Stabilization of austenite in low carbon Cr–Mo steel by high speed deformation during friction stir welding

Abstract

T. Miura). Abstract:Friction Stir Welding (FSW) was applied to a low carbon alloyed steel in order to stabilize the austenite phase which can effectively maintain a preferable toughness. A Cr-Mo steel sheet (0.20wt%C -1.07%Cr -0.16%Mo -0.24%Si -0.61%Mn -bal. Fe) with a ferrite pearlite structure was friction stir welded under the conditions in which the sample was reheated above the A 3 temperature © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 at the maximum. The microstructure observations in the stir zone clarified martensite with several area fractions of retained austenite and elongated ferrite. The retained austenite provides a high ductility with a high strength in the stir zone. In order to clarify the formation mechanism of the austenite, compression tests were conducted at various strain rates up to 10 /s using an ultrahigh-speed compression testing machine. When the sample was compressed at the high strain rate, the elongated ferrite grains and retained austenite were formed. This microstructure was similar to the microstructure in the FSW stir zone, suggesting that the efficient accumulation of plastic strain in the austenite during the FSW plays an important role in obtaining the retained austenite.

Effect of Applied Pressure on Microstructure and Hardness of Linear Friction Welded Martensitic Steel

“…Regardless of the applied pressure, the hardness tended to soften gradually from the base metal to the joint, reaching a minimum value in the HAZ, and then the hardness rapidly increased, reaching a maximum value near the joint. The width of this change in hardness was ± 1 mm in LFW, whereas it was ± 6 mm in the case of FSW, 32) which is also another solid-state joining method. This indicates that the range of hardness change at the joint was very narrow in the case of LFW.…”

Prediction of Material Flow Behavior in Stir Zones of Friction Stir Welded 6％Ni Carbon Steel Based on Texture Analysis

“…摩擦攪拌接合 (FSW: Friction stir welding) は，回転ツールの 摩擦熱と攪拌作用を利用した固相接合技術である．入熱量が 低く，溶接ひずみが小さいなどの優れた特徴を持つことか ら，まずは適用が比較的容易なアルミニウム合金などの低融 点金属 [1][2][3][4][5] で研究が行われ，ツール材質の改良などに伴い鉄鋼 材料などの高融点材料への適用も研究されている [6][7][8][9][10][11][12][13] ． 著者らはこれまでに，低合金鋼 [14][15][16] や Ni-C 合金鋼 17,18)…”

Effects of Carbon Content and Austenite Grain Size on Retained Austenite Fraction in Stir Zone of Friction Stir Welded 6%Ni Carbon Steels

“…Recently, applications for high-melting-point metals such as steels have been investigated owing to improvements in tool materials. [5][6][7][8][9][10][11][12] In many cases involving the FSW of steels, a phase transformation occurs via frictional heating and cooling during and after FSW. In other words, austenite which transforms from ferrite during stirring undergoes dynamic recrystallization, followed by phase transformation from austenite to ferrite and/or martensite during cooling after the stirring stage.…”

“…The authors reported that austenite in the stir zone was stabilized and retained after FSW under appropriate welding conditions, which resulted in an improvement in the strength-elongation balance of the stir zone owing to the transformation-induced plasticity (TRIP) effect in lowalloyed 12,13) and alloyed steels. [14][15][16] Although austenite stability is assumed to be affected by the dislocation density and grain size of the prior austenite and the mobility of the phase transformation interface, 17) the mechanisms affecting austenite stability in terms of concrete welding conditions and fraction of retained austenite have not been elucidated.…”