Patent Application
20180100226
The present invention relates to a part produced by gas nitriding or gas soft nitriding, in particular a part in which wear resistance and pitting resistance are demanded such as a CVT pulley or gear, and a method of gas nitriding and gas soft nitriding used in production of these parts.
Steel parts used in automobiles and various industrial machinery etc. are sometimes required to have fatigue strength at their surfaces. For example, in CVT pulleys for transmissions, wear resistance is demanded, while in gears, the fatigue characteristic of pitting resistance is demanded. For improvement of these characteristics, improvement of the surface hardness of the steel parts is considered effective. For steel materials, nitriding and soft nitriding are being increasingly applied. Nitriding and soft nitriding of steel materials are advantageous in that a high surface hardness is obtained and heat treatment strain is small.
Nitriding is a method of treatment that diffuses nitrogen into the surface of a steel material, while soft nitriding is treatment that diffuses nitrogen and carbon into the surface of the steel material. As the medium used for the nitriding and soft nitriding, there are gases, salt baths, plasma, etc. The transmission parts of automobiles are mainly treated by the excellent productivity gas nitriding and gas soft nitriding.
The hardened layer formed by the gas nitriding and gas soft nitriding is comprised of a nitrogen diffusion layer and a compound layer formed at the surface side from the nitrogen diffusion layer and of a thickness of several μm to several tens of μm. The nitrogen diffusion layer is a layer hardened by diffused nitrogen, solid-solution strengthening by carbon, and the particle dispersion strengthening mechanism of nitrides. It is known that improvement of the hardness and depth of the nitrogen diffusion layer gives rise to an improvement in the pitting resistance. In the past, therefore much research has been conducted into improvement of the hardness and depth of the diffusion layer. The compound layer is comprised of an ϵ phase mainly made of Fe2-3N and also containing carbon or a γ′ phase mainly made of Fe4N. Compared with a steel material, the hardness is extremely high. When the compound layer is formed, the wear resistance is improved.
As conventional findings relating to the compound layer and wear resistance, the following may be mentioned. PLT 1 proposes a gear part which has been nitrided or carbonitrided, has a content of nitrogen from at least the surface down to a depth of 150 μm of 0.2 to 0.8%, has a quenched hardened layer of a mixed structure of martensite and 10 to 40% of residual austenite, and has excellent pitting resistance and wear resistance. PLT 1 has a description relating to the nitrogen content at the steel surface, but has no description relating to the components, composition, and properties of a compound layer formed by nitriding.
Further, PLT 2 proposes a method of treatment using a mixed gas with a residual concentration of NH3 of 45 to 65 vol % for soft nitriding at a gas temperature of 530 to 565° C. for 2 hours to thereby form a compound layer of a thickness of 2 to 12 μm containing pores and improve the pitting resistance, wear resistance, etc. The compound layer described in PLT 2 is comprised of Fe3N (ϵ), Fe4N (γ′), etc.
PLT 1: Japanese Patent Publication No. 7-190173A
PLT 2: Japanese Patent Publication No. 11-72159A
In the above-mentioned PLT 1, a part with excellent pitting resistance and wear resistance is proposed. However, surface hardening by quenching is utilized, so compared with a normal nitrided and soft nitrided part, the heat treatment strain is large and the cost of the later grinding process swells.
In PLT 2, the thickness of the compound layer was considered, but the pores were not optimized. For this reason, sometimes this cannot be applied to parts where high pitting strength is required.
The arts disclosed on the above-mentioned PLTs 1 and 2, as shown in the examples, are arts able to improve the wear resistance, pitting resistance, and other fatigue characteristics. However, the effects of the components, composition, and properties of the compound layer on the wear resistance and pitting resistance have not been studied.
The object of the present invention is to provide a part with excellent wear resistance and pitting resistance which enables demands for reducing the size and lightening the weight of parts and high load capacity to be met. Furthermore, as the means for the same, it also provides the methods of gas nitriding and gas soft nitriding optimally controlling the components and composition of the compound layer.
The components, composition, and thickness of the compound layer can be controlled by the treatment temperature and the nitriding potential (KN) defined by the following formula:
KN=(NH3 partial pressure)/[(H2 partial pressure)3/2] (formula 1)
However, the art of controlling the NH3 and N2 atmosphere in a production scale nitriding furnace has only been established in recent years, so there are still few findings regarding the components, composition, and properties of compound layers of actually produced parts.
Therefore, the inventors controlled the KN to change the compound layer in various ways and investigate the relationship of the compound layer and the wear resistance. As a result, they discovered that the improvement of the wear resistance is affected by the components, composition, thickness, and hardness of the compound layer and further is affected by the volume ratio of the cavities formed by the atomic state nitrogen diffusing into the steel during the nitriding becoming N2 molecules and being released from the steel (below, called “pores”).
