Research on Laser Welding Process and Joint Microstructure of E36 High-strength Steel Plate

E36 high-strength steel plate had been welded by HL5000 CO2 laser. The welding quality of E36 high-strength steel plate had been improved by changing the parameters such as laser power and laser scanning speed .The experimental results indicated that HAZ had the highest hardness after laser welding; When laser power was fixed, as scanning speed increased in a certain range, microstructure was finer, hardness increased obviously; When scanning speed was fixed, with the increase of laser power, microstructure was coarser, hardness gradually decreased; The smallest coefficient of weld formation and the most excellent welding quality was obtained when laser power was 3.1 kW.
Key words：E36 high-strength steel plate; laser welding; mechanical property

With the increase in the tonnage and speed of ships in modern shipping , the structural safety requirements of shipbuilding are getting higher and higher , and the proportion of high-strength marine structural steel plates has increased significantly . The skeleton T -shaped members of ships are generally welded by high-strength ship plate steel , and its welding quality and production efficiency are getting more and more attention. It is called E36 high - strength ship plate steel . C) Impact toughness and welding performance also have higher requirements . The quality level meets the needs of China's ship plate steel [2] . In the process of the development of ships to light weight and large size , the high-strength ship plate steel E36 middle plate has been widely used in the construction of the hull structure [3] . Due to some characteristics of E36 high-strength ship plate steel in terms of processability , hull design and weldability, laser welding is chosen for welding . Laser welding is one of the important aspects of the application of material processing technology , which has been developed since the 1970s . Compared with traditional welding methods , laser welding has high welding energy density , precise control , strong penetration ability , large depth-to-width ratio of weld seam , low heat input , small welding deformation , and can be welded with different welding shielding gases, etc. Advantages , it is considered to be the new hot spot of welding in the 21st century . Especially with the emergence of high-power lasers , the application of laser technology in heavy industry and shipbuilding has opened up new fields .
The chemical composition ( mass fraction, % ) of the E36 high-strength ship plate steel used in the test is :
0.07C , 1.42Mn , 0.20Si , 0.012P , 0.007S , 0.15Ni , 0.02Cr , 0.02Cu .
The HL5000 5kWCO 2 laser was used to weld it, the metallographic structure was observed with the XJG -05 metallographic microscope, and the microhardness was measured with the HX-1000 microhardness tester.

Through the test , the average hardness value of the weld zone is about 480HV , and the average hardness value of the HAZ is 532HV . The hardness peak is in the HAZ , and its value is 568HV .
Combined with the microstructure analysis, it can be seen that the maximum hardness appears in the fully quenched fine-grained zone of the HAZ . From the HAZ to the base material hardness decreased significantly , and the average hardness of the base material was about 273HV . 2.2.2 The effect of laser parameter changes on the microstructure of base metal , HAZ and weld
The columnar grain structure of weld metal is mainly composed of granular bainite and a small amount of MA components . There are thin lath bundles in the bainitic iron chain body , and a large number of thin laths with different orientations greatly increase the grain boundary density of the weld structure and refine the grains of the weld structure . The microstructure of the fully recrystallized zone of the HAZ is mainly composed of granular bainite and a small amount of proeutectoid ferrite , and the grains are fine and evenly distributed . The temperature gradient distribution of the joint reaches the heat-affected transition zone at the austenitization temperature of pearlite with high carbon content , and cooling forms bainite and part of original ferrite composed of spheroidized pearlite . The base material is a ferrite + pearlite mixed structure with uniform grain size , in which white is layered pearlite and black is ferrite .

1. After laser welding of E36 high-strength ship plate steel , the structure of the weld center area is uniform equiaxed grains , the near center area is dendritic grains , the edge area is thick columnar grains , and the fusion zone is slender columnar grains and fine equiaxed grains. . The heat-affected zone is mainly composed of lath martensite , bainite and a small amount of retained austenite, and has the highest hardness .
2. When P=2.8kW , when the scanning speed changes from 100 to 220 mm/min , as the scanning speed increases , the width of the weld zone and HAZ becomes narrower , the structure of the weld zone becomes finer , and the hardness value increases significantly ; when When v=180mm/min , when the laser power changes from 2.6 to 3.2kW , as the power increases , the width of the weld zone and HAZ becomes wider , the structure of the weld becomes thicker , and the hardness value decreases .
3. The penetration depth and penetration width of welds welded with laser power of 2.9 , 3.1 , 3.3 and 3.5kW are not much different , and the shape and size of weld seams are basically the same

In the same way , the weld seam obtained by 3.1kW laser power has smaller fusion width , larger penetration depth , smaller weld seam formation coefficient , and better weld seam quality .

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