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Temperature impact on delayed fracture of creep-resistant steels welds

Skulskyi V., Nimko M.

Journal of Achievements in Materials and Manufacturing Engineering

2018

Vol. 88, nr 1

5--11

Purpose: Phenomenon of delayed fracture (or cold cracks formation) of hardenable steels weldments had been widely investigated. But temperature dependence of cracking susceptibility remained discussable, because there was no strict vision of temperature border for the cracking risk appearance, when joints are cooling after welding completion. The proposed paper aimed at assessment of dangerous temperature range at which delayed fracture, mainly for the steels with martensite formation, becomes most probable. Design/methodology/approach: The “Implant” test, conducted under isothermal conditions at the temperatures selected within the range from 160 to 20°C on cooling of the completed test weld joint, was used. Basing on the obtained thermokinetic characteristics of the cracking, the activation energy E of the fracture process was calculated. Comparing of the found E values with the close values of E for the known processes developing in steels, the explanation of the revealed cracking behaviour at different temperatures was proposed. Findings: Delayed cracking of the martensitic weld joints has started to manifest at the temperatures lower than 140°C. Dependence of the cracking period from the temperature is described by C-type curve with the minimum cracking duration within 80-100°C. Using the approach of the activation energy assessment for different temperature ranges (140 to 100°C and 80 to 20°C), the effect of the diffusible hydrogen and a martensite decay on the cracking thermokinetics was considered. Research limitations/implications: Additional investigations of the fine microstructure after different stages of the low-temperature martensite decay could be necessary for deepening understanding of a role of this process in the low-temperature heterogeneity formation and cracking susceptibility. Practical implications: Results widen data on weldability of actual for industry steels and give a ground for consideration of the technological approaches for their welding. Originality/value: Temperature border of the cold cracking risk is specified for the weldments of some commercial steels.

The Attempt of the Low-Cycle Fatigue Life Description of Chosen Creep-Resistant Steels Under Mechanical and Thermal Interactions

Okrajni J., Marek A.

Archives of Metallurgy and Materials

2017

Vol. 62, iss. 4

2349--2353

The study focuses on the problem of determination of low-cycle fatigue properties for the chosen group of creep-resistant steels used in the power and chemical industries. It tries to find the parameter which would describe well the fatigue life and take into account mechanical loads and temperature. The results of LCF tests have been presented in the paper. New parameter P has been introduced. This parameter joins a plastic strain range, a stress range and temperature. The fatigue life has been predicted versus parameter P. The comparison of the predicted and observed values of fatigue life shows the agreement between these values. The method of fatigue life prediction formulated in this way is expected to describe the behavior of materials under thermo-mechanical fatigue.