JP4054649B2 - Method for producing ferrous sintered alloy exhibiting quenched structure - Google Patents
Method for producing ferrous sintered alloy exhibiting quenched structure Download PDFInfo
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- JP4054649B2 JP4054649B2 JP2002288441A JP2002288441A JP4054649B2 JP 4054649 B2 JP4054649 B2 JP 4054649B2 JP 2002288441 A JP2002288441 A JP 2002288441A JP 2002288441 A JP2002288441 A JP 2002288441A JP 4054649 B2 JP4054649 B2 JP 4054649B2
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- 229910045601 alloy Inorganic materials 0.000 title claims description 59
- 239000000956 alloy Substances 0.000 title claims description 59
- 238000004519 manufacturing process Methods 0.000 title claims description 12
- 230000001747 exhibiting effect Effects 0.000 title claims description 5
- CWYNVVGOOAEACU-UHFFFAOYSA-N Fe2+ Chemical compound [Fe+2] CWYNVVGOOAEACU-UHFFFAOYSA-N 0.000 title 1
- 239000000843 powder Substances 0.000 claims description 74
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 39
- 238000005245 sintering Methods 0.000 claims description 34
- 229910000734 martensite Inorganic materials 0.000 claims description 26
- 238000001816 cooling Methods 0.000 claims description 19
- 229910052742 iron Inorganic materials 0.000 claims description 13
- 239000000203 mixture Substances 0.000 claims description 12
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 10
- 229910017116 Fe—Mo Inorganic materials 0.000 claims description 10
- 229910003296 Ni-Mo Inorganic materials 0.000 claims description 10
- 229910052750 molybdenum Inorganic materials 0.000 claims description 10
- 239000011812 mixed powder Substances 0.000 claims description 8
- 239000002184 metal Substances 0.000 claims description 7
- 229910052751 metal Inorganic materials 0.000 claims description 7
- 239000012535 impurity Substances 0.000 claims description 4
- 239000011148 porous material Substances 0.000 claims description 3
- 238000010438 heat treatment Methods 0.000 claims description 2
- 230000001590 oxidative effect Effects 0.000 claims description 2
- 238000000748 compression moulding Methods 0.000 claims 1
- 239000000463 material Substances 0.000 description 12
