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metallurgy - چدن آستنیتی
جمعه 27 اسفند 1389  10:28 ق.ظ
نوع مطلب: (انواع چدنها ،) توسط: anarch kh

چدن های آستنیتی خانواده ای از چدن ها هستند که به دلیل مقاومت به خوردگی وابستگی شدیدی به ترکیب شیمیایی دارند.بنابراین رعایت حدود تغییرات عناصر در محدوده های تععین شده بسیار مهم است.

از آنجائیکه سیلیسیم از تشکیل کاربیدها جلوگیری نموده وباعث رسوب گرافبت می شود لذا باید با در نظر گرفتن محدوده مجاز استاندارد مقدار ان را به بالا ترین حد ممکن رساند.نوع کاربید تاثیر مهمی بر خواص ریخته گری دارد. تشکیل کاربید بیانگر این نکته است که در هنگام انجماد گرافیت کمتری رسوب می کند.که در این صورت میزان انبساط چدن در قالب کمتر می باشد

به علاوه کاربیدهای یوتکتیک کروم در مراحل انتهی انجماد نمی توانند به خوبی تغذیه شوند ودر نتیجه باعث ایجاد خلل و فرج و همچنین بروز انقباض های بین دندریتی می شوند. بنابراین از نظر انقباض ناشی از انجماد بهتر است که میزان کاربید ها حداقل باشد. در بیشتر چدن های آستنیتی بهتر است کروم وجود داشته دارد که منجر به تشکیل کاربیدهای کروم می شود.

به هر حال کروم مقاومت در برابر خوردگی و اکسیداسیون را افزایش می دهد ودر بعضی موارد برای افزایش مقاومت سایشی زیاد بیش از 3% کروم به آلیاژ اضافه می شود. چدن های آستنیتی مانند چدن های خاکستری غیر آلیاژی نسبت به ضخامت حساس می باشند. بدین معنی که مقاطع نازک تر نسبت به مقاطع ضخیم تر دارای کاربید های بیشتری هستند. همچنین مقذار کاربید با افزایش کربن معادل کاهش می یابد. کربن معادل باید به نحوی انتخاب شودکه از قابلیت ریخته گری و تشکیل تخلخل درونی و استحکام و ساختمان گرافیت مطلوب ترین شرایط به دست آید. چدن آستنیتی قادر به تولید ساختمان های گرافیتی کامل نظیر آنچه که در چدن های غیر آلیاژی دیده می شوند نیستند. کربن معادل مطابق فرمول زیر محاسبه می شود.

CE=%C + 0.33%Si + 0.047%Ni – 0.055Ni * %Si

 

عوامل موثر در ریخته گری قطعات چدن آستنیتی

 

بی عیب بودن ققطعات ریختگی از جنس چدن آستنیتی علاوه بر اینکه به روش های راهگاهی وتغذیه گذاری مربوط است معمولا به عوامل دیگری نیز بستگی دارد که بعضی از آنها از نظر اندازه ی قطعه ی ریخته گری و استحکام قالب وترکیب شیمیایی و درجه حرارت ریختن و جوانه زایی خود با یکدیگر وابستگی دارند. همیشه در نظر داشته باشید که چدن آستنیتی با گرافیت ورقه ای از نظر خصوصیات انقباضی وسایر خصوصیات ریخته گری شباهتی با چدن خاکستری ندراد. برای قطعات چدن آستنیتی بایستی مناسب ترین  مدل ساخته شود. انقباض انجماد مدل از جنس چدن آستنیتی 0.65% می باشد. در حالی که مقدار آن برای قطعاتی از چدن معمولی 0.85% می باشد.

به طور کلی تا آنجا که امکان داشته باشدطراحی یک قطعه چدن آستنیتی باید دارای مقاطع یکنواخت باشد. ودر صورتی که وجود مقاطع ضخیم اجتناب ناپذیر باشد باید تکیه سیستم براین باشد که فلز مذاب به اندازه کافی به مقاطع ضخیم تغذیه گردد. استحکام وسختی زیاد قالب از عوامل بسیار مهم است. قالب بایستی توانایی مقاومت در برابر فشار فرواستاتیکی وفشاری که در اثر رسوب گرافیت به وجود می آید را داشته باشد.

 

 

کاربرد قطعات چدن آستنیتی

 

از چدن آستنیتی برای تولید محصولاتی استفاده می شود که بایستی در محیط های گوناگون در برابر خوردگی و پوسته شدن مقاوم باشند و در درجه حرارت هی بالاتر از 800 درجه رشد وخزش آنها نسبتا کم باشد. البته این آلیاژ ها به خاطر خواص خوب انبساطی و الکتریکی و غیر مغناطیسی بودن وسایر خصوصیات خوب فیزیکی مصارف بسیار زیادی دارد.همچنین ازآن در تولید قطعاتی که در درجه حرارت های زیر صفر به کار گرفته می شوند نیز استفاده می گردد.

کاربرد اصلی چدن آستنیتی در تولید رینگ پیستون های موتور های دیزلی و پمپ ها و شیر های مورد استفاده در صنایع نمک زدایی و شیمیایی و نفت وتمام صنایع که آب وسایر مواد خورنده حمل می نماید. همچنین برای تولید توربوشارژهای لوله های اگزوز و شیر های مقاوم در درجه حرارت های بالا و کمپرسورها و کلید های الکتریکی وپروانه های کشتی ها می باشد. مقاومت خوب چدن های آستنیتی در برابر خوردگی در آب دریا از مهمترین عواملی است که استفاده از این نوع چدن را روز افزون نموده است


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