Microstructure And Properties Of 30MnCrNiMoB Low Alloy Ultra High Strength Steel Welded Joints

Some weldabitity test as maximum hardness test and inclined Y groove crack resistance test on 30MnCrNiMoB of the ultra high-strength low-alloy steel, after these test,30MnCrNiMoB were welded with 11kJ/cm with three different solder wire by pulse gas metal arc welding. Mechanical properties (tensile strength, inpact toughness) and microstructures observation by optical microcopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), find the best combination property in three types solder wire, and welded with 7kJ/cm? 11kJ/cm and 15 kJ/cm with the best property wire by pulse GMAW, and studied on the microstructure and mechanical properties of the Welds and Heat affected zone of the three types welded joints, finally, obtain the best welding consumables and welding procedure.In the maximum hardness test, with the heat inputs increasing from 7kJ/cm, 11kJ/cm to 15kJ/cm, the maximum hardness of HAZ is reduced, but the soften zone of steel increase rapidly, and with the preheat temperature in creasing from 23?, 60? to 100?, the hardening tendency of HAZ also reduced, the width of soften zone increased limited; in the inclined Y groove crack resistance test, T3 wire is better than DM4 of the ability of blocking the delayed crack,although at the high temperature and humidity environment which contain the high moisture. Besides, when the heat input reach the 10kJ/cm, the DM4 wire also emerge the good ability of blocking the delayed crack. The macroscopic observation to the crack, these cracks are beginning in the CGHAZ, the crack arrest when the crack expand into the Weld, it shown that the capacity of delay crack resistance on both wire are better than the base metal. Base on the above results, the welding of the 30MnCrNiMoB steel with T3 or DM4 wire not required the preheating, only insure the heat input pass 10kJ/cm when the DM4 wire used.In the contrast of the three types weld joints used different wire, the Weld which used T3 wire contained the higher content of C, Ni, Mo and so on, showed a higher tensile strength, and ensure the inpact toughness at low remperature. The microstructure of three types Weld is martensite, have carbide separate out between lath of T3 and DM4 welding joint, and dispersion carbide separate out on the lath of T3 Weld. Use the SEM and EDX on the inpact fracture of three type weld joint, the morphology of these fracture roughly the same, the morphology of the beginning of fracture shown quasi-cleavage fracture, the morphology of wxpansion area shown dimple fracture, the inclusion of DM4 and GHS80 is primarily Ti2O3-Al2O3-MnS, T3 is mainly MnS.Use T3 wire weld in three kinds of heat inputs (7 kJ/cm,11 kJ/cm and 15 kJ/cm), with the heat inputs insreasing, the lath of martensite becoming coarsing, the intensify elements of weld metal has burning loss, the tensile strength of weld metal and weld joint to bring down with the heat inputs increasing. With the heat inputs increasing, the hardness of weld metal from 443Hv in average to 369Hv, beacause the cooling time extend so that the hardening tend to be lower. The grain size of HAZ grown up, meanwhile the block of carbide decrease, hardening tendency of HAZ reduce, the soften zone of weld joint increased with the heat inputs increasing dramatically.