JP3996482B2 - Vacuum carburizing method - Google Patents

Description

【００４３】
【表２】

【００４４】
条件２〜条件６によれば、強い脱炭条件のもとで強脱炭処理を実施した。殊に参考例１Ｂ、参考例２Ｂによれば、相対的に強い脱炭条件のもとで強脱炭処理を実施した後に、強脱炭処理よりも相対的な弱い脱炭条件のもとで弱脱炭処理を実施した。
【００４５】
即ち、条件２〜条件６によれば、表１に示すように拡散処理の初期に脱炭性ガスを導入して強脱炭処理をワークに対して実行した。この場合、脱炭性ガスとして機能する空気（酸素含有）を５０リットル（ＮＬ）／分の割合で炉内に導入しつつ炉内ガスをポンプで吸引し、表１に示す所定の炉内圧力に維持した。
【００４６】
図３の特性線Ｍ２は、真空浸炭した後に脱炭を含む拡散処理を実行したときにおけるワークの表面付近の炭素濃度の形態を模式的に示す。特性線Ｍ２に模式的に示すように、ワークの最表面の炭素濃度のピークはかなり下がっており、更に炭素がワークの内部にかなり浸透している。なお例によれば、ワークの表面の炭素濃度の目標値は０．８質量％である。
【００４７】
比較例１については、表１に示すように、拡散処理（１０００℃、１７０分、２Ｐａ）を実行するものの、脱炭性ガスを導入せず脱炭処理を実行しなかった。このため表２に示すように、比較例１に係る試験結果によれば、ワークの平面部の表面の炭素濃度は０．８６質量％と、亜共析鋼組成付近であるにもかかわらず、ワークの角部の表面の炭素濃度は１．９１質量％と過剰に多く、ワークの平面部の表面と角部の表面とで炭素濃度の差が非常に大きく、好ましいものではなかった。更に比較例１によれば、ワークの平面部の表面にはセメンタイトが残留していないものの、ワークの角部の表面においてはセメンタイトが多量に残留しており、好ましいものではなかった。なおワークの炭素濃度はＥＰＭＡで測定したものである。
【００４８】
表１に示すように、参考例３に係る強脱炭処理については、脱炭時間を５０分とし、脱炭性ガスを装入した炉内圧力は４０ＫＰａとかなり高めであり、即ち、脱炭性ガスの量がかなり多めであり、強脱炭条件とした。このため表２に示すように、参考例３に係る試験結果によれば、ワークの角部の表面の炭素濃度は０．９８質量％と亜析鋼組成に近いものとなり、この結果、ワークの平面部の表面の炭素濃度（０．７５質量％）に近いものとなり、更に、ワークの平面部の表面ばかりか、ワークの角部の表面においてもセメンタイトが残留しておらず、良好であった。
【００４９】
なお参考例３によれば、強脱炭条件で強脱炭処理を行うため、１４μｍ程度の酸化物が認められ、粒界酸化が認められた。但し、ＲＸガスを用いる一般的なガス浸炭法（浸炭温度９５０℃、浸炭時間１０時間）によれば、浸炭時間が長くなるため、２０〜２５μｍ程度の酸化物の生成が認められるのが一般的である。従って参考例３は酸化物が認められるものの、一般的なガス浸炭法よりも粒界酸化はかなり少ないものである。
【００５０】
表１に示すように、参考例２Ｂに係る強脱炭処理については、脱炭性ガスを装入した炉内圧力は２５ＫＰａと高めであり、即ち、脱炭性ガスの量が多めであり、脱炭時間も６０分とやや長めとし、強脱炭条件とした。このため表２に示すように、参考例２Ｂに係る試験結果によれば、ワークの角部の表面の炭素濃度（１．０１質量％）は、ワークの平面部の表面の炭素濃度（０．７７質量％）に近いものとなり、更に、ワークの平面部の表面ばかりかワークの角部の表面においてもセメンタイトが残留しておらず、良好であった。なお参考例２Ｂによれば、５μｍ程度の酸化物が認められ、粒界酸化が認められたが、前述同様に一般的なガス浸炭法に比較して粒界酸化は少ないと言える。
【００５１】
表１に示すように、参考例１Ｂに係る強脱炭処理については、脱炭性ガスを装入した炉内圧力は２０ＫＰａと高めであり、即ち、脱炭性ガスの量が比較的多めであり、脱炭時間も６０分とやや長めとし、強脱炭条件とした。このため表２に示すように、参考例１Ｂに係る試験結果によれば、ワークの角部の表面の炭素濃度（１．０７質量％）は、ワークの平面部の表面の炭素濃度（０．７９質量％）に近いものとなり、更に、ワークの平面部の表面にセメンタイトが残留していないし、ワークの角部の表面においてはセメンタイトがほんの僅かに残留しているに過ぎず、良好であった。
【００５２】
表１に示すように、参考例２に係る強脱炭処理については、脱炭性ガスを装入した炉内圧力は２０ＫＰａと高めであり、即ち、脱炭性ガスの量が比較的多めであり、脱炭時間は５０分とやや長めとし、強脱炭条件とした。このため表２に示すように、参考例２に係る試験結果によれば、ワークの角部の表面の炭素濃度（１．２６質量％）は、比較例１に比べて、ワークの平面部の表面の炭素濃度（０．８１質量％）に近いものとなり、更に、ワークの平面部の表面にセメンタイトが残留していないし、ワークの角部の表面においてはセメンタイトが少量残留しているに過ぎず、良好であった。
【００５３】
表１に示すように、参考例１に係る強脱炭処理については、脱炭性ガスを装入した炉内圧力は１１ＫＰａと参考例１Ｂ，２Ｂに較べてやや低めであり、即ち、脱炭性ガスの量が比較的低めであるものの、脱炭時間を１７０分とかなり長めとし、やや強脱炭条件とした。このため表２に示すように、参考例１に係る試験結果によれば、ワークの角部の表面の炭素濃度は１．３２質量％となり、ワークの平面部の表面の炭素濃度は０．６４質量％となり、更に、ワークの平面部の表面にセメンタイトが残留していないし、ワークの角部の表面においてはセメンタイトが少量残留しているに過ぎず、良好であった。
【００５４】
このように参考例１では、強脱炭処理であるものの、脱炭性ガスの濃度が実施例１，２に比較してやや低い（１１ＫＰａ）ため、脱炭時間（１７０分）であれば、ワークの角部の表面においてはセメンタイトが少量であるものの残留してしまう。但し、この程度のセメンタイトであればワークの用途によっては支障がない。
【００５５】
更に参考例１Ｂ、参考例２Ｂについては、表１に示すように、強脱炭処理の後で、強脱炭処理と同じ温度に維持しつつ、強脱炭処理よりも脱炭性ガスの分圧を低めとした弱脱炭処理を７０分実行した。即ち、参考例１Ｂ及び参考例２Ｂに係る弱脱炭処理においては、脱炭温度を１０００℃に維持した状態で、脱炭性ガスとして機能する空気を炉内に１０リットル（ＮＬ）／分の割合で導入しつつ炉内ガスをポンプで吸引し、所定の炉内圧力（参考例１Ｂでは４ＫＰａ、参考例２Ｂでは６ＫＰａ）に維持した。なお、この弱脱炭処理における脱炭性ガスの分圧は、前記した強脱炭処理に比較して１／１０〜１／３の範囲内である。なお、弱脱炭処理における脱炭温度は参考例１Ｂ、参考例２Ｂでは同じ温度とされているが、弱脱炭処理における脱炭温度を強脱炭処理の脱炭温度よりも低めとしても良い。
【００５６】
上記したように参考例１Ｂ、参考例２Ｂによれば、強脱炭処理の後で弱脱炭処理を実行するため、最終製品のワークの角部のセメンタイトを除去しつつ、ワークの平面部の炭素濃度も高めに確保されるため、好ましいものである。
【００５７】
なお、参考例３によれば、強脱炭処理のみであるため、ワークの角部の表面におけるセメンタイトを効果的に脱炭して除去できるものの、ワークの平面部の表面の炭素濃度の低下が比較的大きめとなる傾向がある。また弱脱炭処理のみとすれば、脱炭能力が低いため、ワークの角部の表面に残留するセメンタイトの量が課題となり易い。そこでワークの角部の表面におけるセメンタイトを除去しつつ、ワークの平面部の表面の炭素濃度の低下を抑えるためには、参考例１Ｂ、参考例２Ｂのように強脱炭処理の後で弱脱炭処理を実行することが好ましいと考えられる。
【００５８】
上記した各例を実施したワークについては、真空雰囲気（炉内圧力３Ｐａ）で８５０℃で均熱処理された後に、油焼き入れされた。
【００５９】
（パルス真空浸炭）
表１に示す比較例２はパルス浸炭に相当するものである。パルス浸炭は、浸炭処理の時間及び拡散処理の時間をそれぞれ通常の真空浸炭処理に比較して短時間とし、浸炭処理及び拡散処理を繰り返すものであり、一般的にはセメンタイトの発生を抑えるのに有利と言われている。
【００６０】
比較例２はパルス浸炭に相当するものであり、表１に示すように、浸炭処理（１）を実行した後に、脱炭処理を含まない拡散処理（３）を実行する処理を１サイクルとし、このサイクルを実施例６と同様に７回連続的に繰り返した後に、本拡散処理（拡散温度１０００℃、拡散時間２０分、炉内圧力２Ｐａ）を実行した。表２に示すように、比較例２に係る試験結果によれば、ワークの角部の表面の炭素濃度（１．０６質量％）は、ワークの平面部の表面の炭素濃度（０．８５質量％）に近くなり、更に、ワークの平面部の表面にセメンタイトが残留していないものの、ワークの角部の表面においてはセメンタイトが僅かに残留していた。
【００６１】
また参考例３Ｂ（条件８）によれば、表１に示すように、浸炭処理（１）を実行した後で、初期における脱炭処理（２）を含む拡散処理（３）を実行する処理（（１）（２）（３））を第１サイクルとしている。脱炭処理（２）は表１において強脱炭処理の欄に記載されており、拡散処理（３）は表１において弱脱炭処理の欄に記載されている。そしてこの（１）（２）（３）からなる第１サイクルを７回連続的に繰り返した。その後、本拡散処理（拡散温度１０００℃、拡散時間２０分、炉内圧力２Ｐａ）を実行した。この参考例３Ｂは従来のパルス浸炭の改良として位置づけうる。