Details of the obtained discoveries are summarized in the following (a) to (e):
(a) The compound layer formed by gas nitriding or gas soft nitriding is either of a γ′ single phase, ϵ single phase, and γ′+ϵ phase. The ϵ phase is higher in hardness than the γ′ phase, so to raise the wear resistance, it is effective to make the compound layer which is formed a single phase of the ϵ phase. The ϵ phase is formed in the higher KN region than the γ′ phase, so there is a need to set a lower limit of KN. Further, by raising the amount of carbon in the steel or performing soft nitriding, an ϵ single phase is easily obtained.
(b) The ϵ phase becomes harder the greater the carbon and nitrogen contents. For this reason, to raise the wear resistance of the ϵ phase, raising the amounts of carbon and nitrogen in the ϵ phase is effective. For this reason, it is necessary to raise the amount of carbon of the steel serving as the source of supply of the carbon and employ soft nitriding diffusing carbon so as to further perform nitriding/soft nitriding in the high KN region and raise the amount of nitrogen in the ϵ phase.
(c) If the thickness of the compound layer increases, pores are formed and the wear resistance and pitting strength fall. For this reason, it is necessary to suitably control the thickness of the compound layer. Specifically, the thickness of the compound layer becomes greater the higher the KN, so it is necessary to provide an upper limit of the KN.
(d) In actual gas nitriding, it is difficult to continue to hold the furnace gas atmosphere constant. For this reason, it is necessary to set a range of the KN value where a compound layer satisfying the above (a) to (c) is obtained. On the other hand, right after the start of treatment, the atmosphere becomes particularly unstable. It tends to take about 50 minutes until it stabilizes. For this reason, at minutes 0 to 50 after start of treatment, it is necessary to satisfy the above (a) to (c) and, considering the fact that the atmosphere is unstable, set the range of control of the KN value broader.
Furthermore, the following findings were obtained regarding the effect of the nitrogen diffusion layer on the pitting resistance and the wear resistance.
(e) If there are Mn, Cr, or other nitride forming elements in the steel, the nitrogen diffusion layer changes in hardness and diffusion layer depth. The pitting resistance is improved the higher the diffusion layer hardness and, further, the deeper the diffusion layer, so it becomes necessary to set the optimum ranges of the components of the steel material.
(f) The nitrogen diffusion layer is lower in wear resistance than the compound layer, so if the compound layer is worn away, wear proceeds faster.
Therefore, to improve the wear resistance and pitting resistance of a part utilizing gas nitriding and gas soft nitriding, it is necessary to control the KN and amount of C in the steel to control the amount of carbon and nitrogen in the compound layer and form a compound layer having few pores and having an ϵ single phase of a suitable thickness and hardness and adjust the steel components to increase the thickness of the nitrogen diffusion layer.
Note that, to evaluate the pores quantitatively, a SEM image of the compound layer was used, 50 μm line segments parallel to the surface were drawn every 2 μm from the surfacemost part to the bottommost part of the compound layer, the average value of the rates of the lengths of the pore parts in the line segments was calculated, and this was defined as the “pore volume ratio (%)”. Further, the evaluated value of the compound layer hardness was made the average value of 10 random points of the compound layer measured using a Microvicker's hardness meter at a load of 9.8×10−2N.
The present invention was completed based on the above discoveries and has as its gist the gas nitrided part and gas soft nitrided part shown in the following (1) to (4):
(1) A nitrided part or soft nitrided part made of a steel material comprising, by mass %,
(2) The nitrided part or soft nitrided part according to (1), further containing, by mass %, one or both of Mo: 0.01to less than 0.50% and V: 0.01to less than 0.50%.
(3) The nitrided part or soft nitrided part according to (1) or (2), further containing, by mass %, one or both of Cu: 0.01to less than 0.50% and Ni: 0.01to less than 0.50%.
(4) The nitrided part or soft nitrided part according to any one of (1) to (3) wherein the compound layer includes, by atm %, (C+N)=22% or more.
(5) A method of nitriding a part comprising a steel material having the components according to any one of (1) to (3), comprising heating the part in a gas atmosphere comprising NH3, H2, and N2 to 550 to 620° C. for 1.0 to 10 hours, wherein a nitriding potential KN obtained by the following (formula 1) is 0.3to 2.0in minute 0 to 50 in the nitriding time and is 0.70 to 1.50 from minute 50 on:
KN=(NH3 partial pressure)/[(H2 partial pressure)3/2] (formula 1)
(6) A method of soft nitriding a part comprising a steel material having the components according to any one of (1) to (3), comprising heating the part in a gas atmosphere comprising NH3, H2, N2, and CO2 to 550 to 620° C. for 1.0 to 10 hours, wherein a nitriding potential KN obtained by the following (formula 1) is 0.3to 2.0in minute 0 to 50 in the soft nitriding time and is 0.70 to 1.50 from minute 50 on:
KN=(NH3 partial pressure)/[(H2 partial pressure)3/2] (formula 1)
The nitrided part and soft nitrided part of the present invention are excellent in wear resistance and pitting resistance, so can be utilized for the gears, CVT pulleys, transmission parts, etc. of automobiles and industrial machines.