- 230000007423 decrease Effects 0.000 description 10
- 230000000694 effects Effects 0.000 description 8
- 229910052759 nickel Inorganic materials 0.000 description 8
- 238000000034 method Methods 0.000 description 7
- 238000010791 quenching Methods 0.000 description 6
- 230000000171 quenching effect Effects 0.000 description 6
- 229910001566 austenite Inorganic materials 0.000 description 5
- 238000000465 moulding Methods 0.000 description 4
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 3
- 238000002156 mixing Methods 0.000 description 3
- 229910001563 bainite Inorganic materials 0.000 description 2
- 238000005452 bending Methods 0.000 description 2
- 239000002131 composite material Substances 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 239000010439 graphite Substances 0.000 description 2
- 229910002804 graphite Inorganic materials 0.000 description 2
- 239000000314 lubricant Substances 0.000 description 2
- 238000004663 powder metallurgy Methods 0.000 description 2
- 239000000047 product Substances 0.000 description 2
- 238000003303 reheating Methods 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- XOOUIPVCVHRTMJ-UHFFFAOYSA-L zinc stearate Chemical compound [Zn+2].CCCCCCCCCCCCCCCCCC([O-])=O.CCCCCCCCCCCCCCCCCC([O-])=O XOOUIPVCVHRTMJ-UHFFFAOYSA-L 0.000 description 2
- 229910000640 Fe alloy Inorganic materials 0.000 description 1
- 229910000990 Ni alloy Inorganic materials 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 238000005255 carburizing Methods 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 230000006835 compression Effects 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
Description
【０００１】
【発明の属する技術分野】
本発明は、粉末冶金法に係り、特に焼入れ工程を必要とせずに優れた強度を有する鉄系焼結合金を製造する方法に関するものである。
【０００２】
【従来の技術】
粉末冶金法によって製造された鉄系焼結合金は、経済性に優れているという特徴から、例えば、自動車部品、工作機器、家電製品等に広く利用されている。しかるに、近年の各種製品における低価格化の趨勢に対応するため、焼結部品においても更に低廉化が要求されるようになりつつある。この要求を満たすために、低廉な鉄粉の開発等も行われているが、材料特性が低下するという問題がある。
また、製造工程の連続化や無人化によるコストの低減も行われているが、十分な効果は得られていない。そのような背景により本発明者らは特開平９−８７７９４号公報において焼入れ工程を不要とした焼結合金の製造方法を発明した。しかしながら、最近の更なる高強度化の要求に対しては、この材料を用いても対応できなくなってきた。
【０００３】
【発明が解決しようとする課題】