【００６２】
また参考例４Ｂは参考例３Ｂと基本的に同様である。参考例４Ｂによれば、浸炭処理（１）を実行した後で、初期における脱炭処理（２）を含む拡散処理（３）を実行する処理（（１）（２）（３））を第１１サイクルとしている。そしてこの（１）（２）（３）からなるサイクルを６回連続的に繰り返した。その後、参考例３Ｂに係る浸炭処理（１）を再び実行した後に、脱炭処理を実行せずに拡散処理（拡散温度１０００℃、炉内圧力２Ｐａ、拡散時間２９分（１５分＋１４分：（１５分=参考例３Ｂの脱炭処理に相当する時間）,１４分=参考例３Ｂの拡散処理に相当する時間）を実行した。その後参考例３Ｂと同様に本拡散処理（拡散温度１０００℃、拡散時間２０分、炉内圧力２Ｐａ）を実行した。この参考例４Ｂは従来のパルス浸炭の改良として位置づけうる。
【００６３】
表２に示すように、参考例３Ｂに係る試験結果によれば、ワークの角部の表面の炭素濃度は０．９２質量％であり、比較例２よりも低めであり、ワークの平面部の表面の炭素濃度（０．８３質量％）に近くなり、更に、ワークの平面部の表面にセメンタイトが残留しておらず、ワークの角部の表面においてもセメンタイトが残留しておらず、良好であった。また表２に示すように、参考例４Ｂに係る試験結果によれば、ワークの角部の表面の炭素濃度は０．９３質量％であり、比較例２よりも低めであり、ワークの平面部の表面の炭素濃度（０．８３質量％）に近くなり、更に、ワークの平面部の表面にセメンタイトが残留しておらず、ワークの角部の表面においてもセメンタイトが残留しておらず、良好であった。従って本発明方法はパルス浸炭にも有効であることがわかる。
【００６４】
【表３】

【００６５】
表３は参考例１Ａ、２Ａ、本発明に係る実施例３Ａを示す。参考例１Ａ、２Ａ、実施例３Ａは、浸炭処理を実行した後に拡散処理を実行するものの、拡散処理の全時間にわたり脱炭処理を行うものであり、拡散処理時間=脱炭処理時間となる。即ち、脱炭処理ではワークは高温領域に加熱されているため、ワークにおける拡散現象も生じているためである。
【００６６】
表３に示すように、参考例１Ａによれば、浸炭性ガスとしてアセチレンガスを用い、浸炭温度９５０℃で炉内圧力を３ＫＰａとし、４５分浸炭処理を実行した後に、脱炭性ガスとして空気を１２０リットル（ＮＬ）／分の割合で炉内に装入し、炉内圧力を２０ＫＰａとした。この脱炭処理は、浸炭性ガスを含んでいない空気を炉内に導入して脱炭しているため、強脱炭処理に相当する。
【００６７】
このような参考例１Ａを実施したワークの平面部の表面の炭素濃度は０．５質量％であり、ワークの角部の表面の炭素濃度は０．８質量％であり、角部におけるセメンタイトは無かった。脱炭処理が良好であったためである。なお脱炭性ガスとして空気を脱炭期に炉内に導入した場合には、空気は本来的には脱炭性ガスとして機能できるが、脱炭処理の開始直後には、浸炭処理において炉内に残留していたすすの影響を受けて、空気中の酸素がＣＯを生成するため、脱炭処理の開始直後には浸炭気味となるが、開始直後を経過すれば次第に脱炭ガスとして機能することなる。従って空気を炉内に導入するときには、脱炭を良好に行うためには、一般的には導入時間としては５分以上が好ましい。
【００６８】
表３に示すように、参考例２Ａによれば、浸炭温度９５０℃で炉内圧力を３ＫＰａとし、４５分浸炭処理を実行した後に、脱炭性ガスとして二酸化酸素ガス（ＣＯ_２）と浸炭性ガスであるプロパンガス（Ｃ_３Ｈ_８）とを混合した混合ガスを３４．８リットル（ＮＬ）／分の割合で炉内に装入し、炉内圧力を３０ＫＰａとした。この脱炭処理は、浸炭性ガスを含んでいる脱炭性ガスを炉内に導入して脱炭しているため、弱脱炭処理に相当する。
【００６９】
上記した場合、二酸化酸素ガス（ＣＯ_２）とプロパンガス（Ｃ_３Ｈ_８）との混合比は、体積比で、表３に示すように、二酸化酸素ガス：プロパンガス=３３：４．２とした。即ち、二酸化酸素ガスを３３リットル（ＮＬ）／分で炉内に供給し、プロパンガスを４．２リットル（ＮＬ）／分で炉内に供給している。換言すれば、上記した混合ガスにおいて、脱炭性ガスである二酸化酸素ガス（ＣＯ_２）は、浸炭性ガスであるプロパンガス（Ｃ_３Ｈ_８）よりもリッチとされている。
【００７０】
このような参考例２Ａを実施したワークの平面部の表面の炭素濃度は０．８質量％であり、ワークの角部の表面の炭素濃度は０．９質量％であり、ワークの平面部の表面の炭素濃度とワークの角部の表面の炭素濃度とは接近しており、更に、ワークの平面部ばかりか、ワークの角部においてもセメンタイトは無かった。
【００７１】
このように参考例２Ａにおいてワークの表面の炭素濃度と角部の炭素濃度とが接近している理由としては、混合ガスのカーボンポテンシャル（Ｃｐ値）を０．７〜０．８程度となるように、脱炭性ガスである二酸化酸素ガス（ＣＯ_２）と浸炭性ガスであるプロパンガス（Ｃ_３Ｈ_８）とを所定の混合比で混合した混合ガスを脱炭期に導入しているため、ワークの平面部の上記Ｃｐ値よりも低い炭素濃度が増加してＣｐ値に近づき、且つ、ワークの角部の上記Ｃｐ値よりも高い炭素濃度が減少してＣｐ値に近づき、結果として、ワークの平面部の表面の炭素濃度とワークの角部の表面の炭素濃度とが近づくためと推察される。
【００７２】
従って、脱炭性ガスである二酸化酸素ガス（ＣＯ_２）と浸炭性ガスであるプロパンガス（Ｃ_３Ｈ_８）とを混合した混合ガスを脱炭期に導入し、カーボンポテンシャル（Ｃｐ値）を所定の目標炭素濃度に設定すれば、ワークの平面部の上記目標炭素濃度よりも低い炭素濃度（例えば０．７５質量％）が増加して目標炭素濃度（例えば０．８質量％）に近づき、且つ、ワークの角部の上記目標炭素濃度よりも高い炭素濃度（例えば０．９質量％）が減少して目標炭素濃度（例えば０．８質量％）に近づき、結果として、ワークの平面部の表面の炭素濃度とワークの角部の表面の炭素濃度との双方がそれぞれ目標炭素濃度（例えば０．８質量％）に近づくためと推察される。
【００７３】
表３に示すように、本発明に係る実施例３Ａによれば、浸炭温度９５０℃で炉内圧力を３ＫＰａとし、４５分浸炭処理を実行した後に、脱炭処理として条件１Ａで強脱炭処理を１５分行い、その後に直ちに、条件２Ａで弱脱炭処理を３５分行なった。条件１Ａは、脱炭性ガスとして空気を１２０リットル（ＮＬ）／分の割合で炉内に装入し、炉内圧力を２０ＫＰａとするものである。条件２Ａは、脱炭性ガスとして二酸化酸素ガス（ＣＯ_２）とプロパンガス（Ｃ_３Ｈ_８）とを混合した混合ガスを３７．２リットル（ＮＬ）／分の割合で炉内に装入し、炉内圧力を３０ＫＰａとするものである。
【００７４】
このように強脱炭処理の直後に弱脱炭処理を行なう本発明に係る実施例３Ａを実施すれば、表３に示すように、ワークの表面の炭素濃度は０．８質量％となり、角部の炭素濃度は０．８３質量％となり、ワークの平面部の表面の炭素濃度とワークの角部の表面の炭素濃度とはかなり接近しており、更に、ワークの平面部ばかりか、ワークの角部においてもセメンタイトは無かった。
【００７５】
このように実施例３Ａにおいてワークの平面部の表面の炭素濃度とワークの角部の表面の炭素濃度とが近づいている理由としては、前述同様に、脱炭性ガスである二酸化酸素ガス（ＣＯ_２）と浸炭性ガスであるプロパンガス（Ｃ_３Ｈ_８）とを所定の混合比で混合した混合ガスにおいてカーボンポテンシャル（Ｃｐ値）を０．７〜０．９程度となるように制御できると推察されるため、ワークの平面部の上記Ｃｐ値よりも低い炭素濃度が増加してＣｐ値に近づき、且つ、ワークの角部の上記Ｃｐ値よりも高い炭素濃度が減少してＣｐ値に近づき、結果として、ワークの平面部の表面の炭素濃度とワークの角部の表面の炭素濃度とが近づくためと推察される。なお、参考例１Ａ、２Ａ、実施例３Ａを実施したワークについては、前述同様な条件で均熱処理された後に、油焼き入れされる。
【００７６】
（その他）その他、本発明方法は上記した実施例のみに限定されるものではなく、要旨を逸脱しない範囲内で適宜変更して実施できるものである。
【００７７】
【発明の効果】
以上説明したように本発明に係る真空浸炭方法によれば、生産時間の短縮を図りつつ、ワークの角部におけるセメンタイトを低減または除去できる。脱炭処理は、脱炭性ガスと浸炭性ガスとを含む混合ガスで脱炭作用を抑制しつつ実行される。この結果、前述したように、ワークの平面部の表面の炭素濃度とワークの角部の表面の炭素濃度とを近づけることができ、高品質化に有利である。
【００７８】
次の形態の場合には、特有の効果が認められる。浸炭処理を実行した後に、初期における脱炭処理を含む拡散処理を実行する処理を１サイクルとし、このサイクルを複数回連続的に繰り返す場合には、前述したように、ワークの角部におけるセメンタイトの低減または除去に一層有利である。
【００７９】
