Year

1999

Degree Name

Master of Engineering (Hons.)

Department

Department of Materials Engineering

Abstract

The development of steel chemistries which result in highly weldable niobium -molybdenum micro-alloyed thin walled, high strength X80 grade is resulting in increasing consideration for use by Australian industry, particularly in offshore and pipeline applications. Despite the significant material cost saving through reduced thickness and increased strength, a limiting factor controlling widespread use of X80 is the susceptibility to weld metal cracking. The excellent weldability of this grade of line pipe steel has enhanced the potential for the use of high strength cellulosic consumables like E 9010 and E 8010 in root pass welding, but the risk of hydrogen assisted cold cracking ( HACC ) is also increased.because of the high strength weld metal. To quantify the cracking susceptibility and to alloy guidance for field welding condition, the Welding Institute of Canada ( WIC ) restraint test was used in the present investigation. This thesis outlines the use of two grades (E 9010 and E 6010 ) and five different brands of commercial cellulosic consumable to assess conditions leading to hydrogen assisted cold cracking and solidification cracking in the diluted weld metal. The investigation also involved clarification of the relationship between microstructure, preheat temperature and hardness values for the various weld consumables and their effect on cracking susceptibility.

Tests were carried out using a standard restraint length of 25mm on 8.6mm strip preheated to various temperatures to determine the critical crack free temperature. For E9010 electrodes, a preheat of 40°C or more was found to be effective and For E6010 electrodes, a preheat of 30°C or more was effective in avoiding cold cracks in the weld metal. Most of the cracks found on welds preheated at room temperature were initiated by a defective weld feature such as undercut or lack of penetration. An attempt was made to correlate the hardness values and carbon equivalents ( IIW ) of weld metal to cracking. However, it was found that there was no significant effect of these variables on cracking over the range of welding conditions investigated.

The cracking morphology studies indicated that there are many ways in which the crack can propagate in the weld and HAZ. The modes of cracking observed were microvoid coalescence, quasi cleavage and intergranular. In some cases solidification cracking was detected.

The testing program indicated that despite the high hydrogen content of the weld metal for E 9010 electrodes, crack-free root pass welds can be obtained for 8.6mm X80 steel strip provided a preheat of 40°C or higher is used to ensure a low cooling rate.

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