NO141786B - PROCEDURE FOR STAINLESS STEEL COOLING - Google Patents

Description

The present invention relates to a method for cold working stainless steel, and more specifically a method for wire drawing and cold forging of such steel.

Procedures for cold working stainless steel, and in particular for wire drawing and cold forging of this material, are particularly difficult to implement as they require the use of drawing and forging tools, namely drawing irons and matrices, respectively, of tungsten carbide.

Stainless steel adheres very easily to the carbide base

of the formation of microwelds, and to prevent this adhesion, which can take place very quickly, lubrication is necessary. As the lubricants that are currently used for this purpose, particularly calcium and zinc stearates, do not adhere to stainless steel, it is necessary that the steel is first coated in order to be able to retain and pull the aforementioned lubricants.

Coating methods, such as pre-coppering, phosphating, oxalating, lime treatment and pre-leading, are used in the production of "machine wires" of stainless steel that are produced by hot rolling and are used in cold working processes, but these methods do not make it possible to carry out the subsequent wire drawing or cold forging operations under particularly satisfactory conditions, namely because the coatings achieved are either expensive, such as with pre-coppering,

fragile, such as in the case of phosphating, or dangerous, such as in the case of lead.

Furthermore, they must have stainless steel wires that have been surface treated

in the above-mentioned processes, after a small number of passes through the tools, the coating is freed, which has already become ineffective and has been subject to tarnishing, and a further surface treatment must then be carried out in order to be able to continue the manufacturing process;

which leads to very expensive facilities and treatment processes.

It is also known per se to use lubricants containing carbon fluorides when cold working stainless steel. German publication no. 2,016,040 thus specifies a lubricant which consists of a base material which can be oil or fat, and carbon fluoride with the general formula (CF) n. However, this lubricant has too poor adhesion to the metal surface to be lubricated and is thus not satisfactory for the present purpose.

Furthermore, it is from the magazine "Wire" for November 1963,

pages 1696 - 1700 as well as 1755 and 1756, known to use tetrafluoroethylene polymer as a lubricant when drawing stainless steel. This material has the disadvantage that as a coating on drawn threads it is difficult to remove, either chemically (on

due to the material's weak chemical activity) or mechanically

(due to contamination of the abrasive). PTFE can

possibly used when wire drawing stainless steel, but hardly when cold forging followed by extrusion, as the PTFE coating will then be torn to pieces..

In the study of carbon fluids, however, according to the invention, a product with the formula CF^ has been arrived at,

in which the carbon used for the manufacturing synthesis is available as natural graphite, artificial graphite, activated carbon or coke, and x is a number between 0.4 and 1.1. These derivatives,

whose lubricating properties in and of themselves are known, have surprisingly been shown, unlike other lubricants, to have the special property of adhering to stainless "machine steel",

which is not hot-rolled and has only been subject to a surface treatment of very short duration. These lubricants make it possible to continuously carry out the numerous mechanical operations that it is necessary to perform on a metal to achieve

the desired end product, without having to carry out the expensive intermediate cleaning processes and repeated surface treatments of the threads in question, which is unavoidable with the hitherto known methods.

The invention thus relates to a method for cold working stainless steel in the presence of a lubricant containing fluorides of carbon, the distinctive feature of the method according to the invention being that a dry lubricant is used which contains or consists of a fluoride of carbon with the formula CF^, in which the carbon which used in the manufacturing synthesis consists of natural graphite, artificial graphite, coke or activated carbon, while x lies between 0.4 and 1.1.

Application of such CF, either alone or mixed with fasting

x

lubricants previously known for use in the cold working of stainless steel, and in particular calcium stearate and zinc stearate, as a lubricant in the so-called "soap box" which is placed in front of the first drawing iron as well as in front of a limited number of the drawing irons in the later drawing stages, have made it possible to carry out cold processing of stainless steel wires that have only been subject to a very limited pre-

continuous processing, which considerably reduces the amount of waste material during production.

