JP5793802B2 - Gas carburizing method - Google Patents
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Description
本発明は、ガス浸炭方法に関する。 The present invention relates to a gas carburizing method.
従来のガス浸炭方法として、変成炉で生成された吸熱型の変成ガスを用いるものがあり、前記変成ガスの浸炭力の弱さを補うために、変成ガスに加えてエンリッチガスを炉内に供給して浸炭を行うことで、所定の表面浸炭濃度を確保している（特許文献１参照）。 As a conventional gas carburizing method, there is a method using an endothermic shift gas generated in a shift furnace, and in order to compensate for the weak carburizing power of the shift gas, an enriched gas is supplied into the furnace in addition to the shift gas. By performing carburization, a predetermined surface carburization concentration is secured (see Patent Document 1).
前記方法には、浸炭雰囲気ガスのカーボンポテンシャルが平衡状態図における浸炭処理温度でのγ相の飽和炭素量を超えると、スーティングが発生したり、処理品表面にセメンタイト等の異常組織が生成したりする等の問題がある。 In the above method, when the carbon potential of the carburizing atmosphere gas exceeds the saturated carbon content of the γ phase at the carburizing temperature in the equilibrium diagram, sooting occurs or abnormal structure such as cementite is generated on the surface of the treated product. There are problems such as.
そこで従来、浸炭処理温度でのγ相の飽和炭素量以下のできるだけ高いカーボンポテンシャルを有する雰囲気ガスで浸炭処理を行うのが効率の良い浸炭条件であると考えられている。 Therefore, conventionally, it is considered that carburizing treatment with an atmospheric gas having a carbon potential as high as possible below the saturated carbon amount of the γ phase at the carburizing temperature is an efficient carburizing condition.
しかしながら、前記従来の方法では、浸炭雰囲気ガスのカーボンポテンシャルの上限に制約があるので、浸炭時間を短縮しようとしても限界がある。 However, in the conventional method, there is a limit to the upper limit of the carbon potential of the carburizing atmosphere gas, so there is a limit even if it is attempted to shorten the carburizing time.
本発明は、前記の如き事情に鑑みてなされたものであり、スーティングや異常組織の発生を抑制でき、浸炭時間も短縮できるガス浸炭方法を提供しようとするものである。 The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a gas carburizing method that can suppress the occurrence of sooting and abnormal tissue and can shorten the carburizing time.
前記課題を解決するため、本発明の発明者等は鋭意研究を重ね、その結果として、炭化水素系ガスと空気とを用いて生成された変成ガスからＣＯ２を低減させた組成に相当する組成を有するガスは、前記変成ガスよりもカーボンポテンシャル（ＣＰ値）が高く、且つ、処理品表面に炭素が浸入する速度が従来のエンリッチガスを用いた浸炭雰囲気ガスよりも遅いことを発見した。本発明は、このような知見に基づいてなされたもので、次の点に特徴を有する。 In order to solve the above problems, the inventors of the present invention have made extensive studies and, as a result, a composition corresponding to a composition in which CO 2 is reduced from a modified gas generated by using a hydrocarbon-based gas and air. It has been discovered that a gas having a carbon potential (CP value) is higher than that of the metamorphic gas, and the rate at which carbon enters the surface of the treated product is slower than a carburizing atmosphere gas using a conventional enriched gas. The present invention has been made based on such knowledge, and has the following features.
すなわち、本発明に係るガス浸炭方法は、炭化水素系ガスと空気とを用いて変成ガスを生成する工程と、この工程で得られた変成ガスにエンリッチガスを混合することなくＣＯ ２ の含有量を全体の０．１体積％以下に低減させる工程と、を経ることで、浸炭処理温度において鋼の平衡状態図におけるγ相の飽和炭素量を超える１．３％以上のカーボンポテンシャルを有する浸炭雰囲気ガスを生成し、この浸炭雰囲気ガスを用いて、処理品表面にセメンタイトが析出することがない温度領域で浸炭を行うことを特徴とする（請求項１）。 That is, the gas carburizing method according to the present invention includes a step of generating a modified gas using a hydrocarbon-based gas and air, and a content of CO 2 without mixing an enriched gas with the modified gas obtained in this step. A carburizing atmosphere having a carbon potential of 1.3% or more exceeding the saturated carbon content of the γ phase in the equilibrium diagram of the steel at the carburizing treatment temperature. A gas is generated, and carburization is performed in a temperature range in which cementite does not precipitate on the surface of the processed product using the carburized atmosphere gas (claim 1).
