316L stainless steel is widely used in large-pressure equipment parts in the petroleum and chemical industries. Improving the fatigue performance of parts is critical for maximizing the remaining material value. Directed energy deposition (DED) technology has the advantages of high forming flexibility and a short processing cycle, and is considered one of the best choices for pressure equipment remanufacturing. However, defects such as poor surface finish and the generation of tensile residual stresses occur during the DED process. The heat treatment process is typically used to reduce the internal stress of a material to improve its fatigue performance, which is explained in terms of microscopic grain modulation. However, few quantitative studies have been conducted on the residual stress-induced fatigue performance of parts. Therefore, in this study, the finite element method was used to investigate the quantitative influence of residual stress on fatigue crack propagation (FCP) behavior under different heat treatment processes.

316L stainless steel was employed in this study. Initially, 3D finite element models of DED and compact tensile (CT) specimens were developed. Subsequently, the effect of different heat treatment processes on the FCP rate of the DED 316L stainless steel parts was investigated by comparing it with conventional 316L stainless steel hot-rolled plates using a combination of experiments and numerical simulations. Based on the virtual crack closure technique, the quantified influence rules of the residual stress on the stress intensity factor (SIF) and effective stress ratio were examined. Finally, the specimen fracture morphology was analyzed using scanning electron microscopy.

The residual stress distribution of DED stainless steel parts was accurately calculated using finite element simulation with an error of approximately 5.05% (Fig.4). The simulated specimen fatigue life matched the experimental results well with an error of approximately 4.71% (Fig.5). The effect of the SIF (K_res) caused by tensile residual stress on the minimum SIF (K_min) was more significant than that of the maximum SIF (K_max)(Fig.9). After the heat treatment process, the effective stress ratio of the DED stainless-steel parts decreased from 0.122 to 0.117, which reduced the FCP rate (Fig.10). When the effective stress ratio increased to 0.117 and 0.122, compared to the hot-rolled plate, the FCP rate of DED and heat-treated DED specimens increased by 43.51% and 52.62%, respectively. The FCP rate positively correlated with the effective stress ratio and SIF range, which were fitted using Walker’s formula with good agreement (Fig.11).

The fatigue performance of the conventional 316L stainless steel hot-rolled plate was superior to that of the DED stainless steel parts. The fatigue lives of the DED and heat-treated DED specimens were 63.9% and 69.0% of that of the hot-rolled plate, respectively. The formation of tensile stress zones around the internal dimples of the DED stainless steel parts accelerated crack propagation. After heat treatment, the crack length and the number of cracks at the dimples decreased, which reduced the FCP rate. The SIF (K_res) acts on the FCP rate of DED stainless steel parts by increasing the effective stress ratio (R_eff). Based on Walker’s formula, a well-fitted equation correlating the FCP rate, SIF range, and effective stress ratio was obtained. This equation can be used to provide data support for predicting the FCP rate of 316L austenitic stainless steel.