The effect of microstructure on creep resistance of the low carbon chromium steel X20CrMoV121 after 100‐hours of static‐load test at a temperature of 580 °C and constant stress of 170 MPa was investigated. The specimens for the experiments were extracted from steam pipes of a steam power plant and heat treated. The effect of isothermal annealing on the microstructure and hardness as well as the kinetics of the precipitation of the carbide particles were determined.
The number of carbide particles stringers at boundaries and subboundaries of martensite per unit of surface decreases with the third root of tempering time at 1073 K. The accelerated creep rate at 853 K depends on the number of stringers of carbide particles per unit of surface, and below an inflection point, the creep rate increases strongly. Equations relating for creep rate are discussed in terms of experimental creep rate, number of carbide particles stringers and average particles spacing.
The fracture toughness of some areas in the multi-pass heat affected zone (HAZ) of a high strength low alloy (HSLA) structural steel was analysed in a straightforward way using precracked, cylindrical specimens tested on a conventional tensile machine. The specimens were made from samples with a simulated HAZ microstructure; however, the size of the samples was restricted by the limitations of the Gleeble machine. The brittleness of the samples was an indication of the detrimental effect of welding on their toughness. The specimens were not large enough for a direct K Ic measurement over a wide testing temperature range; it was necessary to modify the results. The low fracture toughness and the substantial shift of fracture transition temperatures suggest that welding of the investigated steel could be a delicate procedure. STWJ/430
A transition layer of width 5 -10 m was found on the boundary between ductile and brittle fracture for Charpy V notch specimens in the transition temperature range of a structural steel having a microstructure of polygonal ferrite -pearlite. The fracture mode in the transition layer was shearing with occasional submicrometre dimples. From tensile tests on notched specimens, the cleavage fracture stress and ow stress by ductile decohesion were determined. Based on the experimental data and the assumption that the volume of metal involved in the plastic deformation during fracture was related to the volume of the dimples, it was deduced that the transition layer width represents the size of the plastic zone immediately before cleavage initiation. The crack opening displacement and the crack tip radius for the change of fracture mode were calculated.M ST/4854
The fracture toughness of the HAZ in welds on a HSLA structural steel was assessed by using two types of precracked small-scale fracture-mechanics specimens. The microstructure of the single-and double-pass HAZs was simulated on a Gleeble machine at two different cooling rates. A direct measurement of K Ic was not possible due to the limited size of both kinds of specimens. A suitable adaptation of the loading diagrams was performed in order to produce relevant data about the fracture properties of the investigated sub-zones of the HAZ.
Two creep resistant steels, P91 and X20, were tempered for 17520 h at 650 8C or 8760 h at 750 8C 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 8C 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 8Á10 À7 s À1 to 5Á10 À9 s À1 . For equal yield stress, the creep rate was slightly lower for the steel P91 than for the steel X20.
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