Method and system for die compensation and restoration using high-velocity oxy-fuel thermal spray coating and plasma ion nitriding

Abstract

A method and system for die compensation and restoration uses high-velocity oxy-fuel (HVOF) thermal spray coating and plasma ion nitriding to compensate for a particular part (damaged part) of a press die that causes formation of fine curves at a door of a vehicle to restore it to its original state. A coating thickness quantification technique may precisely compensate for the damaged part of the die that causes formation of the fine curves at the door of the vehicle in a circular form using HVOF thermal spray coating. A surface of the die may be nitrided using plasma ion nitriding after HVOF thermal spray coating is performed, so as to harden the surface of the die so that wear resistance and fatigue resistance of the die can be greatly improved and the hardfacing or overlay welding efficiency of the die can be increased.

Claims

1. A system for die compensation and restoration, the system comprising: a high-velocity oxy-fuel (HVOF) thermal spray coating unit for forming a ferro-alloy powder coating layer on a damaged part of a press die in which spheroidal graphite east iron is used as a substrate, using a H VOF thermal spray coating; a grinding unit for grinding a surface of the coating layer up to 1000-grit to 2000-grit; a cleaning unit for removing impurities from the coating layer using alcohol ultrasonic cleaning; and a plasma ion nitriding unit for forming a nitriding layer on the ground and cleaned coating layer by nitriding a surface of the coating layer of the press die using plasma ion nitriding.

2. The system of claim 1, further comprising a surface roughness controller for controlling surface roughness of a surface of the damaged part of the die as a pre-treatment process before HVOF thermal spray coating is performed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an image showing an example in which fine curves are formed at a cover panel of a door of a vehicle manufactured using a press die;

(2) FIG. 2 is a schematic view of an exemplary stack structure of a coating layer of a die using high-velocity oxy-fuel (HVOF) thermal spray coating and plasma ion nitriding according to the present invention;

(3) FIG. 3 is a view illustrating surface roughness of spheroidal graphite cast iron for forming a dense interface between a die and a coating layer;

(4) FIG. 4 is images of a stacking example of ferro-alloy powder coated on the die (spheroidal graphite cast iron), an adhesion force between the coating layer and the die and a bonding strength thereof according to surface roughness;

(5) FIG. 5A, FIG. 5B and FIG. 5C are graphs showing examples of quantification of coating thicknesses of ferro-alloy powder according to the present invention

(6) FIG. 6 is a view illustrating an exemplary plasma ion nitriding process according to the present invention;

(7) FIG. 7 is a cross-sectional view of nitrogen diffusion layers of cross-sections of an exemplary die to be repaired after the plasma ion nitriding process is performed according to the present invention;

(8) FIG. 8 is a graph of an exemplary profile of microhardness of a coating cross-section of the die to be repaired after the plasma ion nitriding process is performed according to the present invention; and

(9) FIG. 9 is a schematic view of an exemplary structure of a spray gun used in HVOF thermal spray coating according to the present invention.

DETAILED DESCRIPTION

(10) Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

(11) The present invention provides a coating thickness quantification technique, whereby a damaged part of a press die manufactured of spheroidal graphite cast iron that causes formation of fine curves at a door of a vehicle may be precisely compensated for in a circular form using high velocity oxy-fuel (HVOF) thermal spray coating. The present invention also provides a method and system for die compensation and restoration using HVOF thermal spray coating and plasma ion nitriding, whereby a surface of the die is nitrided using plasma ion nitriding after HVOF thermal spray coating is performed, so as to harden the surface of the die so that wear resistance and fatigue resistance of the die may be greatly improved and the hardfacing or overlay welding efficiency of the die may be increased.

(12) To this end, first, coating powder that is suitable for spheroidal graphite cast iron as a substrate for a press die is selected.

