Abstract
800 MPa high strength steel with a low carbon equivalent has been developed by means of refining ferrite grains. In the welded joints of the 800 MPa steel with ultra-fine ferrite grains (less than 1 μm) and dispersed fine cementite particles, fine ferrite grains coarsen due to the welding heat input, and then remarkable softening occurs in the heat-affected zone.
For the ultra-fine grained steel (UFG steel), ultra-narrow gap gas metal arc (UNG) welding was utilized as a low heat input welding process. By the UNG welding process with the cooling time from 1073 K to 773 K of about 4.5 sec, the width of the softened region in the heat-affected zone (HAZ) of the ultra-fine grained steel can be minimized to below 2 mm.
In order to clarify the softening behavior in the HAZ of UFG steel, the effect of welding thermal cycle on the microstructures of UFG steel was discussed on both the HAZ of the welded joint of UNG welding and its synthetic HAZ, and the results were also compared with the HAZ of SM490 steel.
The peak temperature at each point of HAZ was estimated from thermal conduction theory and the measured data. Slightly coarsened ultra-fine ferrites (fine grain A) and larger polygonal ferrites (fine grain B) can be observed in the regions of the peak temperatures from 920 K to Ac3 (1150 K). It was found that the softening in the HAZ of UFG steel occurs within the peak temperature range of about 920 K (below Ac1=980 K) to 1300 K, and the softening in the regions of the peak temperatures from 920 K to 990 K is dependent on the grain size of the larger polygonal ferrite grains (fine grain B).
M-A constituents begin to form when the peak temperature exceeds Ac1, and its volume fraction increases with the peak temperature. As second hard phase, M-A constituents restrain the softening of the HAZ in the peak temperature range from about 1000 K to 1250 K.
It is known from the results that the hardness in the HAZ has the lowest value at the peak temperature of 1100 K.