本発明によれば、浸炭処理温度でのγ相の飽和炭素量を超えるカーボンポテンシャルを有する浸炭雰囲気ガスを用いて浸炭を行うので、浸炭の効率が良く、浸炭時間が短縮される。 According to the present invention, carburizing is performed using the carburizing atmosphere gas having a carbon potential exceeding the saturated carbon content of the γ phase at the carburizing temperature, so that the carburizing efficiency is good and the carburizing time is shortened.
また、本発明で用いる浸炭雰囲気ガスは、カーボンポテンシャルが１．３％以上と高くても、処理品表面に炭素が浸入する速度が遅く、且つ、比較的高温域において処理することで処理品内部に炭素が拡散する速度が速くなる。本発明の発明者等は、処理品表面にセメンタイトを析出させず、且つ処理品内部に炭素を拡散させ得る温度領域があることを見出した。 In addition, the carburizing atmosphere gas used in the present invention has a slow rate of carbon intrusion into the surface of the processed product even when the carbon potential is as high as 1.3% or more , and the inside of the processed product is processed in a relatively high temperature range. The rate at which carbon diffuses into the substrate increases. The inventors of the present invention have found that there is a temperature region in which cementite is not deposited on the surface of the treated product and carbon can be diffused inside the treated product.
さらに本発明では、処理品表面にセメンタイトが析出することがない温度領域で浸炭を行うので、処理品の品質も良い。エンリッチガスを用いないので、スーティングも起こりにくい。 Furthermore, in the present invention, since carburization is performed in a temperature range in which cementite does not precipitate on the surface of the treated product, the quality of the treated product is good. Since no enriched gas is used, sooting is unlikely to occur.
好適な実施の一形態として、前記温度領域が９００〜１０６０℃である態様を例示する（請求項２）。さらに好ましい前記温度領域として、９２０〜９７０℃を例示する（請求項３）。 As a preferred embodiment, a mode in which the temperature region is 900 to 1060 ° C. is exemplified (Claim 2). A more preferable temperature range is 920 to 970 ° C. (Claim 3).
好適な実施の一形態として、前記浸炭雰囲気ガスの炉内流速が１．５ｍ／ｓ以上の流速である態様（請求項４）、及び、前記浸炭雰囲気ガスの炉内流量が炉内雰囲気置換回数２９回／ｈ以上となる流量である態様（請求項５）を採用することもできる。これらの態様によれば、有効硬化層深さを確実に目標値に到達させることができるとともに、浸炭時間も短縮できる。 As a preferred embodiment, a mode in which the in-furnace flow rate of the carburizing atmosphere gas is a flow rate of 1.5 m / s or more (Claim 4 ), and the in-furnace flow rate of the carburizing atmosphere gas is the number of times the furnace atmosphere is replaced An embodiment (Claim 5 ) having a flow rate of 29 times / h or more may be employed. According to these aspects, the effective hardened layer depth can surely reach the target value, and the carburizing time can be shortened.
以下、添付図面を参照して、本発明の実施の一形態に係るガス浸炭方法について説明する。 Hereinafter, a gas carburizing method according to an embodiment of the present invention will be described with reference to the accompanying drawings.
本発明の実施の一形態に係るガス浸炭方法は、炭化水素系ガスと空気とを用いて生成された変成ガスからＣＯ２を低減させた組成に相当する組成を有する低ＣＯ２変成ガスを雰囲気ガスとして使用する。 In the gas carburizing method according to one embodiment of the present invention, a low CO 2 modified gas having a composition corresponding to a composition obtained by reducing CO 2 from a modified gas generated using a hydrocarbon-based gas and air is used as an atmosphere. Use as gas.