(13) A commonly-used ferro-alloy (stainless steel) group that is a coating material used in HVOF thermal spray coating, represents mutual suitability with spheroidal graphite cast iron as the substrate for the press die and high mechanical characteristics (hardness, wear resistance, and bonding strength) compared to the substrate and is as shown in the following Table 1 in consideration of surface nitriding after coating is performed, may be selected.

(14) TABLE-US-00001 TABLE 1 No. Model Chemical Composition [wt %] Remark 1 FE-101 Fe17Cr12Ni2.5Mo 316 SS 2 FE-206 Fe16.1Cr4.1Ni3.2Cu0.3Nb 17-4 PH [Duplex] 3 FE-108 Fe12.5Cr 410 SS

(15) FE-101 powder among the commonly-used ferro-alloy group selected, as shown in Table 1, is an austenite stainless steel material, has high low-temperature spray coating efficiency, realizes deformation hardening using process control and grain reinforcement using grain refinement, thereby improving coating strength characteristics.

(16) Also, FE-206 powder in Table 1 is a martensite-type precipitation hardening stainless steel material and has the effect of hardening a diffused Cu precipitate, and FE-108 powder is a martensite stainless steel material having high hardening performance.

(17) After a powder material for HVOF thermal spray coating for compensation and restoration of the press die, i.e., iron-base alloy powder is selected, the diameter of ferro-alloy powder should be determined, because it is a significant factor for determining coating performance.

(18) If the diameter of ferro-alloy powder is too small and is less than 15 m, powder is fully molten, and a laval nozzle for HVOF thermal spray coating is clogged so that coating may not be performed.

(19) On the other hand, if the diameter of ferro-alloy powder is too large and is greater than 35 m, gas for HVOF thermal spray coating does not sufficiently accelerate powder particles so that particle coating may not be well performed, and the coated particles form a weak interface between particles due to unmelting and pores so that cracks occur and a coating layer may be peeled off (see FIG. 2).

(20) Thus, in the present invention, the average diameter of ferro-alloy powder used in fine curve compensation of the die may be set in the range of 25 to 35 m.

(21) When a material for powder used in HVOF thermal spray coating for compensation and restoration of the press die is selected and the diameter of the powder material is determined in this way, a process of controlling surface roughness as a pre-treatment process on a coating surface of the die is performed.

(22) The reason why surface roughness of a surface (surface of a damaged part) on which a coating layer of the die is to be formed is controlled is to secure the bonding strength of the coating layer.

(23) To this end, a sand shot-blasting process is performed as a pre-treatment process before HVOF thermal spray coating is performed so that surface roughness for coating of the die may be controlled.

(24) More specifically, a sand shot-blasting process as an essential pre-treatment process for securing adhesion performance between a substrate and the coating layer of the die, a high bonding strength and durability, is performed so that a predetermined bonding strength between the substrate and the coating layer of the die with predetermined surface roughness may be maintained and simultaneously a dense interface therebetween may be formed.

(25) The surface roughness of the substrate (spheroidal graphite cast iron) using the sand shot-blasting process may satisfy the equation Ra=5.630.41 m or more, because in case of Ra=5.630.41 m or less, a relative low bonding strength is maintained and cracks occur between the substrate and the coating layer.

(26) Thus, as a pre-treatment process before coating is performed using HVOF thermal spray coating according to the present invention, surface roughness of the surface of the damaged part of the die is controlled by a surface roughness controller using sand shot-blasting.

(27) As an Experimental Example for controlling surface roughness of the die substrate according to the present invention, the sand shot-blasting process was performed on the die substrate so that surface roughness may satisfy the equations R1=3.810.47 m, R2=5.630.41 m, and R3=9.540.55 m, as shown in FIG. 3, a coating layer was formed on the die substrate using HVOF thermal spray coating, and its result is as shown in FIG. 4.

(28) As shown in FIG. 4, when surface roughness of the substrate (spheroidal graphite cast iron) using the sand shot-blasting process satisfies the equation R1=3.810.47 m, cracks may occur in the interface between the coating layer and the substrate, and on the other hand, when surface roughness of the substrate (spheroidal graphite cast iron) using the sand shot-blasting process satisfies the equation R2=5.630.41 m or more, a dense interface between the substrate and the coating layer may be formed.

