Simulation Study on Fire and Water Rectification for EH36 Ship Plate

The effects of heat-treatment temperature between 800 °C and 1100 °C on the microstructure and mechanical properties of EH36 ship plate were investigated by SEM, EDS, OM, electronic universal material tester and metallic pendulum-type impact tester. The results indicate that the granular bainite precipitates from 800 °C to 850 °C , martensite precipitates at 900 °C, single martensite phase precipitates when heat-treatment temperature is above 1000 °C and the grain size increases accordingly with the heat-treatment temperature increasing. The elongation and impact toughness decrease while strength increases after heat-treatment. The over-temperature of heat-treatment leads to higher tensile strength and lower elongation and higher or lower heat-treatment temperature leads to lower impact toughness, which can't meet the requirements of CCS standard. The appropriate fire and water rectification temperature is from 850 °C to 900 °C.
Key words ： EH36 ship plate; fire and water rectification; microstructure; mechanical properties

With the development trend of large-scale and light-weight shipbuilding , the requirements for mechanical properties and quality grades of steel for hull structures are getting higher and higher [1-2] . EH36 is a steel for high-strength hull structure . It is an ideal hull structure material because of its high strength , good low-temperature impact toughness and plasticity , excellent processing performance , welding performance and corrosion resistance .
However, in the process of ship construction , especially in the process of welding and cutting , deformation beyond the tolerance often occurs . In order to ensure the appearance quality , it needs to be corrected by water and fire . However, EH36 structural steel for high-strength hulls is sensitive to temperature , and different temperatures and cooling rates have a greater impact on its impact toughness . For ships in unlimited navigation areas, excellent impact toughness at low temperatures is required .
At present, the research on EH36 mainly focuses on rolling , normalizing and welding processes. It is of urgent practical significance to study the impact toughness of ship plate after water and fire correction .

 In this paper, water cooling ( water quenching ) after heating in a muffle furnace is used to simulate the worst environment of water cooling in actual industrial and mining, and the effects of different heating temperatures on the microstructure , tensile properties and low-temperature impact toughness of EH36 ship plates are studied . Provide technical reference for EH36 water and fire correction process .

1 Experimental materials and methods
1.1 Experimental materials
The experimental material is EH36 ship plate with a thickness of 14mm . The chemical composition is measured with a spark direct reading spectrometer. The results are shown in Table 1. The material meets the requirements of the CCS specification .
Table 1 Chemical composition of EH36 ship plate ( mass fraction , % ) Tab.1 Chemical composition of EH36 ship plate (wt % )

	C	Si	mn	P	S.	Cr	Ni	Mo	Cu	Ti
practice	0.10	0.28	1.37	<0.010	<0.014	<0.022	<0.110	<0.019	<0.051	<0.020
standard	w	w	0.90~	w	w	w	w	w	w	w
	0.18	0.50	1.60	0.035	0.035	0.20	0.40	0.08	0.35	0.02

1.2 Test method
The samples were kept at different temperatures ( 800 , 850 , 900 , 950 , 1000 , 1050 and 1100 ° C ) for 10 minutes , then cooled quickly in water , and 9 sets of impact samples ( 3 for each set ) were cut from the same steel plate. The sampling direction for portrait . The sample size is 10mm x 10mm x 55mm , the notch type is V -shaped , and the notch depth is 2 mm . The impact toughness is tested at - 40 °C .

 in conclusion

1. EH36 ship plate is heat treated at 800-850 °C , granular bainite is mainly precipitated ; martensite begins to precipitate at 900 °C; when the heat treatment temperature is higher than 1000 °C , it is a single martensite structure , and with the continuous increase of temperature The higher the value , the larger the martensite grains .
2. After heat treatment, the strength of EH36 ship plate increases , but the plasticity and impact toughness decrease . If the heat treatment temperature is too high, the tensile strength will be too high , and the elongation at break will be too low , while the heat treatment temperature that is too low or too high will lead to impact toughness. If it is too low , it cannot meet the requirements of CCS code , and the suitable water and fire correction temperature is 850 ~ 900°C .

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