KR100251396B1 - Valve with hard-facing - Google Patents

Abstract

The present invention is 20 to 24wt% cr, 0 to 8wt% W, 4 to 7wt% A1, 0.2 to 0.55% C, 0 to 2% Hf, 0.1 to 1.5wt% Nb, 0 to 8wt% Mo, 0 to 1.2 It relates to a nickel main alloy used for valve seat material applications of exhaust valves for internal combustion engines, consisting of wt% Si and 0 to 15 wt% Fe and with a sum of W and Mo content of 10 wt% or less. The alloy has good corrosion resistance and ductility corresponding to Alloy 50 even at a working temperature of 500 ° C. in a strong corrosive sulfur containing environment.

Description

[Name of invention]

Surface hardening valve

[Technical Field]

The present application relates to a valve, in particular an exhaust valve for an internal combustion engine, consisting of a movable valve member having a valve seat area of a nickel main component metal alloy.

The material composition selected in the valve seat area in an exhaust valve for an internal combustion engine has played an important role for many years in view of the operational reliability of the engine concerned and the life of the exhaust valve and the extent of the necessary maintenance work.

Combustion in the working cylinder in an internal combustion engine produces coking residue consisting of hard particles to be conveyed through the exhaust valve, which sometimes enters between the closing seats and creates dents on the seat surface. As is known, the marks initially lead to local leakage and then to greater throughburning involving corrosive deterioration of the sheet surface.

[Prior technology]

The valve seat material preferably has a hardness sufficient to prevent or reduce the extent of dents. In large heavy oil-fired diesel engines, the valve seat area has recently been fabricated with a nickel-based surface hardening material, Alloy 50, containing approximately 12% Cr, 3.9% Si, 2.9% Fe, 2.25% B, and 0.5% C as the most important alloy components. I have tried In addition to the desired hardness, Alloy 50 exhibits high temperature corrosion resistance to strong corrosive environments where exhaust valves are exposed in heavy oil diesel engines. In engines with large cylinder inner diameters (ie, 600 to 900 mm), in addition to good corrosion resistance, the mechanical strength of the material used is greatly demanded. Work experience has shown that in some cases radial cracks occur in the surface hardening material Alloy 50, which causes sheet thru-burning or dangerous circumferential crack growth. In terms of eliminating the risk of damage, Stellite 6, a softer surface hardening material, is used for organs of this size. This material is a cobalt-based alloy whose high temperature corrosion resistance is found to be less than Alloy 50 in the real environment, with shorter maintenance intervals during valve condition inspection.

It is known that the hardness of the surface hardening material decreases with temperature rise. Therefore, Stellite 6 has a path of about 298HB at room temperature of about 370HB at 500 ° C and the path of Alloy 50 decreases from about 530HB to 420HB at the corresponding temperature.

Likewise, the high hardness Ni principal component surface hardening materials known to date are known to have poor ductility or poor ductility and thus poor fatigue strength properties.

It is preferable that the hardness required by the Ni main component alloy known so far is obtained by adding B, Si, and C components. In particular, due to the size and arrangement of the metal borides in the alloy structure, the hardened surface has very poor ductility, which leads to the risk of crack formation already during welding or after a short or long working cycle.

If the B content is reduced or completely ignored, it is necessary to increase the C content by a comprehensive carbide precipitation method to increase the alloy hardness, likewise the precipitated carbide network reduces the ductility to a considerable extent.

A common feature of the prior art nickel main alloys is that they must have a significant hardness to achieve high hardness at a working temperature of about 500 ° C. and therefore are brittle at room temperature.

Japanese Patent No. 59-9146 (Kokai) introduces a nickel alloy valve having an A1 content of less than 4.5% to enable welding of the alloy. The combination of C content in excess of 0.55% and the alloy content of Ti, W and Mo makes it possible to increase the alloy hardness by carbide precipitation. The B content of up to 2% described can further contribute to the increase in hardness. However, very hard carbide and boride precipitates in the relatively soft matrix phase result in a change in the micro hardness of the alloy, and as described above the carbide network can further reduce the ductility.

