Relation between microstructure, composition, and hot cracking in Ti-stabilized austenitic stainless steel weldments
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I. INTRODUCTION
HOT cracking during welding is believed to occur through a combination of liquid and solid phases in the solidifying weld metal that causes cracking in the presence of shrinkage or external stresses. The propensity for hot cracking has been identified with certain temperature regimes during welding or casting called the brittleness temperature range (BTR). The BTR is defined[1] as an envelope in temperature-strain space, as shown in Figure 1, following experimental observations that on application of strain, cracking in many materials extends over a definite temperature range. Many materials do not show cracking at low strains and are said to require a threshold strain «min for cracking. On further application of strain, cracking increases until it attains a constant value upon reaching the BTR. Since welding involves a moving solidification boundary, the temperature at a given point is a function of time. Therefore, the temperature differential of strain is equivalent to a strain rate, and a minimum strain rate required to produce cracking «min can be defined. In many materials, such as stainless steels and nickel-base alloys, BTR values derived from hot-cracking tests have been identified with solidification temperatures of various phases that form in the interdendritic regions during solidification.[2,3,4] Hot cracking is considered a problem in the welding of austenitic stainless steels, particularly those that are fully austenitic and that are compositions that contain elements V. SHANKAR, Scientific Officer ‘E,’ T.P.S. GILL, Head, and S.L. MANNAN, Associate Director, Materials Development Group, and A.L.E. TERRANCE, Scientific Officer ‘E,’ Materials Characterization Group, are with the Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. S. SUNDARESAN, Professor, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Madras 600 036, India. Manuscript submitted January 26, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
such as titanium and niobium. These elements are usually added to stabilize the carbon and to prevent the precipitation of chromium carbides at the grain boundaries that causes intergranular corrosion. Austenitic stainless steels stabilized with niobium or titanium additions, such as AISI type 347 or 321, are known to exhibit a greater susceptibility to hot cracking during welding than are the unstabilized varieties.[5] The stabilized grades are preferred for elevated temperature service because of their superior high-temperature mechanical properties and their resistance to intergranular corrosion. In these materials, maintaining a sufficiently high ferrite potential usually alleviates the problem of hot cracking. However, in fully austenitic materials such as alloy 800, this method cannot be used and the composition must be carefully optimized with respect to the levels of impurity and minor elements, to provide good weldability. Alloy D9, a fully austenitic Ti-stabilized steel, corresponding to UNS S38660, has been chosen for th
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