GB2175611A - Processes for lubrication of metal surfaces - Google Patents

Abstract

In order to prepare for cold working a metal surface, generally an unreactive metal surface such as stainless steel, the surface is shot blasted with zinc or zinc alloy particles, is then given a phosphate conversion coating, and is then coated with lubricant.

Description

SPECIFICATION Processes for lubrication of metal surfaces When a metal surface is to be subjected to cold working, i.e., deformation, it is normal to provide a lubricant coating in order to facilitate the cold working.

When the cold working is to be conducted at a high reduction ratio it is generally desirable to modify the metal surface in such a way as to improve the performance of the lubricant, and in particular to improve the adhesion of the lubricant to the metal surface.

For surfaces of iron and many forms of conventional steel it is conventional to prepare the surface for the lubricant coating by applying a phosphate conversion coating.

Unfortunately it is difficult to obtain a satisfactory phosphate conversion coating on surfaces that are relatively chemically inert, for instance stainless steels, and so it has been necessary to find some other process for improving the performance of the lubricant coating when high reduction ratios are required.

It is known to use an oxalate coating on stainless steel, instead of the phosphate coating, but this is not entirely satisfactory and can result in galling during cold working at high reduction ratios.

It is also known to use a lubricating film that includes a resin coating, as this avoids the need for a conversion coating, but the resultant lubricant film tends to have rather poor adhesion to the metal, inferior heat resistance and poor lubrication properties.

Another disadvantage of existing processes is that the treatment of the surface, before the application of the lubricant, can be a rate determining or costly step. For instance the formation of an oxalate coating on the stainless steel can take 10 to 30 minutes at a temperature of 90"C or more.

It would be desirable to devise a process by which it is relatively easy to form on a wide variety of metal surfaces a phosphate coating that will serve as a good base for lubricant.

In the invention a metal surface is coated with zinc or a zinc alloy by shot blasting the surface with particles having a zinc or zinc alloy surface, a phosphate conversion coating is then formed by applying a conversion coating solution and a lubricant coating is formed on the phosphate coating.

The shot blasting with the zinc or zinc alloy particles results in the deposition of a film of the zinc or zinc alloy on the metal surface. If the surface is contaminated with metal oxide then this is cleaned from the surface by the shot blasting.

The particles used forthe shot blasting generally have a diameter of 0.2 to 1.1 mm. In general it is desirable for them to be as hard as possible, especially to promote the removal of oxide film. The particles can consist of zinc or zinc alloy but it is often preferred to use particles having a metal core coated by zinc or zinc alloy.

Preferred particles are prepared by coating zinc on to ferrous fine particles followed by heating so that the zinc coating is alloyed with iron in the ferrous particles. Suitable particles of this type are available from KK Sanpou under the trade name Z-lron Alloyed Shot (hardness 350 to 450).

The weight of the coating of zinc or zinc alloy that is formed on the metal surface as a result of the shot blasting is generally in the range 0.1 to 50 g/m2, often 5 to 20 g/m2.

The weight of the coating can be varied by varying the duration of the shot blasting or by varying the force of blasting.

The amount of blasting the shot is generally followed by spraying shots 10 to 150 mm in thickness.

Although a wide variety of metal surfaces can be subjected to the shot blasting, the invention is of particular value when the metal surface is of a material on which it is difficult or impossible to form a satisfactory phosphate conversion coating by convenient techniques. The invention is of particular value when the metal surface is of titanium, zirconium, nickel, cobalt or molybdenum or an alloy thereof, a stainless steel, Hastelloy, Incolloy, a bearing steel, a spring steel or a high speed tool steel.

The phosphate conversion coating can be formed by the application of a conventional acidic phosphating solution designed for forming a convention coating on a zinc surface. Typical solutions are acid phosphate solutions containing 5 to 100 g/l phosphate ion, 3 to 50 g/l zinc ion, 1 to 100 g/l nitrate ion and 0 to 20 g/l nickel ion, the solution preferably being formulated in the absence of accelerators such as sodium nitrite.

