Controlling of the Softened Region in Weld Heat Affected Zone of Ultra Fine Grained Steels

Abstract

When many additional alloying elements are used for high strength of steels, the weldabilrty is usually pcor. The developmentof high-strength steels of 800MPa class vvith goodweldabilky is preferred to prcceed on the basis of a new concept for innovative materials design which may remarkably reduce the carbon equivalent et steels. The higher-strength steels ot 700 to 800MPa class with the same carbon equivalent as mild steels could be achieved by creating uitra fine ferrite grains ot less than 1um.Efficient and high speedjoining processes with low heat input and narrow wioth of heat afrected zone (HAZ) were developed to obtain high-performance welded structures while preserving ultra fme microstructure. The softened region in the HAZ et ultra fin,~grained steels ean be controlled vvith the newly developed welding process

Development of hyper-interfacial bonding technique for ultra-fine grained steels and microstructural analysis of bonded joints

“…The maximum grain size in hyper-interfacial bonded joints was smaller than that in EBW joints. 5. From the fact that grain growth and thermal damage in the joint were sufficiently suppressed, the hyperinterfacial bonding technique would be a feasible joining method for ultra fine-grained steels.…”

“…On the other hand, the applicability of some welding techniques to UFG steel joining has been investigated. [3][4][5][6][7] The weldability of UFG steel for conventional arc welding and laser welding is generally excellent. However, the material properties of the heat affected zone (HAZ) in UFG steel weldments are poorer than the base metal as a result of grain coarsening and softening by weld heat input.…”

Superior Properties of Ultra-fine-grained Steels

“…Efficient joining processes with low heat input and narrow width of the heat affected zone (HAZ) have to be used to obtain high-performance welded structures while preserving an ultra-fine microstructure. Laser welding [13] and ultra narrow gap GMA welding [14], for example, have been shown to be adequate processes.…”

Newly developed welding process and its applications to ultra-fine-grained steel

“…8 When usual arc welding is executed on this UGF steel, fine ferrite grains induced by welding heat become larger, hence causing the strength in the area from the base material to the heat-affected zone (HAZ) to become lower than that in the base material. This leads to a softening of the HAZ, 9,10 and a big problem in securing the joint strength.…”