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    Study on control of acetylene based vacuum carburizing process

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    https://www.riss.kr/link?id=T17157525

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    다국어 초록 (Multilingual Abstract) kakao i 다국어 번역

    영어
    한국어
    Vacuum carburizing is a high-temperature heat treatment method that improves the mechanical properties of automotive and industrial components by diffusing carbon into the surface of low-alloy steel, followed by quenching to enhance hardness and wear resistance. This process commonly uses acetylene gas which decomposes catalytically to transfer carbon to the steel surface. Vacuum carburizing stands out for its rapid carburizing rate and low CO2 emissions during operation, offering excellent process efficiency and environmental friendliness.
    Due to very high carbon activity during a boost step, vacuum carburizing potentially results in the formation of cementite or soot on the surface and along grain boundaries of austenite. It is essential to carefully design the vacuum carburizing process to achieve the required surface concentration and carburizing depth without abnormal microstructures such as cementite networks or soot. The process recipe should be designed by using a carburizing simulator.
    Therefore, to accurately calculate the carbon concentration within components and validate process control using a mathematical model, it is essential to precisely define the diffusion equation and boundary conditions. A key parameter among the boundary conditions is the external carbon flux, which represents the amount of carbon supplied to the product surface per unit time and surface area from the carburizing atmosphere.
    In this study, the relationship between external carbon flux and carburizing temperature as well as acetylene flow rate experimentally quantified. The effects of various process parameters (flow rate, temperature, pressure, and boost time) on carburizing behavior examined to determine the optimal conditions for the first boost step. Experimentally derived boundary condition was applied to the carburizing simulator, resulting in the flexible of adapting to diverse operating conditions and efficient process development.
    To accurately measure the carbon concentration in carburized components, EPMA equipment was utilized to establish optimal electron beam conditions (accelerating voltage, beam current) and generate calibration curves based on standard specimens. Subsequently, to evaluate the prediction accuracy of the carburizing model, validation tests were conducted under various carburizing conditions at lab-scale and production-scale, comparing the measured and predicted carbon concentration values (lab-scale accuracy: 97.6%; production-scale accuracy: 93.1%).
    Finally, a target carburizing depth was set, and process recipes were derived under various input conditions (external carbon flux, carburizing temperature, concentration distribution method) for numerical test. As the external carbon flux associated with acetylene flow rate increased, acetylene consumption increases significantly. Increasing the minimum carbon concentration set during the diffusion step resulted in a reduction in total carburizing time, while lowering the maximum carbon concentration during the boost step led to decreased acetylene consumption.
    번역하기

    Vacuum carburizing is a high-temperature heat treatment method that improves the mechanical properties of automotive and industrial components by diffusing carbon into the surface of low-alloy steel, followed by quenching to enhance hardness and wear ...

    Vacuum carburizing is a high-temperature heat treatment method that improves the mechanical properties of automotive and industrial components by diffusing carbon into the surface of low-alloy steel, followed by quenching to enhance hardness and wear resistance. This process commonly uses acetylene gas which decomposes catalytically to transfer carbon to the steel surface. Vacuum carburizing stands out for its rapid carburizing rate and low CO2 emissions during operation, offering excellent process efficiency and environmental friendliness.
    Due to very high carbon activity during a boost step, vacuum carburizing potentially results in the formation of cementite or soot on the surface and along grain boundaries of austenite. It is essential to carefully design the vacuum carburizing process to achieve the required surface concentration and carburizing depth without abnormal microstructures such as cementite networks or soot. The process recipe should be designed by using a carburizing simulator.
    Therefore, to accurately calculate the carbon concentration within components and validate process control using a mathematical model, it is essential to precisely define the diffusion equation and boundary conditions. A key parameter among the boundary conditions is the external carbon flux, which represents the amount of carbon supplied to the product surface per unit time and surface area from the carburizing atmosphere.
    In this study, the relationship between external carbon flux and carburizing temperature as well as acetylene flow rate experimentally quantified. The effects of various process parameters (flow rate, temperature, pressure, and boost time) on carburizing behavior examined to determine the optimal conditions for the first boost step. Experimentally derived boundary condition was applied to the carburizing simulator, resulting in the flexible of adapting to diverse operating conditions and efficient process development.
    To accurately measure the carbon concentration in carburized components, EPMA equipment was utilized to establish optimal electron beam conditions (accelerating voltage, beam current) and generate calibration curves based on standard specimens. Subsequently, to evaluate the prediction accuracy of the carburizing model, validation tests were conducted under various carburizing conditions at lab-scale and production-scale, comparing the measured and predicted carbon concentration values (lab-scale accuracy: 97.6%; production-scale accuracy: 93.1%).
    Finally, a target carburizing depth was set, and process recipes were derived under various input conditions (external carbon flux, carburizing temperature, concentration distribution method) for numerical test. As the external carbon flux associated with acetylene flow rate increased, acetylene consumption increases significantly. Increasing the minimum carbon concentration set during the diffusion step resulted in a reduction in total carburizing time, while lowering the maximum carbon concentration during the boost step led to decreased acetylene consumption.

