Effects of heat treatment process and niobium addition
on the microstructure and mechanical properties
of low carbon steel weld metal
Shanping Lu•Shitong Wei•Dianzhong Li•
Yiyi Li
Received: 16 April 2009/Accepted: 2 January 2010/Published online: 20 January 2010
? Springer Science+Business Media, LLC 2010
Abstract
The mechanical properties and microstructure
were evaluated and analyzed by optical microscopy (OM)
and transmission electron microscopy (TEM) for micro-
alloy carbon steel weld metal with and without Nb addi-
tion, respectively, under different heat treatment processes
including stress relief annealing, normalizing, and no
treatment after welding. The strength and elongation of the
weld metal without treatment after welding were improved
with the addition of Nb element, and the impact toughness
was not affected obviously with the Nb addition. After
stress relief annealing, the strength decreased for the
Nb-free weld metal, while the elongation and impact
toughness increased. However, for the Nb-bearing weld
metal, stress relief annealing improved the strength of the
weld metal significantly, and deteriorated the elongation
and impact toughness. In the case of normalizing treat-
ment to the weld metal, it was shown that with the
increase of the holding time at the normalizing tempera-
ture of 920 ?C, for both the weld metals with and without
Nb addition, the microstructure of the columnar grain zone
(CGZ) was transformed from one of columnar grain into
one of equiaxed grain. The grain size of the equiaxed grain
zone (EGZ) increased initially, then remained almost
unchanged with the prolonging of the holding time. The
mechanical properties of the weld metal with and without
Nb addition showed no obvious change with the increas-
ing holding time. With the increase of the normalizing
temperature, the strength of the Nb-bearing weld metal
increased, while the elongation and impact toughness
decreased significantly. OM and TEM analysis found that
the fine NbC particles were precipitated at the normalizing
temperature of 920 ?C, which refined the grains of the
weld metal and increased the impact toughness. With the
increase of the normalizing temperature, the content of
widmanstatten ferrite (WF) in the Nb-bearing weld metal
increased, whereas the quantity of the NbC particles
decreased, which improved the strength and lowered the
impact toughness.
这不是整篇，只是第一段