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The Research of Hot Working Technology of403Steel

Author: WangLiangLong
Tutor: JinMan
School: Shanghai University
Course: Materials Processing Engineering
Keywords: 403steel δ-ferrite hot deformation thermal refining strength andtoughness
CLC: TG142.41
Type: Master's thesis
Year: 2013
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Abstract

403steel is used as the material of hold-down spring in pressurized heavy waterreactors. The strength and toughness of403steel are considered as two of mostimportant mechanical properties remained to be achieved.In order to well match its strength and toughness, the effect of primary alloyelements on transformation temperature of δ-ferrite, hot deformation and Thermalrefining are three pieces of basic work in this paper, by which the evolution ofmicrostructures of403steel that determines its final properties in hot workingprocess was investigated.First, the effect of primary alloy elements on transformation temperature ofδ-ferrite was simulated by a phase diagram software Jmat-Pro. The equation abouttransformation temperature of δ-ferrite and the amount of alloy elements was obtainedby using Matlab to do multiple linear regression:T_δ=1753+1125[C]-45[Si]+34[Mn]-59[Cr]+75[Ni]Furthermore, two kinds of403steel were selected to test the accuracy of equationabove. It shows that δ-ferrite was very sensitive to elevated temperature and theamount or size of this microstructure will rise sharply even after a short stay in thistemperature.Forging is a key process in the shaping of hold-down spring. The hotdeformation of403（modified） steel was carried on Gleeble-3180stimulator, bywhich the flow stress curves were plotted. It shows at a certain strain dynamicrecrystallization is likely to occur with a peak strain shifted to left as temperaturerises or strain rate decreases. A low temperature, a big strain rate or a big strain willcontribute to developing a small size of dynamic recrystallization grain. Under thesame strain, dynamic recrystallization grain size D is related with Z parameter:D=260.08×Z-0.1096, Z=·exp（380150/RT）. The activation energy of dynamicrecrystallization of403steel is Qd=380.15kJ/mol and two kinds of constitutive equations were obtained:（?）=4.65×10¹²[sinh（0.0153σ_p）]^3.33exp（-380150/RT）σ_p=1/0.0153ln[（Z/A）^1/3.33+(（Z/A）^2/3.33+1)^1/2]An optimized hot working technology was recommended, that is, deformedbetween1050℃-1190℃with a strain rate below0.1s-1to ensure a high powerdissipation efficiency, a stable metal flow, uniform DRX grain size as well as a smallamount of δ-ferrite during the hot deformation.Thermal refining plays a vital role in final properties of hold-down spring. Thestrength of403steel rises gradually to an extent as the quenching temperature rises.The amount of lateral expansion also rises until1010℃then it falls. As the soakingtime rises, the strength or the amount of lateral expansion basically shows a trend ofincreasing but not monotonically. What’s more, the soaking time can help decreasethe amount or size of δ-ferrite apparently. As the tempering temperature rises, thestrength of403steel decreases sharply but lateral expansion shows an absolutelyopposite trend. After a series of test, an optimized thermal refining was selected:soaking at990℃for7h then cools down in oil+tempering at650℃for4h thencools down in oil. Finally the strength and lateral expansion of403steel wererespectively achieved to775MPa and1.35mm.

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