A PROCESS TO FORM A SULPHIDE CASE AT THE SURFACE OF A METAL PART

Abstract: The present invention relates to a process and the corresponding equipment, with which a case of sulphide forms at the surface of metal part (5) by heating and gasifying sulphur, the metal part (5) is on the table (4) of cathode in a vacuum reacting chamber (1) of said equipment while a vapourizer (6) is provided in the same vacuum reacting chamber (1), the vapourizer (6) being heated with a low voltage supply (11) and used to hold solid sulphur. Under vacuum condition, solid sulphur will be gasified by heating and the gasified sulphur will be ionized and discharged with glow under the action of a direct electric field at high voltage between the anode (2) and cathode (4), under the glow discharge conditions, the positive sulphur ions will bombard the metal parts (5) which are located on the cathode table (4) and a sulphide case will be formed at the surfaces of parts (5).

Description: The present invention relates to a process for forming a sulphide case at the surface of a metal part or component, and more particularly relates to such a process used to form a sulphide case at the surface of metal parts by means of gas sulphurization. Sulphurization is one of the surface-treating procedures used for metal parts or components. Since a ferrous sulphide case can be formed by sulphurization and ferrous sulphide is a form of solid lubricant, then if there is a sulphide case between two contacting surfaces of friction components, the coefficient of friction between them can be reduced. Furthermore, a sulphide case may separate the underlying metal of two friction components from direct contact with each other and thereby provide the surface of the components with a fine resistance to wear and to seizure. Therefore, in recent years the study of sulphurization has been further developed and had great attention paid to it. Both solid and liquid sulphurization processes have been studied but both still have some disadvantages, such as a long processing duration, great energy consumption, unstable sulphurizing quality, and the production of only a small depth of sulphurized case. From them, a new gas sulphurizing technology - low temperature electrolytical sulphurization has been developed. This technology has been developed rapidly because it features a low treating temperature, less deformation, and the possibility of being able to carry out the sulphurizing process after other traditional processes such as carburizing, nitriding, low-temperature tempering and induction heating-quenching. There was a paper issued at the seminar of IFHT in 1978. The paper introduced a gas-sulphurizing process by means of nitrosulphurizing. Although with this process there is a sulphide case formed at the surface of the part, the constituents of the sulphide case are complex and do not comprise a single unitary sulphide such as ferrous sulphide or iron sulphide, which has a detrimental affect on the coefficient of friction and on the resistance of the part to wear and to seizure. Furthermore, it is difficult and complicated to control the proportion of the nitrogen content to the sulphur content in the case to a desirable and exact degree. Therefore, an uneven composition and unstable quality results from the process. The Huanan Polytechnic College has developed a sulphurizing process using a single gas instead of the said nitrosulphurization in which a sulphide is used as a medium and a special-purpose sulphurizing furnace is used to hold the temperature at 160°C or above. Here, a case containing ferrous sulphide (FeS) and iron disulphide (FeS₂) forms at the surface of the part (see "Heat treatment of metals" Vol. 5, 1983), but there are some disadvantages relating to this process. Because the medium is a sulphide, the sulphurizing atmosphere contains some other components beside sulphur, which has a detrimental influence on the quality of the case. Moreover, some sulphides are poisonous, such as H₂S and not only ferrous sulphide but also iron disulphide is formed in the process , the former being favourable but the latter being detrimental to the resistance of the part to wear. In addition to these disadvantages, the treating temperature used in the process is rather high. In recent years, a sulphurizing process involving direct gasifying of solid sulphur has been developed (see "Textil Transactions" Vol. 2, 1983; "Hubei Machinery" Vol. 4, 1981). With this process, the solid sulphur-containing gas is ionized under the action of an electric field so that the surface of the part is sulphurized. There is a comparatively satisfactory effect resulting from the use of solid sulphur as the sulphurizing medium and thereby the said disadvantages resulting from the use of a sulphide are overcome. But there are still some shortcomings. Firstly, the sulphur-containing gas is ionized under a low-voltage field