Microstructure and Fatigue of Ultrafine‐Grained Ferritic/Martensitic Steel

Abstract

The impact of equal‐channel angular pressing in combination with additional heat treatment on microstructure and fatigue of ferritic/martensitic steel is investigated. It is shown that the formation of both the ultrafine‐grained (UFG) structure with an average grain size of about 0.8 μm and the enhanced share of boundaries with the coincidence site lattice (CSL) relationships (the CSL boundaries) leads to an increase in the fatigue endurance limit of more than 50% compared with samples after tempering. Dynamic strain aging is found in the fracture zone of the UFG of samples after fatigue tests. The influence of structural parameters on the fatigue endurance limit of the UFG samples is discussed.

Enhanced Fatigue Limit in Ultrafine-Grained Ferritic–Martensitic Steel

“…From the microstructure of the samples subjected to CR + requenching ( Table 2 ), numerous microtwins can be observed, which, in this instance, suppresses the overall crack growth as it is splitting into several microcracks. It has recently been demonstrated that the formation of the ultrafine-grained structure and microtwins by using equal-channel angular pressing leads to the enhancement of the fatigue endurance limit [ 16 ]. This work demonstrated that the formation of microtwins can enhance fatigue life in the structures containing elongated grains in a material subjected to CR ( Table 2 ).…”

“…The standard heat treatment procedure for steels with a content of 9–12% Cr is quenching, i.e., heating above the temperature of phase transitions with rapid cooling [ 16 ]. The subsequent high-temperature tempering relieves internal stresses and leads to greater structure equilibrium.…”

“…When the samples were annealed in the temperature range of 1040–1050 °C, almost complete dissolution of carbides occurs, and with further oil quenching, martensite is formed ( Figure 2 b). Subsequent tempering at a temperature of 800 °C leads to the decomposition of martensite ( Figure 2 c) and the precipitation of the M 23 C 6 carbides (50–170 nm in size) and MX carbonitrides (30–50 nm in size) [ 16 ].…”

“…According to [ 3 ], the grain refinement occurring at severe plastic deformation (SPD) increases the fatigue limit; however, the decrease in the plasticity of SPD samples can negatively affect the nature of fatigue failure. In [ 16 ], the authors showed that the formation of UFG structure by equal-channel angular pressing (ECAP) in combination with additional heat treatment above the temperature of the ferrite/austenite phase transition promoted an increase in the fatigue endurance limit by more than 50%, from 475 to 735 MPa. The main features that made it possible to provide such a level of fatigue endurance were a homogeneous UFG structure with an average grain size of 0.8 microns, smaller and uniformly distributed carbide particles of 30–50 nm as well as an increased fraction of boundaries with the CSL boundaries of up to 7%.…”

A State‐of‐Art Review on Prediction Model for Fatigue Performance of Welded Joints via Data‐Driven Method

“…Therefore, several researchers have fully discussed the characteristics of fatigue behavior for welded joints from the perspective of empirical, analytical, and numerical solutions. [22][23][24][25][26][27][28] Due to size effect, most of the samples used in fatigue tests are small-sized samples, and the smallsized samples are difficult to fully reflect the stress concentration and stress gradient of the actual structures, leading to difficulties in understanding the relationship between IFs and fatigue properties. [29,30] Although various classical methods have achieved fruitful results, these methods usually adopt some simplified assumptions, [31,32] which make it difficult to fully consider the impact of comprehensive factors on fatigue performance; thus, a general method for variable service conditions is required.…”