焊后多次正火对超细晶粒钢热影响区组织与硬度的影响

为研究400 MPa级超细晶粒钢焊接接头热影响区粗晶区的晶粒细化行为,对其空冷与水冷接头均进行了3次焊后正火热处理.组织观察表明:第1次正火后粗晶区晶粒明显细化,但细晶区和母材区的晶粒均比正火前的稍微粗大,第2次与第3次正火并不能进一步细化晶粒;冷却条件对热影响区组织的影响并不十分显著.显微硬度测试表明:第1次正火使接头硬度大幅度降低,甚至低于母材的原始硬度,后续正火对硬度的影响较小;在所有试样中,仅水冷接头第1次正火后的热影响区硬度与原始母材及母材区的硬度接近.由于焊后多次正火并不能使热影响区晶粒进一步细化,反而使其有粗化与软化的趋势,故应避免焊后多次正火处理.

Effects of Multiple Post Welding Normalizations on Microstructure and Hardness of Heat Affected Zone of Ultra-Fine Grained Steel

To investigate the refinement of the grains in the coarse grain heat affected zone(CGHAZ) of 400 MPa ultra-fine grained steel joints welded by gas tungsten arc welding(GTAW) with air cooling and water cooling,post welding normalization treatments(PWNT) are performed respectively for three times.The microstructure examinations show that the grains of CGHAZ are remarkably refined after the first PWNT,however the grains in the fine grain heat affected zone(FGHAZ) and base metal are slightly coarsened.The grains can not be further refined after the second and third normalizations.The cooling conditions after welding exert less effect on the microstructure in heat affected zone(HAZ).The results of micro-hardness test indicate that the joints hardness decreases greatly after the first PWNT,so that the whole joints hardness is below the original hardness of base metal.Further PWNTs have less effect on the hardness of HAZ.Among all specimens,after the first PWNT only the HAZ hardness of the joints with water cooling is equivalent to the hardness of the original base metal.Multi-normalization post welding ought to be avoided since the grains in the HAZ can not be refined further,on the contrary,coarsening and softening may arise.