JP4041602B2

JP4041602B2 - Vacuum carburizing method for steel parts

Info

Publication number: JP4041602B2
Application number: JP30654498A
Authority: JP
Inventors: 敬二横瀬; 英樹井上; 英寿十良沢; 淳高橋; 真一武本; 伊孝中広
Original assignee: Dowa Holdings Co Ltd; Dowa Mining Co Ltd
Current assignee: Dowa Holdings Co Ltd
Priority date: 1998-10-28
Filing date: 1998-10-28
Publication date: 2008-01-30
Anticipated expiration: 2018-10-28
Also published as: JP2000129418A

Description

【０００１】
本発明は、鋼部品の浸炭方法、特に減圧下における鋼部品の浸炭方法に関する。
【０００２】
【従来の技術】
従来、減圧下における鋼部品の浸炭方法及び装置として、減圧保持した加熱室内の被処理物を高周波加熱手段により加熱する装置（特開平５９−２５９７４号公報）、浸炭用ガスとして鎖式不飽和炭化水素を使用するとともに、加熱室内を１ｋＰａ以下の真空状態にして浸炭処理を行う方法（特開平８−３２５７０１号公報）等が提案されている。
【０００３】
【発明が解決しようとする課題】
前記のごとき、減圧下における鋼部品の浸炭方法及び装置においては、減圧及び加熱条件等が微妙に鋼部品の浸炭精度に影響することに鑑み、本発明は浸炭むら及びスーティングの発生がなく、精度を高めた減圧浸炭が短時間に可能で、経済的な浸炭方法を提供することを目的とする。
【０００４】
【問題を解決するための手段】
請求項１は、減圧無酸化雰囲気の加熱室に鎖状飽和炭化水素ガスを供給して１．３３〜１３．３ｋＰａの減圧下において鋼部品を高周波加熱手段により１０００〜１２００℃に加熱して浸炭処理を行い、その後、前記鎖状飽和炭化水素ガスの供給を停止及び排出して減圧無酸化雰囲気下で拡散処理を行い、前記浸炭処理及び拡散処理を、鋼部品の表面炭素量がオーステナイト固溶限に到達しない範囲で、各浸炭時間を５秒〜３分として適数回繰り返すものである。
【０００５】
請求項１において、加熱室内を１．３３〜１３．３ｋＰａの減圧下におくことは、１．３３ｋＰａ未満では浸炭むらが生じ、また、１３．３ｋＰａを超えると浸炭に直接関与しない炭素がススとなって、いわゆるスーティングが生じるこおとが確認されたためである。
【０００６】
また、高周波加熱を手段を採用したことは、鋼部品のみを加熱することができるため、加熱室の断熱保温材を必要とせず、加熱室構造を簡略化できるため経済的であり、さらに加熱温度を通常一般の浸炭処理温度より高めたのは浸炭時間の短縮を図るためであり、また、浸炭処理後の拡散処理において鎖状飽和炭化水素ガスの供給を停止及び排出することは、浸炭処理後、残存している鎖状飽和炭化水素ガスを直ちに排出することによりス−ティングの発生を抑えることができるためである。
【０００７】
また、鋼部品の表面炭素量がオーステナイト固溶限に到達しない範囲で前記浸炭処理及び拡散処理を適数回繰り返すことにより、必要とする浸炭深さの調整及び確保が容易となり、浸炭精度を高めることができる。
【０００８】
なお、本発明では、前記適数回繰り返す浸炭処理及び拡散処理時の各浸炭時間を５秒から３分とする。
【０００９】
５秒以下では、鋼部品が十分加熱されずに、しかも一回のサイクルで鋼部品への炭素の供給が不十分となる。また、浸炭のサイクルタイムが短い場合、拡散のサイクルタイムも短くなるが、この時拡散時に必要とする真空度まで時間内に排気することが困難になってしまう。
【００１０】
請求項２は、前記請求項１の工程の後、無酸化雰囲気下において一次冷却、再加熱、均熱保持及びガス焼入れを行うものである。すなわち、高温処理中に粗大化した結晶粒を微細化するために行うものである。
【００１１】
請求項３は、前記請求項１及び２における高周波加熱の出力を１０ｋＨｚ以下としたものである。すなわち、短時間に鋼部品の内部まで十分加熱するためであり、１０ｋＨｚを超える高周波出力で加熱を行なうと、鋼部品の表面のみが加熱され、内部までの加熱に時間がかかるためである。