焼入れを行わずに高強度の部品を得る手法としては、焼入れ性の優れた合金粉末を用いて焼結時の冷却速度でマルテンサイト変態させた材料があるが、通常の焼結炉の冷却速度は５〜２０℃／分であり、この冷却速度でマルテンサイト組織を得るためには、合金元素の添加量が多くなり圧縮性を著しく低下させ、強度は低くなる。
【０００４】
一方、純鉄粉に焼入れ性を改善させるＮｉ、Ｃｕ、Ｍｏ等の粉末を添加したもの、またはこれらを複合合金化したものは、圧縮性に優れているが、焼結体の合金成分が不均一となるため、金属組織の一部分をマルテンサイト組織にすることができる。しかし組織の８５％以上をマルテンサイト組織にすることは通常の焼結条件においては困難である。
【０００５】
本発明は、このような技術状態を考慮して、焼入れを行わずに従来の焼入れ処理材と同等の強度を有する鉄系焼結合金の新規な製造方法を提供することを目的とするものである。
【０００６】
【課題を解決するための手段】
上記のような目的を達成するために、焼入れ性を高め、圧縮性の低下を最小限に抑えるための研究を種々進めた結果、焼入れ性に優れる合金粉末と圧縮性に優れる合金粉末を混合し、さらに焼入れ性を改善させる元素を単味粉末の形態で添加することにより、圧縮性の低下による強度の低下を抑え、通常の焼結炉の５〜２０℃／分の冷却速度で金属組織中の８５％以上がマルテンサイト組織となる鉄系焼結合金が得られることを見出した。なお、通常、マルテンサイト組織は、マルテンサイト結晶間に未変態のオーステナイトが残留、分散した組織を呈するもので、本願はこのような変態も含むものである。
【０００７】
すなわち、本発明の焼入れ組織を呈する鉄系焼結合金の製造方法は、粉末の組成が、質量比で、Ｎｉ：３〜５％、Ｍｏ：０.４〜０.７％、残部：Ｆｅおよび不可避不純物よりなるＦｅ−Ｎｉ−Ｍｏ合金粉末と、粉末の組成が、質量比で、Ｍｏ：０.５〜２％、残部：Ｆｅおよび不可避不純物よりなるＦｅ−Ｍｏ合金粉末との質量比が、５：９５〜７０：３０になるように配合するとともに、全体組成におけるＮｉ量が４〜６質量％、焼結後のＣ量が０.２〜０.７質量％となるようＮｉ粉末と黒鉛粉末をさらに添加した混合粉末を、金型内で圧縮成形し、得られた圧粉体を非酸化性雰囲気中で１１３０〜１３００℃の範囲で焼結した後、焼結炉中で５℃／分以上、２０℃／分以下の速度で冷却して、焼結後の気孔部を除く金属組織を、面積比で、８５％以上がマルテンサイト組織とすることを特徴とする。
【０００８】
なお、Ｎｉ粉末および／またはＣｕ粉末を、前記Ｆｅ−Ｎｉ−Ｍｏ合金粉末および／または前記Ｆｅ−Ｍｏ合金粉末に複合合金化（部分的に拡散して固着）した粉末を用いると好適である。
【０００９】
また、上記焼結後に１００℃以下まで冷却した後、さらに１５０℃以上、３００℃以下の温度に加熱し保持すること、または焼結炉内で冷却中に１５０℃以上３００℃以下の温度で保持するとさらに好適である。
【００１０】
混合粉の主体として合金粉末を使用し、焼入れ性を改善するために必要な元素を単味粉末の状態で添加すると、完全合金粉末を用いた場合に比べて粉末圧縮性が高く、高密度の焼結合金を得ることが容易になる。ただし、合金鉄粉の合金元素の含有量が所定量より少ない場合、あるいは純鉄粉に焼結合金の所定組成になるようにＮｉ粉、Ｍｏ粉およびＣｕ粉を添加した混合粉の場合には、基地全体にそれらの元素を拡散させて目的の焼入れ組織を有する焼結合金を得ることが困難になる。
【００１１】
合金鉄粉中に添加する合金元素の種類としては、焼入れ性の改善に効果があり、しかも圧縮性の低下が少ないＮｉおよびＭｏが有効である。また、その含有量は材料の焼入れ性と圧縮性により決定されるものであり、質量比で、Ｎｉ：３〜５％、Ｍｏ：０.４〜０.７％、残部Ｆｅからなる組成のＦｅ−Ｎｉ−Ｍｏ合金粉末と、Ｍｏ：０.５〜２％、残部ＦｅからなるＦｅ−Ｍｏ合金粉末とを５：９５〜７０：３０の比率で混合して用いると所望の焼入れ組織を有する焼結合金が得られる。
【００１２】
Ｆｅ−Ｎｉ−Ｍｏ合金粉末は、ＮｉおよびＭｏの含有量がそれぞれ３質量％、０.４質量％未満では焼入れ性改善の効果が乏しく、それぞれ５質量％、０.７質量％を越えると粉末が硬くなり圧縮性が低下する。また前記Ｆｅ−Ｍｏ合金粉末は、Ｍｏの含有量が０.５質量％未満では焼入れ性改善の効果が乏しく、２質量％を越えると粉末が硬くなり圧縮性が低下する。
【００１３】
これらのＦｅ−Ｎｉ−Ｍｏ合金粉末とＦｅ−Ｍｏ合金粉未を用い、Ｆｅ−Ｎｉ−Ｍｏ合金粉末とＦｅ−Ｍｏ合金粉未の質量比が５：９５〜７０：３０となるよう添加混合することにより、例えば成形圧力６ｔ／ｃｍ２で、成形密度が６.９ｇ／ｃｍ３以上となり、従来の完全合金粉末以上の圧縮性が得られる。上記のＦｅ−Ｎｉ−Ｍｏ合金粉末の添加量が上限値を越える場合は圧縮性が低下し材料強度は低下する。また、Ｆｅ−Ｍｏ合金粉末の添加量が上限値を越える場合は、焼入れ性を改善する合金元素の単味粉末を添加しても、金属組織中の８５％以上をマルテンサイト組織にすることはできず、材料強度は低下する。
【００１４】
