δ-ferrite transformation mechanism and its effect on mechanical properties of 316H weld metal

The microstructure evolution and mechanical properties of the 316 H stainless steel weld metals with different C contents were studied at the aging temperature of 550℃ for different aging holding time.The transformation behavior of δ-ferrite and precipitation mechanisms of M(23)C6 and σ phase in the as-aged weld metal were investigated.The results indicated that for the as-welded weld metal,with increasing C content,the yield and tensile strengths increased,while the elongation decreased owing to the increase of C solid solution strengthening effect.Moreover,both the high δ-ferrite content in low C weld metal and the precipitated M(23)C6 carbide in high C weld metal deteriorated the impact energy obviously.During the aging process,the rapid precipitation of M(23)C6 carbide occurred in σ-ferrite firstly owing to the high diffusion rate of C.Once the carbon is depleted by precipitation of M(23)C6,the slow formation of σ phase occurred through eutectoid transformation(δ→σ+γ) depending on the diffusion of Cr and Mo.Moreover,increasing C content promoted the formation of M(23)C6 carbides and inhibited the formation of σ phase.Therefore,increasing C content accelerated the transformation of σ ferrite in weld metal during aging process.Furthermore,after a long enough aging time,a transformation from M(23)C6 to σ occurred.The variations of mechanical properties with aging conditions depended to a large extent on the microstructures at different aging conditions.For the low C weld metal aged at 550℃,with the increase of the aging time,fine M(23)C6 first precipitated,then coarsened,after that σ phase formed,which caused that the yield and tensile strengths first increased,then decreased,and finally increased slightly again.For the medium C weld metal,as the aging time increased,first the depletion of the solid solution C as a result the M(23)C6 precipitation deteriorated the strength,and then the formation of σ phase improved the strength.For the high C weld metal,with the increase of the aging time,the depletion of the solid solution C and the coarsening of the M(23)C6 precipitates deteriorated the strength.Furthermore,with increasing aging time,both the precipitation and coarsening of M(23)C6 and increasing σ phase content deteriorated the elongation and impact ene rgy.

Effect of Carbon Content on the Creep Rupture Properties and Microstructure of 316H Weld Metals

Two types of 316 butt welds with carbon contents of 0.016%and 0.062%have been produced using the gas tungsten arc welding process.Theδ-ferrite content decreased from 7.2 to 2.8%in volume as the carbon content increased.The creeprupture strength and creep ductility of the two types of weld metals have been measured at 550℃over the stress range of 290-316 MPa and at 600℃over 230-265 MPa.The microstructure change and precipitation behavior of the weld metals were observed and related to the creep rupture properties.The creep rupture strength of the C2(0.062%C)weld metal was higher than that of the Cl(0.016%C)weld metal at both 550℃and 600℃.At 550℃,as the decrease in the applied stress,the difference of the creep-rupture life between the two weld metals diminished due to the higher depletion rate of carbon by precipitation of M_(23)C_(6) in the C2 weld metal,while at 600℃,the difference enlarged due to the massive precipitation ofσphase and extensive crack formation and propagation alongσ/austenite boundaries in the C1 weld metal.For both the C1 and C2 weld metal,the decrease in ductility was adverse with the transformation percentage and related to products of theδ-ferrite transformation.