【００１２】
【発明の実施の形態】
以下に本発明の実施の形態の一例を説明する。図１は本願方法の発明を実施するに適した減圧浸炭装置の概略断面図が示されている。図中、１は気密性を有する加熱室、２は被処理品としての鋼部品、３は高周波加熱装置、４はポンプ等の減圧排気装置、５は前記加熱室１への鎖状飽和炭化水素ガス供給装置（以下単に「ガス供給装置」と言う。）、６は制御バルブ、７は加熱室１内の圧力制御装置、８は冷却室、９は前記加熱室１と冷却室８との間に設けられた開閉扉、１０はファン、１１はファンモータ、１２は搬送装置である。
【００１３】
つぎに、前記減圧浸炭装置を使用して本発明の減圧浸炭方法は下記のごとく実施される。
【００１４】
（１）加熱室１に鋼部品２がセットされ、減圧排気装置４を稼働させて加熱室１内を１．３３Ｘ１０^-3ｋＰａに減圧し、無酸化雰囲気を保持する。
【００１５】
（２）高周波加熱装置３により、前記鋼部品２を１０００〜１２００℃に急速加熱する。
【００１６】
（３）圧力制御装置７を制御して加熱室１内が１．３３〜１３．３ｋＰａの圧力になるようにガス供給装置５から鎖状飽和炭化水素ガス（プロパン、メタン、ブタンガス・・・以下単に「ガス」と言う。）が供給される。
【００１７】
（４）浸炭処理を一定時間行った後に前記ガスの供給を停止及び排出して減圧無酸化雰囲気で拡散処理を行う。
【００１８】
（５）さらに要求される炭素濃度及び浸炭深さが得られていない場合、その他当初の処理計画に基づき前記（３）及び（４）が交互に繰り返される。
【００１９】
（６）鋼部品２に要求される浸炭深さが得られたことが確認されたら、減圧下で開閉扉９を開き、前記鋼部品 2を冷却室８に移動させ、Ａｒ₃ 変態点以下まで冷却を行う（一次冷却と言う。）。
【００２０】
（７）減圧下で加熱室１に前記鋼部品２を戻し、高周波加熱にてオ−ステナイト化温度以上に再加熱後、所定時間保持する。
【００２１】
（８）減圧下で冷却室８に鋼部品１を移動してガス焼入れを行う（後述図２参照）。
【００２２】
【実施例１】
供試片として、ＪＩＳ．ＳＣｒ４２０Ｈを鍛造成形後に機械加工を施し、リング状に構成した。形状は、外径Φ１９０ｍｍ、内径Φ１４０ｍｍ、厚さ２５ｍｍであり、重量は２．３６ｇ、表面積５３１ｃｍ² である。さらに前記供試片に、径Φ５ｍｍ及びΦ８ｍｍ、深さ１５ｍｍの未貫通穴を加工した（以下「鋼部品２」と言う。）。
【００２３】
前記鋼部品２を前記加熱室１内にセットし、該加熱室１内を減圧排気装置４を稼動させて０．６Ｘ１０^−３ｋＰａ以下に減圧した後、高周波加熱装置３の０．８ｋＨｚの高周波誘導加熱により前記鋼部品２を１１００℃まで急速昇温して３分間均熱保持した。なお、この時、室温から１１００℃到達までの所要時間は３分であった。
【００２４】
つぎに、鎖状飽和炭化水素ガスとしてプロパンガスを前記加熱室１内の圧力が７．９８ｋＨｚになるまで瞬時に導入した後、さらに前記圧力を維持するように、圧力制御装置７により減圧排気装置４とガス供給装置５を制御して浸炭処理を施した。
【００２５】
前記１１００℃における減圧浸炭の鋼部品への炭素供給速度が非常に速いので、１１００℃におけるオ−ステナイトの固溶炭素量及び拡散速度が大きいにもかかわらず、短時間の浸炭処理で鋼部品２の表面炭素量は固溶限に達し、その後セメンタイトの析出及び結晶粒界、さらに粒内に合金炭化物を形成する。
【００２６】
前記合金炭化物の析出により、母相の合金成分、特に肌焼鋼ではＣｒが減少することにより焼入れ性が低下するため、鋼部品２の表面近傍に不完全焼入れ組織としてトル−スタイトを析出し、表面硬さの低下のみならず材料硬度の低下をきたす。
【００２７】
また、粒界に網状に析出したセメンタイトもまた、材料強度を損なう要因となる。さらに鋼部品２の表面炭素濃度が固溶限に達すると、鋼部品２表面にススが析出しやすくなり、浸炭反応を阻害する。
【００２８】
前記のごとき鋼部品２の品質低下を防止するためには、鋼部品２の表面炭素量がオーステナイト固溶限に到達しない範囲で浸炭時間を設定して浸炭処理を施して、続いて拡散処理を施すことが望ましい。このことから、高温減圧浸炭処理においては短時間に浸炭と拡散のサイクルを適数回繰り返すことが好ましい。下記表１には、本実施例１における浸炭と拡散のサイクルが示されている。
【００２９】
【表１】

