IE41504B1 - Process for the cold working of stainless steels - Google Patents

Abstract

1456192 Drawing; stamping PRODUITS CHIMIQUES UGINE KUHLMANN 23 July 1975 [24 July 1974] 30831/75 Headings B3P B3Q [Also in Division C5] In the cold working of stainless steels, are used as lubricants carbon fluorides that are of the formula CF x where x is between 0À4 and 1À1, and have been synthesized from natural graphite, artificial graphite, coke or activated charcoal. The carbon fluoride may be used alone or mixed with known lubricants, e.g. calcium stearate or zinc stearate. Processes referred to are wire drawing and cold stamping. The use of carbon fluoride avoids the need for repeated surface treatments between drawing and stamping steps.

Description

This application relates to a process for the cold working of stainless steels, and more particularly to a process for the wire-drawing and cold stamping of stainless steels.

Cold working processes for working Steel, particularly the operations of wire drawing and stamping, are operations which are particularly difficult to effect and which require the use of drawing and stamping dies of tungsten carbide.

IO Stainless steel seizes fairly readily on the carbide through the creation of microwelds, and in order to prevent this seizing, which may occur very quickly, lubrication is necessary However, since the lubricants used at the present time for these operations, which are mainly calcium and zinc stearates, do not adhere to stainless steel, it is necessary for the steel to be prior coated for the purpose of fixing and entraining the lubricants.

Coating techniques, such as coppering, phosphating, oxalating, liming, and leading, are used for preparing the stainless steel machine wires obtained by hot rolling which are used in cold working processes, but they do not make it possible for subsequent operations of wire drawing or cold stamping to be carried out under really satisfactory conditions, because the coatings obtained are expensive, as in the case of coppering, fragile, as in the case of phosphating, or dangerous as in the case of leading.

Furthermore,after a small number of passes through the dies the stainless steel wires superficially treated by these processes must be freed of the coating which ISO 4 has become ineffective and undergo annealing and further surface treatment in order to enable the processing to be continued, thus leading to very expensive installations and treatments.

In studying the properties of carbon fluoride, a product of the formula CF in which the carbon x, used for the synthesis of the CFX is natural graphite, artificial graphite, activated charcoal, or coke, and in which x is a number between 0.4 and 1.1, we have unexpectedly found that, unlike other lubricants, these derivatives, whose lubricating properties were known, have the particular characteristic of adhering to stainless machine steels which have been hot rolled and have undergone only a Surface preparation treatment of very short duration, and that they make it possible to carry out continuously the numerous operations of mechanical forming of a metal which is necessary to obtain the desired finished product, without having to carry out the expensive intermediate operations of cleaning and retreating the wires which are unavoidable in previously known techniques.

The use of CFx -either alone or mixed with the solid lubricants which it is known to use in the cold working of stainless steels and in particular with calcium stearate and zinc stearate- as lubricant contained in the soap box situated upstream of the first drawing die and then upstream of a limited number of dies in the wire drawing stages, has enabled us to effect the cold working of stainless steel wires which have undergone only a very limited preliminary treatment, while considerably reducing production rejects.

In our technique it is remarkable to note that instead of being obliged to pass the wire through a lubricant before passing through each die, as in the conventional technique, it is sufficient to lubricate it before the first die and then every three or four dies, which demonstrates the adhesion and strength of the film of CFx obtained after passing through the dies whose soap boxes are filled with CFX or with mixtures of CF , and conventional lubricants, such as calcium X stearate ajid/or zinc stearate, containing at least 10% by weight of CFX· The CFx products as marketed by us under the trademark Foracarb are in the form of pulverulent solids with a granulometry between 1 and 80 μ, with a density close to 2.70, while their colour is the lighter, the higher the degree of fluorination.

All carbon fluorides CFx can be used for the cold working of stainless steel, but we have found that any CFx in which the carbon used for the synthesis of the CFx is moderately fluorinated artificial graphite and in which x is between 0.6 and 1, and more particularly between 0.6 and 0.9, is particularly suitable because of its remarkable friction qualities.

The use of carbon fluoride CFX as lubricant makes it possible in particular to effect cold wire drawing operations under very satisfactory conditions, because it makes possible the continuous wire drawing of stainless steel wires previously subjected to oxalation treatment of the flash type, that is to say a treatment which leads to the formation on the wire of a discontinuous deposit of a ferrous oxalate film of a thickness of a few μ; - 4 41B0J or 15 continuous passes through tungsten carbide dies makes it possible, when CFx is used as lubricant, to bring a wire having an original diameter of about 6.6 mm into the form of a wire of a diameter of 1 mm.

