Fatigue strength of low carbon steel with hard boride alloy bonded on.

Abstract

In order to improve the erosion resistance of heat exchanger tubes in fluidized-bed boilers, small blocks of Boride alloys were diffusion bonded on the low carbon steel tubes. It was anticipated that these blocks could reduce the fatigue strength because of the stress concentration at the corner of bonded blocks and the difference of hardness between the tube and the blocks.Fatigue tests were performed on plate and tube specimens made of boiler graded low carbon steels with and without bonded Boride blocks and the fatigue crack initiation behavior was studied experimentally. Low carbon steel tubes with the Boride alloy blocks were found to have the equivalent fatigue strength to the low carbon steel without Boride blocks. Analyzing the test results, it was concluded that the crack initiation behavior of tube specimens was controlled by two counteracting effects, namely beneficial effects o f compressive residual stress and boron diffusion, and detrimental effect of corner notch.It was concluded that low carbon steel tubes with boride alloys can be employed in the fluidized-bed boilers much the same way as usual tubes without boride blocks.