U.S. Patent No. 3,795,510 relates to a nickel alloy containing 20% Cr, 5 ° 5% A1, 2.5% Ti, 7.5% Fe and 0.15% C. Ready-made nickel alloy blanks were friction welded to the remaining valve parts made of carbon steel to produce suitable valves. The alloy is non-contact by conventional methods, and the hardness is lower than the hardness desired herein.

[invent]

It is an object of the present invention to be used for engines of any size and to have a high hardness at temperatures up to 500 ° C., while maintaining sufficient ductility for valve applications that are mechanically loaded and periodically operated. It is to provide a valve made of a surface hardening material which also serves good high temperature corrosion resistance. In the review so far, the valves first mentioned by the present application, in terms of wt%, are present in wt%, 20 to 24% Cr, 0 to 8% W, 4 to 7% Al, 0.2 to 0.55% C, 0-18% Hf 0-1.5 $ Nb, 0-8.0% Mo, 0-1.2% Si and 0-15% Fe, the sum of W content and Mo content is not more than 10% Is characterized.

Alloys of this type are currently sold as surprisingly desirable properties. Previously, welded nickel alloys with high Al content were generally recognized to exhibit extreme thermal cracking propensity in the weld metal with crack formation in the heat affected zone of multilayer welding. Therefore, Ni-Cr alloys containing Al in excess of 3-4% are described in the literature as non-contact.

The weldability of the Ni-Cr-A1-C alloys of the A1 content described above according to the present application makes it possible to utilize the precipitation-hardening mechanism in this type of alloy. That is, the intermetallic phase Ni ₃ A1 (γ ') is precipitated as a coherent hardness increasing phase in the soft nickel-matrix (γ). The γ'-phase can be precipitated at 15-25% tissue mass in the basic γ-tissue to obtain the desired high strength and hardness. The strength and hardness are generally constant at temperature intervals of 20 to 600 ° C. including temperature intervals at which the exhaust valve in normal operation is exposed.

The Cr content of the alloy plays a significant role in the sulfur compound and contributes significantly to the high corrosion resistance of the alloy which is required in the real environment. The A1 content forms a bonded A1 ₂ O ₃ and Cr ₂ O ₃ surface layer in the valve seat, which increases corrosion resistance, especially at temperatures above 750 ° C. In the event of a leak on the seat due to the creation of dents, the improved corrosion resistance especially inhibits rapid deterioration of the valve seat. The leak can locally cause surface temperatures significantly higher than the normal operating temperature of the valve.

Cr content also has a solution-strengthening effect that contributes to the strength increase of the alloy. The solid solution strengthening effect can also be promoted by adding replaceable Mo and W. The total content of W and Mo should not exceed 10%. Otherwise, the carbide arrangement of the alloy will be adversely affected.

The hardness of the alloy is finally determined by adjusting the C content to affect the arrangement and amount of carbonized precipitates. If the C content is less than 0.2%, the desired hardness cannot be obtained. If the C content is more than 0.55%, it is difficult to obtain the desired ductility. A C content in the range of 0.3 to 0.55% was found to provide a combination of hardness and ductility advantageous for valve material applications.

In the manufacture of Hot Isostatic Pressure (HIP) compound exhaust valves, it is also possible to use alloys according to the present application instead of commercially available nickel main surface hardening alloys. This is because the solidus temperature of the alloy is similar to the solidus temperature of the valve main material, which is essential for the use of the HIP method.

The arrangement of carbides has a great influence on the ductility of the alloy, in particular acicular and flaky and flaky carbide precipitates have a negative influence on the ductility. In practical alloys, it has been found that with increasing C content, the propensity for formation of adverse carbide cracks in the form of "Chinese letters" increases, and for this reason the C content should not exceed 0.6%.

