In this study, the effect of austenitizing temperatures and low-temperature isothermal heat treatment (below martensite start temperature) on the microstructure and mechanical properties of AISI 9254 high silicon spring steel has been investigated. Experimental studies show that ultra-fine carbide-free bainite, tempered martensite and carbon enriched retained austenite could be observed in isothermally heat-treated samples where the as-received sample consisted of fine pearlite. A high tensile strength of ~2060 MPa, a total elongation of ~8 %, and absorbed energy of 105 J were achieved in a commercial high-Si steel by austempering below the Ms temperature. A good combination of strength and ductility has been obtained in prolonged austempering below the martensite start temperature (225 °C) from an austenitizing temperature of 870 °C.
This study summarizes research into the effect of high boron additions on the microstructure of cast steel under the certain carbon (0.42 C wt.%) and chromium (11.65 Cr wt.%) content. Varying amounts of boron between 0.48 -4.75 wt.% was added into steel melt. The boron analysis of the alloys was conducted by wet chemical analysis. The microstructure of the specimens was characterized with optical microscope, Scanning Electron Microscope with Energy Dispersive X-Ray Analysis (SEM/EDX) and X-Ray Diffraction (X-RD). The experimental results showed that high boron steel was successfully produced by sand casting with high casting yield. The microstructure of boron alloyed high chromium cast steel consists of three dimensional networks of M 2 B borides surrounding the martensitic matrix in as cast condition. Under the certain carbon and chromium content, boron carbide volume fraction is directly depended on the boron additions. Due to the high amount of chromium and boron, carbides and borides were embedded in each other in all additions. In the specimens having less than 2.44 B wt.%, carbide/boride structure was fishbone type with dendritic distribution over the matrix whereas rod/needle like structure with randomly distribute was observed in specimens having 2.44 -4.75 B wt.%. In all specimens containing boron, M 2 B and M 7 (C, B) 5 type carbides/boride peaks were detected by X-RD analysis.
The effect of isothermal heat treatment temperatures under martensite start (Ms) temperature on the microstructure and mechanical properties of high-silicon commercial spring steel has been investigated. For this purpose, tensile specimens are prepared from AISI 9254 steel isothermally heat-treated under Ms temperatures (225 °C, 250 °C, and 275 °C) for 168 h after austenitizing at 870 °C for 30 min. Optical microscopy, scanning electron microscopy and X-ray diffraction analysis were used to characterize the microstructures of the specimens. Mechanical properties were determined by the tensile and hardness tests. Experimental results revealed that microstructure consists of carbide-free bainite, carbon enriched retained austenite, and martensite in high-silicon spring steel by the isothermal treatment under Ms temperature. The yield and tensile strength were increased by decreasing the isothermal temperature. However, uniform elongation and breaking energy were decreased by decreasing the isothermal temperature. The specimen which was isothermally heat-treated at 250 °C under Ms temperature showed a very good combination of tensile strength and total elongation as 2046 MPa and 8.5%, respectively. Dimples along with cap and cone formation which are evidence of a ductile fracture were observed in fractured surfaces of all isothermally heat-treated specimens.
Bu çalışmanın amacı, akışkan yatağa daldırma yöntemi ile uygulanan poliamid kaplamanın mekanik ve kimyasal özelliklerinin incelenmesidir. Poliamid kaplama uygulaması için çelik test plakaları kullanılmıştır. Spreyleme ile astar uygulanmış test plakalarına, fırında ısıtma işlemi sonrası akışkan yatakta daldırma metoduyla poliamid kaplanmıştır. Kaplamanın mekanik dayanımının incelenmesi için aşınma dayanımı, çizilme testi, tuz testi ve sıcak yağa dayanım testleri yapılmıştır. Ayrıca kaplamanın termal karakteristiğini belirlemek amacı ile DSC, TGA analizleri yapılmıştır. Polimer kaplama ve astarın moleküler yapıları FTIR spektroskopisi ile analiz edilmiştir. Deneysel çalışma sonuçlarına göre, kaplamada yaklaşık olarak %50 su bulunduğu, kaplama yüzeyinin literatür araştırmalarına göre daha düzgün olduğu görülmüştür. Kaplamanın çizilme testi dayanımı 40N üzerindedir. Ağırlık testinde, 1,5kg yüke kadar kaplama yüzeyinde herhangi bir çatlak görülmemiştir. Aşınma testinde 1kg ek yükleme kullanılmasına rağmen, 5000 döngü sonrası poliamid kaplama iyi bir aşınma performansı sergilemiştir. Tuz testi ve sıcak yağ testleri sonrası kaplama yüzeyinde herhangi bir hata veya bozukluk gözlemlenmemiştir. Sonuç olarak akışkan yatağa daldırma metoduyla uygulanan poliamid toz kaplamaların, sert ve aşındırıcı çevrelerde ve mekanik dayanımın gerektiği mekanik parçalarda kullanılması uygundur. Anahtar Kelimeler: Poliamid kaplama, termal karakteristik, yüzey morfolojisi, triboloji.
This research investigates the improvement of impact toughness by changing isothermal heat treatment time for an alloy having high boron/chromium constituents. Different boron contents (0.5–5.00 wt% B) were added into high chromium steel by sand casting. Along with JMatPro program, dilatometry used to calculate critical temperatures and transformation diagrams as well. After austenization at 1050 °C for an hour, at 390 °C high B/Cr alloys isothermally heat treated for three different time intervals (24, 96, and 168 h). According to experimental findings, isothermal heat treatment enhanced hardness and impact toughness of the alloys (excluding 3.10 and 4.75 wt% boron containing alloys) due to the elimination of microsegregations and lowering carboboride sharpness. However, for every alloy wear resistance was not affected by isothermal heat treatment because carbide/boride fraction controls wear properties of the high B/Cr alloys. According to scanning electron microscope microstructures, cleavage fracture, and plowing were observed in isothermally heat treated specimens. It is noted that, isothermal heat treatment time showed no effect in mechanical properties of the high B/Cr alloys because the large amount of alloying elements shifted the phase transformation curve longer times which was more than JMatPro estimations.
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