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    국문 초록 (Abstract) kakao i 다국어 번역

    한국어
    영어
    진공침탄이란 자동차 및 기계 부품의 기계적 특성을 향상시키는 열처리 공정으로 오스테나이징한 저합금강의 표면에 탄소를 확산시킨 후, 소입 공정을 통해 표면의 내마모성 및 강도를 높이는 공정이다. 이 공정은 고온에서 수행되며, 촉매를 이용해 철을 분해하고 탄소를 공급하는 아세틸렌 가스를 주로 사용하기 때문에 매우 빠른 침탄 속도를 가지고 침탄 과정 중에 CO2 배출이 없는 장점이 있다. 따라서, 공정 효율성과 환경 친화성 측면에서 최적의 선택이다. 하지만 이러한 우수한 진공침탄 공정도 몇가지의 문제점들이 있다. 비평형 상태에서 침탄 반응이 일어나기 때문에 침탄 가스를 주입하는 짧은 시간 안에 강철 표면의 탄소 농도가 포화 상태에 도달하여 제품 표면에 시멘타이트 또는 그을음이 형성될 수 있다. 요구되는 표면 탄소농도 및 침탄 깊이를 만족하고 부품 표면에 입계 시멘타이트나 soot과 같은 이상층이 형성되지 않기 위해 진공침탄 공정을 세심하게 설계하는 것이 필요하다.
    따라서, 부품 내의 탄소 농도를 정확히 계산하고 수학적 모델을 통해 공정 제어를 검증하기 위해서는 확산 방정식과 경계 조건을 명확히 설정하는 것이 필수적입니다. 경계 조건에서 핵심 요소는 외부 탄소 플럭스로, 이는 침탄 환경에서 제품 표면으로 단위 시간과 표면적당 공급되는 탄소량을 나타낸다.
    본 연구에서는 침탄 온도와 아세틸렌 유량에 따른 external carbon flux의 관계를 실험적으로 정량화하였다. 또한, 다양한 공정 변수 (유량, 온도, 압력, boost time)가 침탄 거동에 미치는 효과에 대해 조사하여 first boost step의 적절한 조건을 나타내었다. 결국, 실험적으로 얻은 경계조건을 침탄 모델에 적용하여 다양한 작업 조건에 적용할 수 있는 유연성과 공정 개발을 하는데 효율성을 제공하는 침탄 시뮬레이터를 개발하였다.
    침탄 부품의 정확한 탄소농도 측정을 위해 EPMA 장비를 사용하여 적절한 전자빔 조건을 설정하고, 표준시편에 따른 Calibration curve를 도출하였다. 다음으로, 침탄 모델의 탄소농도 예측 정확도를 평가하기 위하여 랩 스케일 및 양산 스케일에서 다양한 침탄 조건으로 실증 테스트를 진행하여 탄소농도 실측값과 예측값을 검증비교하였다 (랩 스케일 정확도: 97.6 %; 양산 스케일 정확도: 93.1 %).
    마지막으로, 목표 침탄깊이를 설정하고 다양한 입력 조건 (external carbon flux, 침탄 온도, 농도 배분 방식)에 따른 공정 레시피를 도출하여 numerical test를 실시하였다. Boost step에서 아세틸렌이 투입되는 유량과 관련된 external carbon flux가 증가할수록 아세틸렌 사용량이 뚜렷하게 늘어났고, 공정 시간은 큰 차이가 없었다. Boost step과 diffusion step으로 구성된 펄스 사이클을 배분하기 위한 농도 제어에 따른 공정 계산을 통해 공정 효율을 분석하였다. Diffusion step에 최소 탄소농도가 높아질수록 총 침탄 시간은 감축되었으며, boost step에서 최대 탄소농도가 낮아질수록 아세틸렌 사용량이 감축되는 결과를 얻었다.
    번역하기

    진공침탄이란 자동차 및 기계 부품의 기계적 특성을 향상시키는 열처리 공정으로 오스테나이징한 저합금강의 표면에 탄소를 확산시킨 후, 소입 공정을 통해 표면의 내마모성 및 강도를 높...