この合金粉末に黒鉛粉末を添加するのみでは、ベイナイト組織しか得られないため、８５％以上をマルテンサイト組織とするためには、焼入れ性を更に改善する必要があり、焼入れ性を改善するための元素を単味粉末として添加することが必要となる。元素としては、Ｃｕ、Ｎｉ、Ｍｎ、Ｃｒ等が考えられるが、焼結性を考慮した場合、焼入れ性の改善にはＮｉが最も効果的で、次いでＣｕが効果的である。Ｃｕ粉末の添加量は、１％未満ではその効果が認められず、３％を越えると衝撃値が低下するため１〜３質量％とする。また、Ｎｉは焼入れ性を改普する効果の他に、Ｃｕによる脆化を抑制する効果がある。Ｎｉ粉末の添加量は全体で４％未満ではその効果が認められず、６％を越えるとＮｉの未拡散相であるオーステナイト組織により、かえってマルテンサイト組織が減少し強度が低下する傾向があるのでその範囲を４〜６質量％とすることが好ましい。
【００１５】
上記のＮｉ粉末とＣｕ粉末は、上記のＦｅ−Ｎｉ−Ｍｏ合金粉末および／またはＦｅ−Ｍｏ合金粉末に部分的に拡散して固着させた部分拡散合金粉末として用いると、圧縮性を損なうことなく、合金元素を基地に拡散させやすいので好適である。
【００１６】
焼結時の冷却速度は、材料のＣＣＴ線図により決定され、マルテンサイトが組織の面積比で８５％以上となるように、５℃／分以上とする。また、冷却速度が２０℃／分を越えると設備上新たな冷却装置が必要となり、焼結費を上昇させることになるので５℃／分以上、２０℃／分以下とする。
【００１７】
Ｃは、黒鉛粉末の形態で添加する方法や焼結雰囲気の浸炭性ガスにより添加する方法があるが、内部まで均一にマルテンサイト組織にするためには、黒鉛として添加する必要がある。その添加量は、焼結後のＣ含有量が０.２〜０.７質量％となるように定めればよい。焼結後のＣ含有量が０.２％未満の場合には、マルテンサイト組織の面積比が８５％のものを得ることは不可能であり、一方、０.７％を越えると残留オーステナイト相が多くなり、更には結晶粒界に沿ってセメンタイトが析出するため強度が低下する。このため焼結後のＣ含有量の範囲を０.２〜０.７質量％とする。黒鉛粉末を添加した後には焼結を行なうので、焼結後のＣ含有量は焼結前よりも幾分減少する。黒鉛粉末の実際の添加量は、発明者らが用いた分解アンモニアガスによる還元性雰囲気の場合、０.４〜０.８質量％が必要であった。なお、減少量は用いる粉末、焼結雰囲気などにより左右されるので、実験によりその量を確認した後、目標のＣ含有量に換算し添加することが必要である。
【００１８】
焼結後の組織は気孔を除く素地全体を１００％として、８５％以上をマルテンサイト組織とすることにより、従来材の熱処理を行ったものと同等の強度のものが得られる。
【００１９】
さらに、焼結および冷却後あるいは冷却の過程において、焼結材を１５０℃以上、３００℃以下の温度に保持することにより、マルテンサイト組織は焼戻しマルテンサイトとなって強靭化し、強度がさらに改善される。また、組織としてはより安定化することにより、経時変化、特に寸法変動を抑制する効果を付与することもできる。１５０℃以上、３００℃以下の温度に保持する方法としては、焼結後に室温まで冷却してから戻し炉で再加熱を行う方法があるが、室温まで冷却せずに１００℃程度の温度から焼戻し炉へ移送し再加熱することにより、エネルギーの省力化を図ることもできる。さらに焼結ヒートパターンにより、焼結炉を１００℃以下に冷却することなく、直接１５０℃以上、３００℃以下の温度に保持することによって、恒温変態が促進され、残留オーステナイトはベイナイトに変態し、マルテンサイトは焼戻されて高い靭性が得られる。また、この手法によれば工程削減によるコストの低減を図ることもできる。なお、上記の温度範囲内に保持する時間（ｈｒ）は、製品の最大肉厚（ｍｍ）×０.０５〜０.１０程度が好適である。
【００２０】
【発明の実施の形態】
特定組成範囲のＮｉおよびＭｏを含む鉄合金粉に、特定量のＮｉ粉、黒鉛粉あるいはさらにＣｕ粉を配合した粉末からなる混合粉末を圧縮成形し、圧粉体を１１３０〜１３００℃の範囲で焼結した後、焼結炉中で所定の冷却速度で冷却することにより、特定の焼入れ組織を有する強度の優れた鉄系焼結合金が得られる。
【００２１】
【実施例】
［実施例１］
質量比で４％Ｎｉ、０.５％Ｍｏ、残部Ｆｅからなる合金粉末Ａと１.５％Ｍｏ、残部Ｆｅからなる合金粉末Ｂに全体組成で４Ｎｉ％、１.５Ｃｕ％になるようにＮｉ単味粉、Ｃｕ単味粉を添加し、さらに焼結後のＣ含有量が０.５質量％になるように黒鉛粉をそれぞれ添加し、成形潤滑剤としてステアリン酸亜鉛粉を０.８質量％配合し、３０分間混合した。混合粉を７００ＭＰａで成形したときの密度、および１２００℃で６０分間分解アンモニアガス中で焼結し、１０℃／分の速度で冷却したときの金属組織中のマルテンサイト組織の面積比と曲げ強さを表１に示す。
【００２２】
【表１】
以上の結果から明らかなように、本発明の焼結材である試料Ｎｏ.３、Ｎｏ.４、Ｎｏ.５の圧縮性は優れており、またマルテンサイト組織の面積比も高いため高強度が得られる。
【００２４】
［実施例２］
