Year

1991

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

A weld joint produced by a fusion welding process consists of the weld metal and affected zone (HAZ). The macro- and microstructures and mechanical properties of these regions determine the weldability of a material. Although the composition of the weld metal can be varied by the choice of filler metal and the extent of dilution, the HAZ has a composition which is essentially the same as the base plate and is an identifiable region because of the structural changes induced by the weld thermal cycle. The HAZ is important because of its potential to develop structures which adversely affect the properties of the joint.

In the present work, the structures and properties of the HAZs produced by various welding processes have been investigated for a commercial structural plate steel. The structure and property gradients across the HAZ have been examined to determine the critical region of the HAZ which governs the properties of the whole HAZ . The effects on the HAZ of weld process type (bead-on -plate submerged arc [BOP SA], bead-on-plate flux cored arc [BOP FCA], and four wire submerged arc [4 wire SA]); and the welding parameters of heat input, welding speed, multi-passes and postweld heat treatment have been analysed in order to assess the weldability of the steel. The mechanical properties of toughness, tensile properties and hardness are of major concern in the present investigation.

The steel investigated was a low carbon, copper-bearing, precipitation hardening (HSLA 80) which has been recently developed by BHP Steel, SPPD, Port Kembla, Australia. It is based on a modified ASTM A710 steel chemical composition and is produced by a thermomechanical control rolling process (TMCP). The high yield strength of this steel (80ksi or 550Mpa for plate ≤ 25mm) is achieved by copper precipitation hardening through an aging heat treatment at 550°C for 1/2 hour after rolling. The reason for the development of this steel was to produce an 80ksi grade steel which can be welded more easily and can be more economically produced than quenched and tempered HY 80 type steels and thus can be qualified as a replacement in various structural applications. The weldability of this new steel is thus of critical importance in proving its advantages over HY 80.

This thesis reports a detailed investigation of the structure and properties of the HAZ , which is widely regarded as a critical region in terms of the weldability of a steel.

As a result of the microstructural gradient which develops across the HAZ, it is to carry out Charpy impact toughness testing on a particular microstructural region of the HAZ . In order to facilitate mechanical testing, especially impact testing, thermal simulation experiments have been conducted to reproduce in "bulk form" structures similar to those of different sub-regions of the actual HAZ . A comparison has been made of the results obtained from actual and simulated HAZs.

The effect of multi-pass welding and postweld heat treatment on the HAZ structure been simulated to assess the response of the HAZ to a series of thermal cycles. A partial ϒ→α continuous cooling transformation diagram for the grain coarsened HAZ region under weld thermal cycle conditions was also obtained by analysing the cooling curves associated with thermal cycles simulating those experienced in the HAZ during welding under different conditions.

The microstructure of the grain coarsened HAZ region for both actual and simulated welds generally consisted of ferrite in the form of grain boundary allotriomorphs, Widmanstatten sideplates and laths, together with martensite-austenite (MA) islands. The dominant constituents were lath ferrite and MA islands. Low carbon lath martensite was also found in the HAZ of some low heat input welds, particularly BOP FCA welds.

A general problem in welding precipitation hardening steels is that the HAZ thermal cycle can destroy the precipitation hardening and reduce the hardness locally to below the level of the base plate. Such a softened HAZ was observed in the present steel for the BOP SA, BOP FCA and the 4 wire S A welds; as well as for the simulated HAZs . The loss of precipitation hardening was found to be due to solution of copper on re-austenitising and the resulting supersaturation of ferrite on cooling. For intercritical heating, a significant part of the softening was due to rapid overaging of copper precipitate particles in the untransformed ferrite.

It was found that the HSLA 80 steel showed a good overall toughness in the HAZ for welding conditions investigated. The toughness of this type of steel in the hot rolled and aged condition is due to its low carbon content (0.055%) and a fine grained structure. In addition, the low carbon equivalent (0.41), relative to the strength, ensured that HAZ toughness generally exceeded the minimum requirements for HY 80 and was similar to that of the base plate. O f the various H A Z sub-zones, the grain coarsened region (GCHAZ) near the fusion line exhibited the lowest toughness and highest hardness values and, therefore, this region is likely to govern the overall HAZ toughness.The heat input did not appear to have a major effect on HAZ toughness, despite the observation that HAZ structural refinement and an increase of HAZ hardness occurred with decreasing heat input.

It was established by simulated multiple weld thermal cycles that multi-pass welding generally refines the HAZ structure and improves the toughness of the HAZ . However, it was found that a second weld thermal cycle to a subcritical peak temperature, consistent with a high heat input, could markedly increase the hardness of an original grain coarsened HAZ region produced by a low heat input, because of precipitation of copper from supersaturated ferrite. This combination of thermal cycles appears to have the potential to reduce the toughness in this local region.

Strengths similar to that of the base plate were obtained from transverse tensile tests on weld joints produced at heat inputs of 2.5 and 5kJ/mm by 4 wire submerged arc welding, despite the softening which occurred in the H A Z . However, for a high heat input of 10kJ/mm, significant degradation of weld strength occurred because of the wide softened HAZ . Varying the welding speed of 4 wire SAW showed little effect on HAZ structure, toughness, hardness and tensile properties.

Postweld heat treatment of the GCHAZ region at 550°C for 1 hour significantly reduced its toughness. This embrittlement was attributed to precipitation hardening by copper which resulted in a considerable increase in hardness. However, postweld heat treatment at 450°C and 650°C were found to improve the toughness and reduce the hardness of the GCHAZ.

Investigation of the ϒ→α transformation temperature of the grain coarsened HAZ region under simulated welding conditions showed that during the cooling part of a weld thermal cycle, austenite begins to transform at temperature between 600-650° C for the equivalent heat input range of 1.9-4.9kJ/mm. A lower transformation temperature was associated with a lower heat input.

Comparison with a reference steel indicated that the copper and nickel additions to HSLA 80 suppressed the HAZ transformation temperature. The major associated microstructural change was the predominance of a nondiffusional second constituent (martensite-austenite islands) rather than a diffusional one (pearlite and/or bainite).

The research investigation makes two main contributions to knowledge in the field physical metallurgy of ferrous alloy welding. The first is the provision of detailed data on the structure and properties of the HAZ of a modified A710 type precipitation hardening steel for welding by flux cored arc and submerged arc processes under various conditions. This characterisation of the structure and properties has allowed definition of welding conditions leading to satisfactory strength and toughness in the HAZ .

The second contribution is a general finding concerning limitations of the Rosenthal analysis of heat transfer during welding which is based on the assumption of a moving point heat source. The implication of this analysis, and a widely accepted view, is that a constant heat input dictates a constant H A Z cooling rate and hence structure. However, structure and properties have been observed to vary in a small but significant way with position around the fusion line of a single weld bead at a given heat input and between welding processes at the same nominal heat input In both cases, variations in weld bead shape affect the local heat transfer conditions and hence the cooling rate.

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