In the method of the invention, it is noteworthy that

instead of the thread necessarily having to be passed through a lubricant before the passage through each drawing iron, as in the conventional methods, it is sufficient to lubricate the thread before the first drawing iron and then before every third or fourth successive iron, which clearly shows the adhesion and strength of the film of CFx obtained after the passage through the drawing irons whose "soap boxes" are filled with CFx or mixtures of this lubricant and conventional lubricants, such as calcium stearate and/or zinc stearate, the respective mixtures should contain at least 10% by weight of CF . The CF x products are marketed under the trade name "FORACARB" and are available as powdered solids with a granule size

of 1-80 yum and a density close to 2.70. The color of the products is brighter the higher they are fluorinated.

All carbon fluids CF x according to the above

definition can be used when cold working stainless steel,

but it has been shown that the compounds of this kind in which the carbon used for the synthesis is a moderately fluorinated artificial graphite, and in which x lies in the range 0.6 - 0.9,

are particularly suitable due to their remarkable frictional properties.

The use of such carbon fluoride as a lubricant does so

possible to carry out cold wire drawing under very satisfactory conditions, for the reason that one can continuously draw rust-

free steel wires which have previously been subjected to an oxalate treatment of the "flash" type, which means a be-

action leading to the formation of a continuous deposit of an iron oxalate film of a thickness of a few ^um on the wire.

10 or 15 continuous passes through tungsten carbide

tool makes it possible, by using the specified CFx as a lubricant, to draw a thread with the original diameter

6.6 mm down to a diameter of 1 mm.

The use of such a lubricant has also proven very beneficial in cold forging with the aim of producing stainless steel screws of large diameter and of various types. In this process, the original stainless "machine" undergoes

steel wire a preliminary treatment, which usually consists of a copper ring or possibly oxalate, and then a light wire drawing through a drawing iron in the presence of lubricants, usually calcium and/or zinc stearates, which reduces the original diameter of the wire by only a few hundredths of a millimeter. This mono-draw, which is usually called "skin-pass", aims to adjust the diameter of the wire and at the same time harden the wire. The process includes a subsequent up-. cutting the wire into pieces and then the forging itself, which takes place in tungsten carbide dies and generally includes the following steps: extrusion to set the diameter, roughing of the screw head, cutting out the head and cutting the threads.

With this method, the ends of the cut screw blanks are not lubricated, and this interruption in lubrication makes the deposited film on the side surfaces particularly fragile during the forging processes, which can lead to a very significant material loss, which can be as high as 10-40% by weight of the amount of steel used. Furthermore, the stainless steel can get stuck in the matrix, which often leads to the destruction of the relevant matrix, which is an extremely expensive production component.

However, using the specified CF x, preferably in its pure state, as a lubricant during cold forging makes it possible to obtain a well-adherent and very strong film, which results in a manufacturing process without play and without the risk of material sticking in the matrices.

The invention will now be illustrated in more detail by the examples given below, of which examples 1, 2, 4, 6, 7 and 9 indicate known methods as a basis for comparisons with the method of the invention.

Example 1: Wire drawing (comparison example).

A wire with a diameter of 6.5 mm and of the usual stainless steel grade Z 2 CN 10 - 18 which has been subject to a "normal" oxalation, i.e. has been covered by a continuous coating of iron oxalate with a thickness of approx. 1/100 mm by passing through a bath consisting of an aqueous solution of oxalic acid, iron (II) oxalate and iron (III) oxalate, as well as heating to a temperature of 65 - 70°C for 12 min., thread is drawn with a speed of 1.75 m/sec. by passing through a bench with 6 drawing irons, whereby a wire with a diameter of 2.9 mm is obtained, as a "soap box" filled with calcium stearate as a lubricant is placed in front of each drawing iron.

After six passes, the surface coating of oxalate is partially destroyed, and in order to be able to continue drawing the wire, it is necessary to subject the wire to a subsequent deoxalation treatment and then a hyper-hardening, which consists of heating the metal to a temperature above 1000° C and an immediately subsequent quench in cold water, followed by a further oxalation in the same way as the original..

