing treatment is large in bending strength at its soldered portion and is.extremely improved in mechanical properties as compared with the untreated material before the brazing treatment.

For a better understanding of this invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 to which reference has already been made, shows, in addition to the known heat treatment curve, a heat treatment curve illustrative of use of the present apparatus, Figures 2 and 3 are graphs showing mechanical properties of a product treated in the present apparatus, of material treated in the known manner, and of untreated material, Figure 4 is a graph showing relationship between a treatment temperature and a tensile strength at a brazed portion both for material treated in the present apparatus, and for material treated in the known manner, Figures 5 and 6 are graphs showing relationships between treatment temperatures and bending properties for both material treated in the present apparatus and material treated in the known manner, Figure 7 is a diagrammatic view of one form of apparatus for brazing and subsequent heat treatment of steel, and Figure 8 is a diagrammatic view of another form of such apparatus.

The present apparatus will be described with reference to a rear fork for a motorcycle, which is made of a steel selected from steels including C in a range of 0.920.25%. This form has applied at its joining portions a copper solder and is introduced into a preliminary heating chamber within a furnace having an oxidation-free atmosphere such as an atmosphere of a converted gas, in which chamber it undergoes a preliminary heating. The fork is then transferred to a main portion of the heating furnace where it is heated to a temperature of 1 100--1 500C for 2-3 minutes, so that the solder becomes molten and flows to fill the joining portions. Next the fork is introduced into a furnace cooling chamber of water jacket type where it is subjected to furnace cooling.The furnace cooling is carried out for a short period and is stopped when the material reaches a desired temperature of 720570OC, which is below then, transformation point in equilibrium phrase diagram of the material. Then, the material is introduced into a rapid cooling chamber wherein it is subjected to a rapid cooling treatment, for example by dipping into water, to complete the treatment. This cycle of heat treatment in one specific example is as shown by solid lines throughout in Figure 1 , the rapid cooling being effected from 6000C.

For comparison, the hardness of steels selected from steels including C within the range indicated above was measured prior to treatment, after treatment in the known manner discussed above (and illustrated by the full line curve as continued by the dotted line curve as in Figure 1), and after treatment in the present apparatus. The results of these measurements were as shown in the following Table 1.

TABLE 1 SPCC SPCE STKM13A 0.07%C 0.06%C 0.10%C Material prior to treatment H.B. 63 H.B. 59.5 H.B. 67 Material after treatment in known manner H.B. 43.5 H.B. 48 H.B. 42 Material after treatment in present apparatus H.B. 69.5 H.B. 65 H.B. 69.5 As is clear from the above Table, the product of the known heat treatment is of lower hardness than the untreated material before treatment, while the product obtained by the present apparatus is improved in hardness as compared with the untreated material and is significantly improved as compared with the product of the known treatment.

Other mechanical properties of material treated in the present apparatus are shown by curves a in Figures 2 and 3, in which Figures, for the sake of comparison, the corresponding mechanical properties of material treated in the known manner and those of untreated material are shown by curves b and c, respectively. In Figure 2 stress in kg/mm2 is plotted as ordinate against strain as abscissa, whilst in Figure 3 load in kg is plotted as ordinate against the vertical deflection at the point of load in mm. The excellence of the material treated in the present apparatus as compared with material treated in the known manner, or untreated is clear from these Figures.

As compared with the known treatment, not only is the treated material obtained improved in various kinds of mechanical properties of the material in itself, but also the portions thereof joined by the solder are extremely high in binding strength. This is shown in Figure 4 which is concerned with the furnace cooling part of the present apparatus in a case where butt jointing portions (0.05 mm gap) of SPCC steel of 20 mm thickness are jointed together by a copper solder. In'the Figure tensile stress in kg/mm2 is plotted as ordinate against heat treatment temperature in OC as abscissa. The tensile stress achieved by the known treatment in which the furnace cooling is not effected is shown at the left-hand side of the Figure.From this Figure it will be seen that a product having a large tensile strength capable of resisting a tensile force of above 30 kg/cm2 can be obtained by treatment in the present apparatus, whereas the product of the known treatment only has a resistance to about 25 kg/mm2 at the maximum.

The fact that in the treatment in the present apparatus the steps, after heating for brazing, of furnace cooling to 720-5700C and then rapid cooling, are especially significant is demonstrated in Figures 5 and 6. Figure 5 relates to comparisons obtained by pipe bend tests carried outforfinding a load value at the time when deflection becomes above 50 mm, in respect of untreated material (extreme left of the Figure); material treated in the known manner (left of the Figure); material treated in the present apparatus including furnace cooling to a temperature within the temperature range indicated above (major part of the Figure); and material furnace cooled from a temperature above that range (extreme left of the Figure).In the Figure tensile stress in kg/mmt is plotted as ordinate against the heat treatment termination temperature in OC as abscissa and it can be seen therefrom that a product extremely large in bending strength can be obtained by the treatment in the present apparatus.

