Microstructure and Properties of Welding Heat Affected Zone of E550 Ship Plate Steel

The microstructure and properties of welding heat-affected zone of high-strength ship plate steel E550 under different welding process conditions were studied by thermal simulation test .

Key words: high-strength ship plate steel ; E550 ; welding thermal cycle; heat-affected zone

With the development of large-scale and lightweight ships and marine structures, the demand for high-strength, high-toughness, easy-to-weld, and corrosion-resistant marine steel plates continues to increase Increase. Welding performance is one of the important performance indicators of ship plate steel. When the steel plate undergoes welding heat cycle, the grain coarsening and microstructure transformation in the welding heat-affected zone will lead to changes in its properties, so the welded joint is the weakest area of the welded structure , has an important influence on the hull quality.

In order to meet the demand for high-strength ship plate steel, high-strength ship plate steel E550 has been developed , which adopts low-carbon, micro-alloyed design, and is produced by quenching and tempering process, with excellent strength and toughness. In order to further understand its weldability, the welding performance of E550 ship plate steel was studied by using welding heat simulation technology, microscopic analysis technology and mechanical performance test .

The microstructure and performance of welding heat-affected zone under different cycle passes and different peak temperatures are analyzed, which provides a reference for its engineering application and on-site welding process specification formulation.

1 Test materials and methods
The test adopts 60 mm thick quenched and tempered ship plate steel E5 50,
The chemical composition is: c = 0. 07 % , si = 0. 22 % , M n = 1. 55 % , p = 0. 015 % , s = 0. 010 % , .
mechanical properties are: K eL = 605 MPa , R m = 670 MPa , A = 22. 9% , A KV ( — 40 °;C )$220 J o
Take the transverse sample from 1/4 of the thickness of the steel plate, use the Gleebl ^ 3800 thermal simulation testing machine to measure the welding continuous cooling transition curve (SHCCT ) and simulate the welding heat affected zone test. The sample size is '10 mmX 80 mm and 10 . 5 mm X 1 0. 5 mm X 80 mm o SHCCT curve measurement Heating to 1 350 C at a speed of 200 °;C/s , holding for 0. 1 s, selected as 5 time is 5 , 10 , 20 , 30 , 40 , 60 , 100 , 150 , 300 s to study the change of structure, hardness and impact toughness according to 8/5 . The process parameters of the welding heat simulation test, in which the different peak temperatures selected for the first thermal cycle correspond to the coarse-grained zone , fine-grained zone , and critical zone of the welding heat-affected zone respectively; the second thermal cycle test simulates the unchanged coarse-grained zone ( Coarse-grained region + coarse-grained region ), transcritical coarse-grained region (coarse-grained region + fine-grained region), critical coarse-grained region (coarse-grained region + critical region ) .

The samples that have undergone different welding heat cycles are processed into impact samples of 10 mm X 10 mm X 55 mm , and the impact test is carried out at 40 C in accordance with the standard of GB /T 229-2007 "Charpy Pendulum Impact Test Method for Metal Materials" ; The hardness test was carried out on a Vickers hardness testing machine Tukon 2100B ; the metallographic sample was corroded with 4% nitric acid alcohol solution after mechanical polishing, and observed on a Carl Zeiss Axio Imager Alm microscope; Backscatter analysis ( EBSD) and fracture observation.

Table 1 Process parameters of welding heat simulation test
Table 1 Parameters of welding thermal cycles

	sample	heating rate/ (°;C・sT )	peak temperature/ C	'^ 8/5 / the s	second peak temperature/ C	^ 8/5 / the s
Coarse grain area	1	200	1350	15	-	-
	2	200	1200	15	-	-
fine-grained region	3	200	1100	15	-	-
	4	200	900	15	-	-
critical section	5	200	800	15	-	-
Unchanged coarse-grained region	6	200	1350	15	1200	15
	7	200	1350	15	1100	15
supercritical coarse-grained region	8	200	1350	15	1000	15
	9	200	1350	15	900	15
critical coarse-grained region	10	200	1350	15	800	15

The results show:
"5 has a great influence on the structure of the coarse-grained zone. When the cut 5 is small, the structure is mainly lath bainite and granular bainite; with the increase of cut 5 , the granular bainite gradually increases, and the impact toughness Gradually decreased; when cutting 5 for 300 s , the structure is dominated by granular bainite.

With the increase of peak temperature, the impact energy of single-pass welding heat-affected zone first increases and then decreases, and the coarse-grained zone and critical zone embrittlement exist, while the impact energy of double-pass welding heat-affected zone gradually increases, and the critical zone The toughness of the coarse-grained region is low.

Microstructure analysis shows that the embrittlement in the coarse-grained zone is mainly caused by coarse granular bainite, while the embrittlement in the critical zone and critical coarse-grained zone is mainly caused by the coarse M - A components distributed along the grain boundaries.

---