滲碳後不同冷卻參數對滲碳鋼材特性之影響

Effect of Different Cooling Parameters on The Property of Carburized Steel.

黃冠議(kang-yi huang)

指導教授 ： 李景恆

國立虎尾科技大學/工程學院/材料科學與工程系材料科學與綠色能源工程碩士在職專班/碩士(2015年)

https://doi.org/10.6827/NFU.2015.00120

摘要

熱處理技術為機械加工技術的基礎。熱處理加熱溫度及冷卻溫度決定鋼材熱處理後的機械性質。鋼材於高溫滲碳後，採用不同淬火-回火方式，將影響其顯微組織的組成，造成鋼材機械性質的差異。
本研究選擇業界常用的JIS-S15C鋼及JIS-SNCM220兩種滲碳鋼材。鋼材先以碳勢1.0 %在925 °C溫度中進行滲碳處理120 min，再降低碳勢至0.8 %進行擴散處理60min，然後降溫至860 °C溫度以細化晶粒、減少碳固溶量並降低熱應力。緊接著將鋼材分別進行四種不同的淬火-回火處理，其中一種採淬入350 °C鹽溫槽浸漬60 min方式進行恆溫回火，另外三種淬火-回火方式為分別淬入210 °C鹽溫槽、100 °C油溫槽及70 °C油溫槽，浸漬20 min後提出空冷，再於350°C回火120 min。
實驗結果顯示，S15C鋼及SNCM 220鋼熱處理後在較低溫（70 °C，100 °C）冷卻淬火後表層得到組織及硬化深度相近。以採70 ℃淬火試片為例，淬火表層滲碳組織為高碳麻田散體及殘留沃斯田體，硬化深度(550HV0.3)分別為0.75mm、1.03mm; 210 °C淬火試片得到的表層滲碳組織為麻田散體及變韌體，硬化深度(550HV0.3)分別為0.32mm、0.55mm。SNCM220鋼雖其含碳量與S15C鋼相近，但因添加Ni、Cr、Mo合金元素增加鋼材的硬化能，故有較大的硬化深度。
兩種滲碳鋼材採70 °C和100 °C淬火冷卻後，再於350°C回火後的組織與性質相近。兩種滲碳鋼材經70 ℃淬火及350°C回火後，表層滲碳組織為回火麻田散體及變韌體，試片的彎曲變形量分別為0.31 mm、0.17 mm，抗拉強度最高，分別為1321MPa、1380 MPa，伸長率最低，分別為1.2 %EL、3.0 ％EL。滲碳鋼材經210 °C鹽浴淬火及350°C回火後，表層滲碳組織為回火麻田散體、雪明碳體及肥粒體混合組織，試片的彎曲變形量最大，分別為0.37mm、0.25mm，抗拉強度分別為1130MPa、1269 MPa，伸長率低分別為2.8%EL、3.9％EL。
兩種滲碳鋼材經350°C恆溫回火後，表層滲碳組織為變韌體及肥粒體，試片的彎曲變形量最低分別為0.02 mm、0.14 mm，伸長率最高分別為12％EL，16.1 ％EL，抗拉強度分別為1265 MPa，1240MPa。
兩種滲碳鋼材採用350°C恆溫方式冷卻，與其它三種冷卻方法比較結果顯示其變形量最低、抗拉強度略低、伸長率最高，且因不需再實施回火，熱處理成本更低。
關鍵字：滲碳、S15C、SNCM220、恆溫回火

關鍵字

滲碳 ； S15C ； SNCM220 ； 恆溫回火

並列摘要

Heat treatment is part of the foundation of machinery manufacturing technology, and the mechanical properties of heat-treated steels are determined by the heating and cooling temperatures. For high-temperature carburized steels, their microstructure composition is influenced by the type of quenching and tempering method, and the microstructure composition in turn affects the mechanical properties of the steels.
The specimens tested in the study were JIS-S15C steel and JIS-SNCM220 steel. These two carburizing steels are commonly used in the industry. The steels were first carburized with carbon potential of 1.0% and at a temperature of 925°C for 120 minutes, followed by carbon potential of 0.8% for diffusion for 60 minutes. Then the temperature was dropped to 860°C to refine the grains, reduce the amount of solid solution carbon, and decrease the thermal stress. Immediately after, the steels were subject to one of the following four quenching and tempering methods: One was salt bath immersion at 350°C for 60 minutes of isothermal tempering, and the other three were 20 minutes of salt bath immersion at 210°C, oil bath immersion at 100°C, and oil bath immersion at 70°C followed by air cooling and another 120 minutes of tempering at 350°C.
It was found from the lower temperature quenching experiment comparing two quenching temperatures, 70°C and 100°C, that steel specimens treated by these two temperatures showed a similar surface structure and depth of hardening, probably because these two quenching temperatures were not much different. Take the S15C steel and SNCM220 steel specimens quenched at 70°C as an example, the carburized structure of the surface was made up of high-carbon martensite with retained austenite, and the depth of hardening (550HV0.3) was 0.75mm for S15C and 1.03mm for SNCM220. For the specimen quenched at 210 °C, the carburized structure of the quenched surface was made up of martensite and bainite, and the depth of hardening (550HV0.3) was 0.32mm for S15C and 0.55mm for SNCM220. Even though the carbon content of SNCM220 is similar to that of S15C, the addition of Ni, Cr, and Mo these alloying elements may increases its hardenability, resulting a larger hardening depth.
When two types of carburized steels were quenched at 70°C and then tempered at 350°C, the surface of the carburized structure was made up of tempered martensite and bainite. The bending deformation was 0.31mm for S15C and 0.21mm for SNCM220. The tensile strength was the highest among all quenching temperatures; 1321MPa for S15C and 1380 MPa for SNCM220. The elongation rate was the lowest among all quenching temperatures; 1.2% EL for S15C and 3.0% EL for SNCM220. When the carburized steels were quenched in the salt bath at 210°C and tempered at 350°C, the surface of carburized structure was composed of a mixture of tempered martensite, cementite, and ferrite. The bending deformation was the highest; 0.37mm for S15C and 0.25 mm for SNCM220. The tensile strength was 1130MPa for S15C and 1269 MPa for SNCM220. The elongation rate was the lowest; 2.8% EL for S15C and 3.9% EL for SNCM220.
When two types of carburized steels were isothermal tempered at 350°C, the surface of the carburized structure was composed of bainite and ferrite. The bending deformation of the specimens was the lowest among all methods tested; 0.02mm for S15C and 0.14mm for SNCM220. The elongation rate was the highest among all methods tested; 12% EL for S15C and 16.1% EL for SNCM220. The tensile strength was 1265MPa for S15C and 1240MPa for SNCM220.
When comparing the cooling methods, the two carburized steels that went through isothermal cooling at 350°C had the lowest deformation, a slightly lower tensile strength, and a highest elongation rate compared to the other three cooling methods. Because no re-tempering is required, this isothermal cooling at 350°C had the lowest heat treatment cost.

Keywords: Carburization, S15C, SNCM220, Isothermal tempering

並列關鍵字

Carburization ； S15C ； SNCM220 ； Isothermal tempering

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