The use of CF* as lubricant has also been found very advantageous for cold stamping operations for the purpose of reproducing stainless steel screws of large diameter and of various types. In this technique, by the conventional process the starting stainless machine steel wire undergoes preliminary treatment, which is usually coppering or sometimes oxalation, and then slight wire drawing through a die in the presence of lubricants, usually calcium and/or zinc stearates, which reduces the original diameter of the wire by a few hundredths of a millimetre. This monodrawing, generally called skin-pass, has the aim of adjusting the diameter of the wire while strain hardening it. The process then comprises the operations of cutting the wire’ into pieces and then the actual stamping, which is effected in tungsten carbide dies and which generally comprises the following succession of operations; extrusion for adjustment of diameter, rough formation of the screw head, cutting out the screw head, and forming the thread.

In this technique the ends of the elements of the cut-out screws are not lubricated, and this discontinuity of lubrication makes the film deposited on the side surface particularly fragile during the stamping operations, which results in very considerable wastage, which may be as high as from 10 to 40% by weight of the stainless steel used; furthermore, the stainless steel may seize in the die, thus not infrequently entailing the destruction of dies, which are particularly expensive parts.

The use of CFx, preferably in the pure state, as cold stamping lubricant makes it possible to obtain a very adherent and very strong film, thus resulting in production without wastage and without the risk of seizure in the dies.

The invention is now further described with reference to the following Examples in which Nos. 1, 2, 4, 6, 7 and 9 illustrate the known processes for the purpose of comparison.

Example 1- Wire Drawing A wire of the diameter of 6.5 mm of the ordinary grade Z 2 CH 18-10 stainless steel, which has undergone normal oxalation, that is to say has been covered with a continuous coating of iron oxalate with a thickness of about 1 hundredth of a millimetre by passing through a bath composed of an aqueous solution of oxalic acid, ferrous oxalate, and ferric oxalate and brought to a temperature of from 65 to 70°C for 12 minutes, is wiredrawn at the rate of 1.75 metres per second by passing through a bank of six drawing dies, thus producing a wire of a diameter of 2.9 mm, each die being preceded by a “soap box filled with calcium stearate as lubricant.

After six passes the surface coating of oxalate has partly deteriorated, and in order to be able to continue the wire drawing it is necessary to subject the wire in succession to de-oxalation treatment and then to hyperhardening, which consists in heating the metal to a temperature above 1000°C and then immediately quenching it in cold water, followed by further oxalation in the same way as the original oxalation. 41S04 The wire drawing is then resumed and by passing the wire at the rate of 1,50 metre per second through a bank of four dies, whose soap boxes are filled with calcium stearate, a wire of a diameter of 1.8 mm is obtained. At this point the surface coating has again deteriorated and all the treatments of deoxalation, hyperhardening, and oxalatlon described above must be carried out again in order to make it possible to obtain the desired wire of a diameter of 1.2 mm by finally passing it at the rate of 1.5 metre per second through a bank of four dies, whose soap boxes are filled with calcium stearate.

Example 2-Wire Drawing The same wire as in Example 1 is coated with a discontinuous layer of oxalate of a thickness of a few microns by so-called flash oxalation treatment for 3 minutes, carried out under the same conditions as the normal treatment of Example 1, but lasting only 3 minutes.

In the first wire drawing bank comprising the six dies having soap boxes filled with calcium stearate, which in Example 1 made it possible to reduce the diameter from 6.5 mm to 2.9 mm, the wire is scratched and breaks, making the continuation of the work entirely impossible.

Example 3-Wire Drawing The same wire as in Example 1 is prepared by the same flash oxalation as in Example 2, and then passed at the rate of 1.75 metres per second through a bank of six dies, the soap boxes of dies 1 and 4 being filled with a pure CF of the formula CF„ o. At the X U* Q - 7 41-S04 outlet of this bank the wire, which has been brought to a diameter of 2.9 mm, is introduced into a second bank, in which it passes at the rate of 1.50 metres per second through four dies whose soap boxes in the first and third dies are filled with the same CFx as in the first bank, thus producing a wire of a diameter of 1.8 mm, which can also be introduced directly into a third bank of four dies, in which dies 1 and 3 are lubricated as previously with CFq θ thus leading directly to a wire of a diameter of 1.2 mm.

It will be noted that because of the lubrication with carbon fluoride CFX it is possible to reduce to a diameter of 1.2 mm a wire of Z 2 CN 18-10 steel with an original diameter of 6.5 mm, by successive passes through three wire drawing banks, without it being necessary to carry out the reconditioning operations for the coating film which were necessary on two occassions in Example 1, where the lubricant used was calcium stearate.

Example 4-VJire Drawing A wire of stainless steel Z 2 CND 18-12, that is to say a quality having greater strength than that of the wires treated in Examples 1 to 3 and more difficult to wire draw, was prepared in the conventional manner by normal oxalation identical to that indicated in Example 1.

In order to bring this wire to a diameter of 2.18 mm, it is necessary to effect in succession a passage through a bank of four wire drawing dies whose soap boxes are i filled with zinc stearate and which reduces the diameter of the wire of 3.5 mm and there-upon, the surface coating having deteriorated, to restore it by the successive - 8 41604 operations of deoxalation, hyperhardening, and oxalation as indicated in Example 1, before passing it through a second bank of four dies lubricated with zinc stearate, which leads to a wire of a diameter of 2.18 mm.