前記変成ガスは、従来公知の変成炉を用いて生成できる。変成炉で生成される変成ガスは、炭化水素系ガス（例えば、１３Ａガス、天然ガス、プロパン、ブタン等）と空気の混合比や、また目的とするガス組成、ＣＰ値によって様々であるが、概ね、ＣＯ：１８〜２５体積％、Ｈ２：３０〜４１体積％、Ｎ２：３５〜５０体積％、ＣＯ２：０．１２〜０．２５体積％、Ｈ２Ｏ：０．６体積％以下、ＣＨ４：０．０４体積％以下であることが好ましい。さらに、炭化水素系ガスとして１３Ａ（都市ガス）を用いた場合の変成ガス組成の一例を挙げると次の通りである（８５０℃、ＣＰ値０．８のとき）。 The shift gas can be generated using a conventionally known shift furnace. The shift gas generated in the shift furnace varies depending on the mixing ratio of hydrocarbon gas (for example, 13A gas, natural gas, propane, butane, etc.) and air, the target gas composition, and the CP value. generally, CO: 18 to 25 vol%, H 2: 30-41 vol%, N 2: 35 to 50 vol%, CO 2: 0.12~0.25 vol%, H 2 O: 0.6% by volume hereinafter, CH 4: is preferably 0.04 vol% or less. Furthermore, an example of the modified gas composition when 13A (city gas) is used as the hydrocarbon-based gas is as follows (at 850 ° C. and CP value of 0.8).
Ｎ２（窒素）： 約３９．８５体積％
Ｈ２（水素）： 約３６体積％
ＣＯ（一酸化炭素）： 約２４体積％
ＣＯ２（二酸化炭素）：約０．１５体積％
前記低ＣＯ２変成ガスは、前記変成炉で生成された変成ガスから、従来公知のＣＯ２吸着器を用いてＣＯ２を吸着分離することによって生成できる。したがって、前記低ＣＯ２変成ガスは、ＣＯ２の割合が前記変成ガスよりも小さくなる。具体的には、ＣＯ２が０．１５体積％未満、さらには０．１体積％以下、０．０５体積％以下であることが好ましい。
N 2 (nitrogen): about 39.85% by volume
H 2 (hydrogen): about 36% by volume
CO (carbon monoxide): about 24% by volume
CO 2 (carbon dioxide): about 0.15% by volume
The low CO 2 converted gas from reformed gas generated in the reforming furnace can be generated by adsorptive separation of CO 2 using a conventionally known CO 2 adsorber. Therefore, the low CO 2 modified gas has a smaller CO 2 ratio than the modified gas. Specifically, CO 2 is preferably less than 0.15% by volume, more preferably 0.1% by volume or less, and 0.05% by volume or less.
なお、本発明において、低ＣＯ２変成ガスとは、ＣＯ２がゼロの場合も含む。 In the present invention, the low CO 2 modified gas includes a case where CO 2 is zero.
低ＣＯ２変成ガスのＣＰ（カーボンポテンシャル）は、前記変成ガスよりもＣＯ２が低減されたことで、前記変成ガスのＣＰよりも高くなり、これを浸炭雰囲気ガスとして用いることで浸炭力が高まる。また、変成ガスの浸炭力の弱さを補うために従来添加されていたエンリッチガスを使用する必要がなくなり、炭化水素系ガスの分解がなく、安定した浸炭雰囲気ガス中で浸炭が行われるので、浸炭むらやスーティングが生じにくい。この利点は、前記低ＣＯ２変成ガスのみを雰囲気ガスとして浸炭を行う場合だけでなく、前記変成ガスと前記低ＣＯ２変成ガスとの混合ガスを雰囲気ガスとして用いて浸炭を行う場合にも得られる。 The CP (carbon potential) of the low CO 2 shift gas is higher than the CP of the shift gas due to the reduction of CO 2 compared to the shift gas, and the carburizing power is increased by using this as the carburizing atmosphere gas. . In addition, it is no longer necessary to use the enriched gas that has been added in the past to compensate for the weak carburizing power of the metamorphic gas, there is no decomposition of hydrocarbon gases, and carburizing is performed in a stable carburizing atmosphere gas. Carburizing unevenness and sooting are less likely to occur. This advantage is obtained wherein only the low CO 2 converted gas not only when performing carburizing as the atmospheric gas, even when performing carburizing with a mixed gas of the low CO 2 converted gas and the reformed gas as the atmosphere gas It is done.