It was found that the addition of 1-2% Hf had a positive effect on the carbide arrangement. By adding Hf, the carbide arrangement changed from flaky and acicular carbides to a more rounded shape that did not reduce the ductility of the alloy to that extent. Unexpectedly, however, when the C content exceeds 5%, carbide precipitation has been found to have a limited effect of Hf addition, in which case the C content can be conveniently adjusted to 0.35 to 0.50%.

Furthermore, it has been surprisingly found that the alloy structure increases in hardness when Nb is added so as not to exceed 1.5%, especially in a slowly solidifying melt. This may be because Nb increases the amount of carbide precipitation and / or changes the carbide composition. By adding Nb to the alloy, it was also confirmed that the carbide arrangement changed to the undispersed metal carbide form, which is expected to have a positive effect on the ductility of the alloy.

When welding the valve seat area, the addition of Si to the alloy improves the welding properties due to the carbonic effect of silicon. Si content can be appropriately set at 0.8 to 1.2%. Surprisingly, however, the Si content was found to lead to the formation of A1, Si, Cr and C rich processes when the A1 content exceeds 5 to 5.5%. Unexpectedly, since the process is significantly less corrosion resistant than the remaining components of the alloy, it is desirable that the phase fraction of the process does not exceed 5%.

The valve member with the valve seek area appropriately is generally made of austenitic stainless steel alloy. When welding the valve seat area by welding, the steel alloy enters the nickel main component filler and mixes slightly, in particular Fe, which does not exceed 15% in the first attached weld layer.

Less than 20% of Fe may have strengthening effect, but at the same time, it reduces corrosion resistance. Since there is already a risk of corrosive degradation at a Fe content of 5%, the Fe content in the final weld layer should not exceed 10%, preferably 5%.

It is advantageous for the alloy to consist of at least 55% Ni in the region close to the valve surface, because lower Ni content can reduce precipitation hardening and reduce the hardness of the alloy.

As described above, the hardness of the alloy is obtained by a combination of precipitation strengthening, and the alloy of high Al content causes an increase in hardness of the nickel main component material itself (Ni-matrix) and carbide precipitation in the base material. The hardness is maintained even at high temperatures. In order to preserve the desired carbide arrangement, the alloy must consist of any minimum content of carbide former, in addition to Cr content. It is therefore advantageous for the Hf, Nb, W and Mo contents to be at least 5%.

[Brief Description of Drawings]

Various embodiments of the present invention will be described in detail in detail with reference to the drawings. 1 to 4 are 320 times magnified photographs of samples polished four different alloys according to the present application.

Preferred Embodiment

Example 1

In order to obtain a comparison of the alloy according to the invention with the previously known valve seat alloy, four geometrically identical valve spindle blanks in the form of valve heads of diameter D = 250 mm and made of austenitic stainless steel were fabricated. After preheating the spindle members, four different alloys were plasma welded to each blank using a transfer arc and the welding parameters were as follows.

The seam is made of three layers, 8mm deep, 25mm wide and 60 ° seam side angle.

An approximate analysis of the alloys made is in Table 1, where Stellite 6 and Alloy 50 are as described above, BW1-50 is likewise a commercially available nickel alloy while I-1 is the alloy according to the present application.

TABLE 1

After welding, the weld seam surface was flipped over and defects were observed with a capillar test.

The blank was subsequently placed in an oven and heated to 250 ° C. and quenched in a water bath at about 40 ° C. The blanks were observed visually and by capilla test, with no cracks found in the sheet material.

All the blanks were put back into the oven and heated to 350 ° C. and then quenched first in a water bath at about 70 ° C. and visually and by capillary tests. Three radial cracks and some residues were found in Alloy 50. Three specimens were flawless.

The temperature choke test was repeated for three complete specimens and heated to 450 ° C. After quenching in a water bath at about 70 ° C., coarse cracked patterns were found in the BW 1-50 alloy, but the valve seat area was not damaged in the remaining two specimens.

These two specimens were then heated to 520 ° C, quenched in a water bath at about 70 ° C and visually and capillary tested to find many small radial cracks in the Stellite 6 alloy and valve seats in the I-1 alloy. One radial crack was found in the area. Unlike the cracks of the other three alloys, the cracks of the I-1 alloy were marked by plastic deformation traces in the form of round crack edges.