脱炭処理における炉内圧力（真空度）を４０ＫＰａ以下に設定した場合には、酸素分圧が低下しているため、ワークにおける粒界酸化を抑制するのに有利である。更に脱炭処理が、脱炭能力が相対的に強い強脱炭処理と、強脱炭処理後に行われ脱炭能力が強脱炭処理よりも相対的に弱い弱脱炭処理との組み合わせで構成されているため、前述したように、ワークの平面部の表面の炭素濃度を確保しつつ、ワークの角部の表面におけるセメンタイトを低減または除去するのに有利である。
【図面の簡単な説明】
【図１】炉を模式的に示す構成図である。
【図２】温度の変化の形態を模式的に示すグラフである。
【図３】真空浸炭直後におけるワークの表面付近の炭素濃度を模式的に示すグラフである。
【符号の説明】
図中、１００はワークの平面部を示し、１０２はワークの角部、２００は炉を示す。BACKGROUND OF THE INVENTION
  The present invention relates to a vacuum carburizing method for carburizing an iron-based workpiece in a reduced pressure atmosphere.
[0002]
[Prior art]
  Conventionally, a carburizing gas is introduced into a furnace maintained at a high temperature (930 to 980 ° C.) higher than a depressurized atmosphere and a carburizable temperature, and carburizing treatment is performed on a workpiece in the furnace, and then the inside of the furnace A vacuum carburizing method is disclosed in which diffusion treatment is performed on a workpiece in a furnace by evacuating and heating in a reduced-pressure atmosphere (Japanese Patent Laid-Open Nos. 06-172960 and 2000-303160).
[0003]
  Gas carburizing uses RX gas as a carrier gas and propane (C₃H₈) Or butane (C₄H₁₀) As an enriched gas and carburizing while adjusting the carbon potential (Cp value). In this case, CO gas by the Boudouard reaction contributes to carburization. The carbon potential (Cp value) indicates an equilibrium relationship between the gas and the austenite carbon concentration of the workpiece. For example, the workpiece is left at a high temperature for a long time in a gas having a carbon potential (Cp value) of 0.8% by mass. Then, regardless of the original carbon concentration of the workpiece, the carbon concentration of the workpiece is basically 0.8% by mass.
[0004]
  In such gas carburizing, since carburizing can be performed while adjusting the gas composition and controlling the carbon potential (Cp value), the carbon concentration is different between the plane portion and the corner portion of the workpiece, although the carburizing depth is different. Basically the same. However, in such gas carburizing, since the carburizing speed is slow, there is a problem that the production time for obtaining the necessary effective hardness depth is considerably long, and the productivity is not always sufficient.
[0005]
  On the other hand, the vacuum carburizing method described above does not use a carrier gas, and a carburizing gas (generally, a hydrocarbon-based gas such as acetylene or ethylene) is introduced into the furnace while the furnace is in a reduced pressure state. To be introduced. In this case, it is said that carbon produced by direct decomposition of hydrocarbon gas enters the workpiece and carburization is performed.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 06-172960
[Patent Document 2]
JP 2000-303160 A
[0007]
[Problems to be solved by the invention]
  According to the vacuum carburizing method described above, the carbon potential (Cp value) is not controlled (because it cannot be performed) compared to gas carburizing in which carburizing is performed while controlling the carbon potential (Cp value). There is a tendency for a large amount of cementite to remain excessively at the corners of the work rather than at the flat part. This is because the corner area of the workpiece has a higher carbon permeation amount because the exposed area per unit volume is larger than that of the plane area. If cementite remains excessively in this way, there is a limit to improving the quality of the workpiece.
[0008]
  According to the above-mentioned publication technique, even if the carburized workpiece is heated and held at a high degree of vacuum and diffusion treatment is performed, a large amount of cementite is present at the corner of the workpiece. Cementite remains.
[0009]
  This invention is made | formed in view of the above-mentioned actual condition, and makes it a subject to provide the vacuum carburizing method which can reduce or remove the cementite in the corner | angular part of a workpiece | work, aiming at shortening of production time.
