See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/263656906 Issues in welding of HSLA steels Article in Advanced Materials Research · January 2012 DOI: 10.4028/www.scientific.net/AMR.365.44 CITATIONS READS 17 2,661 3 authors: Sandeep Jindal Rahul Chhibber Guru Jambheshwar University of Science & Technology Indian Institute of Technology Jodhpur 14 PUBLICATIONS 75 CITATIONS 90 PUBLICATIONS 328 CITATIONS SEE PROFILE N. P. Mehta Maharishi Markandeshwar University, Mullana 32 PUBLICATIONS 248 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: solar selective coatings View project Design and development of electrode coatings for bimetallic welds View project All content following this page was uploaded by Sandeep Jindal on 11 June 2017. The user has requested enhancement of the downloaded file. SEE PROFILE Advanced Materials Research Vol. 365 (2012) pp 44-49 Online available since 2011/Oct/24 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.365.44 Issues in Welding of HSLA Steels Sandeep Jindal 1,a, Rahul Chhibber 2,b and N.P.Mehta 3,c 1 Faculty Member, MM Engineering College/Mech. Engg. Dept., Mullana (Ambala), India 2 Faculty Member, Thapar Univ./Mech. Engg. Dept., Patiala, India 3 Director, MM Engineering College, Mullana (Ambala) cum Pro.V.C. MMU, Mullana (Ambala), India a er.sandeepjindal@gmail.com, brahulchh@gmail.com, cdrnpmehta7@gmail.com Keywords: HSLA steels, Arc Welding, Acicular Ferrite, Hydrogen Induced Cracking (HIC) and Heat Affected Zone (HAZ). Abstract. The application of High Strength Low Alloy (HSLA) steels has expanded to almost all fields viz. automobile industry, ship building, line pipe, pressure vessels, building construction, bridges, storage tanks. HSLA steels were developed primarily for the automotive industry to replace low-carbon steels in order to improve the strength-to-weight ratio and meet the need for higherstrength materials. Due to higher-strength and added excellent toughness and formability, demand for HSLA steel is increasing globally. With the increase of demand; other issues like the selection of filler grade and selection of suitable welding process for the joining of these steels have become very significant. This paper discusses the various issues regarding selection of suitable grade and selection of suitable welding process for joining of HSLA steels and issues concerning the structural integrity of HSLA steel welds. Development of HSLA Steels The Twentieth Century was a period of active development of welding processes and welding materials. Many new welding processes and welding materials had been developed. But arc welding is still the commonly type of welding used in the large group of fusion welding processes [1,2]. The paper [2] reveals that early structural steels were largely of the C-Mn type. These steels were containing relatively high Carbon and Manganese contents which led to welding problems and subsequent structure failures. By microalloying these C-Mn steels such welding problems were greatly reduced [2]. The first microalloying element used was vanadium added to C-Mn steels, then titanium (Ti) for improving the strength and finally microalloyed with niobium which led to the rapid deve1opment of high strength low alloy (HSLA) steels containing the microalloys vanadium, niobium and titanium either singly or in combination [2]. HSLA steels were developed primarily for the automotive industry to replace low-carbon steels in order to improve the strength-to-weight ratio and meet the need for higher-strength constructiongrade materials [3]. During this period, micro-alloyed or HSLA steels became an indispensable class for different applications like automobile industry, ship building, line pipe, pressure vessels, building construction, bridges, storage tanks. The current direction of development of welded structures is a decrease of their weight and energy requirement in fabrication, and improvement of reliability and endurance [4]. Typically, HSLA steels are low-carbon steels containing maximum carbon content 0.2%, containing up to 1.5% Mn, strengthened by small additions of alloys columbium, copper, vanadium, niobium or titanium such that the total alloy content is less than 2% and sometimes by special rolling and cooling techniques and containing [3,5]. Niobium (Nb), vanadium (V), and titanium (Ti) are strong carbide and nitride formers which tend to hinder the movement of grain boundaries, thus reducing the grain size by making grain growth more difficult. The reduction in grain size in HSLA steels increases their strength and toughness at the same time. 