質量比で４％Ｎｉ、０.５％Ｍｏ、残部Ｆｅからなる合金粉末Ａ５０％に質量比で１.５％Ｍｏ、残部Ｆｅからなる合金粉末Ｂを５０％混合した粉末に全体組成が表２に記載した比率となるように、Ｎｉ単味粉、Ｃｕ単味粉、黒鉛粉を添加し、成形潤滑剤としてステアリン酸亜鉛粉を０.８質量％配合し、３０分間混合した。混合粉を７００ＭＰａで成形したときの密度、および１２００℃で６０分間分解アンモニアガス中で焼結し、１０℃／分の速度で冷却したときの金属組織中のマルテンサイト組織の面積比と曲げ強さを表２に示す。
【００２５】
【表２】
以上の結果から明らかなように、本発明の焼結材である試料Ｎｏ.４、Ｎｏ.５のマルテンサイト組織の面積比は高く高強度が得られる。Ｎｏ.６は焼入れ性に優れマルテンサイト組織が全面に見られたが、Ｎｉ粉末の添加量が多く、Ｎｉリッチのオーステナイト相が発生し、マルテンサイト組織の量が低下して高い強度が得られていない。
【００２７】
【発明の効果】
本発明の焼入れ組織を呈する鉄系焼結合金の製造方法によれば、焼結のみで組織の８５％以上がマルテンサイト組織の焼結合金が得られ、焼入れ工程が不要となり、強度の優れた鉄系焼結合金を経済的に製造することが可能となる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a powder metallurgy method, and more particularly to a method for producing an iron-based sintered alloy having excellent strength without requiring a quenching step.
[0002]
[Prior art]
Iron-based sintered alloys manufactured by powder metallurgy are widely used in, for example, automobile parts, machine tools, home appliances, and the like because of their excellent economic efficiency. However, in order to respond to the trend of lower prices in various products in recent years, there is a demand for further reduction in the cost of sintered parts. In order to satisfy this requirement, inexpensive iron powder has been developed, but there is a problem that the material characteristics are deteriorated.
Moreover, although the cost is reduced by continuation of manufacturing processes and unmanned manufacturing, sufficient effects are not obtained. With such a background, the present inventors have invented a method for producing a sintered alloy that does not require a quenching step in Japanese Patent Application Laid-Open No. 9-87794. However, it has become impossible to meet the recent demand for higher strength even if this material is used.
[0003]
[Problems to be solved by the invention]
As a method of obtaining high-strength parts without quenching, there is a material that has been martensitic transformed at the cooling rate during sintering using alloy powder with excellent hardenability, but the cooling rate of a normal sintering furnace Is 5 to 20 ° C./min. In order to obtain a martensite structure at this cooling rate, the amount of alloy element added is increased, the compressibility is significantly reduced, and the strength is lowered.
