Comparison of High Cycle Fatigue in 4340 and 300M Steels Welded with Fiber Laser Andreia de Souza Martins Cardoso 1,a , Antonio Jorge Abdalla 1,b* , Carlos Antonio Reis Pereira Baptista 2,c , Milton Sérgio Fernandes de Lima 1,d 1 Instituto de Estudos Avançados – IEAv Rod.Tamoios, Km 5,5, 12.228-001 – S. J. Campos, Brazil 2 Escola de Engenharia de Lorena, Pólo Urbo-Industrial, 12600-970 Lorena, Brazil a bra_afs@hotmail.com, b ajorgeabdalla@gmail.com, c baptista@demar.eel.usp.br, d msflima@gmail.com Keywords: ultra-high strength steels, laser beam welding, fatigue, microstructural characterization. Abstract. AISI/SAE 4340 and 300M steel samples have been used in severe conditions where high strength and toughness are necessary. Since there is a particular interest on these materials in the aerospace industry, they are being used in aircraft landing gear, rocket engine vessels and satellite launch vehicles. Therefore, the understanding of the weldability of these steel samples has been subject to special interest. This study aims to evaluate and compare the microstructure and mechanical strength of 4340 and 300M steels after bead-on-plate laser welding and tempering heat treatments. The mechanical properties have been assessed by means of tensile and high-cycle fatigue tests. The results showed that the welds have high hardness and small heat affected zones. Nevertheless, there was not a decrease in the uniaxial tensile properties for the 300M and 4340 steels. The decrease in fatigue life due to the welding process, compared to the unwelded condition, was small, indicating that the welding parameters are adequate and that this process is viable. Introduction The SAE 4340 steel can be classified as a HSLA (high strength low alloy) and has been widely used in aerospace and automobile components as well as heavy equipments. The Brazilian studies and developments gave rise to a modified version, called 300M steel, considered as a technological evolution of the 4340 steel. Several properties of 300M steel are similar to 4340, except by for the fact that the higher silicon content and the presence of vanadium increases the hardening depth, hardenability and the solid solution decreases the tendency to softening at elevated temperatures[1]. Convenient mechanical properties, concerning toughness, tensile strength and fatigue life, for 4340 and 300M steels have been obtained by heat treatments. Earlier works, dealing with phase transformation have resulted in the so called dual phase steels and later in the multiphase or complex phase steels [2, 3]. These structures were obtained by means of intercritical or isothermal heat treatments or by thermomechanical treatments and are currently used by the industries [4, 5]. The study of the weldability of these steels has been subject to special interest because there is a loss in mechanical strength and ductility after conventional welding. There is a lack in literature data about welding of these steels, particularly regarding to laser beam welding [6]. The formation of a hard martensite phase due to rapid cooling can cause brittleness. This phenomenon can affect the mechanical properties, especially fatigue. To reduce these effects tempering has been recommended [7, 8, 9]. Welding produces two distinct regions, the weld zone and heat affected zone. In laser welding these regions are relatively small, reducing its negative effects on the microstructure [10, 11]. In this work, results concerning the influence of the laser welding process on the microstructure of the 4340 and 300M steel samples are presented. Advanced Materials Research Vols. 891-892 (2014) pp 1507-1512 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.891-892.1507 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 161.24.244.39-24/02/14,14:20:27)