##### Research on 315 ℃ Low Cycle Fatigue Performance of GH4169 Alloy Bar for Nuclear Power

**nucler power;GH14169; low-cycle fatigue properties**

Abstract: The axial total strain control method was used to study the low cycle fatigue performance ofGH4169 rod at 315 ℃.The results show that the low-cycle cyclic stress-strain behavior of bar GH41 69 conforms to the nonlinear relation. The cyclic strain strengthening index and cyclic strength coefficient are 0.050 2 and 1 438.47, respectively. The low-cycle fatigue strain-analysis life of the bar at 315 ℃ conforms to the relation.

Key words :nucler power;GH14169; low-cycle fatigue properties

GH4169 (International Grade INCONEL718) alloy is a precipitation-hardening type nickel-chromium-iron alloy containing Nb and Mo. The alloy consists of body-centered cubic Ni3Nb-type r “and face-centered cubic Ni3Al-type r. It is a precipitation phase alloy in the range of -253-700 ℃ It has good comprehensive properties. Yield strength below 650 ° C ranks first in deformed superalloys. “The alloy has good fatigue resistance, radiation resistance, oxidation resistance, corrosion resistance, and good processability. It can manufacture various shapes and complex parts. Aviation, aerospace, petrochemical and nuclear power engineering fields are widely used. GH4169 alloy is mainly used for the fastening bolts of core fuel elements in nuclear power plants. The mechanical behavior of the alloy, including monotonic tensile properties at different temperatures and creep durability, have been extensively and extensively studied. In general, the design guidelines for high-temperature fastening are mainly based on the theory of static strength.However, in actual service, in addition to the creep load caused by the tight circumferential force, the bolt must also be subjected to alternating load, making the fastening in actual service Bolts work in a plastic state, resulting in low cycle fatigue damage. Therefore, the low-cycle fatigue of tight-peripheral bolt materials is the focus of engineering materials research. GH4169 alloy, as a nickel-based alloy for domestic turbine disks, has been deeply studied for its low cycle fatigue performance, but most of the research has mainly focused on the fatigue behavior of the material at 650 ℃ and room temperature. 2% is used as a nuclear core fastening material The service temperature is 315 ℃, and the design safety of the material is high. In particular, in order to fully guarantee the intergranular and stress corrosion properties of the material, the amount and distribution of primary carbides in the matrix need to be strictly controlled during the melting and processing of the alloy ingot, so that the amount and distribution of primary carbides in the bar are equal. Or better than the 1.0 level in the figure in GB / T14999.5. This article focuses on the low-cycle fatigue performance of GH4169 rods for fastening bolts of nuclear power fuel elements at 315 ℃, which provides theoretical basis for the application of GH4169 alloy in nuclear power and parts manufacturing.

**1 Experimental materials and methods**

The GH4169 ingot used in the experiment was smelted by the vacuum induction melting + vacuum consumable duplex process. The chemical composition w (%) of the ingot was: 0.029 C. 0.04 Si, 0.7 Al, 1.07 Ti, 9.94 Ni 3.26 Mo18.96 Cr, 5.10 Nb + Ta, 51.68 Ni, the ingot is processed into φ14 mm bar through the processes of homogenization → forging → hot rolling → surface treatment. Finally, the bar was heated at 975 ℃ for 1 hour and water-cooled at 750 ℃ for 8.5 hours, and then cooled at 50 ℃ h to 650 ℃ for 8.5 hours, and then cooled to room temperature with the furnace. Figure 1 shows the microstructure of the bar after heat treatment.

It can be seen that it is mainly composed of fine grains (grain size above 10) and rod-like phases distributed inside and outside the grain boundaries. The 315 ° C low cycle fatigue test for the bar is performed on an electro-hydraulic servo frequency fatigue tester in accordance with the provisions of GB / T15248-2008. The test adopts the total axial strain control, taking Ԑ = 0.5%. 0.6%, 0.8%, 1.0%, 1.2%, a total of 5 strain variables. . In the test, the temperature was controlled by a resistance furnace, and the temperature control accuracy was ± 0.5 ℃. The fatigue specimen is 175 mm long and the gauge length is 8 mm.