The threading is then resumed, and by leading the thread along

a speed -of 1.50 m/sec. through a bench with four drawing irons, whose "soap boxes" are filled with calcium stearate, a wire with a diameter of 1.8 mm is obtained.

At this point the surface feature is again destroyed, and the deoxalation, hyperhardening and oxalation treatment described above must be carried out again to enable the desired wire of 1.2 mm diameter to be obtained by finally feeding the wire at a speed of 1 .5 m/sec. through a bench with four drawbars and "soap boxes" filled with calcium stearate in front of each box.

Example 2: Wire drawing (comparison example).

A thread of the same type as in example 1 is coated with a discontinuous layer of oxalate with a thickness of a few µm in the so-called "flash" oxalation with a duration of 3 min., the treatment conditions below being the same as during the "normal" treatment according to example 1, but has a significantly shorter duration.

In the first wire drawing bench with six drawing irons each equipped with a "soap box" filled with calcium stearate, which in example 1 made it possible to reduce the wire diameter from 6.5 to 2.9 mm, the wire was torn and broke, making it impossible to complete the work process.

Example 3: Thread pulling.

A thread of the same kind as in Example 1 was produced by the same "flash" oxalation as indicated in Example 2, and was then fed at a speed of 1.75 m/sec. through a bench with six drawing irons, where the "soap boxes" assigned to drawing irons 1 and 4 were filled with pure CF with the formula CF 0. At the exit

f x U, o

from this bench, the thread, whose diameter had been reduced to 1.9 mm, was passed through another bench, where, at a speed of 1.50 m/sec. was passed through four draw bars, whose "soap boxes" at the first and third draw bars were filled with the same CFx as in the first bench. By this process it was achieved

a wire with a diameter of 1.8 mm, which can be fed directly into a third bench with four drawing irons, of which the irons 1 and 3, as in d,e above mentioned benches, are lubricated with CFq g, which directly leads to a wire of diameter 1.2 mm.

It will be noted that due to the specified lubrication with carbon fluoride CFx, it was possible to reduce a wire of the said steel type Z 2 CN 10 - 18 with an original diameter of 6.6 mm to a diameter of 1.2 mm by successive passes through drawing benches, without it being required to carry out the repeated application of overcoating film which was necessary in example 1,

where the lubricant used was calcium stearate.

Example 4: Wire drawing (comparison example).

A thread of the stainless steel grade Z 2 CND 18 - i2, which

i.e. a quality with greater strength than the threads used in examples 1 - 3 and thus more difficult to thread, was produced in a conventional way by normal oxalation as described in example 1..

In order to achieve a wire diameter of 2.18 mm, it was necessary

to pass this wire in sequence through a drawing bench with four drawing irons whose "soap boxes" are filled with zinc stearate, thereby reducing the diameter to 3.5 mm, and then, when over-

the surface was broken, to rebuild this by oxalating, hyperhardening and renewed oxalating, as indicated in example 1, before the wire was passed through another drawing bench with four drawing irons, which was lubricated with zinc stearate, which finally led to a wire with a diameter of 2 .18 mm.

Example 5: Thread pulling.

The same wire that was used in example 4 and has been subject to the same normal oxalation treatment as in examples 1 and 4, was passed through a drawing bench with four drawing irons, where the boxes at irons 1 and 3 were filled with 20% CFg g in calcium stearate. The wire leaving this bench had a diameter of 3.5 mm and its surface coating was not broken. It could then be directly transferred to another drawing bench with four drawing irons, all of which were lubricated in the same way as in the first bench and were capable of producing a thread with a diameter of 2.18 mm.

Again, the use of a carbon fluoride CPx as a lubricant made it possible to carry out a wire drawing by guiding the wire without interruption through two benches with drawing irons, without any intermediate treatment being carried out to rebuild the surface coating of the wire, which was necessary in the conventional method which was described in example 4.

Example 6: Wire drawing (comparison example).