表１によれば、当初の浸炭時間は１２０秒であるが、その後の浸炭処理及び拡散処理の繰り返し時の浸炭時間はそれぞれ３０秒とされている。前記表１に示した浸炭と拡散処理を終了した後、高温処理中に粗大化した結晶粒を微細化するために一次冷却、再加熱、均熱保持、焼入れを無酸化雰囲気下にて行った。
【００３０】
前記一次冷却は、最終サイクル拡散後の０．６７Ｘ１０^−３ｋＰａに減圧された加熱室１から、０．６７Ｘ１０^−３ｋＰａに減圧排気された冷却室８に鋼部品２を移動してから加熱室１との間の開閉扉９を閉め、１．０１Ｘ１０^２ｋＰａ（大気圧）のＮ_２ガスを導入し、ファン１０を用いて鋼部品２の温度が５００〜６００℃になるまで行った。
【００３１】
つぎに鋼部品２を８５０℃に再加熱し、３分間均熱保持した後、焼入れを行った。該焼入れは前記一次冷却と同様に、０．６７Ｘ１０^-3ｋＰａに減圧排気された無酸化状態にて加熱室１から冷却室８へ鋼部品２を移送後、冷却室８にＮ₂ ガスを７．９８Ｘ１０² （６ｂｅｒ）導入し、ガス焼入れにて行った。前記減圧浸炭処理結果が下記表２に示されている。
【００３２】
【表２】

表２は前記供試片、すなわち、鋼部品２のΦ５ｍｍ穴及びΦ８ｍｍ穴の１５ｍｍ底部における浸炭深さを鋼部品２の外周面における浸炭深さに対する比率で表し、各浸炭処理圧と穴の径別に表したものである。すなわち、比率が１に近いほど、複雑な形状の鋼部品２でも浸炭むらがないことを意味する。
【００３３】
前記表２から明らかなように、６．６５〜１３．３ｋＰａの浸炭処理圧の場合に各穴の深部まで均一な浸炭が行なわれることが確認され、かつスーティングも認められなかった。また、１．３３〜６．６５ｋＰａに至らない浸炭処理圧においては若干の浸炭むらが認められたが、スーティングの発生なしに処理が可能であることが確認された。
【００３４】
なお、浸炭処理圧が１３．３ｋＰａを超えた場合は、鋼部品２及び加熱室１内部にスーティングが発生し、連続処理に不都合をきたすばかりでなく、浸炭むらが発生する。これはワーク表面に析出したスス量が非常に多くなり浸炭反応を阻害するためである。
【００３５】
【実施例２】
ＪＩＳ．ＳＣｒ４２０を鍛造成形後に加工したリングギアを用いて処理を行った。リングギアは外径２１８ｍｍ、モジュ−ル２．４、歯巾３３ｍｍ、重量３．４ｋｇである。
【００３６】
前記リングギアを１ｋＨｚの高周波誘導加熱により加熱した。その他の処理条件は前記実施例１と同様である。図２にはその処理工程全体が示されている。図中、Ａは一次昇温工程、Ｂは均熱保持工程、Ｃは浸炭−拡散処理サイクル、Ｄは一次冷却工程、Ｅは再加熱工程、Ｆは均熱保持工程、そしてＧがＮ₂ ガス焼入れである。前記Ｃの浸炭−拡散処理サイクルは下記表３の通りである。
【表３】

