Effect of multi-pass friction stir processing on microstructure and mechanical properties of cast A216 alloy

One of the main problems in oil and gas pipelines is abrasion corrosion on the edges of the flow channel in the plug and ball valves. Under normal conditions, the gas is moving at a pressure of about 145 bar and an approximate speed of 70 feet per second; The suspended particles in the gas collide with the edges of the flow channel and cause severe erosion on them. Abrasion resistance of steels depends mainly on their surface properties and can be increased by increasing the surface hardness by friction stir processing (FSP). In this study, A216-WCB steel, which is used in the manufacturing of casting parts for valves, flanges and fittings, was processed using a friction stir process for one and three passes. The microstructure, hardness and wear properties of the processed area were investigated. The results showed that two distinct zones, the stir zone (SZ) and the thermo-mechanical affected zone (TMAZ), are formed in the processed zone. Due to the friction stir process, the ferrite grain size in the stirring region decreased from 25 microns to about 3 microns. The hardness of the stir zone increased from 165 Vickers to about 780 HV. Also, the abrasion resistance of the stirring area increased up to 2.5 times.