[0010]
[Means for Solving the Problems]
  In the vacuum carburizing method according to the present invention, a carburizing gas is introduced into a furnace maintained at a high temperature equal to or lower than a reduced pressure atmosphere and a carburizable temperature to perform a carburizing process on a workpiece in the furnace,
  Thereafter, in the vacuum carburizing method of performing diffusion treatment on the work in the furnace by evacuating the furnace and heating and holding in a reduced pressure atmosphere,
  At least in the initial stage of the diffusion treatment, a decarburizing gas is introduced into the furnace to decarburize the surface of the work in the furnace, and a vacuum carburizing method for reducing or removing cementite on the surface of the work,
  The decarburization process is performed in the presence of a decarburization gas, and the decarburization conditions are relatively strong, and the decarburization process is performed after the strong decarburization process and the decarburization gas is present. The decarburization conditions are relatively weaker than the strong decarburization treatment, and is composed of a weak decarburization treatmentAnd
  Among the decarburization processes, the weak decarburization process is a mixed gas containing carbon dioxide gas, which is a decarburizing gas, and propane gas, which is a carburizing gas, so that the carbon concentration can be increased in excess decarburized parts. It is executed while suppressing the actionIt is characterized by this.
[0011]
  According to the vacuum carburizing method according to the present invention, a carburizing gas is introduced into a furnace maintained at a high temperature equal to or higher than a reduced pressure atmosphere and a carburizable temperature, and a carburizing process is performed on a workpiece in the furnace. For this reason, as compared with gas carburizing, a large amount of carbon penetrates into the surface of the workpiece in a short time, and a large amount of cementite which is iron carbide is generated. In this case, in general, a large amount of cementite, which is an iron carbide, is generated not only at the corners of the work but also at the flat parts of the work. This is thought to be because carbon produced by direct decomposition of the hydrocarbon gas penetrates into the workpiece. In addition, as carbon concentration of the surface of the corner | angular part of a workpiece | work immediately after carburizing process (before spreading | diffusion process), it may be 1.0 mass% or more, and 1.5 mass% or more depending on the case, and 2.0 mass% or more. it can.
[0012]
  According to the vacuum carburizing method according to the present invention, at least in the initial stage of the diffusion treatment, a decarburizing gas is introduced into the furnace, and the surface of the workpiece in the furnace is decarburized. As a result, the carbon produced by the decomposition of cementite on the surface of the workpiece is decarburized by the decarburizing gas in the furnace and is reduced or removed. Carbon produced by the decomposition of cementite becomes CO gas and is considered to be decarburized. As a result, the carbon produced by the decomposition of cementite, in particular the carbon present in the corners of the workpiece, is decarburized by the decarburizing gas in the furnace, and is reduced or removed.
  The decarburization process is performed in the presence of a decarburization gas, and the decarburization conditions are relatively strong, and the decarburization process is performed after the strong decarburization process and the decarburization gas is present. The decarburization conditions are relatively weaker than the strong decarburization treatment, and is composed of a weak decarburization treatmentIn the decarburization process, the weak decarburization process is performed while suppressing the decarburization action with a mixed gas containing carbon dioxide gas which is a decarburizing gas and propane gas which is a carburizing gas..
[0013]
  The corner of the work has a larger exposed area per volume than the flat part of the work, so the amount of carbon permeated during the carburizing process increases and is excessively carburized. The amount of charcoal increases and tends to be decarburized in large quantities.
[0014]
  When the above decarburization process is completed, the decarburized gas is exhausted from the furnace, and the workpiece is diffused by heating and holding. As a result, carbon existing on the surface of the workpiece diffuses inside, and an effective hardening depth is ensured after quenching.
[0015]
  According to the vacuum carburizing method according to the present invention, carbon is excessively permeated into the workpiece by carburizing treatment to perform excessive carburization and then decarburize a large amount. The production time for obtaining can be shortened.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
  According to the method of the present invention, at least in the initial stage of the diffusion treatment, a decarburizing gas is introduced into the furnace, and the surface of the workpiece in the furnace is decarburized. The decarburization process may be performed only in the initial stage of the diffusion process, or the decarburization process is performed by introducing a decarburizing gas into the furnace during the entire or most of the diffusion process or intermittently. May be performed.
[0017]
  As the iron-based workpiece according to the method of the present invention, the carbon content before carburizing treatment can be 0.7% by mass or less, and generally 0.05 to 0.5% by mass, 0.1% -0.4 mass% and 0.1-0.3 mass%. The workpiece can contain other alloy elements (at least one of chromium, silicon, manganese, aluminum, vanadium, tungsten, nickel, etc.).
[0018]
  The carbon concentration on the surface of the workpiece after carrying out the method of the present invention can be a eutectoid steel composition, but in some cases, it can be a hypoeutectoid steel composition or a hypereutectoid steel composition. The eutectoid steel composition is generally said to have a carbon concentration of 0.8% by mass. Therefore, the carbon concentration on the surface of the work after the method of the present invention is carried out can be 0.5 to 1.3% by mass, 0.6 to 1.1% by mass, 0.7 to 0.00%. It can be 9 mass%.
[0019]
  According to the method of the present invention, as a gas introduced into the furnace as the carburizing gas during the carburizing process, generally, a hydrocarbon-based gas, methanol gas, or the like can be employed. Examples of the hydrocarbon gas include acetylene gas, ethylene gas, propane gas, and methane gas. As the gas introduced into the furnace as a decarburizing gas during the decarburization treatment, generally, oxygen, air (including oxygen), water vapor, CO₂Gas can be used.
[0020]
  According to the method of the present invention, the furnace pressure (absolute pressure) in the carburizing process may be any pressure that can be vacuum carburized, but if it is too low, the carburization is not sufficient, and if it is too high, the cementite becomes excessive. Therefore, as the furnace pressure (absolute pressure) in the carburizing process, generally 0.01 to 40 KPa, 1 to 20 KPa, and particularly 1 to 10 KPa can be employed.
[0021]
  The pressure in the furnace (absolute pressure) in the decarburization process may be any pressure that can be decarburized, but if it is too low, the decarburization time becomes long and the decarburization becomes insufficient. Lower than necessary, grain boundary oxidation becomes excessive, and other alloy elements are excessively oxidized. Therefore, as the furnace pressure (absolute pressure) in the decarburization treatment, generally 0.5 to 100 KPa, 1 to 80 KPa, particularly 2 to 60 KPa can be employed.
[0022]
  According to the method of the present invention, a mode in which the furnace pressure (decompression degree) in the decarburization process is set to 40 KPa or less, for example, can be exemplified. As a result, the ratio of decarburizing gas in the decarburization treatment can be reduced, so that the decarburization ability may be slightly reduced, but the grain boundary oxidation in the work can be suppressed, and the alloy elements (chromium, Oxidation of at least one of silicon, manganese, aluminum and the like can be suppressed.
[0023]
  As the furnace pressure (absolute pressure) in the diffusion treatment (not including the period of decarburization treatment), in the case of diffusion treatment in a reduced pressure atmosphere, generally, 20 KPa or less, 7 KPa or less, 1 KPa or less can be adopted, Moreover, 20 Pa or less, 10 Pa or less, 0.01 Pa or more can be employ | adopted, and when carrying out the diffusion process in inert gas atmosphere, atmospheric pressure may be sufficient.
[0024]
  According to the method of the present invention, after performing the carburizing process, the first cycle is a process for performing the diffusion process including the decarburizing process in the initial stage, and the embodiment includes a step of continuously repeating the first cycle a plurality of times. can do. In this case, since the first cycle is continuously repeated a plurality of times, the time per carburizing process and the time per initial decarburizing process are shortened. This can be positioned as an improvement of pulse carburization. In this case, even if carburizing is performed, an advantage that the amount of cementite generated in the workpiece can be suppressed is obtained. Thus, since decarburization is performed in a state in which the amount of cementite is suppressed in the carburizing process, residual cementite is further suppressed. The number of times the first cycle is repeated a plurality of times can be 2 to 100 times, 2 to 15 times, or 2 to 10 times, although it varies depending on the workpiece.
[0025]
  Further, according to the method of the present invention, the cycle for performing the carburizing process, performing the diffusion process including the decarburizing process in the initial stage is a first cycle, and performing the carburizing process, including the decarburizing process. When the second cycle is a cycle in which no diffusion process is performed, the first cycle is repeatedly repeated a plurality of times, and the second cycle is executed after the first cycle is repeated. it can.