condition (300-500 V). Secondly, the solid sulphur, which is loaded in a steel box, is also located on the cathode table along with the parts being treated so that it is difficult to control the sulphur-gasifying temperature and the gasifying quantity. Thirdly, the parts and the solid sulphur are heated by an inert gas discharge, the inert gas being pumped out from the vacuum furnace after the temperature has been raised to the desirable level in order to leave the sulphur-containing gas in the furnace, thus making the operation complicated and expensive. The process of the present invention is an improved and developed version of the solid sulphur-gasifying and sulphurizing process mentioned above. One of the objects of the present invention is to carry out a sulphurizing process in which the solid sulphur is directly heated to gasify it, which overcomes the shortcomings mentioned above and produces a better sulphurizing quality than that resulting from other processes with a uniform and thick case comprising a unitary constituent. According to the present invention there is provided a process for sulphurizing a metallic substrate held within a vacuum chamber, comprising the steps of: (a) evacuating the vacuum chamber, (b) vaporizing solid sulphur and then (c) subjecting said vaporized sulphur to an ionization environment within said vacuum chamber to create a plasma of ionized sulphur particles for sulphurizing the said substrate, wherein said sulphurizing is carried out under a high voltage electric field at a pressure of about 1 torr (133.3 Pa), and wherein said substrate is held at a temperature of between 140°C and 300°C, characterized in that (d) the solid sulphur is vaporized by means of a low voltage supply after the pressure has been lowered to about 10⁻² torr (1.3 Pa), (e) said sulphurizing is carried out in the absence of any gas inert or other substance, wherein the temperature of the metallic substrate is controlled through said high voltage electric field, and in that (g) the low voltage supply and high-voltage field are controlled independently from each other. The keypoint of the gas sulphurizing is to produce a sulphur-containing gas. The advantages resulting from the present process in which the sulphur-containing gas is made from pure solid sulphur, not from a sulphide by direct heating are that the component of the gas is unitary, and does not contain other components like a sulphide thus making the control and regulation of the process simple, and that the component (ferrous sulphide) of the sulphurized case at the surface is also unitary so that there is no influence or disturbance from other elements. Sulphur is in a solid state under normal atmospheric pressure and has a boiling point of 441°C. However, the boiling point is lowered at low pressure, for example the boiling point is 188.8°C at 1 torr (133.3 Pa), and is even less when the pressure decreases further. Hence, parts or components located on a cathode table of a vacuum furnace can be sulphurized by means of heating and gasifying sulphur at low pressure (1 - 10⁻² torr)(133.3 - 1.3 Pa), applying a direct voltage field to ionize the gasified sulphur and bombarding the cathode with the high-speed sulphur ions, thus sulphurizing the parts on the table. In the process of the present invention, the sulphur-containing gas is glow discharged under the action of a high voltage field of preferably 450 V - 1500 V and the duration of the discharge is preferably between 1 hour and 4 hours. Because of the glow discharge, the sulphurizing process is speeded up, the sulphurized case depth is increased and, in addition, the parts are heated solely by means of electron bombardment. Generally, the temperature of the parts is controlled between 140°C and 300°C and is regulated by regulating the field voltage but in the prior art when the low voltages are used the parts are heated only by the inert gas in the chamber. In addition, because of the glow discharging, the means of heating and gasifying the sulphur is variable. For example, it may be carried out by means of a low voltage supply when sulphur is located on the cathode table, as in the prior art, or sulphur can be located in the furnace but not on the table, or it can be located outside the furnace. The present invention also has the advantage that the gas-sulphurizing operation can be carried out following a carburizing, nitriding or carbonitriding process when the furnace temperature is lowered to the sulphurizing level. Thus the working hours and the energy resource consumption of the furnace can be reduced. The invention will now be described in detail with reference to the accompanying drawing in which: Figure 1 is a schematic diagram of equipment for carrying out a process according to the present invention. A part 5 to be treated is located on a cathode table 4 in a reacting chamber 1 of a vacuum furnace. The temperature of