本発明の実施の一形態では、前記低ＣＯ２変成ガスであって、浸炭処理温度において鋼の平衡状態図（Ｆｅ−Ｃ系平衡状態図）におけるγ相の飽和炭素量を超えるカーボンポテンシャルを有するものを浸炭雰囲気ガスとする。そして、この浸炭雰囲気ガスを用いて、処理品表面にセメンタイトが析出することなく処理品内部に炭素が拡散する温度領域で浸炭・拡散処理を行う。 In one embodiment of the present invention, the low-CO 2 metamorphic gas has a carbon potential that exceeds the saturated carbon content of the γ phase in the steel equilibrium diagram (Fe-C system equilibrium diagram) at the carburizing temperature. Use carburizing atmosphere gas. And using this carburizing atmosphere gas, carburizing / diffusion treatment is performed in a temperature range where carbon diffuses inside the treated product without precipitation of cementite on the treated product surface.
本発明の発明者等は、ＣＯとＨ２とＮ２の３種のガスを所定の割合（ＣＯ：２４体積％、Ｈ２：３６体積％、Ｎ２：約４０体積％）で混合して、前記低ＣＯ２変成ガスに相当する組成のガスを生成し、ＳＣｒ４２０の被処理体（ワーク）に対してガス浸炭処理を行った。試験炉はゴールドファーネス、浸炭実施時間は２時間である。その浸炭条件（浸炭温度、ＣＰ値）と浸炭雰囲気ガス組成を表１に、浸炭結果を表２及び表３に示す。
表１〜３において、ＣＰ値は、「（ＣＯ濃度）２／（ＣＯ２濃度）×Ｋｐ（平衡定数）×飽和炭素量（浸炭温度での鋼の飽和炭素量）」の計算式で求めた。ＣＰ値は、１．６％、６．５％である。 In Tables 1 to 3, the CP value was determined by a calculation formula of “(CO concentration) 2 / (CO 2 concentration) × Kp (equilibrium constant) × saturated carbon amount (saturated carbon amount of steel at carburizing temperature)”. . The CP values are 1.6% and 6.5%.
表１において、ＣＯ２、Ｈ２Ｏ、ＣＨ４は、反応により微量生成されたものと考えられる。 In Table 1, CO 2 , H 2 O, and CH 4 are considered to be produced in a trace amount by the reaction.
表２に示すように、比較例１〜３、実施例１〜３ともに、使用した浸炭雰囲気ガスは、浸炭処理温度でのγ相の飽和炭素量を超えるカーボンポテンシャルを有する。 As shown in Table 2, the carburizing atmosphere gas used in Comparative Examples 1 to 3 and Examples 1 to 3 has a carbon potential that exceeds the saturated carbon content of the γ phase at the carburizing temperature.
表２，３に示すように、比較例１〜３では、ワークの表面炭素濃度のピーク値が高くなりすぎ、処理品表面にセメンタイトが析出した。これに対し、実施例１〜３では、ワークの表面炭素濃度のピーク値が適正値に抑制され、処理品表面にセメンタイトの析出はなく、良好な浸炭が行えた。いずれの例でもスーティングの発生はなかった。 As shown in Tables 2 and 3, in Comparative Examples 1 to 3, the peak value of the surface carbon concentration of the workpiece was too high, and cementite was deposited on the treated product surface. On the other hand, in Examples 1 to 3, the peak value of the surface carbon concentration of the workpiece was suppressed to an appropriate value, and there was no precipitation of cementite on the treated product surface, and good carburization could be performed. There was no sooting in any of the examples.
