Microstructure, hardness and residual stress distribution in maraging steel gas tungsten arc weldments P. Venkata Ramana* 1 , G. Madhusudhan Reddy 2 and T. Mohandas 2 The distribution of residual stresses due to welding has been studied in maraging steel welds. Gas tungsten arc welding process was used and the effect of filler metal composition on the nature of residual stress distribution has been investigated using X-ray diffraction technique with Cr K a radiation. Three types of filler materials were used, they include: maraging filler, austenitic stainless steel and medium alloy medium carbon steel filler metal. In the case of maraging steel weld, medium alloy medium carbon filler, the residual stress at the centre of the weld zone was more compressive while, less compressive stresses have been identified in the heat affected zone of the parent metal adjacent to the weld metal. But, in the case of austenitic stainless steel filler the residual stresses at the centre of the weld and heat affected zone were tensile. Post-weld aging treatment reduced the magnitude of stresses. The observed residual stress distribution across the weldments has been correlated with microstructure and hardness distribution across the weld. Keywords: Maraging steel, Microstructure, Hardness, Residual stress Introduction Residual stresses are introduced in a structural member as a result of non-uniform relaxation of plastic defor- mation. Uneven cooling after welding may cause this relaxation. The effect of residual stresses can be either beneficial or detrimental depending upon their magni- tude, sign (tensile or compressive), and distribution with respect to service induced stresses. It is well known that it is necessary to minimise or eliminate residual stresses so as to overcome assembly related problems during manufacturing stage and dimensional stability or stress corrosion cracking related problems in service. As a consequence, knowledge of the residual stresses and their distribution is an important input to an overall structural integrity assessment. Fusion welding is a reliable and efficient joining process in which the coalescence of metals is achieved by fusion. This form of welding is widely employed in diverse industries such as aerospace, shipbuilding, nuclear and defence. Residual stresses in weldments are caused by non-uniform expansion and contraction of both the weld metal, the heat affected zone (HAZ), and the adjoining parent material. The major contribut- ing factors to these are the temperature of the weld area (heat input), yield strength of the material, thermal characteristics (coefficient of thermal expansion and conductivity of the material), and material transforma- tions that occur in the welding thermal cycle involving solidification, post solidification cooling and solid state phase transformations. Several factors may contribute to the formation of residual stress. 1–7 The plastic deformation produced in the parent metal is a function of structural, material and fabrication parameters. The structural parameters include the geometry, thickness and joint design. The material parameters reflect the metallurgical condition of the parent material and the weld metal. Fabrication parameters include the welding process, procedure, parameters and the degree of restraint. In addition when weldments are subjected to mechanical surface treatments like shot blasting, grit blasting, peening, surface grinding and milling etc., the surface residual stress pattern gets altered to a considerable extent. 8 Hence the final stress state of a weldment is also dependent on mechanical surface treatment applied besides the stress induced due to welding. This study is focused on the distribution of residual stresses in maraging steel welds. Maraging steels are a special class of ultra high strength steels that are age hardened by the precipitation of intermetallic com- pounds. The unique combination of toughness and strength of maraging steels has made them desirable for critical applications in strategic technologies. In addition to the above mentioned properties, good weldability and easy machinability and very high dimensional stability during aging make this material an ideal choice for rocket motor casings in aerospace industry. 9 Such 1 Mahatma Gandhi Institute of Technology, Gandipet, Hyderabad 500 075, India 2 Defence Metallurgical Research Laboratory, Hyderabad 500 058, India *Corresponding author, email pvramana04@yahoo.com ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 28 January 2008; accepted 27 February 2008 DOI 10.1179/174329308X300091 Science and Technology of Welding and Joining 2008 VOL 13 NO 4 388