Year

2005

Degree Name

Master of Engineering

Department

Faculty of Engineering

Abstract

The current standard practice when welding 2.25Cr-1Mo steel is to use preheat and post welding heat treatment (PWHT) to improve weldability. The research outlined in this thesis is an investigation of the microstructure and mechanical properties of weldments of 2.25Cr-1Mo steel before and after the application of preheat and/or post weld heat treatment. Trial welds and simulated coarse grained heat affected zone (CGHAZ) material were mechanically tested and etallurgically examined before and after welding and PWHT. The pre-heat investigation involved 3.8 mm and 7.1 mm thick tubes, welded with pre-heats of 25 °C, 50 °C and the standard 150 °C. These samples were then mechanically tested to AS3992. The PWHT investigation was carried out on 12 mm thick tube and 16 and 26 mm thick plate samples. After welding, the samples were heated to a simulated service temperature of 550 °C for up to 10 h and PWHT at 700 °C. These samples were then compared in terms of microstructure and hardness. To enable mechanical testing of HAZ material, simulated HAZ material was produced. These controlled cooling samples also enabled monitoring of the transformation behaviour to produce a CCT diagram and comparison of the transformation products with the trial welds. The results of this experimental program showed that pre-heating thin tubes in the range 25-150 °C had no noticeable effect on their properties. It is however expected that the influence of the pre-heat temperature would be limited to the root pass, as the higher interpass temperature would override any effect of the pre-heat temperture. The welds made in 12 mm and 16 mm thick material also showed negligible difference in their response to short-term ageing and PWHT. The results from the 26 mm sample however showed higher hardness and a different microstructure to those of other plate thicknesses. Thus no evidence based on mechanical properties and microstructures was found to support the specification in the Australian Standard AS4458 for pre-heat 7 temperatures above 25 °C when welding tubes up to 7.1 mm thick. Furthermore, although the Australian Standards do not require PWHT for 12 mm tube, but specify PWHT for 16 mm plate, the present results do not provide a basis for the need for PWHT in either case. These conclusions are drawn within the context that the experimental welds were made under low restraint conditions and high restraint in industrial practice could affect weldment integrity through the development of high residual stresses, distortion and/or HAZ cracking.

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