前記処理を施した前記リングギアの歯面部及び歯底部の硬さ測定結果が図３に示され、図４に歯面部、図５に歯底部の断面組織写真（Ｘ４００）が示されている。
【００３７】
前記、図３から明らかなように、歯面部及び歯底部の硬度分布もほぼ同様であり、また、図４及び図５から明らかなように歯面部及び歯底部の組織及び浸炭濃度もほぼ同一であり、さらに粒界酸化層も全く確認されなかった。
【００３８】
本願方法の発明によれば、浸炭むら及びスーティングを発生させることなく、精度を高めた減圧浸炭が短時間に可能で、経済的である効果が得られる。
【図面の簡単な説明】
【図１】本発明を実施する減圧浸炭装置の実施の一形態を示す概略断面図である。
【図２】実施例２の処理時間及び温度曲線図である。
【図３】実施例２のリンクギア歯面部と歯底部の硬さの測定曲線図である。
【図４】実施例２の処理後のリンクギアの歯面部の金属組織を示す顕微鏡写真（Ｘ４００）である。
【図５】実施例２の処理後のリンクギアの歯底部の金属組織を示す顕微鏡写真（Ｘ４００）である。
【符号の説明】
１加熱室
２鋼部品
３高周波加熱装置
４減圧排気装置
５鎖状飽和炭化水素ガス供給装置
７圧力制御装置
８冷却装置[0001]
The present invention, carburizing how the steel part, about the carburizing how the steel part under reduced pressure, especially.
[0002]
[Prior art]
Conventionally, as a carburizing method and apparatus for steel parts under reduced pressure, an apparatus (Japanese Patent Laid-Open No. 59-25974) for heating an object to be processed in a heating chamber held under reduced pressure by a high-frequency heating means, and chain unsaturated carbonization as a carburizing gas. There has been proposed a method (Japanese Patent Laid-Open No. 8-325701) and the like that use hydrogen and perform a carburizing process in a vacuum state of 1 kPa or less in the heating chamber.
[0003]
[Problems to be solved by the invention]
Said such as, in the carburizing method and apparatus of the steel components under reduced pressure, in view of the fact that vacuum and heating conditions, etc. are slightly affects the carburizing accuracy of the steel parts, the onset Ming no occurrence of uneven carburization and sooting , can in a short period of time under reduced pressure carburization with improved accuracy, and to provide an economical carburizing how.
[0004]
[Means for solving problems]
According to the first aspect of the present invention, a chain saturated hydrocarbon gas is supplied to a heating chamber in a reduced pressure non-oxidizing atmosphere, and the steel parts are heated to 1000 to 1200 ° C. by high frequency heating means under reduced pressure of 1.33 to 13.3 kPa, and carburized. After that, the supply of the chain saturated hydrocarbon gas is stopped and discharged, and diffusion treatment is performed in a reduced pressure non-oxidizing atmosphere, and the carburization treatment and diffusion treatment are performed in such a manner that the surface carbon content of the steel part is austenite solid solution. within a range not reaching the limit, in which the carburizing time and minutes 5 seconds to 3 are repeated several times applied to.
[0005]
In claim 1, placing the heating chamber under a reduced pressure of 1.33 to 13.3 kPa causes uneven carburization at less than 1.33 kPa, and carbon that does not directly participate in carburization exceeds 13.3 kPa This is because it has been confirmed that so-called sooting occurs.
[0006]
In addition, the use of high-frequency heating means is economical because only the steel parts can be heated, so that no heat insulation and heat insulation material is required in the heating chamber, and the heating chamber structure can be simplified. The temperature was raised from the normal carburizing temperature in order to shorten the carburizing time, and the chain saturated hydrocarbon gas supply was stopped and discharged in the diffusion process after carburizing. This is because the generation of sooting can be suppressed by immediately discharging the remaining chain saturated hydrocarbon gas.
[0007]
Further, by repeating the carburizing treatment Ri及 beauty diffusion process to the extent that the surface carbon content of the steel parts do not reach the austenite solubility limit applied several times, easy to adjust and secure the carburized depth in need, and the carburizing accuracy Can be increased.
[0008]
In the present invention, it

