Improvement of Structural Design Rule for Efficient Use of Heat Resistant Materials

确定材料许用应力的安全系数要求尽可能小，以便在尽可能高的应力下利用材料。然而，由于高温下材料性能的不确定性，安全系数通常保持在较高的值。本研究对耐热钢长期蠕变性能不确定性的原因进行了研究。1长期蠕变性能预测：蠕变持久寿命的温度依赖性往往由于蠕变断裂机制的改变而改变。由于这种变化，我们经常高估钢的长期蠕变断裂寿命。在SU316不锈钢中，材料性能的热-热变化、温度变化引起的误差和数据的固有离散性对长期性能不确定性的贡献估计为3：3：2。2高铬铁素体钢的早期断裂：高铬铁素体钢在短期断裂条件下通常表现为穿晶断裂模式，而在长期断裂条件下其断裂模式转变为沿晶断裂。由于断裂模式的改变，不能准确地预测其长期性质。沿晶断裂是通过蠕变空洞的长大和聚合而发生的。空洞是在粗大的Fe2W、M&lt；23和Gt；C6等析出物上形成的，析出物的长大很好地反映了断口形态的变化。3高铬铁素体钢焊接件的早期失效：焊接件的IV型失效导致高铬铁素体钢焊接接头严重的早期失效。蠕变空洞在粗大的MC_6颗粒上形核，并在两相HAZ区累积的蠕变应变的帮助下长大。MC_6颗粒的长大是IV型失效发生的一个很好的量度。

The safety factor for determining allowable stress of a material is required to be as small as possible, so that the material can be utilized at as high stress as possible. However, the safety factor is usually remained at a high value because of the uncertainties of material properties at elevated temperatures. This research studied the cause of the uncertainty of long term creep properties of heat resistant steels.1 Long-term Creep Property Prediction : Temperature dependence of creep rupture life often changes due to a change in creep rupture mechanism. Because of this change we often overestimate long term creep rupture life of steels. The contributions in the uncertainty of the long term property from heat-to-heat variation of the material property, error introduced by the change in temperature dependence and the inherent scatter in data are estimated to be 3:3:2 in SU316 stainless steel.2 Premature Failure of High Cr Ferritic Steels : High Cr ferritic steels usually shows a transgranular fracture mode under short term rupture condition, but their fracture mode changes to intergranular fracture under long term rupture condition. One cannot accurately predict the long term property due to the fracture mode change. The intergranular fracture occurs by the growth and coalescence of creep cavities. The cavities are formed at coarse precipitates such as Fe_2W, M_<23>C_6, and growth of the precipitates is a good measure of the fracture mode change.3 Premature Failure in Weldment of High Cr Ferritic Steels : The type IV failure of weldment causes serious premature failure of welded joint of high Cr ferritic steels. Creep cavities are nucleated at coarse MC_6 particles and grow with the assistance of creep strain accumulated in the dual phase HAZ region. Growth of the MC_6 particles is a good measure of the occurrence of type IV failure.