Global Casting Magazine Volume 9, Number 1

Page 40

图11:来自0.78%锰系列 的两种不同硫含量的B棒的 应力-应变曲线,表明在较 高硫含量下断裂伸长率显著 降低。

Fig. 11. Stress-strain curves for B bars of two different sulfur levels from the 0.78%Mn series illustrating a significant reduction in elongation at fracture at a higher sulfur level.

deviations observed. One would expect to find higher hardness in the cell boundaries, but the locations of the microhardness readings were random and could not be specifically located in the cell boundary or cell center. The wide variation in matrix hardness further supports the hypothesis that sulfur is segregating during eutectic solidification and influencing the combined carbon content of the pearlite matrix in the last metal to freeze. Past research indicated that as sulfur increases (and the Mn:S ratio decreases) the combined carbon content in cast iron increases.

Evidence of Embrittlement Probably the most compelling data generated in this study were the stress-strain properties of the alloys. The reduction in strength associated with increasing sulfur was associated with a reduction in the elongation at fracture. That is, premature fracture was observed in the stress-strain plots (Figs. 9-11). Premature fracture in the tensile test was observed in every manganese series and in every section size. The data in Table 3 shows the elongation at fracture decreased on average 37% and was as much as 53%. The greatest reduction in fracture elongation occurred in the 0.28% Mn series. Once again, it is notable the series also exhibited the highest free sulfur contents. Scanning electron microscopy on some of the fractured tensile bars revealed changes in the fracture mode in samples of high and low strength. As strength and elongation decreased, the fracture mode changed from ductile tearing to transgranular cleavage, a brittle fracture mode. With transgranular cleavage fracture, there is very limited plastic deformation occurring during crack propagation. It is proposed that the reduction in strength with increasing sulfur was caused by an embrittlement of the alloy. The embrittlement may well be due to the presence of spikey graphite and IC carbides in the cell boundaries. Clearly, the transgranular cleavage fracture mode is associated with the pearlitic matrix; and the harder the pearlite, the more likely the fracture mode will be by transgranular cleavage. The original hypothesis was that changes in manganese and sulfur produced changes in the graphite structure, which in turn produced a reduction in strength. The findings of this investigation indicate the reduction in strength is due to an embrittlement phenomenon. It now appears that the degradation in strength with increasing sulfur may be more a function of high free sulfur contents and sulfur segregation during solidification, rather than to a modification of the graphite structure. The roles of MnS precipitation and the influence of MnS inclusions on the microstructure are less clear. ■ 38

显微硬度值的位置是随机的并且不能特异性地位于细 胞边界或细胞中心。基体硬度的大幅变化进一步支持 了这样的假设:硫在共晶凝固过程中偏析并影响最后 凝结金属中珠光体基体的化合碳含量。过去的研究表 明,随着硫含量的增加(以及锰:硫比率降低),铸 铁中的碳含量增加。

脆裂的实验证明 本研究中,最引人注目的数据可能是合金的应力应变特性。与硫增加相关的强度降低与断裂伸长率的 降低有关。也就是说,在应力-应变图中观察到过早 断裂(图9-11)。在每个锰系列和每个截面尺寸中观 察到拉伸试验中的过早断裂。表3中的数据显示,断 裂伸长率平均降低37%,并且高达53%。断裂伸长率 的最显著降低发生在0.28%锰系列中。而且,值得注 意的是该系列还表现出自由硫含量最高。 在一些断裂拉伸试样上,扫描电子显微镜检查显示 出高强度和低强度样品中的断裂模式的变化。随着强 度和伸长率的降低,断裂模式从韧性撕裂变为穿晶断 裂,即脆性断裂模式。对于穿晶断裂,在裂纹扩展期 间发生非常有限的塑性变形。对此,可推断随着硫的 增加导致强度的降低是由合金的脆化引起的。脆化很 可能是由于细胞边界中存在针状石墨和胞状晶间碳化 物。显然,穿晶断裂模式与珠光体基体有关;并且珠 光体越硬,穿晶断裂的断裂模式越可能发生。 最初的假设是,锰和硫的变化会在石墨结构中产生 变化,从而导致强度降低。实验结果表明,强度的降 低是由于脆化现象造成的。现在看来,随着硫含量的 增加,强度的降低可能更多的是凝固过程中自由硫含 量高和硫偏析的作用,而不是石墨结构的改性。硫化 锰析出的作用和硫化锰夹杂物对微观结构的影响还不 太清楚。 ■

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