minutes

3 each carburizing time during carburization and diffusion treatment is repeated the applied several times from 5 seconds.
[0009]
If it is 5 seconds or less, the steel part is not sufficiently heated, and the supply of carbon to the steel part is insufficient in one cycle. In addition, when the carburizing cycle time is short, the diffusion cycle time is also shortened, but at this time, it becomes difficult to exhaust the exhaust gas to the required vacuum level during the time.
[0010]
In the second aspect, after the process of the first aspect, primary cooling, reheating, soaking and gas quenching are performed in a non-oxidizing atmosphere. That is, it is performed in order to refine the crystal grains coarsened during the high temperature treatment .
[0011]
In the third aspect, the output of the high frequency heating in the first and second aspects is 10 kHz or less. That is, it is for heating sufficiently to the inside of a steel part in a short time, and when heating is performed at a high frequency output exceeding 10 kHz, only the surface of the steel part is heated and it takes time to heat the inside.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
An example of the embodiment of the present invention will be described below. Figure 1 is a schematic cross-sectional view of a vacuum carburization apparatus suitable for carrying out the invention of the gun method is shown. In the figure, 1 is an airtight heating chamber, 2 is a steel part as a product to be treated, 3 is a high-frequency heating device, 4 is a vacuum exhaust device such as a pump, and 5 is a chain saturated hydrocarbon to the heating chamber 1. Gas supply device (hereinafter simply referred to as “gas supply device”), 6 is a control valve, 7 is a pressure control device in the heating chamber 1, 8 is a cooling chamber, and 9 is between the heating chamber 1 and the cooling chamber 8. , 10 is a fan, 11 is a fan motor, and 12 is a transport device.
[0013]
Next, vacuum carburization process of the present invention the pre-Symbol decreased leaching coal device use is carried out as follows.
[0014]
(1) The steel part 2 is set in the heating chamber 1, the vacuum exhaust device 4 is operated, and the pressure in the heating chamber 1 is reduced to 1.33 × 10 ⁻³ kPa to maintain a non-oxidizing atmosphere.
[0015]
(2) The steel part 2 is rapidly heated to 1000 to 1200 ° C. by the high-frequency heating device 3.
[0016]
(3) A chain saturated hydrocarbon gas (propane, methane, butane gas... Below) from the gas supply device 5 so as to control the pressure control device 7 so that the pressure in the heating chamber 1 becomes 1.33 to 13.3 kPa. Simply called "gas").
[0017]
(4) After performing the carburizing process for a predetermined time, the supply of the gas is stopped and discharged, and the diffusion process is performed in a reduced pressure non-oxidizing atmosphere.
[0018]
(5) Further, when the required carbon concentration and carburization depth are not obtained, the above (3) and (4) are alternately repeated based on other initial treatment plans.
[0019]
(6) When it is confirmed that the required carburizing depth for the steel part 2 is obtained, the door 9 is opened under reduced pressure, the steel part 2 is moved to the cooling chamber 8, and the Ar ₃ transformation point or less is reached. Cooling is performed (referred to as primary cooling).
[0020]
(7) The steel part 2 is returned to the heating chamber 1 under reduced pressure, and is kept for a predetermined time after reheating to the austenitizing temperature or higher by high-frequency heating.
[0021]
(8) Gas quenching is performed by moving the steel part 1 to the cooling chamber 8 under reduced pressure (see FIG. 2 described later).
[0022]
[Example 1]
As a test piece, JIS. SCr420H was machined after forging and formed into a ring shape. The shape is an outer diameter of Φ190 mm, an inner diameter of Φ140 mm, a thickness of 25 mm, a weight of 2.36 g, and a surface area of 531 cm ² . Further, non-through holes having a diameter of Φ5 mm, Φ8 mm, and a depth of 15 mm were processed in the specimen (hereinafter referred to as “steel part 2”).
[0023]
Set the steel part 2 before Symbol pressurized hot chamber 1, the pressure was reduced to ^{0.6 × 10} -3 kPa below the the heating chamber 1 is operated vacuum evacuation device 4, the high-frequency heating apparatus 3 0.8 kHz The steel part 2 was rapidly heated to 1100 ° C. by high-frequency induction heating and kept soaked for 3 minutes. At this time, the time required to reach 1100 ° C. from room temperature was 3 minutes.
[0024]
Next, after introducing propane gas as chain saturated hydrocarbon gas instantaneously until the pressure in the heating chamber 1 becomes 7.98 kHz, the pressure control device 7 further reduces the pressure so that the pressure is maintained. 4 and the gas supply device 5 were controlled to perform carburizing treatment.
[0025]
Since the carbon supply rate to the steel part of reduced pressure carburizing at 1100 ° C. is very high, the steel part 2 can be obtained by a short time carburizing treatment despite the large amount of dissolved carbon and diffusion rate of austenite at 1100 ° C. The amount of surface carbon reaches the solid solubility limit, and thereafter cementite precipitates and crystal grain boundaries, and alloy carbides are formed in the grains.
[0026]
Due to the precipitation of the alloy carbide, since the hardenability deteriorates due to the reduction of Cr in the alloy component of the parent phase, particularly the case-hardened steel, torsutite is precipitated as an incompletely hardened structure near the surface of the steel part 2, Not only the surface hardness is lowered but also the material hardness is lowered.
[0027]
Further, cementite precipitated in a network form at the grain boundaries is also a factor that impairs the material strength. Further, when the surface carbon concentration of the steel part 2 reaches the solid solubility limit, soot is likely to precipitate on the surface of the steel part 2 and inhibits the carburization reaction.
[0028]
In order to prevent the deterioration of the quality of the steel part 2 as described above, the carburizing time is set in the range where the surface carbon amount of the steel part 2 does not reach the austenite solid solubility limit, and then the diffusion treatment is performed. It is desirable to apply. Therefore, it is preferable to repeat several times apply the cycle and diffusion carburizing in a short period of time at high temperature under reduced pressure carburization process. Table 1 below shows the carburization and diffusion cycle in Example 1.
[0029]
[Table 1]