[0004]
On the other hand, a powder obtained by adding powders such as Ni, Cu, and Mo that improve hardenability to pure iron powder, or a composite alloy of these powders is excellent in compressibility, but the alloy component of the sintered body is poor. Since it becomes uniform, a part of metal structure can be made into a martensite structure. However, it is difficult to make a martensitic structure 85% or more of the structure under normal sintering conditions.
[0005]
The present invention is intended to provide a novel method for producing an iron-based sintered alloy having a strength equivalent to that of a conventional quenching material without quenching in consideration of such a technical state. is there.
[0006]
[Means for Solving the Problems]
In order to achieve the above objectives, various studies have been conducted to improve hardenability and minimize the decrease in compressibility. As a result, alloy powders with excellent hardenability and alloy powders with excellent compressibility are mixed. Further, by adding an element for improving hardenability in the form of a simple powder, a decrease in strength due to a decrease in compressibility is suppressed, and in a metal structure at a cooling rate of 5 to 20 ° C./min in a normal sintering furnace. It was found that an iron-based sintered alloy having a martensite structure of 85% or more can be obtained. In general, the martensite structure exhibits a structure in which untransformed austenite remains and is dispersed between martensite crystals, and the present application includes such a transformation.
[0007]
That is, in the method for producing an iron-based sintered alloy exhibiting a quenched structure according to the present invention, the composition of the powder is, by mass ratio, Ni: 3 to 5%, Mo: 0.4 to 0.7%, the balance: Fe and The mass ratio of the Fe—Ni—Mo alloy powder made of inevitable impurities and the composition of the powder is, by mass ratio, Mo: 0.5 to 2%, the balance: Fe—Mo alloy powder made of Fe and inevitable impurities, Ni powder and graphite so that the amount of Ni in the overall composition is 4-6% by mass and the amount of C after sintering is 0.2-0.7% by mass The mixed powder to which the powder is further added is compression-molded in a mold, and the obtained green compact is sintered in a non-oxidizing atmosphere in the range of 1130 to 1300 ° C., and then in a sintering furnace at 5 ° C. / Min. Or more, cooling at a rate of 20 ° C./min or less, the metal structure excluding the pores after sintering, by area ratio, 5% or more is characterized in that a martensitic structure.
[0008]
It is preferable to use a powder obtained by forming a Ni alloy and / or a Cu powder into a composite alloy (partially diffused and fixed) to the Fe—Ni—Mo alloy powder and / or the Fe—Mo alloy powder.
[0009]
In addition, after cooling to 100 ° C. or lower after the sintering, it is further heated and held at a temperature of 150 ° C. or higher and 300 ° C. or lower, or held at a temperature of 150 ° C. or higher and 300 ° C. or lower during cooling in a sintering furnace. This is more preferable.
[0010]
When alloy powder is used as the main component of the mixed powder and elements necessary for improving the hardenability are added in the form of a simple powder, the powder compressibility is higher and the density is higher than when using the complete alloy powder. It becomes easy to obtain a sintered alloy. However, when the content of alloying elements in the alloy iron powder is less than a predetermined amount, or in the case of a mixed powder in which Ni powder, Mo powder and Cu powder are added to pure iron powder so as to have a predetermined composition of the sintered alloy. Therefore, it becomes difficult to obtain a sintered alloy having a desired quenched structure by diffusing these elements throughout the base.
[0011]
As the types of alloy elements added to the alloy iron powder, Ni and Mo which are effective in improving hardenability and have little decrease in compressibility are effective. Further, the content is determined by the hardenability and compressibility of the material, and in terms of mass ratio, Ni: 3 to 5%, Mo: 0.4 to 0.7%, the balance of Fe Fe composition -Ni-Mo alloy powder and Fe-Mo alloy powder composed of Mo: 0.5-2% and the balance Fe are used in a ratio of 5: 95-70: 30. Bonds are obtained.
