为阐明焊接过程中，复杂力、热作用导致焊区晶粒粗化进而影响焊件残余应力分布的机理，本工作结合XRD与中子衍射，研究了电子束焊接不锈钢样品焊缝区域晶粒尺寸分布及粗/细晶相互作用。结果表明，焊缝区晶粒尺寸差异与残余应力分布存在明显相关性，晶粒尺寸极差小(约32 μm)的样品横向残余应力起伏小(幅度约为160 MPa)，晶粒尺寸极差大(约53 μm)的样品横向残余应力起伏大(幅度约为316 MPa)。晶界及内部的高位错密度使细晶具有比粗晶更强的抗变形能力，细晶的晶面间距大于粗晶，即细晶具有较小的形变量，对粗晶产生局部挤压作用；粗/细晶尺寸差异越大，两者的横向应力差也越大，从而导致残余应力非均匀分布。粗/细晶尺寸差异是决定焊接残余应力分布的关键微观参量，通过调控晶粒尺寸分布均匀性可改善局部应力集中，进而实现焊接构件残余应力及结构稳定性的优化控制。

Stainless steel components are widely used in aerospace, marine engineering, and railway transportation owing to their outstanding corrosion resistance and high mechanical strength. Welding is a key joining technique for stainless steel structural components. However, substantial residual stresses are inevitably introduced into components owing to the complex coupling effects of force and heat during welding. Because of the critical role of residual stress in the performance and service life of components, considerable attention has been given to its experimental measurement and theoretical modeling, as well as to the effects of microstructure (e.g., phase transformation and grain size) on residual stress. However, the microscopic mechanism through which grain size, especially the interaction between coarse and fine grains, influences residual stress distribution remains an open question. To clarify this mechanism, this study investigates grain-size distribution and interactions between coarse and fine grains in the weld zone of stainless steel samples prepared via electron beam welding using a combination of X-ray and neutron diffraction. A clear correlation is observed between grain-size variation in the weld zone and residual stress distribution. Samples with a narrow grain-size distribution (~32 μm) exhibit minimal transverse residual stress fluctuations, with a stress amplitude of ~160 MPa. However, samples with a wide grain-size distribution (~53 μm) show substantial transverse residual stress fluctuations, with a stress amplitude of ~316 MPa. Owing to the high dislocation density at grain boundaries and within grains, fine grains possess greater deformation resistance than coarse grains. The interplanar spacing of fine grains is larger than that of coarse grains, indicating that fine grains undergo less deformation and exert an extrusion effect on coarse grains. The size difference between coarse and fine grains is a key microscopic parameter that determines the distribution of welding residual stresses. By regulating the uniformity of grain-size distribution, local stress concentration can be reduced, enabling effective control of residual stress and enhancing the structural stability of welded components.