Nya Svetsaren 1, 1999. - Luleå University of Technology

Consumables and weldingmodified 9 Cr-1 Mo steelby Eva-Lena Bergquist, Esab AB, Göteborg, SwedenTo meet the demands ofthe power generatingindustry with its aim ofincreasing steam temperaturesand pressures, thedevelopment of morecreep-resistant ferriticsteels started in the mid-1970s. Ferritic steels werepreferred to austeniticsteels because of theirlower coefficient of thermalexpansion and theirhigher resistance to thermalshock (1). Of thesesteels, the modified 9%Cr1%Mo steel known asP91/T91 or, in Europe, asX10CrMo VNb9-1, hasbeen used in a wide rangeof industrial applications.As Table 1 shows, theroom-temperature strengthof the modified variant issuperior to that of theconventional variant.In addition to improved roomtemperaturetensile strength, themodified variant also has increasedcreep strength, lowerductile-to-brittle transition temperaturesand higher upper-shelfenergy. The improved mechanicalproperties are explained by smalldifferences in the chemical compositionconsisting of controlledamounts of Nb, N and V in theP91 steels, see Table 2.P91 is primarily used in normalisedand tempered condition.During heat treatment, a fine dispersionof Nb(C, N) and M23C6is precipitated. Through the mechanismof precipitation strengthening,this gives rise to the enhancedmechanical properties.Steel grade R p (MPa) R m (MPa) A 5 (%)conventional9Cr-1Mo (P9) min 205 min 415 min 30X10CrMoVNb9-1 min 415 min 585 min 20(P91)Table 1. Mechanical properties of modified and conventional 9Cr-1Mo steel.Steel grade C Si Mn P S Cr Mo Ni Nb V Al NP9 max 0.25- 0.30- max max 8.0- 0.9-0.15 1.00 0.60 0.025 0.025 10.0 1.10P91 0.08- 0.20- 0.30- max max 8.0- 0.85- max 0.06- 0.18- max 0.03-0.12 0.50 0.60 0.02 0.01 9.5 1.05 0.40 0.10 0.25 0.04 0.07Table 2. The specified chemical composition of conventional 9Cr-1Mo (denotedP9) compared with the composition of P91 (X10CrMoVNb9-1). (wt%).P91 is generally used in steampiping, headers and super heaterpiping where the advantages overconventional CrMo steels can beused either for weight reductionsor to permit an increase in operatingtemperature. Arav and vanWortel (2) made a comparison ofthe pipe-wall thickness requiredfor different creep-resistant materialsin certain operating conditionsand demonstrated that the wallthickness can be reduced by threequartersif P91 is chosen instead of2 1/4Cr1Mo steel (Figure 1).Welding is one of the most essentialfabrication processes forcomponent manufacture. Theweldability of P91 steels is verygood. Due to the high alloyingcontent, a relatively high preheatingtemperature (200-350°C)must be used. The exact choice ofpreheating temperature also dependson the material thicknessand the amount of restraint. Alow hydrogen content is naturallynecessary and only the basic typeof consumables should thereforebe used.2 1 / 4 Cr1MoX20347HP91Figure 1. Comparison of the wall thicknessrequired for different creep-resistantmaterials. Assumed operating conditions250 bar and 600°C. Note the largereduction in wall thickness which ispossible when P91 steel is chosen.From Arav and van Wortel (2).When designing a welded component,it is important to considernot only the mechanical propertiesof the base material but inparticular the strength and propertiesof the weldments. For componentsdesigned for high-temperatureservice, the creep propertiesmust be regarded as central.22 Svetsaren No.1–2 1999