High pressure pipe fittings: P91 steel pipe welding process

　The choice of 
　　P91 steel preheating temperature Ms point of approximately 400 ° C, the preheating temperature 200 to 250 ° C general election. The preheating temperature is not too high, otherwise the connector cooling rate is reduced, may cause the grain boundary carbide precipitation and the formation of the ferrite structure of the welded joint, thereby greatly reducing the impact toughness of the welded joints of steel at room temperature. The lower limit of the preheating temperature can be obtained from the the implant test of Harbin Boiler Factory did a very good description. 

　　Bolt test bars using P91 steel, diameter 8 mm deep 0.5 mm, base plate 13CrMo steel, thickness 20 mm, the test without preheating, preheat to 150 ° C preheated 200 ° C preheated 250 ° C conditions. Electrode using J707. Welding current of 165 to 170 A, the arc voltage 21 - 267 V, the test results are shown in Table 2. 
　　Table 2 P91 latch test results 
　　test 
　　conditions the sample 
　　No. stress level 
　　/ MPa rupture time 
　　/ min 
　　without preheating 1 303.8 99 
　　2 186 8 237 
　　3 176.4 8.3 1440 is not broken 
　　preheated to 150 ° C 4 421.4 8.1 1260 
　　354.8 120 off 
　　pre- a heat of 200 ° C for 6 465.2 8.6 1440 is not broken 
　　7 482.7 　　preheated 250 ℃ 9 539 8.2 1440 8.1 438 
　　8539 7.9 313 　　10 600 8.0 1440 
is not broken 
off. 
　　known by the above test results, without preheating conditions, P91 steel welded joint critical stress to 176.4 MPa; preheated to 150 ° C, the critical stress is 354.8 MPa, 85.4% of the yield limit of 415 MPa for the P91 steel at room temperature; preheated above 200 ° C, the critical stress greater than 460 MPa, more than P91 steel at room temperature to yield limit. Thus, in order to avoid cold cracking is generated when the P91 steel welding, preheating temperature must not be lower than 200 ° C, Germany, the preheating temperature of 180 ° C to 250 ° C, CE provides a preheat temperature of 120 ~ 205 ° C. 
　　4.2 interlayer temperature selection 
　　layer temperature not lower than the lower limit of the preheating temperature, but as the selection of preheating temperature, interpass temperature is not too high. P91 welding layer temperature is controlled at 200 to 300 ° C. France provides: the layer temperature does not exceed 300 ° C. The provisions of the United States: and interpass temperature located between 170 to 230 ° C.

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