Investigation of Different Carbon Ratio and Temperature Effects on Impact Toughness Properties of Nickel Added Powder Metallurgy Steels

Öz

In this study, impact toughness properties were investigated by adding nickel element to Powder Metallurgy (PM) steels with different carbon ratios. For this purpose, 0.4% - 0.8 - 1.2% by weight natural graphite, 2% nickel and 0.5% zinc stearate as lubricant were added to the pure iron powder, respectively. Impact test samples were produced by shaping the prepared powder mixtures at room temperature and 700 MPa pressing pressure in a single-acting mold. Then, it was sintered at 1200 ºC in a vacuum atmosphere of 5x10-2 Pa for 20 minutes. After the sintering process, impact toughness values were determined by applying the impact test to powder metal steels at different temperatures. Depending on the chemical composition of the alloy, SEM microstructure and fracture surface analyzes were performed and the relationships between macrohardness values and impact toughness were investigated. It has been determined that nickel-rich ferritic solid solution forms in the microstructure of nickel-added PM steels and suppresses the precipitation of pearlite. All samples had very low impact toughness at -20 °C, while the impact test values at +40 °C decreased with increasing carbon content.

Anahtar Kelimeler

• Powder metallurgy
• impact toughness
• fracture surface
• nickel addition steel

Nikel İlaveli Toz Metalurjisi Çeliklerin Darbe Tokluk Özelliklerine Farklı Karbon Oranı ve Sıcaklık Etkilerinin İncelenmesi

Öz

Bu çalışmada farklı karbon oranlarına sahip Toz Metalurjisi (TM) çeliklere nikel elementi ilave edilerek darbe tokluk özellikleri incelenmiştir. Bu amaçla saf demir tozuna sırasıyla ağırlıkça % 0,4 - 0,8 - 1,2 oranlarında doğal grafit , % 2 oranında nikel ve yağlayıcı olarak ise % 0,5 çinko stearat ilave edilmiştir. Hazırlanan toz karışımları, oda sıcaklığında ve tek etkili kalıpta 700 MPa presleme basıncında şekillendirilerek darbe test numuneleri üretilmiştir. Ardından 1200 ºC sıcaklıkta 5x10-2 Pa vakum ortamında 20 dk süre sinterlenmiştir. Sinterlenme işlemi sonrasında toz metal çeliklere farklı sıcaklıklarda darbe testi uygulanarak darbe tokluk değerleri belirlenmiştir. Alaşımın kimyasal bileşimine bağlı olarak SEM mikroyapı ve kırık yüzey analizleri yapılarak, makrosertlik değerleri ile darbe toklukları arasındaki ilişkiler incelenmiştir. Nikel ilaveli TM çeliklerin mikroyapılarında nikelce zengin ferritik katı çözeltinin oluşarak perlit çökelmesini baskıladığı belirlenmiştir. -20 °C’de tüm numuneler çok düşük darbe tokluğuna sahipken, +40 °C’de yapılan darbe testi değerleri artan karbon oranıyla azalmıştır.

Anahtar Kelimeler

• Toz metalurjisi
• darbe tokluğu
• kırık yüzey
• nikel ilaveli çelik

Kaynakça

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