Figure 6 illustrates, for the same conditions as Figure 5, the relation between load deflection movement in mm (plotted as ordinate) and it can be seen therefrom that a product which is remarkably excellent in bending strength against loads as large as above 200 Kg can be obtained by the treatment in the present apparatus.

It is to be noted that the gradient of the rapid cooling is so sharp if it is carried out from a temperature above 7200C that control of the rapid cooling becomes difficult and strain of the treated material and that of the soldered portion thereof become large, and thus this is not desirable.

It has been found that a treated material having extremely excellent mechanical properties can be obtained if a steel of 0.10.8% in C content is furnace cooled from any desired heat treatment temperature for brazing, that is, usually from a temperature of 11 200C (that is within the range 11 000C to 11 500C mentioned above) down to a temperature of 6500C (that is within the range 720 to 5700C also mentioned above) within a comparatively short period, that is, usually between 10 to 5 minutes, and is then rapidly cooled.- From many experiments carried out for various steels containing various amounts of C, it has been learned that any steel containing C within a range of 0.020.25% provides especially appropriate tensile strengths below 80 kg/mm2, but that steels containing C above this amount are liable to become too large in hardness.

The rapid cooling treatment is carried out by, for example, dipping the material in a coolant liquid such as water, or the coolant liquid is poured onto the material, of the material is passed through a gaseous coolant atmosphere, and it is preferably that, in the case of liquid, the liquid is kept in a liquid temperature range of 5 to 450C.

When the material to be treated is to undergo a ground coating treatment it is advantageous that a ground coating liquid such as phosphate solution or the like is used as the coolant liquid so that the ground coating is effected simultaneously with the cooling treatment.

The form of apparatus will now be described with reference to Figure 7, in which numeral 1 denotes a conveyor arrangement through which trays 2 are conveyed, in order, intermittently, tray by tray first along an upper run of the conveyor arrangement from left to right (as indicated by an arrow) by a pusher. At the left-hand end the conveyor arrangement is provided with an elevator device 1-a for lifting the trays 2 from a lower run of the conveyor arrangement, whilst at the right-hand end there is another elevator device 1 -b for lowering the trays 2 back down to the lower run, there being at this end another pusher 1 for returning the trays to the left-hand end. A lengthy heating furnace 3 is provided over the upper run extending longitudinally therealong.A left-hand portion of this furnace is formed as a preliminary heating chamber 5. The furnace is heated by electric resistance heaters. A gas conduit 4 for introducing into the furnace 3 any desired oxidation-free gas such as any converted gas such as decomposed ammonia gas, nitrogen gas or reducting gas is connected to the heating furnace 3 so that the preliminary heating and the heating for brazing the material to be treated may be effected under the oxidation-free atmosphere. A furnace cooling chamber 6 of water jacket type is provided connected to the right-hand end of the heating furnace 3 via a shutter arrangement 7 alternately opening and closing the furnace 3 and the cooling chamber 6 in conjunction with tray feeding. Numerals 8, 8 denote gas exhaust pipes, the furnace cooling of the material to be treated also being effected under the oxidationfree atmosphere.A cooling chamber 9 is connected to the right-hand end of the furnace cooling chamber 6, a cooling tank 10 containing a coolant liquid such as water being provided below the chamber 9. A gas inlet pipe 11 for introducing an oxidation-free gas and a gas exhaust pipe 8 are connected to the interior of the chamber 9, and there is provided at the right-hand end thereof a shutter arrangement 12 that is opened and closed in conjunction with tray feeding. Thus, the material to be treated is subjected to a cooling treatment under the oxidation-free gas atmosphere. Numeral 1 3 denotes an elevator device whereby each tray 2 received thereon is raised and lowered so that the material within the tray 2 is dipped for a predetermined time in the coolant liquid to undergo a rapid cooling treatment.Numeral 14 denotes a supply tank having a pump 1 5 connected to the coolant liquid tank 10 so that the liquid within the tank 10 may be partly replaced by a fresh liquid, as occasion demands, so that the liquid temperature thereof can be kept preferably at 5-450C. If any other coolant liquid for ground coating, such as aqueous phosphate solution is used instead of water, a subsequent coating step, that is, a phosphate pickling step can be omitted.

In a particular use of the apparatus a rear fork for a motorcycle made of STKM 1 3A steel containing C below 0.25%, having applied at its respective joining portions a hard solder such as copper solder, is placed on each tray 2, and a number of trays so loaded are fed one by one into the heating furnace 5 by being pushed intermittently along the upper run from left to right. Each rear fork is thus moved toward the centre of the furnace after subjection to a preliminary heating in the preliminary heating chamber 5, and at the centre is subjected to heating of 1100-11 500 C, in the particular case under consideration to a maximum of 11 200 C, for 2-3 minutes, whereby the solder is made molten and flows to fill each joining portion.After this heat treatment to effect brazing, the rear fork is conveyed to the furnace cooling chamber 6 to be furnace cooled. The furnace cooling is effected until a temperature within the range 720--5700C, in the case under consideration 6500C is reached, the temperature reached being below the A1 transformation point in equilibrium phase diagram, thermal deformation being small.

