高碳合金钢中未饱和碳的过冷奥氏体的转变

Transformation Behavior of Carbon-unsaturated and Super-cooled Austenite in a High Carbon Alloy Steel

研究了高碳合金钢A(SAE 52100,1.02%C、0.21%Si、0.35%Mn、1.44%Cr、0.20%Ni,质量分数)的相变,并与另一种钢B(0.53%C、0.27%Si、0.35%Mn、0.66%Cr,质量分数)进行了比较。结果表明,两种钢过冷奥氏体中的碳和铬的含量均随着奥氏体化温度的升高而提高。当加热温度为工业中常用的840℃时,A钢奥氏体中的含碳量远低于相图中表明的数值,仅达到中碳钢的水平。这可能是由于奥氏体中碳和铬的不均匀分布所致。业已证明,用萃取法测定的A钢奥氏体中的含碳量只是奥氏体中的平均含碳量,不能准确反映奥氏体中不同区域的含碳量。因此,碳和合金元素含量相同时,由于A钢奥氏体中碳的不均匀分布改变了TTT图的性质,从而使其比B钢奥氏体更容易发生转变。当采用通常的淬火和低温回火工艺时,与TTT图相比的这些改变促进了在奥氏体的高碳区形成含有先共析渗碳体(Fe3C)的珠光体,而在其低碳区形成含有先析铁素体(α-Fe)的贝氏体。这就是高碳合金钢具有优良综合力学性能的原因。

The transformation behavior of a high carbon alloy steel A （ SAE 521000） （ 1.02% C ,0.21% Si ,0.35% Mn, 1. 44% Cr,0.20% Ni, mass fraction）was investigated and compared to another steel B（0.53% C ,0.27% Si, 0.35% Mn and 0.66% Cr, mass fraction）. It is shown that both the carbon and chromium contents in the super- cooled austenite increase with an increase in the austenitizing temperature. At a typical commercial heating temperature （ 840 ℃ ）, the carbon content of the austenite in steel A is much lower than that indicated by the phase diagram, only reaching the level of a medium carbon steel. It is believed that the above result can be attributed to the inhomogeneous distribution of carbon and chromium in the unsaturated austenite. It is shown that the carbon content measured by an extraction method in austenite for steel A is only an average content of carbon, rather than the precise values in the different regions. Hence, for similar carbon and alloying elementcontents, the austenite in steel A is easier to transform than in steel B since the inhomogeneous carbon distribution in the austenite of steel A changes the nature of the transformation-temperature-time （TTT） diagram. These modifications to the TTT diagram promote the formation of pearlite with the proeutectoid Fe3C in the high carbon region, and bainite with the proeuteetoida-Fe in the low carbon region of austenite, when the commercial quenching and low temperature tempering process was applied. This expains why some high carbon alloy steels have good overall mechanical properties.