**2 test results and discussion**

**2.1 Cyclic stress-strain behavior**

The cyclic stress-strain performance of metal materials is an important aspect of low-cycle fatigue research, which reflects the true stress-strain characteristics of materials under low-cycle cyclic conditions. This curve can be measured by the stable stress-strain hysteresis curve of the number of half-life cycles. According to ASTME606, the cyclic stress-strain relationship can be generally expressed as:

ΔԐ1 is the total strain amplitude,Δσt is the total stress amplitude, E is the Young’s modulus, K ‘is the cyclic strength coefficient,n’ is the ordinate intercept on the curve, and n’ is the cyclic strain hardening index, which is the slope of the curve . Figure 2 is the cyclic stress-total strain relationship curve of GH4169 alloy bar.

In addition, the cyclic stress-strain curve can also be expressed by the stress-plastic strain amplitude:

Based on the cyclic stress-plastic strain curve of the bar shown in Figure 3, the logarithmic coordinate regression analysis was performed to determine the values of the cyclic strength coefficient K ‘= 1 438.47 and the cyclic strain hardening index n’ = 0.0502. The cyclic stress-plastic strain of GH4169 alloy bar can be expressed as:

**2.2 Low-cycle fatigue strain-life analysis**

Figure 4 is the high temperature and low cycle fatigue strain amplitude-fatigue life curve of GH4169 alloy at 315 ° C. The three relationship curves including,ΔԐt/2-Nf, ΔԐt/2-Nr

andΔԐt/2 ΔԐp/2 are obtained from the stress-strain hysteresis curve at half-life. It can be seen from Fig. 4 that the three relationship curves are logarithmic straight lines.

In general, the strain-fatigue life of an alloy can be expressed using the Manson-Coffin equation meter, that is:

n the formula, the first term is the elastic part, which reflects the relationship between elastic strain amplitude and life. σ fatigue strength coefficient, b is the fatigue strength index,Lg(Δσ/2)Lg(2Nf),Lg(Ԑp/2)-Lg(2Nt) is the slope of the curve, and the second part is the plastic part, which reflects the plastic strain amplitude and The relationship between life, Ԑ’ffatigue ductility coefficient, Lg(Ԑp/2)-Lg(2Nt) is the weave coordinate intercept on the curve 2Nt=1. c Slope of the fatigue yield index,Lg(ΔԐp/2)Lg(2Nt) curve.2Nr is the number of reversals at failure. The low-cycle fatigue strain-life test data of the bar is fitted, and the strain-fatigue characteristic parameters and fatigue transition life of the material are shown in Table 1. Therefore, the strain-life relationship equation of the GH14169 alloy bar is obtained as

For the Manson-Coffin equation, in the long life interval, the elastic strain amplitude is dominant, and the effect of the plastic strain amplitude can be ignored, which indirectly reflects the fatigue life characteristics of the alloy under stress control conditions. This is in contrast to Basquin, which reflects high-cycle fatigue performance. The relationship Δσt/2=σ’f(2Nf) is consistent. Therefore, the Δσt/2-Nfof GH4169 bar, the equation isΔσt/2=1616.06*(2Nf)in the short life interval, mainly plastic strain amplitude, the influence of elastic strain amplitude can be ignored, which is consistent with the Manson-Coffin low-cycle strain fatigue formulaΔσt/2=Ԑ’f(2Nf) . Therefore, the Δσt/2-Nf equation of GH4169 bar isΔσt/2=0.6521*(2Nt) , which indirectly reflects the fatigue life characteristics of the alloy under stress control conditions. According to [10-11], it can be seen that theΔԐp/2-Nf of GH4169 alloy at room temperature exhibits bilinear characteristics, that is, a polyline composed of two straight lines. The slope of the GH4169 alloy, that is, the fatigue ductility index is different under different strain amplitudes. The slope of the plastic line is smaller at the strain amplitude, and the slope of the plastic line is larger at the lower strain amplitude. However, the research in this paper shows that the alloy does not exhibit bilinear characteristics at 315 ℃, which is very different from the bilinear characteristics of the Manson-Coffin plastic line at room temperature. It can be seen that temperature has an important influence on the bilinear behavior of GH4169 alloy.

**3 conclusions**

(1) The cyclic stress-strain behavior of GH4169 rods for nuclear power at 315 ℃ is consistent. The cyclic strain strengthening index and cyclic strength coefficient of the bar are 1438.47 and 0.0502, respectively

(2) Low-cycle fatigue strain-life curve of GH4169 bar at 315 ℃ does not have bilinear characteristics at room temperature.