Then, the rear fork, furnace cooled to this temperature, is further conveyed intermittently to be introduced into the rapid cooling chamber 9, where it is rapidly cooled by being dipped, for 30-60 seconds, in the coolant liquid tank 10 under the oxidation-free atmosphere, each tray being lowered by the elevator device 1 3 and after completion of the rapid cooling treatment, elevated again to the original position. When it has been so elevated again, the shutters 7, 1 2 are opened and the tray 2 is moved out outside the chamber 9 and at the same time a fresh tray 2 is introduced into the chamber 9. After the shutters 7, 12 have again closed, the elevator device 1 3 is operated to repeat the immersion treatment.

The so-far treated fork is taken out from the tray 2 and moved to the next treatment stage whilst the emptied tray 2 is lowered to the lower run by the elevator device 1 -b and at this position is supplied with a fresh fork to be treated. It is then conveyed to the left-hand end and is elevated to the upper run by the elevator device 1 -a, and the operations described above are repeated.

If, in the rapid cooling treatment, a ground coating liquid such as an aqueous phosphate solution, for instance is used, then simultaneously with the rapid cooling treatment of the material, a coating can be formed on the surface thereof at a reaction temperature during the cooling progress. As occasion demands, the aqueous phosphate solution is partly or wholly replaced by a fresh liquid in the supply tank 14, whereby adjustment in temperature, liquid density, PH value and so on can be made. The rapid cooling can be also carried out, instead of by dipping, by pouring or spraying. In this case, instead of the liquid tank, liquid is supplied from a spreading or spraying apparatus provided on the upper surface of the chamber 9. Alternatively, the rapid cooling can be effected by projecting a gaseous agent in the chamber 9. In the foregoing heat treatment, the furnace cooling from 11 200C to, say 6000C is effected in 6-7 minutes.

Figure 8 shows another form of apparatus. At the inlet and outlet openings for the material there are provided, instead of the shutters described above, gas curtain producing devices 1 6, 1 6 producing an oxidation-free gas, and the cooling tank 10 is provided in such a manner that the surface of the liquid contained therein is overlapped or covered partly with the gas curtain flow from the right-hand end outlet, so that the material can be introduced from the furnace cooling chamber 6 in the cooling tank 10 without being exposed to the external air in order to undergo rapid cooling treatment or rapid cooling and coating treatments. The trays 2 are conveyed continuously by an endless belt conveyer 1 so that a continuous treatment may be carried out. Numeral 1 7 denotes a belt conveyer for taking out the products after dipping.

Claims (2)

1. An apparatus for brazing and subsequent heat treatment of steel such as a soft steel, the apparatus comprising a heating furnace for heating for brazing a steel such as a soft steel having solder applied at joining portions to a temperature above the melting point of the solder in an oxidation-free atmosphere, a furnace cooling section for furnace cooling the steel after this heating for brazing to a temperature in the range 720-5700C which is below the At transformation point of the steel in an oxidation-free atmosphere, a rapid cooling section for rapid cooling the steel after the furnace cooling, and a conveying means for conveying the steel through the heating furnace, the furnace cooling section and the rapid cooling section.

2. An apparatus for carrying out brazing and subsequent heat treatment of steel such as a soft steel, substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

2. An apparatus as claimed in claim 1 wherein the rapid cooling section comprises a dip tank or pouring or spraying means for a ground coating treatment liquid.

3. An apparatus for carrying out brazing and subsequent heat treatment of steel such as a soft steel, substantially as hereinbefore described with reference to Figure 7 or Figure 8 of the accompanying drawings.

New claims or amendments to claims filed on 25.6.81.

Superseded claims 1-3.

New or amended claims:

1. An apparatus for brazing and subsequent heat treatment of steel such as a soft steel, the apparatus comprising a heating furnace for heating for brazing a steel such as a soft steel having solder applied at joining portions to a temperature above the melting point of the solder in an oxidation-free atmosphere; a furnace cooling section for furnace cooling the steel after this heating for brazing to a temperature in the range 720-5700C which is below the A1 transformation point in equilibrium phase diagram of the steel in an oxidation-free atmosphere; a rapid cooling section comprising a cooling liquid tank for rapidly cooling the steel after the furnace cooling; and a conveying means for conveying the steel through the heating furnace, the furnace cooling section and the rapid cooling section; the heating furnace and the furnace cooling section being between gas curtain producing devices, one of which is disposed such that the surface of cooling liquid in the cooling liquid tank is overlapped or covered partly by the gas curtain flow from this device.