The present invention relates to the use of transition metal halogenides, especially FeCl ₃ , as lubricants at cold flow processing of stainless steel-

Cold flow processing of metals is a process by which the metal (often steel) is placed in a press tool, as for example a gear wheel die, after which the metal and the press tool are placed in a hydraulic press. By means of a piston a very high pressure is applied, whereby the metal passes its yield point and undergoes plastic deformation.

Typically, various types of process steel are used for the manufacture of workpieces by cold flow processing. Applica¬ tion of stainless steels has also begun lately, which results in finished workpieces of very high quality, but which in the processing presents great problems with cold welding.

For a long time various chlorinated, organic compounds have been used as lubricants in the manufacture of stainless steels, such as punching and sheet manufacture. The good properties of these lubricants in steel manufacture are based on the fact that they react with the iron in the steel alloy while forming FeCl ₂ as shown in the reaction scheme:

R-Cl _χ (1) + x/2 Fe (s) « XJ 2 FeCl ₂ (I)

where R-Cl _χ is the chlorinated organic compound, R being for example a mineral oil.

The good lubricating properties are due to the formation of FeCl ₂ , because especially FeCl ₂ is described as the EP (extreme pressure) lubricant that is generated under tribo- logic conditions (J. Yates et al. in "Fundamentals of

Friction": Macroscopic and Microscopic Processes, 313-322, edited by I.L. Singer and H.M. Pollock, Kluwer Acad. Publ. (1992)) . Other authors, too, have lately established that FeCl ₂ has good lubricating properties under extreme condi- tions (see, for example, P.V. Kotvis et al. in "Highly Chlorinated Methanes and Ethanes on Ferrous Surfaces": Surface Science Investigations in Tribology, ACS Symp. Series 485, Am. Chem. Soc. , Washington (1992)).

Such chlorinated organic solvents, however, are unsuitable for cold flow processing of steel, because the adhesion to the steel surface is very poor. This causes the lubricant to be pressed away during the process, which causes direct, metallic contact between the workpiece and the surface of the press tool. This direct contact causes cold welding with consequent extreme wear of the tool surface.

During the cold flow process, quite considerable surface expansion occurs, and steel surfaces which are not covered with lubricant will therefore be continuously exposed. The ideal lubricant must therefore be able to "flow with" the surface, so that the surface is continuously covered, or the lubricant must be able to react with the newly formed surfaces. The latter possibility is the more attractive one, because it ensures that there is no direct, metallic contact between the workpiece and the tool surfaces.

Among the most used lubricants today in cold flow process¬ ing of stainless steel are porous oxalate coatings with either a metallic soap or an aqueous solution of MoS ₂ absorbed in the pores. These lubricants are not optimal, however, since at large degrees of deformation and/or complicated geometries, decomposition of the lubricant film occurs with consequent cold welding problems.

An aqueous lubricant for cold flow processing of metals which contains both a solid lubricant such as MoS ₂ , a

metallic soap and a surface active agent, a colloidal titanium compound and water, is known from US patent publi¬ cation No. 5 116 521. The colloidal titanium compound is a solution obtained by neutralisation of a titanium-sulphu- ric-acid solution or a titanium-phosphoric-acid solution with a base, such as caustic soda or the like. The content of colloidal titanium in the aqueous lubricant is 10-5000 pp , preferably 50-3000 pp Ti, which is present in the form of negatively loaded, colloidal micelles. In extreme situations this lubricant is not optimum, either.

It has also been attempted to use other chemical substances as lubricants in connection with cold flow processing. Thus the US patent publications No. 3 978 702 and No. 3 992 303 describe a lubricant in the form of a film of a chlorine- containing, film-forming polymer or copolymer and a destabilising agent, which may be among other things a transition metallic salt. The chlorine-containing, film- forming polymer may be a polymer or copolymer of such monomers as vinyl chloride, vinylidene chloride, and epi- chlorhydrin. Chlorinated polymers may also be used such as chlorinated polyethylene and other chlorinated polyolefins. The transition metallic salt may be a halogenide, among others.

