Toughness Of Simulated Heat-affected Zone Research Articles

Microstructure and Toughness of Simulated Heat-Affected Zone of Laser Welded Joint for 960 MPa Grade High Strength Steel

The microstructure and toughness of coarse grain zone (CGZ) and mixed grain zone (MGZ) for laser welded 960 MPa grade high strength steel joints were investigated by thermal simulation with a Gleeble-3500 thermal simulator. The results show that microstructure of the stimulated CGZ mainly consists of uniform interweaved lath martensite, and grain growth is not severe upon increasing the cooling time (t 8/5). Microstructure of the stimulated MGZ presents strip-like in low peak temperature, and small block martensite is formed on the grain boundary. However, in high peak temperature, the strip-like microstructure disappears and small block martensite presents net-like structure. The lath character for MGZ and CGZ is very obvious under TEM observation, and the average lath thickness of BM, MGZ, and CGZ is 100, 150 and 200 nm, respectively. The impact energy and microhardness of CGZ are higher than MGZ and reduce with increasing the cooling time. The fracture toughness deteriorating drastically for MGZ may be related with the formation of the mixture microstructure, in which the small block martensite is distributed in the shape of a network.

Effect of weld thermal cycle on microstructure and fracture toughness of simulated heat-affected zone for a 800 MPa grade high strength low alloy steel

In the present investigation, thermal simulated specimens were used to investigate the effect of welding cooling time and peak temperature on characteristic fracture toughness and microstructure feature of heat-affected zone (HAZ) for an 800 MPa grade high strength low alloy (HSLA) steel. It is found that the fracture toughness is the best for the simulated coarse-grained HAZ, when the cooling time of t 8/5 is 18 s. In addition, the size of prior austenite grain, and the volume fraction of bainitic ferrite and M/A constituent increase with increasing the cooling time. However, the volume fraction of martensite decreases with increasing the cooling time. Remarkable decrease of toughness is observed with increasing the size of austenite grain and the volume fraction of M/A constituent. Moreover, there exists the effect of orientation on fracture toughness for the specimens subjected to weld thermal cycle. Generally, the fracture toughness of simulated HAZ with L–T orientation is higher than that with T–L orientation. The reason may be related to that the strip structure formed during rolling is remained after the thermal simulation. Furthermore, the investigation shows that the toughness of coarse-grained zone is higher than that of fine-grained zone for the simulated HAZ. The reason may be related with the microstructure evolution of the HAZ during the complete thermal cycle used in the simulation. For the fine-grained HAZ, the shorter cooling time of t 8/5 may be not benefit for the self-tempering and decomposition of M/A constituents, then the toughness of the fine-grained HAZ is lower. For the coarse-grained HAZ, however, the longer cooling time of t 8/5, may promote the decomposition of M/A constituents, then the toughness of coarse-grained HAZ is improved. The fracture toughness deteriorated drastically for the partly phase transformed HAZ may be related with the formation of mixture microstructure, in which the M/A constituent is distributed in shape of network.

Effect of sulfur content on the microstructure and toughness of simulated Heat-Affected zone in Ti-Killed steels

Four Ti-killed steels were made to study the specific influence of sulfur on the inclusion, microstructure, and toughness of a simulated heat-affected zone (HAZ). The HAZ toughness was mainly determined by the volume fraction of intragranular acicular ferrite (IAF) which was closely related to the supercooling required to initiate austenite to ferrite transformation. The extent of supercooling was strongly influenced by the composition of grain boundary and inclusions. Sulfur addition up to 102 ppm caused a segregation of sulfur to the grain boundaries and a change of inclusion phase from predominantly Ti-oxides to Ti-oxysulphides and MnS. This behavior, in turn, suppressed the formation of polygonal ferrite and promoted the formation of IAF. Further addition of sulfur elevated transformation temperature and promoted the formation of polygonal ferrite due to the refinement of grain size and the increase of grain boundary associated inclusions. A methodology was proposed to evaluate the intragranular nucleation potential of inclusions, and the results showed that Ti-oxysulphides possessed better nucleation potential for IAF than Ti-oxides and MnS. With the lowest transformation temperature and most effective nuclei, the best HAZ toughness can be obtained at sulfur content of 102 ppm due to the achievement of the maximum volume fraction of IAF.