The nickel main alloy according to the present invention thus shows surprisingly good weldability and ductility, and crack resistance, which is significantly better than nickel alloys Alloy 50 and BW I-50, and completely Stellite 6 level.

A radial cutout was made from the seat area of each valve spindle specimen, and a hardness test (HV20) of approximately the middle of the weld was performed at room temperature, at various distances from the seat surface. The results are shown in Table 2.

TABLE 2

The hardness of the nickel main alloy is sensitive to the mixing of the valve spindle material in the seat material, and the hardness of the outer half of the seat material changes within the limits expected to prevent complete equalization of the alloy composition due to rapid solidification of the melt. appear.

Example 2

In order to measure the alloy hardness with increasing temperature, a bar blank was prepared from the starting material powder using the HIP method (hot isostatic compression molding). The rod blank was 30 mm in diameter and 160 mm in length from which 8 mm thick flakes were cut out for hardness measurement. The blanks were made of alloy Stellite 6 and nickel main alloy corresponding to I-1 of Table 1. The measured hardness (HB 10/3000/15) is shown in Table 3, whereby stellite 6 shows a significant decrease (28%) at high temperature hardness of 500 ° C., while the hardness of the alloy according to the present application is only slightly reduced ( 3%).

TABLE 3

In order to compare with the hardness of Table 2, the hardness (HV20) of each of three slices of Stellite 6 and I-1 was measured at room temperature. The results were as follows.

TABLE 4

The hardness 473HV at approximately 20 ° C of the Alloy 50 sheet material measured in Table 2 decreases approximately 20% to 378HV at the working temperature of the sheet material, while the hardness of alloy I-1 is only about 3% at 413HV at the working temperature. Decreases.

Example 3

The alloy according to the present application was manually TIG welded to make a blank of austenitic stainless valve steel with a diameter of 80 mm and a thickness of 20 mm. An approximate analysis of the alloys is in Table 5.

Hardness measurements were made for each alloy at 20 ° C and about 500 ° C.

TABLE 5

It can be seen that as heating from room temperature to about 500 ° C., the rate of decrease in hardness increases from about 2% for I-2 alloys to about 7% for I-5 alloys.

Polishing samples of each alloy were prepared, and FIGS. 1 to 4 are sample photographs of alloys I-2 to I-5.

In Alloy I-2 of FIG. 1, elongated bright precipitates of aluminum-free metal carbides are seen, with dark nodular precipitates, presumably composed of carbide Perovskite.

In alloy I-3 of FIG. 2, a clear dendrite structure is seen with the cell in which the material has a uniform crystal lattice orientation. There are some Perovskite precipitates and metal carbide precipitates between the dendritic arms. This alloy is expected to have high temperature hardness with good ductility.

Alloy I-4 in FIG. 3 has a somewhat less uniform dendritic structure, with very little perovskite precipitate. In the alloy I-5 of FIG. 4, the perovskite precipitate was almost disappeared.

Example 4

A hardness test corresponding to the hardness test described in Example 2 was conducted in a corresponding analysis for I-1 of Tables 1 and 3. However, before testing the hardness, the blank was subjected to a heat treatment consisting of solution treatment for 2 hours at a temperature of 1150 ° C., followed by precipitation hardening at a temperature of 750 ° C. for at least 2 hours. The measured hardness (HB / 10/3000/15) is shown in Table 6 and (HV20) Table 7.

TABLE 6

TABLE 7

Although the high temperature hardness was slightly reduced (7%), the hardness of approximately 460 HB at 500 ° C. was much higher than that obtained from the surface hardening alloy of the prior art.

If the Cr content of the alloy is less than 20%, the high temperature corrosion resistance becomes too meaningless. If the Cr content exceeds 24%, the strength properties of the alloy are affected in an adverse direction, and the weldability is also deteriorated.