[0026]
  Alternatively, according to the method of the present invention, the cycle for performing the carburizing process, performing the diffusion process including the decarburizing process in the initial stage is a first cycle, and performing the carburizing process, including the decarburizing process. When the second cycle is a cycle in which no diffusion process is performed, the first cycle and the first cycle are repeated in the middle of the repetition of the plurality of first cycles after continuously repeating the plurality of first cycles. A mode including a step of executing the second cycle can be adopted. In this case, the second cycle is preferably performed after N / 3 and in the latter half of N / 2 and thereafter, where N is the number of repetitions of the first cycle. This is to eliminate or reduce grain boundary oxidation. In the case described above, the slight grain boundary oxidation generated during the decarburization treatment in the first cycle can be reduced by the second cycle, which is advantageous in eliminating or reducing the grain boundary oxidation. For example, if the atmosphere is 1000 ° C. × 3 Pa, Cr oxide is also reduced.
[0027]
  In other words, in the case described above, the second cycle advantageous for eliminating the grain boundary oxidation needs to be performed after the first cycle is executed at least once. Here, after the first cycle is executed a plurality of times, a mode in which the second cycle is executed at least once after the first cycle can be adopted. In this way, it is advantageous to eliminate or reduce the grain boundary oxidation when the second cycle advantageous for eliminating or reducing the grain boundary oxidation is executed on the last side. Further, when the first cycle is repeatedly executed a plurality of times, the second cycle may be executed between the first cycle and the first cycle as described above.
[0028]
  According to the method of the present invention, the temperature of the workpiece is generally 880 to 800 in the carburizing treatment, although it varies depending on the material of the workpiece, the required characteristics, the composition of the carburizing gas, the composition of the decarburizing gas, and the like. 1150 degreeC, 900-1100 degreeC, 900-1050 degreeC, or 930-1050 degreeC is employable. The temperature can be higher than the carburizing temperature in gas carburizing, but depending on the case, it may be the same as or lower than that of gas carburizing. About the temperature of a decarburization process, generally 880-1150 degreeC, 900-1100 degreeC, 900-1050 degreeC, or 930-1050 degreeC can be employ | adopted. About the temperature of a diffusion process (a decarburization process is not included), generally 880-1150 degreeC, 900-1100 degreeC, 900-1050 degreeC, or 930-1050 degreeC is employable. In the carburizing process, the decarburizing process, and the diffusion process (not including the decarburizing process), the temperature of the workpiece may be basically the same or different.
[0029]
  According to the method of the present invention, the decarburization process is a strong decarburization process having a relatively strong decarburization capacity, and a weak decarburization process that is performed after the strong decarburization process and is relatively weaker than the strong decarburization process. The form comprised by the combination with a process is employ | adopted. If only strong decarburization treatment is used, the decarburization capability is relatively strong, so that cementite existing in large quantities on the surface of the corner of the workpiece can be reduced and removed well, but the carbon concentration of the surface of the plane of the workpiece is reduced. There is a risk that the reduction will increase, and in some cases, the carbon concentration on the surface of the planar portion of the workpiece may be reduced more than necessary. On the other hand, if only weak decarburization treatment is used, the decarburization ability is relatively weak, so the carbon concentration on the surface of the flat part of the work can be secured, but the ability to remove cementite on the surface of the corner of the work is reduced. There is a risk.
[0030]
  Therefore, as described above, as described above, after performing the strong decarburization process with relatively strong decarburization capacity, the weak decarburization process with relatively weaker decarburization capacity than the strong decarburization process is performed. By doing so, it is possible to ensure the ability to remove cementite from the surface of the corner of the workpiece while suppressing a decrease in the carbon concentration of the surface of the flat portion of the workpiece. Strong decarburization treatment can adopt a form in which the decarburization gas is introduced into the furnace and the pressure in the decarburization period exceeds 6 KPa, and in particular, the decarburization gas alone (not including the carburizing gas). Thus, it is possible to adopt a configuration in which the pressure inside the furnace in the decarburization period exceeds 6 KPa when introduced into the furnace. Weak decarburization is a form in which carburizing gas and decarburizing gas are mixed and introduced into the furnace.Adopt. When the same decarburizing gas is introduced into the furnace in the strong decarburizing process and the weak decarburizing process, the partial pressure of the decarburizing gas in the weak decarburizing process is 1 in comparison with the strong decarburizing process. It can be in the range of / 15 to 1/2 or in the range of 1/10 to 1/3.
[0031]
  The decarburization temperature in the weak decarburization process may be the same as the decarburization temperature in the strong decarburization process, or may be relatively lower than the decarburization temperature in the strong decarburization process. Therefore, the temperature of strong decarburization treatment can illustrate 880-1150 degreeC, or 900-1100 degreeC, and the temperature of weak decarburization process can illustrate 870-1150 degreeC, or 890-1100 degreeC, It is not limited to this. The time for strong decarburization treatment can be exemplified by 5 to 700 minutes, and the time for weak decarburization treatment can be exemplified by 3 to 500 minutes, but it is not limited to this because it differs depending on the decarburization conditions. Of course.
[0032]
  According to the method of the present invention,Weakness ofThe decarburization process adopts a form in which the decarburization process is performed while suppressing the decarburization action with a mixed gas containing a decarburizing gas and a carburizing gas.TheIn this case, as shown in the examples described later, excess cementite is generated at the corners of the workpiece by carburization, and the difference between the carbon concentration at the surface of the workpiece and the carbon concentration at the surface of the workpiece is large. Even if it becomes, the carbon concentration of the surface of the plane part of a workpiece | work and the carbon concentration of the surface of the corner | angular part of a workpiece | work can be brought close by decarburization processing. That is, even if the difference between the carbon concentration on the surface of the flat part of the workpiece and the carbon concentration on the surface of the workpiece corner due to carburization increases, The advantage is that the carbon concentration on the surface of the portion can be made close to the target carbon concentration. As decarburizing gasCarbon dioxide gasAnd as carburizing gaspropane gasAdoptTo do. ThisIn this case, the decarburization process is performed immediately after executing the strong decarburization process having a relatively strong decarburization capacity. Using a mixed gas containing reactive gas and carburizing gasTheIn addition, oxygen dioxide gas (CO₂)Formed withDecarburized gas and propane gas (C₃H₈)Formed withWhen using a mixed gas with a carburizing gas, the mixing ratio should be a decarburizing gas: carburizing gas = (20-2): 1 or (10-4): 1 by volume. Can do.
[0033]
  In addition, according to the method of the present invention, the quenching process is generally performed after the diffusion process including the carburizing process and the decarburizing process is performed, but the quenching process may not be performed in some cases. The quenching process may be water quenching, oil quenching, or die quenching depending on circumstances. When the workpiece is soaked before the quenching treatment, the soaking temperature may be 780 to 1100 ° C., but is not limited thereto.
[0034]
【Example】
  Hereinafter, examples of the present invention will be described.Reference examples andA specific example will be described together with a comparative example.
[0035]
  (Vacuum carburizing)
  According to the present embodiment, a furnace maintained at a high temperature equal to or higher than a reduced pressure atmosphere and a carburizable temperature is used. Iron-based workpieces are charged in the furnace. FIG. 1 schematically shows a furnace 200 to which a vacuum pump 300 is connected. Reference numeral 100 denotes a plane portion of the workpiece, and reference numeral 102 denotes a corner portion of the workpiece. Since the corner portion 102 has a large exposed area per volume, the carburization amount and the decarburization amount are large. The carbon content of the workpiece before carburizing is 0.2% by mass. The material of the workpiece used in this example is a material equivalent to JIS-SCM420 and is Fe-C-Cr-Mo type, and the basic composition is Cr 1 mass%, Mo 0.23 mass%, and Mn. Including 0.7% by mass.
[0036]
  FIG. 2 schematically shows changes in temperature under conditions 2 to 6. In this case, after introducing a carburizing gas (acetylene gas) of a hydrocarbon-based gas into a furnace heated to a predetermined temperature and setting the furnace pressure to a predetermined pressure, the iron-based workpiece in the furnace Carburizing treatment was performed. Then, the inside of the furnace was evacuated, the inside of the furnace was made into a reduced pressure atmosphere, and the diffusion treatment was performed on the work in the furnace by heating in the reduced pressure atmosphere.