Application of the solution to the metal surface may be by spraying or immersion for a suitable period which is generally from 5 seconds to 10 minutes and at a suitable temperature which is generally from room temperature to 80"C. Thus it is noticeable that the coating conditions can be relatively fast and convenient, compared to the very slow coating conditions that were often required previously.

To accelerate further the formation of the phosphate coating it is often preferred to subject the zinc coating that is to be phosphated to a conditioning solution, generally a solution containing colloidal titanium.

Suitable conditioning solutions of this type are well known.

The phosphate coating may be subjected to a water rinse and may be dried or used without rinsing and/or drying.

A lubricant coating is applied. It may be of soap, lubricating oil or solid lubricant.

An alkaline soap treatment can involve immersing the metal surface in an aqueous solution containing 40 to 100 g/l of a fatty acid soap such as sodium stearate at a temperature of 70 to 90 C for 1 to 10 minutes followed by drying.

A metallic soap film can be formed by applying a metallic salt or salt mixture of a fatty acid, such as calcium stearate, barium stearate or zinc stearate, in the form of a powder or a dispersion in a solvent.

Lubricating oils that may be applied include both oils and fats and may comprise a synthetic oil or a mineral oil, for instance as a base oil that may include also extreme pressure additives such as sulphur compounds, phosphorous compounds or chlorine compounds.

Other solid lubricants that may be used include molybdenum disulphide, tungsten disulphide, fluorocarbon resin, graphite or wax, generally applied in the form of powder or a dispersion in a solvent.

Resin may be incorporated in order to improve the adhesion of the lubricant to the substrate, especially when the lubricant is a solid lubricant.

A particular advantage of the invention is that the surface texture of the phosphate coating provides an excellent base for the lubricant. This surface texture is probably due, at least in part, to the penetration of the blasting particles into the surface or their partial adhesion to the surface to form the zinc or zinc alloy coating.

The phosphate coating is formed of fine crystals. Its texture imparts good lubricant retention properties to it and the overall results of the combination of zinc or zinc alloy coating with the phosphate coating and the subsequent lubricant layer demonstrate a synergistic effect.

The coated metal surface can then be subjected to cold working operations such as wire drawing, cold forging, roll forging or tube drawing.

The following are some examples.

Example I A metallic zinc film weighing 15 g/m2 was formed on a pure titanium panel by shot blast spraying with zinc particles having a diameter 0.3 to 0.7 mm. The panel was then immersed for 10 minutes in an acidic zinc phosphate solution designed for coating zinc substrates. The particular solution used was formed of 90 g/l Bonderite (trade mark) L3664 and 20 gil Additive 4801, both from Nihon Parkerizing Co. Ltd. The solution was held at 62 to 680C. The resultant zinc phosphate coating had a coating weight of 9.8 g/m2.

The coated surface was water rinsed and then immersed for 5 minutes in a lubricant solution based mainly on sodium stearate and containing 70 g/ I Bonderlube (trade mark) 235 from Nihon Parkerizing Co.Ltd. The solution was held at 72 to 88"C. The coated surface was then dried. The resultant lubricant coating had a weight of 7 g/m2 as sodium soap.

The lubrication performance was evaluated using a Bauden type friction-wear testing machine at a load of 5 kg, a stroke length of 10 mm, at room temperature and using, as the pressure balls, a 5 mm steel ball made from material SUJ2.

As a comparison the process was repeated except that the shot blasing step was omitted and the phosphate coating was not formed.

The product of example 1 had a coefficient of friction of 0.08 and the number of sliding times on the machine prior to seizure or galling was 1,200. The product of the comparison had a coefficient of friction of 0.1 and the number of times of sliding was only 5.

Example 2 Fine particles of zinc-iron alloy coated metal (Trade name Z-lron Alloyed Shot, particle diameter 0.2 to 0.3 mm, manufactured by KK Sanpou) were sprayed by shot blasing for 5 minutes onto the surface of SUS 304 type stainless steel wire having a diameter of 2.3 mm. The wire was initially contaminated with oxide film but this was removed by the shot blasting and the wire acquired a coating of 10 g/m2 zinc-iron alloy.

