加熱過程中H2O(g)對(duì)55SiCr彈簧鋼脫碳的影響

加熱過程中H2O(g)對(duì)55SiCr彈簧鋼脫碳的影響Effect of H2O(g) on decarburization of 55SiCr spring steel during the heating process

本文設(shè)計(jì)氣氛混合裝置，控制爐氣成分以及H2O(g)含量，利用真空管式加熱爐對(duì)彈簧鋼鑄坯試樣進(jìn)行加熱保溫。采用激光多點(diǎn)共聚焦顯微鏡研究了混合氣氛下H2O(g)對(duì)彈簧鋼表面脫碳的影響。結(jié)果表明，試樣總脫碳層厚度隨溫度升高而增加；混合氣氛下，650℃出現(xiàn)輕微鐵素體脫碳，700~950℃有明顯鐵素體脫碳層，850℃為鐵素體脫碳峰值溫度，與空氣條件相比，表面鐵素體脫碳更加嚴(yán)重，脫碳開始溫度有所降低。低溫區(qū)表面脫碳的發(fā)生與C濃度梯度驅(qū)動(dòng)下珠光體片層中滲碳體的溶解有關(guān)，鐵素體脫碳層晶粒形態(tài)呈現(xiàn)為尺寸細(xì)小、無明顯取向。H2O(g)破壞防止脫碳的保護(hù)機(jī)制，對(duì)試樣脫碳程度的加深有重要作用。

Spring steel is an important steel widely used in the national economic construction. Its application environment is quite harsh, so it strongly demands for a high quality of the surface. However, the decarburization behavior on the surface seriously affects the surface quality and reduces the fatigue performance of materials. Decarbonization is caused by chemical reaction between oxidizing atmosphere and steel surface. Hydrosphere, as a component of the oxidizing atmosphere, could lead to a significant influence on decarburization process. Our experimental materials, taken from continuous casting billet, were polished carefully by abrasive paper in order to remove stains, original scale and decarburization layer. Components of furnace gas were controlled by mixing units designed independently. Spring steel samples were heated by a tube vacuum furnace in 600~950℃. The mixed atmosphere contains 15%~20% CO2, 2%~4% O2, hydrosphere of different content and N2 in balance. With the condition of the mixed atmosphere, influence of hydrosphere on surface decarburization of spring steel 55SiCr were investigated by 3D Measuring Laser Microscope. The results show that, with the influence of the mixed atmosphere, decarburization is able to happen during low temperature interval, 650℃~Ac1(Ac1: Starting temperature of austenitization during slow heating). The thickness of total decarburized layer increases correspondingly with the temperature. In the mixed atmosphere, slight decarbonization occurs at 650℃ and obvious ferrite decarburization layer can be detected within a temperature range of 700 ~ 950℃, which is more serious than that in the air. The peak temperature of ferrite decarbonization is 850℃ under both mixed atmosphere and air. The surface decarbonization in low temperature region is related to the dissolution of cementite in pearlite lamellae driven by carbon concentration gradient. When the decarburization degree deepens as time goes, partial pearlite colony begins to shrink, much line form cementite dissolves gradually, and the line cementite becomes short and punctate. The grain morphology of ferrite decarbonization layer is different from that who generates within α+γ phase region, which is small in size and without a strong orientation. Hydrosphere in mixed atmosphere could increases porosity of oxidation layer and destroys the important protection mechanism of preventing decarbonization by compact scale. It is deduced that existing of hydrosphere offers a chance for low temperature decarburization process occurring and hydrosphere plays an important role in deepening the decarbonization degree of samples.

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