[0037]
  UpIn the initial stage of the diffusion treatment described above, a decarburizing gas was introduced into the furnace, and the surface of the workpiece in the furnace was decarburized to reduce or remove cementite on the surface of the workpiece. Air was adopted as the gas to be charged into the furnace as a decarburizing gas. And after performing the diffusion process including a decarburization process, as shown in FIG. 2, the soaking process was performed for 30 minutes at 850 degreeC, and the oil quenching was performed. The target of the effective hardening depth (surface depth up to Vickers hardness Hv513) of the workpiece (after quenching) in which the manufacturing method according to this example is performed is 1.80 to 1.85 mm.
[0038]
  Shown in Table 1The target value of the processing time is the sum of the carburizing process and the diffusion process including the decarburizing process, and is set to around 240 minutes in consideration of productivity. As shown in Table 1, the time for carburizing treatment and the time for diffusion treatment including decarburization treatment are set so as to be within the range of the treatment time.
[0039]
  Comparative Example 1 corresponds to Condition 1 in Table 1. Reference Example 1 corresponds to Condition 2 in Table 1, Reference Example 2 corresponds to Condition 3 in Table 1,Reference Example 1BCorresponds to condition 4 in Table 1,Reference Example 2BCorresponds to condition 5 in Table 1, and Reference Example 3 corresponds to condition 6 in Table 1.
[0040]
  As shown in Table 1, in both the conditions 2 to 6 and the comparative example 1, the carburizing temperature was set to 1000 ° C., the carburizing time was set to 70 minutes, and the furnace pressure (decompression degree) was set to 3 KPa. When vacuum carburizing was performed in this manner, excessive cementite was generated on the entire surface of the workpiece. That is, excessive cementite was generated on the surface of the planar portion of the workpiece and the surface of the corner portion of the workpiece.
[0041]
  A characteristic line M1 in FIG. 3 schematically shows the form of the carbon concentration near the surface of the workpiece when vacuum carburizing is performed. In the case of vacuum carburizing, carbon permeates excessively on the surface of the workpiece as compared with gas carburizing, so the carbon concentration near the surface of the workpiece is considerably high, which is assumed to exceed 1.0% by mass. Is done. In particular, the carbon concentration on the surface of the corner of the workpiece having a large exposed area relative to the volume is estimated to be 1.5% by mass, 2.0% by mass, and in some cases exceeding 3.0% by mass. Is done.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
  According to the conditions 2 to 6, the strong decarburization treatment was performed under strong decarburization conditions. EspeciallyReference Example 1B, Reference Example 2BAccording to the above, after carrying out the strong decarburization treatment under relatively strong decarburization conditions, the weak decarburization treatment was carried out under relatively weak decarburization conditions than the strong decarburization treatment.
[0045]
  That is, according to the conditions 2 to 6, as shown in Table 1, the decarburizing gas was introduced at the initial stage of the diffusion process and the strong decarburization process was performed on the workpiece. In this case, while introducing air (oxygen-containing) functioning as a decarburizing gas into the furnace at a rate of 50 liters (NL) / min, the furnace gas is sucked with a pump, and the predetermined furnace pressure shown in Table 1 is obtained. Maintained.
[0046]
  A characteristic line M2 in FIG. 3 schematically shows the form of the carbon concentration in the vicinity of the surface of the workpiece when a diffusion process including decarburization is performed after vacuum carburization. As schematically shown in the characteristic line M2, the peak of the carbon concentration on the outermost surface of the workpiece is considerably lowered, and further, carbon penetrates considerably into the workpiece. In additionFor exampleAccording to this, the target value of the carbon concentration on the surface of the workpiece is 0.8% by mass.
[0047]
  About Comparative Example 1, as shown in Table 1, although the diffusion treatment (1000 ° C., 170 minutes, 2 Pa) was performed, the decarburization gas was not introduced and the decarburization treatment was not performed. For this reason, as shown in Table 2, according to the test result according to Comparative Example 1, the carbon concentration of the surface of the plane portion of the workpiece is 0.86% by mass, despite being near the hypoeutectoid steel composition. The carbon concentration at the surface of the corner of the workpiece was excessively high at 1.91% by mass, and the difference in carbon concentration between the surface of the flat portion of the workpiece and the surface of the corner was very large, which was not preferable. Further, according to Comparative Example 1, although cementite did not remain on the surface of the flat portion of the workpiece, a large amount of cementite remained on the surface of the corner portion of the workpiece, which was not preferable. The carbon concentration of the work is measured by EPMA.
[0048]
  As shown in Table 1, with respect to the strong decarburization treatment according to Reference Example 3, the decarburization time was set to 50 minutes, and the furnace pressure charged with the decarburizing gas was as high as 40 KPa, that is, decarburization. The amount of sex gas was rather large, and strong decarburization conditions were adopted. For this reason, as shown in Table 2, according to the test result according to Reference Example 3, the carbon concentration at the surface of the corner of the workpiece is 0.98% by mass, which is close to the segregation steel composition. It was close to the carbon concentration (0.75% by mass) of the surface of the flat part, and further, cementite remained not only on the surface of the flat part of the work but also on the surface of the corner of the work. .
[0049]
  According to Reference Example 3, since strong decarburization treatment was performed under strong decarburization conditions, oxides of about 14 μm were observed and grain boundary oxidation was observed. However, according to a general gas carburizing method using RX gas (carburizing temperature 950 ° C., carburizing time 10 hours), the carburizing time becomes longer, so that it is generally recognized that oxides of about 20 to 25 μm are formed. It is. ThereforeReference example 3Although an oxide is observed, grain boundary oxidation is considerably less than that of a general gas carburizing method.
[0050]
  As shown in Table 1,Reference Example 2BWith regard to the strong decarburization treatment, the pressure inside the furnace charged with decarburized gas is as high as 25 KPa, that is, the amount of decarburized gas is large, and the decarburization time is slightly longer at 60 minutes. Strong decarburization conditions were used. For this reason, as shown in Table 2,Reference Example 2BAccording to the test results, the carbon concentration (1.01% by mass) on the surface of the workpiece corner is close to the carbon concentration (0.77% by mass) on the surface of the flat surface of the workpiece. No cementite remained on the surface of the corner portion of the workpiece as well as the surface of the flat portion, and it was good. In additionReference Example 2BAccording to the above, oxides of about 5 μm were recognized and grain boundary oxidation was observed, but it can be said that the grain boundary oxidation is small as compared with the general gas carburizing method as described above.
[0051]
  As shown in Table 1,Reference Example 1BFor the strong decarburization treatment, the pressure inside the furnace charged with the decarburized gas is as high as 20 KPa, that is, the amount of the decarburized gas is relatively large, and the decarburization time is slightly 60 minutes. Long and strong decarburization conditions. For this reason, as shown in Table 2,Reference Example 1BAccording to the test results, the carbon concentration (1.07% by mass) of the surface of the corner of the workpiece is close to the carbon concentration (0.79% by mass) of the surface of the flat surface of the workpiece. There was no cementite remaining on the surface of the flat portion, and only a slight amount of cementite remained on the surface of the corner portion of the workpiece.
[0052]
  As shown in Table 1, in the strong decarburization process according to Reference Example 2, the pressure inside the furnace charged with the decarburizing gas is as high as 20 KPa, that is, the amount of the decarburizing gas is relatively large. Yes, the decarburization time was a little longer, 50 minutes, and strong decarburization conditions. For this reason, as shown in Table 2, according to the test results according to Reference Example 2, the carbon concentration (1.26% by mass) of the surface of the corner portion of the workpiece is higher than that of Comparative Example 1 in the plane portion of the workpiece. It is close to the surface carbon concentration (0.81% by mass). Furthermore, no cementite remains on the surface of the flat part of the workpiece, and only a small amount of cementite remains on the surface of the corner of the workpiece. ,It was good.
[0053]
  As shown in Table 1, with respect to the strong decarburization treatment according to Reference Example 1, the furnace pressure charged with the decarburizing gas was 11 KPa.Reference examples 1B and 2BHowever, the decarburization time was set to 170 minutes, and the decarburization condition was set to be slightly stronger, although the amount of decarburizing gas was relatively low. For this reason, as shown in Table 2,Reference example 1According to the test results, the carbon concentration at the surface of the workpiece corner is 1.32% by mass, the carbon concentration at the surface of the workpiece flat is 0.64% by mass, and the surface of the workpiece flat surface is No cementite remained on the surface of the workpiece, and only a small amount of cementite remained on the surface of the corner of the workpiece.
