δ -ferrite in 9Cr-1Mo weld metal
This paper by Sibusiso Mahlalela and Pieter Pistorius from the University of Pretoria’s SAIW Centre for Welding Engineering in the Department of Materials Science and Metallurgical Engineering in South Africa, outlines an SAIW-funded investigation into the welding of 9Cr- 1Mo (P91) steels using different electrodes to determine how to prevent retained δ -ferrite content forming in the weld metal. An investigation of δ -ferrite content in weld metal of modified 9Cr-1Mo electrodes using thermodynamic modelling and quenching experiments
D uring fabrication of modified 9Cr-1Mo steels, δ -ferrite formed in the weld metal and heat-affected zone may not transform completely to austenite during subsequent cooling. The influence of changes in weld metal composition on δ -ferrite content was investigated using weld pads produced using basic coated P91 electrodes from four different manufacturers. The weld pads were designated Electrodes 1, 2, 3, and 4 accord- ing to the different manufacturers. Electrodes 1 and 3 fully complied with the EN ISO 3580-A CrMo91 specification. Electrode 2 had very low nickel and high niobium contents, and Electrode 4 had carbon content above the maximum allowable limit. Thermo-Calc results showed that the temperature range over which a mixture of δ -ferrite and austenite is stable (the Ae 4 to Ae 3 temperature range) was smaller by more than 100 °C for Electrode 2 when compared with those of the other three electrodes. The limited Ae 4 to Ae 3 temperature range, the high ferrite factor and the chromium-nickel balance value of Electrode 2 were asso- ciated with an increase in the δ -ferrite content of the weld metal. Metallography results confirmed a significant amount of δ -ferrite in the as-welded microstructure of Electrode 2. Thermo-Calc estimates for the amount of δ -ferrite at high temperatures were supplemented by experimental anneal heat treatment on the weld metal. High-temperature anneal heat treat- ments were carried out at 1 320 °C and 1 420 °C. The amount of δ -ferrite in the high-temperature annealed and quenched samples was significantly less than predicted by Thermo- Calc property diagrams. Introduction P91 steel is a ferritic-martensitic steel in the 9% to 12%Cr family that is modified by nitrogen, niobium and vanadium additions. Modified 9Cr-1Mo (P91) steel has attractive properties, such as high creep rupture strength, good resistance to stress corrosion cracking, a low thermal expansion coefficient and high thermal conductivity, which makes it suited to long-term elevated-temperature applica- tion in power-generating plants [1].
For adequate performance of modified 9Cr-1Mo steels, the alloy design and thermomechanical processes during manufacturing should be such that a fully martensitic microstructure is achieved that is free from delta ( δ ) ferrite. The presence of δ -ferrite, even in small quantities in the final microstructure, has a detrimental effect on mechanical properties [2], especially creep rupture strength in long-term high-temperature applications [3]. Production of P91 base metal always includes austenitisation at temperatures of 1 040-1 150 °C to dissolve any retained δ -ferrite, followed by air cooling and tempering [4]. During fabrication, aus- tenitisation is not a feasible post-weld heat treatment, so δ -ferrite formed in the weld and heat-affected zone may not transform completely to austenite during subsequent cooling. It is under these circumstances that retained δ -ferrite is often observed in the final microstructure of the weld metal [5]. A strict balance between austenite- and ferrite-forming ele- ments in P91 is necessary to ensure that no δ -ferrite is present in the weld metal. The retention of δ -ferrite in the weld metal is often predicted from the chemical composition using modifications of the Schaeffler, Schneider, Kalten-hauser, and Newhouse empirical formulas [6]. To obtain weld metals free from δ -ferrite, Onoro [7] stated that a Schneider chromium equivalent (Cr eq ) value lower than 13.5 and the difference between the chromium (Cr eq ) and nickel equivalents (Ni eq ) – referred to as the ferrite factor (FF) – lower than 8 are necessary. The Schneider formulas are as follows: Cr eq = Cr+2Si+1.5Mo+5V+1.75Nb+0.75W (1) Ni eq = Ni+0.5Mn+30C+25N+0.3Cu (2) Honda et al [8] reported that having Cr eq and Ni eq lower than the proposed limits does not always prevent the formation of δ -ferrite. Roberts et al [9] suggested using the chromium–nickel balance (CNB), given by Eq. 3: CNB = Cr+6Si+4Mo+1.5W+5Nb+9Ti+11V+12Al−40C−30N−2Mn−4Ni −1Cu (3) Swindeman et al [4] reported that if the CNB is less than 10, δ -ferrite is not usually present; while for CNBs above 12, significant quantities of δ -ferrite are observed. In the current investigation, shielded metal arc welding was performed using four coated P91 electrodes from different manu- facturers. The main objective was to investigate the influence of compositional differences on δ -ferrite content in the weld metal and δ -ferrite/austenite ( δ / γ ) phase-transformation temperatures. Thermo-Calc modelling results for the amount of δ -ferrite at high temperatures were supplemented by experimental anneal heat treatment on the weld metal. The annealing was performed at a temperature range in the equilibrium phase diagram where both δ -ferrite and austenite are stable. Additionally, the annealing experiments also enabled us to study the kinetics of delta-ferrite/ austenite transformation during cooling.
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Figure 1: (a) A photograph of a sectioned weld pad on the P91 base metal; (b) Stereoscope images of the weld pad cross section at 10× magnification.
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November 2022
AFRICAN FUSION
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