(29) Thus, the sand shot-blasting processing is performed so that surface roughness of the die substrate (spheroidal graphite cast iron) may satisfy the equation R2=5.630.41 m or more.

(30) Next, a process of forming the coating layer is performed on the surface of the damaged part of the die substrate having predetermined surface roughness using HVOF thermal spray coating.

(31) That is, an operation of forming a ferro-alloy powder coating layer on the damaged part of the press die in which spheroidal graphite cast iron is used as a substrate, is performed using an HVOF thermal spray coating method performed by an HVOF thermal spray coating unit.

(32) To this end, an optimum coating process condition for repairing the press die should be established.

(33) That is, in the HVOF thermal spray coating method, the flying speed and temperature of powder is controlled by controlling pressures and flows of fuel and gas so that stacking efficiency of coating may be determined and coating fine tissue characteristics, such as adhesion performance between the coating layer and the substrate and air porosity thereof, may be determined. Thus, in order to form a die compensation coating layer having excellent characteristics, process optimization on type, pressure and flow conditions of fuel and gas should be established, and simultaneously, optimized process condition suitable for mass production for forming the coating layer should be established.

(34) In this case, equipment JP-5000 manufactured by the TAFA company was used in the HVOF thermal spray coating method, and in order to draw optimum process parameters, as shown in the following Table 2, coating was performed by increasing/decreasing an oxygen flow and a fuel flow based on process parameters (condition C2) of coating powder that is provided to technical data of TAFA that is a manufacturer of HVOF thermal spray coating equipment JP-5000.

(35) TABLE-US-00002 TABLE 2 Parameters C1 C2 C3 Gun barrel 4 4 4 Spray distance 14 [355 mm] 14 [355 mm] 14 [355 mm] Spray speed 300 mm/s 300 mm/s 300 mm/s Spray pitch 5 mm 5 mm 5 mm Spray rate 76 g/min 76 g/min 76 g/min Oxygen flow 1700 scfh 1800 scfh 2000 scfh Fuel flow 5.1 gph 5.1 gph 6 gph Carrier gas [N.sub.2] 20 2 scfh 20 2 scfh 20 2 scfh

(36) In the HVOF thermal spray coating method according to the present invention, kerosene is used as a fuel, powder is heated and accelerated using a high-temperature and high-velocity gas that is generated when kerosene is mixed with oxygen and is combusted, and power collides with the die, thereby performing coating.

(37) Referring to FIG. 9, the HVOF thermal spray coating method is performed using a spray gun in which a path on which fuel and oxygen are transported and a path on which metal powder (see Table 1) together with a nitrogen carrier gas is transported are formed.

(38) Thus, after powder is heated and accelerated by the high-temperature and high-velocity gas generated when kerosene is mixed with oxygen and is combusted, powder is sprayed through the laval nozzle of the spray gun and simultaneously collides with the die, thereby forming a coating layer.

(39) Also, nitrogen is used as a carrier gas while the HVOF thermal spray coating method is performed, and cooling of the substrate of the die is performed in an air-cooled manner without an external cooling device.

(40) Thus, as shown in FIG. 2, a ferro-alloy powder coating layer is formed on the surface of the substrate of the die manufactured of spheroidal graphite cast iron as a coating layer formed using the HVOF thermal spray coating method.

(41) In this case, a fine tissue of the coating layer after the HVOF thermal spray coating method has been performed, includes splat in which well-molten particles are re-coagulated, extend long in a curve form and form a layer-shaped structure, unmolten particles, particles, of which surface is partially molten, pores, and debris having a fine grain shape that is divided into many parts due to collision when thermal spray coating is performed.