The coated wire was then immersed for 5 minutes in a zinc phosphate solution of 60 gll Bonderite 3663 and 20 g/l Additive 4801, at 75 to 80 C. The resultant zinc phosphate coating weighed 12 g/m2. It was water rinsed and dried.

A calcium stearate coating was applied prior to wire drawing by use of calcium stearate powder in the dye box. Drawing was conducted at a draw speed of 50 m/min with reduction ratios of 27.4% in the first pass and 20% in each of the second and third passes. The results were good, with no sign of galling even after the third pass.

As a comparison the process was repeated except that the shot blasting was omitted, and as a result the zinc phosphate coating was not formed. Galling occurred after a singie pass through the wire drawing machine.

As another comparison, instead of shot blasting and applying zinc phosphate, an oxalate coating of 9 g/m2 was formed by a process comprising pickling at room temperature for 10 minutes in a nitric acid-hydrogen fluoride solution, rinsing in tap water, applying an oxalate conversion coating using the material Ferrbond A (trade mark) at 35 gll,90 C, and 15 minutes, followed by rinsing in tap water and hot air blow drying at 100"C for 30 minutes. Galling occurred during the second pass.

Example 3 The same zinc-iron shot as in Example 2 was sprayed by shot blasting for 10 minutes on to the surface of an SCM3 bar having a diameter of 12 mm. As a result the bar acquired a coating of 15 g/m zinc-iron alloy. The bar was immersed for 5 minutes in a zinc phosphate solution as in Example 1 at 85"C, and the resultant zinc phosphate coating had a weight of 10 g/m2. It was water rinsed, treated at 75"C for 5 minutes with a lubricant solution containing 70 g/l Bonderlube 235 and subsequently dried. The resultant lubricant layer was formed of stearate soap and had a coating weight of 7 g/m2.

Evaluation of the lubrication performance of the bar was conducted by way of drawing using a draw bench. The drawing was conducted in accordance with the conditions as shown in Table 1 and the result was fairly good with no galling even passed 3 times and showed low drawing force.

Table 1 Drawing 17.8m/min speed Ratio of 1st pass: 12.0 10.0, Reduction Ratio 30.0% 2nd pass: 10.0 8.5, Reduction Ratio 27.7% Reduction 3rd pass: 8.5 7.0, Reduction Ratio 32.2% As a comparison, the process of Example 3 was repeated except that the shot blasting step was omitted and the conversion coating treatment was conducted for 10 minutes instead of 5 minutes, to give a zinc phosphate coating weight of 11 g/m2. After only 5 minutes treatment the coating weight had been 7 gim2 but microscopic examination showed that the coating was incomplete.

The lubricant performance was assessed in the same manner as in Example 3, and the results are shown in Table 2.

Table 2 Test Number of Drawing pass Drawing Force 1 st pass 3090 Example 3 2nd pass 2369 3rd pass 2339 1 st pass 3170 Comparison 2nd pass 2412 3rd pass 2420 This demonstrates the lower drawing force, and thus the superior lubricant properties, of the coating obtained in the invention and also demonstrates the quicker phosphating treatment that is required to provide this coating.

Claims (6)

1. A process in which a metal surface is coated with zinc or a zinc alloy by shot blasting the surface with particles having a zinc or zinc alloy surface, a phosphate conversion coating is then formed by applying a conversion coating solution, and a lubricant coating is formed on the phosphate coating.

2. A process according to claim 1 in which the metal surface is selected from titanium, zirconium, nickel, cobalt, molybdenum, alloys thereof, stainless steel, Hastelloy, Incolloy, bearing steel, spring steel and high speed tool steel.

3. A process according to claim 1 or claim 2 in which the particles are metal particles having a diameter of 0.2 to 1.1 mm and which are either formed of zinc or zinc alloy or are formed of metal coated with zinc or zinc alloy.

4. A process according to any preceding claim in which the particles are of iron coated with zinc or zinc alloy.

5. A process according to any preceding claim in which the lubricant coating is formed of a soap, lubricating oil or a solid lubricant.

6. A process according to any preceding claim in which the metal surface carrying the lubricant coating is subjected to cold working.