This design example was carried out with a stainless steel

steel of the type Z 10 CN 30 - 10, which is a very brittle steel, which is considered by all experts to be very difficult to draw.

A 6.1 mm diameter wire of this steel, which had been treated by normal oxalating as in Example 1, was introduced into a first wire drawing bench, the "soap boxes" of which were filled with calcium stearate, causing the wire to tear and break, so that continued threading became completely impossible.

Example 7: Wire drawing (comparison example).

A wire of the same type as in example 6 was subjected to a pre-treatment by immersion in molten lead. The thread was then pulled at a speed of 0.30 m/sec. led into a first drawing bench with two drawing irons, whose "soap boxes" were filled with calcium stearate.

On the output side of this bench, the wire diameter was reduced

to 4.95 mm, and when the surface coating was therefore destroyed, it was necessary to carry out a de-lead treatment and then hyper-hardening as well as a renewed lead application.

The wire was then fed into another drawing bench with two drawing irons, whose "soap boxes" were filled with calcium stearate, reducing the diameter to 4.10 mm.

Example 8: Thread pulling.

Thread of the same type as in example 6 was produced by the same normal oxalation treatment as in example 1. During the experiment, the thread was fed to a drawing bench with four drawing irons,

whose "soap boxes" were all filled with 20% CFQ g5 in calcium stearate. In this bench it was possible to obtain a thread with a diameter of 4.10 mm directly without intermediate treatment.

Example 9: Cold forging (comparison example).

A wire with a diameter of 13.66 mm of type stainless steel

Z 2 CN 18 - 10 was used for the production of TH screws with the dimension 14 x 80, which means screws with a diameter of 14 mm and a length of 80 mm.

Oxalation of the wire roll used in this work operation was clearly deficient, and when this wire was exposed to normal treatment, it stuck during the first forging operations, which were carried out after lubrication with mineral oil.

A passage through a drawbar fitted with a "soap box"

filled with zinc stearate makes it possible to achieve a small reduction in the wire diameter, but this does not improve the subsequent passage through the screw manufacturing bench.

Example 10: Cold forging.

The wire roll in example 9 was placed in a disposable drawing bench,

where the thread was subject to a very slight diameter reduction of a few hundredths of a millimetre, when using pure

CFQ g as a lubricant. This wire was then transferred to a screw manufacturing bench equipped with the necessary tungsten

carbide tools for the production of TH screws with dimensions 14 x 18. The production took place without difficulty and there was no sticking or tearing of the metal used at any time.

Example 11: Cold forging.

Four tonnes of wire with a diameter of 13.66 mm and of the stainless steel

steel type Z R CN 18 - 20, was subjected to a "flash" oxalation in the same manner as indicated in Example 2, before the wire was passed through a wire drawing iron fitted with a "soap box" filled with pure CF^ g,., whereupon the wire was used for the production of screws of varying lengths from 50 to 100 mm. The four tons of steel on display were processed without difficulty and without material-

spillage, while during normal production in a factory, where zinc stearate was used as a lubricant, a quantity of spillage appeared.

of approx. 40% under the same conditions for this type of steel, which is considered difficult to pre-process by cold forging.

Claims (4)

1. Procedure for cold working stainless steel in the presence of a lubricant containing fluorides of carbon, characterized in that a dry lubricant is used which contains or consists of a fluoride of carbon with the formula CF x, in which the carbon used in the manufacturing synthesis consists of natural graphite, artificial graphite, coke or activated carbon, while x lies between 0.4 and 1.1. 2. Procedure as stated in claim 1, characterized in that the lubricant used is a carbon fluoride CF x in which the carbon used for the manufacturing synthesis is artificial graphite, and the value x lies between 0.6 and 0.9. 3. Method as stated in claim 1 or 2, characterized in that the lubricant used is a mixture of CFx and a solid lubricant and with a content of at least 10% CF. x 4. Method as stated in claim 3, characterized in that the solid lubricant used in mixture with CFx is calcium stearate or zinc stearate.