the part 5 is measured and controlled by a thermocouple 7 and a temperature meter 12. Solid sulphur is loaded into a vaporizer 6 and the temperature of the vaporizer 6 is measured and controlled by a thermocouple 8 and a temperature meter 10. The vaporizer 6 is heated by electric energy supplied from a low-voltage supply 11. The vaporizer 6 shown in Figure 1 is located outside the reacting chamber 1 of the furnace but it could alternatively be located on the table 4 or still within the chamber 1 but off the cathode table 4. The steps involved in the sulphurization are as follows. Firstly, a vacuum pump 16 is started and the air in the reacting chamber 1 is pumped out. When the pressure reaches about 10⁻² torr (1.3 Pa), the low voltage supply 11 is turned on and thus the temperature of the vaporizer 6 starts to rise. Because the pressure is lowered to 10⁻² torr, solid sulphur will be gasified when the vaporizer 6 is heated to more than 140°C (the gasifying temperature of the solid sulphur ranges from 140°C to 200°C in this example). As the gasified sulphur enters the reacting chamber 1, the pressure in the chamber 1 gradually rises, the temperature of the 6 vaporizer being controlled by regulating the current supplied from the low-voltage supply 11. In this way, the quantity of gas resulting from the sulphur gasifying process is regulated and controlled so that the pressure in the reacting chamber 1 is also controlled. Of course, the pressure in the reacting chamber 1 is also controlled by regulating the relationship between the vaporizing speed and the pumping speed (i.e. the vacuum pump still keeps running). When the pressure in the reacting chamber 1 is steady and within the desirable range (about 1 torr (133.3 Pa) in the present example) a high voltage direct supply 3 (ranging between 0 and 1500 V) is turned on and the voltage controlled between 450 V and 1500 V. Thus, a high voltage field is formed between the cathode table 4, the part 5 and an anode 2, which forms a plasma of ionized gaseous sulphur so that positive sulphur ions will bombard the cathode table 4 and the part 5 to create a glow discharge which raises the temperature of the part 5. Because of the physical and chemical reactions existing between the gaseous sulphur and the part 5, a case of sulphide is formed at the surface of the part 5. Generally, the duration of the glow discharge is controlled between 1 hour and 4 hours and the temperature of the part 5 is controlled between 140°C and 300°C. The temperature-controlling means is conventional. A rolling bearing which was sulphurized according to this process was found to give the following performance when tested. The sulphurized rolling bearing was a 204 single-row radial ball bearing. The bearing was subjected to a life test with a model 7501-type lever tester. The testing conditions were: 2400 r/min, radial load 190 kg, no lubricant agent. The bearing tested ran continuously for 200 hours without fatigue damage. Whereas the same kind of bearing parts, ball, outer races, inner races and cages, not subjected to the sulphurizing process were all burnt when they ran for only 80 minutes, the shortest life being only 35 minutes. It was also found that the case depth of the sulphurized bearing was 120 µm, which is much greater than the case depth formed by other processes. Other examples of parts sulphurized according to the present invention are described as follows: (1) sliding bearings made of grey iron, spheroidized iron, or 20C, 445C, 40Cr steel: After gas sulphurizing, the life of the bearings was extended up to 1 to 3 times under a no oil or less oil condition. (2) shafts such as straight shafts made of 45C steel, crankshafts made of steel or iron, reverting shafts (45C) for glassbottle-making machines: After sulphurizing and the formation of FeS, the wear was decreased to 20-70% of the usual wear under lubricated conditions and to 20% under a less oil condition. (3) pistons, piston rings and cylinder sleeves made of steel or iron for internal combustion engines, air compressors, or hydraulic presses: After sulphurizing, the life of the parts was extended by 25% and more. (4) cutting tools such as high-speed steel tools and pinion shaped cutters: The fineness of the machining can be increased with a sulphurized cutter. In addition, the life of the cutter used for cutting stainless steel parts was increased by about 20%. (5) travellers: A traveller of 20C steel which had already been subjected to carbonitriding had its life extended by 100%-200% after sulphurizing. The life of an ordinary traveller may be extended by 60%-100% after sulphurizing. (6) gears: With high precision gears and worms which have been subjected to sulphurizing, their liability to seizure disappears. Hence, bronze gears or worms which are used to prevent seizure can be replaced with those made of steel or iron which have been subjected to sulphurizing. When