[0012]
Fe-Ni-Mo alloy powder has poor effect of improving hardenability when the content of Ni and Mo is less than 3% by mass and less than 0.4% by mass, respectively, and when the content exceeds 5% by mass and 0.7% by mass, respectively. Becomes hard and compressibility decreases. Further, the Fe—Mo alloy powder has a poor effect of improving the hardenability when the Mo content is less than 0.5% by mass, and if it exceeds 2% by mass, the powder becomes hard and the compressibility is lowered.
[0013]
Using these Fe—Ni—Mo alloy powder and Fe—Mo alloy powder not added, mixing is performed so that the mass ratio of Fe—Ni—Mo alloy powder and Fe—Mo alloy powder is 5:95 to 70:30. Thus, for example, at a molding pressure of 6 t / cm 2 , the molding density becomes 6.9 g / cm 3 or more, and compressibility higher than that of a conventional complete alloy powder can be obtained. When the added amount of the Fe—Ni—Mo alloy powder exceeds the upper limit, the compressibility is lowered and the material strength is lowered. In addition, when the addition amount of the Fe-Mo alloy powder exceeds the upper limit, even if a simple powder of an alloy element that improves hardenability is added, 85% or more of the metal structure can be martensitic. It cannot be done and the material strength decreases.
[0014]
Only by adding graphite powder to this alloy powder, only a bainite structure can be obtained. Therefore, in order to obtain a martensite structure of 85% or more, it is necessary to further improve the hardenability, and to improve the hardenability. It is necessary to add the element as a simple powder. As the element, Cu, Ni, Mn, Cr, and the like can be considered. When considering the sinterability, Ni is the most effective for improving the hardenability, and then Cu is the most effective. If the amount of Cu powder added is less than 1%, the effect is not recognized. If the amount exceeds 3%, the impact value decreases, so the amount is set to 1 to 3% by mass. In addition to improving the hardenability, Ni has an effect of suppressing embrittlement due to Cu. When the total amount of Ni powder is less than 4%, the effect is not recognized, and when it exceeds 6%, the martensite structure tends to decrease and the strength tends to decrease due to the austenite structure that is an undiffused phase of Ni. The range is preferably 4 to 6% by mass.
[0015]
When the Ni powder and Cu powder are used as a partial diffusion alloy powder partially diffused and fixed to the Fe-Ni-Mo alloy powder and / or Fe-Mo alloy powder, the compressibility is not impaired. It is preferable because the alloy element is easily diffused into the base.
[0016]
The cooling rate at the time of sintering is determined by the CCT diagram of the material, and is 5 ° C./min or more so that martensite is 85% or more in terms of the area ratio of the structure. Further, if the cooling rate exceeds 20 ° C./min, a new cooling device is required on the equipment and the sintering cost is increased. Therefore, the temperature is set to 5 ° C./min or more and 20 ° C./min or less.
[0017]
C may be added in the form of graphite powder or by a carburizing gas in a sintering atmosphere, but it must be added as graphite in order to obtain a uniform martensite structure. The addition amount may be determined so that the C content after sintering is 0.2 to 0.7% by mass. When the C content after sintering is less than 0.2%, it is impossible to obtain a martensite structure with an area ratio of 85%. On the other hand, when the C content exceeds 0.7%, the residual austenite phase is not obtained. In addition, since cementite precipitates along the grain boundaries, the strength decreases. For this reason, the range of C content after sintering shall be 0.2-0.7 mass%. Since the sintering is performed after the graphite powder is added, the C content after the sintering is somewhat less than before the sintering. In the case of the reducing atmosphere with the decomposed ammonia gas used by the inventors, the actual amount of graphite powder required was 0.4 to 0.8 mass%. The amount of reduction depends on the powder used, the sintering atmosphere, and the like. Therefore, after confirming the amount by experiment, it is necessary to convert to the target C content and add it.
[0018]
The structure after sintering is 100% of the whole substrate excluding pores and the martensite structure is 85% or more, so that a structure having the same strength as that obtained by heat treatment of the conventional material can be obtained.
