In the USA in the early 1980s, tubing of P91 (9Cr-1Mo) steel was introduced into the superheaters of power boilers as a replacement for 300 series austenitic stainless steels [1]. P91 steel is a high strength alloy in which austenite nor- mally transforms completely to marten- site during air cooling [2]. Cr-Mo ferritic steels are widely used in the fabrication of steam pipes in fossil, steam generator materials, and nuclear power plants due to their adequate mechanical strength at service temperatures, excellent medium temperature strength as well as good oxidation resistance, and easy control of microstructure through heat treatment [3-6]. The 9Cr-1Mo steel is one of the best alloys in the family of Cr-Mo steels for elevated temperature application. With the ever-increasing demand for high effi- ciency in the power generation industry, the operation temperature is also steadi- ly increasing. The modified 9Cr-1Mo steel is a relatively new structural alloy that was originally developed for use in steam generators for the advanced nu- clear and power generation industries [7]. The use of modified 9Cr-1Mo steel is found not only to reduce the total weight of steam pipes, but also to be cost effec- tive in construction [8 - 9]. In this appli- cation, welding is one of the most com- monly used joining techniques. There- fore, weldability and the quality of welds in 9Cr-1Mo steels have attracted exten- sive attention during steel development. The behavior of a welded joint de- pends upon the properties of the base metal, heat affected zone, and weld met- al. For the latter, because of the signifi- cant microstructural changes associated with melting and rapid cooling, the weld properties are generally different from those of the base metal. The microstruc- ture of weld metal and its relation with properties is important to the devel- opment and application of modified 9Cr-1Mo steels [10]. MP 324 MATERIALS TESTING IN JOINING TECHNOLOGY An examination of the mechanical properties of dissimilar steel welds © Carl Hanser Verlag, München MP Materialprüfung Jahrg. 47 (2005) 6 A.Volkan Uygun, Ali Durmus ¸, Ali Bayram and Sedat Ülkü, Bursa, Turkey Microstructure, hardness, tensile, three-point bending and fracto- graphy properties were investigated for the following dissimilar steel welds: P91-P22 (W1), P91-P11 (W2), and P91-P1 (W3). These mate- rials were first joined at the root by gas tungsten arc welding; the second, third, and fourth passes were deposited manually by shield- ed metal arc welding. After welding, these pairs were tempered 760, 735, and 700°C for 1, 2, and 3 hours, respectively (i.e. T1, T2, and T3). The highest hardness values were revealed in the heat-affected zone of P22 for the W1-T2 pair. The lowest hardness was observed in the base metal-P91 for the W2-T2 and W2-T3 pairs. Fracture failure in tensile tests occurred in base metals-P22, P11, and P1. In a few samples, failure took place during three-point bending tests. Cracks which formed as a result of bend testing were within tolerance limits. All weld regions were martensitic, with an acicular structure. The base metal P91 steel revealed a martensitic structure, whereas P22, P11, and P1 base metals had a ferrite-pearlite structure. Cr Mo C Si Mn P S V Nb P91 8.63 0.91 0.052 0.35 0.52 0.008 0.004 0.16 0.072 P22 2.3 1.08 0.08 0.45 0.52 0.009 0.006 – – P11 1.2 0.56 0.09 0.64 0.54 0.011 0.005 – - P1 - 0.52 0.13 0.38 0.47 0.010 0.007 – – ER 90S-B3 2.2 1.0 0.07 0.3 0.6 – – – – ER 9018-B3 2.7 1.1 0.1 0.6 1.0 – – – – Table 1. Chemical composition of the materials investigated and the electrodes used in the present study (wt.%)