"Performance assessment of full strength and tensionof laser welds of nitrogen content, austenitic and martensitic steels".
S.D. Voronchuk, V.I. Krivorotov, L.V. Shamova NTO «IRE-Polus»
Fryazino, Moscow region, Russia sVoronchuk@ntoire-polus.ru
M.V. Kostina, S.O. Muradjan, Yu. Petrov Laboratory of structural steels and alloys
A.A. Baikov IMET RAS Moscow, Russia mvk@imet.ac.ru Abstract — The report contains the results of the evaluation
of full strength and tension of laser welds of nitrogen content, austenitic and martensitic steels.
Keywords — laser welding, joint weld, microstructure, nitrogen content steel, microhardness, full strength, tension.
Introduction.
One of the major requirements in the development of new structural materials, in particular nitrogen content steels, is investigation and evaluation of their weldability; which is important for industrial application of the steels.
Currently, the application of laser welding for critical structures of new nitrogen content steels is of high importance.
However, publications on the subject are very few; as for publications concerning welding properties of laser welding, they can hardly be found.
Methods
There were investigated the properties of joint welds (JW), performed with the help of single-pass laser welding without steel additives: 05Х22АГ15Н8М2Ф (in DIN-system - Х3 CrMnNiMoVN 22-15-8-2) (JW-1) and 05Х16Н5АБ (in DIN-system – X5 CrNiN 16-5) (JW-2). Welding was performed on the plate rolled steel specimens sized 100х50х3 mm. To select welding conditions the method of multifactor experiment was used. Welding conditions were varied: power P, kW = 1...6; welding speed Vw, m/min = 1…7; focus depth measured from the surface Δf, mm = +3...-3.
As a shielding gas, commercial purity Ar was used. Gas flow rate was 10…20 liters/min.
In this work we investigated and evaluated weldability, strength properties and tension of JW, made with high-power ytterbium fiber laser of NTO “IRE-Polus” production. We carried out metallographic examination, measured microhardness (Нμ) and coercive field (Нс).
Microstructure and structural properties of JW are described in the report "Structure and characteristics of thin sheet laser welded joints of nitrogen content austenitic and martensitic steels"
(M.V. Kostina, S.O. Muradjan, E.V. Blinov, Yu. Petrov S.D., A.A. Baikov IMET RAS Voronchuk, V.I. Krivorotov, L.V.
Shamova, NTO “IRE-Polus”).
Mechanical properties and tension of JW were evaluated in accordance with the standardized interrelations (specified in GOSTs) between physical quantities and strength properties of steels under investigation, using methods proposed by Grigorovich V.К., Del G.D., Bogacheva N.D. etc. The results of measurements of Нμ, hardness and Нс were processed using methods of mathematical statistics.
Нμ was measured to find the correlation between its Vickers hardness (HV) and Нμ, as JW size did not allow to make full-scale HV examination of parent metal (PМ), joint weld (JW) and weld adjacent zone (WAZ). Basing on the interrelation between HV and Brinell hardness (HB), as well as relation between ultimate tensile strength (
σ
в)
and HB according to GOST 22761 we evaluated mechanical characteristics of JW as it was impossible to evaluate mechanical properties of JW by means of static tension test.The tension was evaluated with the help of charts, showing the results of measurement. As a primary information parameter we used Нс values, calculated and measured before welding of samples and afterwards.
Investigation results.
Provided the mentioned modes, JW with full penetration had the following dimensions: width of face side of weld–
0,8…1,2 mm; width of root side of weld – 0,3 …0,5 mm;
reinforcement 0,05…0,3 mm. Weld geometry factor К=Вav./Н=0,15…0,35, which is the evidence of good formation.
The measured average value Нс of JW-2 specimens prior to welding was 34,4 А/cm, and calculated average value according to Bogacheva method was 37,5 А/cm. Obviously the results are similar.
The relation Нс~A*(ρ)-1/2(1), which shows dependence of Нс from dislocation density (ρ) and graphing ε ~ Нμ ~
σ
(2), defined tension of JW-1 and JW-2. In the formula (1) and (2):A - coefficient, depending on the physical properties and the structural state of steel; ε – degree of deformation and
σ –
tension.
It was found that after laser welding tension in JW-1 and JW-2 increases 8% 12,6% respectively, i.e. tension increase of JW in JW-2 is 1,5 higher, than JW-1, which can be due to the formation of specific micro- and macrostructure during crystallization process caused by laser welding.
Conclusion.
There were defined mechanical properties and tension of laser JWs with the help of computational and analytical methods as well as with the help of metallographic examination,
microhardness testing and magnetic structurescope analysis.
There was detected a 1.5 times increase of internal tension in respect of the initial values of PM in laser JW of steel (JW-2) in comparison with (JW-1).