Microstructure and mechanical properties of titanium-containing steel weld metal with varying molybdenum and nickel levels before and after thermal cycling

Abstract

The purpose of this research was to investigate the effects of Ni and Mo on the mechanical properties of Ti-containing weld metal. Alloyed weld metals of varying chemical compositions were produced with a unique weld-alloying technique developed for the purposes of this research. The weld-alloying technique was used to make controlled changes to the chemical composition of commercially available high-strength low-alloy (HSLA) steel weld metal. The technique involves the deposition of beads of alloying elements on steel plates by means of laser metal deposition (LMD), followed by the deposition of commercially available welding wire. This technique was used to produce nine Ti-containing welds with varying weld metal compositions with regard to Ni and Mo. The responses of Ni and Mo in the presence of Ti have not previously been investigated. These welds were produced by depositing LMD beads of pure Ni, Mo, and Ti on low-carbon steel plates. Submerged arc welds (SAWs) were deposited over the LMD beads to produce alloyed welds. Ti was added because it is increasingly being used in HSLA-steel weld wires to promote acicular ferrite formation. Ti was maintained at 0.019 mass%–0.030 mass%; the Ni content was varied between 0.01 mass%–2.44 mass% whilst the Mo content was varied between 0.01 mass%–0.03 %. When welds with the same intended alloy content were produced using this procedure, the alloy content of those welds, in the best case, varied within a range of 12 mass% (expressed as a percentage of the intended alloy content). This is comparable with ranges of alloy content measured in welds made with conventionally alloyed welding wire. However, this range increased to as high as 50 mass% when the intended alloying content decreased. This indicates that there is a lower limit of alloying element content below which this technique becomes unstable. The weld-alloying technique requires refinement, but shows promise as a method for altering submerged arc weld metals composition for research purposes. Results from the literature indicated that multi-pass welds may have different mechanical properties to comparable single-pass welds. This indicated that it may be necessary to investigate the response of the nine welds to thermal cycling with regard to both mechanical properties and microstructure. To do this, the as-welded samples were exposed to thermal cycling, which was intended to simulate the intercritically reheated (IR) heat-affected zone (HAZ), the fine-grained (FG) HAZ and the coarse-grained (CG) HAZ in the weld metal of each weld. The peak temperature required for the thermal cycling of each weld was determined by measuring the individual AC1 (as the lowest temperature at which austenite can form upon heating) and AC3 (the temperature at which all metal has transformed to austenite upon heating) temperatures of each weld metal and accordingly calculating the requisite peak temperature. This is an improvement on conventional thermal cycling, which usually uses the same peak temperature for all weld compositions studied. Simulation of the HAZ was carried out by exposing the welds to thermal cycles using a Gleeble dynamic thermomechanical testing instrument. Mechanical testing and microscopy were performed on welds in the as-welded condition and on each of the simulated HAZ regions. Impact testing was done using half-size Charpy impact test coupons at a testing temperature of −40°C. Hardness testing was done by done by using a Vickers hardness tester and a mass of 10 kg. Microscopy included optical and scanning electron microscopy on mounted samples and fractography of Charpy impact test fracture surfaces using a low-voltage high-resolution scanning electron microscope (SEM). Such equipment has not been previously used to study the microstructure of HSLA-steel weld metal. Hardness results indicated that the addition of both Ni and Mo increased the hardness of welds and that there was an interaction between the two elements. Multivariant linear regression analysis on the effects of Ni content, Mo content, and the product of the Ni and Mo contents was used to predict the hardness of individual welds with an accuracy of 82 mass%. The average hardness of the welds was 240 HV10 with a range of 208 HV10 to 271 HV10. The impact toughness of welds in the as-welded condition showed that, individually, neither Ni nor Mo had a strong effect on impact energy. Of the nine weld metal compositions tested, two had a significantly lower impact energy than the rest. The impact energies of the two outliers were 29 J and 31 J. These welds contained 0.12 mass% Mo, 2.34 mass% Ni and 0.23 mass% Mo, 2.44 mass% Ni, respectively. The average impact energy of the remaining seven welds was 43 J, with a range of 39–49 J. Quantitative microscopy showed that increases in both Ni and Mo led to a reduction in the fraction of grain-boundary products. The equation derived to predict grain boundary product content of welds was 86 % accurate when the Mo content, Ni content, and product of the Mo and Ni contents were used as independent variables. No other trends with regard to the influence of chemical composition on microstructure were detected. An etch effect was observed during optical microscopy, which indicated that alloying elements segregated to the prior austenite grain boundaries of the two Mo- containing high-Ni welds. SEM and energy-dispersive X-ray spectroscopy confirmed that the etch effect was a result of segregation of Ni to prior austenite grain boundaries. SEM revealed that regions of coalesced bainite had formed on and directly adjacent to the PAGBs of the Mo-containing high-Ni welds in which Ni and Mn segregation had been confirmed. Fractography of these samples revealed that fracture occurred along the regions of coalesced bainite. The low impact energy values of the Mo-containing high-Ni welds were attributed to these networks of coalesced bainite. Segregation of these elements is believed to have contributed to the formation of coalesced bainite. Segregation is attributed to the fact that a significant amount of the weld metal solidified as austenite. The results of thermal cycling indicated that reheating of the weld metal did not have a strong effect on hardness; however, the impact energies of welds that contained Mo appeared to be sensitive to thermal cycling. With the exception of one weld, the largest decrease in impact energy seen across the three thermally cycled samples from each weld exceeded 10 J. The exception was a low-Mo high-Ni weld that did not experience any significant reduction in impact energy after thermal cycling. The results of this study clearly show that thermal cycling associated with multi-pass submerged arc welding may significantly affect the impact energy as measured in single-pass welds of Ti-containing HSLA steel. Hence, single-pass welds should preferably not be used in isolation to evaluate the influence of chemical composition on impact energy of SAWs. It is recommended that future studies on the influence of chemical composition on HSLA-steel welds include an evaluation of both as-welded and thermally cycled weld metal. Thermal cycling by means of Gleeble simulation increases the likelihood of detecting brittle zones that may be more difficult to detect in actual multi-pass SAWs. Although the impact energy of thermally cycled weld metal may prove useful in detecting brittle HAZ regions, it cannot be used to estimate the impact behaviour of multi-pass welds and should be used in tandem with actual multi-pass welding to accurately evaluate the influence of varying chemical composition on the impact energy of such welds. Lastly, the results indicate that the presence of Ti in the weld metal did not affect the way in which the weld metal reacted to changes in Ni and Mo levels. In the as-welded state, both Ni and Mo increase the strength without significantly affecting impact energy. An interaction between Ni and Mo was noted, but its nature could not be identified. When weld metal was thermally cycled, the presence of Mo made it susceptible to mechanisms that reduced the impact energy of the welds.