A resistance spot welding system can join two polymeric workpieces and includes a power supply. The power supply has a positive terminal and a negative terminal. The resistance spot welding system further includes a welding electrode assembly electrically connected to the power supply. The welding electrode assembly includes a housing, a first electrically conductive pin and a second electrically conductive pin. The first and second electrically conductive pins both protrude from the housing. The first electrically conductive pin is electrically connected to the positive terminal of the power supply, and the second electrically conductive pin is electrically connected to the negative terminal of the power supply. The second electrically conductive material is electrically insulated from the first electrically conductive pin. The first and second electrically conductive pins are at least partly made of a material having a hardness ranging between 50 HRC and 70 HRC.