Example 5-Wire Drawing The same wire as in Example 4, which has undergone the same normal oxalation treatment as in Examples 1 and 4, passes through a bank of four drawing dies, in which dies 1 and 3 are filled with a 20% CFQ g in calcium stearate. The wire leaving this bank has a diameter of 3.5 mm, its surface coating has not been spoilt, and it can be introduced directly into a second bank of four dies lubricated in the same way as the first bank, thus leading to a wire having a diameter of 2.18 mm.

Here again the use of a carbon fluoride CFx as lubricant makes it possible to effect the wire drawing by successive passage through two banks of drawing dies, without having to carry out the intermediate treatments for the reconditioning of the surface coating, as is necessary in the case of the conventional process according to Example 4.

Example 6-Wire Drawing This Example was performed with a stainless steel Z 10 CN 30-10, whioh is a very brittle steel acknowledged by all specialists to be very difficult to wire draw.

A wire of this steel, with a diameter of 6.1 mm, treated and prepared by normal oxalation as in the case of the wire of Example 1, introduced into a first wire drawing bank in which the soap boxes of the dies are filled with calcium stearate, is scratched and broken, making the continuation of the wire drawing quite impossible.

Example 7-Wire Drawing The same wire as in Example 6 is subjected to leading pre-treatment by dipping in molten lead. In the course of the test it is introduced at the rate of 0.30 metres per second into a first wire drawing bank comprising two dies whose soap boxes are filled with calcium stearate. At the outlet of this bank its diameter is 4.95 mm, and since its surface coating has been spoilt it is necessary to subject it to deleading treatment, and then to hyperhardening and releading.

The wire is then introduced into a second bank comprising two dies whose soap boxes are filled with calcium stearate, thus bringing its diameter to 4.10 mm.

Example 8-Wire Drawing The same wire as in Example 6 is prepared by the same normal oxalation treatment as in Example 1. In the course of an experiment it is introduced into a wire drawing bank comprising four dies whose soap boxes are all four filled with 20% CFQ θ5 in calcium stearate. This bank makes it possible to obtain a wire of a diameter of 4.10 mm direct.

Example 9-Cold stamping A wire of a diameter of 13.66 mm of stainless steel Z 2 CN 18-10 is Used to produce TH 14x8 screws, that is to say screws of a diameter of 14 mm and a length of 80 mm. The oxalation of the wire coil used for this operation is manifestly defective, and when subjected to the normal process this wire seizes during the first stamping operations carried out after lubrication with mineral oil. Passing through a monopass drawing die provided with a soap box filled with zinc stearate enables the diameter of this wire to be reduced slightly, but does 41604 not improve its subsequent passage through the screw production bank.

Example 10-Cold stamping The coil of wire of Example 9 is mounted on a monopass wire drawing bank in which the wire undergoes very slight reduction of diameter by a few hundredths of a millimetre, using pure CF0 g as lubricant. This Wire is then passed to a screw production bank equipped with the tungsten carbide dies necessary for the production of TH 14x80 screws. Production proceeds without difficulty and no seizing or scratching of the metal used is noted at any time.

Example 11- Cold stamping Four metric tons of wire of a diameter of 13.66 mm, of stainless steel Z 2 CN 18-10, was subjected to flash11 oxalation as in Example 2 before passing through a wire drawing die provided with a soap box filled with pure CFQ θ^, and1then being used to produce screws of a length varying from 50 mm to 100 mm.

The four metric tons of wire are treated without difficulty and without rejects, whereas in normal production in a workshop where zinc stearate is used as lubricant a proportion of rejects close to 40¾ is obtained under the same conditions for this grade of steel, which is considered as being difficult to work by cold stamping.

Claims (6)

1. CLAIMS:1. A process for the cold working of stainless steels, characterised by using as lubricant a carbon fluoride of the formula CF^ which has been synthesized from 5 carbon in the form of natural graphite, artificial graphite, coke or activated charcoal, in which formula x is between 0.4 and 1.1.

2. A process according to Claim 1, in which the lubricant used is a carbon fluoride CF X in which the carbon 10 used for the synthesis of the CF x is artificial graphite and in which x is between 0.6 and 0.9 .

3. A process according to either of Claims 1 or 2, in which the lubricant used is a mixture of CF x and a solid lubricant containing at least 10% of carbon 15 fluoride CF x .

4. A process according to Claim 3, in which the solid lubricant used in a mixture with CF x is calcium stearate or zinc stearate.

5. A process for the cold working of stainless steel 2q according to Claim 1, substantially as hereinbefore described with reference to any one of Examples 3, 5, 8, 10 and 11.

6. Cold worked stainless steel produced by the process of any one of Claims 1 to 5.