Below, the requirements of the present invention will be explained in detail. Note that the “%” showing the contents of the components of the elements in the steel material used as the material and the concentration of elements at the surfaces of the parts means “mass %”.
(A) Regarding Chemical Composition of Steel Material Used as Material
C: 0.05 to 0.3%
Si: 0.05 to 1.5%
Mn: 0.2 to 1.5%
P: 0.025% or less
S: 0.003 to 0.05%
Cr: 0.5 to 2.0%
Al: 0.01to 0.05%
N: 0.003 to 0.025%
The following are optional elements.
Mo: 0.01to less than 0.50%
V: 0.01to less than 0.50%
Cu: 0.01 to 0.50%
Ni: 0.01 to 0.50%
(B) Gas Nitriding and Gas Soft Nitriding Temperature
(C) Gas Nitriding and Gas Soft Nitriding Time
(D) KN Control During Gas Nitriding and Gas Soft Nitriding
To control the KN for nitriding, for example, there is the method of seasoning the part, before nitriding, by holding the inside of the furnace in a high NH3 atmosphere, then adjusting the flows of NH3, H2, and N2 to give the target KN, while for gas soft nitriding, further adjusting the flow of CO2, then introducing the part into a furnace. However, the method of control of KN of the present invention is not limited to this.
Note that, the atmosphere for performing gas nitriding and gas soft nitriding sometimes includes oxygen or other unavoidable impurities. In gas nitriding, the total of NH3, H2, and N2, while in gas soft nitriding, the total of NH3, H2, N2, and CO2 is preferably made 99.5% (vol %) or more.
(E) Identification of Compound Layer
(F) Hardness of Compound Layer
It is known that the wear resistance greatly depends on the hardness of the part from the surface down to several tens of μm. The inventors measured the Vicker's hardness of the compound layer based on “Vicker's Hardness Test—Test Method” described in JIS Z 2244 (2003).
The inventors compared and studied the results of a wear test using a roller pitting test machine. As a result, it became clear that to make the depth of wear after a repeated 2×106 cycles at a surface pressure of 1600 MPa 15 μm or less, the compound layer has to be 680 HV or more in hardness,
(G) Volume Ratio of Pores in Compound Layer
(H) Ratios of Components in Compound Layer
Steels “a” to “z” having the chemical components shown in Table 1 were melted in a 50 kg vacuum melting furnace, then were cast to form ingots. Note that, in Table 1, “a” to “q” are steels having the chemical components prescribed in the present invention. On the other hand, the steels “s” to “z” are steels of comparative examples with at least one or more elements outside the chemical components prescribed in the present invention.
0.02
0.17
0.060
0.40
0.01
0.092
0.18
0.45
0.060
0.01
1.80
*1Balance of chemical components is Fe and impurities.
Each ingot was hot forged to a diameter 35 mm rod. Next, each rod was annealed, then machined to fabricate a plate-shaped test piece for evaluation of the type, thickness, hardness, and volume ratios of pores of the compound layer. The plate-shaped test piece was made a vertical 20 mm, horizontal 20 mm, and depth 2 mm one. Further, a small roller for roller pitting test use was fabricated for evaluating the wear depth and pitting strength. The small roller had a diameter of 26 mm and a length of 130 mm.
Next, gases of NH3, H2, N2 (and, in case of gas soft nitriding, CO2) were introduced into the gas nitriding furnace. The part was gas nitrided and gas soft nitrided under the conditions shown in Table 2, then was oil cooled using 80° C. oil. In the gas nitriding and gas soft nitriding, the H2 partial pressure in the atmosphere was measured using a heat conducting type H2 sensor directly attached to the gas nitriding furnace. The difference in heat conductivity between the standard gas and measured gas was measured converted to the gas concentration. The H2 partial pressure was measured continuously during the gas nitriding. Further, the NH3 partial pressure was measured with a manual glass tube type NH3 analysis meter attached to the outside of the furnace. At the same time as measuring the partial pressure of the residual NH3 every 10 minutes, the nitriding potential KN was calculated and the flow rate of NH3 and flow rate of N2 were adjusted to make it converge to the target value. The nitriding potential KN was calculated every 10 minutes of measurement of the NH3 partial pressure and the flow rate of NH3 and flow rate of N2 were adjusted to make it converge to the target value.
