Elsevier

Acta Materialia

Volume 97, 15 September 2015, Pages 380-391
Acta Materialia

Heat affected zone microstructures and their influence on toughness in two microalloyed HSLA steels

https://doi.org/10.1016/j.actamat.2015.05.055Get rights and content

Abstract

Microstructures and Charpy impact properties have been examined in two microalloyed steels following heat treatments to simulate weld heat affected zone (HAZ) structures over a range of heat input conditions, characterised by the cooling time from 800 to 500 °C (Δt8/5). The base materials were low carbon structural steel plates microalloyed with vanadium and nitrogen (V–N) and niobium (Nb), respectively. The toughnesses of the HAZs displayed remarkably different behaviours as shown by their impact transition temperatures. For the V–N steel, the toughness improved with increasingly rapid cooling (low heat input conditions) whereas the Nb steel showed an opposite trend. Some of this behaviour could be explained by the presence of coarse ferrite grains in the slowly cooled V–N steel. However, other conditions where all the structures were bainitic and rather similar in optical micrographs gave widely different toughness values. The recently developed method of five dimensional boundary analysis based on electron backscattering diffraction has been applied to these cases for the first time. This showed that the lath boundaries in the bainite were predominantly on {1 1 0} planes of the ferrite and that the average spacing of these boundaries varied depending on steel composition and cooling rate. Since {1 1 0} is also the slip plane in ferrite, it is considered that close spacing between the lath boundaries inhibits general plasticity at stress concentrations and favours initiation of fracture. The differences between the two steels are believed to be due to their transformation behaviours on cooling where precipitation of vanadium nitride in austenite accelerates ferrite formation and raises the temperature of the phase transformation in V–N steels.

Introduction

Among the primary requirements of structural steels are good mechanical properties in connection with welding, notably toughness. In recent decades, great advances have been made through developments in steelmaking, with significant reductions in the contents of detrimental elements such as carbon, sulphur and phosphorus. At the same time, strength levels have increased so that resistance to fracture remains an important issue. The properties of the weld metal itself are possible to control through the use of consumables having suitably designed alloying additions but the heat affected zones adjacent to welds (HAZs) are often the most critical regions with regard to potential failures. Although the steel chemistry is not affected in the HAZ, exposure to very high temperature and variable cooling conditions give rise to microstructures that deviate widely from the ideal ones existing after controlled processing and are often undesirable with respect to toughness. When reviewing the situation of welding microalloyed steels, Hart [1] identified three main problem areas. These were (i) the existence of hard zones with the associated risk of hydrogen cracking, (ii) the toughness of coarse grained heat affected zones (CG HAZ) close to the fusion line, especially in the case of single pass welds and (iii) toughness of these CG HAZ when they have been exposed to reheating into the inter-critical temperature range (IC CG HAZ) in successive welding passes. The present work concentrates on the second of these, toughness of the CG HAZ and uses thermal treatments to simulate effects of welding heat.

A number of metallurgical factors are known to affect CG HAZ properties. Prior austenite grains close to the fusion line can become very large which may affect the toughness directly as well as through their effect on the nature and scale of transformation products in the microstructure after cooling. The cooling rate also controls the transformation products and this depends on the heat input that is used in relation to the thickness of the material being welded. Low heat input conditions lead to higher cooling rates, often specified in terms of the time for cooling through the transformation range from 800 to 500 °C, Δt8/5. Microalloying of steels with aluminium, vanadium, niobium or titanium generally has a favourable influence since carbide and nitride particles help to restrain austenite grain growth, especially TiN [2]. These elements also assist in binding up ‘free’ nitrogen which is known to have a deleterious influence on toughness [3]. Steel specifications are typically more tolerant with respect to nitrogen content in microalloyed than in plain steels, e.g., [4]. Nevertheless, different steel chemistries can lead to very different results of HAZ properties depending on the welding conditions and, in particular, the heat input parameter. Where possible, it is desirable to employ low heat input welding for economy, minimised distortion and a better working environment as, for example, in newer welding methods such as laser, laser-hybrid and pulsed arc processes [5], [6].

Very high heat input welding generally has a deleterious effect on HAZ toughness because the slow cooling rates, e.g., Δt8/5 > 100 s, give rise to coarse grained ferrite-pearlite microstructures. The situation with regard to low and medium heat inputs seems to be more complex, especially in the case of V-microalloyed steels where higher nitrogen contents are normally used to maximise the strengthening effect of vanadium [7]. Several reports [7], [8], [9], [10], [11] have shown that HAZ toughness of V-steels or V–Nb-steels with low nitrogen contents (N < ∼0.005%) is not very sensitive to heat input conditions in this range. However, with higher nitrogen levels (N > ∼0.008%) there appears to be a transition from inferior toughness for higher heat inputs to superior toughness for lower, with the transition occurring in the vicinity of Δt8/5  30 s. Work by Zajac et al. [7], [9] showed that the reduced toughness in the HAZ of V–N-microalloyed steels following slower cooling could be associated with coarse pro-eutectoid ferrite grains lining the prior austenite grain boundaries. For this reason, the fracture surfaces sometimes gave an impression of intergranular cleavage failure. However, the cause of high toughness in other situations could not be assigned. The main aim of the present work was to investigate this phenomenon in more detail, to extend the measurements to faster cooling conditions appropriate to some modern welding processes, and to use detailed metallographic examinations to better understand the relationships between HAZ microstructures and toughness.

Section snippets

Experimental procedure

The present work was carried out on two commercially manufactured plates of HSLA steels, one coded A with V–N microalloying and the other, D, containing Nb. Their chemical compositions are given in Table 1. Steel A was provided as 6 mm strip manufactured via a thin slab casting route while steel D was a 10 mm product from a conventional hot strip mill. In order to make the comparison as valid as possible, both steels were milled to 5 mm and this dimension was used in sub-size Charpy tests. Because

Results and discussion

Apart from their microalloy contents the steel chemistries are quite similar, with D having slightly higher carbon and manganese levels. As is normally the case, the steel D microalloyed with Nb has a low nitrogen content of 0.004% whereas the V-steel contains more nitrogen at 0.014%. Both steels are rather similar in strength in the as-received conditions so, as potential competitor materials, their response to welding makes an interesting comparison. Values of Vickers hardness after the

Conclusions

The two steels examined here are of comparable strength but respond very differently to heat treatments that simulate conditions in the coarse grained heated affected zones associated with welding. The main difference between the steels is that one (A) was microalloyed with vanadium and nitrogen whereas the other (D) contained niobium. With increasing cooling rate (or decreasing Δt8/5 from 40 to 2 s) the toughness of the V–N steel increases remarkably with a lowering of the impact transition

Acknowledgements

The authors thank Vanitec for financial support as well as the Australian Research Council for the work at Deakin University. Particular thanks go to David Milbourn and Peter Hodgson for their support and to colleagues Eva Lindh-Ulmgren, Karl Fahlström and Tadeusz Siwecki for advice and assistance. A part of this work was carried out with the support of the Deakin Advanced Characterisation Facility.

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