S31008 (06 cr25ni20) is a kind of high cr-ni austenitic stainless steel, heat-resistant for strong corrosion resistance and resistance to high temperature, the welding is easy to produce hot crack, such as welding technology to master is bad, will affect the safe use of equipment.The coefficient of thermal conductivity of the S31008 is about 1/3 of the low carbon steel, linear expansion coefficient is about 1.5 times of low carbon steel.0 ~ 100 ℃, the average S31008 coefficient of thermal conductivity is 14.9 W/(m ℃), average linear expansion coefficient of 15.8 x 10-6 mm/mm;S30408 (06 cr19ni10)) of the average coefficient of thermal conductivity is 17.3 W/(m ℃), average linear expansion coefficient of 16.3 x 10-6 mm/mm.
S31008 (06 cr25ni20) as follows: the chemical composition of omega (C) : 0.04 ~ 0.08%, omega (Si) : 1.5%, omega (Mn) : 2.00%, omega (P) : 0.035%, omega (S) : 0.020%, omega (Ni) : 20.00 ~ 22.00%, omega (Cr) : 24.00 ~ 26.00%.
S31008 (06 cr25ni20) has the following salient features:
(1) excellent corrosion resistance performance.The mass fraction of chromium and nickel is very high, has a much better than ordinary stainless steel corrosion resistance and oxidation resistance, especially corrosion resistance in oxidizing medium is very good.
(2) good heat resistance.Have a certain high temperature strength, high temperature than ordinary stainless steel of high strength, also because of its high temperature oxidation resistance, was widely used in high temperature circumstance (using the temperature can reach 1000 ℃).
(3) than ordinary stainless steel welding performance is poor, in the weld zone and heat affected zone are prone to hot crack, its main reasons are as follows:
(a), high nickel.Nickel is low melting eutectic element, Ni + Ni3S2 eutectic melting point (645 ℃), the Ni + Ni3P eutectic melting point (880 ℃), the weld area between the liquid layer forms.Weld the alloy composition is complicated, easy to form a dual or multiple eutectic, segregation, easy to produce hot crack, nickel mass fraction is higher, the easier when welding thermal cracks;
(b), high chromium., because of the high chromium content does not need too high temperature is easy to form sigma precipitation of brittle phase, thus cause heat crack;
(c) and linear expansion coefficient of thermal conductivity.S31008 linear expansion coefficient is big, and coefficient of thermal conductivity is poor, local heating and cooling in the welding conditions, welding heat to the surrounding, austenitic steel is easy to form a strong directional columnar crystal weld organization.As a result, the weld metal in the process of crystallization is tensile stress and tensile strain, easy to produce hot crack.
The measures to prevent welding hot crack mainly has: control of the parent metal and weld material sulfur, phosphorus mass fraction and other harmful impurities quality score, improve the quality of the weld material Mn grade;A variety of welding process measures, strict control of the welding heat process, taking smaller welding line energy.