热处理工艺对P91耐热钢中δ-铁素体和冲击性能的影响

P91耐热钢热加工(轧制、焊接、热处理)过程中易产生δ-铁素体,且其形态、数量和分布与热加工温度关系密切。通过设计P91耐热钢热处理工艺,在1 150℃、1 250℃、1 300℃温度下正火获得马氏体+δ-铁素体混合组织,并对1 300℃正火组织进行1 050℃(油冷)+760℃(空冷)。采用金相显微镜、显微硬度计和扫描电子显微镜等技术研究δ-铁素体数量、形态、分布的变化,并测试各热处理状态下的冲击韧性和失效模式。结果表明,P91钢随着正火温度升高,δ-铁素体数量增加;形态呈细条状、细条状+块状和多边形块状分布;1 050℃正火+760℃回火不能消除在1 300℃正火时产生的δ-铁素体,但能减少其数量、改变其形态与分布。随δ-铁素体含量增加冲击功减小,冲击断口形貌从韧/脆混合断裂转变为脆性断裂,边界平直的块状多边形δ-铁素体较条状形态更不利于冲击韧性。

Effect of Heat Treatment on Delta-ferrite and Impact Toughness of P91 Heat-resistant Steel

Delta-ferrite,whose morphology,content and distribution depended on hot processing temperature,readily genera-ted in P91 heat-resistant steel during hot processing,such as hot rolling,welding and heat treatment.The mixed microstructure,in-cluding martensite and delta-ferrite,was obtained by normalizing at different temperatures (1 150 ℃,1 250 ℃,1 300 ℃)in accor-dance with the designed heat treatment processing,and the normalizing microstructure (1 300 ℃)undertook oil cooling (at 1 050℃)+air cooling (at 760 ℃).The content,morphology and distribution of delta-ferrite were observed by OM(optical microscope), SEM(scanning electron microscope)with EDS (energy-dispersive spectrometry),and the microhardness,impact toughness and fracture failure mode of that were tested.The results demonstrated that delta-ferrite content of P9 1 steel increased along with the in-crease of normalizing temperature,and its morphology presented thin strip,strip and bulk,and bulk polygon.The delta-ferrite generated due normalizing at 1 300 ℃ could not be eliminated by normalizing at 1 050 ℃ and tempering at 760 ℃,while the content decreased,morphology and distribution of delta-ferrite altered.The impact energy reduced with increase in delta-ferrite content,and the impact fracture failure mode converted to brittle fracture from ductile-brittle fracture.In contrast with strip morphology,the bulk polygon of delta-ferrite at boundaries was not conducive to impact toughness.