[0054]
  Thus, in Reference Example 1, although it is a strong decarburization treatment, the concentration of the decarburizing gas is slightly lower (11 KPa) than in Examples 1 and 2, so if the decarburization time (170 minutes), A small amount of cementite remains on the surface of the corners. However, if it is such a cementite, there is no problem depending on the use of the workpiece.
[0055]
  MoreReference Example 1B, Reference Example 2BAs shown in Table 1, after the strong decarburization treatment, while maintaining the same temperature as the strong decarburization treatment, weak decarburization with a lower partial pressure of the decarburizing gas than the strong decarburization treatment The process was run for 70 minutes. That is,Reference Example 1B and Reference Example 2BIn the weak decarburization process according to the above, while the decarburization temperature is maintained at 1000 ° C., air that functions as a decarburizing gas is introduced into the furnace at a rate of 10 liters (NL) / min. Suction with a pump and a predetermined furnace pressure (Reference Example 1BThen 4KPa,Reference Example 2BThen, it was maintained at 6 KPa). In addition, the partial pressure of the decarburizing gas in this weak decarburization process is in the range of 1/10 to 1/3 compared with the above-described strong decarburization process. The decarburization temperature in the weak decarburization process isReference Example 1B,Reference Example 2BHowever, the decarburization temperature in the weak decarburization process may be lower than the decarburization temperature in the strong decarburization process.
[0056]
  As mentioned aboveReference Example 1B,Reference Example 2BAccording to the present invention, since the weak decarburization process is performed after the strong decarburization process, the carbon concentration in the flat part of the work is secured high while removing the cementite at the corners of the work of the final product, which is preferable. Is.
[0057]
  In addition, according to the reference example 3, since it is only a strong decarburization process, although the cementite in the surface of the corner | angular part of a workpiece | work can be effectively decarburized and removed, the fall of the carbon concentration of the surface of the plane part of a workpiece | work is reduced. There is a tendency to be relatively large. Further, if only weak decarburization treatment is performed, the amount of cementite remaining on the surface of the corner portion of the work tends to be a problem because the decarburization capability is low. Therefore, in order to suppress the decrease in carbon concentration on the surface of the flat part of the workpiece while removing the cementite on the surface of the corner of the work,Reference Example 1B,Reference Example 2BThus, it is considered preferable to perform the weak decarburization process after the strong decarburization process.
[0058]
  AboveExamplesThe workpieces subjected to the above were subjected to soaking at 850 ° C. in a vacuum atmosphere (furnace pressure 3 Pa), followed by oil quenching.
[0059]
  (Pulse vacuum carburizing)
  Comparative Example 2 shown in Table 1 corresponds to pulse carburization. In pulse carburizing, the time for carburizing treatment and the time for diffusion treatment are set to a short time compared to normal vacuum carburizing treatment, respectively, and the carburizing treatment and diffusion treatment are repeated. It is said to be advantageous.
[0060]
  Comparative Example 2 corresponds to pulse carburization, and as shown in Table 1, after executing the carburization process (1), the process of executing the diffusion process (3) not including the decarburization process is one cycle, After repeating this cycle seven times in the same manner as in Example 6, the main diffusion treatment (diffusion temperature 1000 ° C., diffusion time 20 minutes, furnace pressure 2 Pa) was performed. As shown in Table 2, according to the test results according to Comparative Example 2, the carbon concentration (1.06% by mass) on the surface of the corner portion of the workpiece is equal to the carbon concentration (0.85 mass on the surface of the plane portion of the workpiece). Furthermore, although cementite did not remain on the surface of the flat portion of the workpiece, cementite remained slightly on the surface of the corner of the workpiece.
[0061]
  AlsoReference Example 3B (Condition 8)As shown in Table 1, after performing the carburizing process (1), the process ((1) (2) (3) including the initial decarburizing process (2) and the diffusion process (3) is performed. )) Is the first cycle. Decarburization process (2) is described in the column of strong decarburization process in Table 1, and diffusion process (3) is described in the column of weak decarburization process in Table 1. And this (1) (2) (The first cycle consisting of 3) was repeated 7 times continuously. Thereafter, the main diffusion treatment (diffusion temperature 1000 ° C., diffusion time 20 minutes, furnace pressure 2 Pa) was performed. thisReference Example 3BCan be positioned as an improvement over conventional pulse carburizing.
[0062]
  AlsoReference Example 4B is Reference Example 3BAnd basically the same.Reference Example 4BAccording to the above, after executing the carburizing process (1), the process ((1) (2) (3)) for executing the diffusion process (3) including the decarburizing process (2) in the initial stage is set as the eleventh cycle. Yes. The cycle consisting of (1), (2) and (3) was continuously repeated 6 times. afterwards,Reference Example 3BAfter performing the carburization process (1) according to the above, the diffusion process (diffusion temperature 1000 ° C., furnace pressure 2 Pa, diffusion time 29 minutes (15 minutes + 14 minutes: (15 minutes = 15 minutes =Reference Example 3BEquivalent to the decarburization process),14 minutes =Reference Example 3BThe time corresponding to the diffusion process) was executed. afterwardsReference Example 3BThe diffusion treatment (diffusion temperature 1000 ° C., diffusion time 20 minutes, furnace pressure 2 Pa) was performed in the same manner as above. thisReference Example 4BCan be positioned as an improvement over conventional pulse carburizing.
[0063]
  As shown in Table 2,Reference Example 3BAccording to the test results, the carbon concentration at the surface of the corner of the work is 0.92% by mass, which is lower than that of Comparative Example 2, and the carbon concentration at the surface of the flat part of the work (0.83% by mass). In addition, no cementite remained on the surface of the flat part of the work, and no cementite remained on the surface of the corner of the work. As shown in Table 2,Reference Example 4BAccording to the test results, the carbon concentration at the surface of the corner of the workpiece is 0.93% by mass, which is lower than that of Comparative Example 2, and the carbon concentration at the surface of the plane portion of the workpiece (0.83 mass%). In addition, no cementite remained on the surface of the flat part of the work, and no cementite remained on the surface of the corner of the work. Therefore, it can be seen that the method of the present invention is also effective for pulse carburizing.
[0064]
[Table 3]

[0065]
  Table 3 shows reference examples 1A, 2A,According to the present inventionExample 3A is shown. In Reference Examples 1A, 2A, and Example 3A, the diffusion process is performed after the carburization process is performed. However, the decarburization process is performed over the entire time of the diffusion process, and the diffusion process time is equal to the decarburization process time. That is, in the decarburization process, since the workpiece is heated to a high temperature region, a diffusion phenomenon occurs in the workpiece.
[0066]
  As shown in Table 3, according to Reference Example 1A, acetylene gas was used as the carburizing gas, the furnace pressure was 3 KPa at a carburizing temperature of 950 ° C., and after performing the carburizing process for 45 minutes, air was used as the decarburizing gas. Was charged into the furnace at a rate of 120 liters (NL) / min, and the furnace pressure was 20 KPa. This decarburization process corresponds to a strong decarburization process because air containing no carburizing gas is introduced into the furnace for decarburization.
[0067]
  The carbon concentration of the surface of the flat part of the workpiece subjected to such Reference Example 1A is 0.5% by mass, the carbon concentration of the surface of the corner of the work is 0.8% by mass, and the cementite at the corner is There was no. This is because the decarburization treatment was good. In addition, when air is introduced into the furnace as a decarburizing gas during the decarburization period, the air can function as a decarburizing gas in nature, but immediately after the start of the decarburizing process, Under the influence of soot remaining in the atmosphere, oxygen in the air produces CO, so it becomes carburized immediately after the start of the decarburization treatment, but gradually functions as a decarburization gas after the start of the decarburization. It will be different. Therefore, when introducing air into the furnace, in order to perform decarburization satisfactorily, the introduction time is generally preferably 5 minutes or more.
[0068]
  As shown in Table 3, according to Reference Example 2A, the carburization temperature was 950 ° C., the furnace pressure was 3 KPa, and after performing the carburizing process for 45 minutes, oxygen dioxide gas (CO₂) And propane gas (C₃H₈) Was charged into the furnace at a rate of 34.8 liters (NL) / min, and the furnace pressure was 30 KPa. This decarburization process corresponds to the weak decarburization process because the decarburization gas containing the carburizing gas is introduced into the furnace for decarburization.
