High strength and toughness steel structures by friction stir welding

Abstract

Provided are steel structures methods of making such steel structures including structural steel components bonded by friction stir weldments with advantageous microstructures to yield improved weldment strength and weldment toughness. In one form of the present disclosure, the steel structure includes: two or more structural steel components produced by conventional melting or secondary refining practices and friction stir weldments bonding faying surfaces of the components together, wherein the chemistry and grain size of the starting structural steel satisfies one or more of the following criteria: a) 0.02 wt %<Ti+Nb<0.12 wt %, b) 0.7<Ti/N<3.5, c) 0.5 wt %<Mo+W+Cr+Cu+Co+Ni<1.75 wt %, d) 0.01 wt %<TiN+NbC+TiO/MgO<0.1 wt %, e) average grain size of at least 2 microns, wherein the friction stir weldments have a prior austenite grain size of between 5 and 60 microns and less than 50 vol % of martensite-austenite constituent, and wherein the friction stir weldment strength is greater than the starting structural steel and the friction stir weldment toughness as measured by the crack tip opening displacement test at less than or equal to 0° C. is greater than or equal to 0.05 mm or by the Charpy V-notch impact test at less than or equal to 0° C. is greater than 40 J. The steel structures find application in linepipe for oil and gas production.

Dependencies are inferred from the claim text.