In order to identify the effect of total strain amplitudes on the fatigue property of 12Cr1MoV steel, low-cycle fatigue experiments with different total strain amplitudes under a mixed chloride salt environment, a mixed sulfate salt environment, and an air condition were carried out at 600 °C.
Figure 2 shows the cyclic stress as a function of cyclic numbers. As seen in
Figure 2, the effect of total strain amplitude on the cyclic stress response of the alloy is approximately the same under three different deformation conditions. For all samples, with the increase of total strain amplitude, the cyclic stress values increased and the fatigue life decreased significantly. This is a very normal and reasonable result. On the other hand, with the increase of the cyclic numbers, all of the samples presented an increment of the cyclic stress, especially at the strain amplitudes of ±0.3% and ±0.2%. Therefore, it can be concluded that the alloyed steel mainly exhibited cyclic hardening during the loading. This phenomenon is consistent with the data calculated from the uni-axial tensile test, as shown in
Table 3. Generally, cyclic hardening and softening of material can be determined by the data of the uni-axial tensile test. When σ
b/σ
0.2 > 1.4, the material exhibits cyclic hardening; when σ
b/σ
0.2 < 1.2, the material exhibits cyclic softening; and when 1.4 > σ
b/σ
0.2 > 1.2, it is impossible to determine whether the material is cyclic hardening or cyclic softening [
12]. Based on the data of the tensile test at 600 °C, it can be calculated that: σ
b/σ
0.2 = 544/311 = 1.749 > 1.4. According to the calculation, cyclic hardening should occur during the fatigue test at 600 °C for the 12Cr1MoV steel, which is consistent with the curves drawn from the experiments.
The Holomon expression is used to depict the relationship between the magnitude of the stress and the amplitude of the plastic strain [
13], as shown in Equation (1):
where
K′ is the cyclic strength coefficient, and
n′ is the strain hardening exponent. After taking logarithms on both sides, the relationship between the total strain amplitude and stress of the samples under different deformation environments was plotted in
Figure 3. In this figure, Type A represents mixed alkali metal salt samples, and Type B represents mixed sulfate samples. It can be seen that for all of the samples, the three fitting lines are almost parallel to each other. The
n′ value is the slope of a straight line, and the
K′ value is the intercept of a straight line on the longitudinal axis. It can be concluded the values of the cyclic strength coefficient
K′ and the strain hardening exponent
n′ in the three environments are almost the same. After extensive research on high-strength materials commonly used in industry, Landgra [
14] proposed that the strain hardening exponent
n′ can be used to evaluate the effect of cyclic strain on material properties. When
n′ < 0.1, the material behaves as cyclic softening, when
n′ > 0.1, the material exhibits cyclic hardening or cycle stability. After fitting, the strain hardening exponent is obtained, as shown in
Table 4, with
= 0.1083 > 0.1,
= 0.1102 > 0.1,
= 0.1107 > 0.1. According to this standard, 12Cr1MoV steel mainly exhibits cyclic hardening or cycle stability, which is consistent with the results of the monotonic tensile determination and the cyclic stress response determination. In addition, all of the samples show a cyclic softening before the final fracture, with the cyclic stress decreasing rapidly during the last several cycles. The reason is that the fatigue crack becomes unstable and propagates rapidly and fractures eventually after the nucleation and coalescence.