(42) When the melting temperature of powder particles is in an optimum condition (process condition C2 of Table 2), the powder particles may collide with the substrate at high velocity and simultaneously may be properly diffused to form a lamella structure or splat.

(43) On the other hand, when the melting temperature of the powder particles is higher than the optimum condition (process condition C2 of Table 2), i.e., in case of process condition C1 of Table 1, or when the melting temperature of the powder particles is lower than the optimum condition (process condition C2 of Table 2), i.e., in case of process condition C3 of Table 1, the powder particles have a fine structure with internal defects.

(44) When the melting temperature of the powder particles is higher than the optimum condition (process condition C2 of Table 2), i.e., in case of process condition C1 of Table 1, phase transformation occurs due to an undesirable reaction, such as oxidation, in a high-temperature gas flow field so that an oxide, such as Fe.sub.3O.sub.4, is dominantly formed on the coating layer.

(45) Since oxides that are dominantly formed on the coating layer constitute weak interfaces between the oxides and the powder particles due to a difference in thermal expansion coefficients during cooling, mechanical characteristics (microhardness and bonding strength) that are not uniform and weak in the coating layer, are generated. Also, since the instant fully-molten particles collide with the substrate and the fully-molten particles are widely diffused, the stacking efficiency of the coating layer (coating thickness compared to spray pass number) is not good, as indicated by C1 of FIGS. 5A through 5C.

(46) On the other hand, when the melting temperature of the powder particles is higher than the optimum condition (process condition C2 of Table 2), i.e., in case of process condition C3 of Table 1, no sufficient heat is supplied to the powder particles and the unmolten particles collide with the surface of the die substrate and are stacked. Thus, an adhesion force between the particles is weak, and cracks are grown between weak interfaces of the particles so that the coating layer may be peeled off (see FIG. 4).

(47) Thus, in the present invention, the HVOF thermal spray coating method is performed according to the process condition C2 (condition in which the melting temperature of the powder particles is optimized) shown in the above Table 2 so as to optimize the fine tissue of the coating layer formed using the HVOF thermal spray coating method and the stacking efficiency of powder.

(48) More specifically, the HVOF thermal spray coating method is performed according to the process condition C2 (condition in which the melting temperature of the powder particles is optimized) including barrel of 4 of the spray gun, spray distance of 14 with respect the die substrate, spray speed of 300 mm/s, spray pitch of 5 mm, spray rate 76 g/min, oxygen flow of 1800 standard cubic feet per hour (scfh), fuel flow of 5.1 gallon per hour (gph), and carrier gas (N.sub.2) of 202 scfh.

(49) Next, a surface hardening process of the coating layer coated on the damaged part of the die is performed using plasma ion nitriding.

(50) That is, the surface of the coating layer of the die is nitrided using plasma ion nitriding performed by a plasma ion nitriding unit so as to perform surface hardening. Thus, an operation of forming a nitriding layer on the coating layer may be performed.

(51) Referring to FIG. 6, plasma ion nitriding for surface hardening and improving wear resistance of the coating layer coated on the surface of the compensated die, i.e., the damaged part of the die includes pumping in a nitriding reaction chamber, heating, sputter cleaning, plasma nitriding, and cooling.

(52) The above processes will now be described in detail.

(53) First, before plasma ion nitriding is performed, after the surface of the coating layer of the die compensated for using the optimum HVOF thermal spray coating method (process C2 of Table 2) is finely ground by a grinding unit using a silicon carbide (SiC) sandpaper up to #1000 to #2000, impurities are removed from the coating layer using alcohol ultrasonic cleaning by using a cleaning unit for 10 minutes.

(54) Subsequently, after the die is loaded into the reaction chamber, the reaction chamber is pumped in a high vacuum state, voltage is applied to the surface of the die, the pressure of the reaction chamber is checked that it is decreased less than 1 torr and then, the reaction chamber is heated at 300 for 30 minutes.