US patent publication No. 4 517 029 describes a cold flow process where the steel workpiece is first treated with a phosphating solution in order to form a phosphate film (Zn/Ca) , whereupon the film is coated with a lubricant of the soap type. The US Patent publications No. 4 983 229 and No. 5 234 509 disclose the use is known of phosphates in the form of aqueous solutions from which the phosphate is released to the metal surface by means of hydroxyla ine. However, these lubricants are applied only in connection with carbon steel, not in connection with stainless steel.

Finally, DE Patent specification No. 13 03 203 describes a lubricant containing one or more embedment compounds with layer-lattice structure, which compounds have been obtained by reaction of graphite, montmorillonite and/or MoS ₂ with FeCl ₃ , FeBr ₃ and/or CuBr ₂ .

All these known lubricants are complicated to produce and/or use, and their effects are not optimum under tribol- ogic conditions. Therefore there is still a need for find- ing lubricants which can be used with optimum results in cold flow processing of stainless steel.

Surprisingly, it has now proved that a transition metal halogenide, especially FeCl ₃ , in the form of a coating that is applied either in melted form or in the form of a satu¬ rated or diluted aqueous solution, is excellently suited as a lubricant in cold flow processing of stainless steel, and that such a lubricant results in lower friction than attainable by the often used oxalatation combined with soap lubrication.

The invention is based on the fact that the preferred transition metal halogenide FeCl ₃ reacts with free iron while forming FeCl ₂ according to the equilibrium:

2 FeCl ₃ + Fe ** 3 FeCl ₂ (II)

As stated above, FeCl has excellent lubricating properties under tribologic conditions, and since FeCl ₃ may react with newly formed surfaces, thereby preventing the direct, metallic contact between workpiece and tool, which is the cause of cold welding, FeCl ₃ is a suitable lubricant.

Anhydrous ferri(III)chloride, FeCl ₃ , is a strongly hygro- scopic compound with a layered crystal structure. When heated in an anhydrous environment, the compound decomposes into FeCl ₂ and Cl ₂ . In a chlorine atmosphere, FeCl ₃ melts

at approx. 280°C. FeCl ₃ can absorb up to six molecules of crystallisation water, whereby the melting point drops to 35-38°C.

Also other transition metal halogenides than FeCl ₃ can react with free iron and are therefore applicable according to the invention. Thus the transition metal may be chosen among Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Sn, and Pd, and the halogen may be fluorine, chlorine or bromine. Thereby a large number of transition metal halogenides become potentially applicable. Among these may be mentioned especially CrCl ₃ .6H 0, SnCl ₄ .4H 0, CuCl .2H 0 and FeBr ₃ ,6H ₂ 0.

At the described use as a lubricant, the transition metal halogenide is applied to the surface of the workpiece in two ways as mentioned above: Partly based on a melt, and partly based on an aqueous solution.

For example, a melt of FeCl ₃ .xH ₂ 0 (x = 0-6) may be applied to the previously cleaned surface of the workpiece by immersion or spraying. The resulting thickness of the coat of lubricant will be approx. 20-250 g/m ² depending on the temperature of the melt and the retention time. According to P.H. Hansen, N. Bay & P. Christensen, 16th NAMRC, 41-47 (1988), a friction factor of approx. 0.05 at approx. 200 g/m ² is attained (measured by a simple ring test as described by A.T. Male et al., J. Inst. Metals jn, 38-46 (1964-65)). Briefly described this test consists in deter- mination by a special procedure of the friction factor f between the tool and the metal workpiece. The procedure involves only measuring of the geometric change of a plane ring during the deformation process, but not measuring of the mechanical properties of the metal workpiece, and it is therefore suitable in connection with a large number of different test conditions, including evaluation of lubricants. The ring test also provides very reliable

determination of the frictional variation during plastic deformation, because a higher value of D (the reduction percentage of the internal diameter of the ring) yields a higher friction factor.

No seizing or pick-up occurs, and the wear of the tool surfaces is therefore minimal. The lubricants mentioned are also efficient in cup pressing, because wear and friction are less here than with traditional lubricants.