such gears ran under a poor lubrication condition, their life was raised by 50%-200%. (7) dies such as wire-drawing dies: Because the sulphurizing process may improve the lubricity of the surface of the parts, the quality of the worked parts is improved. (8) parts of sewing machines: After forming FeS at the surface of a part, its life may be raised by 50% and more. (9) guiding rail: After sulphurizing according to the present process, the quality of the rails may be improved. (10) magnetic head: After gas sulphurizing, the life was raised without any reduction in sound reproduction quality. The gas sulphurizing process and the equipment used for the ten examples mentioned above are the same as previously described. Ferrous sulphide is formed at the surface of the parts by means of the process and the case depth is generally between 100 and 120 µm. The equipment described above is the basic equipment required. However, in order to make the equipment function properly, it should be equipped with some devices such as a vacuum gauge 9 for controlling pressure, a flow meter 19 for controlling flow, valves 13,14, and 18, a waste gas filter 17, a cold trap 15 for condensing reactant, a gas cylinder 20, and an observing window 21, which would all be known to those skilled in the art. A detailed description of one embodiment of the invention has been described but the present invention is not to be limited to this embodiment. The present invention may be applied not only to parts made of steel or iron to form a ferrous sulphide case at the surface but also to parts made of other metal to form a different sulphide case at the surface, for example, to form NiS, NiS₂ at the surface of parts made of Ni or Ni alloy, to form MoS₂ at the surface of parts made of Mo and Mo alloy, to form CuS, CuS₂ at the surface of parts made of Cu and Cu alloy, and to form CoS at the surface of parts made of Co and Co alloy. In summary, the advantages of the process according to the present invention are as follows: 1. As sulphur is heated directly, gasified and thereby glow discharged in the high voltage field, the sulphurizing process is speeded up while an even and thick sulphurized case can be attained. 2. It is unnecessary to introduce inert gas for heating the parts to be sulphurized because the parts are heated by being bombarded with sulphur ions in their glow discharge condition. 3. The vaporizer containing solid sulphur can be heated by a low-voltage supply. It is thus easy to control the heating temperature of the vaporizer exactly and the vaporizer can be located either on the cathode table or beside it. 4. Sulphide cases with different constituents and depths may be formed by regulating the pressure in the reacting chamber, the working-gas density, the temperature of the part being sulphurized, the duration of the sulphurizing process, and the electric current or voltage between the cathode and anode. 5. The process of the present invention has many uses mad may be used not only for parts made of steel or iron but also for parts made of other metals.

Claims: A process for sulphurizing a metallic substrate (5) held within a vacuum chamber (1), comprising the steps of: (a) evacuating the vacuum chamber (1), (b) vaporizing solid sulphur and then (c) subjecting said vaporized sulphur to an ionization environment within said vacuum chamber to create a plasma of ionized sulphur particles for sulphurizing the said substrate, wherein said sulphurizing is carried out under a high voltage electric field at a pressure of about 1 torr (133.3 Pa), and wherein said substrate is held at a temperature of between 140°C and 300°C, characterized in that (d) the solid sulphur is vaporized by means of a low voltage supply (11) after the pressure has been lowered to about 10⁻² torr (1.3 Pa), (e) said sulphurizing is carried out in the absence of any gas inert or other substance, wherein the temperature of the metallic substrate is controlled through said high voltage electric field, and in that (g) the low voltage supply (11) and high voltage field are controlled independently from each other. A process as claimed in Claim 1, characterized in that the voltage of the high-voltage field during the sulphurization of the said substrate is controlled within the range of 450 V to 1500 V. A process as claimed in Claim 1, characterized in that the solid sulphur is heated to a temperature between 140°C and 200°C to vaporize it. A process as claimed in Claim 1, characterized in that the solid sulphur is heated and vaporized outside the vacuum chamber (1). A process as claimed in Claim 1 or Claim 2, characterized in that the duration of the subjection of said vaporized sulphur to an ionization environment to create the plasma for sulphurizing the substrate is between 1 hour and 4 hours.

Inventors: ZHANG, YIFEI
Applicants: ZHANG, YIFEI
Application year: 1986
Main classification: C23C14
IPC codes: C23C8