(55) Next, in the sputter cleaning operation, when a voltage of 250 V is applied at a mixture gas atmosphere of Ar and H.sub.2, plasma is formed, and a stable oxide layer, such as Cr.sub.2O.sub.3, formed on the coating layer may be removed by etching.

(56) A nitriding process using plasma is performed using a mixture gas of H.sub.2 and N.sub.2, a process pressure of 1.6 torr, a fixed current of 30 A or more at 550 for 10 hours, and cooling is slowly performed in a vacuum state.

(57) Through the above processes, a nitriding layer (including a nitrogen diffusion layer and a nitrogen compound layer) having a thickness of about 17 to 50 m is formed on the coating layer (ferro-alloy powder coating layer) coated by the HVOF thermal spray coating method, as shown in FIG. 7.

(58) In addition, the nitrogen diffusion layer has been checked from a die product, of which surface is hardened by the nitriding process, using an electron probe micro analyzer (EPMA). Thus, as shown in FIG. 7, an N-rich region as a nitrogen compound layer may be checked from an upper part of the nitrogen diffusion layer.

(59) That is, it may be checked that the N-rich region exists in the surface of the die and the nitrogen diffusion layer having a thickness of 17 to 50 m is formed on a depth part of the coating layer according to steel types 316 SS, 17-4 PH, and 410 SS.

(60) Referring to FIG. 8, as a dense compound layer (N-rich region that is the nitrogen compound layer) including nitrides, such as CrN, Fe.sub.4N, and Fe.sub.2-3N, is formed on the upper part of the nitrogen diffusion layer, an excellent nitrogen hardening layer with Hv 1100 or more is formed, and hardness is increased due to the nitrogen diffusion layer formed according to the depth of the coating layer.

(61) Although a method of forming a surface hardening layer using plasma ion nitriding has been described for surface hardening and improving wear resistance of the damaged part (repaired part) of the die according to the present invention, a surface to be restored of the repaired die using plasma ion nitriding may be used in various environments and conditions by adjusting time, temperature, voltage, and gas ratio that determine the tissue and depth of the nitriding layer.

(62) As described above, the present invention provides the following effects.

(63) According to the present invention, a particular part of a die is hardfacing or overlay welded by stacking ferro-alloy powder on a particular part (damaged part) of a press die formed of spheroidal graphite cast iron using an HVOF thermal spray coating technique, and the coating thickness of a coating layer is quantified and controlled in units of micron so that repair numbers for a precise dimensioning work can be reduced and production efficiency can be improved and production costs can be reduced.

(64) In addition, in the HVOF thermal spray coating method, relatively low temperature stacking can be performed compared to a welding technique according to the related art, and thermal deformation of a substrate when the die is repaired can be minimized, unlike in arc welding according to the related art.

(65) In particular, a nitriding layer including a nitrogen compound layer on the surface of the die and a nitrogen diffusion layer with the depth of the coating layer is formed by performing surface hardening on the repaired die using plasma ion nitriding so that wear resistance and fatigue resistance of the die can be improved, damage of the die can be suppressed and the usage life span of the die can be extended.

(66) In addition, in plasma ion nitriding, a nitrogen gas can be ionized due to glow discharge. Thus, the die can be nitrided at a low temperature, and ammonia (NH.sub.3) gas and nitrous oxide (N.sub.2O) are not used so that eco-friendly nitriding can be performed.

(67) In addition, it is efficient in nitriding aluminum, stainless steel, and cast iron that are metals that are not easily nitrided by performing oxide decomposition and surface activation of the surface of an object to be processed using a sputtering effect during a plasma ion nitriding process.

(68) In addition, in plasma ion nitriding, the phase and thickness of a nitride formed in various process conditions (temperature, time, pressure, gas ratio) can be changed so that the surface characteristics of the die can be selectively changed according to characteristics and usage of the repaired die and the repair and restoration efficiency of the die can be improved.

(69) For convenience in explanation and accurate definition in the appended claims, the terms upper and etc. are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures.

(70) The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.