Doctor of Philosophy
Faculty of Engineering
Kuzmikova, L, An Investigation of the weldability of high hardness armour steels, Doctor of Philosophy thesis, Faculty of Engineering, University of Wollongong, 2013. https://ro.uow.edu.au/theses/3853
The main objective of this PhD research was to develop safe highly optimised welding procedures for high hardness armour (HHA) steel welded with austenitic stainless steel consumable. This involved optimisation of currently used flux-cored arc welding (FCAW) process, adjustment of the procedure for fully automated welding in combination with metal-cored consumable and finally assessment of feasibility of the advanced laser-gas metal arc welding (laser-GMAW) hybrid welding using solid consumable for potential future replacement of the current process. The aim was to significantly improve economics of welding fabrication while ensuring weldment with desired properties.
The alloy design of the HHA has been altered prior to commencement of this research adopting novel low manganese approach. To develop welding procedures with optimised parameters a deep understanding of parent material properties was crucial. Hence, the initial effort focused on detailed property and weldability characterisation of the modified HHA steel. This included comprehensive metallography investigation, hardness testing and also development of continuous cooling transformation diagrams directly applicable to welding.
The optimisation of preheat temperature was required as the industry recently adopted austenitic stainless steel consumable that replaced the conventional ferritic filler and because AS 1554 does not differentiate between austenitic and ferritic fillers the level of preheat employed in industry practice remained as per procedure developed for ferritic filler. Further, AS 1554 does not provide any guidelines for determination safe interpass temperature range which has a significant effect on productivity particularly for robot controlled welding thus upper interpass temperature threshold was also determined experimentally.
The main concern when welding high strength Q&T steels is hydrogen assisted cold cracking (HACC) and heat affected zone (HAZ) softening. Preheating is carried out to eliminate the risk of HACC therefore the process of determination of lower preheat temperature threshold considered all three factors contributing to this phenomenon.
The level of diffusible and residual hydrogen present in the HAZ and weld metal were measured employing collection of hydrogen over mercury and inner gas melt extraction methods for all three consumable types (flux-cored, metal-cored and solid). Magnitude of residual stresses resulting from different levels of preheat were evaluated by measuring the angular distortion. Susceptibility of HAZ microstructure formed under variety preheat conditions was assessed using hardness testing and colour etching technique to determine volume fraction of various microstructural constituents. The degree of HAZ softening resulting from different levels of interpass temperature was evaluated utilising colour etching and hardness testing.
Feasibility of laser-GMAW hybrid welding was investigated employing 3.5 kW diode laser coupled with GMAW torch in combination with solid austenitic stainless steel consumable. The procedure for typical joint of a land platform was developed significantly reducing the number of welding passes compared to weldments produced using conventional GMAW. The joint properties were assessed using the same techniques as employed for weldments produced with metal-cored arc welding (MCAW) and FCAW.
The objectives of this work have been achieved resulting in optimised welding procedures for HHA steel. Additionally, from the entire family of high strength Q&T steels HHA possesses a highest carbon content, carbon equivalent, hardenability and thus it is the most difficult to weld hence the findings of this research work can be applied to all high strength Q&T steels. Further, the proposed approach based on the difference in transformation temperature of WM and HAZ can be used as an effective tool for future assessment of hydrogen control in steel welding.
Findings of this research regarding optimised preheat and interpass temperatures for welding of high strength Q&T steels are being reviewed by DSTO and Australian Standard Committee to be incorporated into standard AS/NZS 1554.4 - Structural steel welding - Welding of high strength quenched and tempered steels and also relevant defence standard.