According to Table 1, the initial carburizing time is 120 seconds, but the carburizing time when the subsequent carburizing treatment and diffusion treatment are repeated is 30 seconds. After finishing the carburizing and diffusion treatment shown in Table 1, primary cooling, reheating, soaking and quenching were performed in a non-oxidizing atmosphere in order to refine the coarsened crystal grains during the high temperature treatment. .
[0030]
The primary cooling is performed after moving the steel part 2 from the heating chamber 1 decompressed to 0.67 × 10 ⁻³ kPa after diffusion of the final cycle to the cooling chamber 8 evacuated to 0.67 × 10 ⁻³ kPa. close the door 9 between 1, _{N 2} gas was introduced in 1.01X10 ² kPa (atmospheric pressure) was carried out until the temperature of the steel part 2 is 500 to 600 ° C. using a fan 10.
[0031]
Next, the steel part 2 was reheated to 850 ° C., kept soaked for 3 minutes, and then quenched.該焼insertion, like the primary cooling, after transferring the steel part 2 from the heating chamber 1 into the cooling chamber 8 in a non-oxidizing state of being evacuated to 0.67X10 ^-3 kPa, the N ₂ gas in the cooling chamber 8 7 .98 × 10 ² (6 bar) was introduced and gas quenching was performed. The results of the reduced pressure carburizing treatment are shown in Table 2 below.
[0032]
[Table 2]

