Experimental study on residual stresses in stud welding based on X-ray diffraction and the blind-hole method

2.1. X-ray diffraction method for measuring stud welding residual stress

The X-ray diffraction method is based on Bragg’s Law and the $\mathrm{s}\mathrm{i}\mathrm{n}²\psi$ method for residual stress measurement. The experiment used a Cr-Kα radiation source, targeting the ferritic (211) crystal plane with a diffraction angle (2$\theta$) of approximately 156°. $\psi$ angles were set at 0°, 25°, 35°, and 45° for measurement. The stress value was calculated by linearly fitting the $2\theta -{\sin }^2\psi$ curve. To improve accuracy, each measurement point was tested three times and averaged. A standard stress-free sample was used to calibrate instrument error. Before measurement, the sample surface was subjected to electropolishing to eliminate surface stresses induced by mechanical processing, ensuring that the results accurately reflected welding-induced residual stresses.

The test material used was Q345B steel. The testing equipment included an X-ray stress analyzer. At room temperature, the elastic modulus of Q345B steel is 200 GPa, its Poisson’s ratio is 0.3, density is 8000 kg/m³, and yield strength is 345 MPa. The stud welding process is illustrated in Fig. 1. Initially, the stud is in contact with the base metal. Subsequently, the stud is lifted to draw an arc, which melts both the stud and the surface of the base metal. After the molten weld pool is formed, the stud is plunged into the pool. The welded joint is formed uponcooling.

The X-ray diffraction measurement scheme is summarized in Fig. 2. Radial and circumferential residual stresses were measured along four orthogonal directions from the weld toe. Eight measurement points were spaced at 5-75 mm in each direction to cover the weld zone, heat-affected zone, and base material.

Fig. 1Stud welding process

Fig. 2Welding residual stress measurement scheme via X-Ray diffraction method. All photographs were taken by Yuhao Liu in Guangzhou, China, on September 15, 2025

a) Measurement direction

b) Measurement points layout

c) Measured welding residual stress

2.2. Residual stress measurement in stud welds using the blind-hole method

A three-element strain gauge rosette (0°-45°-90° layout) was used, with strict alignment to the center of each measurement point. A high-speed pneumatic drilling device with a drill bit diameter of 1.8 mm was employed. The drilling depth was controlled at 2.0 mm, executed in four steps (0.5 mm per step), with strain release values recorded at each increment. Strain data were acquired using a static strain instrument with a sampling frequency of 1 Hz to ensure stability. Stress calculation followed the ASTM E837 standard formula:

where $A$ and $B$ are calibration coefficients related to the material’s elastic modulus and Poisson’s ratio.

The primary steps for measuring stud welding residual stress using the blind-hole method are as follows: (1) The measurement points are ground, cleaned, and a strain gauge rosette is attached and connected to a strain data acquisition system. (2) A blind hole is drilled at the center of the rosette, and the released strain is recorded. (3) The residual stress at all measurement points is subsequently calculated through data processing. The test material used in this study was Q345B steel. The testing equipment is listed in Fig. 3.

The blind-hole measurement scheme is summarized in Fig. 4. Radial and circumferential residual stresses were measured along one direction at four distances (5, 20, 35, and 50 mm) from the weld toe.

Fig. 3Residual stress testing equipment for the blind-hole method. All photographs were taken by Yuhao Liu in Guangzhou, China, on September 20, 2025

a) Blind-hole drilling unit

b) Strain gauge demodulation equipment

Fig. 4Welding residual stress measurement scheme via the blind-hole method

a) Measurement points layout

b) Measured welding residual stress

3.1. Results of radial welding residual stress measurement

The radial welding residual stresses obtained from the X-ray diffraction method and the blind-hole method are shown in Fig. 5, where the black line represents the results from the X-ray diffraction method and the red line represents the results from the blind-hole method.