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: 117.200.55.129-23/12/11,01:37:44) Advanced Materials Research Vol. 365 45 The author [6] has divided HSLA steels into six categories, out of these categories micro alloyed ferrite-pearlite steels is widely used; which uses additions of alloying elements such as niobium and vanadium to increase strength (and thereby increase load-carrying ability) of hot-rolled steel without increasing carbon and/or manganese contents. Carbon content thus could be reduced to improve both weldability and toughness because the strengthening effects of niobium and vanadium compensated for the reduction in strength due to the reduction in carbon content. Where these high strength steels are used for fabrication by welding, care must be exercised in the selection of grade and in the details of the welding process. The main tendencies of the optimisation of properties of HSLA steels are a decrease in the content of alloying elements; an increase in the number of combinations of microalloying elements; a decrease in the content of impurities in the form of carbon, hydrogen, nitrogen, oxygen residual elements like sulphur and phosphorus; improvement of the homogeneity, improvement of level of mechanical properties. Welding Processes for HSLA Steels The present trend in fabrication industries is the use of automated welding techniques to obtain high production rates and high precision. Among all the welding techniques, arc welding is still main type of welding in the large group of fusion welding processes [7-10]. The author [11] has concluded from the comparison of welding techniques; Friction Stir Welding (FSW) and SAW on the HSLA-65 steel (ASTM A 945) that there was no difference in longitudinal distortion between the two weldments whereas Packer et.al. [12] have determined that longitudinal distortion is dependent on FS welding parameters; thus, longitudinal distortion could likely be improved by proper selection of process parameters. Trials on short welds in HSLA-65 showed that FSW resulted in less angular distortion than single-pass, gas metal arc welds (GMAW) [13]. The author [14] studied the weld metal deposited with different welding processes and consumables for A-36 plates and concluded that the oxygen level of weld metals deposited with self-shielded FCAW (110 ppm) was the lowest as compared to other processes. The nitrogen content in Weld metals deposited with the GMAW process was minimum. Among arc welding processes, plasma transferred arc is an efficient method [15] for hard facing or cladding with the advantages such as very high quality deposition, high-energy concentration, narrow heat affected zone (HAZ), less weld distortion and with certain disadvantages, namely low deposition rates, overspray, and very high equipment cost. But SAW cladding also offers very high fusion efficiency, compatible with automation, heavy section work, cladding of large areas. The mechanical properties of any weldment depend on its chemical composition; microstructures of weld metal which further depend upon solidification history and post-weld heat treatment. During SAW, weld-metal chemistry is determined mainly by welding consumables and operating variables, other secondary factors being joint design, heat input of welding and weld thermal history [16, 17]. Generally, in SAW, the mechanisms are primarily affected by three factors: dilution of weld pool by the base plate [18-21], environmental contamination [22-25] and the transfer of elements to or from the slag [19,21,26]. So flux and welding parameters are the two main variables in SAW process [27]. In the tensile testing of aluminum welds [5]; it was seen that the formation of fine equiaxed grains in the fusion zone help to reduce the susceptibility of the weld metal to solidification cracking and improve the mechanical properties of the weld in the case of steels and stainless steels. Therefore, efforts have been made to try to grain refine the weld fusion zone. It has been concluded that among different microstructures; increasing amount of acicular ferrite (AF) (Refer Fig. 1) [5] provides both increased toughness and improved strength in low carbon steel weld metals [28-31]. To a certain extent the HAZ size reflects on the grain coarsening and toughness; a larger/wider HAZ indicates larger grains in the HAZ and thus poor toughness and a narrower HAZ indicates finer grain size and better toughness [32]. An ideal fusion welding process is the one where a maximum percentage of supplied energy is utilized in fusing the filler/electrode and base metal, i.e. creating the weld, and minimum energy is wasted in forming the heat affected zone [32]. 