IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 12, Issue 2 Ver. VI (Mar - Apr. 2015), PP 05-10 www.iosrjournals.org DOI: 10.9790/1684-12260510 www.iosrjournals.org 5 | Page Influence of Microstructure on Mechanical Properties of Martensitic Stainless Steel Welds A. Rajasekhar Prof. Mechanical Engg. St.Marten Engg College, Kompally, Ranga Reddy, Telangana, India. Abstract: The mechanical properties of welds in martensitic stainless steels are critically dependent upon the microstructure developed in the weld metal and heat affected zone. Hence it is essential to understand the transformations that take place during the welding cycle. The purpose of the present paper is to present an overview of the phase transformations which take place in the weld metal or heat affected zones of martensitic stainless steels and their influence on mechanical properties of the welds. Keywords: Martensitic stainless steel; mechanical properties, delta ferrite; retained austenite I. Introduction: Fusion welding involves localized heating, melting and cooling with the thermal cycle experienced varying over a wide range depending on the thickness of the material being weld and the welding process employed. In practice, all the metals and alloys which experience welding display some metallurgical change between the base metal and weld and HAZ region and hence the joint properties of these regions will differ significantly from those of the base metal. Metallurgical transformation will take place during both heating and cooling, however for weld metal it is only the cooling cycle that is of concern. The metallurgical changes during welding may be mainly of two types, the first one is the major phase changes that occur and secondly the involvement of second phase particles either dissolution on heating or precipitation on cooling. Because the properties of welds in steels are critically dependent upon the microstructure developed in the weld metal and heat affected zone, it is essential to understand the transformations that take place during the welding cycle. The purpose of the present paper is to present an overview of the phase transformations which may take place in the weld metal or heat affected zones of martensitic stainless steels and their influence on mechanical properties of the welds. II. Phase Transformations Phase transformations take place during the original solidification process of the weld metal, and solid state transformations may occur in both the weld metal and heat affected zone. The predominant phase transformation in the martensitic stainless steel welds is the austenite-to-martensite transformation that occurs in the fusion zone and regions of the HAZ that have been heated into the austenite phase field. 2.1 Fusion Zone The fusion zone of martensitic stainless steel with a nominal 11 to 14 wt% Cr and 0.1 to 0.25 wt% C solidify as delta ferrite. Segregation of C and other alloying elements during solidification can in some cases result in the formation of austenite, or a mixture of ferrite and austenite, at the end of solidification. As the weld metal cools in the solid state, the δ-ferrite transforms into fully austenite structure below about 1100 0 C. The austenite will transform to martensite upon further cooling. This transformation is represented by the following sequence. Transformation path 1: fully martensitic microstructure L→ L + Fp→ Fp +A → A → martensite Where, Fp is primary ferrite, and A is austenite. However complete transformation to austenite will be influenced by i. Segregation during solidification resulting in the formation of ferrite, which remains stable during cooling and remain at dendritic axes [1]. On the other hand, some of the ferrite stabilizers are also rejected into the liquid during solidification, so that the inter dendritic regions also can become ferrite, with some of the ferrite remaining at room temperature [2]. The amount of ferrite will depend on the ratio of ferrite to austenite promoting elements and the solidification conditions. This transformation is represented by the following sequence.