図１は、前記比較例１、２、前記実施例１、２の各処理品において、処理品表面からの距離（深さ）と炭素濃度との関係を示したグラフである。同様に、図２は、前記比較例３の処理品において、処理品表面からの距離（深さ）と炭素濃度との関係を示したグラフ、図３は、前記実施例３の処理品において、処理品表面からの距離（深さ）と炭素濃度との関係を示したグラフである。 FIG. 1 is a graph showing the relationship between the distance (depth) from the treated product surface and the carbon concentration in each treated product of Comparative Examples 1 and 2 and Examples 1 and 2. Similarly, FIG. 2 is a graph showing the relationship between the distance (depth) from the surface of the treated product and the carbon concentration in the treated product of Comparative Example 3, and FIG. 3 is the treated product of Example 3, It is the graph which showed the relationship between the distance (depth) from the surface of a processed goods, and carbon concentration.
これらの図を参照すれば、前記比較例と前記実施例における処理品表面の炭素濃度のピーク値の違いがより一層明らかとなる。 Referring to these figures, the difference in the peak value of the carbon concentration on the surface of the treated product in the comparative example and the example will become more apparent.
また、図１〜図３より明らかなように、処理品内部への炭素の拡散の深さが、比較例１〜３のものよりも実施例１〜３のものの方が深くなっている。よって、実施例１〜３のものは、比較例１〜３のものよりも、処理品の内部にまで良好な浸炭が行われたといえる。 As is clear from FIGS. 1 to 3, the depth of carbon diffusion into the treated product is deeper in Examples 1 to 3 than in Comparative Examples 1 to 3. Therefore, it can be said that the examples 1 to 3 were carburized better to the inside of the treated product than the comparative examples 1 to 3.
図４は、前記比較例１〜３、前記実施例１〜３において、浸炭処理温度と表面炭素濃度のピーク値との関係、及び、浸炭処理温度とγ相の飽和炭素量との関係を示すグラフである。 FIG. 4 shows the relationship between the carburizing temperature and the peak value of the surface carbon concentration and the relationship between the carburizing temperature and the saturated carbon content of the γ phase in Comparative Examples 1 to 3 and Examples 1 to 3. It is a graph.
図４から分かるように、ＣＰ値１．６％の雰囲気ガスで処理を行う場合（比較例３、実施例３の場合）、浸炭処理温度を約９２０℃以上とすれば、処理品表面にセメンタイトが発生せず、浸炭深さも十分な処理品が得られる。同様に、ＣＰ値６．５％の雰囲気ガスで処理を行う場合（比較例１，２、実施例１，２の場合）、浸炭処理温度を約９３０℃以上とすれば、処理品表面にセメンタイトが発生せず、浸炭深さも十分な処理品が得られる。いずれの場合も、浸炭処理温度を高くしすぎると、粒界酸化の発生や処理品の軟化が起こるおそれがあるので、浸炭処理温度の上限は１０６０℃前後とする。 As can be seen from FIG. 4, when the treatment is performed with an atmospheric gas having a CP value of 1.6% (in the case of Comparative Example 3 and Example 3), if the carburizing treatment temperature is about 920 ° C. or higher, cementite is formed on the treated product surface. And a processed product with sufficient carburization depth can be obtained. Similarly, when processing is performed with an atmospheric gas having a CP value of 6.5% (in the case of Comparative Examples 1 and 2 and Examples 1 and 2), if the carburizing temperature is about 930 ° C. or higher, cementite is formed on the surface of the processed product. And a processed product with sufficient carburization depth can be obtained. In any case, if the carburizing temperature is too high, grain boundary oxidation or softening of the treated product may occur, so the upper limit of the carburizing temperature is about 1060 ° C.
ＣＰ値が１．６％より低い場合（例えば１．３％の場合等）には、浸炭処理温度は約９００℃以上であればよいと考えられる。ＣＰ値が低すぎると浸炭能力が低いため、本願発明においては１．３％以上が好ましく１．４％以上がより好ましい。 When the CP value is lower than 1.6% (for example, 1.3%), it is considered that the carburizing temperature may be about 900 ° C. or higher. If the CP value is too low, the carburizing ability is low, so in the present invention 1.3% or more is preferable and 1.4% or more is more preferable.
