Investigation of HAZ Softening in Laser Welding of AHS/High Hardness Steels

Advanced high strength (AHS) steels have been produced by rapid thermal processing (Flash Processing) of low alloy steels.  A common weldability problem with this steel and other AHS steels is related to the softening in the heat affected zone (HAZ).  In this research, we investigated the weldability of flash processed AISI 4130 steel.  The study includes dilatometry, hardness mapping, light optical and electron microscopy,  gas metal arc welding, laser welding, and simulated HAZ thermal cycles.  To provide a reference, similar weldabiltiy analyses was performed in high hard (HH) armor steel.  Initial GMAW results showed extensive softening of the HAZ in the FP 4130 steels, while HAZ of HH steel showed both softening and hardening.  The above phenomenon was evaluated by performing HAZ simulations in a Gleeble 3800.  The results indicated that hardness of these samples depended on peak temperature, chemical composition, and prior austenite grain size.  The most significant softening occurred on heating the samples to a peak temperature in between the A₁ and A₃ temperatures.  The data indicated a rapid thermal cycle would reduce the softening in the HAZ.  To validate this hypothesis, [low heat input, high speed] laser welding was performed with a 15kW Ytterbium Fiber Laser with a 333-µm spot size.  The welds were made [bead on plate and zero gap]on both FP4130 and HH steels.  Microhardness testing was performed to evaluate the hardness throughout the samples and compare it to other welding processes.  The extent of softening in both steels was reduced significantly.  Practical application of such high-speed fiber-laser welding for AHSS will be presented.