## 2 Experimental Details
However, this equilibrium is unstable because any external action (further processing or operational loading) induces new stresses that add algebraically to the pre-existing ones and may lead to undesirable effects such as additional plastic deformation, cracking, or rupture, depending on the total stress level and the mechanical properties of the material. Hence, the need arises to reduce or eliminate residual stresses in parts that are to be further processed or used in service--an objective achieved through stress-relief annealing. In principle, the elimination of residual stresses is based on transforming the accompanying elastic deformations into plastic ones. According to Hooke's Law, the value of residual stresses is proportional to the elastic modulus. Therefore, if the elastic modulus is lowered, the residual stress level will also decrease. Lowering the elastic modulus is possible by heating the metallic material, since the modulus is a measure of the bonding forces between atoms, which weaken as atoms move further apart with increased temperature. Moreover, Hooke's Law states that residual stress cannot exceed the material's yield strength, as this would lead to plastic deformation and dimensional changes in the part. Thus, an inequality is established between the residual stress and the yield strength:  
\[\sigma_{\text{res}}\leq\text{R}_{\text{P}0.2}\]  
From this inequality, it follows that reducing the yield strength leads to a decrease in residual stress to a new value corresponding to the lowered \(\text{R}_{\text{P}0.2}\). It is well known that as temperature increases, the yield strength decreases, meaning that heating also helps reduce residual stresses via this mechanism. Additionally, it is known that if a metallic material subjected to a stress below its yield strength is heated for a period at a certain temperature, the stress decreases over time due to the phenomenon of stress relaxation through creep. From the above, it is clear that stress relief is achieved through three complementary mechanisms: reduction in the elastic modulus, reduction in the yield strength, and relaxation via creep--all of which are favored by increasing temperature. According to the heat treatment cycle diagram shown in Figure 2, heating to the process temperature (650-870 \({}^{\circ}\)C) [25] is performed at a constant, low rate of approximately 100 \({}^{\circ}\)C/h to avoid introducing high thermal stresses that could add to the existing residual stresses and potentially deform or crack the parts. The heating temperature was established based on data provided by the alloy's T.T.T. (time-temperature-transformation) diagram in order to avoid the precipitation of carbides and other intermetallic phases [21]. The holding time, calculated using the relation \(\text{t}_{\text{hold}}=\text{g}_{\text{max}}/25\) (where \(\text{g}_{\text{max}}\) is the maximum thickness in mm), is essential for achieving the desired degree of stress relief. The cooling rate must also be kept low to avoid inducing thermal stresses in the already stress-relieved parts. Typically, a rate of 50-60 \({}^{\circ}\)C/h is used. The values of the mechanical properties obtained following the application of this heat treatment are presented in Table 3.  
The cavitation test specimens (Figure 3) were machined from a cylindrical bar with a diameter of 25 mm, which had previously undergone stress-relief annealing. The specimens are equipped with a threaded section to ensure secure attachment to the head of the cavitation vibrator. After the turning operations, the cavitation-attack surface was ground and polished to a surface roughness of Ra = 0.2-0.5 \(\upmu\)m.  
For cavitation testing, the specimens were submerged in a container filled with industrial supply water. Since the temperature of the testing liquid significantly influences the degree of cavitation erosion [25; 26; 27], the water temperature in the container was kept constant at 22 \(\pm\) 1 \({}^{\circ}\)C. Two sets of 3 specimens each were tested at successive intervals of 15 and 60 min.