なお、エンリッチガスを変成ガスに添加して行う従来の浸炭では、浸炭雰囲気ガスのカーボンポテンシャルを、浸炭処理温度でのγ相の飽和炭素量より低く、且つ浸炭処理温度でのγ相の飽和炭素量に近い値にするのが、浸炭速度を上げるための常識であった。そうしなければ処理品表面にセメンタイトが発生してしまうからである。 In the conventional carburizing performed by adding the enriched gas to the metamorphic gas, the carbon potential of the carburizing atmosphere gas is lower than the saturated carbon amount of the γ phase at the carburizing temperature, and the saturated carbon of the γ phase at the carburizing temperature. It was common sense to increase the carburization rate to make the value close to the amount. Otherwise, cementite will be generated on the treated product surface.
これに対し、本実施の形態では、低ＣＯ２変成ガスの計算上のＣＰ値は例えば１．６〜６．５％と高い値であり、浸炭処理温度でのγ相の飽和炭素量を大きく上回っている。にもかかわらず、浸炭温度を９００℃以上や９２０℃以上、さらに好ましくは９３０〜９７０℃前後と、比較的高い温度領域で浸炭処理を行うことで、セメンタイト等の異常組織が生成せず、且つ、処理品の内部にまで適正な浸炭が行われる。 In contrast, in the present embodiment, the calculated CP value of the low CO 2 metamorphic gas is a high value, for example, 1.6 to 6.5%, and the saturated carbon content of the γ phase at the carburizing temperature is increased. It has exceeded. Nevertheless, the carburizing temperature is 900 ° C. or higher, 920 ° C. or higher, more preferably around 930 to 970 ° C., and by performing the carburizing treatment in a relatively high temperature region, no abnormal structure such as cementite is generated, and Appropriate carburization is performed even inside the treated product.
本発明の発明者等は、連続浸炭炉を使用して、本発明の実施の一形態に係るガス浸炭方法と、エンリッチガスを添加する従来のガス浸炭方法とをシミュレーションにより検討し、その結果を比較した。浸炭条件とその結果を表４に示す。
表４において、比較例４，５が、従来のエンリッチ浸炭の例であり、実施例４が、本発明の実施の一形態に係るガス浸炭方法の例である。 In Table 4, Comparative Examples 4 and 5 are examples of conventional enriched carburizing, and Example 4 is an example of a gas carburizing method according to an embodiment of the present invention.
表４に示すように、実施例４では、比較例４，５と比べて浸炭時間が大幅に短縮（約２０％短縮）される。処理品の品質は実施例、比較例とも同等である。 As shown in Table 4, in Example 4, the carburizing time is significantly shortened (about 20% shorter) than Comparative Examples 4 and 5. The quality of the processed product is the same in both the examples and comparative examples.
以上述べたところから明らかように、本発明の実施の形態によれば、浸炭処理温度でのγ相の飽和炭素量を超えるカーボンポテンシャルを有する浸炭雰囲気ガスを用いて浸炭を行うので、浸炭の効率が良く、浸炭時間が短縮される。また、本発明で用いる浸炭雰囲気ガスは、カーボンポテンシャルが高くても、処理品表面に炭素が浸入する速度が遅いので、処理品表面にセメンタイトが析出しにくい。さらに、処理品表面にセメンタイトが析出することなく処理品内部に炭素が拡散する温度領域で浸炭・拡散処理を行うので、処理品の品質も良い。エンリッチガスを用いないので、スーティングも起こりにくい。 As is clear from the above description, according to the embodiment of the present invention, carburizing is performed using a carburizing atmosphere gas having a carbon potential exceeding the saturated carbon content of the γ phase at the carburizing temperature, so that the carburizing efficiency is increased. And carburizing time is shortened. Further, even if the carburizing atmosphere gas used in the present invention has a high carbon potential, the rate at which carbon enters the surface of the treated product is slow, so that cementite hardly precipitates on the surface of the treated product. Furthermore, since the carburizing / diffusion treatment is performed in the temperature range where carbon diffuses inside the treated product without precipitation of cementite on the treated product surface, the quality of the treated product is good. Since no enriched gas is used, sooting is unlikely to occur.