When using an aqueous solution of FeCl ₃ with a minimum concentration of 30% FeCl ₃ (w/w) , the layer thickness is typically 25-125 g/m ² . In this case a friction factor of 0,15 is obtainable (measured by the ring test described above) at approx. 39.5 g/m ² .

The invention is illustrated in more detail by the follow¬ ing examples.

Before being coated, all the workpieces have been degreased and cleaned. Seizing or pick-up has not occurred during the tests.

EXAMPLE 1

A cleaned ring of stainless austenitic steel was heated to 70°C and then immersed in a 70°C hot melt of FeCl ₃ .6H ₂ 0 for 5 seconds. The resulting layer thickness of the lubricating film was 204 g/m ² . The ring was subsequently subjected to a ring test, and the friction factor was found to be 0.01.

EXAMPLE 2

A cleaned ring of stainless austenitic steel was immersed in a 70°C hot melt of FeCl ₃ .6H ₂ 0 for 5 seconds. The result¬ ing layer thickness of the lubricating film was 175 g/m .

The ring was subsequently subjected to a ring test, and the friction factor was found to be 0.12.

EXAMPLE 3

A cleaned ring of stainless austenitic steel was immersed in a saturated, aqueous solution of FeCl ₃ at 50°C for 3 min. The resulting thickness of the lubricating film was 39.5 g/m ² . The ring was subsequently subjected to a ring test, and the friction factor was found to be 0.15.

The following examples 4-12 describe examinations of lubricant coatings of FeCl ₃ .6H ₂ 0 made on the basis of melts, aqueous solutions, and water/ethanol solutions of FeCl ₃ .6H ₂ 0.

EXAMPLE 4

A saturated, aqueous solution of FeCl ₃ .6H ₂ 0

Saturated, aqueous solutions of FeCl ₃ .6H 0 at 50°C were made by dissolving FeCl ₃ .6H ₂ 0 (in excess) in water and cooling the solution to 50°C. The rings were then brought in contact with the solution, and the retention time varied.

As it appears, the friction factor is relatively constant (f=0-18±0.03) , but also relatively high, which may be due to hydrolysis of FeCl ₃ .6H ₂ 0 into Fe(H ₂ 0) _χ (0H) _y Cl _z .

EXAMPLE 5

50% saturated solution of FeCl ₃ .6H ₂ 0 in water

The solution is made by diluting a saturated solution of FeCl ₃ .6H ₂ 0 which has been filtered at 50°C. The test temperature was 50°C, and the retention time varies.

In these tests the friction remains unchanged (f=0.18+0.03) in relation to the saturated solution, but it is seen that the layer thickness increases (to an average of 87 g/m ² as compared with 49 g/m ² in example 4) . It is also seen that the variation of the layer thickness is much greater when the solution is not saturated.

EXAMPLE 6

25% saturated solution of FeCl ₃ .6H ₂ 0 in water

The friction is seen to have increased in relation to the examples 4 and 5 (f=0.23 against f=0.18), whereas the layer thickness compared to example 4 is practically unchanged. Examples 4-6 show that the increasing dilution results in increased friction, probably owing to increased hydrolysis.

EXAMPLE 7

Saturated solution of FeCl ₃ .6H ₂ 0 in water/ethanol (50:50)

In order to have a saturated solution of FeCl ₃ .6H ₂ 0 with a lower concentration, a test has been made with a mixture of H ₂ 0 and ethanol. A saturated, aqueous solution of FeCl ₃ .6H ₂ 0 was filtered and diluted with ethanol. The test was carried out at 70°C with varied contact times.

A noticeable drop occurs in the layer thickness, which must be assumed to be the primary cause of the relatively high friction (f=0.32+0.03) , since there is a general tendency

towards increasing friction at decreasing layer thickness, However, hydrolysis may also contribute to the friction factors found.

EXAMPLE 8

Aqueous solution of CrCl ₃ and FeCl ₃ .6H ₂ 0

Since CrCl ₃ does not hydrolyse in an aqueous solution, tests have been made with addition of this compound. CrCl ₃ and FeCl ₃ .6H ₂ o were suspended/dissolved in water at 50°C, and the contact time was 60 seconds.