[0019]
Furthermore, by maintaining the sintered material at a temperature of 150 ° C. or higher and 300 ° C. or lower after sintering and cooling or in the cooling process, the martensite structure becomes tempered martensite and is toughened, and the strength is further improved. The Further, by stabilizing the structure, it is possible to impart an effect of suppressing a change with time, particularly a dimensional variation. As a method for maintaining the temperature at 150 ° C. or more and 300 ° C. or less, there is a method of cooling to room temperature after sintering and then reheating in a return furnace, but tempering from a temperature of about 100 ° C. without cooling to room temperature. Energy saving can also be achieved by transferring to a furnace and reheating. Furthermore, by maintaining the sintering furnace directly at a temperature of 150 ° C. or more and 300 ° C. or less without cooling the sintering furnace to 100 ° C. or less by the sintering heat pattern, the isothermal transformation is promoted, and the residual austenite is transformed into bainite, Martensite is tempered to obtain high toughness. Moreover, according to this method, the cost can be reduced by reducing the number of processes. In addition, as for the time (hr) hold | maintained in said temperature range, the maximum thickness (mm) of product * 0.05-0.10 is suitable.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
A mixed powder composed of a powder obtained by blending a specific amount of Ni powder, Ni powder, graphite powder or further Cu powder into iron alloy powder containing Ni and Mo in a specific composition range is compression molded, and the green compact is in the range of 1130 to 1300 ° C. After sintering, by cooling at a predetermined cooling rate in a sintering furnace, an iron-based sintered alloy having a specific quenched structure and excellent strength can be obtained.
[0021]
【Example】
[Example 1]
The alloy powder A and the alloy powder B consisting of 4% Ni, 0.5% Mo and the remaining Fe in mass ratio and the alloy powder B consisting of the remaining Fe and Ni to have a total composition of 4Ni% and 1.5Cu%. A simple powder and a simple Cu powder are added, and graphite powder is added so that the C content after sintering is 0.5 mass%, and 0.8 mass of zinc stearate powder as a molding lubricant. % And mixed for 30 minutes. Density when mixed powder is molded at 700 MPa, and area ratio and bending strength of martensitic structure in metal structure when sintered in decomposed ammonia gas at 1200 ° C. for 60 minutes and cooled at a rate of 10 ° C./min Table 1 shows the length.
[0022]
[Table 1]
As is clear from the above results, the compressibility of Samples No. 3, No. 4, and No. 5, which are the sintered materials of the present invention, is excellent, and since the area ratio of the martensite structure is high, high strength can get.
[0024]
[Example 2]
Table 2 shows the total composition of a powder obtained by mixing 50% of an alloy powder B consisting of 4% Ni, 0.5% Mo and the balance Fe, 50% by weight, and 1.5% Mo and the balance Fe consisting of the balance Fe. Ni simple powder, Cu simple powder, and graphite powder were added, and 0.8 mass% of zinc stearate powder was blended as a molding lubricant and mixed for 30 minutes. Density when mixed powder is molded at 700 MPa, and area ratio and bending strength of martensitic structure in metal structure when sintered in decomposed ammonia gas at 1200 ° C. for 60 minutes and cooled at a rate of 10 ° C./min Table 2 shows the length.
[0025]
[Table 2]
As is clear from the above results, the area ratio of the martensitic structures of samples No. 4 and No. 5 which are the sintered materials of the present invention is high, and high strength is obtained. No. 6 was excellent in hardenability and a martensite structure was observed on the entire surface, but the amount of Ni powder added was large, a Ni-rich austenite phase was generated, the amount of martensite structure was reduced, and high strength was obtained. Not.
[0027]
【The invention's effect】
According to the method for producing an iron-based sintered alloy exhibiting a quenched structure according to the present invention, a sintered alloy having a martensite structure of 85% or more of the structure can be obtained only by sintering, and a quenching step is not required and the strength is excellent. An iron-based sintered alloy can be produced economically.
Claims (4)
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