| Developing high-capacity set with high efficiency and high parameter is the main trend of thermal power generation in the world today. Higher requirement on boiler steel is being brought forward for the continual increasing of steam parameter. Accordingly, the manufacture of P91 steel has epoch-making significance. Till later 90s' last decade, steam temperature parameter of power station boiler has reached 593℃and 610℃. During the 40 years from early 50s' to later 90s', the qualitative transformation occurred to power station boilers just for the application of heat-resistant steel. The progress has been made in welding techniques consequently.The time of introduction of P91 steel is comparatively short in China. And manufacture technology, in particular, welding technique is not so perfect. In theprocess of welding using P91 steel, there appear following problems—lowtoughness of weld joint, being very sensitive to cold crack, area of coverage being soften, aging tendency, appearance of IV-type crack in area of coverage.According to the characteristic of P91 steel, especially based on the analysis on its welding behaviors, a series of experiments have been completed. They are, welding property test, impact of interlayer temperature and heat treatment after welding on performance of welding joints, impact of weld line energy on the mechanical property of joint, adaptability test of domestic P91 welding rod, as well as the evaluation of welding techniques. Based on all the experiments above, the following conclusion can be summarized.(1). Welding property test.As the welding preheating temperature varies within the period of 100～150℃, P91 shows evident welding crack appearance. Between 150～200℃, the behavior of welding crack is eliminated obviously. Between 200~250℃, there is no any crack on the surfaces, interfaces and roots of the sample. That is, preheating before welding, P91 steel is beneficial to reduce the possibility of cold cracks. And the most preferential temperature is between 200～250℃. (2). The interlayer temperature.Experiments show that comparatively high interlayer temperature with in a certain range is beneficial to prevent cold crack. But excess temperature will thicken the martensitic structure, that the grains can not be detailed fined. Such the microalloy elements as Nb etc, are still solid-soluted in base mental. The toughness of welding beads are comparatively lowered because these elements can not be sufficiently separated out. Thus the reasonable technical parameter which determines the interlayer temperature is the point for ensuring toughness of beads. For P91 steel, the recommended interlayer temperature is 200～250℃. Additionally, the interlayer temperature has no obvious impact on the mechanical properties of joints.(3). Heat-treatment after welding.The immediate heat-treatment after welding will bring about such a result that partial austenites have not finished the transition to martensites, and that the components composed of martensites and austenites are formed. Although the left austenites will transit to martensite structure successively in the following process of heat-treatment, the toughness of welding bead mental is inevitably reduced.If the welded steel is cooled to 100 centigrade firstly, all the austenites can transit to martensites. And the following reheating at the temperature 350±50℃for about 2h has two advantages. One is that hydrogen can sufficiently diffuse and superfuse, so that the hydrogen-deduced delayed cracks can be avoided as the bead mental is cooled to room temperature. The other is that the area of heat-coverage and bead structures can be malleableized. At last, a process of tempering heat-treatment is designed at the temperature between 730～750℃to obtain martenite structures with fine beads properties.It is also shown in the experiments that the martenites can be sufficiently tempered as could as possible and the toughness of the beads can be promoted by means of increasing the tempering temperature and prolonging the tempering time. But it has to be pointed out that the tempering temperature can not more than the Acl temperature of the bead mental as it rises. Also, the economic cost should be acceptable.To summarize, welded P91 steel is not suitable to immediate heating. The proper process is cooling to 100℃at first, reheating secondly, cooling to room temperature after that, tempering at a high temperature between 750～770℃.(4). The energy of weld lines.The energies of weld lines have no evident impact on strength of extension Rm, yield strength Re and elongation ration after truncation A of the P91 weld joints, whereas it influnces the impact toughness obviously.A lower line energy is beneficial to decrease the size of grain of the joint structures and to decrease the width of martenite panels. As a result, the mechanical properties of joints can be improved.In this experience, the lower line energies are always controlled in the scope from 16～22kJ/cm. The thickness of weld layers is no more than the diameter and the width of weld passage no more than 3 times of diameter. (5). The adaptability of domestic welding rods and the evaluation of welding techniques.The domestic welding rods can be substitution of imported rods as the mechanical properties of welding joints fulfilling the demands which can be realized as the appropriate techniques are adopted.In this experiments, TIG-R71 sticks and R717 rods are elected as the samples. The preheating temperature is 200～250℃, and the process of de-hydrogenation lasted about 2h at 350±20℃. The heating speed of tempering is required no more than 150 centigrade per hour over 300℃. The speed of tempering cool is required slow cooling to 300℃, after that air-cooling. Heating of tempering is at the temperature 760±10℃, and the length of thermal retardation is about 5～5.5h.With the introduction and generalization of P91 steel, we have obtained a series of fabrication techniques as combining practical production and the applicable technology of the overseas trade. All the experimental results such as welding, heating treatment techniques, etc., have been applied successfully in Longxing Power Equipment Factory of Changchun.
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