Table 2 shows the carburization depth at the bottom of 15 mm of the Φ5 mm hole and Φ8 mm hole of the steel part 2 as a ratio to the carburization depth on the outer peripheral surface of the steel part 2, and each carburizing pressure and the diameter of the hole. It is shown separately. That is, as the ratio is closer to 1, it means that there is no uneven carburization even in the steel part 2 having a complicated shape.
[0033]
As apparent from Table 2, it was confirmed that uniform carburization was performed up to the deep part of each hole in the case of carburizing treatment pressure of 6.65 to 13.3 kPa, and sooting was not recognized. Further, although carburizing unevenness was slightly observed at a carburizing treatment pressure not reaching 1.33 to 6.65 kPa , it was confirmed that the treatment was possible without the occurrence of sooting.
[0034]
When the carburizing pressure exceeds 13.3 kPa , sooting occurs in the steel part 2 and the heating chamber 1, causing not only inconvenience in continuous processing, but also uneven carburizing. This is because the amount of soot deposited on the workpiece surface becomes very large and inhibits the carburization reaction.
[0035]
[Example 2]
JIS. Processing was performed using a ring gear obtained by processing SCr420 after forging. The ring gear has an outer diameter of 218 mm, a module of 2.4, a tooth width of 33 mm, and a weight of 3.4 kg.
[0036]
The ring gear was heated by high frequency induction heating at 1 kHz. Other processing conditions are the same as those in the first embodiment. FIG. 2 shows the entire processing process. In the figure, A is the primary temperature raising process, B is the soaking process, C is the carburizing-diffusion treatment cycle, D is the primary cooling process, E is the reheating process, F is the soaking process, and G is the N ₂ gas. It is quenching. The C carburization-diffusion treatment cycle is as shown in Table 3 below.
[Table 3]

The hardness measurement results of the tooth surface portion and the tooth bottom portion of the ring gear subjected to the processing are shown in FIG. 3, FIG. 4 shows a tooth surface portion, and FIG. 5 shows a cross-sectional structure photograph (X400) of the tooth bottom portion.
[0037]
As apparent from FIG. 3, the hardness distributions of the tooth surface portion and the tooth bottom portion are substantially the same, and the tissue and carburization concentration of the tooth surface portion and the tooth bottom portion are substantially the same as is apparent from FIGS. 4 and 5. In addition, no grain boundary oxide layer was observed.
[0038]
According to the invention of the present process, carburization unevenness and without generating soot, can in a short period of time under reduced pressure carburization with improved accuracy, the effect is obtained is economical.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing an embodiment of a reduced pressure carburizing apparatus for carrying out the present invention.
2 is a treatment time and temperature curve diagram of Example 2. FIG.
FIG. 3 is a measurement curve diagram of the hardness of a link gear tooth surface part and a tooth bottom part of Example 2.
4 is a micrograph (X400) showing a metallographic structure of a tooth surface portion of a link gear after the processing of Example 2. FIG.
5 is a photomicrograph (X400) showing the metal structure of the tooth bottom portion of the link gear after the processing of Example 2. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Heating chamber 2 Steel parts 3 High frequency heating device 4 Depressurization exhaust device 5 Chain saturated hydrocarbon gas supply device 7 Pressure control device 8 Cooling device

Claims

A chain saturated hydrocarbon gas is supplied to a heating chamber in a reduced pressure non-oxidizing atmosphere, and the steel parts are heated to 1000 to 1200 ° C. by a high frequency heating means under a reduced pressure of 1.33 to 13.3 kPa, and then carburized. The supply of the chain saturated hydrocarbon gas is stopped and discharged to perform a diffusion treatment in a reduced pressure non-oxidizing atmosphere, and the carburization treatment and the diffusion treatment are performed in a range where the surface carbon amount of the steel part does not reach the austenite solid solubility limit. in vacuum carburizing method of the steel parts, characterized in that repeated several times apply the carburizing time 5 seconds to 3 minutes to.

A chain saturated hydrocarbon gas is supplied to a heating chamber in a reduced pressure non-oxidizing atmosphere, and the steel parts are heated to 1000 to 1200 ° C. by a high frequency heating means under a reduced pressure of 1.33 to 13.3 kPa, and then carburized. The supply of the chain saturated hydrocarbon gas is stopped and discharged to perform a diffusion treatment in a reduced pressure non-oxidizing atmosphere, and the carburization treatment and the diffusion treatment are performed in a range where the surface carbon amount of the steel part does not reach the austenite solid solubility limit. in the respective carburizing time and minutes 5 seconds to 3 repeated several times applied to, then, the primary cooling in the absence of an oxidizing atmosphere, reheating, soaking and vacuum carburizing of steel parts and performing gas quenching Method.

The low-pressure carburizing method for steel parts according to claim 1 or 2, wherein the output of the high-frequency heating is 10 kHz or less.