    진공침탄이란 자동차 및 기계 부품의 기계적 특성을 향상시키는 열처리 공정으로 오스테나이징한 저합금강의 표면에 탄소를 확산시킨 후, 소입 공정을 통해 표면의 내마모성 및 강도를 높이는 공정이다. 이 공정은 고온에서 수행되며, 촉매를 이용해 철을 분해하고 탄소를 공급하는 아세틸렌 가스를 주로 사용하기 때문에 매우 빠른 침탄 속도를 가지고 침탄 과정 중에 CO2 배출이 없는 장점이 있다. 따라서, 공정 효율성과 환경 친화성 측면에서 최적의 선택이다. 하지만 이러한 우수한 진공침탄 공정도 몇가지의 문제점들이 있다. 비평형 상태에서 침탄 반응이 일어나기 때문에 침탄 가스를 주입하는 짧은 시간 안에 강철 표면의 탄소 농도가 포화 상태에 도달하여 제품 표면에 시멘타이트 또는 그을음이 형성될 수 있다. 요구되는 표면 탄소농도 및 침탄 깊이를 만족하고 부품 표면에 입계 시멘타이트나 soot과 같은 이상층이 형성되지 않기 위해 진공침탄 공정을 세심하게 설계하는 것이 필요하다.
    따라서, 부품 내의 탄소 농도를 정확히 계산하고 수학적 모델을 통해 공정 제어를 검증하기 위해서는 확산 방정식과 경계 조건을 명확히 설정하는 것이 필수적입니다. 경계 조건에서 핵심 요소는 외부 탄소 플럭스로, 이는 침탄 환경에서 제품 표면으로 단위 시간과 표면적당 공급되는 탄소량을 나타낸다.
    본 연구에서는 침탄 온도와 아세틸렌 유량에 따른 external carbon flux의 관계를 실험적으로 정량화하였다. 또한, 다양한 공정 변수 (유량, 온도, 압력, boost time)가 침탄 거동에 미치는 효과에 대해 조사하여 first boost step의 적절한 조건을 나타내었다. 결국, 실험적으로 얻은 경계조건을 침탄 모델에 적용하여 다양한 작업 조건에 적용할 수 있는 유연성과 공정 개발을 하는데 효율성을 제공하는 침탄 시뮬레이터를 개발하였다.
    침탄 부품의 정확한 탄소농도 측정을 위해 EPMA 장비를 사용하여 적절한 전자빔 조건을 설정하고, 표준시편에 따른 Calibration curve를 도출하였다. 다음으로, 침탄 모델의 탄소농도 예측 정확도를 평가하기 위하여 랩 스케일 및 양산 스케일에서 다양한 침탄 조건으로 실증 테스트를 진행하여 탄소농도 실측값과 예측값을 검증비교하였다 (랩 스케일 정확도: 97.6 %; 양산 스케일 정확도: 93.1 %).
    마지막으로, 목표 침탄깊이를 설정하고 다양한 입력 조건 (external carbon flux, 침탄 온도, 농도 배분 방식)에 따른 공정 레시피를 도출하여 numerical test를 실시하였다. Boost step에서 아세틸렌이 투입되는 유량과 관련된 external carbon flux가 증가할수록 아세틸렌 사용량이 뚜렷하게 늘어났고, 공정 시간은 큰 차이가 없었다. Boost step과 diffusion step으로 구성된 펄스 사이클을 배분하기 위한 농도 제어에 따른 공정 계산을 통해 공정 효율을 분석하였다. Diffusion step에 최소 탄소농도가 높아질수록 총 침탄 시간은 감축되었으며, boost step에서 최대 탄소농도가 낮아질수록 아세틸렌 사용량이 감축되는 결과를 얻었다.

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    목차 (Table of Contents)

