Study of Property Degradation of T23 Heat‐resistant Steel based on Microstructural Evolution during Creep

In order to study the microstructural evolution and the effect on property degradation of T23 heat‐resistance steel (2.25Cr‐1.6W‐V‐Nb‐B‐N) during creep, creep rupture specimens were investigated at 823K, 873K and 923K. The microstuctural evolution was examined by optical, scanning and transmission electron microscopy. It has been noted that the creep property degradation of T23 is related to the decrease of dislocation density due to the recovery and recrystallization of the bainitic ferrite matrix and the martensite in the carbon‐rich islands, the coarsening of M₂₃C₆ carbides, and even the transformation from M₂₃C₆ to M₆C. Coarsening of M₂₃C₆ is the dominating effect during short‐term creep whereas recovery and recrystallization is the key factor for long‐term creep. Property degradation is advanced at higher temperature due to the quicker recovery and recrystallization.     

Hardness Variation in P92 Heat‐Resistant Steel based on Microstructural Evolution during Creep

In order to optimize the strength of P92 heat‐resistant steel, the variation in hardness and microstructural evolution during creep were investigated. The results show that before crept for 1429 h at 873 K, the coarsening of M₂₃C₆ is the main factor to decrease the hardness. When the creeping time prolongs from 1429 to 6063 h, the increase of hardness is mostly attributed to the precipitation of a large amount of Laves phase. Thereafter, the coarsening of Laves phase leads to the decline in hardness. The precipitation hardening resulting from MX drops distinctly in spite of the slight growth during creep. It is important to control the growth of MX, decrease the coarsening velocity of M₂₃C₆ and keep Laves phase to be fine.     

Dependence of Accelerated Creep Rate at 580 °C and Room Temperature Yield Stress for Two Creep Resistant Steels

Two creep resistant steels, P91 and X20, were tempered for 17520 h at 650 °C or 8760 h at 750 °C to study the growth and redistribution of carbide precipitates in martensite. On specimens annealed for a different time, yield stress at room temperature and accelerated creep rate at 580 °C were determined. With increasing yield stress in the range from 350 to 650 MPa the accelerated creep rate decreased continuously by about 2 orders of magnitude from 8·10^?7 s^?1 to 5·10^?9 s^?1. For equal yield stress, the creep rate was slightly lower for the steel P91 than for the steel X20.     

低碳含钒钢组织变化及V(C,N)析出规律

通过热模拟试验,研究了0．058％C-0．92％Mn-0．51％V钢在变形后等温过程中的组织变化规律及显微硬度变化规律,并用透射电镜研究了V(C,N)的沉淀析出规律。结果表明,变形后在650、700、750℃等温1130s时可获得单相铁素体组织,从而基本消除珠光体组织。等温转变过程中,铁素体先是主要沿奥氏体晶界形核,随后逐渐变为晶内形核。V(C,N)的沉淀析出存在两种形式：相间析出和随机析出。V(C,N)的相间析出消除了C元素在奥氏体中的富集,是获得单相铁素体组织的重要条件。     

Isothermal Microstructural Transformations of the Heat‐treatable Steel 42CrMo4 during Heat‐treatment following Hot‐forming

Apart from reducing the processing energy, hardening and tempering of near‐net shape forged components from their forging heat primarily promises shortened conventional process sequences with reduced manufacturing costs. In this case, the time‐temperature‐transformation diagrams (T‐T‐T diagrams) found in the literature can only be used to a limited extent for determining the microstructural transformations during the heat‐treatment. The reasons for this are that firstly, the deformation influences the transformation kinetics and secondly, the forming temperatures at which austenitising takes place are comparatively high. For this reason, isothermal deformation T‐T‐T diagrams for forging temperatures from 1200 °C and deformation levels of 0.3 and 0.7 were determined for the heat‐treatable steel 42CrMo4 (1.7225). These diagrams were subsequently modelled for simulating the heat‐treatment and implemented in the FE‐software ANSYS®.     

Effects of Nb and V on Microstructural Evolution,Precipitation Behavior and Tensile Properties in Hot-rolled Mo-bearing Steel

Microstructural evolution,precipitation behavior,and tensile properties of four experimental Mo-bearing steels were studied to elucidate the effects of Nb and V on microstructural properties.The results indicated that the microstructure of hot-rolled steels consisted of polygonal ferrite and degenerate pearlite,and the morphology remained same after holding at 600 ℃for 1h.The smallest grain size was obtained in Nb-V-Mo containing steel,followed by Nb-Mo and V-Mo steels.Precipitation was less in the hot-rolled Mo-bearing steel.The maximum volume fraction of precipitates was obtained in Nb-V-Mo bearing steel,and the Nb-V-Mo complex carbonitrides were highly stable at 600℃.In the Mo-bearing steel,the yield ratio was lowest at room temperature but highest at 600℃,which was attributed to the precipitation of Mo carbides and drag effect of Mo solute on dislocation movement.     

Research on Microstructure and Mechanical Property of S30432 Supreheater Pipe Aged at 700°C

The microstructure and performance of S30432 supreheater pipe were investigated before and after it is aged 3000 h at 700°C. A lot of twins disappeared after aging, M₂₃C₆ particles precipitated along the grain boundary and MX particles precipitated near the dislocation. After the endurance experiment of the aged pipe, M₂₃C₆ precipitations with the size of about 50 nm pinned dislocations and the cavity appeared on the grain boundary. The aged S30432 supreheater pipe showed the ductile fracture. Increasing temperature made the inter‐granular fracture transfer the quasi‐cleavage fracture and the equiaxed morphology change the parabola morphology. The strength of the untreated specimen decreased with the temperature increasing. The hardness and the impact energy of the aged specimen were larger than those of the untreated specimen. Tensile strength and elongation of untreated specimen were larger than that of aged specimen.