Size Effects on Static Tensile Properties of Welded Joints including Soft Interlayer

Abstract

In the welded joint, chemical composition and structure of weld metal or heat-affected zone which is adjacent to it are different from those of parent metal. Hence, the mechanical properties, for example stress-strain relation, of the welded joints must also be different from those of parent metal and change continuously. In this way, considering macroscopically, the welded joints are not homogeneous nor isotropic. Concerning such welded joints which are heterogeneous, it matters a great deal to investigate their strength and ductility. The welded joints are fundamentally idealized as models which are joints consisting solely of parent metal and a soft or hard interlayer. By "soft interlayer" is meant apart of welded joint in which yield point is lower than that of parent metal.
In the present report, such model specimens were made by flash butt welding. The parent metal was grade S35C, the soft interlayer grade S10C. The specimens were given post-weld heat treatment (850°C×10 min. water quenching and 500°C×30 min. tempering) to cause an abrupt change of the mechanical properties between parent metals and soft interlayer. Static tensile test was made for four kinds of specimen diameter, 3, 6, 10 and 15 mm, at room temperature.
In the joints including a soft interlayer under tension, plastic flow will begin at first in the parts of a soft interlayer. The transverse plastic flow of interlayer at the contact surfaces and the neighbouring zone will be held in check by the stronger parent metal, and the interlayer will be in a triaxial stress state analogous to that in the neck of tension specimen. The triaxiality will be severe with a decrease in the thickness of soft interlayer. The ultimate tensile strength of the joints exceeds that of the material of interlayer, if the values of thickness of interlayer are sufficiently low. The present paper describes theoretical and experimental investigations on the effect of thickness of a soft interlayer or specimen diameter on mechanical properties of welded specimens under static tension. The results obtained are as follows.
(1) The yeild stress, ultimate tensile strength, plastic constraint factor and reduction in area of the welded joint including a soft interlayer are represented as functions of the relative thickness X, or the ratio of thickness of soft interlayer to specimen diameter, regardless of the specimen diameter.
(2) The yield stress and ultimate tensile strength increase and the elongation and reduction in area decrease as the relative thickness decreases. Fracture mode becomes brittle for a sufficiently small value of X, although the material of interlayer possesses high ductility under unidirectional tension at room temperature.
(3) The test results on ultimate tensile strength agree with the results of the authors' theoretical analysis for a comparatively large value of X.