The effects of major alloying elements on the Ac, temperature in P92 steel weld metal were evaluated by Thermo- Calc, and a formula relating Ac1 to the content of major alloying elements was developed using multiple regression method. Results show that both C and N reduce Ac, temperature in weld metal, the effect of N on Ac, is greater than that of C, but their influence on Act is not so significant when they individually vary in the specified chemical composition ranges. Si, Cr, Mo and W increase the Acl temperature, and the descending order of their effects is determined as Si, Mo, W, Cr. Mn and Ni decrease the AcI in weld metal, the decreasing effect is especially remarkable when the （ Mn ＋ Ni） ≥ 1%. The effect of Co is moderate and is much smaller compared to Ni. The variations of Acl for the ranges of micro-alloying elements Nb and V are also evaluated, the effect of V is greater compared to Nb and the effect of Nb could be ignored in the specified chemical composition range. A prediction formula for Ac, temperature in P92 steel weld metal within the specified chemical composition ranges was developed based on the comprehensive consideration of the calculated Ac, temperatures and the experimentally measured results.
The effects of the addition of O - 1.2wt. % Ni and 0 - 2wt. % Co in F92 steel weld metal on its A L temperature was analyzed using computational thermodynamic software （ Thermo-CalcTM ） in this paper. The results indicate that both Ni and Co decrease the A, of P92 .steel weld metal, and each elements has a pronounced linear relationship with the At. The il^fluence of Ni on A,I is much larger than that of Co. In order to evaluate effects of Co and Ni in combination on AI of P92 .steel weld metal, an equation Ni = Ni ＋ O. 096Co was developed. According to the formula, the decreaing effect of Co on A., is jun about 1 / 10 of that of Ni. For welding consumables of P92 steel, the .substitution of Co for Ni （part or whole） could be an appropriate way to avoid the strong reduction of A and improve the properties of weld metal.