46 Future Materials Engineering and Industry Application Fig. 1: Acicular ferrite and inclusion particles in a low-carbon, low-alloy steel weld [5]. Structural Integrity Issues in Welding of HSLA steels Structural Integrity Issues in Welding of HSLA steels are associated with different problems encountered in welding of these steels. These problems are cracking, residual stresses, distortion and fatigue damage. Among all the problems encountered during welding; cracking is the most common. The other important problem in welding HSLA steels is to prevent brittle fracture of welded joints due to increased strength of HSLA steels. Brittle fracture is caused by structural transformations in the welded joint and the heat-affected zone (HAZ) and also by impurities dissolved in the metal mainly due to hydrogen. Hydrogen can affect the soundness of the weld significantly by inducing cracks and delayed fracture of welded joints [1, 33]. Cracks occur because stress at that point in the weldment exceeds the ultimate tensile strength or ultimate shear strength of the weld metal. Cracks are classified into two categories: (i) Hot Cracks (ii) Cold cracks Hot Cracks mainly occur in the weld bead but sometimes they may develop in the HAZ. When located in the weld metal they are referred to as Solidification cracks whereas while in the HAZ they are called Liquation Cracks [34]. Solidification cracking, shown in Fig. 2 is intergranular, that is, along the grain boundaries of the weld metal [35]. It occurs during the terminal stage of solidification, when the tensile stresses developed across the adjacent grains exceed the strength of the almost completely solidified weld metal [36-38]. Cold cracks or Hydrogen Induced Cracking (HIC) or delayed cracking is the most serious and the least understood of all weld cracking problems particularly in the welding of high strength low alloy steels [34] (Refer Fig. 3). Fig. 2: Solidification cracking in GMAW Fig.3: Hydrogen-induced cracking in HY-80 steel Advanced Materials Research Vol. 365 47 Hydrogen-assisted cracking in HSLA steel welds is promoted by conditions viz. unacceptable diffusible hydrogen content, high restraint tensile stress, high hardness or a susceptible microstructure, and a temperature ranging between –100 and 100°C [39,40]. To prevent hydrogen induced cracking, advanced welding technologies used for steels of this type are based on preheating of components and heating during welding [1], avoid cellulose-type electrode coverings, and hydrogen-containing inert gases, dry the electrode covering and flux to remove moisture and clean the filler wire and workpiece to remove grease, adjust the composition of the consumables if feasible and using postweld heating to help hydrogen diffuse out of the weld [41]. Preheating of components and heating during welding operations are energy and labourintensive and require high technological culture of production hence practically not much efficient. Because of the high temperature of the structures resulting from preheating, the operating conditions for the workers working in assembling operations are greatly impaired [1]. Then fabrication of structures from HSLA steels without preheating is one of the main problems of arc welding at present. To decrease the hydrogen content of initial and completed welding materials, it is recommended to apply heat treatment of electrodes and fluxes, with drying of shielding gases, to use the methods of decreasing the hydroscopic complicity of coatings and vacuum packing of electrodes and increasing the CaF2 content in the electrode covering or the flux also reduce the weld hydrogen content [42-43]. Whilst technological advances in the control of HIC, such as the use of lower carbon and hydrogen levels, have been made in the development of welding consumables they have not kept pace with developments in the production of the so-called 'preheat free' steels [44-46]. Summary Literature is summarized as: • With the increase of demand of HSLA steels globally; selection of grade and welding process for the joining of these steels is more important. • Further work may be carried on the development of aluminium high strength alloys, alloyed titanium alloys and other types of new structural materials. • The advances in the development and production of welding materials are linked to the tendencies in the development of structural materials and optimization of the systems of alloying the weld metal in relation to the structure and properties of the parent metal, minimize the content of impurities in the form of carbon, hydrogen, nitrogen, oxygen residual elements like sulphur and phosphorus, to reduce the content of harmful impurities, decrease the carbon content in high-alloy wires and development of new composition of fluxes for the these materials. • Methods of decreasing the high content of hydrogen, nitrogen and other harmful impurities in the weld metal, decreasing preheat temperature, preventing the formation of different cracks, including those induced by hydrogen should be devised. 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