If the A1 content is less than 4%, the high temperature hardness is too low. If the A1 content is more than 7%, the perovskite precipitates deteriorate the corrosion resistance and ductility of the alloy.

The alloy can also be used to weld the valve seat area to the valve member, in which case the alloy should contain Si, and the content of Y which easily oxidizes should be kept as low as possible. Or it can also be used to manufacture the valve member by the HIP method.

Nickel alloys composed of 20 to 23% Cr, 4 to 5.5% Al, 0 to 5% Fe, 0.3 to 0.5% C, and 5 to 7.5% W and / or Mo not only have good ductility but also high hardness. It has been found that if the valve seat area is attached by welding, it is 20 to 23% Cr, 4 to 5.5% Al, 0.3 to 0.5% C, 0.8 to 1.2% Si, and 5 to 7.5% of W and / Or by adding a nickel main component filler composed of Mo.

Alloys consisting of 22.5 to 23.5% Cr, 4.0 to 5.0% A1, 0.40 to 0.45% C, 1.0 to 0.5% Hf, and 5.5 to 6% W and / or Mo are those where very high crack resistance is required It could be used as well.

If the alloy is manufactured by the HIP method, Y may be included in the alloy composition, thereby effecting high temperature resistance.

In the expression format, the individual components of the alloy are all described in wt%.

Claims (12)

20 to 24 wt% Cr, 0 to 8 wt% W, 4 to 7 wt% Al, less than 0.2 to 0.55 wt% C, 0 to 2 wt% Hf, 0 to 1.5 wt% Nb, 0 to 8.0 wt% Mo, excluding impurities A movable valve member having a valve seat area of a nickel main component alloy comprising 0 to 1.2 wt% Si and 0 to 15 wt% Fe, wherein the sum of the W content and the Mo content does not exceed 10 wt% Exhaust valve for internal combustion engine. The alloy of claim 1, wherein the alloy consists of 20 to 23 wt% Cr, 4 to 5.5 wt% A1, 0 to 5 wt% Fe, 0.3 to 0.5 wt% C, and 5 to 7.5% wt% of W and / or Mo. An exhaust valve for an internal combustion engine, characterized in that. The alloy of claim 1, wherein the alloy consists of 22.5 to 23.5 wt% Cr, 4 to 5 wt% A1, 0.40 to 0.45 wt% C, 1 to 1.5 wt% Hf, and 5.5 to 6 wt% W and / or Mo. An exhaust valve for an internal combustion engine, characterized in that. The exhaust valve according to any one of claims 1 to 3, wherein the alloy is composed of 1 to 2 wt% Hf and 0.35 to wt% C. The exhaust valve according to any one of claims 1 to 3, wherein the alloy is comprised of 0.3 to 1.5 wt% Nb. The exhaust valve according to any one of claims 1 to 3, wherein the alloy is comprised of 0.6 to 4.0 wt% of Mo and W. 4. The exhaust valve according to any one of claims 1 to 3, wherein an alloy content of at least one of the components Hf, Nb, and Mo amounts is about 0 wt%. The exhaust valve according to any one of claims 1 to 3, wherein the alloy is composed of 0.6 to 1.2 wt% of Si. The exhaust valve according to any one of claims 1 to 3, wherein the alloy contains 5 wt% or less of Fe near the surface of the valve seat region. The exhaust valve according to any one of claims 1 to 3, wherein the total alloy content of Hf, Nb, W, and Mo is at least 5%. The exhaust valve according to any one of claims 1 to 3, wherein the alloy near the valve surface comprises at least 55% Ni. 20 to 23 wt% Cr, 4 to 5.5 wt% Al, 0.3 to 0.5 wt% C, 0.8 to 1.2 wt% Si, 0 to 2 wt% Hf, 5 to 7.5 wt% W and / or Mo, excluding impurities A valve seat region is welded by adding a filler in the form of a nickel alloy composed of a residual amount of Ni during welding and mixing with a valve member material to form a nickel main component alloy having the composition according to claim 1. A method of making a valve of any one of claims 1 to 3, wherein the valve is attached thereto.