0.20
0.35
3.00
1.60
0.10
0.20
0.65
630
20
25
650
19
31
530
20
29
1700
33
650
19
18
1700
620
19
35
660
18
16
1700
16
1600
11
1650
16
1650
630
18
1700
23
1650
1750
640
21
21
Test Nos. 1 to 25 are examples of the nitriding and soft nitriding of the present invention. After the nitriding and soft nitriding, the C-cross-section of each plate shaped test piece (drawing direction) was polished to a mirror finish, etched by a 3% Nital solution for 20 to 30 seconds, then measured for thickness of the compound layer and the volume ratio of the pores by SEM.
The compound layer was photographed at 2000×. From five fields of the photograph of the structure (field area: 2.4×102 μm2), the thicknesses of five points of the compound layer were measured at 10 μm intervals. The average value of the total 25 points was obtained as the compound thickness. Furthermore, 50 μm line segments parallel to the surface were drawn every 2 μm from the surfacemost part to the bottommost part of the compound layer, the ratios of length including the pores in the line segments were calculated using the following formula (2), and the average value of the five fields was used as the volume ratio of the pores.
Volume ratio of pores (%)=Length including pores (μm)/50 (μm)×100 formula (2)
Further, a cross-section polisher was used to polish the C-cross-section and an SEM (scan type electron microscope) was used to photograph the structure. The EBSD attached to the SEM was used to judge the phases formed in the compound layer. The compound layer was photographed at 2000×. Using five fields in the photograph of structure (field area: 2.4×102 μm2), 50 μm line segments parallel to the surface were drawn every 2 μm from the surface most part to the bottommost part of the compound layer, and the ratios of the length in the line segments where the CI value of Fe2-3N was 0.05 or less were calculated using the following formula (3). The case where the average value of five fields was less than 10% was judged to be the ϵ single phase.
Length where CI value of Fe2-3N is 0.05 or less (μm)/50 (μm)×100 formula (3)
Next, the Vicker's hardness was measured by the following method based on the “Vicker's Hardness Test—Test Method” in JIS Z 2244 (2003). That is, the average value of 10 points of Vicker's hardness at positions near the center of the compound layer in the thickness direction was defined as the hardness of the compound layer. The hardness of the compound layer was measured with a test load of 9.8×10−2N. The Vicker's hardness (HV) was measured at 10 points of each field and the average of the total 50 points was obtained.
Next, a small roller for roller pitting test use was finally worked at the grip part for the purpose of relieving the heat treatment strain, then was used as a roller pitting test piece. The shape after the final processing is shown in
Table 3 shows the test conditions when evaluating the wear depth. The test was stopped after a repeated 2×106 cycles. A roughness meter was used to run the wear part of the small roller along the main shaft direction then measure the maximum wear depth. The number N was made 5 to calculate the average value of the wear depth. The parts of the present invention were formed targeting a wear depth of 15 μm or less.
Further, Table 4 shows the test conditions for evaluation of the pitting strength. The test cutoff was made 107 showing the fatigue limit of general steel. The maximum surface pressure when the number of tests reached 107 without pitting occurring in the small roller test piece was defined as the fatigue limit of the small roller test piece. Pitting was detected by a vibration meter attached to the test machine. After vibration occurred, the rotations of both the small roller test piece and large roller test piece were made to stop. The occurrence of pitting and speed were confirmed. In the parts of the present invention, a maximum surface pressure at the fatigue limit of 1800 MPa or more was targeted.
The results are shown in Table 2. From Table 2, in Test Nos. 1 to 25 satisfying all of the conditions prescribed in the present invention, it is clear that the amount of wear and the pitting strength both reach the targets and good wear resistance and pitting resistance were obtained. Further, in the tests using steel containing at least one of Mo, V, Cu, and Ni as well, both the amounts of wear and pitting strengths reached the targets and it is clear that both excellent wear resistance and pitting resistance were obtained. On the other hand, Test Nos. 26 to 40 outside the conditions prescribed in the present invention are comparative examples. It is clear that either or both of the wear resistance and pitting resistance do not reach the target. Test Nos. 26, 27, 30, 36, and 40 are examples where ϵ single phases are not formed, but this is because the amount of C in the steel was not satisfied or the KN value was low or both were not satisfied. Test Nos. 28 and 29 are examples where the upper limit of the KN value during treatment became too high, so the ϵ phase became too large in thickness or cavity volume ratio. Test No. 31 is an example of a ϵ single phase material satisfying the above thickness and cavity volume ratio, but where the KN value during the treatment was too low, so the amount of (C+N) in the ϵ phase was low and the hardness was insufficient. Test Nos. 32 to 39 are examples where the chemical components of the steel are not optimized.
The gas nitrided part and gas soft nitrided part of the present invention are excellent in wear resistance and pitting resistance, so can be utilized for the transmission parts of automobiles or industrial machines etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-062803 | Mar 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/059489 | 3/24/2016 | WO | 00 |
