Doctor of Philosophy
Department of Materials Engineering
Chen, Liang, Characterisation of transverse cold cracking in weld metal of a high strength quenched and tempered steel, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 2000. http://ro.uow.edu.au/theses/1485
Hydrogen induced cold cracking in weldments is the most serious problem limiting the use of high strength low alloy steel in the structural industry. Improvement of the weldability of structural steel has significantly reduced the risk of cold cracking in the heat affected zone. Consequently, the avoidance of weld metal cold cracking becomes a major task since the risk of hydrogen induced cold cracking in the HAZ is greatly diminished in modem low carbon steels. Hydrogen induced cold cracking in weld metal is caused by the complex interaction of the diffusible hydrogen content, residual stress and susceptible microstructure. Depending on the joint geometry which induces stresses, cold cracking in weld metal can be of two types with respect to the welding direction: longitudinal cracking which is parallel to the welding direction, and transverse cracking which is perpendicular to the welding direction. Weld deposits for high strength low alloy steels can be susceptible to transverse cold cracking.
The investigation described in this thesis involved the use of the gapped bead-on-plate (G-BOP) test to examine the effect of shielding gas mixture and preheat temperature on weld metal transverse cold cracking behaviour in a flux cored arc welded BIS812EMA quenched and tempered steel. The minimum yield stress of BIS812EMA steel 1s 690MPa. The consumable used in welding was a low hydrogen flux cored wire.
Literature relevant to the characteristics of weld metal cold cracking has been reviewed, with emphasis on the factors affecting hydrogen induced cold cracking. The process of flux cored arc welding, the weld thermal cycle and the properties of weld metal have been examined. The main types of cold cracking and relevant weldability tests have been discussed.
Two different Ar-C02 shielding gas mixtures (Ar-5%C02 and Ar-20%C02) and five different preheat temperatures (20°C, 40°C, 60°C, 80°C and 100°C) were used at an aim heat input of 1. 7 kJ/mm to investigate the effect of welding conditions on the transverse cold cracking behaviour of the weld metal deposited on BIS812EMA steel. The microstructures, hardness values and fracture surfaces of the weld metals, have been examined in detail using optical and electron microscopy and microhardness testing. The characteristics of non-metallic inclusions in the weld metals have been determined using scanning electron microscopy and automatic image analysis. The continuous cooling transformation diagrams of the base metal and the weld metals have been developed using dilatometry and microstructural observation. Additionally, precipitation hardening by alloy carbides has been studied by tempering of weld metals.
It was found that, below a critical preheat temperature, weld metal transverse cracks developed in the self restrained G-BOP test at least five minutes after welding and initiated below 150°C, confirming that the weld metal transverse cracking was cold cracking.
Shielding with Ar-20%C02 mixture resulted in significantly lower susceptibility to transverse cold cracking than shielding with Ar-5%C02 mixture. However, the microstructures of the weld metals deposited with the two different Ar-C02 shielding gas mixtures were similar and mainly consisted of acicular ferrite and bainitic ferrite. Comparing the two weld metals, the higher oxygen potential of the Ar-20%C02 shielding gas mixture caused reduction of hardenability elements such as carbon, boron, manganese and silicon in the weld metal; and the higher oxygen absorption during welding resulted in a higher non-metallic inclusion volume fraction and a slightly larger mean particle size and number of particles per unit volume. These factors led to phase transformation during cooling at higher temperature in weld metal deposited with Ar-20%C02, a slightly coarser microstructure with more bainitic ferrite, and a significantly lower hardness than the weld metal deposited with Ar-5%C02. In addition, the weld metal deposited with Ar-20%C02 shielding gas mixture had a lower diffusible hydrogen level and less nitrogen in solid solution. These characteristics combined with the lower hardness value of the weld metal are considered to be the main factors contributing to the lower susceptibility to transverse cold cracking in the weld metal deposited with Ar-20%C02 compared with Ar-5%C02 under the same welding conditions.
Three types of fracture morphologies were found to be associated with the weld metal transverse cold cracking: microvoid coalescence (MVC) fracture, quasi-cleavage (QC) fracture and intergranular (IG) fracture. IG was more prevalent in the weld metal deposited with Ar-5%C02 shielding gas mixture while MVC dominated in the weld metal deposited with Ar-20%C02. These trends are consistent with the difference in weld metal hardness (strength).
The susceptibility to weld metal transverse cold cracking, based on the G-BOP test, decreased with increasing preheat temperature for both shielding gas mixtures. No cracking was observed for the weld metal deposited with Ar-20%C02 shielding gas mixture for a preheat of 40°C, but a preheat of 100°C was required for crack-free welding using Ar-5%C02 shielding gas mixture. However, it is concluded that the finer microstructure of the weld metal deposited with Ar-5%C02 is capable of both higher strength and lower cold cracking susceptibility than the weld metal deposited with Ar- 20%C02 provided the hardnesses are similar and below a critical level of about 290 HV.