Fracture analysis on dissimilar metal welding zone of vessel CPP power pipe

Highlights

• The failure analysis case has important reference significance for failure analysis and improvement of dissimilar metal welding between copper alloy and steel.

• The process of copper invading into steel and the fracture morphology due to grain boundary weakening are clearly revealed due to the difference of melting point and composition.

• The fatigue crack begins in the intergranular brittle crack zone caused by welding copper penetration, which results in grain boundary weakening and intergranular crack.

Abstract

The power oil pipe of a vessel adjustable pitch propeller fractured after running for about 25,000 h. The fracture causes were analyzed by macro fracture morphology, micro fracture morphology, chemical composition, metallographic analysis, numerical simulation analysis, mechanical property and hardness test. Fractographic analysis shows that the fracture mode of oil pipe is high cycle fatigue fracture. The crack starts from the intergranular brittle fracture zone in the heat affected zone of dissimilar metal welding. The intergranular crack is mainly formed in the welding process of dissimilar metals, which is caused by intergranular embrittlement due to the deep diffusion of low melting point welding fluid mainly composed of Cu along the grain boundary in the heat affected zone of steel. The temperature gradient of the welding zone is analyzed by finite element method, and the reason why the low melting point welding fluid can diffuse to the farthest depth of 1 mm on the steel side is explained. The fracture mechanics analysis shows that when the crack depth reaches 1 mm, the crack has a greater probability to continue to expand forward, which leads to the risk of unstable fracture. Therefore, both experiments and numerical simulation show that the intergranular brittle crack formed by the diffusion of liquid metal to the heat affected zone of the steel side is the cause of fatigue crack initiation and propagation.

Introduction

Many literatures have studied dissimilar metal welding between steels of different material grades, such as dissimilar welding between ferritic martensitic steel and austenitic steel [1], welding of alloy 617 and A387 steel [2], welding of P22 and F69 steel [3], etc. It is found that the difference of composition and microstructure leads to the problem of harmful phase and carbon migration in the welding zone. The filler wire with higher nickel content can reduce the degree of carbon migration and martensite formation in the transition zone due to its intermediate physical and mechanical properties, and improve the performance of dissimilar metal welding zone to a certain extent. However, there are few studies on the dissimilar welding of Cu alloy and steel. Different from the studies in the above literatures, the welding of Cu alloy and steel with copper wire shows more diffusion of Cu with low melting point into the heat affected zone of steel. This is because the difference between the two melting points is more than 400 °C, Cu can keep liquid for a long time in the welding process, and has a longer diffusion distance, so the harm is much greater, this paper studies the fatigue fracture failure of dissimilar metal weldment used in vessel propulsion device.

The performance of vessel propulsion device directly affects the maneuverability and navigation safety of the vessel. The controllable pitch propeller has excellent operation efficiency, maneuverability and maneuverability under different working conditions, and has been widely used in the main propulsion equipment of various vessels. The hydraulic system is the key structure of the whole CPP device, which plays an important role in ensuring the stable power output of the CPP. The outer oil pipe is an important carrier of power transmission in hydraulic system. Two functions are realized in the work. One is to adjust the distance, which can bear the maximum hydraulic stress of 6 MPa for a short time and 1.6 MPa for a long time in the steady state. The other is to transfer the position of screw pitch. At this time, the oil pipe overcomes the friction between the supporting strip and the inner wall of the shaft hole and makes a linear reciprocating motion in the shaft hole. The maximum clearance between the supporting bar and the inner wall of the shaft hole is 0.05 mm. Although the supporting bar is generally linear in motion, it will have a small amplitude of up and down jump. After running for about 25,000 h, the oil pipe broke from the welding position. In this paper, the fracture cause analysis of the composite is carried out.