さらに、本発明の発明者は、本発明の方法を実施する際の、浸炭雰囲気ガスの好ましい流速及び流量について探求した。 Furthermore, the inventors of the present invention have sought for preferred flow rates and flow rates of carburizing atmosphere gases when carrying out the method of the present invention.
まず、実験用の装置として、既存のゴールドファーネス炉に内管を取替え挿入できるものを作製し、該内管内を炉として用いる。前記内管として、様々な内径を有するものを準備し、前記内管を取り替えて管内径を変更することで、前記内管内を流れる浸炭雰囲気ガスの流速や流量が変わるようにする。 First, as an experimental apparatus, an apparatus capable of replacing and inserting an inner pipe into an existing gold furnace furnace is prepared, and the inside of the inner pipe is used as a furnace. As the inner pipe, pipes having various inner diameters are prepared, and the inner pipe is changed to change the inner diameter of the pipe so that the flow rate and flow rate of the carburizing atmosphere gas flowing in the inner pipe are changed.
図５は、この実験における処理工程図である。図５に示すように、まず、窒素ガスを炉内（内管内）に供給し、１０分で９５０℃まで昇温した。昇温後１０分間保持した後ＣＯ２ガスを供給し、Ｎ２＋ＣＯ２の雰囲気でさらに５分間保持した。次に低ＣＯ２変成ガス（ガス組成は表５参照）を炉内に供給し、雰囲気を安定させるために３０分間保持した。その後、試験片ＴＰ（テストピース：ＳＣｒ４２０）を炉内に挿入して７０分間保持し、前記低ＣＯ２変成ガスを連続的に供給しながら浸炭処理を実施した。浸炭処理後には、拡散処理１４分、降温処理１４分を実施し、その後焼入れ処理を実施した。様々な内径を有する内管（表６の管内径を参照）を用いることで、浸炭処理中における低ＣＯ２変成ガスの流速と流量を様々な値に変えることができる。
前記実験の結果を表６に示す。
表６のうち、上から三つの例、すなわち、比較例１と実施例１，２が、流速を変更した場合（流量一定）の実験結果である。それ以下の例、すなわち、実施例３と比較例２〜４が、流量を変更した場合（流速一定）の実験結果である。 In Table 6, three examples from the top, that is, Comparative Example 1 and Examples 1 and 2, are the experimental results when the flow velocity is changed (the flow rate is constant). The following examples, that is, Example 3 and Comparative Examples 2 to 4, are experimental results when the flow rate is changed (the flow rate is constant).
表６を参照して、浸炭雰囲気ガスの好ましい流速と流量について考察する。 With reference to Table 6, the preferred flow rate and flow rate of the carburizing atmosphere gas will be considered.
＜流速について＞
ＣＰ値の高い前記低ＣＯ２変成ガスで浸炭しても、比較例１のように流速が小さいと本実験の目標としたカーボン濃度が０．３ｗｔ％である試験片表面からの深さ（位置）０．７５ｍｍ以上に達しないことが分かる。これに対し、実施例１，２より、流速が３．０４ｍ／ｓ、５．９８ｍ／ｓの場合には目標とする十分な浸炭深さを得られ、従来よりも早く浸炭できる。この試験結果を用いて流速とカーボン濃度が０．３ｗｔ％である試験片表面からの深さ（位置）との関係をグラフにしたものが図６である。図６より、目標とする深さ（０．７５ｍｍ）を得るためには、浸炭雰囲気ガスの流速を１．５ｍ／ｓ以上にすればよいことが分かる。
<About flow velocity>
Even when carburizing with the low CO 2 gas having a high CP value, if the flow rate is small as in Comparative Example 1, the depth (position) from the specimen surface where the carbon concentration targeted in this experiment is 0.3 wt% It can be seen that it does not reach 0.75 mm or more. On the other hand, from Examples 1 and 2, when the flow velocity is 3.04 m / s and 5.98 m / s, a target sufficient carburization depth can be obtained, and carburization can be performed faster than before. FIG. 6 is a graph showing the relationship between the flow velocity and the depth (position) from the surface of the test piece having a carbon concentration of 0.3 wt% using this test result. FIG. 6 shows that the flow rate of the carburizing atmosphere gas should be 1.5 m / s or more in order to obtain the target depth (0.75 mm).