Ring No. Concentration of Layer thickness Friction

Fe/Cr (g/ml) (g/m ² ) factor (f)

1 0.5/0.07 42 0.25

2 II 51 0.18

3 0.5/0.3 62 0.16

4 II 66 0.16

In comparison with examples 4-6, addition of CrCl ₃ seems to cause decreasing friction.

EXAMPLE 9

Melt of FeCl ₃ .6H ₂ 0

FeCl ₃ .6H ₂ 0 is melted and heated to 70°C. The test is carried out in a dry atmosphere (glove box) in order to prevent the melt from absorbing moisture during the test, The retention time is varied.

In these tests both layer thickness and friction vary some¬ what, but there is a tendency towards reduction of the fric¬ tion when the layer thickness increases.

EXAMPLE 10

Melt of anhydrous FeCl ₃

In order to investigate the lubricating qualities of FeCl ₃ , four workpieces were prepared by vapour deposition of anhydrous FeCl ₃ in a sealed glass ampoule under an atmosphere of el-

The layer thickness was 140-180 g/m ² and the friction factor f=0.26+0.02. However, the shear stress of anhydrous FeCl ₃ is so high that the lubricant "compacts" during the pressing.

EXAMPLE 11

Melt of anhydrous NaFeCl ₄

A portion of anhydrous NaFeCl ₄ is prepared, because this com¬ pound can be melted without decomposing into FeCl ₂ and Cl ₂ . NaFeCl ₄ is melted at 240°C, and contact times of 5 and 30

seconds are used. After a contact time of 5 seconds the coating pealed off, and no pressing was therefore made.

The layer thickness varied from 200 to 400 g/m ² , and a fric¬ tion factor of f=0.25+0.02 was found. At the pressing the lubricant "compacted" considerably, which is due most likely to the non-layered structure of NaFeCl ₄ , or to the fact that the compound has an excessive shear yield point and therefore is not sufficiently deformable during pressing.

EXAMPLE 12

Melt of FeCl ₃ .6H ₂ 0 mixed with graphite

Tests were made with a melt of FeCl ₃ .6H ₂ 0 mixed with 10 per cent graphite by weight at 50°C. The immersion time was 10 seconds. The layer thickness achieved was between 20 and 55 g/m ² , and a friction factor f=0.14+0.02 was found.

EXAMPLE 13

A solid cylinder of austenitic, stainless steel was immersed in a 50°C hot melt of FeCl ₃ .6H ₂ 0. The resulting lubricating film was subsequently tested by cup pressing, and a height/diameter (h/d) ratio of 1,5 was found, which is better than the ratio obtained by prior art.

The test was repeated with a 70°C hot melt, and the same result was found.

EXAMPLE 14

The procedure from example 13 was followed with a 50° hot melt of FeCl ₃ .6H ₂ 0. Also here a h/d ratio of 1.5 was obtained.

EXAMPLE 15

The procedure from example 13 was followed with a 50°C hot melt of FeCl ₃ .6H ₂ 0 mixed with 10 per cent graphite by weight. A h/d ratio of more than 2 was obtained.

EXAMPLE 16

Melt of SnCl ₄ .4H ₂ 0

Tests were made with a melt of SnCl ₄ .4H ₂ 0 at 50°C. After a retention time of 5 minutes a layer thickness of 45 g/m ² was found, and a friction factor of f=0.30.

EXAMPLE 17

Saturated solution of CuCl

After a retention time of 5 minutes a saturated, aqueous solution of anhydrous CuCl ₂ at 60°C gave a layer thickness of 22 g/m ² and a friction factor of f=0.60.

EXAMPLE 18

Melt solution of CrCl ₃ .6H ₂ 0

Tests were made with a melt of CrCl ₃ .6H ₂ 0 at 50°C. After a retention time of 5 minutes a layer thickness of 200 g/m ² and a friction factor of f=0.37 were found.