[0069]
  In the above case, oxygen dioxide gas (CO₂) And propane gas (C₃H₈)), And the volume ratio was, as shown in Table 3, oxygen dioxide gas: propane gas = 33: 4.2. That is, oxygen dioxide gas is supplied into the furnace at 33 liters (NL) / min, and propane gas is supplied into the furnace at 4.2 liters (NL) / min. In other words, in the mixed gas described above, oxygen dioxide gas (CO₂) Is a carburizing gas, propane gas (C₃H₈) Is richer than.
[0070]
  The carbon concentration of the surface of the flat part of the workpiece on which the reference example 2A is implemented is 0.8% by mass, the carbon concentration of the surface of the corner of the work is 0.9% by mass, The carbon concentration at the surface and the carbon concentration at the surface of the corner of the workpiece were close to each other, and there was no cementite not only at the flat portion of the workpiece but also at the corner of the workpiece.
[0071]
  As described above, in Reference Example 2A, the carbon concentration at the surface of the workpiece and the carbon concentration at the corner are close to each other so that the carbon potential (Cp value) of the mixed gas is about 0.7 to 0.8. In addition, oxygen dioxide gas (CO₂) And propane gas (C₃H₈) Is introduced at a predetermined mixing ratio in the decarburization period, the carbon concentration lower than the Cp value of the plane part of the workpiece increases and approaches the Cp value, and the angle of the workpiece It is inferred that the carbon concentration higher than the Cp value of the part decreases and approaches the Cp value, and as a result, the carbon concentration of the surface of the flat part of the work and the carbon concentration of the surface of the corner of the work approach.
[0072]
  Therefore, oxygen dioxide gas (CO₂) And propane gas (C₃H₈)) Is introduced in the decarburization period, and the carbon potential (Cp value) is set to a predetermined target carbon concentration. 75% by mass) increases and approaches the target carbon concentration (for example, 0.8% by mass), and the carbon concentration (for example, 0.9% by mass) higher than the target carbon concentration at the corner of the workpiece decreases. It approaches the target carbon concentration (for example, 0.8% by mass), and as a result, both the carbon concentration on the surface of the plane portion of the workpiece and the carbon concentration on the surface of the corner portion of the workpiece are respectively the target carbon concentration (for example, 0.8 mass). %).
[0073]
  As shown in Table 3,According to the present inventionAccording to Example 3A, the carburization temperature is 950 ° C., the furnace pressure is 3 KPa, the carburizing process is performed for 45 minutes, then the carburizing process is performed for 15 minutes under the condition 1A, and immediately thereafter, the condition 2A The weak decarburization treatment was performed for 35 minutes. Condition 1A is that decarburizing gas is charged with air at a rate of 120 liters (NL) / min and the pressure in the furnace is 20 KPa. Condition 2A is an oxygen dioxide gas (CO2) as a decarburizing gas.₂) And propane gas (C₃H₈) Is introduced into the furnace at a rate of 37.2 liters (NL) / min, and the furnace pressure is set to 30 KPa.
[0074]
  Thus, the weak decarburization process is performed immediately after the strong decarburization process.According to the present inventionWhen Example 3A is carried out, as shown in Table 3, the carbon concentration on the surface of the workpiece becomes 0.8 mass%, the carbon concentration on the corner becomes 0.83 mass%, and the carbon on the surface of the plane portion of the workpiece The concentration and the carbon concentration at the surface of the workpiece corner were very close, and there was no cementite at the workpiece corner as well as the workpiece flat.
[0075]
  Thus, in Example 3A, the reason why the carbon concentration of the surface of the plane portion of the workpiece is close to the carbon concentration of the surface of the corner portion of the workpiece is the oxygen dioxide gas (CO2) that is a decarburizing gas as described above.₂) And propane gas (C₃H₈) With a predetermined mixing ratio, the carbon potential (Cp value) can be controlled to be about 0.7 to 0.9. The low carbon concentration increases and approaches the Cp value, and the carbon concentration higher than the Cp value at the corner of the workpiece decreases and approaches the Cp value. It is presumed that the carbon concentration at the surface of the corners of the corners approaches. In addition, about the workpiece | work which implemented Reference Example 1A, 2A, and Example 3A, after carrying out the soaking | uniform-heating process on the same conditions as the above-mentioned, it quenches with oil.
[0076]
  (Others) In addition, the method of the present invention is not limited to the above-described embodiments, and can be implemented with appropriate modifications without departing from the scope of the invention.
[0077]
【The invention's effect】
  As described above, according to the vacuum carburizing method of the present invention, it is possible to reduce or remove cementite at the corners of the workpiece while shortening the production time.The decarburization process is executed while suppressing the decarburization action with a mixed gas containing a decarburizing gas and a carburizing gas. As a result, as described above, the carbon concentration on the surface of the planar portion of the workpiece can be brought close to the carbon concentration on the surface of the corner portion of the workpiece, which is advantageous for high quality.
[0078]
  In the case of the following form, a specific effect is recognized. After performing the carburizing process, the process of performing the diffusion process including the decarburizing process in the initial stage is one cycle. When this cycle is continuously repeated a plurality of times, as described above, the cementite at the corners of the workpiece is More advantageous for reduction or elimination.
[0079]
  When the furnace pressure (degree of vacuum) in the decarburization process is set to 40 KPa or less, the oxygen partial pressure is reduced, which is advantageous for suppressing grain boundary oxidation in the workpiece. In addition, the decarburization process consists of a combination of a strong decarburization process with relatively strong decarburization capacity and a weak decarburization process that is performed after the strong decarburization process and is relatively weaker than the strong decarburization process. Has beenForAs described above, it is advantageous for reducing or removing cementite on the surface of the corner portion of the workpiece while ensuring the carbon concentration of the surface of the flat portion of the workpiece.
[Brief description of the drawings]
FIG. 1 is a configuration diagram schematically showing a furnace.
FIG. 2 is a graph schematically showing a form of temperature change.
FIG. 3 is a graph schematically showing the carbon concentration near the surface of a workpiece immediately after vacuum carburization.
[Explanation of symbols]
  In the figure, 100 indicates a plane portion of the workpiece, 102 indicates a corner portion of the workpiece, and 200 indicates a furnace.

Claims (4)

Carburizing gas is introduced into the furnace maintained at a high temperature that is higher than the reduced-pressure atmosphere and carburizable temperature, and the carburizing process is performed on the iron-based workpiece in the furnace.
Thereafter, in the vacuum carburizing method of performing diffusion treatment on the workpiece in the furnace by evacuating the furnace and heating and holding in a reduced pressure atmosphere,
In a vacuum carburizing method in which decarburizing gas is introduced into a furnace at least in the initial stage of the diffusion treatment to decarburize the surface of the work in the furnace, and cementite on the surface of the work is reduced or removed. There,
The decarburization process is performed in the presence of a decarburizing gas, and a strong decarburization process with relatively strong decarburization conditions, and is performed after the strong decarburization process and the decarburizing gas is present. The decarburization conditions are performed in a state where the decarburization process is relatively weaker than the strong decarburization process ,
Among the decarburization processes, the weak decarburization process is a mixed gas containing carbon dioxide gas, which is a decarburizing gas, and propane gas, which is a carburizing gas, and is desorbed so that the carbon concentration can be increased in excess decarburized parts. A vacuum carburizing method which is performed while suppressing charcoal action . In Claim 1, the cycle which performs the carburization process, performs the diffusion process including the decarburization process in the initial stage is made into the 1st cycle, and the process of repeating the 1st cycle several times continuously is included, Vacuum carburizing method. The diffusion process according to claim 1, wherein a cycle in which the carburizing process is performed and a diffusion process including the decarburizing process in the initial stage is a first cycle, and the carburizing process is performed and the decarburizing process is not included. When the cycle to execute is the second cycle,
The first cycle is repeated a plurality of times continuously, after the end of the repetition of the first cycle, or during the repetition of the plurality of first cycles, between the first cycle and the first cycle, A vacuum carburizing method comprising a step of executing a second cycle. The vacuum carburizing method according to any one of claims 1 to 3, wherein a degree of vacuum in the decarburization process is set to 40 KPa or less.
[0001]

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