Comparison of Performance and Life of Threaded Holes under Different Forming Processes

Abstract

This paper systematically compares the differences in microstructure, mechanical properties, and fatigue life of non-magnetic drill collar threaded holes fabricated by two forming processes: cold extrusion and machining. The research primarily involved material characterization through microscopic observation, hardness testing, and mechanical property analysis. Fatigue tests were conducted to obtain life data under various stress levels and to construct S-N curves, with particular attention to the inflection points and fatigue limits. High-frequency three-point bending fatigue tests were performed on full-scale bolt specimens produced by both machining and cold extrusion, under a stress ratio of R=0.1 and frequencies ranging from 10 to 30 Hz. The tests employed rectangular cross-section specimens with a span of 160 mm, subjected to sinusoidal wave loading using an Instron electro-hydraulic testing machine, while crack initiation at the threaded holes was closely monitored. Based on the experimental data, the fatigue life, scatter, and allowable design stress of the two processes were compared. The experimental parameters included thread forming method (cold extrusion/machining), microhardness, surface roughness, and tensile strength. The results indicate that the fatigue performance of the cold extrusion process is significantly superior to that of the machining process. At a stress amplitude σ_a= 345.6 MPa, the fatigue life of the extruded specimens was approximately 4.1 times that of the machined specimens, with a notably lower scatter band factor (1.42 vs. 8.69), indicating superior performance stability. Fitting using the Basquin equation yielded a fatigue strength coefficient σ'_f = 1132.6 MPa and exponent b = -0.1097 for the machining process, compared to σ'_f = 3033 MPa and b =-0.1777 for the extrusion process. At a design life of 10⁵ cycles, the allowable stress amplitudes for the machined and extruded threads were 270 MPa and 300 MPa, respectively, corresponding to actual safety factors of 1.28 and 1.15. This suggests that fatigue failure in these cases is dominated by local plastic strain. The extrusion process effectively reduces the fatigue notch factor by introducing surface compressive residual stress and improving the material state around the notch, thereby extending fatigue life. In summary, the cold extrusion process offers significant advantages in high-cycle fatigue applications. It is therefore recommended for use in the design of critical load-bearing components, and that stress concentration effects and process-induced performance variations be fully accounted for in fatigue life assessment to enhance structural reliability and service life.