    • List of figures --------------------------------------------------------------------------------------------------------------------- iv
    • List of tables ----------------------------------------------------------------------------------------------------------------------- xi
    • Abstract in English ------------------------------------------------------------------------------------------------------------ xii
    • 1. Introduction ------------------------------------------------------------------------------------------------------------------- 1
    • 1.1. Carburizing ------------------------------------------------------------------------------------------------------------ 1
    • List of figures --------------------------------------------------------------------------------------------------------------------- iv
    • List of tables ----------------------------------------------------------------------------------------------------------------------- xi
    • Abstract in English ------------------------------------------------------------------------------------------------------------ xii
    • 1. Introduction ------------------------------------------------------------------------------------------------------------------- 1
    • 1.1. Carburizing ------------------------------------------------------------------------------------------------------------ 1
    • 1.1.1. Definition of carburizing ------------------------------------------------------------------------------ 1
    • 1.1.2. Gas carburizing process -------------------------------------------------------------------------------- 4
    • 1.1.3. Acetylene based vacuum carburizing process ------------------------------------------ 7
    • 1.1.4. Historical development of vacuum carburizing technology ------------------- 10
    • 1.2. Control of vacuum carburizing process: Simulator ----------------------------------------- 14
    • 1.3. Research problems-------------------------------------------------------------------------------------------------- 18
    • 1.4. Purpose and outline of present study ----------------------------------------------------------------- 19
    • 2. Theoretical background ----------------------------------------------------------------------------------------------- 21
    • 2.1. Diffusion equation and boundary condition ----------------------------------------------------- 21
    • 2.1.1. Carbon diffusion coefficient ------------------------------------------------------------------------ 23
    • 2.1.2. External carbon flux -------------------------------------------------------------------------------------- 26
    • 2.1.3. Surface carbon concentration ---------------------------------------------------------------------- 32
    • 2.2. Partitioning method ---------------------------------------------------------------------------------------------- 35
    • 2.3. Numerical method ------------------------------------------------------------------------------------------------- 38
    • 3. Experimental Method---------------------------------------------------------------------------------------------------- 41
    • 3.1. Materials ----------------------------------------------------------------------------------------------------------------- 41
    • 3.2. Vacuum carburizing test --------------------------------------------------------------------------------------- 44
    • 3.2.1. Carburizing furnace -------------------------------------------------------------------------------------- 44
    • 3.2.2. Process conditions ----------------------------------------------------------------------------------------- 46
    • 3.3. Microstructure observation ---------------------------------------------------------------------------------- 49
    • 3.3.1. Optical microscope (OM) ---------------------------------------------------------------------------- 49
    • 3.3.2. Scanning electron microscope (SEM) -------------------------------------------------------- 49
    • 3.3.3. X-ray diffractometer (XRD) ------------------------------------------------------------------------ 50
    • 3.3.4. Raman spectrometer -------------------------------------------------------------------------------------- 50
    • 3.4. Carbon mass gain -------------------------------------------------------------------------------------------------- 51
    • 3.5. Hardness test ---------------------------------------------------------------------------------------------------------- 51
    • 3.6. Electron probe microanalyzer (EPMA)-------------------------------------------------------------- 52
    • 3.7. Calibration curve method ------------------------------------------------------------------------------------- 53
    • 4. Influence of process conditions on carburizing behavior --------------------------------------- 54
    • 4.1. Introduction ------------------------------------------------------------------------------------------------------------ 54
    • 4.2. Results and discussion ------------------------------------------------------------------------------------------ 57
    • 4.2.1. Effect of acetylene flow rate ------------------------------------------------------------------------ 57
    • 4.2.2. Effect of boost time -------------------------------------------------------------------------------------- 64
    • 4.2.3. Effect of pressure ------------------------------------------------------------------------------------------ 69
    • 4.2.4. Evaluation of carbon flux ----------------------------------------------------------------------------- 73
    • 4.3. Summary ---------------------------------------------------------------------------------------------------------------- 78
    • 5. Surface characteristics according to the steel types ------------------------------------------------ 79
    • 5.1. Introduction ------------------------------------------------------------------------------------------------------------ 79
    • 5.2. Results and discussion ------------------------------------------------------------------------------------------ 81
    • 5.3. Summary ----------------------------------------------------------------------------------------------------------------- 89
    • 6. Verification of carbon profile prediction ------------------------------------------------------------------ 90
    • 6.1. Introduction ------------------------------------------------------------------------------------------------------------ 90
    • 6.2. Results and discussion ------------------------------------------------------------------------------------------ 93
    • 6.2.1. Effect of beam conditions ---------------------------------------------------------------------------- 93
    • 6.2.2. Establishment of calibration curve ------------------------------------------------------------- 98
    • 6.2.3. Carburizing tests for verification of simulator------------------------------------------- 100
    • 6.3. Summary ----------------------------------------------------------------------------------------------------------------- 109
    • 7. Numerical test of vacuum carburizing simulator ----------------------------------------------------- 110
    • 7.1. Introduction ------------------------------------------------------------------------------------------------------------ 110
    • 7.2. Results and discussion ------------------------------------------------------------------------------------------ 112
    • 7.2.1. Structure of a vacuum carburizing simulator -------------------------------------------- 112
    • 7.2.2. Effect of external carbon flux --------------------------------------------------------------------- 116
    • 7.2.3. Effect of carburizing temperature --------------------------------------------------------------- 121
    • 7.2.4. Partitioning method (Cmin-Cmax)------------------------------------------------------------------- 125
    • 7.3. Summary ----------------------------------------------------------------------------------------------------------------- 131
    • References -------------------------------------------------------------------------------------------------------------------------- 132
    • Abstract in Korean ------------------------------------------------------------------------------------------------------------ 142
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