なお、浸炭炉の浸炭室或いは拡散室内のワーク設置範囲内において浸炭雰囲気ガスの流速を１．５ｍ／ｓ以上とするために、浸炭室或いは拡散室にガス攪拌用のファンを上下、左右に必要に応じて設置して流速のバラツキを抑制することが好ましく、これにより安定した浸炭処理を実現することができる。 In order to make the flow rate of the carburizing atmosphere gas 1.5 m / s or more within the work installation range of the carburizing furnace or diffusion chamber of the carburizing furnace, fans for gas agitation are required vertically and horizontally in the carburizing chamber or diffusion chamber. Therefore, it is preferable to suppress the variation in the flow rate, and a stable carburizing process can be realized.
ガス流速（風速）は風速計で測定することができる。また、流速を上げることで炭素移行係数（β値）が増加し、流速増加は流量（物質供給量）増加との関係はない。 The gas flow velocity (wind velocity) can be measured with an anemometer. In addition, increasing the flow rate increases the carbon transfer coefficient (β value), and the increase in flow rate is not related to the increase in flow rate (substance supply amount).
＜流量について＞
比較例２〜４より、浸炭雰囲気ガスの流量が小さい場合、すなわち、炉内ガス置換回数が１時間当たり５．１〜１９回、流速約０．１ｍ／ｓのときは、前記目標とする浸炭深さが得られないことが分かる。これに対し、実施例３のように流量が大きい場合は、目標とする浸炭深さが得られ、従来よりも早く浸炭できる。この試験結果を用いて流量とカーボン濃度が０．３ｗｔ％である試験片表面からの深さ（位置）との関係をグラフにしたものが図７である。図７より、目標とする深さ（０．７５ｍｍ）を得るためには、浸炭雰囲気ガスの流量を炉内雰囲気置換回数２９回／ｈ以上となる流量にすればよいことが分かる。
<About flow rate>
From Comparative Examples 2 to 4, when the flow rate of the carburizing atmosphere gas is small, that is, when the number of gas replacements in the furnace is 5.1 to 19 times per hour and the flow rate is about 0.1 m / s, the target carburizing is performed. It can be seen that the depth cannot be obtained. On the other hand, when the flow rate is large as in Example 3, the target carburizing depth is obtained, and carburizing can be performed earlier than in the past. FIG. 7 is a graph showing the relationship between the flow rate and the depth (position) from the surface of the test piece having a carbon concentration of 0.3 wt% using this test result. From FIG. 7, it can be seen that in order to obtain the target depth (0.75 mm), the flow rate of the carburizing atmosphere gas may be set to a flow rate that is 29 times / h or more in the furnace atmosphere replacement.
なお、炉内雰囲気置換回数は、炉内に供給する１時間あたりのガス流量を炉内体積で割った値である。炉内雰囲気置換回数を大きくするために、炉内に連続的に新しいガスを供給しても良いが、炉内ガスを循環させて炉内に供給することがコスト面から好ましい。また、流量（置換回数）を増加させると炭素移行係数（β値）が低下することがわかり、浸炭効率を向上させるためにはガス流量よりもガス流速を増加させる方が好ましい。 The number of atmosphere replacements in the furnace is a value obtained by dividing the gas flow rate per hour supplied into the furnace by the furnace volume. In order to increase the number of times the atmosphere in the furnace is replaced, new gas may be continuously supplied into the furnace, but it is preferable from the viewpoint of cost that the furnace gas is circulated and supplied into the furnace. Moreover, it turns out that a carbon transfer coefficient ((beta value)) will fall when a flow rate (number of substitutions) is increased, and in order to improve carburizing efficiency, it is more preferable to increase a gas flow rate than a gas flow rate.
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