As can be seen from Fig. 5, the radial welding residual stress exhibits a trend of first increasing and then decreasing as the distance from the weld ring increases. The maximum radial welding residual stress, recorded at –170 MPa, occurs at a distance of 5 mm from the weld ring. As the distance from the weld ring increases further, the compressive stress gradually transitions into tensile stress, with a maximum tensile stress of 40 MPa. Subsequently, with increasing distance from the weld center, the radial welding residual tensile stress gradually transforms back into compressive stress. This “compression-tension-compression” three-stage distribution pattern is significantly different from the distribution characteristics of the weld area in the classical welding residual stress theory, which mainly shows tensile stress.

3.2. Results of circumferential welding residual stress measurement

The circumferential welding residual stresses obtained via the X-ray diffraction method and the blind-hole method are presented in Fig. 6, where the black line and red line denote the results from the X-ray diffraction method and the blind-hole method, respectively.

As observed in Fig. 6, the circumferential welding residual stress consistently exhibits a compressive stress distribution. Furthermore, its magnitude demonstrates a trend of initially increasing, then decreasing, and subsequently increasing again as the distance from the weld ring increases. The maximum circumferential welding residual stress reaches –170 MPa at a location 5 mm from the weld ring. With increasing distance from the weld ring, the compressive stress first increases and then decreases. Beyond this point, as the distance from the weld ring continues to increase, the circumferential welding residual compressive stress gradually rises again, eventually stabilizing at approximately -100 MPa, which indicates that even if the material is far away from the heat source, there is still non-negligible residual compressive stress in the material.

Fig. 5Distribution of radial welding residual stress

Fig. 6Distribution of circumferential welding residual stress

3.3. Stress distribution mechanism analysis

Analysis of the welding residual stress measurement results reveals a compressive stress distribution in both the region immediately adjacent to the weld ring and the base metal area far from the weld, with a tensile stress region in between. The weld zone and its immediate vicinity exhibit high compressive stress (–170 MPa), while areas further away show low tensile stress (40 MPa). This pattern is opposite to the classic welding residual stress distribution, where the weld zone typically experiences tensile stress.

The reasons for this phenomenon may be as follows:

(1) A significant solid-state phase transformation (primarily the transformation of austenite to martensite or bainite) occurred in the weld metal and the nearby heat-affected zone during welding. During rapid cooling, the transformation of austenite to martensite or bainite induces a volumetric expansion. This expansion effectively counteracts, or even reverses, the tensile strain generated by prior thermal contraction, consequently resulting in the formation of compressive stresses after cooling.

(2) The base metal region far from the weld was less affected by the welding heat and thus did not undergo phase transformation. The presence of compressive stress in this area is likely attributable to the initial rolling stresses present in the steel plate.

(3) Welding residual stress is static and self-equilibrating, meaning it can exist in an isolated system at a constant temperature without any external load. Since compressive stresses are concentrated near the weld and in the base metal region, the balancing tensile stresses are necessarily distributed in the intermediate zone. This creates the overall “compressive-tensile-compressive” stress distribution pattern.

Based on the above analysis, it is recommended to introduce preheating and post-weld heat treatment during welding. Preheating reduces temperature gradients and thermal stress, while post-heat treatment [12] (e.g., annealing at 350 °C for 2 hours) facilitates stress relaxation, particularly alleviating compressive stress concentration near the weld and improving stress uniformity.

The compressive residual stresses observed near the weld and in the base material are beneficial to structural durability, as compressive stresses tend to inhibit the initiation and propagation of fatigue cracks. However, even in the presence of compressive stress, the toe area of the weld can still become a crack initiation site due to stress concentration effects under cyclic loading. Therefore, recommendations for preheating and post-weld heat treatment remain applicable to further reduce local tensile stress peaks and ensure long-term fatigue performance.

Although the high compressive stress zone near the weld inhibits crack initiation, the tensile stress zone (peak value of 40 MPa) may become a source of fatigue cracks. It is advised to avoid placing critical welds or openings in tensile stress zones during structural design. Alternatively, surface strengthening techniques such as shot peening or laser shock processing can be applied to introduce compressive stresses and offset the detrimental effects of tensile stresses.