Formability of in-situ Austempered Transformation-induced Plasticity Steels After Laser Beam Welding

Soldagem & Inspeção. 2018;23(3):402-412 https://doi.org/10.1590/0104-9224/SI2303.09 ISSN 1980-6973 (Online) ISSN 0104-9224 (Print) Technical Papers Formability of in-situ Austempered Transformation-induced Plasticity Steels After Laser Beam Welding Vagner Braga1, Raquel Alvim de Figueiredo Mansur1, Rafael Humberto Mota de Siqueira1, Milton Sergio Fernandes de Lima1 1 Instituto de Estudos Avançados – IEAv, São José dos Campos, SP, Brasil. Recebido: 17 Jul., 2018 Aceito: 08 Nov., 2018 E-mail: (MSFL) Abstract: Laser welded Transformation-induced plasticity (TRIP) steels are known to generate martensite in the fusion (FZ) and the heat-affected (HAZ) zones. To solve this issue, the present study proposes a high temperature (HT) welding to avoid decay below the martensite start temperature after laser welding. Therefore, an inductive heating has been used to reach 500 °C before laser weld of superposed 1.6mm thick TRIP steel class 750. After welding the temperature was kept constant at 500 °C for more 10 minutes in order to austemper. The microstructures of the welds at high temperature are composed of bainite and residual austenite with an FZ hardness up to 300 HV, compared to 450 HV of the ambient temperature (AT) weld. The HT values of hardness are slightly higher than a traditional post-weld heat treatment (TW), 300 HV compared to 250 HV, because of the tempering kinetics in each case. Erichsen cup indentation tests shown the HT coupons presents better formability compared to the AT or TW conditions. The present contribution highlights a possible solution to the intrinsic brittleness during cold forming of laser welded TRIP 750 steel by applying an inductive in-situ austempering. Key-words: Laser welding; Transformation-induced plasticity steels; Post-welding heat treatments. Conformabilidade de Aços com Plasticidade Induzida por Deformação Após a Solda a Laser Seguida de Austêmpera in-situ Resumo: A solda a laser de aços com plasticidade induzida por transformação (TRIP) são conhecidos por gerar martensita nas zonas de fusão (ZF) e afetadas pelo calor (ZAC). Para resolver este problema, o presente estudo propõe uma soldagem em alta temperatura (HT) para evitar a transformação abaixo da temperatura inicial de martensita após a soldagem a laser. Assim, um aquecimento indutivo foi usado para atingir 500 °C antes da solda a laser da classe 750 de aço TRIP sobreposto de 1,6 mm de espessura. Após a soldagem, a temperatura foi mantida constante a 500 °C por mais 10 minutos para austemperar. As microestruturas das soldas em alta temperatura são compostas por bainita e austenita residual com dureza na ZF de até 300 HV, contra 450 HV da solda em temperatura ambiente (AT). Os valores da dureza HT são ligeiramente superiores ao tratamento térmico tradicional pós-soldagem (TW), 300 HV em comparação com 250 HV, devido à cinética de têmpera em cada caso. Os testes de embutimento Erichsen mostraram que as amostras HT apresentam melhor conformabilidade em comparação com as condições AT ou TW. A presente contribuição destaca uma possível solução para a fragilidade intrínseca durante a conformação a frio do aço TRIP 750 soldados a laser através da aplicação de uma austempera indutiva in situ. Palavras-chave: Soldagem a laser; Aços de plasticidade induzida por transformação; Tratamentos térmicos pós-soldagem. 1. Introduction This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License which permits unrestricted non-commercial use, distribution, and reproduction in any medium provided the original work is properly cited. Transformation-induced plasticity (TRIP) steels are usually manufactured in two steps. The first one requires a hot rolling at temperatures around 850 °C to partially transform the primary austenite to ferrite. In a second step, the rolled product, usually as a coil, is placed in a furnace at temperatures around 400 °C growing bainite from the retained austenite. This well balanced microstructure is responsible for the excellent toughness of the steel and, in particular, for the continuous transformation of retained austenite to martensite during plastic deformation. This characteristic is interesting for rapid energy absorption in parts such as the cars security cell. Although laser beam welding is the preferable joining technique for the automotive structures, it presents a weakness. The rapid cooling of the Formability of in-situ Austempered Transformation-induced Plasticity Steels After Laser Beam Welding fusion (FZ) and heat-affected (HAZ) zones creates an almost fully martensitic structure, responsible for an intrinsic brittleness and stress concentrator near to the fusion line of the blanks. Tailored blanks for automotive components were usually produced by assembly two or more steel sheets by means of laser beam welding and die forming [1]. A metallurgical issue of the laser welded blanks remains on the brittleness of the fusion zone (FZ) and heat-affected zone (HAZ) of the advanced high strength steels (AHSS). The rapid cooling inherent to the laser weld produces an almost 100% martensite microconstituent in the grains, which may break during mechanical working. Mujica et al. [2,3] and López Cortéz et al. [4] reported martensite formation in FZ and HAZ of Transformation Plasticity (TRIP) steels after laser welding. The hardness typically attains 500 HV in these cases, thus rendering the weldment very brittle for post-processing. Correard et al. [5] showed that, under very controlled conditions, it is possible to generate bainitic grains in the FZ of dual phase (DP) steel, but in the TRIP case always a quantity of martensite appears. Bainite is always desirable in the place of martensite for blanks because of the good toughness and formability [6]. Lima et al. [7] proposed a method for high-temperature laser welding applicable to the 22MnB5 steel. According to the authors, the welds produced at 455 °C per 10 minutes presented 66% of bainite in an austenite matrix with consequent reduction in FZ hardness and increased toughness compared to the ambient temperature weld. As both bainite and residual austenite have an important role in TRIP steels mechanical behavior, it could be interesting to apply high temperature during the weld of these alloys as well. The current paper proposes an in-situ austempering treatment subsequent to the laser welding of TRIP 750 steels by using an inductive furnace in the laser workstation. The aim is to produce bainite instead of martensite in the FZ grains. For comparison purposes, a standard procedure of annealing after the weld is also presented. As one of more stringent requirements for the welded blanks is to resist to severe plastic deformation during conformation [8], the current work access the formability obtained after instrumented Erichsen tests. 2. Material and Methodology Transformation Plasticity (TRIP) steel blanks, class 750, were furnished in as-annea (...truncated)